SiPM based technology with Time-of-Flight capability
What is the benefit of Time-of-Flight?
PETsys Electronics technology is based on the TOFPET ASIC (Figure 1). It provides a cost effective way to use Time-of-Flight (ToF) to improve the “equivalent sensitivity” of PET systems. The gain in sensitivity implies sharper images with substantially less noise (see Figure 2). The ratio of the patient torso diameter to the uncertainty of the location of the gamma rays emission along the Line-of-Response (line joining the interaction points of the two gammas in the detector) determined by the ToF measurements, has been shown to be representative of the sensitivity gain. The signal-to-noise ratio (SNR) of the PET images is improved by the square-root of the sensitivity gain factor.
Figure 2 - Monte Carlo simulation of a uniform cylinder with spheres of diameter 10, 13, 17 and 22 mm at a contrast ration of 4:1 and two spheres of diameter 28 and 37 mm filled with water with no activity: high statistics, 356×106 counts in the total volume (top); low statistics, 10×106 counts (bottom). Filtered back-projection was used for non-TOF reconstruction (left), for TOF reconstruction with 600 ps time resolution (centre), and for TOF reconstruction with 300 ps time resolution (right). Figure taken from M Conti, Eur J Nucl Med Mol Imaging (2011) 38:1147–115
What is the ToF performance of PETsys detectors?
PETsys Detector Modules, based on LYSO crystal arrays (3x3x15 mm pixels), MPPC silicon photomultipliers and PETsys TOFPET ASICs, have Coincidence Time Resolution (CTR) of ~300 ps FWHM (Full Width Half Maximum) – see Figures 3 and 4. The corresponding uncertainty of the emission point is about 4.5 cm FWHM. Knowing that the torso diameter of a typical patient is ~30 cm we conclude that PETsys technology allows a gain in sensitivity of a factor 6.5, and a gain in SNR of a factor 2.5. This also means that the same image quality as with non-ToF PET is achieved with shorter exam time or with lower radiation dose to the patient by a factor 6.5.
Figure 4 : Coincidence time resolution (CTR) measured with two 8x8 LYSO crystal arrays (crystal pixels with 3x3x15 mm) coupled to a Hamamatsu 8x8 SiPM14161-3050HS SiPM array at room temperature. The figure shows the time difference distribution summed over all lines of response for a NA22 point source approximately in the middle between the two arrays.
Application to small animal or breast PET
In some applications, e.g. in small animal PET systems or breast PET, the sensitivity gains from ToF are smaller but a better coincidence time resolution allows the reduction of random coincidences noise. This is an important consideration in high rate applications. Also in this case scanners based on SiPM will lead to a more compact and cost effective solution.
Straightforward integration of SiPM-based PET scanners
PETsys state of the art data acquisition technology based on high-speed point-to-point links allows the straightforward integration of SiPM-based PET scanners. The technology is scalable to systems with several tens of thousand channels and allows for a staggering total rate of singles in the PET scanner of 1 Giga hit per second.