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
1
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
2
Beam Filtration
Detector Characteristics
X-ray fan beam
Filter
Shaped beam
Patient
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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
3
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
5
Filtered Backprojection
4
Spatial Resolution
X-Y Voxel Size
Ability to detect a small object easily distinguished
from background
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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
5
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
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Image thickness
Patient size
Reconstruction filter
Viewing conditions
6
ACR Phantom - Low Contrast Section
Viewing Conditions - Contrast
• Distance
• Ambient (room) lighting
– Cannot see the stars in the daytime
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•
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)
7
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
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-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
8
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
9
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
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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
10
Channels (or data channels)
Detector Configuration
detector
4 x 1.25 mm
4 x 2.5 mm
11
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.
12
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
13
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
14
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
15
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
16
Arc visible, but
faint
Arc visible, but
faint
17
Helical or Windmill Artifact
MSCT Helical Artifact
• Occurs at high subject contrast interface
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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
18
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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