The application of a masked orbiting transmission source for attenuation correction in PET

A new technique for attenuation correction in positron emission tomography is introduced and evaluated. Transmission scans are performed with a point source of 68Ge encapsulated in a lead collimator that masks the source into a fan beam in the scanning plane. The source orbits the patient section at the edge of the slice defining collimator. Only events acquired by detector pairs that are collinear with the source are used to calculate the attenuation coefficients. Events from detector pairs that are nearly collinear are rejected, while those from detector pairs that are far from collinear may be used to acquire a simultaneous emission scan. The coincident event rate per unit source activity is over twice that of rod and ring sources. This technique is compared with calculated outline and ring source attenuation correction techniques in a pie phantom. The linear attenuation coefficient for water was measured as 0.096 cm-1, and 0.094 cm-1 when the water contained 12 kBq/cc 68Ga, compared with 0.085 cm-1 for a ring source. Cerebral glucose utilization rates in a normal volunteer reconstructed with transmission scans performed pre- and postinjection of fluorodeoxyglucose show no significant differences. However, values of cortical glucose utilization average 12% above those measured with the fitted outline method in the highest cuts because of the obliqueness of the skull to the planes examined.