Location: Children's Nutrition Research CenterTitle: Whole body counter calibration using Monte Carlo modeling with an array of phantom sizes based on national anthropometric reference data Author
Submitted to: Physics in Medicine and Biology (London)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2011
Publication Date: 4/13/2011
Citation: Shypailo, R.J., Ellis, K.J. 2011. Whole body counter calibration using Monte Carlo modeling with an array of phantom sizes based on national anthropometric reference data. Physics in Medicine and Biology (London). 56:2979-2997. Interpretive Summary: Whole body gamma counters (WBC) are used in body composition research for measuring total body potassium (TBK). TBK is used to represent the metabolically active portion of fat-free mass. TBK measurements are clinically useful, since illness or physiological stress may result in a loss of body potassium. Having built a new WBC at the Children’s Nutrition Research Center (CNRC), we needed to calibrate the instrument to translate the measured gamma activity to grams of potassium. These days, more and more WBCs are calibrated using Monte Carlo modeling programs. The Monte Carlo method uses statistical sampling to find answers to mathematical problems. We made a detailed mathematical virtual model of our WBC that could predict how it responded to potassium signals coming from objects on its scanning bed, and to interfering signals coming from outside sources. We then designed a series of virtual human-shaped models that were based on national reference data. The human-shaped models spanned a wide range of sizes and shapes, and included an outer layer of fat that also varied in thickness. We found that the WBC responded differently depending on the size and shape of an individual. The outer fat layers also affected the WBC, and the effect varied as the fat layer thickness changed. Interference from external sources had a great effect on the WBC calibration, so accounting for this outside interference is critical in obtaining accurate counting results. Improving our WBC calibration techniques can increase WBC accuracy, which is important in studies of obesity.
Technical Abstract: During construction of the whole body counter (WBC) at the Children’s Nutrition Research Center (CNRC), efficiency calibration was needed to translate acquired counts of 40K to actual grams of potassium for measurement of total body potassium (TBK) in a diverse subject population. The MCNP Monte Carlo n-particle simulation program was used to describe the WBC (54 detectors plus shielding), test individual detector counting response, and create a series of virtual anthropomorphic phantoms based on national reference anthropometric data. Each phantom included an outer layer of adipose tissue and an inner core of lean tissue. Phantoms were designed for both genders representing ages 3.5 to 18.5 yrs with body sizes from the 5th to the 95th percentile based on body weight. In addition, a spherical surface source surrounding the WBC was modelled in order to measure the effects of subject mass on room background interference. Individual detector measurements showed good agreement with the MCNP model. The background source model came close to agreement with empirical measurements, but showed a trend deviating from unity with increasing subject size. Results from the MCNP simulation of the CNRC WBC agreed well with empirical measurements using BOMAB phantoms. Individual detector efficiency corrections were used to improve the accuracy of the model. Nonlinear multiple regression efficiency calibration equations were derived for each gender. Room background correction is critical in improving the accuracy of the WBC calibration.