Transmission Imaging in a PET Scanner Based on Forward-Scattered 662-keV Photons

Abstract

The use of LSO background radiation (gamma rays from naturally occurring 176Lu) has been studied in the past as a means of generating a PET attenuation map without CT. However, the low flux of background radiation has fundamentally limited this approach, requiring a long scanning time. In this work, a different approach is proposed, increasing the flux of gamma rays and thereby shortening the required scanning time. The proposed method is based on the principle of 137CS sources positioned behind the PET detectors. In this method, 662-ke V photons from the sources can enter the detectors and scatter in a forward direction across the scanner's field of view. Although multiple sources could be used, the proposed method was tested with a single 115 MBq 137CS source. A 6-ring Biograph Vision PET/CT scanner with LSO scintillators and SiPM -based detectors was used. Two-dimensional energy histograms were derived from list-mode PET events and so were sinograms, including blank scans with nothing in the field of view and a transmission scan with the patient bed in the field of view. Energy histograms showed the forward-scattered coincidence events. A transmission sinogram of the bed was created as the ratio of the transmission and blank sinograms. A method was introduced to estimate the sensitivity gain, compared to the LSO background method, of a proposed configuration with 38 137CS sources. The estimated gain was 6.5 in the case of 30 MBq per source, leading to the prediction that transmission scan times could be 6.5 times faster.