Energy-based scatter correction for 3-D PET scanners using NaI(T1) detectors

Earlier investigations with BGO positron emission tomography (PET) scanners showed that the scatter correction technique based on multiple acquisitions with different energy windows are problematic to implement because of the poor energy resolution of BGO (22%), particularly for whole-body studies. We believe that these methods are likely to work better with NaI(TI) because of the better energy resolution achievable with NaI(TI) detectors (10%). Therefore, we investigate two different choices for the energy window, a low-energy window (LEW) on the Compton spectrum at 400-450 keV, and a high-energy window (HEW) within the photopeak (lower threshold above 511 keV). The results obtained for our three-dimensional (3-D) (septa-less) whole-body scanners [axial field of view (FOV) of 12.8 cm and 25.6 cm] as well as for our 3-D brain scanner (axial FOV of 25.6 cm) show an accurate prediction of the scatter distribution for the estimation of trues method (ETM) using a HEW, leading to a significant reduction of the scatter contamination. The dual-energy window (DEW) technique using a LEW is shown to be intrinsically wrong; in particular, it fails for line source and bar phantom measurements. However, the method is able to produce good results for homogeneous activity distributions. Both methods are easy to implement, are fast, have a low noise propagation, and will be applicable to other PET scanners with good energy resolution and stability, such as hybrid NaI(TI) PET/SPECT dual-head cameras and future PET cameras with GSO or LSO scintillators.     

Dedicated PET scanners for breast imaging

We have used computer simulations to compare two designs for a PET scanner dedicated to breast imaging with a whole-body PET scanner. The new designs combine high spatial resolution, high sensitivity, and good energy resolution to detect small, low-contrast masses. The detectors are position sensitive NaI(Tl) scintillators. The first design is a ring scanner surrounding the breast and the second consists of two planar detectors placed on opposite sides of the breast. We have employed standard performance measures to compare the different designs: contrast, percentage standard deviation of the background, and signal-to-noise ratios of reconstructed images. The results of the simulations show that both of the proposed designs have better lesion detectability than a whole-body scanner. The results also show that contrast is higher in the ring breast system but that the noise is lower in the planar breast system. Overall, the ring system yields images with the best signal-to-noise ratios, although the planar system offers practical advantages for imaging the breast and axilla.     

Triple energy window scatter correction technique in PET

A practical triple energy window technique (TEW) is proposed, which is based on using the information in two lower energy windows and one single calibration, to estimate the scatter within the photopeak window. The technique is basically a conventional dual-window technique plus a modification factor, which can partially compensate object-distribution dependent scatters. The modification factor is a function of two lower scatter windows of both the calibration phantom and the actual object. In order to evaluate the technique, a Monte Carlo simulation program, which simulates the PENN-PET scanner geometry, was used. Different phantom activity distributions and phantom sizes were tested to simulate brain studies, including uniform and nonuniform distributions. The results indicate that the TEW technique works well for a wide range of activity distributions and object sizes. The comparisons between the TEW and dual window techniques show better quantitative accuracy for the TEW, especially for different phantom sizes. The technique is also applied to experimental data from a PENN-PET scanner to test its practicality.     

Reducing edge effects and improving position resolution in positionsensitive NaI(TI) detectors

Large two-dimensional position sensitive NaI(Tl) crystals used in positron emission tomographs and elsewhere normally have gaps or inactive, unusable areas at the edges. Experiments aimed at reducing these edge effects have been performed. Unencapsulated crystals have been used to test the feasibility of optically coupling crystals together to decrease gap size. Other experiments increased the sampling of the scintillation light at the edges in order to obtain better position sensitivity. In addition, the edges were treated to reduce unwanted reflections and increase the position sensitive area. Experiments aimed at improving the position resolution throughout the crystal, as well as at the edges, were performed