A direct measurement of skull attenuation for quantitative SPECT

The attenuation of 140-keV photons was measured in three empty skulls by placing a line source inside each one and acquiring projection data. These projections were compared to projections of the line source alone to determine the transmission through each point in the skull surrounding the line source. The effective skull thickness was calculated for each point using an assumed dense bone attenuation coefficient. The relative attenuation for this thickness of bone was compared to that of an equivalent amount of soft tissue to evaluate the increased attenuation of photons in brain SPECT (single photon emission computed tomography) relative to a uniform soft tissue approximation. For the skull regions surrounding most of the brain, the effective bone thickness varied considerably, but was generally less than 6 mm, resulting in a relative attenuation increase of less than 6%