multi-slice ct or multidetector ct (mdct) 1991 · slice thickness: single detector array scanners...

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Multi-slice CT or MultiDetector CT (MDCT) 1991

Multi-slice CT or MultiDetector CT (MDCT) 1991

Multiple rows of fan beam detectors Wider fan beam in axial direction Table moves much faster Substantially greater throughput

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Multi-slice CTMulti-slice CT

Multi-slice CT

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Multiple detector arrays

Set of several linear detector arrays, tightly abutted

Use solid-state detector arrays Slice width is determined by the detectors, not

by the collimator (although collimator does limit the beam to the total slice thickness)

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Multiple detector arrays (cont.)

3rd generation multiple detector array with 16 detectors in the slice thickness dimension and 750 detectors along each array uses 12,000 individual detector elements

4th generation scanner would require roughly 6 times as many detector elements; consequently currently planned systems use 3rd generation geometry

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Slice thickness:single detector array scanners

Determined by the physical collimation of the incident x-ray beam with two lead jaws

Width of the detectors places an upper limit on slice thickness

For scans performed at the same kV and mAs, the number of detected x-ray photons increases linearly with slice thickness

Larger slice thicknesses yield better contrast resolution (higher SNR), but the spatial resolution in the slice thickness dimension is reduced

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Slice thickness:multiple detector array scanners

In axial scanning (i.e., with no table movement) where, for example, four detector arrays are used, the width of the two center detector arrays almost completely dictates the thickness of the slices

For the two slices at the edges of the scan, the inner side of the slice is determined by the edge of the detector, but the outer edge is determined either by the outer edge of the detector or by the collimator penumbra, depending on collimator adjustment

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Slice thickness: MDA (cont.)

In helical mode, each detector array contributes to every reconstructed image Slice sensitivity profile for each detector array needs to be

similar to reduce artifacts Typical to adjust the collimation so that the focal spot –

collimator blade penumbra falls outside the edge detectors Causes radiation dose to be a bit higher (especially for small slice

widths) Reduces artifacts by equalizing the slice sensitivity profiles

between the detector arrays

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Detector pitch/collimator pitch

Pitch is a parameter that comes into play when helical scan protocols are used

In a helical scanner with one detector array, the pitch is determined by the collimator

Collimator pitch = table movement (mm) per 360-degree rotation of gantry / collimator width (mm) at isocenter

Pitch may range from 0.75 (overscanning) to 1.5 (faster scan time, possibly smaller volume of contrast agent)

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Pitch (cont.)

For scanners with multiple detector arrays, collimator pitch is still valid

Detector pitch = table movement (mm) per 360-degree rotation of gantry / detector width (mm)

For a multiple detector array scanner with N detector arrays, collimator pitch = detector pitch / N

For scanners with four detector arrays, detector pitches running from 3 to 6 are used

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Multi-detector planes

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Multi-detector planesNew Technology

GE QXi (multi-detector CT) acquires four interweaving helices simultaneously.e.g., 4 x 5 mm slice = 20 mm total scan width

4-slice in one rotation

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Definitions of Pitch

Old definition: Table travel per rotation

P = slice thickness

New definition:

Table travel per rotationP’=

Total nominal scan width

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GE QXi High Quality (HQ) vs High Speed (HS)

Pitch = 15mm/20 mm =0.75

Pitch = 30mm/20 mm =1.5

20 mm

15 mm table travel

30 mm table travel

20 mm

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Typical characteristics of CT

1972 1980 1990 2000

Minimum scan time 300 s 5-10 s 1-2 s 0.3-1s

Data acquired per 360° 57.6 kB 1 MB 2MB 42 MB

Data per spiral sequence - - 24-48 MB 200-500 MB

Image matrix 802 2562 5122 5122

Power (generator) 2 kW 10 kW 40 kW 60 kW

Slice thickness 13 mm 2-10 mm 1-10 mm 0.5-5 mm

Toshiba Aquilion ONE CT

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320-slice (320 x 0.5 mm), 16cm gantry rotation, Year product introduced: 2007, 7.5 MHU, more than a $1 million

Toshiba Aquilion ONE Vision Edition

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640-slice. 0.275 sec rotation, 16cm gantry rotation, Year product introduced: 2012

Micro CT A miniaturized design The X-rayed measuring field, usually as small as 2cm3

for material testing and analysis, medical applications are on their way to taking center stage (analysis of trabecular structures in bones)

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Dual Energy CT Single Source or Dual Source

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Dual Energy CT

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Dual Source CT

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Dual Source CT

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Dual Source CTDetector 2 x Stellar detectorNumber of slices 2 x 128Rotation time 0.28 s‐1

Temporal resolution 75 ms-1, heart-rate independentGenerator power 200 kW (2 x 100 kW)kV steps 70, 80, 100, 120, 140 kVIsotropic resolution 0.33 mmCross-plane resolution 0.30 mmMax. scan speed 458 mm/s1 with Flash SpiralTable load up to 307 kgGantry opening 78 cm

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SPECT-CT

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SPECT-CT

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PET-CT

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PET-CT

4D PET-CT Image

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PE

T-C

T

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Measure Intensity of a Pencil Beam

X-Ray Source

Radiation Detector

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Principle of X-Ray CT

In one plane, obtain set of line integrals for multiple view angles

Reconstruct cross-sectional views

Detector

Linear scan

Angular scan

Object

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Pixels & Voxels

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Digital ImageDigital Image

2-dimensional array of individual image points calculated

each point called a pixel picture element

each pixel has a value value represents x-ray

transmission (attenuation)

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Pixels & Voxels

Pixel is 2D component of an image

Voxel is 3D cube of anatomyVolume Element

CT reconstruction calculates attenuation coefficients of Voxels

CT displays CT numbers of Pixels as gray shades

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Pixel & Voxel Size

Voxel depth same as slice thickness

Pixel dimension field of view / matrix size

FOV = 30 cm256 pixels 30 cmPixel size = ------------

256 pixels

Pixel size = 0.117 cm = 1.17 mm

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Attenuation Equation forMono-energetic Photon Beams

I = Ioe-x

I = Exiting beam intensityIo = Incident beam intensitye = constant (2.718…) = linear attenuation coefficient

•property of•absorber material•beam energy

x = absorber thickness

MaterialIo

I

x

For photons which are neither absorbed nor scattered

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Example Beam Attenuation

Using equation to calculate beam intensity for various absorber thicknesses ( = .223)

1cm100 80

I = Ioe-x

100*e-(0.223)(1) = 80-20%

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1cm 1cm100 80 64

I = Ioe-x

100*e-(0.223)(2) = 64

-20% -20%

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1cm 1cm 1cm100 80 64 51

I = Ioe-x

100*e-(0.223)(3) = 51

-20% -20% -20%

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1cm 1cm 1cm 1cm100 80 64 51 41

I = Ioe-x

100*e-(0.223)(4) = 41

-20% -20% -20% -20%

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More Realistic CT Example Beam Attenuation for non-uniform Material 4 different materials 4 different attenuation coefficients

#1 #2 #3 #4

1 2 4

Io I

x

I = Ioe-(+++)x

xk

k

eII

0

IIx

kk

0ln

IIdxx 0ln)(

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Reconstruction:Solve for ’s

16 22 11 1017

22

12

10

15

13

11 12 13 14

21 22 23 24

31 32 33 34

41 42 43 44

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Real Problem Slightly More Complex

11 12 13 14

21 22 23 24

31 32 33 34

41 42 43 44

24 13 15 22 16

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13

22

9

14512 values

512values

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Effect of Beam Energy on Attenuation

Low energy photons more easily absorbed High energy photons more penetrating All materials attenuate a larger fraction of low

than high energy photons

Material100 80

Higher-energymono-energeticbeam

30Material

Lower-energymono-energeticbeam

100

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Mono vs. Poly-energetic X-ray Beam Equations below assume Mono-energetic x-

ray beam

#1 #2 #3 #4

1 2 4

Io I

x

I = Ioe-(+++)xI = Ioe-x

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Mono-energetic X-ray Beams

Available from radionuclide sources Not used in CT because beam intensity much

lower than that of an x-ray tube

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X-ray Tube Beam High intensity Produces poly-energetic beam

#1 #2 #3 #4

1 2 4

Io I

x

I = Ioe-(+++)xMono-energetic beam equation!

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Beam Hardening Complication Attenuation coefficients n depend on beam energy!!! Beam energy incident on each block unknown Four ’s, each for a different & unknown energy

1 2 4

1cm 1cm 1cm 1cm

I = Ioe-(+++)x

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Beam Hardening Complication

Beam quality changes as it travels through absorber greater fraction of low-energy photons removed from

beam Average beam energy increases

1cm 1cm 1cm 1cm

Fewer PhotonsBut higher avg

kV than A


kV than B

A B


kV than C

C D


kV than D

E

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Reconstruction

Scanner measures “I” for thousands of pencil beam projections

Computer calculates tens of thousands of attenuation coefficients one for each pixel

Computer must correct for beam hardening effect of increase in average beam energy from one side of

projection to other

I = Ioe-(++++)x

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CT Number (The Hounsfield Unit)

Calculated from reconstructed pixel attenuation coefficient

t - W)HU= CT # = 1000 ------------

W

Where:t = linear attenuation coefficient for tissue in pixelW = linear attenuation coefficient for water

Caculate CT # for Water. Answer: 0Caculate CT # for Air. Answer: -1000

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CT Numbers for Special Stuff

Bone: +1000 Water: 0 Air: -1000

t - W)CT # = 1000 ------------

W

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The Hounsfield scale

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Digital Image MatrixDigital Image Matrix

125 25 311 111 182 222 176

199 192 85 69 133 149 112

77 103 118 139 154 125 120

145 301 256 223 287 256 225

178 322 325 299 353 333 300

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Numbers / Gray ShadesNumbers / Gray Shades

Each number of a digital image corresponds to a gray shade for one pixel

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Digital to Analog Conversion(D to A)

Computer reconstructs digital image set of numbers

Computer displays analog image

125 25 311 111 182 222 176

199 192 85 69 133 149 112

77 103 118 139 154 125 120

145 301 256 223 287 256 225

178 322 325 299 353 333 300

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Analog vs. Digital Images

Analog continuous gray

shade information Digital

Discrete gray shade information

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Digital Image FormationDigital Image Formation

Clinical ImageScreen Wire Mesh

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Digital Image Formation:Sampling


Place mesh over image

Assign each square (pixel) a value based on density

Pixel values form the digital image

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-10

-650

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Each pixel assigned a value

Value averages entire pixelAny spatial variation

within a pixel is lostThe larger the pixel,

the more variation120

-10

-650

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Digital Image FormationDigital Image Formation The finer the mesh (sampling), the more accurate the

digital rendering

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What is this?What is this?

12 X 9 Matrix

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Same object, smaller squaresSame object, smaller squares

24 X 18 Matrix

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48 X 36 Matrix

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96 X 72 Matrix

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192 X 144 Matrix

multi-slice ct or multidetector ct (mdct) 1991 · slice thickness: single detector array scanners...

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