Michael Wayson

Hybrid computational phantoms for medical dose reconstruction

As outlined in NCRP Report No. 160 of the US National Council on Radiation Protection and Measurements (NCRP), the average value of the effective dose to exposed individuals in the United States has increased by a factor of 1.7 over the time period 1982–2006, with the contribution of medical exposures correspondingly increasing by a factor of 5.7. At present, medical contributors to effective dose include computed tomography (50% of total medical exposure), nuclear medicine (25%), interventional fluoroscopy (15%), and conventional radiography and diagnostic fluoroscopy (10%). An increased awareness of medical exposures has led to a gradual shift in the focus of radiation epidemiological studies from traditional occupational and environmental exposures to those focusing on cohorts of medical patients exposed to both diagnostic and therapeutic sources. The assignment of organ doses to patients in either a retrospective or a prospective study has increasingly relied on the use of computational anatomic phantoms. In this paper, we review the various methods and approaches used to construct patient models to include anthropometric databases, cadaver imaging, prospective volunteer imaging studies, and retrospective image reviews. Phantom format types—stylized, voxel, and hybrid—as well as phantom morphometric categories—reference, patient-dependent, and patient-specific—are next defined and discussed. Specific emphasis is given to hybrid phantoms—those defined through the use of combinations of polygon mesh and non-uniform rational B-spline (NURBS) surfaces. The concept of a patient-dependent phantom is reviewed, in which phantoms of non-50th percentile heights and weights are designed from population-based morphometric databases and provided as a larger library of phantoms for patient matching and lookup of refined values of organ dose coefficients and/or radionuclide S values. We close with two brief examples of the use of hybrid phantoms in medical dose reconstruction—diagnostic nuclear medicine for pediatric subjects and interventional fluoroscopy for adult patients.

An Approach for Balancing Diagnostic Image Quality with Cancer Risk: Application to Pediatric Diagnostic Imaging of 99mTc-Dimercaptosuccinic Acid

A recent survey of pediatric hospitals showed a large variability in the activity administered for diagnostic nuclear medicine imaging of children. Imaging guidelines, especially for pediatric patients, must balance the risks associated with radiation exposure with the need to obtain the high-quality images necessary to derive the benefits of an accurate clinical diagnosis. Methods: Pharmacokinetic modeling and a pediatric series of nonuniform rational B-spline–based phantoms have been used to simulate 99mTc-dimercaptosuccinic acid SPECT images. Images were generated for several different administered activities and for several lesions with different target-to-background activity concentration ratios; the phantoms were also used to calculate organ S values for 99mTc. Channelized Hotelling observer methodology was used in a receiver-operating-characteristic analysis of the diagnostic quality of images with different modeled administered activities (i.e., count densities) for anthropomorphic reference phantoms representing two 10-y-old girls with equal weights but different body morphometry. S value–based dosimetry was used to calculate the mean organ-absorbed doses to the 2 pediatric patients. Using BEIR VII age- and sex-specific risk factors, we converted absorbed doses to excess risk of cancer incidence and used them to directly assess the risk of the procedure. Results: Combined, these data provided information about the tradeoff between cancer risk and diagnostic image quality for 2 phantoms having the same weight but different body morphometry. The tradeoff was different for the 2 phantoms, illustrating that weight alone may not be sufficient for optimally scaling administered activity in pediatric patients. Conclusion: The study illustrates implementation of a rigorous approach for balancing the benefits of adequate image quality against the radiation risks and also demonstrates that weight-based adjustment to the administered activity is suboptimal. Extension of this methodology to other radiopharmaceuticals would yield the data required to generate objective and well-founded administered activity guidelines for pediatric and other patients.

Hybrid Patient-Dependent Phantoms Covering Statistical Distributions of Body Morphometry in the U.S. Adult and Pediatric Population

Hybrid computational phantoms offer unique advantages for the construction of diverse anthropomorphic models. In this paper, a methodology is presented for the construction of patient-dependent phantoms built around anthropometric distributions of the U.S. adult and pediatric populations. The methodology relies on the flexibility of hybrid phantoms to match target anthropometric parameters as determined from National Center for Health Statistics databases. Target parameters as defined in this paper include the primary parameters such as standing height, sitting height, and total body mass; and secondary parameters such as waist, buttocks, arm, and thigh circumference. As a demonstration of this methodology, the UF hybrid adult male (UFHADM) and UF hybrid 10-year-old female (UFH10F) were selected as representative anchor phantoms for this study and were subsequently remodeled to create 25 different adult male and 15 different pediatric female patient-dependent phantoms. The phantoms were evaluated based on appearance and internal organ masses. Aesthetically, the phantoms appear correct and display characteristics of a diverse population including variability in body shape and standing/sitting height. Organ masses display several general trends, including a gradual increase with both standing height and subject weight. Selected organ masses from the UFHADM series were also compared with published correlations taken from a 2001 French-based autopsy study. The organ masses were located well within the statistical deviation presented in the autopsy study and followed similar trends when correlated with both standing height and body mass index.

Internal Photon and Electron Dosimetry of the Newborn Patient – A Hybrid Computational Phantom Study

Estimates of radiation absorbed dose to organs of the nuclear medicine patient are a requirement for administered activity optimization and for stochastic risk assessment. Pediatric patients, and in particular the newborn child, represent that portion of the patient population where such optimization studies are most crucial owing to the enhanced tissue radiosensitivities and longer life expectancies of this patient subpopulation. In cases where whole-body CT imaging is not available, phantom-based calculations of radionuclide S values--absorbed dose to a target tissue per nuclear transformation in a source tissue--are required for dose and risk evaluation. In this study, a comprehensive model of electron and photon dosimetry of the reference newborn child is presented based on a high-resolution hybrid-voxel phantom from the University of Florida (UF) patient model series. Values of photon specific absorbed fraction (SAF) were assembled for both the reference male and female newborn using the radiation transport code MCNPX v2.6. Values of electron SAF were assembled in a unique and time-efficient manner whereby the collisional and radiative components of organ dose--for both self- and cross-dose terms--were computed separately. Dose to the newborn skeletal tissues were assessed via fluence-to-dose response functions reported for the first time in this study. Values of photon and electron SAFs were used to assemble a complete set of S values for some 16 radionuclides commonly associated with molecular imaging of the newborn. These values were then compared to those available in the OLINDA/EXM software. S value ratios for organ self-dose ranged from 0.46 to 1.42, while similar ratios for organ cross-dose varied from a low of 0.04 to a high of 3.49. These large discrepancies are due in large part to the simplistic organ modeling in the stylized newborn model used in the OLINDA/EXM software. A comprehensive model of internal dosimetry is presented in this study for the newborn nuclear medicine patient based upon the UF hybrid computational phantom. Photon dose response functions, photon and electron SAFs, and tables of radionuclide S values for the newborn child--both male and female--are given in a series of four electronic annexes available at stacks.iop.org/pmb/57/1433/mmedia. These values can be applied to optimization studies of image quality and stochastic risk for this most vulnerable class of pediatric patients.

Suggested reference values for regional blood volumes in children and adolescents

Estimates of regional blood volumes (BVs) in humans are needed in dosimetric models of radionuclides and radiopharmaceuticals that decay in the circulation to a significant extent. These values are also needed to refine models of tissue elemental composition in computational human phantoms of both patients and exposed members of the general public. The International Commission on Radiological Protection (ICRP) in its Publication 89 provides reference values for total blood content in the full series of their reference individuals, to include the male and female newborn, 1 year-old, 5 year-old, 10 year-old, 15 year-old, and adult. Furthermore, Publication 89 provides reference values for the percentage distribution of total blood volume in 27 different blood-filled organs and tissues of the reference adult male and adult female. However, no similar distribution values are provided for non-adults. The goal of the present study is to present a volumetric scaling methodology to derive these values for the same organs and tissues at ages younger than the reference adult. Literature data on organ-specific vascular growth in the brain, kidneys, and skeletal tissues are also considered.

WE-E-BRE-01: An Image-Based Skeletal Dosimetry Model for the ICRP Reference Adult Female - Internal Electron Sources.

PURPOSE Limitations seen in previous skeletal dosimetry models, which are still employed in commonly used software today, include the lack of consideration of electron escape and cross-fire from cortical bone, the modeling of infinite spongiosa, the disregard of the effect of varying cellularity on active marrow self-irradiation, and the lack of use of the more recent ICRP definition of a 50 micron surrogate tissue region for the osteoprogenitor cells - shallow marrow. These limitations were addressed in the present dosimetry model. METHODS Electron transport was completed to determine specific absorbed fractions to active marrow and shallow marrow of the skeletal regions of the adult female. The bone macrostructure was obtained from the whole-body hybrid computational phantom of the UF series of reference phantoms, while the bone microstructure was derived from microCT images of skeletal region samples taken from a 45 year-old female cadaver. The target tissue regions were active marrow and shallow marrow. The source tissues were active marrow, inactive marrow, trabecular bone volume, trabecular bone surfaces, cortical bone volume and cortical bone surfaces. The marrow cellularity was varied from 10 to 100 percent for active marrow self-irradiation. A total of 33 discrete electron energies, ranging from 1 keV to 10 MeV, were either simulated or modeled analytically. RESULTS The method of combining macro- and microstructure absorbed fractions calculated using MCNPX electron transport was found to yield results similar to those determined with the PIRT model for the UF adult male in the Hough et al. STUDY CONCLUSION The calculated skeletal averaged absorbed fractions for each source-target combination were found to follow similar trends of more recent dosimetry models (image-based models) and did not follow current models used in nuclear medicine dosimetry at high energies (due to that models use of an infinite expanse of trabecular spongiosa).

Evaluation of image quality using Channelized Hotelling observer for pediatric diagnostic imaging of 99mTc-dimercaptosuccinic acid

Radiation dose is of special concern in pediatric patients for higher sensitivity in radiation and longer timeframe to manifest stochastic effects. Since body habitus can affect image quality in pediatric patients, it is important to understand the tradeoff between image quality, radiation dose, and body habitus. In this study, B-spline based mathematical phantoms were used to simulate the anatomy of two 10-year old girls having the same weight but different body habitus. Literature data was used to obtain organ uptakes of 99mTc-dimercaptosuccinic acid. Projection data for administered activities (AA) ranging from 0.25 to 1.5 times a standard AA were simulated using an analytic projector modeling attenuation, scatter, and the collimator-detector response followed by simulation of Poisson noise. Kidney function defects were created at several locations in each kidney with varying activity concentration ratios. The projections were reconstructed using filtered-backprojection (FB) followed by 3D Butterworth filter with various cutoff frequencies to find an optimal cut-off frequency. Channelized Hotelling observer and receiver operating characteristics (ROC) methodologies were applied to the reconstructed images for the task of defect detection. Areas under the ROC curve (AUC) were computed to assess the changes in the lesion detection. At higher, non-optimal cutoffs results showed different trade-offs for the lesion detectability between the 2 phantoms indicating that body habitus, and not just weight, is a significant factor in determining image quality for a given AA. However, use of the optimal cutoff resulted in little difference in the image quality versus AA tradeoff for the two phantoms. A population of phantoms spanning the range of pediatric height has been generated and the results of this study will ultimately be extended to the full population in order to provide data needed to establish optimal pediatric dosing guidelines.

MO‐F‐213CD‐02: A Series of NURBS and MicroCT‐Based Reference Skeletal Dosimetry Models of Pediatric and Adolescent Skeleton

Purpose: The hematopoietically active tissues of the skeleton are an important target tissue for dosimetric analysis, both in terms of diagnostic risk optimization and evaluating treatment efficacy. In the work presented here, a recently published dosimetry model of the adult is extended to all pediatric ages of the ICRP reference series. Methods: NURBS/PM‐based computational phantoms of the ICRP 89 reference newborn, 1‐year, 5‐year, 10‐year, and 15‐year male and female were constructed from image segmentation of age and gender‐matched CTimages. Bone samples were subsequently acquired from autopsy harvest of two female newborns and one 18‐year male subject. Individual bones were collected and segmented following high‐resolution ex‐vivo CT to yield fractional volumes of cortical bone and spongiosa. Cored samples of spongiosa were later imaged under microCT to yield fractional volumes of bone trabeculae and marrow tissues and to provide a 3D geometry for radiation transport. Previously acquired pathlength distributions of trabecular spongiosa for a 1.7‐year and 9‐year child were used to supplement the dataset. Results: A comprehensive set of absorbed fractions of energy for internally emitted electrons are presented for active and shallow marrow targets in all bones, all ages, and over the energies 1 keV to 10 MeV. These electron absorbed fractions were then used to assemble photon fluence‐to‐dose response functions permitting detailed marrow dosimetry for both externally incident (e.g., CT) and internally emitted (e.g., nuclear medicine)photons by bone site and subject age. Techniques and issues for patient‐specific adjustments are discussed. Conclusions: Marrow dosimetry is a critical component to nuclear medicine risk assessment and therapy treatment planning. This work provides state‐of‐the‐art methods for pediatric marrow dosimetry that supplants those developed previously for simpler stylized models of the pediatric skeleton. R01 CA116743, R01 CA96441, DE‐FG07‐06ID14773

SU‐E‐I‐56: Computational Internal Dosimetry Methods as Applied to the UF Series of Hybrid Phantoms

Purpose: To compute specific absorbed fractions for monoenergetic photons and electrons for all UF hybrid ICRP‐reference phantoms. Ultimately, radionuclide S values will be calculated for all radionuclides of interest in nuclear medicine. Methods: Voxelized versions of the UF newborn hybrid computational phantoms were used in conjunction with the radiation transport code MCNPX v2.6 to determine the absorbed fraction of energy per unit mass (SAF) for a variety of source‐target organ combinations for 21 photon and electron energies. A method known as a fluence‐to‐dose response function was used, for the first time in a model of the newborn child, to determine the absorbed dose to active marrow and total shallow marrow, the radiosensitive tissues of the skeleton. A novel method for minimizing the poor statistics associated with electron transport was also introduced. First, a simulation was completed which only tracked primary electrons, giving the dose contribution solely attributed to collisional energy losses. Next, a simulation was completed which recorded the initial energy of any photons produced by the primary electrons. Monoenergetic photon SAFs, determined previously, were then weighted according to each monoenergetic electron bremsstrahlung energy spectrum so as to procure low uncertainty photon dose from the primary electrons. Results: The novel method for electron dosimetry was benchmarked and found to be an accurate and effective way of reducing the uncertainty associated with electron cross‐dose. S values were generated for Tc‐99m and compared to estimates provided by OLINDA/EXM 1.0. Noticeable differences were seen for both self‐dose and cross‐dose scenarios. Conclusion: The work performed represents a state‐of‐the‐art internal dosimetry model, and once dosimetry has been performed on the entire UF series of phantoms, the results will be incorporated into an online accessible software package which can be of use in imaging and dose optimization for pediatric nuclear medicine patients.