Digital ImagingDigital ImagingDigital ImagingDigital Imaging

CHAPTERS 1, 4-7CHAPTERS 1, 4-7

CARTERCARTER

CHAPTER 1• Conventional radiography• Film/screen system• Light exposes film• Film processed with chemicals• Film taken to radiologist for

interpretation

Let’s compare rooms for

• Conventional (Film)• CR• DR

Let’s compare image formation/acquisition

• Film

• CR

• DR

Let’s compare image processing

• Film

• CR

• DR

CR- Chapter 4IP layers

• Protective • Phosphor/active• Reflective• Conductive• Color• Support• Backing• barcode

• Protects phosphor• PSP-barium fluorohalide• Sends light forward when

released in the reader• Absorbs/reduces static

electricity• Absorb stimulating light

(laser)/reflects emitted light• Protects the back of

cassette• Match image with patient

Reading the IP• Red laser light scans in a raster

pattern at 2 eV • Laser scans multiple times as IP

moves through reader =translation

• Light produced –detected by photomultiplier

DIGITIZING• PHOSPHOR STORAGE CENTER IS

SCANNED• RELEASED ELECTRON ENTER DIGITIZER

DIVIDES THE ANALOG SIGNAL INTO SQUARES (MATRIX).

• EACH SQUARE IS ASSIGNED A NUMBER BASED ON THE BRIGHTNESS OF THE SQUARE

• SQUARE IS CALLED A PIXEL

SPATIAL RESOLUTION• Film screen = 10 line pairs per mm• CR =2.55 to 5 line pairs per mm

(lp/mm)• Less detail in CR but more tissue

densities seen given the appearance of better detail

• Wider dynamic recording range

speed• Film – determined by size and

layers of crystals and phosphors• CR – amount of photostimulable

luminescence given off = 100 film speed screen (approx)

EXPOSURE –CASSETTE BASED-chapter 5

• What is the relationship between selecting the correct body part and computer interpretation of the image?

• Too much kVp (above 120) and too little (below 45) can over excite or produce too little excitation of the phosphors

• Does the pixel size of a 2000 x 2000 matrix change when using an 8 X10 vs

a 14 x 17 CRcassette? • How does the change in pixel size impact

resolution?

MOIRE PATTERN• Grid lines from a stationary grid

can cause a wavy artifact known as a moire pattern. The grid lines and the scanning laser run parallel

Exposure Indicators• Exposure indicator number• Fuji, Philips, Konica – S number-

indirect relationship• Kodak –Exposure index (EI)-direct

relationship• Page 88 tables 5-1 and 5-2

HISTOGRAM• Graphic representation of the

numerical tone (grays/blacks/whites) of an x-ray exposure

• More when we move on to Chapter 7

OTHER ARTIFACTS• Plate artifacts

– Adhesive tape residue– cracks

• Plate reader artifacts– Line patterns– Plate reader loads multiple IP in one cassette

• Image processing artifacts– Incorrect erasure (ghosting)– moire

• Printer artifacts– White lines

CASSETTELESS -Chapter 6

• Direct and indirect• DR plate (amorphous selenium)

or(a-Se)-converts radiation into an electrical signal –CCD or silicon detectors

• Signal stored in a Thin film transmitter (TFT)

INDIRECT DIRECT• Two step process• X-rays convert to

light• Light converted

to electrical signal

• Sent to TFT

• X-rays converted directly to an electrical signal

• Sent to TFT

DQE• Detective Quantum Efficiency• Ability to convert an x-ray signal

into a useful image• Of the following-which do you think

has the most efficient DQE?• Film or CR or indirect capture DR or

direct capture DR?

Digital Radiographic Digital Radiographic Image Processing and Image Processing and

ManipulationManipulation

Digital Radiographic Digital Radiographic Image Processing and Image Processing and

ManipulationManipulation

Chapter 7Chapter 7

CR image *sampling (*conversion from analog to

digital)• Histogram• X axis = amount of

exposure(number of grays)

• Y axis = number of pixels for each exposure

• Low kVp= wider histogram????

• High kVp = narrower histogram???

• Histogram is anatomy specific

histogramhistogramhistogramhistogram

http://http://bloggingradiography.blogspot.cobloggingradiography.blogspot.co

m/2007/08/anatomy-of-m/2007/08/anatomy-of-histogram.htmlhistogram.html

NYQUIST THEOREM• When sampling(converting from analog to

digital) a signal, the sampling frequency must be greater than the bandwidth of the input signal

• http://images.google.com/imgres?imgurl=http://www.cs.cf.ac.uk/Dave/Multimedia/nyquist.gif&imgrefurl=http://www.cs.cf.ac.uk/Dave/Multimedia/node149.html&h=420&w=529&sz=6&hl=en&start=10&um=1&tbnid=zyDOLJNUH6mM8M:&tbnh=105&tbnw=132&prev=/images%3Fq%3Dnyquist%

Sample? Try this at home

• http://www.pcguide.com/art/soundSampleRate-c.html

ALIASING• When the spatial frequency is

greater than the Nyquist frequency and the sampling occurs less than twice per cycle, information is lost causing a moire effect.

Automatic Rescaling• Fixing the image• Why is this dangerous?

LUT and Windowing• Automatic rescaling to achieve

appropriate contrast on an image

• http://www.sprawls.org/resources/DIGPROCESS/module.htm#13

LATITUDE• Find the percentage the exposure

can be greater or less than before it impacts the image in CR

IMAGE MANIPULATION cont.

• Window- how light or dark an image should be

• Level-contrast• Background removal or shuttering

– Removing the unexposed borders or to blacken the white borders

MTF• Modular Transfer function• Reproducing the spatial resolution

of an object as a diagnostic image• 100% of the spatial resolution of the

object can never be perfectly reproduced – even with DR and CR – why? -See Bushong – pgs 451-454

acronyms1. PSP2. CRT3. ADC4. IP5. CR6. DR7. PACS8. SNR9. CNR

10. CCD11. FOV12. LUT13. DICOM14. RIS15. HIS16. TFT17. DQE

1. Photostimulable phosphor-europian activated barium fluorohalide

2. Cathode-ray tube or computer monitor3. Analog to digital converter4. Imaging plate5. computed6. /digital radiography7. Picture archiving communication systems

8. SIGNAL TO NOISE RATIO PG 410 –BUSHONG The higher the signal the less the noise.

9. CONTRAST TO NOISE RATIO can be manipulated until noise

becomes too apparent-limited by the SNR 10. Charge coupled device- crystal silicon- small, replaces

vidicon in fluoro, device used in DR

11. Field of View –how much of the patient is imaged in the matrix

12. Look up Table – plotting grays on a scale

13. Digital imaging and communications in medicine -blending PACS and other imaging modalities

14. Radiology Information system15. Hospital Information system

16.Thin Film Transmitters- indirect and direct conversion detector in DR. Stores electronic charge (from converted light) before computer processes it.

17.Detective Quantum Efficiency converting x-ray intensities into a radiographic image