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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
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
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
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
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
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
Dual‐source geometry
▸ Two tubes/generators allow simultaneous collection of dual‐kVp data
Low kVpHigh kVp
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
Silva et al, Dual‐Energy (Spectral)CT: Applications in Abdominal Imaging. Radiographics 2011
85 keV40 keV 120 keV
Virtual mono‐energetic images (VMI)
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
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
SSDE = fsize x CTDIvol
AP
LAT
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
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
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and 28 guests and consultants
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1 6 3 1 P r i n c e S t r e e t , A l e x a n d r i a , V A 2 2 3 1 4 | 5 7 1 - 2 9 8 - 1 3 0 0 | w w w . a a p m . o r g
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