Spiral CT Protocol Optimization

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Spiral CTProtocol Optimization Quality
Assurance
Ge Wang, Ph.D.Department of RadiologyUniversity
of IowaIowa City, Iowa 52242, USA
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Spiral CTProtocol Optimization Quality
Assurance

• Protocol optimization
• Techniques parameters
• Imaging model
• Optimal pitch
• Quality Assurance
• Monthly test
• Dose measurement
• Future

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Techniques

• Tube voltage
• 137 kV - 100 (20 cm water)
• 120 kV - 40
• 80 kV - 20
• Product of tube current scan time (mAs)
• The higher mAs, the weaker noise
• Reconstruction filters display settings
• The higher-pass filter, the sharper image, the
  stronger noise
• Window level, window width

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Filters
P(??t)
P(??t)
P(??t)
f(x,y)
f(x,y)
g(x,y)
Filtering in the projection domain P,
fFiltering in the image domain P, g Smooth
- Standard - SharpSoft tissue - Standard - High
- Ultrahigh
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Image Display
CT Number - Hounsfield unit Air -1024
Water 0 Bone 175 to 3071 Viewing
Parameters Window level (L) Window width
(W) Zoom factor
0
255
-1024
3071
W
L
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Parameters

• Detector collimation
• Table feed
• Scanning time
• Reconstruction interval

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Detector Collimation

• 1 mm temporal bone
• 2-3 mm lung nodule, renal arteries
• 5 mm neck, kidney, pancreas
• 8 mm chest, liver

Source
Detector
Collimation
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Pitch

• Pitch Table Feed / Collimation
• In current practice
• Generally, pitch1
• Not to exceed pitch2

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Slice Thickness Formula
SSP
D
T
s
? Standard deviation of the SSP D Detector
collimation T Table feed
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Slice Thickness Formula
? Standard deviation of the SSP D Detector
collimation T Table feed

• For reduction of slice thickness
• Reduction of D is more effective than
• reduction of T
• When pitch goes from 1 to 2,
• slice thickness is degraded by about 40

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Number of Reconstructed Slicesper Collimation
n slice number per collimation p pitch defined
as ratio of table feed over collimation
To avoid aliasing the MTF, about 2-3
transverse slices should be reconstructed per
collimation. Wang and Vannier Medical Physics
241635-1639, 1997
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Image Noise

• Due to quantum and other random effects
• Depending on
• Collimation
• Tube voltage
• mAs product
• Patient Size
• HVL for water 3.6 cm
• Voxel size
• Algorithm

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Image Noise
h Standard deviation of the noise I Tube
current D Detector collimation
h Standard deviation of the noise N Number of
X-ray photons
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Maximum Scanning Time Range
t
t Max scanning time I Tube current L Max
scanning length T Table increment
I
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Imaging Model
D detector collimation T table increment p pitch,
T/D ? SSP standard deviation ? image noise
standard deviation I tube current t maximum
scanning time L maximum scanning range
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Optimization Problems

• Given slice thickness and image noise,
• maximize scanning range
• Given scanning range and image noise,
• minimize slice thickness
• Given scanning range and slice thickness,
• minimize image noise
• Given scanning range, minimize
• product of slice thickness and image noise,
• that is, maximize signal-to-noise ratio

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Optimal Pitch
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Spiral CTProtocol Optimization Quality
Assurance

• Protocol optimization
• Techniques parameters
• Imaging model
• Optimal pitch
• Quality Assurance
• Monthly test
• Dose measurement
• Future

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CT Performance Phantom
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Monthly Test
For the consistency of the system MTF of a
thin wire 2 Slice sensitivity
profile Full-Width-at-Half-Maximum (FWHM)
Image noise Homogeneity Contrast CTwater /
(CTwater - CTair) Position of the light-beam
indicator Exactness of the table top position
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Spiral CTProtocol Optimization Quality
Assurance

• Protocol optimization
• Techniques parameters
• Imaging model
• Optimal pitch
• Quality Assurance
• Monthly test
• Dose measurement
• Future

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Radiation Dose
Dose - radiation energy transferred to an
anatomic structure during X-ray scanning The
unit of dose is Gray (Gy) sometimes Rad (0.01
Gy) Typical values for a CT transaxial scan are
in the range of 30 to 50 mGy
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Radiation Profile Single Scan
Ideal profile
Real profile
Radiation spreads outside the designated slice
due to scattering
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CT Dose Index
CTDI CT dose index T slice thickness D(z)
local dose z longitudinal coordinate
T
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Radiation Profile Multiple Scans
Real profile
Radiation dose from multiple scans
are accumulated in the central slice
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Multiple Scan Average Dose
I
MSAD multiple scan average dose I inter-slice
distance N Number of scans
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Dose Measurement

• Cylindrical phantoms of 16 cm 32 cm
• Pencil ionization chamber
• Dosimeter

16 or 32 cm
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MSAD Estimation

• MSAD is
• directly proportional to mA
• directly proportional to scan time
• increases with kVp
• as compared to dose at 120 kVp
• 0.2-0.4 times less at 80 kVp
• 1.2-1.4 times more at 140 kVp
• increases slightly with decreasing slice
  thickness
• similar at the iso-center and near surface for
  head
• significantly less at the iso-center than
• near surface for body

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Spiral CTProtocol Optimization Quality
Assurance

• Protocol optimization
• Techniques parameters
• Imaging model
• Optimal pitch
• Quality Assurance
• Monthly test
• Dose measurement
• Future

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Cone-Beam Spiral CT

• Simultaneous
• Source rotation
• Table translation
• Data acquisition
• 2D detector array

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Cone-BeamImage Reconstruction
(From AMIL, SUNY/Buffalo)
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Image Analysis

• Visualization analysis
• 3D, 4D
• Networked, PC-based
• Image fusion
• Computer aided diagnosis
• Image-based surgery

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References
T. S. Curry III, J. E. Dowdey, R. C. Murry Jr.
Christensens physics of diagnostic Radiology
(4th edition), Lea Febiger (for residents) G.
Wang, M. W. Vannier Computerized tomography.
Encyclopedia of Electrical and Electronics
Engineering, edited by Webster JG, to be
published by John Wiley Sons (for
engineers) http//dolphin.radiology.uiowa.edu/ge
(on-line slides handouts in the Teaching
section)