Development of a method for calibrating in vivo measurement systems using magnetic resonance imaging and Monte Carlo computations.

Research efforts towards developing a new method for calibrating in vivo measurement systems using magnetic resonance imaging (MRI) and Monte Carlo computations are discussed. The method employs the enhanced three-point Dixon technique for producing pure fat and pure water MR images of the human body. The MR images are used to define the geometry and composition of the scattering media for transport calculations using the general-purpose Monte Carlo code MCNP, Version 4. A sample case for developing the new method utilizing an adipose/muscle matrix is compared with laboratory measurements. Verification of the integrated MRI-MCNP method has been done for a specially designed phantom composed of fat, water, air, and a bone-substitute material. Implementation of the MRI-MCNP method is demonstrated for a low-energy, lung counting in vivo measurement system. Limitations and solutions regarding the presented method are discussed.

Calibration of a 241Am wound-monitoring system using Monte Carlo techniques.

Monte Carlo techniques have been used to establish calibration factors and to predict gamma spectra for well-defined measurements. These techniques are routinely used to predict shielding requirements and critical specifications. The Lawrence Livermore National Laboratory is researching the feasibility of using Monte Carlo techniques to establish calibration factors for in vivo measurement systems. A pilot study was conducted to demonstrate the use of the Monte Carlo technique to calibrate in vivo measurement systems, to predict the efficiency of a wound measurement system and compare the predicted efficiency with the measured efficiency, and to investigate the effects of the source geometry and the detector size on the measured efficiency. Results of this study demonstrate good agreement between the Monte-Carlo-predicted efficiency and the measured efficiency for a wound calibration phantom. The effects of the source geometry and the detector size tend to conform to the physical processes that govern the measurement process. These results demonstrate that the Monte Carlo technique accurately predicts the in vivo measurement efficiency if the characteristics of the attenuating material and the Monte Carlo source geometry are properly established.

A comparison of techniques in the assessment of chest wall thickness and composition.

The thickness and composition of defined regions of the anterior chest wall are important factors in the assessment of pulmonary plutonium by low-energy x ray counting. Estimates of these quantities are reported for seven male subjects investigated by three laboratories using ultrasonic methods and by a fourth laboratory using magnetic resonance imaging. No important bias was found in any one laboratory's estimates of chest wall thickness relative to those of the others, but differences of up to 6 mm were noted for individual subjects. The discrepancies are believed principally to reflect the different sampling regimes adopted to reach a representative mean chest wall thickness over the region of interest from measurements at selected points. The adipose-tissue component was consistently found to be lower when assessed by magnetic resonance imaging compared with estimates by ultrasound, but the differences were unimportant in the context of plutonium assessment.

Improved ultrasonic measurement techniques applied to assay of Pu and other transuranics in lung.

At the Lawrence Livermore National Laboratory, we are developing a system that will more accurately measure fat, muscle, and bone content from ultrasonic images of the chest wall. This paper describes a procedure that will allow chest-wall thickness to be determined to within +/- 1.5 mm (compared with the +/- 3-6 mm from current techniques) and may allow absolute errors in chest-wall composition to be reduced to +/- 4%.