# Performance Evaluation of the Current Birmingham PEPT Cameras
## 3 Development of Positron Cameras at Birmingham  
### History of Birmingham PEPT Systems  
PEPT was first developed at Birmingham in the 1980s using a \"positron camera\" comprising a pair of position sensitive multiwire proportional chambers (MWPCs) [4]. The major weakness of this system was its low sensitivity--each MWPC had a quantum efficiency of only 7% for detecting 511 keV photons, so that the overall efficiency for detecting a pair of such photons, both incident on detectors, was less than 0.5%. The maximum useful data rate from this camera was only around 3 kHz. Moreover, because each detector had multiple interaction planes, the data included a significant fraction of events which had scattered in a previous plane before detection, so even for a bare source \(f\) was less than 0.4 [1]. Figure 1: The solid angle for detecting pairs of back-to-back photons is maximum for a source at the centre of the FOV (**left**). As the source moves transverse to the detectors, the angular acceptance falls approximately linearly (**centre**). Movement of the source along the axis between the detectors leads to a smaller reduction in angular acceptance, which is limited by the solid angle of the farther detector (**right**). In 1999, this camera was replaced by a Forte camera (ADAC Laboratories) [5] comprising a pair of gamma camera heads, each containing a crystal of sodium iodide 16 mm thick and with a quantum efficiency for detecting 511 keV photons of approximately 23%. The Forte, which is still in regular use, has a similar geometry to the original MWPC system, consisting of a pair of rectangular detectors, with active area 51 cm \(\times\) 38 cm, which are mounted either side of the system of interest with a face-to-face separation adjustable between 25 cm and 80 cm. A typical operating separation of 50 cm corresponds to a geometric acceptance of approximately 0.3 for a central source, but as discussed above, this falls off linearly as the source moves off centre. Most medical PET scanners use rings composed of high efficiency scintillation crystals to image a relatively narrow slice of a human patient or a laboratory animal. Crystals are grouped into modules (\"buckets\") each with its own set of electronics. Since these systems are inherently modular, it is in principle straightforward to reconfigure them to give a geometry more suitable for PEPT studies of engineering systems. In the early 2000s, the Birmingham group started reconfiguring the detectors from early CTI/Siemens ECAT PET scanners to develop PEPT systems. A modular positron camera was developed comprising 16 modules, each containing four scintillator detectors [6]. The detectors were blocks of bismuth germanate (BGO), each approximately 50 \(\times\) 60 mm\({}^{2}\) in area and 30 mm thick. Each block was cut into an 8 \(\times\) 8 array of crystal elements, each 6.3 \(\times\) 6.8 mm\({}^{2}\), and was coupled to four photomultiplier tubes (PMTs) in such a way that comparing the four PMT outputs allowed the crystal element to be uniquely identified. The capability of this system was compared to that of the Forte when operated in a similar geometry [6]: the boxes were stacked two wide by four high giving two rectangular detector arrays each approximately 66 \(\times\) 35 cm\({}^{2}\) and separated by 50 cm. Despite the much higher efficiency of the BGO crystals, because of the gaps between blocks, the sensitivity in this geometry is only a little higher than for the Forte. Much more recently, a large modular array (LaMA) for PEPT was constructed comprising 48 buckets. The great advantages of these modular systems are their transportability and flexible geometry, enabling tracking to be performed on large-scale industrial equipment, but at the cost of reduced sensitivity due to the gaps between blocks. It has been clear for some time that much better results would be obtained by developing a dedicated high sensitivity system with a field of view large enough to accommodate laboratory-scale equipment.