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CT Organ Dose - Part 1 Estimating organ dose for patients

CT Organ Dose - Part 1 Estimating organ dose for patients. Michael F. McNitt-Gray, Ph.D., DABR David Geffen School of Medicine at UCLA. CTDI vol and DLP. CTDI vol currently reported on the scanner (though not required in US) Is Dose to one of two phantoms (16 or 32 cm diameter)

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CT Organ Dose - Part 1 Estimating organ dose for patients

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  1. CT Organ Dose - Part 1Estimating organ dose for patients Michael F. McNitt-Gray, Ph.D., DABR David Geffen School of Medicine at UCLA

  2. CTDIvol and DLP • CTDIvol currently reported on the scanner • (though not required in US) • Is Dose to one of two phantoms • (16 or 32 cm diameter) • Is NOT dose to the patient • Does not tell you whether scan was done “correctly” or “Alara” without other information (such as body region or patient size) • MAY be used as an index to patient dose with some additional information

  3. Scenario 1: No adjustment in technical factors for patient size 100 mAs 100 mAs 32 cm phantom 32 cm phantom CTDIvol = 20 mGy CTDIvol = 20 mGy The CTDIvol (dose to phantom) for these two would be the same

  4. Scenario 2: Adjustment in technical factors for patient size 100 mAs 50 mAs 32 cm phantom 32 cm phantom CTDIvol = 10 mGy CTDIvol = 20 mGy The CTDIvol (dose to phantom) indicates larger patient received 2X dose

  5. Did CTDI Really Tell Us Patient Dose? For same exposure parameters, the smaller patient absorbs more dose (think “Image Gently” here) • Scenario 1: CTDI is same but dose to smaller Patient is higher • Scenario 2: CTDI is smaller for smaller Patient, but dose is closer to equal for both. • CTDI DOES describe system output (Reduced for small patient?) • CTDI DOES NOT describe patient dose • At least not without other info. such as patient size, body region, etc.)

  6. Radiation Dose Basics: Organ Dose • BEIR VII report (2005) • Risk based on radiation dose to organ, age, gender, etc. • ICRP 103 (2007) • Calculates “effective dose” based on weighted sum of organ dose • All use dose to radiosensitive organs as a basis for estimating metrics that relate to risk

  7. Effective Dose (mSv) • Tissue ICRP 60 weighting factor (wT) ICRP 103wT • Gonads 0.20 .08 • Red Bone Marrow 0.12 .12 • Colon 0.12 .12 • Lung 0.12 .12 • Stomach 0.12 .12 • Bladder 0.05 .04 • Breast 0.05 .12 • Liver 0.05 .04 • Esophagus 0.05 .04 • Thyroid 0.05 .04 • Skin 0.01 .01 • Bone Surface 0.01 .01 • Brain (Remainder) .01 • Salivary Glands (Remainder) .01 • Remainder (Adrenals, etc.) 0.05 .12

  8. Can We Get There? • W/o doing detailed analysis on EACH patient?

  9. Monte Carlo Simulation Methods for Estimating Radiation Dose Monte Carlo methods Used in CT for some time NRPB report 250 (1990) GSF (Zankl)

  10. Background • These early reports used: • Detailed Models of Single Detector, Axial Scanners • Idealized (Nominal) collimation • Standard Man Phantom

  11. Background • These form the basis for: • CT Dose computer program • CT Expo • ImPACT dose calculator • k factor approach (Effective dose = k* DLP), which was derived from NRPB simulated data

  12. Current Approaches • Model Scanner (e.g MDCT) in detail • Model Patient (Geometric, Voxelized) • Simulate Scan • Tally Organ Dose

  13. Modeling the CT scanner • Spectra • Geometry • Beam Collimation • Filtration • Tube Current Modulation Scheme • x-y only, z-only, x-y-z, etc.

  14. Long Axis Modulation Lung Region Breast Tissue Shoulder Region Abdomen 180 degrees (LAT) 90 degrees (AP)

  15. Modeling the Patient • Geometric • Voxelized Models

  16. Modeling (Parts of) the Patient • Embryo/Fetus • Breast, Lung, Kidney, Liver, Spleen

  17. Original Image Contoured Image Voxelized Model Early Gestation Late Gestation

  18. Contoured Image Threshold Image Voxelized Model Original Image

  19. Simulating the Scan • Select Technical Parameters • Select Anatomic Region • Translate this to: • Start/stop location -> Source Path

  20. Organ Dose Independent of Scanner

  21. Organ dose (in mGy/mAs) and effective dose (in mSv/mAs) for GSF model Irene resulting from a whole body scan with similar parameters for each scanner

  22. Organ dose and effective dose normalized by measured CTDIvol for GSF model Irene resulting from a whole body scan.

  23. Normalized Organ Dose as function of Pt. Size(Abdomen Scans for each Patient) Turner et al RSNA 2009

  24. Tube Current Modulation 800 a) detailed TCM function from raw data b) discrete TCM function from image data c) single tube current value (average mA across the scan) 700 600 500 Tube Current in mA 400 300 200 100 0 0 50 100 150 200 250 300 350 400 Table Position in mm Tube Current Modulation Actual TCM Approx TCM (DICOM header) Average mAs

  25. Future of Dosimetry? Patient Size info • Patient Organ Dose • Accounting for scanner and acquisition parameters • Accounting for anatomic region • Accounting for patient size Size Coefficients CTDIvol (or TG 111)

  26. Summary Estimating Organ Doses Part 1 Organ Doses are meaningful indicators of Dose More informative than CTDI

  27. Summary Estimating Organ Doses Part 1 • Demonstrating concept and feasibility of NOT doing detailed analysis on each Patient • Not quite ready for implementation • A path to estimate organ doses that takes into account: • Scanner • Acquisition parameters (including TCM) • Anatomic Region • Patient Size

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