A direct measurement of skull attenuation for quantitative SPECT

The attenuation of 140-keV photons is measured in three empty skulls by placing a ^99mTc line source inside each one and acquiring projection data. These projections are compared to projections of the line source alone in order to determine the transmission through each point in the skull surrounding the line source. The effective skull thickness is calculated for each point using an assumed dense bone attenuation coefficient. The relative attenuation for this thickness of bone is compared to that of an equivalent amount of soft tissue to evaluate the increased attenuation of photons in brain single photon emission computed tomography (SPECT) relative to a uniform soft tissue approximation. For the skull regions surrounding most of the brain, the effective bone thickness varies considerably, but is generally less than 6 mm, resulting in a relative attenuation increase of less than 6%