AITENUATION CORRECTION IN POSITRON TOMOGRAPHY by: SIU

Accurate attenuation correction is a prerequisite for the determination of precise regional radioactivity concentrations in positron tomography. Attenuation correction can be performed using an external source of radiation and two measurements: a blank scan performed with no subject in the tomograph, and a transmission scan performed with the subject in the field of view. The ratio of blank to transmission counts gives the appropriate attenuation correction factor for each line of response. In theory. this provides a perfect correction for photon attenuation. but in practice the technique is limited by noise due to limited counting statistics and scattered radiation in the measured transmission data. In the present work. 137CS is proposed. as a suitabl~ radiation source ·for transmission measurements in 'singles' mode, a technique that substantially increases the statistical accuracy of the transmission data. 137Cs can be used without any ~calibration of the tomograph, and the spatial resolution is comparable to that obtained using 6KGe. Since 137Cs emits a monoenergetic gamma ray at 662 keV, and emission data are acquired by detecting annihilation photons of energy 511 keY, a simple extrapolation method is developed to extrapolate the attenuation coefficients measured at 662 keV to 511 keV. To eliminate scatter contamination in the transmission data, a dual-energy-window scatter correction technique is developed whereby correction can be made on-the-fly during data acquisition. Using the developed extrapolation method and dual energy scatter correction method, the linear attenuation coefficients measured in 'singles' mode using lJ7es agree well with the expected values.