Positron emission tomography

Positron emission tomography (PET) is the most powerful and extensively used system to probe the human physiology. It is also a diagnostic tool used to detect physiological modifications in tissues. Cancer prevention would call for early detection of such changes, which strongly depends on the spatial resolution. A key factor to improve the resolution relies on the background reduction. The frequency of accidental almost-back-to-back photons, which mimics a genuine electron-positron annihilation followed by two 511 keV photons at 180 degrees, is directly related to the time window within which the photons are considered as emitted by the same process. By substantially increasing the resolution on the arrival time of the photons, one can tight the window thus reducing the number of spurious events. The commercial PET systems dispatched resolutions somewhat smaller than 1 nsec, which, considering the light speed, corresponds to 30 cm in the position of the annihilation along the line-of-view. As a first approximation, the signal-to-noise ratio (SNR) scales linearly with the time resolution. With this project we consider to improve the time resolution by an amount close to two orders of magnitude. We will observe a dramatic improvement according to two aspects. The image quality will considerably enhance, due to the decrease of the uncertainty of the position of the two photons emission which will be somewhere between 3 to 5 mm, hitting the natural bound due to the positron range (1-2 mm) in Water. The other, more important issue, is the sensibility to detect even marginal physiological changes mainly related to early cancer lesions. The SNR rise directly impacts the sensibility. With the above considerations in mind, we claim that the 4D detector we are proposing will be greatly beneficial to the PET performances in the investigation capabilities.