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Facilities Overview and History

The PET Camera

The WA PET Service is currently equipped with one whole body PET camera. It is an Allegro GSO scanner that was purchased in August 2002 and has been fully operational since November 2002.

The Allegro is a cutting-edge PET camera manufactured by Philips Medical Systems. Unlike its predecessors, C-PET and PENN-PET which use superseded NaI(Tl) technology, the Allegro employs gadolinium orthosilicate (GSO) crystals to achieve faster scans and better image quality resulting in more accurate diagnosis. GSO crystal technology has an advantage over NaI(Tl) because GSO is a denser crystal material, with a higher Z number and superior scintillation properties. This results in higher sensitivity, good energy resolution and higher count rate capabilities.

The Allegro consists of 17864 GSO crystals mounted in 29 rings. Each crystal is a little cube with the dimensions of 4mm x 6mm x 20mm. These crystals are mounted on 420 photomultiplier tubes (PMTs). The detection of electrical signals from the PMTs is turned on using trigger modules, which use 15 PMTs each in an overlapping arrangement. The electrical signal is shaped and digitised and if it recognised as a valid count it is written on the coincidence rebinning board (CRB). The Allegro at the WA PET Service has an upgraded CRB, which permits better dynamic study capabilities.

The Allegro is a 3D camera. PET 3D imaging maximises sensitivity thus reducing scanning time. PET 3D cameras do not use interplane septa which limit the detection of photons in directions oblique to the axial plane. The Allegro uses a rotating Cs-137 source for attenuation correction. To optimse scanning times the transmission and emission scans may be performed in two separate passes rather than the emission and transmission scans being interleaved on a position-by-position basis. Because PET is a tomographical imaging technique, a wide variety of reconstruction software is available. In current use at the WA PET Service is the advanced 3D iteration algorithm RAMLA 3D, which gives the best reconstructed image quality.

The WA PET Service performs comprehensive quality control tests as required. Daily tests include energy checks, PMT gain tests and baseline collections. The industry standard NEMA 2002 tests are performed every six months.