Euromedim 2006 :1st European Conference on Molecular Imaging Technology

Sensitivity of limited angle TOF PET systems

11 May 2006, 14:00

1h

Palais du Pharo, Marseille

poster • Status of animal and clinical PET, SPECT and CT (biomedical and technical) Poster session : Imaging systems, Molecular Imaging

Speaker

Dr stefaan Vandenberghe (Ugent)

Description

Due to the availability of scintillators (LSO, LaBr3) with good timing resolution, sufficient stopping power and good energy resolution, different 3D TOF PET scanners are now under development. Recently we have shown that improved timing resolution allows to reduce the number of angles needed to reconstruct TOF-PET data. Histoprojections from TOF data can be seen as an asymmetrically blurred image with noise. Therefore image reconstruction from histoprojections can be seen as a problem of image restoration (earlier discussed in a paper by Grangeat) and this reconstruction method was implemented to reconstruct the data. Using these reconstruction techniques a full ring configuration is not needed to allow reconstruction. As it has been shown before for non-TOF PET systems a full ring configuration is not optimal from a sensitivity perspective for a given number of detectors. This has led to development of half ring systems like the ECAT ART. The disadvantage of these PET systems was the need for rotation to obtain complete data. This is however not needed for limited angle TOF PET systems. Methods and results We quantified the changes in sensitivity and scatter fraction between different configurations by Gate Monte Carlo simulations. A full ring system with either 60 rings or 120 rings of LaBr3 crystals (4x4x30 mm) was modeled. A limited angle system with the same number of detectors as the 60 ring system was constructed by only using half of the ring. Different activity distributions where simulated to determine the change in sensitivity, Most objects for PET imaging will at least cover the whole axial FOV of the camera. An 5cm radially off center line source was used as a representative source for the volume sensitvity. The sensitivity increases for a full 120 ring system compared to the 60 ring system with 298 %. The semi ring (two opposed curved detectors) with the same amount of detectors as a full ring system has 99% more sensitivity. For a line source in a 70 cm long phantim the scatter fraction increases with 3 % when going from the complete 60 ring system to a limited angle 120 ring system. For small objects (less than about 25 cm axial extent of the 60 ring system) the situation is different. Here we used a central point source for estimating the sensitivity. When the number of rings of a complete ring system is doubled, the point source sensitivity increases by a factor of 99.5 %. So there is not an increase in sensitivity for a limited angle system with the same number of detectors. For small objects it is however possible to reduce the radius of the two parts of the limited angle system. We have quantified this increase in sensitivity. Conventional ring systems have a radius of about 45 cm. When we reduce the radius of a limited angle system to 40 cm we gain 34 % sensitivity, at 35 cm it is 86 % and at 30 cm the increase it 103%. Conclusions: A limited angle system with the same number of detectors has a doubled volume sensitivity and the same point source sensitivity. By modifying the radius of the scanner for small objects the sensitivity can be increased. This makes it possible to use a limited angle TOF PET system for brain or breast imaging without the disadvantage of conventional whole body PET systems.