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

Transcription

1 AAPM ACTIVITIES WITH RESPECT TO CT IMAGING Cynthia McCollough, PhD, DABR, FAAPM, FACR, FAIMBE 1631 Prince Street, Alexandria, VA

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

3 LEARNING OBJECTIVES 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

4 ORGANIZATIONAL STRUCTURE

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

6 EQUILIBRIUM DOSE IN CT

7 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

8 TG233: PERFORMANCE EVALUATION OF COMPUTED TOMOGRAPHY SYSTEMS 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

9 Model observers Evaluate low contrast detection performance Template Choice 1 Choice 2 10 mm HU 9.5 mm 6.3 mm 4.8 mm 4 mm 3.2 mm 2.4 mm 9

10 Based on noise, 33% of original dose is OK with IR FBP IR Mild IR Std IR Str Noise(33% dose) Noise(100% dose) 10

11 With similar image noise, detection performance has been compromised with IR IR 25% dose FBP 100% dose 11

12 Model observer predicts degraded performance at 25% dose -Δ17% AUC 100% dose 25% dose 12

13 Model observer predicts comparable performance at 78% dose AUC 78% dose 100% dose 13

14 With comparable AUC, detection performance is close with IR IR 78% dose FBP 100% dose 14

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

16 Clinical Motivation CT number depends on x ray attenuation Physical density (g/cm 3 ) [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

17 1.0E+02 Iodine Bone X-ray absorption 1.0E E+00 Strong increase Weak increase 1.0E Energy / kev

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

19 Current Acquisition Methods for Multi Energy CT: Single Tube Potential

20 Split Beam Filtration Single spiral acquisition over entire scan volume One spectrum lags the other by half a rotation Au filter Sn filter

21 Dual Layer Detectors X-rays Reflectors Photodiode Photodiode Scintillator Scintillator Low energy spectrum High energy spectrum

22 Photon Counting Detectors (PCD) X-rays Semiconductor detector directly converts x-ray to charge (e. g. CdTe) Signals are binned according to energy level Two or more energy levels * Courtesy Ken Taguchi, John Hopkins Low Energy Bin High Energy Bin

23 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

24 Current Acquisition Methods for Multi Energy CT: Dual Tube Potential

25 Slow kvp switching Consecutive scans of entire scan volume Axial Spiral Inter scan delay = scan time + table move time Unacceptable motion misregistration for most cases May be acceptable for large volume acquisitions (entire volume scanned in one rotation) Low kvp High kvp

26 Slow kvp switching Consecutive scans of one anatomic section Axial Spiral X Inter scan delay = rotation time + kv switching time Motion misregistration will limit many applications Low kvp High kvp

27 Rapid kvp switching Tube potential switched between successive views Low kvp High kvp

28 Dual source geometry Two tubes/generators allow simultaneous collection of dual kvp data Low kvp High kvp

29 Clinical Applications of Multi Energy CT

30 Color coded stones from in vivo study Qu et al., Eur Radiol (2013) 23: UA UA CYS APA COX/BRU/STR

31 High density material in soft tissues within and surrounding joints consistent with tophaceous deposits Courtesy of Dr. Katie Glazebrook

32 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

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

34 Courtesy of Dr. Amy Krambeck

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

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

37 DECT Virtual non Ca MRI Single energy CT McCollough et al., Radiology 276: 3 (2015)

38 Virtual mono energetic images (VMI) 40 kev 85 kev 120 kev Silva et al, Dual Energy (Spectral)CT: Applications in Abdominal Imaging. Radiographics 2011

39 Standard Image VMI (105 kev) Courtesy of Dr. Lifeng Yu

40 TG299: QUALITY CONTROL IN MULTI-ENERGY COMPUTED TOMOGRAPHY (MECT) Develop a quality control program for performance evaluation of MECT systems Define the appropriate tests, frequency and tolerance limits for MECT system evaluation.

41 TG293: TASK GROUP ON SIZE SPECIFIC DOSE ESTIMATES (SSDE) FOR HEAD CT Determining the conversion factors for head that parallel those produced in Report 204 for the torso.

42 CTDI VOL IS NOT PATIENT DOSE CTDI quantifies scanner radiation output Patient size must be considered to estimate patient dose McCollough, et al, Radiology, May 2011

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45 SSDE Estimates Mean dose Center of scan range Specific size Closer to real patient dose

46 SSDE = CTDIvol Conversion factor SSDE DEFINITION SSDE = f size x CTDI vol

47 SIZE SPECIFIC DOSE ESTIMATION (SSDE) SSDE = f size x CTDI vol AAPM Report 204

48 HOW TO DETERMINE SSDE Patient dimension such as anterioposterior thickness (AP) lateral width (LAT) AP+LAT LAT AP Tabulated conversion factors, f size * AAPM TG Report SSDE = f size x CTDIvol

49 CT Radiograph (Dw)

50 SAMPLE CALCULATION 5.40 mgy = CTDI vol SSDE = 5.4 mgy 2.29 SSDE = 12.4 mgy ± 20%

51 32 CM CTDI VOL CONVERSION FACTORS Effective diameter in Report 204 = Water equivalent diameter in Report 220 (Dw) (Dw) (Dw) (Dw)

52 16 CM CTDI VOL CONVERSION FACTORS Effective diameter in Report 204 = Water equivalent diameter in Report 220 (Dw) (Dw) (Dw) (Dw)

53 WATER EQUIVALENT DIAMETER (D W ) Diameter of a water cylinder that would absorb the same dose as the irradiated cross-section of the patient Huda et al. Effective doses to patients undergoing thoracic computed tomography examinations. Med Phys, 2000 Menke. Comparison of different body size parameters in body CT of adults. Radiology, 2005; 236:565 71

54 WATER EQUIVALENT DIAMETER (D W ) circle of water with equal ATTENUATION AP lateral Dw lateral Dw

55 BODY CT DATA (N=801) CTDIvol depended on patient size Patient size explained 42% of the variation in CTDIvol Christner et al. Radiology 265(3) 2012 SSDE was independent of patient size Slope decreased 9 fold Patient size explained <1% of variation in SSDE

56 CAVEAT 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

57 TG300: IMAGE QUALITY REGISTRY FOR CT 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.

58 WGCTNP: ALLIANCE FOR QUALITY 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.

59 and 28 guests and consultants 1631 Prince Street, Alexandria, VA

60 1631 Prince Street, Alexandria, VA

61 If you ve not visited this site, please explore it. There is a lot of great information here Prince Street, Alexandria, VA

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

63 IEC MAINTENANCE AND PROJECT TEAMS 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

64 IEC ED. 2.0 ACCEPTANCE 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.

65 IEC PT ED. 1.0 (METHODS FOR CALCULATING SIZE SPECIFIC DOSE ESTIMATE (SSDE) ON COMPUTED TOMOGRAPHY) CDV finished September 2017 Will receive the comments before the Spring meeting in 2018.

66 AAPM MEDICAL PHYSICS PRACTICE GUIDELINE 1.A: CT PROTOCOL MANAGEMENT AND REVIEW PRACTICE GUIDELINE Up for review and revision Send comments and suggestions to Dianna Cody (U of Texas, MD Anderson)

67 SAMS QUESTION Size specific dose estimates (SSDEs) a. are reported on newer CT scanners b. are constant across patient sizes c. can be calculated for all body parts d. allow size-specific diagnostic reference levels e. are calculated according to IEC standards

68 Size specific dose estimates (SSDEs) SAMS QUESTION 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

69 CLOSING THOUGHTS 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!