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Light conversion, S/N characteristicsof x-ray phosphor screens
Item Type text; Thesis-Reproduction (electronic)
Authors Lum, Byron Kwai Chinn
Publisher The University of Arizona.
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LIGHT CONVERSION, S/N CHARACTERISTICS
OF X-RAY PHOSPHOR SCREENS
by
Byron Kwai Chinn Lum
A Thesis Submitted To the Committee on
COMMITTEE ON OPTICAL SCIENCES (GRADUATE)
In Partial Fulfillment of the Requirements for the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
1 9 8 0
STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. ,
Brief quotations from this thesis are allowable without special
SIGNED: ' K . C . / -
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
OltiUV I 4-HANS ROEHRIG DateAdjunct Associate Professor
of Radiology
ACKNOWLEDGMENTS
I would like to express my deep appreciation to my advisor.
Dr. Hans Roehrig, whose guidance and patience made this thesis
possible. I am also very grateful for the efforts of Ms. Betty
Porter and Ms. Delia Bryant in the preparation of the final copy
of this thesis.
This work was sponsored under the project "Evaluation of
PEID Systems for Radiology", awarded through the Bureau of
Radiological Health, Food, and Drug Administration under Grant
No. 5R01FD00804-04RAD.
TABLE OF CONTENTS
PageLIST OF ILLUSTRATIONS...................................... v
LIST OF TABLES .......... viii
■ A B S T R A C T ....................... . . . ix
1. INTRODUCTION ...................................... 1
Justification .......................................... 2Physical Processes of X-ray Induced Emission ......... 9Characteristic X-ray Reabsorption................ . . . 14Properties of New and Traditional X-ray Phosphors . . . 20
2 NOISE ...................... . 26
Introduction .................................... 26Statistics of Screen Amplification ......... 34Scintillation D Q E .............. 38Simulation of Screen Statistical Processes . 40Some Conclusions . . . . . . . . . . . . 44
3. EXPERIMENTAL PROCEDURE AND S E T U P ........................... 45
Introduction................... 45The Photomultiplier . . . . . . . . .................... 47The X-ray Source ..................... 35Electronics............. ... 36.Computer Analysis .............. 38
4. RESULTS AND CONCLUSIONS......................... 39
Results................. ... ........................... 39Analysis .......... 72Conclusions .................. . . . 79
REFERENCES............ ........ .................... .. 81
iv
LIST OF ILLUSTRATIONS
Figure Page1. Screen-film combination (Messier 1973) 3
2. Theoretical and experimental values for radiographic noise;Experimental-theoretical (Rossmann, 1962) . ............. . 6
3. PEID, using fluorescent screen optically coupled tointensified video camera tube ................................ 7
4. Schematic of x-ray intensifier video camera system . . . . . 7
5. Cross section for interaction in a calcium tungstate(CaWO^) screen (Vybomy, 1978) . .......... 11
6 . X-ray absorption processes in a Csl phosphor and the resulting absorbed energy spectrum for monochromaticx-rays (Swank, 1973) . . . .................................. 12
7. Inner-electron transitions for characteristic x-rayemission (Weidner and Sells, 1973) . ........ 15
8. Characteristic x-ray spectrum for a thulium (Tm)secondary t a r g e t ........................................ 15
9. X-ray attenuation as a function of energy for a Baphosphor (Vybomy, 1 9 7 8 ) ........................... ........ 16
10. Characteristic x-ray re absorption diagram (Vybomy, 1978) . . 16
11. CsBr:(Tl)1, CsI:Na(2), ZnCdS:Ag(3), and CsI:Tl(4) ......... 22
12. BaS04 :Eu2+ (5), BaFCl:Eu2+ (6), and CaW04(7) . . ............. 22
13. Gd„(LS:Tb(8) , La„CLS:Tb(9), and Y 0„S :Tb(10) .(Stevens, 1975) 7 7 .............. 23
14. Radiographic mottle (Shaw, 1976) 27
v
viLIST OF ILLUSTRATIONS--Continued
Figure Page
15. Components of radiographic density fluctuations(Rossmann, 1962) . 27.
16. Spatial Frequency content of noise in radiographicimages (Rossmann, 1962). . . . . . . . . . . . . .... . . . 29
17. Random fluctuations in density (SPSE Handbook ofPhotographic Science and Engineering, 1 9 7 3 ) . . . . . . . . . 30
18. Processes in the variation of x-ray screen scintillations. . 32
19. Schematic for the serial combination of two statisticaldevices (RCA Photomultiplier Handbook, 1970) . . . . . . . . 34
20. Simulation screen probability distributions . . . . . . . . 41
21. Simulation results for Fig. 20(a), P^ - P^ = 0.5 . . . . . . 42
22. Simulation results for Fig. 20(a), P^ - 1/4, P^ = 3/4 . . . 42
23. Simulation results for Fig. 20 . = . 43
24. Block Diagram of U of A evaluation facility . . . . . . . . 46
25. Schematic of PMT pulse counting method . . . . . . . . . . . 46
26. RCA 8850 photomultiplier pulse height spectrum . . . . . . . . 48
27. Photomultiplier output corresponding to a Poissondistribution . . . . . . . . . . . . . . . . 49
28. Phosphor screen output decay characteristics . . . . . . . . 50
29. Decay time constant for a CaWO^ screen . . . . . . . . . . . 50
30. Calibration of photomultiplier counting efficiency . . . . . .51
31. Counting efficiency of the system . . . . . 53
32. Variable energy x-ray source . . . . . . . . . . . 55
33. System electronics . . . . . . . . . . . . . . . . . . . . 57
viiLIST OF ILLUSTRATIONS--Continued
Figure Page
34. Measured probability distributions (P ) for a ZnCdS screens:Cd K-edge: 26.7 keV . . . . . . . . f . . . . . . . . . . . . 60
35. Measured probability distributions (P ) for a CaWO. screen;W k-edge = 69.5 k e V ............... f .............. 61
36. Measured probability distributions (P ) for a BaSO. screen;Ba K-edge: 37.5 k e V ...............7 .............. 62
37. Measured probability distributions (P ) for a La.O S-Gd^O Sscreen; La k-edge: 38.9 keV Gd k-edge: 50.2 k e V ............. 63
38. Measured probability distributions for a Csl x-ray imageintensifier; 1 k-edge: 33.2 keV, Cs k-edge: 35.9 keV . . . . 64
39. Bremstrahlung spectrums for different amounts offiltrations.......... 65
■40. Measured output emissions for a CaWO^ screen ................. 66
41. Measured output emissions for a BaSO^ screen ................. 67
42. Measured output emissions for a series of ZnCdS screens . . . 6 8
43. Measured output parameters for a La^OgS-GdgO^S screen........... 69
44. Measured output parameters for a Csl x-ray imageintensifier .......... 70
45. Measured signal to noise ratios from the output of Csl x-ray image intensifier experimental values below 10 absorbed x-ray photons in error due to the fact that the threshold counter did not sample continuously, but was triggered by signalpulses ............................. 73
46. Measured signal to noise ratios from the output of a CaWO^screen; 44 keV incident x-ray photons ............... . . . . 74
47. Csl image intensifier output distributions . . ............... 77
48. CaWO^ screen output distributions . . . . . ' ..................... 78
LIST OF TABLES
Table Page1. Probability of barium k reabsorption in a pair of barium
strontium sulfate screens versus incident x-ray energy . . . . 19
2. Reabsorption probabilities forthe K x-rays emitted by theprincipla phosphor elements in the screens studied .......... 19
3. Basic properties of phosphors ................................ 21
4. X-ray source characteristics .................................. 53
5. Measured output efficiencies of some phosphor screens . . . . 71
viii
ABSTRACT
The variations in gain or amplification are measured for a
variety of x-ray phosphor screens and for a Csl image intensifier as
a function of incident x-ray energy. These variations result in a
reduction of the output SNR (signal to noise ratio) by a factor of
/DQEsc^nt• The scintillation detective quantum efficiency, DQ^scint'
is evaluated theoretically and experimental results are presented.
The findings show that the newer rare earth phosphor screens possess
a higher gain than do the traditional calcium tungstate (CaWO^)
screens and that the values for DQEsc^nt, do not vary considerably
for a different phosphor materials.
CHAPTER 1
INTRODUCTION
Since the incident x-ray photons follow Poisson statistics, the
input SNR (signal to noise ratio) of an x-ray imaging system is readily
known. However, for systems in which x-ray intensifying phosphor
screens are utilized, the output SNR following screen amplification
will be degraded due to variations in the amplification or gain of the
phosphor screen. In this thesis, the average gain and the associated
variations for various phosphor materials are measured and its effect
on the SNR is investigated theoretically and experimentally. The
experimental measurements are done with a photon counting system
with which individual x-ray absorption events may be detected and
analyzed.
The remainder of this chapter will concern itself with the
justification for this study and with the basic concepts of x-ray
induced fluorescence. Also included at the end of the chapter is a
brief run-down of the various phosphor materials that are, or will be
available for diagnostic radiology. Chapter 2 deals with the theo
retical analysis of the noise that is associated with x-ray intensify
ing screens and includes a simulation study of the screen's statistical
processes. The term "scintillation detective quantum efficiency,"
DQEscint’ introduced and formally defined. In Chapter 3, a detailed
description of the photon counting system is given, where the principal
components are the x-ray source, photomultiplier, electronics, and
the digital computer. Finally, the experimental results are presented
in Chapter 4 and comparisons are made between the theory and these
results.
Justification
For the past fifty years medical radiology has consisted of the
transmission of ah x-ray beam through the patient and the recording of
this beam on a screen-film combination. A screen-film combination is
basically a sheet of film sandwiched between two fluorescent intensi
fying screens (Fig. 1). The phosphor in the screens has been tra
ditionally CaWO^, which emits UV and visible light photons with the
absorption of a single x-ray photon. The phosphor screens thus pro
vides a gain mechanism in the process. Quite naturally, in the
interest of the reduction of patient dosage, one would want the
screens to possess good x-ray absorption characteristics and high
light output. Recent years have seen the advent of new rare-earth
screens which have relatively higher x-ray absorption capabilities
and better x-ray to light photon conversion ratios.
However, a higher signal does not necessarily imply a better
image. What are also important are the signal to noise relation
ship and the spatial resolution capability. For instance, if the
larger signal is characterized with even larger fluctuations, and if
detail in the image is smoothened out as a result of this gain in
signal, then the effort would seem hardly worthwhile. Past papers
3Screen Base
deflective Coating X-Ray Excited Phosphor
Protective Coating
‘X-Ray Film Base
lulsion
W W W J M i M M
Fig. 1. Screen-film combination (Messier 1973).
(Cleare et al., 1962; Rossmann, 1962; Rao and Fatouras, 1979) have
given some considerations to the noise aspect of phosphor screens,
but these studies seem to totally ignore the physical characteristics
of the intensifying screens. For instance, they do not consider the
various interactions that may occur between the incident x-ray photons
and the screen material. These interactions may cause variations in the
scintillations of a screen and thus affect the noise characteristics.
One paper of particular interest (Rossmann, 1962) deals with
the comparison of theoretically and experimentally determined noise
characteristics of a particular x-ray radiograph. In the derivation
of the theoretical expression, Rossmann first assumes that the ex
posure E determined by the number of x-rays absorbed by the screen
in a given area in a given time is n^a, where a is the film area of
interest and n is the number absorbed per unit area. If the fluc- xtnations in this number is Poisson limited, then the standard
deviation would be given by:
ox = ^ - i CD
In order to relate this quantity to the density fluctuations in the
film, one must consider the characteristic curve of the film derived
from a plot of the density D versus the logarithim of the exposure
E, i.e.
D = Y log10E + C (2)
where y is called the film gamma and is the gradient of the H and
D curve and C is the density due to fog and base. Differentiating,
we obtain
AE ' 0xAD = 0.43 y — - . = 0.43y ----
E n a (3)x
Through substitution and taking into account film granularity, the total
density fluctuation is
(0.43a)2'^a(D)total = {a (D)grain + --------- ^ (4)
nxa
What now remains is to take into consideration the modulation transfer
function (MTF) of the screen-film combination and of the scanning aper
ture. If we assume that the scanning aperture is circular, its MTF is
given by
where d is the diameter of the aperture and is a first order Bessel
function. Denoting the MTF of the screen-film combination as A (v),
the measured Wiener spectrum of the mottle is given by:
W t l e M = | A # ( v )|2 (6)
where K is a constant. Assuming that the MTF of the screen-film system
is of the form
A#(v) = exp(-27r^p^v) (7)
and since
o2(D) = ^mottle^Vx^Vy (8)
we have as the total density fluctuation
2 (0.43y)2F2 ,total - {" (D1 grain + ( ’
nxawhere F i s a function that varies between zero and one. Figure 2 shows
the comparison between the calculated fluctuations and the measured
fluctuations. The film was Kodak Blue Brand Medical x-ray film with
Kodak fine-grain screens. The tube voltage was 80 KVp with a % mm Cu
filter at the tube.
It is clear that the correlation between theory and experiment
is quite good, and one can conclude, therefore, that the noise is
essentially Poisson limited, assuming the film granularity is negli
gible. . Note that no considerations were made of the screen character
istics nor of the beam quality. A recent study done by Rao
6
.01
.005e>
800 1200
d (microns)Fig. 2. Theoretical and experimental values for radiographic noise;
Experimental-Theoretical (Rossmann, 1962).and Fatouras (1979) reveals, however, that there are some notable
discrepancies between theoretical and experimental values, contrary
to the conclusions. These results suggest that other secondary
sources of noise, such as gain variations with the phosphor screen,
may contribute to the final noise output.Of recent interest to diagnostic radiology has been the
application of photoelectronic imaging devices (Nudelman, 1976,
Beckmann, 1978). In order to evaluate the performance of these
devices, in particular the signal to noise ratio, one has to con
sider the gain and losses of the signal as it works its way through
the system. For instance. Figure 3 shows an intensifying screen
optically coupled directly to a video camera tube. In Figure 4,
the output from an x-ray image intensifier is optically coupled to
the camera. In calculating the signal incident upon the camera,
for both cases, one takes into account the gain of the image
CRTDISPLAYI M T f H S IF IC D
CAMERA TUUEAMPLIFIERX-RAY
SOURCE
Fig. 3. PEID, using fluorescent screen optically coupled to intensified video camera tube.
m: i Ki n i.A ionCs_Sb Re
P720«A»
t IS AVI: MAK
Mill. IHINIiASI; NISISIIW lAI'ACITAW.’ti
UISINIINIIHIN
SNR = /n .P.m2A .tc D( Escint
Fig. 4. Schematic of x-ray intensifier video camera system.
intensifier, the transmission of the optics, and the numerical aperture
of the optics. If the variations in the gain of the image intensifier
is taken into account, then the SNR at the output of the video camera
[Fig. 4) can be shown to be approximately:
2 DQE h(SNR)VID “ (NxPxM Vf ' ----— ) (10)
2where = x-ray photon flux (photons/cm - sec)
n^ = x=ray image intensifier quantum absorption efficiency
m = overall system magnification
Ag = video camera tube pixel area
tj. = video camera tube frame time
a = factor associated with secondary electron emission
due to the landing beam
DQEscint = scintillation detective quantum efficiency
The quantity DQE^^^. will be defined in the following chapter, but for
the present, it would suffice to say that it is a measure of the vari
ation in the intensifier1s gain. Furthermore, if the collection effi
ciency of the optics is very low, then the gain of the intensifier or of
the screen must be high enough so that there may be a detectable signal
for the camera. This same principle can be applied to a simple radio-
graphic screen-film system, in that the gain of the intensifying screens
must be high enough to account for the low quantum efficiency of the
film.
Presently, much attention has been given to the subject of
human utilization of the information present in the radiographic image
(Wagner, 1977). Quantitative characteristics of the imaging system
such as resolution, gain, noise, etc. are needed to be incorporated
with the human-eye brain system in order that the human response to
the final image may be predicted.
From the above discussion, it is not difficult to realize the
importance in the evaluation of radiographic imaging systems. A
system that may actually "count" the number of light photons emitted
by a particular device would be highly desirable in achieving this end.
It would be externely useful for the determination, of the gain of an
x-ray image intensifier or of a specific phosphor screen, notably the
"brighter" rare-earth screens as compared to the CaWO^ screens. A
photon counting technique is also very desirable for a signal to noise
evaluation since it permits an accurate measurement of signal fluc
tuations in terms of photon quanta. This method is highly suitable
for Poisson processes which deal with discrete variables.
Physical Processes of X-Ray Induced Emission
The gain of an x-ray phosphor screen consists of the conversion
of a single highly energetic x-ray photon into several hundred to sev
eral thousand less energetic light photons. Thus, in the case of a
screen-film system, many silver halide grains are made developable in
comparison to only one or two if the x-ray photon was to be absorbed
directly by the film. The phosphor screen also offers two advantages
over film; its ingredients offer a larger mass absorption coefficient
that silver and it is thicker than film, thus providing better
10
absorption efficiency. These two factors of phosphor gain and better
quantum efficiency combined can result in a gain of approximately 50
when considering a double film-screen system.
For the standard medical x-ray energy ranges, an x-ray beam is
attenuated by the phosphor screen through three interactions; 1) photo
electric absorption, 2) Compton or incoherent scatter interaction, and
3) Rayleigh or coherent scatter interaction. The cross sections for
these processes are shown in Figure 5, indicating that the dominant
interaction is the photoelectric effect. Intensifying screens are
usually comprised of inorganic phosphor crystals that contain elements
with high atomic numbers. As a result, an x-ray absorption causes
the ejection of an electron from the K or L shell of the host atom.
The kinetic energy of the electron is the difference between the
absorbed photon energy and the binding energy of the atom. These
energetic electrons> through inelastic collisions, ionize other host
atoms, thus producing secondary electron-hole pairs. In addition,
the initially excited host atom, which now has K or L shell vacancies,
will eventually relax, resulting in the emission of secondary, less
energetic, x-ray photons or Auger electrons. These may then be
absorbed by the phosphor medium, thus contributing to the electron-
hole pair production. In the final step of the process, the second
ary electrons excite still other host atoms or activator impurities
into higher energy states. These excited atoms eventually decay,
resulting in light fluorescence.
11
CaWO
10*
worocLEcrmc
20 30 *0 30 60photon energy (keV) so 90 too
Fig. 5. Cross section for interaction in a calcium tungstate (CaWO ) screen (Vyborny, 1978).
The energy conversion efficiency, as one would expect, is by no
means 100%. As mentioned above, the atom with a K-shell vacancy will
produce a characteristic K-shell x-ray or an Auger electron. The
Auger electron is most often reabsorbed, but the K x-ray has a
relatively high probability of escape from the screen and thus does not
contribute to the fluorescence process. This particular facet will be
discussed in further detail later since it also plays a major role
in the detective quantum efficiency of phosphor screens. A diagram
illustrating these various processes is shown in Figure 6 .
12
INCIDENT X-RAYS
CD
<
N(E)
SHELLLM...
SHELLSLK. . .
SHELLSSHELL
Fig. 6 . X-ray absorption processes in a Csl phosphor and the resulting absorbed energy spectrum for monochromatic x-rays (Swank, 1973).
13
Another mechanism that can result in lost energy involves the
production of photons. Present literature (Kingsley, 1975) states that
lattice vibrations contribute very little to energy loss until the
secondary electron-hole pairs have energies of about 10 to 15 eV.
A generally accepted empirical rule states that it takes on the
average an amount of energy that is three times the bandgap energy
in order for a single electron-hole pair to be created. As a result,
2/3 of the available energy is converted into heat (Kingsley, 1975).
Three other sources contribute to reduced energy conversion.
First an electron emission reduces the efficiency of electron-hole pair
production, and secondly, the final luminescence process has an
efficiency of about 90%. The third source stems from the fact that
the photons must travel through the screen before it can be emitted.
Approximately half of the photons can be lost in this final process.
The above processes can be exemplified by considering the case
of a zinc sulfide (ZnCdS) screen, which has a bindgap energy of 3.3 eV.
Taking into account losses through lattice vibrations, it would require
approximately 10 eV to create a single electron-hole pair. If a 60 keV
x-ray photon is absorbed, then about 6000 electron-hole pairs are pro
duced. Since the luminescence efficiency is about 90%, this would result
in about 5400 light photons. If the impurity activator is silver,
then the emitted photons would have an energy of = 2.8 eV, which is
in the blue region of the visible spectrum. The energy of conversion
efficiency is therefore in the order of:
5400 x 2.8 eV _ _ 0 60 keV
The highest intrinsic efficiency found for any phosphor is in the
neighborhood of this value (Kingsley, 1975).
Characteristic X-Ray Reabsorption
When the absorption of an x-ray photon occurs above the k-edge
of the phosphor element, approximately 90% of the excited phosphor
atoms decay radiatively (Vybomy et al., 1978). This radiative decay
results in the production of Ky and Kg x-rays whose energies are lower
than that of the incident x-ray energy. This process is illustrated
in Figure 7, with the resultant characteristic spectrum in Figure 8.
Since the energies of the k x-rays are slightly below the k-edge
of the phosphor element, as illustrated in Figure 9 for the case of
a Ba phosphor, many may escape the screen entirely due to the low
absorption coefficient.
Theoretical calculations for the probability of k-escape have
been done by Vybomy et al. (1978) and the analytical derivations are
shown in the following discussion. We first divide the screen into
N slices and the total solid angle subtended from the center of the
slice into R pieces. For a given nth slice as shown in Figure 10,
the average mean free path for a characteristic x-ray emitted within
the rth solid angle is:W
150N
M
L
K
Fig. 7. Inner-electron transitions for characteristic x-ray emission (Weidner and Sells, 1973).
Counts
J
49.8 50.7 57 5 X-Ray Energy(keV)
Fig. 8. Characteristic x-ray spectrum for a thulium (Tm) secondary target.
16
1.00.90.80.70
.1020 30 40 50 60 70 80 90100
PHOTON ENERGY (keV )
Fig. 9. X-ray attenuation as a function of energy for a Ba phosphor (Vyborny, 1978).
FRONTSCREEN
Fig. 10. Characteristic x-ray reabsorption diagram (Vyborny, 1978)
coser
where cose^ is the average value of cos8^ within this particular solid
angle, expressed as\ 2 cos0sin6d03rl (12)5r2 sinOde
-°rl
Assuming that scatter is of no consequence in the reabsorption
process, then the characteristic absorption probability P will bean, rgiven by
Pa,n,r = {1"exP I " (Pa/p) (N-y Wp/Ncos0r | } (13)
where is the absorption coefficient of the material for an x-ray
energy of and p is the density of the screen material. The average
probability for the entire slice is then obtained through summing
over the R solid angles and dividing by R,
Rp = _ i y p (14)a,n R a,n,rr=l
These probabilities must now be weighted by the probability ,
which is the relative probability of having a k emission in the nth
slice for an incident x-ray energy of E . is simply the number
of photons incident upon the nth slice multiplied by the total number
of absorbed in that particular slice and finally divided by the total attenuation of the screen:
18exp{- (n-1) | W p ( ( y T+y T) / p ) |/N}{l-exp| - ( ( y +y ) / p ) W p / N | }
G - ----;------------- — ----------------- (15){1-exp| - ( ( y I + y ^ I ) / p ) W p | }
Note that the above expression takes into account incoherent and
coherent scatter. The total characteristic probability of k-reabsorp-
tion is thus given by:
Np T = y g Tpal L . nl an n=l
N ( 1 6 )
P6I = L l ^ S "
Vybomy has carried out the calculations for various phosphor screens,
including ones that contain two principal absorption edges. Their
results are shown in Tables 1 and 2. One conclusion that can be made
is that the reabsorption probability is independent of incident x-ray
energy. Secondly, the probabilities range in values from - 0.2 to
0.6, indicating that a significant portion of the incident energy is
lost. Besides reducing the energy conversion efficiency, this partic
ular phenomenon also plays a large role in the signal to noise
considerations of x-ray screen systems. This topic will be covered
in more detail in the following chapter. However, since many of the
interactions involve the production of k x-rays, a large portion of
the energy absorbed by the screen is through k-reabsorption whenever
the incident energy is above the k-edge of the phosphor.
\19
Table 1. Probability of barium K reabsorption in a pair of Barium Strontium Sulfate screens versus incident x-ray energy.
Incident x-ray energy, keV X-Omatic Regular
37.4 0.49540 0.49750 0.50060 0.50080 0.501
100 0.501
Table 2. Reabsorption probabilities for the K x-rays emitted by the principal Phosphor elements in the screens studied.
Screen Pal P61 Pa2 VPar 0.21 0.16Hi-Plus 0.32 0.24Lightning-Plus 0.40 ,0.32X-Omatic Regular 0.50 0.40Alpha-4 0.48 0.37 0.40 0.32Lanex-Regular 0.64 0.54 0.53 0.44
20Properties of New and Traditional X-Ray Phosphors
In recent years, research on phosphor screen development has
brought forth a large number of new rare-earth phosphors whose
characteristics appear to be an improvement over the traditional
CaWO^ and ZnCdS phosphors. Table 3 lists the basic properties of
these phosphors, while Figures 11-13 show their emission spectra.
The term "luminescent radiant efficiency" is simply the ratio
of the energy emitted by the screen to the energy absorbed by the
screen. One can immediately see that the k-edges of these
materials are at lower energies than the tungsten k-edge. This latter
characteristic can be shown to result in higher absorption coeffi
cients for the range of x-ray energies' in diagnostic radiology.
Also note that the emission spectra of some of the newly developed
screens are different from that of CaWO^, being shifted towards the
green portion of the spectrum. This may pose some problems in terms
of detector adaption. The following paragraphs will provide some
brief comments on each of the above phosphors.
CaWO^ (Coltman, 1947) is the traditional phosphor used in
x-ray intensifying screens and has well established manufacturing
procedures and properties. The other traditional phosphor, ZnCdS: Ag
(Ludwig and Prener, 1972) is used mainly as a direct viewing (fluoro-
scope application) screen and as the input and output phosphor of
image intensifiers.
Table 3. Basic properties of phosphors
PHOSPHOR *Z n**
BaFCL:Eu2+ 56 13BaSo^:Eu2+ 56 6CaW04 74 3.5CsBr:T1 35/55 8Csl:Na 53/55 10Csl:T1 53/55 11Gd202S:Tb 64 15La202S:Tb 57 12Y202S:Tb 39 18ZnCdS:Ag 30/40 18
*Z = atomic no. of principal absorber**n = luminescent radiant efficiency (%)
22
100
80
60
40
f »Jo300 400 500 600 700
A(nm)———♦
Fig. 11. CsBr:(Tl) 1, Csl:Na(2), ZnCdS:Ag(3), and Csl:T1(4).
oo80
40
300 350 400 450 500
Fig. 12. BaS04 :Eu2+(5), BaFCl:Eu2+(6), and CaW04(7).
23
toor5 o
8
lii.. 400 500 600 700A(nm) ►
100
♦ 509
— ... \ ,I i 1400 500 600 700A(nm)— »
XX)
50
500 600 700
Fig. 13. Gd O S:Tb(8), La O S:Tb(9), and Y 0 (Sfevens, 1975).
2S:Tb(10)
242BaFCLrEu is being considered as an intensifying screen due to
its higher conversion efficiency and absorption capabilities. It has2 2 however an "afterglow" drawback. BaSO^Eu is similar to BaFCL:Eu ,
except it has a lower conversion efficiency. Both of these screens
possess the advantage of having their emission spectra towards the
blue, which is compatible with the traditional blue-sensitive films
(Messier and Wolfe).
Cdl: Na and Csl: T1 have been used in replacing ZnCdS: Ag
as the input phosphor of image intensifier tubes (Bates, 1969), due
mainly to the element's higher mass absorption coefficients and
density. This property is very important since the input phosphor
thickness of intensifiers are typically on the order of only 10
mils (Ludwig and Prener, 1972).
Gd202S:Tb (Buchanan, 1972) is a screen being proposed as the
input screen for image intensifiers, as an intensifying screen in
filmscreen combinations, and as a direct viewing screen. However,
the densities of Gd202:Tb screens are very low and thus possess the
same disadvantage as ZnCdS:Ag screens, Its other disadvantage stems
from its emission spectrum which is at around 550-600 nm, making the
coupling to traditional blue sensitive films difficult. La.202S:Tb
has the prospect of being an intensifying screen and was at one time
a contender for the input screen of x-ray intensifiers. Its emission
spectrum is similar to that of Gd202:Tb. Two other practical screens
however, which have emission spectra in the blue are Gd202Br
and La.202Br.
Despite its relatively high conversion efficiency, YgOgSzTb
is a rather poor absorber when compared to Gd^OgSzTb and La^OgBr.
However, its emission can be shifted from green to blue, and thus may
be used with standard x-ray film. For these reasons, ^C^StTb has
the potential of becoming an intensifying screen for a film-screen
combination (Alves, Buchanan, 1973).
CHAPTER 2
NOISE
Introduction
If one were to examine a uniformly exposed radiographic
image, it would be fairly easy to notice the granularity or mottle
present (Fig. 14). A first logical choice as for the cause of these
density fluctuations might be film granularity, but experiments show
that this plays a relatively minor role (Cleare et al., 1962).
Images that were obtained with varying distances between the screen
and the film show that the mottle pattern changes dramatically. Only
when the screen-film distance is very large do the high spatial
frequency fluctuations that are associated with film granularity
become dominant. A second choice might be the structural inhomo
geneities in the phosphor coating of the screens, which is appro
priately termed "structure mottle." Two identical successive x-ray
exposures were made on the same area of an intensifying screen in
hopes of finding some correlation between the two images. However,
no such correlation was ever found, suggesting that structure mottle
also plays a very minor role.
In view of the above findings, one might safely conclude that
the density fluctuations of the film are primarily associated with
the statistical distribution of x-ray photons that are absorbed by
26
27
Fig. 14. Radiographic mottle (Shaw, 1976).
the screen. This third component of the radiographic mottle is referred
to as "quantum mottle." Quantum mottle easily explains why the mottled
appearance decreases as one goes from a fast film to a slow film.
The slower film would require more absorbed x-ray photons in order
to achieve the desired density, thus increasing the signal to noise
ratio in the image. Similarly, screens with higher intrinsic con
version efficiencies would theoretically increase radiographic mottle.
Figure 15 is an illustration of the above discussion.
Radiographic mottle
Screen mottle
Quantum mottle Structure mot 11e grain!ness
Fig. 15. Components of radiographic density fluctuations (Rossmann, 1962).
28However, in terms of the "appearance" of the mottle, purely
statistical considerations cannot account for the results of some
radiographic images. For instance, it was mentioned earlier that the
mottle is "smoothened out" as the screen-film distance is increased,
whereas the quantum mottle explanation would have given identical
images regardless of the distance. This observation suggests that
mottle is highly dependent upon the optical properties of the imaging
system. This is very important in view of the fact that the screen-
film combination isn't simply an amplifier, but also an imperfect
imaging system. This concept is rather obvious when we consider the
input to be Poisson limited (i.e. white noise) and that some of this
noise will be eliminated due to the limited spatial bandwidth of
the system. As derived in the section on the justification, the total
density fluctuation of a film-screen combination is given by:
(0.43Y)2F2 ^“ (“hotal - ™ g r a i n * — =------n a x
The fluctuation F carries the information about the system's modulation
transfer function (MTF), where
0 poor MTFF = (17)
1 perfect imaging
Figure 16 shows the spatial frequency content of the noise typically
found in radiographic images. The above expression clearly explains
the behavior of the mottle pattern as the imaging properties of the
system are varied.
29
perfect
totai/ic.
l/)O>
c
grain
v (cycles/mm)Fig. 16. Spatial frequency content of noise in radiographic images
(Rossmann, 1962).
At this point of the discussion, it would be wise to formally
define such terms as "noise" and "detective quantum efficiency."
Again, consider a sheet of radiographic film that had been uniformly
exposed. As described previously, the developed film will contain
random fluctuations in density as illustrated in Figure 17. Noise
is defined as simply the measure of the random fluctuations about a
mean signal. In this case, it would be equal to o^, implying a
signal to noise ratio (SNR) of
SNR = Do/o d (18)
30
>I
Distance
Fig. 17. Random fluctuations in density (SPSE Handbook of Photographic Science and Engineering, 1972).
where = average density. Detective quantum efficiency (DQE)
compares the input SNR and the output SNR of a particular system or
device. It is defined as:
(SNR)2outDQE = r (19)
(SNR) in
A device that does not degrade its input SNR has a DQE of 100%.
The input signal to noise ratio of the system is defined as
the square root of the number of x-ray photons that are absorbed by
the intensifying screen. Note that the SNR_^ is determined by the
number of absorbed photons, not incident photons. With this defini
tion of an input SNR, the quantity "scintillation DQE" may now be
defined. It simply represents the degradation in the SNR due solely
31to the variation in the gain of the phosphor screen. It does not
take into account the decrease in SNR due to the absorption of the
screen. If one were to take into account the absorption, then
the term "screen DQE" might be more appropriate. These two defi
nitions are related as in Equation 20,
screen " A x ^ s c i n t C20)
where A = absorption of the screen. This discussion and later
evaluations will deal only with the "scintillation DQE".
It is not very obvious as to what happens to the signal to
noise ratio after (phosphor screen) absorption and amplifications.
If one were to simply count the number of absorbed x-ray events in
the screen, then the output signal to noise ratio should essentially
be identical to that of the input. Unfortunately, most detectors are
integrators and consequently do not simply count photon events.
Due to the various processes that occur in the screen, it should
be obvious that the signal output from each absorbed x-ray event will
vary, and it is this variation that degrades the signal to noise
ratio.
The variation in the signal pulse from an individual absorbed
x-ray photon is due mainly to three factors: 1) the incident x-ray
energy distribution, 2) the screen’s energy absorption distribution
for a given monochromatic input energy, and 3) the screen output signal
distribution for a given amount of absorbed energy. The first factor .
arises from the fact that the incident x-ray energy distribution for
32
a standard radiographic x-ray generator is a Bremstrahlung distribution.
The second factor is related to the absorption properties of the screen,
e.g. k-escape or k-reabsorption and phonon losses. Lastly, the third
factor deals with the optical properties of the screen where the light
pulse from a given x-ray photon absorption is attenuated as it prop
agates through the screen before it is emitted. These three processes
can be represented schematically as shown in Figure 18. The final
pulse height distribution is given by the integral:
Ps(E) = | H(E,E1) {F(EM)G(E’,EM)dE”}dE' (21)
where
F(E") = input energy spectrum
G(E',E") = absorption spectrum
H(E,E*) = optical pulse spectrum
Ps(E) = screen probability distribution
F(E") H(E,E ')G(E',E") ► Ps (E)
Fig. 18. Processes in the variation of x-ray screen scintillations.
Thus the signal from an absorbed x-ray photon, for all practical pur
poses, will not be constant and will have a probability distribution
as a function of output scintillation energy, P^(E).
33In order to illustrate how P (E) affects the signal to noise
ratio, let us assume a hypothetical situation where we have a mono
chromatic x-ray source and that the variations in the optical absorp
tion are negligible. Furthermore, assume that the screen has a
k-edge at 50 keV and that there are 100 60 keV x-ray photons
absorbed within a given area within a given time. Also, for the sake
of clarity, let us suppose that the intrinsic conversion efficiency
is identical for every absorbed x-ray photon. In the first example,
it is assumed.that no energy is lost through k-fluorescence. Con
sequently, the total output signal will be proportional to 100 x 60
KeV = 6 MeV and the fluctuation in this signal will be proportional
to 100 x 60 KeV = 0.6 MeV. The resultant signal to noise ratio
is therefore 6 MeV/0.6 MeV = 10, which is equal to the input signal
to noise ratio.
Now consider the case in which k-escape occurs half of the
time. Assuming that the k x-ray photon that escapes the screen has
an energy of 40 keV, the energy output would be proportional to
50 x 20 keV + 50 x 60 keV = 4.0 MeV and the fluctuation proportional
to (/5(T x 60 keV) + (/SO" x 20 keV) = .566 MeV. The output signal
to noise ratio is therefore 7.07, corresponding to a screen detective
quantum efficiency (DQE) sc£n1: of = 50%.
From the above examples, one can conclude therefore that the
signal to noise ratio degradation occurs not from energy losses, but
from the fact that the output signal per absorbed x-ray photon is not
34
constant. It is easy to see that the DOE . would still be 100%scinteven in the case where k-escape occurs 100% of the time. Finally,
if one were to allow also for variations in the intrinsic conversion
efficiency for each absorption event, the output signal to noise
ratio would be further degraded.
Statistics of Screen Amplification
A more formal approach towards analyzing the screen's
contribution to noise can be made with the aid of Figure 19. Device
A would represent the input Poisson distribution for the absorbed
DEVICE ->• DEVICEA "A B
— 2nA,aA ‘ o 2 "
v V— A —
nAB= V nBr°AB '("o' + n
Fig. 19. Schematic for the serial combination of two statistical devices (RCA Photomultiplier Handbook, 1970).
incident x-ray photons while Device B represents the screen's
probability distribution P^(m). The various parameters are defined
as follows:
n^ = average number of absorbed x-ray photons
within a given area and time
= Variance in the number of absorbed x-ray
photons = n^
35rig = average screen scintillation output per
absorbed x-ray photon2
Og = variance in the screen's average scintil
lation output per absorbed x-ray photon.
Before continuing any further, some basic statistical
principles ought to be reviewed. For a general probability function
P(n), where n is the number of particles for a given event, one has
the basic property of:
n=o
With a given probability distribution, it is possible to create what
is called its generating function, defined as
where s is an auxiliary variable. This particular function is useful
in that it has the following properties:
NI?(n) = 1 (22)
NQ(s) = Is11 P(n) , (23)
n=o
(24)
3Q(s) as s=i
n (25)
92Q(s )3s2 s=l (26)
36Thus, given a generating function that describes the statistical
properties of a system's output, its mean, standard deviation, and
probability distribution can, in theory, be derived.
Applying these concepts in evaluating the output characteristics
of an intensifying screen, the first step would be to find the generat
ing function of this system. For a serial combination of statistical
processes, the generating function is given by
s W s ) } • C” )
where = generating function for a Poisson distribution
Qb (s) = generating function for the screen's probability
distribution.-
Making the appropriate substitutions, the result is
N M n
ABQaR = I [ 1 smp (m) PP ^ (28)n=o m=o
where P^(m) = screen probability function
P (n) = Poisson distribution PUpon differentiating, one arrives at
3Qab N _ M n-1 M in-1= I n I smPc(m) I ms Ps(m) Pp (n) (29)
9s 5n=o m=o iti= o
37Evaluating at s = 1
3Q-AB N N n-1 M
9s s=i = I nl I- PSWn=o in=o
I mPs(m) _ Pp (n)m=o
v (29)
MBut P (m)=l, and thus
m=o s
9QAB
9s
N M= I I nPp (n) I I I mPs(m) \
s=l n=o m=o(30)
Since
^AB, N
9s= I nP(n) = n ,
s=l n=o
the final result for the average is
9QAB9s nAB nAnB
s=l(31)
with a little manipulation, the expression for the variance of
the above system is:IT T O I T ! f I ? O ! O’ab2 - % <v + Vs * Vs ■ (y (v (32)
where the primes indicate differentiation with respect to s and2evaluation at s = 1. With a little algebra, a can be written as
2The result for the system’s total noise, a , is not at all very
surprising since noises do add in quadature. In this case, the first
term represents the photon noise multiplied by the gain of the screen
and the second term the noise of the screen multiplied by the
average number of absorbed photons in the measurement interval.
The average output n^g is also as expected, being the product of
the average number of absorbed photons and the average gain per
absorbed photon.
Scintillation DQE
With the above results, the can be given by
(SNR)2 out nAnB(34)
2Note that if the variance in the screen's gain, cR was zero, then
DQEscint = 1, as one would expect.
39Swank (1973) has shown that a screen's DQE may be
calculated if the moments of the screen's probability distribution
were known. With this motivation, assume that P On) was obtained
by observing the output for N absorbed x-ray photons. The moments
can then be expressed as
M = Yn = N o x n n
M = Yn n1 n n
M2 = lNnn2 (35)n
where = the number of absorbed x-ray photons that resulted in the
output of n optical photons. If N is large enough, then DQEi may
be represented by
N 2
DQE_. + = - ^ 5- ---- (36)nscint N
I - — 2n Nn
where use was made of the relation
With a little algebra, the final result is
^ s c i n t = d Nnn)2 / N J N^n2n n
M12
M M o 2(38)
Simulation of Screen Statistical Processes
A rather interesting prospect lies in the possibility of
obtaining in principle, an analytical output probability curve for
a phosphor screen for a given input Poisson probability distribution
and a given screen probability distribution. Upon inspection of
Equations 22 and 28, it is evident that a problem arises when one
tries to evaluate the quantity:
The above involves a summation of a large number of terms, each
term having a different coefficient, with the entire summation then
taken to the nth power. For a measurement interval that corresponds
to a Poisson average of 10, for instance, n may be as high as 15.
However, some basic insights can be gained if one were to
consider a screen distribution that is composed of only several dis
tinct points. The summation will thus consist of only a few terms
and the analysis could then be done easily with a computer. Two
distributions were chosen for the simulation, as shown in Figure 20.
The distribution in Figure 20 (a) is useful in that it somewhat
approximates a screen distribution that exhibits k-escape. Dis
tribution 20 (b) represents a scaled version of a CaWO^ distribution.
The probabilities P^ and P^ were varied in the case of Figure 20 (a)
and the Poisson input average of n^ was varied for both distributions.
41
105
Cl. 0.2 0.2
63 129
No. of photons/event No. of photons/event
Fig. 20. Simulation screen probability distributions.
A Fortran program was utilized in carrying out the calculations.
The results are presented in Figures 21-23.
From the results, the following observations may be made:
1. The screen possesses a DQEscint> which degrades the signal
to noise ratio as described in the previous section.
2. The DOE . decreases as the spread in the probabilitiesscintP. is increased.i
3. The DQEscint is independent of the Poisson average n^.
4. The output probability functions P ^ are rather as sym
metric for small values of n^, but gradually approaches
a symmetric form for larger n^'s. This may be explained
by considering the shape of a Poisson distribution.
5. The probability of obtaining zero output in the measure
ment interval is relatively high for small values on n^.
Pro
bab
ilit
y
Pro
ba
bil
ity
A V G . - I5 .0 0 S T . D E V . - I I . 1 SNR - 1.342
10 20Counts
j40
n - 6AAVG. -45 .00 ST.DEV.- 1 9 . 3 6 SNR - 2.324
00-i— 1------ *»
Counts
nA - l °AVG. -75 .00 ST.DEV.- 2 4 .99 SNR = 3 . 0 0
40 80Count s
Fig. 21. Simulation results for Fig. 20(a), = 0.5.
v2A V G . - I 7 .5 0 ST.DEV.- 1 2 . 7 5 SNR - 1.373
lln40
- 1—
801--1--T~*
Counts
XI
2o
Jl
V 6AVG. -52 .50 ST.DEV.- 2 2 . 0 7 SNR - 2 .370
40 801------1------ r
Counts
X)
JSo
40 80
n - 10 AAVn. -87 .50 ST.DEV.- 2 8 . 7 4 SNR - 3 .045
Counts
Fig. 22. Simulation results for Fig. 20(a), P^ -1/4, P^ - 3/4.
Pro
ba
bil
ity
P
rob
ab
ilit
y
1 8 .30 ST.DEV. - 11 .86 SNR - 1.543
12.20AVGST.DEV.- 9 .685 SNR - 1 .260
AVG
10 20 10 2 0Counts Counts
AVG.- 6 . 1 0 ST.DEV. - 6 .85 SNR - 0 .8907
10 20Counts
nA ■ 5AVG.- 2 4 . 4 0 ST.DEV.- 1 3 . 7 0 SNR - 1.781
AVG.- 3 6 . 6 0 ST.DEV.- 1 6 . 7 7 SNR - 2 .182
O-o_
10 20 10 20 10 2 0Counts Counts Counts
Fig. 23. Simulation results for Fig. 20.
-p*01
Some Conclusions
In summary, three important points should be reiterated. One,
the signal to noise ratio of the output of an intensifying screen
is not as predicted by Poisson statistics alone. Instead, it is
degraded due to the variations in the scintillation output for a
given absorbed x-ray photon. Secondly, if there are no variations
in the scintillation output, the gain of an intensifying screen
does not play a role in the output signal to noise ratio. However,
gain is important in the consideration of the imaging devices that
detect the screen's output, since noise and background from these
devices may conceal the signal if the gain is too low. Thirdly, a
screen's may be easily obtained if its probability distri
bution is known.
CHAPTER 3
EXPERIMENTAL PROCEDURE AND SETUP
Introduction
The basic experimental objective was to measure the photon
emissions from x-ray intensifying screens and thus be able to obtain
the statistical parameters of these emissions. As briefly touched
upon in the introduction of Chapter 1, the experimental setup is
a photon counting system consisting of a fast photomultiplier, wide
bandwidth electronics, and a digital computer for data analysis and
processing. An overall description of the system is shown in Figures
24 and 25 as is described by Roehrig et al., 1979.
The phosphor screen is placed directly against the window of
the photomultiplier in order to maximize the collection efficiency
for the emitted light photons. The intensity of the x-ray source
is low enough such that the overlapping of x-ray events is avoided.
The absorption of a single x-ray photon results in the emission of
several hundred to several thousand light photons, each light photon
having a probability p of being absorbed by the photocathode. If a
light photon is absorbed, a photoelectron is emitted from the photo
cathode, which is then amplified by the subsequent dynode chain of7the photomultiplier. Amplification can be on the order of 10 ,
resulting in a measurable current pulse at the output of the
45
46wwite m irz irxuAtmN «iaci
- dD
Fig. 24. Block Diagram of U of A evaluation facility.
PHOTOELECTRON FASTSCOPE
PUTPULSEHEIGHTDISTRIBUTION
15 ns
■^VX< 50 o
30 pF
PHOTOMULTIPLIERSCREENCOLLIMATOR OYNODES COUNTABLEPULSES
NOISEPULSES
DISCRIMINATORLEVELPHOTOCATHODE
QUANTUM E FF IC IE N C Y nTIME
t 50PHOTOMULTIPLIER PULSES
DUE TO THE ABSORPTION OF A FOLLOWING SINGLE X-RAY PHOTON I X-RAY PHOTON " 11 PMT PULSES
PHOTOMULTIPLIER PULSES DUE TO THE ABSORPTION OF
A SINGLE X-RAY PHOTON 1 X-RAY PHOTON - 13 PMT PULSES
Fig. 25. Schematic of PMT pulse counting method.
47photo-tube'. Thus an absorbed x-ray photon will lead to a train of
current pulses, each pulse to be amplified and counted by the
associated electronics.
The Photomultiplier
The first and probably the most important stage of the system
is the photomultiplier tube. It is an RCA 8850 with a bialkali
photocathode and a GaP first dynode. The spectral response of the
photocathode extends from 300 nm to 60 nm, and thus is coupled
fairly well to the spectral outputs of the intensifying screens,
particularly with the blue-emitting phosphors. The photomultiplier
also has a high gain first dynode and an output pulse width in the
order of several nanoseconds, both characteristics being necessary
for photon counting measurements..
Details are given in RCA Photomultiplier Handbook (1970) con
cerning the noise characteristics of photomultiplier tubes. One of the
conclusions drawn from that discussion states that a photomultiplier
provides noise free gain if the gain of the first stage is large, since
most of the noise is from the first stage. The RCA 8850's GaP first
dynode has a gain of 30 as compared to a gain of 3 for the remaining dy-
nodes. Figure 26 is a plot of the pulse height spectrum of the photomul
tiplier operated at 2140V. The illumination is provided with a green LED
whose intensity is low enough for the resolution capabilities of the
tube, i.e., the probability for multi-photoelectron events is negligi
ble. As one can see, a single photoelectron peak is clearly
48
SINGLEELECTRON
100 rILLUMINATION: CREEN LEO
NOISE
DISCRIMINATORSETTING
OF 130 nV
200LOVER LEVEL (nV)
Fig. 26. RCA 8850 photomultiplier pulse height spectrum,
resolvable. The discriminator level of the electronics is set at the
valley separating the noise and the single photoelectron peak. The
dark count of the photomultiplier is on the order of 160 counts per
second, which is about an order of magnitude lower than the signal
count rates that are obtained with the light outputs of the inten
sifying screens. Thus, with a noise free gain and single photoelectron
detection resolvability, the noise measured at the output of the detec
tor is essentially that of the input. Figure 27 is a plot of the
number of photoelectrons counted in a time interval of 100 msec with
a 600 nm irradiance. The signal to noise ratio observed is that
predicted by Poisson statistics, as expected.
Another factor that must be considered is the count rate
capability of the photomultiplier. In order to know what count rates
Avg Mo of Counts per Measurement Interval
= 2206Stand. Dev. in the Mo of Counts = 47.6
2206 = 4?
PMT CountsFig. 2 7. Photomultiplier output corresponding to a Poisson distribution.
are necessary, the decay characteristics of the light output from a
screen should be examined. Assuming that the screen output decays
as in Figure 2 8 and if the electronics to observe the decay is a
simple RC circuit (Fig. 29), then the equation for solving the voltage
output is
dV -t/r+ C = N e o (40)
R dt
Upon solving the differential equation, the voltage is given by
50
Screen Output
NoDecay Characteristics
> " t/T . where t = time constant
Time
Fig. 28. Phosphor screen output decay characteristics
TIK E ( j s )
10 20 30 40 so 60 70 80 30 100
ICO
200
5006007oo800
220 p f
V ( t ) - - e " 17 ' )
7 • d e c a y t im e o f CaUO^ lu m in escen ce
Fig. 29. Decay time constant for CaWO^ screen.
51For RC>>x, the rise time of the voltage signal is governed by the decay
time t , whereas the fall time is determined by the RC time constant.
Figure 29 also gives the plot of the voltage signal from a CaWO^
screen scintillation as a function of time, indicating a decay time
constant of approximately 7 psec. The photon emission rate can be
approximated with
dt
At t = 0, and supposing that Nq = 500 as suggested by Coltman, (1947) ,
a rate of ~ 70 MHz is obtained which would result in a 10 MHz count
rate for detection efficiencies of 10-20%. The pulse width of a single
photoelectron pulse at the output of the photomultiplier has been
measured with the aid of a fast oscilloscope, giving a width of ~ 10- 8 -1nsec. The corresponding bandwidth is then given by Af = (2.10 sec)
= 50 MHz. The counting rate capability of the tube is thus suitable
for this measurement.
The counting efficiency of the photomultiplier and its
associated electronics was calibrated as shown in Figure 30. The
UNIVERSITY or AUlZOVAFILTER
ECO RADIOMETER
WWWFULSE COUNT
CAMHA SCIENTIFIC STANDARD SOURCE
PMT
Fig. 30. Calibration of photomultiplier counting efficiency.
52source was a Gamma Scientific Standard Source RS10. With the aid
of a set of 10 nm wide interference filters (Melles Griot No. 031FS005)
the photocathode was illuminated with light of a known wavelength and
radiant intensity.
The intensity of the emitted x-ray flux was calibrated with a
Hyperpure Germanium Detector (Ortec Model 1513) and its associated
electronics (Ortec Amplifier 572 and Single Channel Analyzer 550).
The measured intensities of the source are listed in Table 4 for the
3.2 mm colimator-aperture. Note that the intensities are low enough
to prevent overlapping of the absorbed x-ray pulses and that even
lower intensities are possible with smaller apertures.
Finally, a typical radiographic x-ray generator Bremstrahlung
source was also available. A Bremstrahlung spectrum is produced in
this case and it is capable of being modified through varying amounts
of irradiance was measured with an EG§G Radiometer-Photometer Model
550. Calculating the photon incidence rate at the photocathode and
counting the number of pulses at the output of the photomultiplier-
elect ronics system, the counting efficiency was obtained. This
procedure was carried out for wavelengths of 500, 550, and 600 nm,
with the efficiency for other wavelengths in the 350-600 nm range
obtained through extrapolation. The extrapolation was done with the
aid of a typical quantum efficiency curve for the RCA 8850. The
result is shown in Figure 31. The effective counting efficiencies
for the various emission spectra of the intensifying screens are
Table 4. X-ray source characteristics.
Target Mo Ag Du Ce Tb Tm
ka (kcV) 17.4 22 32 34.5 44 50
Nko (counts/sec) 16.9 27.6 39.1 38.1 64.5 70
kg (keV) 20 25 36.8 39.7 50 58
Nkg (counts/sec) 3.2 5.5 10.7 10.5 18.4 18.4
V Nka ' .19 .20 .27 .28 .29 .26
COUNTINGtFFICIiUNCY
c.x'c.cff
w
0.05
*5
s 0.0Jow
0.02sI£v 0.01
B
60CO5500
>-
Fig. 31. Counting efficiency of the system.
54
obtained by evaluating
[ E XT)c X dX-------- (43)^c,eff =EAdX
where = screen emission spectra in energy density per unit
wavelength
ncA - counting efficiency of the system as a function of wave
length.
Table 5 gives the values of nc e££ for the various screens that were
tested. The emission spectra in Figures 11-13 were utilized for these
calculations.
55
The X-Ray Source
The x-ray source is an Amersham Variable Energy X-Ray Source
as illustrated in Figure 32. It contains a radioactive source, Am
241, whose gamma emissions strike secondary targets and cause
characteristic x-ray fluorescence. Table 4 lists the six secondary
targets and their respective characteristic x-ray spectra. Note that
the emitted characteristic x-rays for a given target is not monochro
matic, containing both k and k energies.
filtration.
Rotary taryet holder (5 targets)
X-ray aperture
O '
23
X.208Dimensions in m m
Fig. 3 2. Variable energy x-ray source.
Electronics
The associated electronics consist of an Ortec Amplifier 9302
with a gain of 20, an EG§G fast Discriminator TDlOl/N, and EG§G
Logic Interface L 1380/NL, and a threshold counter that was built
inhouse. Figure 3 3 illustrates the basic order for these components.
After a current pulse from the photomultiplier is amplified and con
verted into a voltage pulse by the amplifier, the discriminator deter
mines whether this pulse is a signal pulse (Fig. 26). If the pulse
is a signal pulse, it is then sent through the interface and into the
threshold counter, where it is recorded and sent to the computer for
analysis. The threshold counter is triggered by the rapid arrival of
a series of signal pulses and counts these pulses within a given gate
time. This gate time interval is selected such that it is greater
than the total decay time of the phosphor screen for a single
absorbed x-ray photon. The threshold counter may be also triggered
independently by a pulse generator. This feature is useful in doing
samplings that are to be independent of the signal.
The bandwidth of the electronics must also be large in order
to resolve the photoelectron pulses from the photomultiplier. The
pulse-pair resolution for the Ortec 20X Amplifier is on the order of
9 nsec. The output pulse width from the differential discriminator
is set to - 10.8 nsec. The bandwidth of the threshold counter is on
the order of 20 MHz. Taking these components as a whole, the overall
bandwidth of the electronics would be on the order of a little less
than 20 MHz, sufficient for the 10 MHz requirement.
Signal and Noise Amplified Amp Iified Reshaped
PMTOr tec AmplIfier 9302 20X
ThresholdCounter
EGGG Logic Interfact
L1330/NL
EG&G DIfferentI a I Discr imi nator
TDI01/N
Current Pulses Signal and Noise Signal PulsesVoltage Pulses Voltage Pulses
Fig. 33. System electronics.
Computer Analysis
After the threshold counter counts all of the pulses resulting
from an absorbed x-ray photon, the computer records this as an x-ray
event and assigns to it the number of pulses that were counted. This
is carried out for each observed x-ray event, the data being stored in
a one-dimensional array, HIST(K). The variable K is the number of
pulses counted in an individual event and HIST(K) is the number of times
an x-ray event resulted in K pulses. HIST(K) is therefore essentially
the screen probability function P , as described in Chapter 2. The
computer is also capable of recording the dark pulses from the photo
multiplier and subtracting it from the above distributions. This
ensures that all of the pulses that are counted and used in the
analysis did indeed originate from an x-ray absorption event.
CHAPTER 4
RESULTS AND CONCLUSIONS
Results
The probability distributions P^(m) for a single absorbed x-ray
photon were obtained for a set of phosphor screens with varying
incident x-ray energies and are shown in Figures 34-38. In the corner
of each curve, the parameters of the distribution are displayed. They
are the zeroth, first and second moments, the number of utilized x-ray
events, the average number of counts per x-ray event, the standard
deviation in the number of counts per event, and the calculated
scintillation DQE. Also listed is the incident x-ray energy for each
case (ignoring the k lines for the time being). For the case of thepCsl input phosphor of an image intensifier, the incident x-ray flux
was produced with the x-ray generator with 2.6 and 8 mm of copper
filtration. These particular spectrums of the input x-ray flux are
shown in Figure 3 9.
Figures 40-44 are plots of some of the above mentioned
parameters as a function of incident x-ray energy, with the k-edge
of some of the phosphors also being noted. Figure 4 2 is a plot for a
series of ZnCdS screens with varying thicknesses.
Table 5 lists the conversion properties of each of the screen
as obtained with the above data. The counting efficiency of the
59
.O<ti
o6<u>•H■P(dHfl)IX
MO - 8129.00Ml • 556379.50M2 • 4 5357976 .00MO. OF UTILIZED EVENTS
- 8129AVC. NO. OF COUNTS/EVENT
- 68.47STAND. DLV. IN THE NO. OFCOUNTS - 29.86THE SCINTILLATION DOE.
• 0.8402
17 k,V
~r~50
rloo3lllLlUiL
PMT Counts T~50
MO » 10053.00Ml • 66)025.50M2 - 85541736.00NO. OF UTILIZED EVENTS
- 10053 1AVC. NO. OF COUNTS/EVENT- 05.85
STAND. DEV. IN THE NO. OKCOUNTS - 33.75THE SCINTILLATION DOE.
■ 0 .6661
22 keV
T100 PMT Counts
•H(dO14( X<D>•H4JOJr-l<Dpr;
\ --I
MO - 12653.00 Ml - 1266431. 5 0 M2 • 156612672.00 NO. OF UTILIZED EVENTS
■ 12653 AVC. HO. OF COUNTS/EVENT - 1 0 0 .1
STAND. DEV. IN THE NO. OF COUNTS - 6 8 . 5 8 THE SCINTILLATION DQ2.
0.8094
32 keV
50T100 PMT Counts
• HrH•r-1•s&o&0)>• HPcdr Ha>a;
MO ■ 11418.00 Ml - 1194394.00 M2 - 151907630.00 NO. OF UTILIZED EVENTS
- 11438AVC. NO. OP COUNTS/EVENT
- 104.4STAND. DEV. IN THE NO. OF COUNTS - 48.75 THE SCINTILLATION DQE.
0.8210
34 k.V
~T~50
I--------------100 PMT Counts
mo • 1 0 0 7 0 . 0 0 mi - 1 3 2 2 2 7 3 00M2 - 195506544.00 NO. OF UTILIZED EVENTS
■ 10870 AVO. NO. OF CCUNTS/tVENT
- 121.6 STAND. DEV. IN THE NO. OF COUNTS - 56.53 THE SCINTILLATION DQE.
- 0.8224
44 keV
I50
I100 PMT Counts
MO " 9 8 5 6 . 0 0 Mi - 1305801.00 M2 - 219445468.00 NO. OF UTILIZED EVENTS - 9856AVC. NO. OF COUNTS/EVENT
■ 132.5 STAND. Dr.V. Ill THE NO. OF COUNTS - 68.64 THE SCINTILLATION D3E.
0.7884
50 keV
100 200 PMT Counts
Fig. 34. Measured probability distributions (P ) for aZnCdS screen; Cd K-edge: 26.7 keV. o
Relative
Probability
, Relative
Prob
abil
ity
it
KO - 6050,00 III ■ 1M D'iO.91 M2 • •»<i6')995. 50 NO. OK UTI LI7tO SiWtlirj
- 6050A VC. NO. OF COUflfJ/tVtNT - 23.36 STAND. DtV. IN THE NO. OF COUNTS - 13.89 THE SCINTILLATION 0Q2.• 0.7388
17 keV
50
&Ir I •H•so&O)>•H■PcdrH0)
MO • 70*111.00Ml • 2y973.3lM2 - 0070913.00NO. OK UTILIZtU EVENTS
■ 70'i*»A VC. NO. OF CCUHTSAVLNT
■ 30.21STAND. DtV. IN THE NO. OFCOUNTS * 13.92Tilt SCINTILLATION DQE.
- 0.02*49
22 k«V
tMjuPMT Counts 50 PMT Counts
FO ■ 9961.00 Ml - *416761 .22 M2 - 19951300.00 NO. OK UTILIZED EVENTS
■ 9963AVC. NO. OK COUNTS/EVENT
- *41.83 STAND. DEV. IN THE NO. OF COUNTS - 15.90 niE SCINTILLATION DQE.
■ 0.8738
32 k v
PMT Counts
MO ■ 8*460.eoMl • 3*165*49.13 112 • 2033*1696 .00 HO. OK UTILIZED EVENTS
■ 8*460AVC. NO. OF COUNTS/EVENT
- *45.69STAND. DtV. IN THE NO. OFCOUNTS - 17.79THE SCINTILLATION DQE.
- 0.8684
3*4 k V
MO • 6026.00 Ml • 332503.00 M2 - 2*11 20o*i6 . 00 NO. OF UTILIZED EVENTS
■ 6026AVC. NO. CF COUNTS/EVENT
=» 56.0*4 STAND. DEV. IN THE NO. OF COUNTS » 19.66 THE SCINTILLATION DQE.
- 0.8084
PMT Counts
44 k#V
i?•HrH• r l
td0 &<D>•H01 rH 0) K
MO - 7141.00 Ml - 433095.41 M2 = 30931 39*4.00 NO. OK UTILIZED EVENTS
«■ 7141AVC. NO. CK COUNTS/EVENT
- 61.35 STAND. DtV. IN THE NO. OF COUNTS = 23.83 THE SCINTILLATION DQE.
• 0.6689
50 keV
PMT Counts 50 PMT Counts
Fig. 35. Measured probability distributions (Pg) for aCaWO^ screen; W k-edge - 69.5 keV.
Relative
Probability
, Relative
Prob
abil
ity
MO • <J??6.00hi » 2'ioro5. ooHZ ■ 16502332.00 NO. or UTILIZED EVENTS
- EZ26A VO. 110. OF COUNTS/EVENT
« 56.80 STAND. DcV. Ill TH£ NO. OK COUNTS - 26.33 m SCINTILLATION DOE.
• 0.8234
17 k.v
. t. a ...1------ 150 100 PMT Counts
MO • 4'3'»).00 Ml - 153437.59 M2 • JO'/'iV/lli.OO NO. or UTILIZED EVENTS
- 4963AVC. NO. OK COUNTS/EVENT
- 72.42STAND. UuV. IN THE NO. OfCOUNTS = 31.48THE SCINTILLATION DQE.
| - 0.8411
22 k*V
iL luii,1 I "50 100 PMT Counts
i?
MO - 3740.00 Ml - 336573.59 M2 • 3f»552024.OO NO. OK UTILIZED EVENTS
■ 3740AVC. NO. OF COUNTS/EVENT
• 89.99 STAND. DcV. IN THE NO. OF COUNTS . 33.76 THE SCINTILLATION DQE.
0.8766
32 keV
I | H '.I50 100 PMT Counts
I
NO - 4217.00Ml - 360743.22M2 ■ 37533500.00NO. 01’ UTILIZED EVENTS
- 4217AVC. NO. or COUNTS/EVENT
- 85.54STAND. DEV. IN THE NO. OF COUNTS * 39-78 THE SCINTILLATION DQE.
0.8222
34 k V
1 r50 100 PMT Counts
MO - 6473.00 Ml - 546000.00 M2 • 59065140.00 NO. OK UTILIZED EVENTS - 6473 AVC. NO. OF CCUNTS/EVENT
• 84.47 STAND. DEV. IN THE NO. OF COUNTS " 44.61 THE SCINTILLATION DQE.
■ 0.7820
44 keV
. . . , :i l?!
1 r50 100 PMT Counts
MO - 7466.00 Ml • 695070.44 M2 - 79596976.00 NO. Or UTILIZED EVENTS
» 7466AVC. NO. OF COUNTS/EVENT
- 93.21 STAND. DI.V. Ill THE NO. OF COUNTS - 44.42 THE SCINTILLATION DQE.
* 0.8148
iiWiiLiiti!
50 keV
100 PMT Counts
Fig. 36. Measured probability distributions (P ) for a BaSOscreen; Ba k-edge: 37.5 keV. 5 4 O'
to
Re
lativ
e
Pro
ba
bil
ity
««ioie 04H i* I t l l . I . N ooV) o» U l l i u i s I - a * I t v ia is .
WC.. MJ LO UIIIS /AIVViMe VO AI
t x K lX T I tU I IO * DQC IS • .? !« «
i
i0 150 PJfT Count*SO 100
'II. 11644.00H I. M lV ? 114 0 . . 6 4 :6 4 . .4 00« i oo : i u i : M i i - * a i lA iM is *
f I IHllk/AC.'Wtl !• I HAT <in tie vi ur tmihT* 'suuv,
i t s:t ie 'X ik iii.u tiu N i«;i is 4 :111
2 2 .2 kcV
I. i -
»> 46,14 00HI. .111160 .»>11. t4:i:„o .wvi of imii:in s.mi ikivr,. 0-4 A ll.. V I . I l f o U lN rs /A B V IfflO % fA I .
siAvi oiv in no vo of toons .14. *4Tl* JCI.VTIUATI04 tA}t IS « |;|l
SO PSTT Counts
M l. 1 :6 1 4 HI M l. 41 40 .4 6 .. 0,1SO. Of U T H C ID ( A l l t . t .x tS • |>e
AM. ‘01 Of -OUMTS/11X16414 I A l l •
nv\». dii in no, «. of cot its •rn SCINTIllAtlON uqi IS 4 lit*
34.7 lev
; L;',; " L_...Ly0 SO 100 ISO POTCowt,
NO 01 0(111:14 «AAI (VINTS.Avti. m o f c o u in i/» 6 6 u * i io 1 m i
SIAMl DLV. 1:1 TIE MW Of rownsTIE Sf IMTILLATION IfH IS 4 '6*4
4 4 .S kcV
all*./"0 iso100so
M l. 14011,, 04 M :.|is :4 i * : oo
TIE SCII.TILLATION 001 IS 4 1:14
5
0 so 100 ISO
Fig. 37. Measured probability distributions (Pg) for a La^CLS-Gd-O S screen; La k-edge: 38.9 keV,Gd k-edge: 50.2 keV.
ON04
Relative
Prob
abil
ity
k-Cscape Ttak I
Mil10 20
ooof uTin:te i-a*f tvfvri.
STAND III* IN (IU NO. I# COUNTS
■5I
the sti.HitiATiow oqe IS a.fits
22.2 koV
3fi*irr Count
««HO- Sill DOhi. i » : o o t iHI. SII90So 7SHO. OF I7TILI:C0 «-M* 6VCVTS •*»C. SO. Of COUNTVAIStieitO l UT •
I I .*4STAND DC* IN TH£ NO. Of COUNTS -n i f s c i n t i i u t i o n oyc i s o i ;o <
34.7 kcV
llllllll.ll3l>>rr Counts
Jll
SIS7 00 Ml. 11700$1C* SSSfill ISNO Of VMUUO I-UT I TINTS . si I 00IVC. NO Of COUNTS/AFSONHJ S-OA* 21.11STAND DC*. IN Till NO. 01 COUNTS I IS*
T ie SCINTILLATION 001 IS a SO.*
32.2 kcV
10 20 3f,Illillll.U
PSfT Counts
*AS07.'0 oo Of UTILCCD I SAT STINTS
A*C. NO Of COUNTS/AtsoaICO I-SATST AND. DC* IN Tie M . Of COUNTS
t
THC i:MTIl.lATI0X DQC IS 0 IS.2
44.S kcV
"55
23 ^-MTCounts0
I
HO- 74*1.00Ml. :I9»S0 IIM;. 7413610 DONO Of UTILCCD I SAT CVCXTS-I 00A VC. NO Of COUNTS/ASSOf ICO : SAT :* ;*$tvo. on :n me no or counts • 11.71
Tie SCINTILLATION DQC IS 0 1*27
SO . 7 keV
3S'HT Counts
Fig. 38. Measured probability distributions for a Csl x-ray image intensifier; I k-edge: 33.2 keV, Cs k-edge:35.9 keV.
O'■t.
Re 1 a
t i vc
In tens i
Ly65
80 kVp
U-K
50 kVp 2.6mm Cu
4020 60 80 100
X-Ray Energy (keV)
Fig. 39. Bremstrahlung spectrums for different amounts of filtration.
66
No. of Counts per Event
TmQ6 0
40
20Mo
0
Absorbed X-Ray Energy(keV)
Fig. 40. Measured output emissions for a CaWO screen.
67
No. of Counts per Event
150Tm
Tb
BaoTm k-Escape100
Tb k-Escape
6040200Absorbed X-Ray Energy
(keV)
Fig. 41. Measured output emissions for a BaSO. screen.
68
No. of Countsper Event
200
150
100
Cd k-edge
3010 50Absorbed X-Ray Energy
(keV)
Fig. 42. Measured output emissions for a series of ZnCdS screens.
>.mocixtn4-fcnoo
Mean Count Averaged over Full
Di sir i but ion
100
80
60
<i0
20
0 Counts at Full tncrgy Peak
a Counts at k-oscape peak
// o
//
/
k-Escapc
/o/
k Escape /o (Tb) /
DQE/ scint
La k-edgcz
20 30 A0 50 60X-Ray Energy (XcV)
Fig. 43. Measured output parameters for a La^O^S-Gd^O^S screen.
o'-O
Scinti H
at ion
DQE
PMT
Coun
ts/E
vent >\0
30
20
10
Thompson CSF Intens i f ier
Scinti11 at ion DQE
Mean Counts/Event Averaged over
Full Di sir i but ipn- tpo"
0
o Counts/Event from Full Energy Peak
□Counts/Event from k-Hscape Peak
Cs-Edgc"Edge IJ.
10 20i-
/|0,Kay Energy (KeV)
.9
.8
.7
-fo-
Fig. 44. Measured output parameters for a Csl x-ray image intensifier.
o
Scin
till
atio
n DQ
E
Table 5. Measured output efficiencies of some phosphor screens.
Phosphor No. of Counts absorbed keV £c,eff"o
No of photons emitted
absorbed keV
Aeff(nm)
Energy Emitted Energy Absorbed
%
Luminescent Radiant Efficiency***
ZnCdS 3.25 4.7 69.15 535 16.0 18.0
CaW04 1.14 11.6 9.83 435 2.80 3,5,5.4**
BaS04 2.88 15.7 18.34 388 5.86 6
(La202SGd202S) 2.10 2.3 91.3 572 19.8 15*
* For a GdO^ screen
** Intrinsic Conversion Efficiency (Coltman, 1947)
*** Stevels, 1975
72system is taken into account and the results are compared with those
that are available in the current literature. Finally, Figures 45-46
are plots of the signal to noise ratio as a function of the average
number of absorbed x-ray photons in the measurement interval. The
data was taken with the Csl image intensifier and with a CaWO^ screen.
The x-ray input for the Csl phosphor was as described above, whereas
the input for the CaWO^ screen was obtained with a high intensity
44 keV source that is similar in nature to the variable energy source.
Analysis
Upon inspection of the distributions, it is rather obvious
that the nunber of UV and light photons emitted is not constant for
each absorbed x-ray photon. There are also many cases where there are
more than one peak in the distribution, the cause being the k-fluores
cence of the phosphor material. Note that a double-peak distribution
always occurs when the energy of the incident x-ray photon is above
the k-edge of the phosphor material. Of particular interest are the
distributions obtained with the BaSO^ screen. The principal absorber
in this case is Ba, which locates the k-edge at 37 keV. Note that
when the secondary target of the source is Ce, there is still a second
peak although the k^ line for Ce is at 34 keV. Obviously the k-fluor
escence is caused by the k^ line of Ce, which is at 38 keV and is
therefore above the k-edge of Ba.
From the relative heights of the two peaks for the various
distributions, it appears that a significant portion of the k x-rayS
Signal
to Noise
Rati
oIdeal S/N, assuming Poisson Distribution
10
Experimental S/N Obtained from Distribut i ons
j i_ j j .j _10 100
Estimated No. of Absorbed X-Ray Photons
Fig. 45. Measured signal to noise ratios from the output of a Csl x-ray image intensifier experimental values below 10 absorbed x-ray photons in error due to the fact that the threshold counter did not sample continuously but was triggered by signal pulses.
-jIzJ
74
SNR
4
3
Poi sson
2
Measu red SNR
6 82 10
No. of Events/Interva1
Fig 46. Measured signal to noise ratios from the output of a CaWO screen; 44 keV incident x-ray photons.
Threshold counter sampled continuously, triggered by pulse generator.
75do escape from the screen (as predicted by Vyborny in Chapter 1) and
thus reduces the efficiency of the screen. The positions of the peaks
have been calibrated, indicating that the peak to the right corresponds
to the case of the x-ray being reabsorbed and thus represents the full
utilization of the absorbed energy. The peak to the left is a
consequence of a k-escape and therefore represents only partial
utilization. The calibrations are plotted in Figures 40-44.
The results and comparisons presented in Table 5 are somewhat
encouraging. First of all, one must realize the difference between
intrinsic conversion efficiency and output efficiency, the latter taking
into account optical losses within the screen whereas the former does
not. The data presented for the CaWO^ screen appears to agree quite
well with that proposed by Coltman (1947). The measured light output
indicates an output efficiency of 2.8%, which in turn represents an
intrinsic conversion efficiency of ~ 5.6% assuming optical losses of2~ 50% as suggested by Coltman for a screen of thickness 84 mg/cm .
The data for the other screens also agree quite well with the data
available in current literature. The only notable discrepancy appears
to be with the La^OgS-GdgO^S screen. It is noted however, that the
counting efficiency of the system is quite low for this particular
screen, being at the tail end of the system’s counting efficiency
curve (Fig. 30). Errors in the calibration measurements can therefore
easily account for the above discrepancy. Future investigations are
obviously needed in order to remedy this situation.
76One of the more important pieces of information obtainable
from this data is the DOE . . values for these various screens.scantTwo conclusions can be drawn. First, the effect of having the phosphor
k-edge energy is to lower the scintillation DQE, as one would expect.
Apparently, the attempt to enhance the absorption capabilities of
intensifying screens has resulted in a slight reduction of the scin
tillation DQE. Secondly, one can conclude that the observed ^Q^sc^n.(-s
are all rather high, the lowest being 73% and the highest 90%. This
would indicate that the degradation of the signal to noise ratio after
absorption and amplification is relatively small.
In an attempt to observe the effects of the scintillation DQE,
the data as shown in Figures 45-48 was obtained. The phosphor screen
was irradiated with a high incident x-ray flux and the light output
of the screen was sampled continously with a certain sampling time
and area. If the screen had a DQE of 100%, then the set of sampled
data should correspond to that predicted by Poisson statistics. The
average number of absorbed x-ray photons in the sampling time is
varied through altering the rate of absorption and by fixing the dura
tion of the sampling time. For the data measured with the Csl image
intensifier (Figs. 45, 47) the results are as predicted when the
number of absorbed x-ray photons in the sampling interval is high.
However, the results are not as good when the number of absorbed x-ray
photons is low. This can be accounted for by considering the trig
gering of the threshold counter as discussed in the section of Chapter
3 concerning the electronics. At the lower rates of absorption, the
0 200
M l
ML). 4-110.00M l. 4 0 14 :4 SO (42 * 4 4 .0 1 0 :0 .0 0>0 O f U l l lU E O 1 -IA T (VESTS*
4VC° NO O f COUNTS/A4S04ILD l - I A T •
S T * . * * 0 O tV . IN TIH » - O f COUNTS *
4 1 . l t
2 X-«;iy PhutOHS
PMT Counts
IN TIC NO. O f COUNTS •
l 4 X -R a y Photons
200 4 0 00 PMT Counts
M l* i t u u i t o m : * ; j 7 i , * : s ' * . o o NO. O f UTILCIO 1 -IA T tVIVTS*4 00*1 6 NO Of COUNTS/USONlfO 1 -IA T • 11*.fSTAND 01V. IN TIC NO Of COUNTS • 77 **
6 X-Ray Photons
4 0 0
tilm iitii
/■>• Uvi'> VUM l- #744.14, W)
:.0 01 U T IL K IU K #AV SViHTSe
ivc;. NO or COUNTS/IkMlMtrO X -M f •
STAND DCV. IN T i lt NO Of- CJJKTS •
PMT Counts
8 X -R a y P h o to n s
400 PUT Counts
Mil- 1191. iNImi* i : 4 4 : v i so N:*s:'(.s4-i„ ,N(.OO 0i 0T' l:;tD '-VT MINTS. 11*
or c o a N T s /iisa L ir i, v m i .
ST.VIH DfV IN n it VO C f COUNTS .
10 X -R a yP h o to n s
.Ai-Liu-i200 4 0 0
PMT C o u n ts
Fig. 47. Csl image intensifier output distributions.
h i HO • 3079.001 HI • 140000.00 fi
H2 e 12412084.00 .r4NO. OF UTILKID EVtSTS H
• 3079 •'"IId AVC. NO. OF COUyTS/EVENT X]& - 42.47 <do stand, dev. ix me no. or "t-1 COUNTS • 44.11 ,
(d
c> >4Jcvl(U _
ft: A *
50 100 I NiT Counts
MO ■ 2630.00 HI • $43481.7$M2 « 119114640.00 NO. OF UTILIZED EVENTS
• 2680AVC. NO. OF COUNTS/EVENT
• 191.11staSo . d e v. in the n o . orCOUNTS • 101.71
$ •H I—I •Hn).0ok0)>• f-l4Jctir~ta>n:
<100 ri.1T Counts
Fig. 48.
HO • 3110.00 Ml - 316666.00 H2 • 47S647T6.00 NO. OF UTILIZED EVENTS • 1110 AVC. NO. OF COUNTS/EVENT
- 94.27 STAND. DEV. IN THE NO. OF COUNTS - 70.60
.ItltiMldlllLuL.J
HO • 2956.00 Ml • 453011.00 M2 • 91510123.00 NO. OF UTILIZED EVENTS
• 2956AVC. NO. OF COUNTS/EVENT
• 145.19STAND. DEV. IN THE NO. OP COUNTS - 87.17
200 lUT Counts 300 HIT Counts
MO - 2280.00 Ml • 584619.19 M2 ■ 180251160.00 NO. OF UTILIZED EVENTS
• 2230 AVC. NO. OP COUNTS/EVENT
> 240.64 STAND. DEV. IN THE NO. OF COUNTS • I IS. 26
MO • 2400.00 Ml • 766744.00 M2 • 286421872.00 NO. OP UTILIZED EVENTS
• 2400AVC. NO. OF COUNTS/EVENT
• 102.14STAND. DEV. IN THE NO. OP COUNTS • 111.25
PMT CountsLiLilLU
HIT Counts
CaWO^ screen output distributions. -~i00
79counter was not triggering continuously and independently of the signal,
but was triggered whenever an x-ray photon was absorbed and detected
- thus the signal to noise ratios obtained would not be governed
by Poisson statistics at all. This problem was remedied by trigger
ing the counter independently with a pulse generator. A CaWO^
screen was utilized and the results are shown in Figures 46 and 48.
The signal to noise ratio is indeed lowered as predicted by the
scintillation DQE. Output distribution plots (P^g) relating to
both sets of data are displayed in Figures 21-23. There appears to
be a very good correlation between these experimental distributions
and those that were created with the computer simulations as de
scribed in Chapter 2.
Conclusions
On the basis of the theoretical and experimental analysis thus
presented, three basic conclusions may be arrived at:
1. The output efficiencies of the newer rare earth screens
are indeed superior to that of the more traditional CaWO^
screens.
2. The observed DQE . . values of the screens are high,scintindicating only a small degradation in the signal,to
noise ratio. .
3. The decrease in the DQE . . of most of the newer screensscintdue mainly to k fluorescence is rather small, differing
from the DQE values for CaWO^ by only several percentage points.
As a final remark, a very important parameter in the
analysis of image formation has been neglected in the discussion thus
far. There is still the need to evaluate the modulation transfer
functions (MTF) of the newer screens. It can be an interesting and
important factor to consider since the k-fluorescence which decreased
the DQEsG^nt may also decrease the resolution capabilities. If the
degradation in resolution is found to be small, then it may be
unimportant since the readiologist is the final component in the
radiographic process and very fine spatial ,detail may not be that
important for the diagnosis (Rossmann, 1974).
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