Technology Assessment Initiative: Summit on CT Dose
Translating Protocols Between
Scanner Manufacturer and Model
Cynthia H. McCollough, PhD, FACR, FAAPM
Professor of Radiologic Physics
Director, CT Clinical Innovation Center
Department of Radiology
Mayo Clinic, Rochester MN
Technology Assessment Initiative: Summit on CT Dose
Disclosures
Research Support:
NIH:
EB 079861
DK 083007
DK 059933
EB 004898
RR 018898
Siemens Healthcare
Off Label Usage
None
Technology Assessment Initiative: Summit on CT Dose
Disclosures
• Our team currently supports 25 CT systems.
Presently they are from only two manufacturers
(GE and Siemens)
– 10 distinct scanner models (all multi-slice)
– 12 distinct multi-slice models in our practice since 1998
• Examples come from the systems I know best
• Am an “equal opportunity” critic, so if I know
about weaknesses (or strengths) of other systems,
I’m happy to share those also.
Technology Assessment Initiative: Summit on CT Dose
The first thing you need is a “dictionary”
Technology Assessment Initiative: Summit on CT Dose
http://mayoresearch.mayo.edu/ctcic/educational-resources.cfm
Technology Assessment Initiative: Summit on CT Dose
Clarity, Transparency, and Uniformity
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Collimation vs. Slice Width
Technology Assessment Initiative: Summit on CT Dose
Technology Assessment Initiative: Summit on CT Dose
Technology Assessment Initiative: Summit on CT Dose
The next thing you need is data
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When is a 5-mm not a 5-mm?
Which pitch values give best images?
Which collimations are most/least dose efficient?
Which reconstruction algorithms/kernels have
“special features” or alter CT number accuracy?
• Which bowtie is used for which scan modes?
• Which focal spot is used when?
• etc.
Technology Assessment Initiative: Summit on CT Dose
Measured width of “5 mm” image
Full vs. Plus reconstruction option
Technology Assessment Initiative: Summit on CT Dose
Full or Plus Mode
• Full mode will retain the prescribed slice thickness
• Plus mode will give you a thicker slice thickness
than prescribed (about 20% thicker, e.g. a 5 mm
becomes a 6 mm). Correspondingly, noise level
will be about 10% lower.
Technology Assessment Initiative: Summit on CT Dose
Pitch vs. Image Quality
• In spiral CT, image noise is dependent on pitch
– To compensate, mAs must be changed as pitch is
changed
– Relationship is linear on some systems, but not all
• Relationship is different for cardiac reconstructions
– Noise is INDEPENDENT of pitch in cardiac CT
• Image width can be affected by pitch
• Windmill and cone beam artifacts affected by pitch
– Windmill artifacts discussed in talks by Gupta, Morin
Technology Assessment Initiative: Summit on CT Dose
Study to evaluate cone beam artifacts
• A thin-walled object with edges at an angle to the scan plane
• Rate of change of funnel shape is constant along the z-axis
• Scanned in air, the funnel has high contrast (~ 500 HU)
Courtesy D. Platten et al. ImPACT (RSNA 2003)
Technology Assessment Initiative: Summit on CT Dose
Example images
• Single slices through the
funnel appear as rings
• MIP image of many slices
results in a wider ring
• If perfect the images should
be uniform
Courtesy D. Platten et al. ImPACT (RSNA 2003)
Technology Assessment Initiative: Summit on CT Dose
Cone-beam algorithm on and off
• Low pitch (0.5), Siemens Sensation 16
Standard
Cone-beam (AMPR)
Courtesy D. Platten et al. ImPACT (RSNA 2003)
Technology Assessment Initiative: Summit on CT Dose
Cone-beam algorithm on and off
• High pitch (1.5), Siemens Sensation 16
Standard
Cone-beam (AMPR)
Courtesy D. Platten et al. ImPACT (RSNA 2003)
Technology Assessment Initiative: Summit on CT Dose
Cone-beam algorithm on and off
• High pitch (1.5), Philips Mx8000 IDT
Standard
Cone-beam (COBRA)
Courtesy D. Platten et al. ImPACT (RSNA 2003)
Technology Assessment Initiative: Summit on CT Dose
Cone-beam algorithm on and off
• High pitch (1.5), Toshiba Aquilion 16
Standard
Cone-beam (TCOT)
Courtesy D. Platten et al. ImPACT (RSNA 2003)
Technology Assessment Initiative: Summit on CT Dose
Cone-beam algorithm with pitch
• GE LightSpeed 16, cone-beam reconstruction always on
0.562
0.938
1.375
1.735
Courtesy D. Platten et al. ImPACT (RSNA 2003)
Technology Assessment Initiative: Summit on CT Dose
Clinical relevance
Standard
Courtesy D. Platten et al. ImPACT (RSNA 2003)
Cone-beam
Technology Assessment Initiative: Summit on CT Dose
Inclined (60°) Teflon rod
• High pitch (1.5), Siemens Sensation 16
Standard
Cone-beam (AMPR)
Cone-beam
Standard
13 cm off center
13 cm (AMPR)
off center
Courtesy D. Platten et al. ImPACT (RSNA 2003)
Technology Assessment Initiative: Summit on CT Dose
Dose Efficiency vs. Collimation
Siemens Sensation 16
Technology Assessment Initiative: Summit on CT Dose
GE Recon Algorithms
Soft
Standard
Detail
Lung
Bone
Edge
Bone Plus
Technology Assessment Initiative: Summit on CT Dose
CT Number Accuracy
• Some edge-enhancing algorithms/kernels can alter CT
numbers
– E.g. GE Lung and Bone Plus
• Boedeker et al. Emphysema: Effect of reconstruction
algorithm on CT imaging measures. Radiology 2004
• Zhang, McCollough, et al. Selection of Appropriate
Computed Tomographic Image Reconstruction
Algorithms for a Quantitative Multicenter Trial of
Diffuse Lung Disease. JCAT 2008
Technology Assessment Initiative: Summit on CT Dose
Boone: Presampled MTF in CT
(Med Phys 2000)
Technology Assessment Initiative: Summit on CT Dose
Siemens Recon Kernels
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B10  B90 Body
H10  H90 Head
U30  U90 Ultra High Resolution
T20  T81 Topogram
Lower number smoother
Higher number sharper
Multiples of 10 are the “basic” kernels
In between values are “special” kernels
Technology Assessment Initiative: Summit on CT Dose
Siemens Recon Kernels
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B18
B20
B25/B26 - cardiac
B30
B31 – finer grain noise
B35/36 - calcium
B40
B41 – finer grain noise
B45
B46 - cardiac/lung
B50
B70
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H30
H31– finer grain noise
H32 – no PFO
H37 – GE like
H40
H41– finer grain noise
H42 – no PFO
H47 – GE like
H48 – GE like but sharper
U70
Technology Assessment Initiative: Summit on CT Dose
Special Body Kernels
• B25 and B26 are for cardiac with edge-preserving
noise reduction.
• B35 and B36 are for Ca scoring without edge
enhancement.
• B45 is intermediate sharpness between B40 and B50
(e.g. “not very special”)
• B46 is designed specifically for accurate assessment
of inside coronary stents with 3D edge preserving
noise reduction techniques.
• B75 is comparable to B70 in sharpness but used 2D
edge-preserving noise reduction
Technology Assessment Initiative: Summit on CT Dose
B10
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B20
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B25
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B26
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B30
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B31
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B35
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B36
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B40
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B41
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B45
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B46
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Noise and Noise Uniformity
• B31/41 is like B30/B40 but with finer grain noise
and a milder edge enhancement. Noise more
uniform over FOV.
Technology Assessment Initiative: Summit on CT Dose
Special Head Kernels
• H21, H31, H41 are like H20, H30, H40 but with finer
grain noise and a milder edge enhancement.
• H22, H32, H42 don’t include iterative beam
hardening correction (PFO). Reconstruction speed is
faster, but the reconstructed images may contain
significant beam hardening artifacts.
• H37 is comparable to GE Soft
• H45 is intermediate sharpness between H40 and H50
• H47 is comparable to GE Standard
• H48 is like H47 but a bit sharper
Technology Assessment Initiative: Summit on CT Dose
H10
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H20
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H21
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H22
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H30
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H31
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H32
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H37
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H40
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H41
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H42
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H45
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H47
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H48
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H50
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H60
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H70
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Have I made your head spin yet?
Technology Assessment Initiative: Summit on CT Dose
GE bowtie and focal spot selection
(once upon a time)
Technology Assessment Initiative: Summit on CT Dose
Moral of the story
• There are many good reasons to invoke special
features and characteristics
• Manufacturers often tie these features to protocols
where they make sense
• There are many ways to get these features when you
don’t want them or to not find them when you do
• Often users are not even educated about them
• Don’t stop having good ideas and features
– But make them transparent so user knows what they do
and when they are used
Technology Assessment Initiative: Summit on CT Dose
Lastly, you need “deliverables”
• To design or translate a protocol, you need to know
what the final product needs to look like
– Scan time (total) and per image (temporal resolution)
– Slice width(s) and image plane(s) required
• The thinnest image width determines the detector
configuration
• Coronals and sagitals require thinner collimation
– Image sharpness or smoothness
– Noise level
– Target anatomy/patient (pediatric, obese, cardiac, etc)
– Diagnostic reference level (CTDIvol)
Technology Assessment Initiative: Summit on CT Dose
Knowing terminology, operation, features
(quirks) and performance of your system(s)
• You can translate across manufacturer (make) and
model to yield the desired “deliverables”
• Usually not one way to accomplish the same results
• Usually not a lot of ways
• Evaluate options as quantitatively as possible on
phantoms and then form a WIP prootcol for clinical
evaluation/refinement
– Some differences between seemingly similar options can
show up only in patients, where motion and specific
diagnostic criteria (like noise texture or subtle
enhancement of small structures) come into play
Technology Assessment Initiative: Summit on CT Dose
Routine Chest
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Routine Chest
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Routine Chest
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Routine Chest
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Routine Chest
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Routine Chest
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Routine Chest
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Mayo CT Clinic Innovation Center and Dept. of Radiology
J. Kofler, L. Yu, S. Leng, M. Bruesewitz, T. Vrieve
http://mayoresearch.mayo.edu/CTCIC