# Performance Evaluation of the Current Birmingham PEPT Cameras
## 4 Materials and Methods  
As a simple characterisation of the three scanners, data were measured for sources mounted at the centre of the FOV in each case. This represents a \"best case\" scenario, but is useful as a basis for comparing their performance.  
Figure 4: MicroPEPT geometry: the 42 cassettes are mounted in two rings, above and below the central mounting plate, with the detectors in the upper ring staggered relative to those in the lower ring.  
Figure 3: MicroPET cassette as used in constructing MicroPEPT: the four LSO crystals on the left of the picture are connected via light guides to position sensitive PSDs on the right.  
The sensitivity and count rate performance of the three cameras were investigated by making repeated measurements using a centrally mounted \({}^{11}\)C source. \({}^{11}\)C sources were produced by activating small pieces of graphite using a beam of 28 MeV protons from the Birmingham MC40 Cyclotron. The initial activity of each source was measured using a Capintec CRC-15R Radioisotope Calibrator, and subsequent activities were calculated based on the known half-life of 20.38 min, where the elapsed time was measured to within 10 s. The principal uncertainty on all these measurements arises from the accuracy of the CRC-15R's calibration, which should be correct to within 2%. Note that any error in calibration will affect all the measurements in the same way, so comparison of the relative sensitivities of the systems is unaffected.  
For each set of measurements, a \({}^{11}\)C source of initial activity around 50 MBq was mounted at the centre of the FOV of the camera under study. In each case, its location was determined using the Birmingham PEPT algorithm and was confirmed to lie within 5 mm of the central point. As the \({}^{11}\)C source decayed over a period of up to 3 h (down to an activity of approximately 100 kBq), 10 s data files were recorded at intervals, and were then analysed to determine the data rates.  
Both MicroPEPT and SuperPEPT simultaneously record both the usual prompt coincidences and also \"delayed\" coincidences in which the signal from one of the two detectors has been delayed by significantly more than the resolving time before checking for the presence of a coincidence. These delayed coincidences correspond only to random coincidences, whereas the prompt coincidences comprise both true coincidences and random coincidences. Hence the rate of true coincidences can be determined by subtracting the delayed coincidences from the prompt coincidences. In the case of the ADAC Forte only prompt coincidences are recorded, so information on the random coincidence rate is not explicitly available.  
The active region of the \({}^{11}\)C sources was somewhat extended (up to 5 mm in diameter) so to assess the tracking performance of each camera point, sources containing \({}^{18}\)F were used. \({}^{18}\)F was produced in solution by irradiating water enriched in \({}^{18}\)O with a beam of 10.8 MeV protons from the Birmingham MC40 Cyclotron and was then absorbed into a bead of ion-exchange resin \(<\)1 mm in diameter. The activity of each particle labelled in this way was measured using the Capintec CRC-15R. The source was then mounted in the FOV and the PEPT algorithm used to confirm that it was within 5 mm of the centre of the the FOV (within 2 mm in the case of MicroPEPT). Data were recorded for different levels of activity and were analysed using the original Birmingham PEPT algorithm [1] taking the sample size \(N\) as 1000 and varying the value of \(f\) so as to minimise the 3D standard deviation of the set of resulting locations.