More than half the 511 keV photons in BGO crystals undergo Compton sca
ttering at least once prior to photoelectric interaction within the de
tector. In a PET scanner, this can result in mis-positioning of annihi
lation events. As crystal dimensions are made smaller, the fraction of
mispositioned events increases. We have studied the coincidence apert
ure function (CAF) of 25 mm x 10 mm BGO crystals with thicknesses vary
ing from 1 mm to 3 mm in steps of 0.5 mm. By sandwiching the active cr
ystal between two other BGO crystals not in contact with the photomult
iplier, we have studied the effect of Compton scatter upon CAF. By all
owing the annihilation photons to be incident along the 25 mm axis and
the 10 mm axis, we have studied the effect of detector depth upon the
CAF, with and without Compton scatter. The CAF increases linearly wit
h crystal width, ranging from 1.3 mm for the 1 mm wide crystal to 2.15
mm for 3 mm crystal. The lines joining the FWHM of the CAFs as a func
tion of crystal width appear to converge indicating that no further im
provement in resolution can be achieved by reducing crystal width. The
CAFs for 10 mm and 25 mm long crystals without Compton scatter is ess
entially the same. The presence of neighbouring crystals results in an
increase in the CAF which is greater for long crystals compared with
short ones of the same width. Using 1 mm wide and 10 mm long crystals
sandwiched between two uncoupled BGO crystals, we have achieved spatia
l resolution of 1.64 mm FWHM for a PMT separation of 34 cm. Recent exp
eriments with PMT separation of 11.5 cm have yielded 1.23 mm FWHM CAF
in the same setup.