Contrast improvement by reduction of the scatter contribution in a PET system using TOF information

The time of flight information (TOF) is used in ''TTVOl'', a PET system built by the French LETI group and installed in Orsay, where clinical studies and physical measurements are performed. Given the TOF sampling interval (12 mm), experiments showed that in three cases out of four a point source located in a scattering medium yielded the same TOF value for both scattered and true events. One can thus assume that the scatter component is mainly due to single scattering of one of the two photons occurring near the positron source. This scatter fraction was approximated by a 2D function which is position independent except for sources near the edge of the object. This function was added to the TOF response of the system and the deconvolution filter was derived from the modified response. It was tested on two 68-Ga filled cylindrical phantoms (24 and 30 cm in diameter). The first one contained three cold cylinders (4, 5, 7 cm in diameter) placed 6 cm off center and the second one a lucite cylinder (6 cm in diameter) placed in its center. The contrast was defined as the ratio of the measured activity in a small ROI inside the cold spots over the measured background activity. It improved by about 15% and dropped below 10% correct images. The grey to white matter ratio of regional /sup 15/O-CO/sub 2/ concentration obtained in a CO/sub 2/ continuous inhalation image was improved by this procedure from 2.4 to 3. This compensation technique can be included in the standard reconstruction procedure (18 sec for an image reconstructed from 500,000 events) and does not require any additional computing time