Hybrid computational phantoms for medical dose reconstruction

Abstract

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.

Figure 1

Graphical examples of a stylized, voxel, and hybrid adult male phantom.

Figure 2

Schematic displaying the joint attributes of hybrid phantoms.

Figure 3

Steps needed to create both a hybrid-NURBS/PM and hybrid-voxel phantom.

Figure 4

Representative transaxial slices through one of the UF pediatric voxel phantoms.

Figure 5

Conversion of the left lung of the UF voxel newborn phantom to a hybrid-voxel format.

Figure 6

(A) Skull models in the KTMAN2 and MAX voxel phantoms as viewed in Rhinoceros™. (B) Repair of the cranial model of the UF 1-year reference hybrid phantom. (C) Demonstration of the use of 3D-Doctor™ to “repair” holes in the originally segmented model of the 1-year cranium.

Figure 7

Perspective view of the UF Family of reference hybrid phantoms.

Figure 8

Flowchart relevant to the creation of a library of patient-dependent hybrid phantoms for patient matching in a medical dose reconstruction project.

Figure 9

Scatter plots from the NHANES database for 5-year, 10-year, and 15-year children showing the relationship between both standing heights (upper grouping) and sitting heights (lower groupings) as a function of subject body mass.

Figure 10

Corresponding data on adult female and adult male standing and sitting heights as a function of body mass from the NHANES database.

Figure 11

Anterior (A and B) and posterior (C and D) views of the UF 15-year female hybrid phantom (left) and the corresponding ORNL stylized phantom (right). Liver (green), stomach (red), ovaries (white), and bladder (yellow).