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CT BasicsDianna Cody, Ph.D.
Professor & Chief, Radiologic PhysicsU.T. M.D. Anderson Cancer Center
“Pure” CT
• Information regarding attenuation correction with CT AND
• Information regarding how CT is partneredInformation regarding how CT is partnered with PET
• Will be covered later in the workshop
Axial Platforms
first generation second generation
Axial/Helical Platforms
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CT X-ray Tube DesignEvacuated glass or metal envelope
Oil for insulation and heat dissipation
Lead housing absorbs unwanted x-rays
Port for useful beam
X-ray Beam Characteristics
• Polyenergetic beam– Bremsstrahlung
radiationCh i i– Characteristic radiation
• Max photon energy depends on kVp
• Min photon energy depends on filtration
Beam Collimation• Pre-patient
collimators define width of beam in z (all systems)
• “Detector” collimators reduce scatter at detectors (some CTs)
Beam Filtration
• Removes low energy x-rays from beam– Low E photons contrib to dose, not image– Filter reduces beam-hardening artifacts
• Shapes energy distribution across beam– Removes more low energy from edges– Results in more uniform beam hardening after
passing through filter and patient
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Beam Filtration
X-ray fan beam
Filter
Shaped beamShaped beam
Patient
Uniform output
Detector Characteristics
• Efficiency• Response time• Dynamic range• High reproducibility• Electronic stability
Solid State Detectors
• Photodiode multipliers (no PMT)
• CdWO4 crystalsCdWO4 crystals99% conversion and
capture efficiency• Ceramics
99% absorption, 3X conversion
Back Projection – 1st Generation CTObject = Rod in air
Beam direction black arrow 1 (angle 1)
Tube detector scans across
Image Reconstruction Process
Tube-detector scans across (red arrows)
Data (Profile 1) recorded w/ detector position
Repeat for Angle 2 to get Profile 2, etc
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Backprojection Reconstruction Filters
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3 4
1 Object
2 Projection data
3 Recon filter
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4 Backprojection of filtered data
5 Backprojection of filtered data for two angles
Filtered Backprojection
Filtered profileFiltered Backprojection
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Spatial ResolutionAbility to detect a small object easily distinguished
from background• Display Field of View (DFOV) size• Reconstruction filter (algorithm, kernel)• X-ray tube focal spot sizey p• Image thickness (blurs edges of objects)• Pitch (blurs edges of objects)• Patient motion• Image zoom Voxel size = DFOV/512
512 50 cm DFOV
X-Y Voxel Size
512 pixels
pixelsPixel = ~ 1 mm
Effects of Recon Filters on Resolution & Noise
Std Recon Soft Recon
Effects of Recon Filters on Resolution & Noise
Std Recon Bone Recon
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Effects of Recon Filters on Resolution & Noise
Std Recon Detail Recon
Effects of Recon Filters on Resolution & Noise
Std Recon Edge Recon
Effects of Recon Filters on NoiseRecon Filter Std Dev
Water ImgSoft 3.8
Standard 4.7Lung 19.6Detail 6.5Bone 18.8Edge 35.8
Bone Plus 27.0
Contrast Resolution Ability to see a small object not easily distinguished
from background (NOISE)
• Effective mAsmA * time / pitch
• Image thickness• Image thickness• Patient size• Reconstruction filter• Viewing conditions
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ACR Phantom - Low Contrast Section
120 kVp, 1600 mAs 120 kVp, 192 mAs
Viewing Conditions - Contrast
• Distance• Ambient (room) lighting
Cannot see the stars in the daytime– Cannot see the stars in the daytime• Monitor brightness• Reflections• Viewing angle (flat screens)• [Age of eyeballs…]
Pixels and Image Matrices Pixels and Image Matrices
222 220 200 146 103
200 158 127 96 73
207 131 103 82 86
202 126 112 124 133
Pixel Values (HU)
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CT Number
• Pixel bit-depth of 212 = 4096 values• Contrast scale
HU = Constant (µ µ ) / µHU = Constant (µm – µwater) / µwater– CT number for water = 0 at all energies– CT number range –1024 to +3072
• CT number affected by kVp– Reduce kVp, increase contrast
Typical CT Numbers• Air• Lung• Fat
-1024~ -700
~ - 120 to ~ -80• Water• Brain• Soft Tissue• Bone• Metal
0 +/- 5~ 40
~ 40 to ~ 100200 to > 600
> 1000
Select CT#’s with WW WL Slip-ring
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Pitch for Single-Slice CT
• Image and beam width are same for conventional CT
• Pitch = table travel ÷ beam width
• Typical pitch values are 0.7 to 1.5
Conventional Helical CT Detectors
Image width determined by beam thicknessbeam thickness
Pitch = table mm / beam mm
z
PitchImagine a CT Scanner with a spray paint can in place of the x-ray tube.
Pitch Definition
• Pitch = distance table travelswidth of x-ray beam
• Pitch = distance table travelswidth of spray paint
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PitchHelical Interpolation
Collect data (black dots)
Rebin to estimate the 180° data (blue squares)
I t l t t ti tInterpolate to estimate image between collected and rebinned data
Helical CT needs fast computers
Multi-Detector Concept
• Acquisition of multiple images per scan• Electronic post-patient collimation• Faster volume acquisition times• Better bolus tracking and thin slices for 3DBetter bolus tracking and thin slices for 3D
MSCT detectors
z
64 x 0.625 mm
General Electric 4 & 64 & 16 channel detectors
1.25 mm
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Channels (or data channels) Detector Configuration
detector
4 x 1.25 mm 4 x 2.5 mm
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4 x 3.75 mm4 x 5 mm
MSCT Detectors
z z
chan
nel
chan
nel
chan
nel
chan
nel
z
THIRD Gen.
MSCT Detectors
Image width determined by output channel
Pitch = table mm / beam mmPitch table mm / beam mm
Pitch = table mm / n * T where n = no. of channels and T = channel thickness
z
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MSCT Faster ScanningDetector Beam Thick.
(mm)# rotations Total scan
time (sec)1 x 1.25 1.25 160 128
4 x 1.25 5 40 32
8 x 1.25 10 20 16
16 x 1.25 20 10 8
64 x .625 40 5 4
1.25mm images and 20cm scan length at 0.8sec rotation and 1.0 pitch
Artifact Sources• Scanner
– Detector imbalance– Obstruction of beam– Pitch and detector configuration
• Patient– Motion– Implants (dental, prosthetics, etc.)– Non-uniformity of normal “ingredients”
Ring Artifact3rd generation and MSCT
Detector imbalance
Material in beam IV contrast
‘gunk’ factor
0.625mm image
Axial acquisition
16 x 0.625
16-slice CT
Image number 7of group with 16 images
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Axial acquisition
16-slice CT
Image number 8of group with 16 images
Axial acquisition
16-slice CT
Image number 9of group with 16 images
Recall Helical Interpolation
Collect data (black dots)
Rebin (blue squares)
Interpolate for image
Multislice CT
• Helical non-planar data• Data from multiple channels
1 2 3 41st rotation 2nd rotation
1 2 3 43
direct data
complementary data
Longitudinal direction
00
1800
3600
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MSCT Ring Artifact
• Imbalance of detector causes ring in those axial images that are same width as detector element
• Images from “binned” detector elements may not h ishow ring
• Helical MSCT images have arc instead of ring
• Arc artifact might not show in images thicker than the element size (depends on pitch and recon alg)
0.625mm image
Helical scan
16 x 0.625
Pitch = 0.562
16-slice CT
Pitch 0.562
Image number 5of group with 16 images
Helical scan
Image number 6of group with 16 images
Helical scan
Image number 7of group with 16 images
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Helical scan
Image number 8of group with 16 images
MSCT Arc Artifact
• Might not be visible in each image due to overlying anatomy
• Easiest to find when viewing images inEasiest to find when viewing images in “stack” mode
• Lower pitch, longer arc• Visibility affected also by WW/WL
1.25mm image
Pitch = 1.375
(16 x 0.625)
16-slice CT
Position of arc inferred from other images in series
Visibility depends on local anatomy
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Arc visible, but faint
Arc visible, but faint
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Helical or Windmill Artifact
• Occurs at high subject contrast interface– Bone and soft tissue (ribs, skull)– Air and soft tissue– Air and Ba contrast– i.v. contrast in tubing
• Varies with – Angle of interface w.r.t. scan plane– Pitch and image width (combined)
MSCT Helical Artifact
High-contrast objects at angle to scan planeReducing Helical Artifact
• Increase z-axis sampling• Change pitch, if possible• Change detector configuration, if possible
• For Prospective study with thin retros – check all image thicknesses at several pitches
(scan a phantom)– choose optimal pitch for all desired image
thicknesses
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Prospective images at 5mm
Scanner: 16-channel
Detector: 8 x 2.5
Pitch = 0.875
Retrospective images at 2.5mm
Same as patient study
Pitch: 0.875, Detector: 8×2.5mm, Beam: 20mm
SE 2, IM 2, 5mm SE 3, IM 3, 2.5mm
Change detector (incr. Z sampling), retain beam widthPitch: 1.375, Detector: 16×1.25mm, Beam: 20mm
Effective mAs = 109 (decreased from 171)SE 10, IM 2, 5mm SE 11, IM 3, 2.5mm
Z-axis Sampling Summary
• In general, use smallest detector spacing possible!
• More powerful than decreasing pitch to reduce helical artifactsreduce helical artifacts
• Beam width may change with detector configuration
• Changes in beam width and/or pitch will affect total scan acquisition time
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End
• Please score this section on evaluation sheetsheet.
• Thanks!!