Translating Protocols Between Scanner Manufacturer and … · Translating Protocols Between Scanner...

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


Disclosures

Research Support:

EB 079861

DK 083007

DK 059933

EB 004898

RR 018898

Siemens Healthcare

Off Label Usage

None

NIH:


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.


The first thing you need is a “dictionary”


http://mayoresearch.mayo.edu/ctcic/educational-resources.cfm


Clarity, Transparency, and Uniformity


Collimation vs. Slice Width




The next thing you need is data

• 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.


Measured width of “5 mm” imageFull vs. Plus reconstruction option


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.


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


Courtesy D. Platten et al. ImPACT (RSNA 2003)

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)


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



Cone-beam algorithm on and off• Low pitch (0.5), Siemens Sensation 16

Standard Cone-beam (AMPR)



Cone-beam algorithm on and off• High pitch (1.5), Siemens Sensation 16

Standard Cone-beam (AMPR)



Cone-beam algorithm on and off• High pitch (1.5), Philips Mx8000 IDT

Standard Cone-beam (COBRA)



Cone-beam algorithm on and off• High pitch (1.5), Toshiba Aquilion 16

Standard Cone-beam (TCOT)



Cone-beam algorithm with pitch• GE LightSpeed 16, cone-beam reconstruction always on

0.562 0.938 1.375 1.735



Clinical relevance

Standard

Cone-beamCourtesy D. Platten et al. ImPACT (RSNA 2003)


Inclined (60°) Teflon rod• High pitch (1.5), Siemens Sensation 16

Standard Cone-beam (AMPR)Standard

13 cm off center

Cone-beam (AMPR)

13 cm off center



Dose Efficiency vs. Collimation

Siemens Sensation 16


GE Recon Algorithms

Soft

Standard

Detail

Lung

Bone

Edge

Bone Plus


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


Boone: Presampled MTF in CT (Med Phys 2000)


Siemens Recon Kernels

• 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


Siemens Recon Kernels

• B18

• B20

• B25/B26 - cardiac

• B30

• B31 – finer grain noise

• B35/36 - calcium

• B40

• B41 – finer grain noise

• B45

• B46 - cardiac/lung

• B50

• B70

• 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


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


B10


B20


B25


B26


B30


B31


B35


B36


B40


B41


B45


B46


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.


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


H10


H20


H21


H22


H30


H31


H32


H37


H40


H41


H42


H45


H47


H48


H50


H60


H70


Have I made your head spin yet?


GE bowtie and focal spot selection(once upon a time)


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


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)


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


Routine Chest


Routine Chest


Routine Chest


Routine Chest


Routine Chest


Routine Chest


Routine Chest


J. Kofler, L. Yu, S. Leng, M. Bruesewitz, T. Vrieve

Mayo CT Clinic Innovation Center and Dept. of Radiology

http://mayoresearch.mayo.edu/CTCIC

Date post:	27-Mar-2019
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Translating Protocols Between Scanner Manufacturer and … · Translating Protocols Between Scanner...

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