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Determination of attenuation maps using a Ba-133 source and medium energy collimators in SPECT

Philips' AXIS/IRIX SPECT systems have an optional "BEACON" transmission imaging system that currently generates accurate non-uniform attenuation maps only with low energy collimators. However, medium energy collimators are used for In-111 and Ga-67 SPECT. Inspection of BEACON data taken with different collimators and an anthropomorphic phantom suggest a way to improve the medium energy attenuation maps: 1. Extend the three stage acquisition to four stages; 2. Compute a one-dimensional "scaling profile" that will modify the medium energy air flood profiles such that they match the low energy air flood profiles; 3. Apply this scaling profile to the transmission data; and 4. Two-dimensionally smooth both the corrected patient transmission data and the air flood data to control noise. Three-dimensional smoothing of the resultant attenuation map affords further noise control. Finally, scale the attenuation map to account for the difference between the attenuation coefficient for water at 140 keV and that for the emission energy of interest. With an additional calibration acquisition and a simple correction method, it is possible to obtain acceptable attenuation maps with medium energy collimators and Philips' BEACON system.