Future instrumentation in positron emission tomography

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Abstract
The solid angle of current cylindrical PET systems and hence the overall sensitivity can be increased by incorporating additional rings of block detectors. However, for a given scintillator volume of LSO, we may optimize the number of detectors and thus the number of rings of blocks in terms of point source and absolute sensitivity (NEMA NU-2 2001). The constant LSO volume restriction implies in the extreme a large axial FOV system with a thin LSO layer or a narrow axial FOV system with a thick LSO layer. The sensitivity values can then be converted into volume and planar sensitivities based on count rates, acquired with a particular phantom. The HiRez detector (Siemens Molecular Imaging) with a varying LSO thickness has been used as the basis for adding extra rings of detectors. The diameter of the system assumed for this study is 85.5 cm and the axial extent ranged from 16.4 cm, that of the current Biograph TruePoint system, up to a maximum of 111 cm. The calculated planar and volume sensitivity values are based on simple geometrical arguments, verified by Geant4 simulations. The calculated NEC values for one set of LSO volume data are from actual experimental data from the Siemens Biograph TruePoint 3 ring scanner and then scaled based on the ratio of the calculated absolute sensitivity to the measured TruePoint absolute sensitivity. Sensitivity data have been obtained for 10, 16 and 23 liters of LSO. The planar sensitivity generally peaks for ring configurations with around 2 cm deep LSO pixels. For the 16 liter LSO data the planar sensitivity peaks for a 33 cm axial FOV while the volume sensitivity peaks at around 65 cm axial FOV. However, the volume sensitivity value at 33 cm is around 75% of the maximum volume sensitivity at 65 cm, suggesting that a 33 cm axial extent may be a better choice for a 16 liter LSO system.

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