aapm activities with respect to ct...

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AAPM ACTIVITIES WITH RESPECT TO CT IMAGINGCynthia McCollough, PhD, DABR, FAAPM, FACR, FAIMBE

President-elect designate, AAPM Chair, CT subcommittee Grant recipient, Siemens Healthcare

DISCLOSURES

At the completion of this presentation, the learner should be able to • Name at least 3 activities of the AAPM CT

subcommittee• Identify at least 2 methods of acquiring

dual-energy CT data• Describe how to calculate SSDE

LEARNING OBJECTIVES

ORGANIZATIONAL STRUCTURE

Translates the dosimetry framework developed in Report 111 into a practical set of tests and phantoms.

Gives practical recommendations for the acceptance and subsequent periodic testing of computed tomography machines.

Includes the design of phantoms and the development of testing methodology.

TG200: CT DOSIMETRY PHANTOMS AND THE IMPLEMENTATION OF AAPM REPORT NUMBER 111

EQUILIBRIUM DOSE IN CT

PHANTOM FOR MEASURING REFERENCE VALUES FOR EQUILIBRIUM DOSE

3 sections 30 cm diameter 60 cm total length

high density polyethylene

NOT INTENDED FOR ROUTINE QC

Briefly summarizes existing performance evaluation tests in CT

Introduces a series of advanced image quality assessment techniques, including• task-based assessment• performance evaluation of iterative

reconstruction techniques • performance assessment of automatic tube

current modulation techniques

TG233: PERFORMANCE EVALUATION OF COMPUTED TOMOGRAPHY SYSTEMS

9

Model observers

• Evaluate low‐contrast detection performance

Template Choice 1 Choice 2

10 mm

9.5 mm

6.3 mm

4.8 mm 4 mm

3.2 mm 2.4 mm

21 14 7 HU

Based on noise, 33% of original dose is OK with IR

FBPIR MildIR StdIR Str

Noise(33% dose) ≈ Noise(100% dose)

10

IR – 25% dose FBP– 100% dose

11

With similar image noise, detection performance has been compromised with IR

Model observer predicts degraded performance at 25% dose

100% dose

25% dose

-Δ17% AUC

12

Model observer predicts comparable performance at 78% dose

78% dose

100% dose

≈ AUC

13

With comparable AUC, detection performance is close with IR

IR– 78% dose FBP– 100% dose

14

Prepare a comprehensive educational primer on multi-energy CT

Topics include • rationale for and fundamental physics of

multi-energy CT• different commercial implementations of

dual-energy CT• dual-energy post-processing• dosimetric considerations

TG291: TASK GROUP ON EDUCATIONAL REPORT ON MULTI-ENERGY CT

Clinical Motivation

▸ CT number depends on x‐ray attenuation– Physical density (g/cm3) [electron‐density]– Atomic number (Z)

▸ Different materials can have the same CT number if atomic number differences are offset by appropriate density differences

▸ Multi‐energy CT– Allows separate determination of density and Z – Can provide material composition information

1.0E-01

1.0E+00

1.0E+01

1.0E+02

10 30 50 70 90 110 130 150Energy / keV

X-ra

y ab

sorp

tion

IodineBone

Strong increase

Weak increase

McCollough et al., Radiology 276: 3 (2015)

Current Acquisition Methods for Multi‐Energy CT:

Single Tube Potential

Split Beam Filtration

▸ Single spiral acquisition over entire scan volume

▸ One spectrum lags the other by half a rotation

Au filterSn filter

Dual Layer Detectors

ScintillatorPhotodiode

ScintillatorPhotodiode

X-rays

Reflectors

Low energy spectrumHigh energy spectrum

Two or more energy levels

Semiconductor detector directly converts x-ray to charge (e. g. CdTe)

X-rays

Photon Counting Detectors (PCD)

Low Energy BinHigh Energy Bin

Signals are “binned” according to energy level

* Courtesy Ken Taguchi, John Hopkins

In vivo PCD results▸ 63 year old female (30 cm lateral width at kidney)

▸ Non‐contrast‐enhanced CT of the abdomen

Mixed DSCT PCD‐CT ‐ Tlow

Courtesy of Dr. J.G. Fletcher

Current Acquisition Methods for Multi‐Energy CT:

Dual Tube Potential

Slow kVp switching

▸ Consecutive scans of entire scan volume

Axial Spiral

Inter‐scan delay = scan time + table move time

Unacceptable motion misregistration for most casesMay be acceptable for large volume acquisitions (entire volume scanned in one rotation) Low kVp

High kVp

Slow kVp switching

▸ Consecutive scans of one anatomic section

Axial Spiral

Inter‐scan delay = rotation time + kV switching time

Motion misregistration will limit many applications

X

Low kVpHigh kVp

Rapid kVp switching

▸ Tube potential switched between successive views

Low kVpHigh kVp

Dual‐source geometry

▸ Two tubes/generators allow simultaneous collection of dual‐kVp data

Low kVpHigh kVp

Clinical Applications of Multi‐Energy CT

Color‐coded stones from in vivo study

UA CYS

COX/BRU/STR

UA

APA

Qu et al., Eur Radiol (2013) 23:1408–1414

High density material in soft tissues within and surrounding joints consistent with tophaceous deposits

Courtesy of Dr. Katie Glazebrook

April December

Before & after images demonstrate 90% reduction in volume of uric acid crystals over 8 months after receiving multiple infusions of rasburicase.

Courtesy of Dr. Katie Glazebrook

Courtesy of Dr. Amy Krambeck

DECT Virtual non‐Ca MRI Single energy CT


Silva et al, Dual‐Energy (Spectral)CT: Applications in Abdominal Imaging. Radiographics 2011

85 keV40 keV 120 keV

Virtual mono‐energetic images (VMI)

Standard Image VMI (105 keV)

Courtesy of Dr. Lifeng Yu

Develop a quality control program for performance evaluation of MECT systems

Define the appropriate tests, frequency and tolerance limits for MECT system evaluation.

TG299: QUALITY CONTROL IN MULTI-ENERGY COMPUTED TOMOGRAPHY (MECT)

Determining the conversion factors for head that parallel those produced in Report 204 for the torso.

TG293: TASK GROUP ON SIZE SPECIFIC DOSE ESTIMATES (SSDE) FOR HEAD CT

CTDI quantifies scanner radiation output Patient size must be considered to estimate patient dose

CTDIVOL IS NOT PATIENT DOSE

McCollough, et al, Radiology, May 2011

Estimates • Mean dose• Center of scan range• Specific size

Closer to “real patient dose”

SSDE

SSDE DEFINITIONSSDE = CTDIvol × Conversion factor

SSDE = fsize x CTDIvol

SIZE SPECIFIC DOSE ESTIMATION (SSDE)

AAPM Report 204


Patient dimension such as • anterioposterior thickness (AP)• lateral width (LAT)• AP+LAT

Tabulated conversion factors, fsize

HOW TO DETERMINE SSDE

* AAPM TG Report 204. 2011


AP

LAT

CT Radiograph

(Dw)

5.40 mGy = CTDIvol

SSDE = 5.4 mGy × 2.29

SSDE = 12.4 mGy

± 20%

SAMPLE CALCULATION

32 CM CTDIVOL CONVERSION FACTORS

(Dw) (Dw) (Dw) (Dw)

Effective diameter in Report 204 = Water equivalent diameter in Report 220

16 CM CTDIVOL CONVERSION FACTORS

(Dw) (Dw) (Dw) (Dw)

Effective diameter in Report 204 = Water equivalent diameter in Report 220

Diameter of a water cylinder that would absorb the same dose as the irradiated cross-section of the patient

WATER EQUIVALENT DIAMETER (DW)

Huda et al. Effective doses to patients undergoing thoracic computed tomography examinations. Med Phys, 2000Menke. Comparison of different body size parameters … in body CT of adults. Radiology, 2005; 236:565‐71

circle of water with equal ATTENUATION

WATER EQUIVALENT DIAMETER (DW)

Dw

AP

lateral

Dwlateral

BODY CT DATA (N=801)

• CTDIvol depended on patient size

• Patient size explained 42% of the variation in CTDIvol

• SSDE was independent of patient size

• Slope decreased 9‐fold• Patient size explained <1% of

variation in SSDEChristner et al. Radiology 265(3) 2012

The relationship between SSDE and patient size depends on how aggressively dose is adjusted as patient size varies• There is no fundamental reason to expect that

SSDE should be the same across patient sizes• It just happens to be that way for this specific

automatic exposure control system• Diagnostic image quality requirements should

dictate how to adjust dose as patient size varies, and SSDE will be whatever that dictates

CAVEAT

Establish a framework for creating a national computed tomography (CT) image quality index registry.

Recommend specific image quality indices to be collected and methods of data collection, storage, access, analysis and reporting.

TG300: IMAGE QUALITY REGISTRY FOR CT

Longstanding working group under the Technology Assessment Committee

Provides routine reports to the CT Subcommittee.

Produces • educational documents • protocols for common CT exams• terminology lexicon • links to manufacturer education information.

WGCTNP: ALLIANCE FOR QUALITY CT


and 28 guests and consultants


If you’ve not visited this site, please explore it. There is a lot of great information here.

CT Subcommittee has regular interactions with the individuals working on CT-related DICOM standards

New CT standards include:• DICOM standard on CT protocols • DICOM standard on multi-energy CT

DICOM WG 21 SUPPLEMENTS ON CT PROTOCOLS AND MULTI-ENERGY CT

CT subcommittee regularly communicates with AAPM members serving on the maintenance and project teams of the International Electrotechnical Committee

IEC: An international standards organization that creates standards that affect all of the medical imaging and therapy devices

IEC MAINTENANCE AND PROJECT TEAMS

CDV finished September 2017 Will receive the comments before the

Spring meeting in 2018.

IEC 61223-3-5 ED. 2.0ACCEPTANCE AND CONSTANCY TESTS –

IMAGING PERFORMANCE OF COMPUTED TOMOGRAPHY X-RAY EQUIPMENT

CDV finished September 2017 Will receive the comments before the

Spring meeting in 2018.

IEC PT 62985 ED. 1.0 (METHODS FOR CALCULATING SIZE SPECIFIC DOSE ESTIMATE

(SSDE) ON COMPUTED TOMOGRAPHY)

Up for review and revision Send comments and suggestions to

Dianna Cody (U of Texas, MD Anderson)

AAPM MEDICAL PHYSICS PRACTICE GUIDELINE 1.A: CT PROTOCOL MANAGEMENT AND REVIEW

PRACTICE GUIDELINE

Size specific dose estimates (SSDEs) …

a. are reported on newer CT scannersb. are constant across patient sizesc. can be calculated for all body partsd. allow size-specific diagnostic reference levelse. are calculated according to IEC standards

SAMS QUESTION

Size specific dose estimates (SSDEs) …

a. are reported on newer CT scanners – FALSE. No manufacturer has this yet. They are waiting for the IEC standard to be completed.

b. are constant across patient sizes – FALSE. Depending on the automatic exposure control system parameters, there can still be a dependence of SSDE on patient size. There is no fundamental physics principles that would mandate that this is desirable, especially for pediatrics.

c. can be calculated for all body parts – FALSE. There is no reason that they can’t be. The report specifying the conversion coefficients is simply not done yet.

d. allow size-specific diagnostic reference levels – TRUE. As shown in the Kanal paper, this allows values to be more similar across patient sizes

e. are calculated according to IEC standards – FALSE. This is work in progress and a year or so off still.

References: AAPM Report 204; AAPM Report 220; Christner et al. Radiology 265(3) 2012; Kanal et al. Radiology 284(1) 2017

SAMS QUESTION

AAPM is responsible for innumerable contributions to our field

What scientific, professional or educational activities are you passionate about?

Do you know how to propose an idea for action within the AAPM committee system?

How are you willing to contribute? The AAPM is all of us! What we do together

moves our profession forward!

CLOSING THOUGHTS

aapm activities with respect to ct...

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