A new anthropometric phantom for calibrating in vivo measurements of stable lead in the human leg using x-ray fluorescence

Fabrication and control of CT number through polymeric composites based on coronary plaque CT phantom applications

Biomedical phantoms are commonly used for various medical imaging modalities to improve imaging quality and procedures. Current biomedical phantoms fabricated commercially are high in cost and limited in the specificity of human environments and structures that can be mimicked. This study aimed to control the measurable computed tomography (CT) number in Hounsfield units through polymeric biomedical phantom materials using controlled amounts of hydroxyapatite (hA). The purpose was to fabricate CT phantoms capable of mimicking various coronary plaque types while introducing a fabrication technique and basis for a numerical model to which the technique may be applied. The CT number is tunable based on the controlled material properties of electron density and atomic numbers. Three different polymeric matrices of polyethylene (PE), thermoplastic polyurethane (TPU), and polyvinylidene fluoride (PVDF) were selected due to their varied specific densities and ease of fabrication acting as integral properties for CT phantom fabrication. These polymers were processed together with additions of hA in mass percentages of 2.5, 5, 10, and 20% hA as well as a 0% hA as a control for each polymeric material. By adding hA to PE, TPU, and PVDF an increasing trend was exhibited between CT number and weight percent of hA.

Comparison of two leg phantoms containing (241)Am in bone

Three facilities (CIEMAT, HMGU and HML) have used their in vivo counters to compare two leg phantoms. One was commercially produced with (241)Am activity artificially added to the bone inserts. The other, the United States Transuranium and Uranium Registries' (USTUR) leg phantom, was manufactured from (241)Am-contaminated bones resulting from an intake. The comparison of the two types of leg phantoms showed that the two phantoms are not similar in their activity distributions. An error in a bone activity estimate could be quite large if the commercial leg phantom is used to estimate what is contained in the USTUR leg phantom and, consequently, a real person. As the latter phantom was created as a result of a real contamination, it is deemed to be the more representative of what would actually happen if a person were internally contaminated with (241)Am.

The influence of bone-density on in vivo K x-ray fluorescence bone-lead measurements

Mathematical simulations and benchmark measurements were performed to assess the impact that normal variations in human calcium content have on in vivo K x-ray fluorescence measurements of lead in bone. Four sets of cortical bone tissue simulants were fabricated containing from 20.8% to 23.8% calcium (by weight) for measurement in a surrogate (phantom) of the human leg. The net counts detected in the coherent backscatter peak at 88.034 keV using a Cd source indicate a positive trend, with a variability of up to 17% over the range of assessed calcium content. Mathematical simulations confirm this trend and also demonstrate that the contribution of 87.3 keV Pb Kβ2 counts, which are unresolved in measurements, do not contribute significantly to the coherent peak at low levels of bone-lead content. Both measurements and simulations confirm that calcium is a statistically significant parameter in predicting the K-XRF response and suggest that lead levels may be over-predicted for individuals having low bone density compared to the calibration matrix. Simulations identify a 4.5% negative bias in measured lead values for each 1% increase in calcium weight percent in the bone matrix as compared to the calibration matrix. It is therefore important to accommodate this uncertainty when performing epidemiological studies of populations having a wide range of bone densities.

Monte Carlo simulation of an anthropometric phantom used for calibrating in vivo K-XRF spectroscopy measurements of stable lead in bone

An anthropometric surrogate (phantom) of the human leg was defined in the constructs of the Monte Carlo N Particle (MCNP) code to predict the response when used in calibrating K x-ray fluorescence (K-XRF) spectrometry measurements of stable lead in bone. The predicted response compared favorably with measurements using the anthropometric phantom containing a tibia with increasing stable lead content. These benchmark measurements confirmed the validity of a modified MCNP code to accurately simulate K-XRF spectrometry measurements of stable lead in bone. A second, cylindrical leg phantom was simulated to determine whether the shape of the calibration phantom is a significant factor in evaluating K-XRF performance. Simulations of the cylindrical and anthropometric calibration phantoms suggest that a cylindrical calibration standard overestimates lead content of a human leg up to 4%. A two-way analysis of variance determined that phantom shape is a statistically significant factor in predicting the K-XRF response. These results suggest that an anthropometric phantom provides a more accurate calibration standard compared to the conventional cylindrical shape, and that a cylindrical shape introduces a 4% positive bias in measured lead values.

Modification to the Monte Carlo N-particle code for simulating direct, in vivo measurement of stable lead in bone

Monte Carlo N-Particle version 4C (MCNP4C) was used to simulate photon interactions associated with in vivo x-ray fluorescence (XRF) measurement of stable lead in bone. Experimental measurements, performed using a cylindrical anthropometric phantom (i.e., surrogate) of the human leg made from tissue substitutes for muscle and bone, revealed a significant difference between the intensity of the observed and predicted coherent backscatter peak. The observed difference was due to the failure of MCNP4C to simulate photon scatter associated with greater than six inverse angstroms of momentum transfer. The MCNP4C source code, photon directory, and photon library were modified to incorporate atomic form factors up to 7.1 inverse angstroms for the high Z elements defined in the K XRF simulation. The intensity of the predicted coherent photon backscatter peak at 88 keV using the modified code increased from 3.50 x 10(-9) to 8.59 x 10(-7) (roughly two orders of magnitude) and compares favorably with the experimental measurements.