SINGLE-PHOTON TRANSMISSION MEASUREMENTS IN POSITRON TOMOGRAPHY USING CS-137

Transmission measurements are an essential step in the quantification of radioisotope distributions in vivo using positron tomographic techn iques. The development of a new technique for measuring transmission d ata that relies on the detection of photons in 'singles' mode rather t han 'coincidence' mode no longer restricts the choice of transmission sources to those that decay by positron emission. The motivation for u sing the 'singles' mode of operation is the substantial increase in co unt rate that can be achieved. This corresponds to a great increase in the statistical accuracy of the transmission data. We propose Cs-137, a suitable isotope for this purpose. Cs-137 is more economical than G e-68, the traditional source used for transmission measurements, in te rms of longer half-life and lower financial cost. Cs-137 can be used f or transmission measurements without any recalibration of the tomograp h, and the estimated spatial resolution is comparable to that obtained using annihilation photons. A simple extrapolation method is develope d, which allows extrapolation of the attenuation coefficients measured at 662 keV to 511 keV. A dual-energy-window technique, whereby correc tion can be made on-the-fly during acquisition, is used for scatter co rrection. The measured linear attenuation coefficients agree with pred icted values.