A spheroid positron emission tomograph for brain imaging: a feasibility study.
It has been long recognized that the primary advantage of imaging the brain with a positron emission tomography using GSO scintillation detectors placed on a spheroid surface is the large solid angle of acceptance for annihilation radiation, which results in improved system sensitivity and image signal-to-noise ratio. In the present study, we investigated spheroid system geometry, detector design and contribution of scattered coincidences.


METHODS
Scintillation detector distribution on a spheroidal surface was investigated by approximating the surface by polygons. Finding a suitable crystal for this purpose led to the development of an experimental GSO block-type detector. The fraction of scattered coincidences was experimentally evaluated using phantoms and detector pairs in conjunction with a testing platform, and the relationship between scattered fraction and phantom volume was obtained.


RESULTS
Spheroid geometry was best implemented with a polyhedron consisting of a series of consecutive rings formed by trapezoids. An experimental block-type detector with 36 GSO scintillators and four 14-mm-diameter photomultiplier tubes, together with custom electronics, yielded a spatial resolution of 3.4 mm FWHM and an energy resolution of 18% FWHM. Using nearly "ideal" scintillation detectors with a 350-keV threshold, we found the scatter fraction to be 0.32 for a 20-cm uniform phantom, 0.22 for a 15-cm phantom and closely proportional to the square root of the phantom volume.


CONCLUSIONS
For cerebral studies, a spheroid PET using GSO scintillators has several advantages: optimized geometry for sensitivity, a dead-time fivefold smaller than an equivalent BGO system, and appreciably better light output for improved energy resolution and detector identification. The construction of such a system is within the capabilities of present technology.
Ficke
D C
DC
Mallinckrodt Institute of Radiology, Department of Chemistry, Washington University School of Medicine, St. Louis, Missouri.
Hood
J T
JT
Ter-Pogossian
M M
MM
D001921:Brain / Q000000981:diagnostic imaging*; D004867:Equipment Design; D005240:Feasibility Studies; D006801:Humans; D019047:Phantoms, Imaging; D012542:Scattering, Radiation; D014055:Tomography, Emission-Computed / Q000295:instrumentation* / Q000379:methods*