Statist ical Models for Randoms - Precorrec ted P E T Scans

1 I n t r o d u c t i o n In P E T measuremen t s , acc identa l coincidence (AC) events are a p r i m a r y source of backg round noise. AC events occur when pho tons t ha t arise f rom sepa ra t e ann ih i l a t ions are mis t aken ly regis tered as having ar isen f rom the same annih i la t ion. In t r ansmiss ion scans the pho tons t h a t o r ig ina te f rom different t r ansmis s ion sources ( rod or sector sources ro t a t i ng a round the pa t i en t ) cause AC events. The ra t io of t o t a l AC events to "true" events is usual ly smal l in t r ansmiss ion scans c o m p a r e d to emiss ion scans. Nevertheless, the effect of AC events becomes severe for regions of high a t t e n u a t i o n coefficients, because p ro jec t ions t h rough such regions resul t in low t rue coincidence rates . These low count ra tes can become c o m p a r a b l e to AC rates . Thus es t ima tes of the AC events are needed. One c a n use the "singles" m e t h o d [1] for this purpose , however th is app roach is not This work was supported in part by NIH grants CA-60711 and CA-54362.