AN ATTENUATION MEASUREMENT TECHNIQUE FOR ROTATING PLANAR DETECTOR POSITRON TOMOGRAPHS

This paper presents a new attenuation measurement technique suitable f or rotating planar detector positron tomographs. Transmission measurem ents are made using two unshielded positron-emitting line sources, one attached to the front face of each detector. Many of the scattered an d accidental coincidences are rejected by including only those coincid ences that form a vector passing within a predetermined distance of ei ther line source. Some scattered and accidental coincidences are still included, which reduces the measured linear attenuation; in principle their contribution can be accurately estimated and subtracted, but in practice, when limited statistics are available (as is the case with the multi-wire Birmingham positron camera), this background subtractio n unacceptably increases the noise. Instead an attenuation image havin g the correct features can be reconstructed from the measured projecti ons. For objects containing only a few discrete linear attenuation coe fficients, segmentation of this attenuation image reduces noise and al lows the correct linear attenuation coefficients to be restored by ren ormalization. Reprojection through the segmented image may then provid e quantitatively correct attenuation correction factors of sufficient statistical quality to correct for attenuation in PET emission images.