CT Basics Dianna Cody, Ph.D. Professor & Chief, Radiologic Physics U.T. M.D. Anderson Cancer Center
“Pure” CT • Information regarding attenuation correction with CT AND • Information regarding how CT is partnered with PET • Will be covered later in the workshop
Axial Platforms
first generation
Axial/Helical Platforms
second generation
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CT X-ray Tube Design Evacuated glass or metal envelope Oil for insulation and heat dissipation Lead housing absorbs unwanted x-rays Port for useful beam
Beam Collimation • Pre-patient collimators define width of beam in z (all systems) • “Detector” collimators reduce scatter at detectors (some CTs)
X-ray Beam Characteristics • Polyenergetic beam – Bremsstrahlung radiation – Characteristic Ch i i radiation
• Max photon energy depends on kVp • Min photon energy depends on filtration
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
Detector Characteristics X-ray fan beam Filter Shaped beam Patient
• • • • •
Efficiency Response time Dynamic range High reproducibility Electronic stability
Uniform output
Solid State Detectors
Image Reconstruction Process Back Projection – 1st Generation CT Object = Rod in air
• Photodiode multipliers (no PMT) • CdWO4 crystals 99% conversion and capture efficiency
• Ceramics 99% absorption, 3X conversion
Beam direction black arrow 1 (angle 1) Tube detector scans across Tube-detector (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 1 Object 2 Projection data
1 2
3 Recon filter
3
4
4 Backprojection of filtered data 5 Backprojection of filtered data for two angles
Filtered Backprojection Filtered profile
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Filtered Backprojection
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Spatial Resolution
X-Y Voxel Size
Ability to detect a small object easily distinguished from background • • • • • • •
Display Field of View (DFOV) size Reconstruction filter (algorithm, kernel) X-rayy tube focal spot p size Image thickness (blurs edges of objects) Pitch (blurs edges of objects) Patient motion Image zoom Voxel size = DFOV/512
512 pixels
50 cm DFOV Pixel = ~ 1 mm
512 pixels
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
Effects of Recon Filters on Resolution & Noise
Detail Recon
Std Recon
Edge Recon
Contrast Resolution Ability to see a small object not easily distinguished from background (NOISE)
Effects of Recon Filters on Noise Recon Filter Soft Standard Lung Detail Bone Edge Bone Plus
Std Dev Water Img 3.8 4.7 19.6 6.5 18.8 35.8 27.0
• Effective mAs mA * time / pitch
• • • •
Image thickness Patient size Reconstruction filter Viewing conditions
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ACR Phantom - Low Contrast Section
Viewing Conditions - Contrast • Distance • Ambient (room) lighting – Cannot see the stars in the daytime
• • • • 120 kVp, 1600 mAs
Monitor brightness Reflections Viewing angle (flat screens) [Age of eyeballs…]
120 kVp, 192 mAs
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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Typical CT Numbers
CT Number • Pixel bit-depth of 212 = 4096 values • Contrast scale HU = 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
Select CT#’s with WW WL
• • • • • • • •
-1024 Air ~ -700 Lung ~ - 120 to ~ -80 Fat Water 0 +/- 5 Brain ~ 40 Soft Tissue ~ 40 to ~ 100 200 to > 600 Bone Metal > 1000
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
Conventional Helical CT Detectors Image width determined by beam thickness Pitch = table mm / beam mm
• Typical pitch values are 0.7 to 1.5
Pitch Imagine a CT Scanner with a spray paint can in place of the x-ray tube.
z
Pitch Definition • Pitch = distance table travels width of x-ray beam • Pitch = distance table travels width of spray paint
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Pitch
Helical Interpolation Collect data (black dots) Rebin to estimate the 180° data (blue squares) I t Interpolate l t to t estimate ti t image between collected and rebinned data Helical CT needs fast computers
MSCT detectors
Multi-Detector Concept
64 x 0.625 mm
• • • •
Acquisition of multiple images per scan Electronic post-patient collimation Faster volume acquisition times Better bolus tracking and thin slices for 3D
z 1.25 mm
General Electric 4 & 64 & 16 channel detectors
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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 5 mm
channel channel channel channel
4 x 3.75 mm
MSCT Detectors z
z
MSCT Detectors Image width determined by output channel Pitch = table mm / beam mm
z
Pitch = table mm / n * T where n = no. of channels and T = channel thickness
z
THIRD Gen.
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MSCT Faster Scanning Detector
# rotations
1 x 1.25
Beam Thick. (mm) 1.25
160
Total scan time (sec) 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
Ring Artifact
Artifact Sources • Scanner – Detector imbalance – Obstruction of beam – Pitch and detector configuration
• Patient – Motion – Implants (dental, prosthetics, etc.) – Non-uniformity of normal “ingredients”
16-slice CT
3rd
generation and MSCT Detector imbalance
0.625mm image Axial acquisition 16 x 0.625
Material in beam IV contrast ‘gunk’ factor
Image number 7 of group with 16 images
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16-slice CT
16-slice CT
Axial acquisition
Axial acquisition
Image number 8 of group with 16 images
Image number 9 of group with 16 images
Multislice CT
Recall Helical Interpolation Collect data (black dots)
• Helical non-planar data • Data from multiple channels
Rebin (blue squares) Interpolate for image
2nd rotation
1st rotation 1
2
3
4
1
2
3
4 Longitudinal direction
00 complementary data
1800 direct data
3600
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MSCT Ring Artifact • Imbalance of detector causes ring in those axial images that are same width as detector element
16-slice CT 0.625mm image Helical scan 16 x 0.625
• Images from “binned” detector elements may not show h ring i • Helical MSCT images have arc instead of ring
Pitch = 0.562
Image number 5 of group with 16 images
• Arc artifact might not show in images thicker than the element size (depends on pitch and recon alg)
Helical scan
Helical scan
Image number 6 of group with 16 images
Image number 7 of group with 16 images
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Helical scan
Image number 8 of group with 16 images
MSCT Arc Artifact
16-slice CT 1.25mm image
• Might not be visible in each image due to overlying anatomy • Easiest to find when viewing images in “stack” mode • Lower pitch, longer arc • Visibility affected also by WW/WL
Pitch = 1.375 (16 x 0.625) 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
MSCT Helical 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)
High-contrast objects at angle to scan plane
Reducing 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
Same as patient study Pitch: 0.875, Detector: 8×2.5mm, Beam: 20mm
Detector: 8 x 2.5 Pitch = 0.875
SE 2, IM 2, 5mm
SE 3, IM 3, 2.5mm
Retrospective images at 2.5mm
Change detector (incr. Z sampling), retain beam width
Z-axis Sampling Summary
Pitch: 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
• In general, use smallest detector spacing possible! • More powerful than decreasing pitch to reduce 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 sheet. sheet • Thanks!!
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