Attenuation correction in PET using single photon transmission measurement.

The use of single photon transmission measurement with a rotating rod source has been evaluated to measure the attenuation correction factors in positron emission tomography (PET). The singles projections are resampled into the coincidence geometry using the detector positions and the rod source location. A nonparalyzable dead time correction algorithm was developed for the block detectors used in the McMaster PET scanner. This enables accurate attenuation correction factors (ACFs) to be computed using a wide range of source strengths for transmission scanning. Transaxial resolution is approximately 6 mm, which is comparable to emission scanning performance. Axial resolution is about 25 mm, with only crude source collimation. ACFs are underestimated by as much as 10% due to increased cross-plane scatter, compared to coincidence transmission accuracy. The response of the correction factors to object density is within 15%, when comparing singles transmission measurement to current coincidence transmission measurement. The major advantage of using singles transmission measurement is a dramatically increased count rate. A factor of 7 increase in count rate over coincidence scanning is possible with a 2-mCi transmission rod source. Uniformity of 2% in the transmission images is possible with this source strength and a 2-min acquisition. There are no randoms counted in singles transmission scans, which makes the measured count rate vary linearly with source activity. Singles detector dead time losses are approximately 6% in the detectors opposite a 2-mCi rod source.