Adjustment of chest wall thickness of voxel phantom for numerical efficiency calibration of lung counters.

A modification method was implemented to adjust the chest wall thickness of human voxel phantom for the purpose of numerical efficiency calibration of lung counters. This method was based on two basic mathematical morphology operations (dilation and erosion) and combined with proper structure elements. The voxel model of LLNL torso phantom was used as a reference to validate the modification process. The chest wall was mathematically modified from the starting phantom with no overlay plate to the target with an overlay plate of 24-mm thickness. The influences of different structure elements on detection efficiency were discussed. When diamond or square structure elements were used, the calculated efficiency was overestimated or underestimated by ∼40 % for 17.5-keV photons, compared with that of target phantom. In contrast, it was shown a good agreement with a deviation of <3 % when the hybrid structure elements were adopted. A similar adjustment method can be used for 3D modification of human voxel phantom to keep the body's anatomical geometry without distortion during the adjustment process.