8.3 Detection of Annihilation Photon

Annihilation photons interact within the detector through Rayleigh, Compton and photoelectric effects: only the latter two release energy to the detector, making the photons "detectable". Rayleigh scattering is not usually considered at typical PET energies.

In most detection systems in the Field of Nuclear Medicine, the response is not constant over the whole detection area: the efficiency varies from a maximum value at the centre of the FOV down to a minimum near the edges. There is also non-uniformity in the sensitivity: the signal shape of the response to a reference source varies with its position. This behaviour can be quantified by putting a linear source of activity in various positions in the FOV: the resulting response constitutes the Line Spread Function (LSF) of the system, and indicates how much the source image spreads (Fig. 8.12). In a PET system, the linear source is moved along the central line between the two detectors (radial sampling), and along the two orthogonal directions (transaxial and axial sampling). Differently, PET systems show notable response uniformity: the FWHM value of the LSF is constant along most part of the detection volume.

Figure 8.12: Response uniformity and LSF shape for PET detection elements. ( a) Circular detectors show a Gaussian LSF, which is practically uniform in the FOV. ( b) Square detectors have a trapezoidal LSF, from triangular in the centre to rectangular near the edges; the response is uniform in the central zone only (from Ref. ^[5]