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Rsna2012 pediatric dosemapvstdphantom

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Rsna2012 pediatric dosemapvstdphantom

  1. 1. DME Bardo, MD1, KA Feinstein, MD2, D Pettersson, MD1, J Wiegert, PhD3, JH Yanof, PhD4 Philips Research3, Philips Healthcare4 Comparison of Patient-Specific & Reference-Phantom Methods for CT Dose Estimation in the Pediatric Population 1 2
  2. 2. Purpose  Compare the characteristics of a) age-based reference phantoms used with Monte Carlo simulation to estimate organ doses with b) patient-specific phantoms based on CT data sets.  Compare the use of reference-phantom and patient-specific dose distribution maps to estimate organ doses.  Describe how differences in organ dose distribution affect the estimation of effective dose.
  3. 3. Content Organization Reference phantom & Patient specific phantoms Methods for estimating organ dose Effective dose calculations Standard From geometrically defined organs in stylized phantoms Patient Specific Using segmentation of organs in patient-specific dose maps Standard CTDI and stylized phantoms Patient Specific Voxelized models based on patient data sets Standard Regression with DLP & E (k factors) Patient Specific Weighted sum of dose map organ doses
  4. 4. Morphology of standard reference phantoms used for CT dose estimation can differ greatly from the anatomy of an individual patient A standard reference (stylized, mathematical) phantom (ORNL, Cristy) is compared with CT images of a 5-6 month old patient. Striking anatomical differences between reference and the patient can effect the estimation of dose by dosimetry simulation (Monte Carlo).
  5. 5. Limitations of standard dose estimates k factor – AAPM 95.6 Table 3 Volume CTDI & DLP are based on PMMA cylindrical phantoms and are not intended to be estimates of patient dose. They do not account for individual patients body habitus, attenuation characteristics, or specific scanner dosimetry. PMMA CTDIvol phantoms DLP conversion factors for Effective dose (reviewed later) – are based stylized phantoms with fixed geometry (modified ORNL set for newborn, 1, 5, and 10 YO as shown). [1,2,3]
  6. 6. The x-ray beam has more attenuation as it traverses a larger patient (yellow to red to blue) in comparison with a smaller patient (yellow to red). Thus, the average beam intensity and dose, as the tube rotates, tends to increase with decreasing size for the same scan parameters Dose (mGy) 10 20 30 40 10 mGy CtrLG 20 mGy, PrphryLG 40 mGy CtrSM 40 mGy Prphry,SM Background Major factors that affect CT dose include size/diameter & tissue/material absorption
  7. 7. Background Average patient dose tends to increasewith decreasingpatient size/diameter The Size Specific Dose estimates (SSDE) [6] also show that absorbed dose increases with decreasing size. A regression model (above) relates dose to effective diameter. The model data was based on a range of dosimetry methods (measured and simulated), four sets of phantoms, and four scanner vendors. All phantom sets included pediatric sizes. “Child” Infant Child Anthropomorphic CTDI Voxelized (GSF) Cylinders Adult
  8. 8. In contrast to a standard reference phantoms (left), a patient’s CT data set can be used to create a patient-specific (virtual) phantom (middle). In addition to patient-specific dosimetry, this enables and individualized dose maps (right). Standard reference vs. Patient-specific Phantoms and Dosimetry Voxelized Phantom Patient Data setStandard reference phantom Patient specific dose map
  9. 9. Dose maps can also be displayed in units of CTDI normalized absorbed dose. In this case, dose map pixels are divided by the simulated dose absorbed by the CTDI phantom. The basic trend of CTDI-normalized average dose increasing with decreasing patient size (relative to the CTDI phantom) tends to agree with the SSDE correction factors. Monte Carlo simulations with patient-specific phantoms: CTDI-Normalized Dose Maps 15 y/o CTDIvol 32 = 6.3 mGy Average dose map value Dose to 32 cm CTDI phantom
  10. 10. A Monte Carlo tool is used to simulate the dose absorbed by the patient specific “virtual phantom” instead of the CTDI phantom. This results in a patient specific dose map (in units of mGy). This example shows that the scan parameters for a 13 day old resulted in less absorbed relative to a 15 year old. Monte Carlo simulations with patient-specific phantoms: Individualized dose maps in mGy 13 day old CTDIvol=1.5 mGy 15 year old CTDIvol = 6.3 mGy
  11. 11. The CTDI of 2.5 mGy for a 13 day old and 6.3 mGy for a 15 year old would yield approximately the same patient specific-dose map. Monte Carlo simulations of patient-specific phantoms can be used to simulate new dose maps 13 day old CTDIvol= 2.5 mGy 15 year old CTDIvol = 6.3 mGy Simulated dose maps
  12. 12. Morphological differences between patient-specific & standard reference phantoms for dosimetry simulation (Monte Carlo) CT data sets are used to form patient specific virtual phantom (previous slide). The size and shape of organs and tissues can have wide variation. Virtual phantoms do not extend beyond reconstructed image volume. Organs are represented by patient generic, fixed (stylized) geometric shapes. Anatomy extends caudal-cranially, enabling dose simulation and scatter beyond scan range. Oak Ridge National Lab Phantoms (Cristy)Virtual Patient (“Voxelized) Phantoms newborn 1 YO 5 YO 10 YO 10 YO 5 YO 1 YO newborn
  13. 13. Acquisition of CT images 3 4 Monte Carlo simulations were performed on voxelized image sets. Dose Maps displaying CTDIvol normalized absorbed dose. Organ doses can be segmented from dose maps CT image voxels were classified as 1 of 5 tissues based on attenuation 5 21 Background Flowchartfor generation of dose maps: CT data acquisition, creationof voxelized phantom,dose simulation 3
  14. 14. Dose Map: Select radiosensitive organ or tissue Average value of pixels segmented in the organ are used for organ dose estimation (in mGy) Organ segmentation Background Patient-specificorgan doses can be estimatedwith dose maps In standard reference phantoms such as those used for the DLP conversion factors, organs are defined with fixed geometry using mathematical equations.
  15. 15. age 6 days 13 days 26 days 2 months Lung dose 1.78 1.96 2.43 2.42 (mGy/mGy) show patient-specific variability in organ doses that cannot be shown in the Oak Ridge National Lab (ORNL) references phantoms Dose maps Estimated lung dose (CTDI normalized) from patient specific dose maps varies from 1.78 to 2.43 mGy/mGy. CTDIvol normalized organ doses simulated in the ORNL infant phantom (right) (new born) would not have any variability.
  16. 16. NRPB reference phantom (center [3]) extended the ORNL phantom concept (lower right) to include gender-based organs. The NRPB phantoms were used by Jessen et al. (with IRCP 60 organ weighing factors) to determine widely used DLP conversion factors [5,11] Standard Reference Phantoms Can be modified to include additionalradiosensitiveorgans
  17. 17. Patient-specific voxelized phantoms have featuresnot included in standard reference phantoms Breast dose can be measured (green contours) in dose maps -- this tissue is represented in the NRCP phantoms (previous slide). 14 y/o female (left) and 15 y/o male (right) with approximately the same effective diameter (~27 mm). Bismuth shields (arrows) were used in both exams (not represented in standard phantoms). Average lung dose is higher for the female patient due to relatively larger lung parenchyma (lower beam attenuation).
  18. 18. Comparison of patient-specific & reference phantom methodsused for effectivedose calculation Effective Dose, Reference Phantom LEGEND: CTDI = Computed Tomography Dose Index DLP = Dose Length Product ED = Effective Dose ODi = Organ Dose wi = tissue weighting factors, ICRP 60 compare Effective Dose, Patient-Specific Phantom ODi From Organ Segmentation Dose Map Monte Carlo Simulation (Scanner Specific) Patient Data Set CTDI, DLPscan ED = k x DLPscan Reference Phantom MonteCarlo Simulation ODi From Organ Compartments Dose Map CTDI, DLP AAPM Report 96.5 k
  19. 19. Background Estimation of effectivedose for organ weighting factors ED = ∑ wi x Odi where OD is the individual organ dose measured from non-patient specific mathematical phantom or patient specific dose map w is organ/tissue weighting factor (ICRP 60 or 103) i = 13 (13 most radiosensitive organs) The effective dose equivalent, therefore, represents a total body dose.
  20. 20. Relative Organ Dose Sensitivities, Wi 0 0.05 0.1 0.15 0.2 0.25 Relative Organ Sensitivities ICRP (Used by both Pt. Specific and Std. Ref. Phantoms) ICRP 60 ICRP 103 ICRP 60 and NRPB phantoms were used for DLP conversion coefficients (Jessen). Patient-specific phantoms Segmenting all the listed organs and tissues for each individual voxelized phantom can pose a challenge. Standard reference phantoms The NRPB phantoms do not include all organs and tissues listed. And they would need to be revised if the ICRP adds new organs to the list.
  21. 21. Background Determinationof DLPconversionfactors Each body and age specific DLP conversion factor (k factor in units of mSv mGy-1 cm-1) was determined by dosimetry simulations with varying DLP. They are based on linear regression analysis of body-region specific effective dose (from the simulations, y-axis) and DLP (x-axis). In this method, effective dose is assumed to be linearly proportional to DLP, i.e., E = DLP x k . DLP is linearly proportional to irradiated scan length (includes helical over-ranging). For pediatrics, DLP is based on CTDI 16. Also, K-factors (i.e., DLP conversion) represent an average over scanner types and are not gender specific. The organ dose weighting factors were described on the previous slide. DLP input to Dose Simulation Chest newborn Slope = k, for each age Chest 1 YO Chest 5 YO Chest 10 YO EffectiveDose asweightedsumoforgandoses
  22. 22. Effective dose using ICRP weightingfactorscannot be patient-specific • The organ sensitivity (weighting) factors are based on population data from survivors of the atomic blast, where the sum of the weighting factors is one. • A 0.12 value for lung tissue implies that the relative likelihood of developing lung cancer in the population of blast survivors, in comparison with other listed organs, is 12%. Therefore, any estimation of effective dose that uses these population based weighting factors – patient-specific dose maps or DLP conversion k factors – cannot be patient-specific. Although effective dose is not patient specific, dose maps enable patient specific organ dose estimates (next slide) and these increase the relative patient (as well as scanner) specificity in comparison with DLP conversion factors.
  23. 23. Partial irradiation of an organ tends to decrease the organ dose estimate. This is because organ dose is defined as the average over the entire organ. An advantage for the reference phantoms is that the caudal- cranial range is not limited to a reconstructed scan volume as with the voxelized phantom. This can help estimate absorbed dose to partially irradiated organs. Basing organ dose on only the fully irradiated voxels will tend of overestimate the estimated organ dose. Organ doses partial irradiation, ICRPweighting factors ICRP weighting factors are based on full-body irradiation. Tissues that have wide distribution throughout the body such as red bone marrow are almost certainly partially irradiated in a CT examination. partial irradiation of liver scanlength
  24. 24. Summary comparison dose maps and standard reference phantoms Dose Map Method Standard Reference Phantom Method Representation Voxelized Phantom Based on Data Set Four pre-defined geometric representation of organs Morphology Patient-specific Not Patient Specific Organs Organs must be segmented. Organs pre-defined mathematically Caudal Cranial End-effects Not modeled (easily) Extends beyond scan length to model partial organ irradiation Computation Computed for each patient and each examination Can be pre-tabulated for set of examinations and stored for future use Material Models CT Numbers are mapped to ICRU 44 ICRP Publication 89 Effective Dose Pt. specific organ dose can be used to estimate eff. dose Generic organ doses are used to determine DLP conversion coefficients.
  25. 25. Estimation of CT dose is evolving … 10 cm CTDI phantom Dose map sequence (z-axis) based on Monte Carlo simulation with infant CT data set
  26. 26. Summary  Patient specific voxelized phantoms can represent complex, patient specific anatomy and materials that are not easily represented in standard reference phantom.  Organ doses estimated from patient specific dose maps ARE patient specific. Patient-specific dose maps demonstrate the variability of organ doses and highlight a key limitation of standard methods for estimating effective dose.  Use of more patient-specific methods to estimate organ and effective doses could lead to better metrics and reporting for CT dose management. Effective doses estimated from ICRP wt. factors and NOT patient specific, but EDose Maps is more patient- and scanner-specific than EDLP.
  27. 27. Clinical Relevance  Patient-specific doses estimated by applying dosimetry simulations to voxelized phantoms may have advantages when patient morphology significantly differs from the reference phantom.  Quantitative evaluation of patient-specific dose maps are underway. This will lead to a better understanding how more accurate dose estimate methods will impact CT radiation dose management.
  28. 28. References1. Cristy M . Mathematical phantoms representing children of various ages for use in estimates of internal dose. Report no. ORNL/ NUREG/TM-367. Oak Ridge, Tenn: Oak Ridge National Laboratory, 1980 . 2. Cristy M , Eckerman KF . Specifi c absorbed fractions of energy at various ages from internal photon sources. I. Methods. Report no. ORNL/TM-8381/V1. Oak Ridge, Tenn: Oak Ridge National Laboratory, 1987 . 3. A Khursheed, Phd, M C Hillier, P C Shrimpton, Phd And B F Wall, Bsc, Influence of patient age on normalized effective doses calculated for CT examinations 4. Maria Zankl , Handbook of Anatomical Models for Radiation Dosimetry Edited by Xie George Xu and Keith F Eckerman , 3] Taylor & Francis 2009 5. American Association of Physicists in Medicine. The measurement, reporting and management of radiation dose in CT. Report 96. AAPM Task Group 23 of the Diagnostic Imaging Council CT Committee. College Park, MD. American Association of Physicists in Medicine, 2008. 6. American Association of Physicists in Medicine. Size-specific dose estimates (SSDE) in pediatric and adult body CT examinations. Report 204. AAPM Task Group 204. College Park, MD. American Association of Physicists in Medicine, 2011. 7. McCollough CH, et al., CT Dose Index and Patient Dose: They Are Not the Same Thing. Radiology: Volume 259:(2) 311-316. 8. Morgan HT., Dose reduction for CT pediatric imaging, Pediatr Radiol. 2002 Oct;32(10):724-8; discussion 751-4. Epub 2002 Aug 29., 9. Adam C. Turner1 and Michael McNitt-Gray, The feasibility of patient size-corrected, scanner-independent organ dose estimates for abdominal CT exams, Med Phys. 2011 Feb;38(2):820-9. 10. Boone JM, Strauss KJ, Cody DD, McCollough CH, McNitt-Gray MF, Toth TL, Goske MJ, Frush DP. Size-specific dose estimates (SSDE) in pediatric and adult body CT examination. Report No. 204. 2011 11. Cynthia H. McCollough et al. How Effective Is Effective Dose as a Predictor of Radiation Risk?, AJR:194, April 2010
  29. 29. air adipose tissue lung tissue soft tissue cortical bone A patient-specific (tomographic) virtual phantom (i.e., model) is created by “voxelizing” and automatically segmenting patients CT dataset. Each voxel is assigned one of five material types based on an a priori, global HU classification intervals (ICRU 44). These material types are also assigned mass density to compute absorbed dose. The resulting virtual patient phantoms are used for dose simulation (Monte Carlo) and the results are referred to as Dose Maps (next slide). CT image for 6 day old Virtual patient phantom Appendix I Creation of a Patient-specificVoxelized Models
  30. 30. Limitations of CTDIvol The CTDIvol reports scanner output based on a standard, fixed-sized phantom (32 cm for body), not patient-specific dose. Therefore, dose is over- and underestimated for patients significantly larger or smaller (respectively) than the phantom. (see AAPM report 201) EstimatedCTDIvol[mGy](120kV) 24 32 50 15 10 5 0 Patient diameter [cm] Actual dose for 24 cm diameter patient Reported dose Actual dose for 50 cm diameter patient
  31. 31. DLPand Effective Dose Effective dose (ED) parameter shown here is also based on the plastic CTDI phantoms. It is a risk-related quantity used to indicate equivalent whole body exposure that includes DLP as well as other factors such as the radiation sensitivities of the various organs in the body, age, and gender. Notes: 1) Effective dose using DLP conversion coefficients are estimated with averages over gender and age and therefore do not estimate risk for an individual patient. 2) Reference for ED are based on estimates for annual background radiation (3 mSv). 3) Another method to compute ED is based on the summation of organ dose estimates. CTDI (mGy) Dose Length Product (mGy *cm) Effective Dose (mSv) Equation or dose calculation method CTDIvol CTDIvol is presently measured with 16 and 32 cm phantoms DLP = Irradiated Scan Length x CTDIvol Helical scan length: the reconstructed scan length plus helical over-ranging Axial scan length: the reconstructed scan length for one “axial shot” * number of axial “shots”. (The CTDIvol accounts for “over- beaming”). k = conversion coefficient for the DLP method of estimation
  32. 32. Dose map reconstructions Dose maps show representations of: Absorbed dose map [mGy] Typical range: 0-20 mGy Absorbed dose map/CTDIvol [mGy / mGy] Typical range: 0-2.5 mGy/mGy Energy imparted map [Joules] Typical range: 10-5 J/pixel

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