if the brain is assumed to be a sphere with diameter 20 cm, and the largest dimension of the body to be 40cm, what are the respective values of the timing resolution necessary to reduce the noise in TOF PET compared to conventional PET?
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Answer:
The signal in PET is produced by the annihilation of an emitted positron with an electron in the surrounding medium or tissue. Positron annihilation leads to the production of two 511 keV photons emitted almost back-to-back that are detected in time coincidence by the surrounding PET detectors to form a line-of-response (LOR). The emission distance along the LOR (d) is determined by d=c*(t2−t1)/2, where c is the speed of light, and t1 and t2 are the arrival times of the two photons (Figure 1A). In conventional PET the difference in arrival time (t2−t1) of these two photons is not measured precisely enough to localize the emission point along the LOR. By collecting all possible LORs around the object (full angular coverage) and assuming uniform probability of the emission points lying along the full length of the LORs (and within object boundary), it is mathematically possible to reconstruct the emission object accurately. Knowledge of emission point locations along the LORs is not necessary to reconstruct the emission object. However, by assuming uniform probability of event location along the full LOR length, noise from different emission events gets forward and back projected during image reconstruction over many image voxels leading to increased noise correlation. Hence, the image signal-to-noise ratio (SNR) gets reduced