Abstract-The ability of differential time-of-flight (TOF) information to reduce the statistical noise variance in PET reconstructions has been known since the 1980's. Since then, the technology and applications of PET have evolved, warranting a reconsideration of the estimated improvements of TOF with respect to modern PET. For example, whereas 2D cardiology or neurology studies were once the only options, 3D clinical whole-body oncology imaging is becoming more common. The augmented sensitivity, change in object size and shape, as well as the accompanying changes in isotope and dose, result in different relative amounts of scattered, random, and true coincidences than were seen in the past. Thus in an analysis of the TOF gain for modern PET, it is useful to consider the separate effects of varying these fractions. We present a simulation study investigating the relative amount of TOF contrast-to-noise gain for a range of levels of scattered and random coincidences. We demonstrate that both increased scatter and increased randoms noticeably enhance the TOF gain, but that the higher randoms fraction introduces the most drastic improvement. These results are encouraging for modern PET, where there is a greater random/scatter fraction than in the PET of the 1980's.
Abstract-Multi-modality imaging studies are becoming more widely utilized in the analysis of medical data. Anatomical data from CT and MRI are useful for analyzing or further processing functional data from techniques such as PET and SPECT. When data are not acquired simultaneously, even when these data are acquired on a dual-imaging device using the same bed, motion can occur that requires registration between the reconstructed image volumes. As the human torso can allow non-rigid motion, this type of motion should be estimated and corrected.We report a deformation registration technique that utilizes rigid registration for bony structures, while allowing elastic transformation of soft tissue to more accurately register the entire image volume. The technique is applied to the registration of CT and MR images of the lumbar spine. First a global rigid registration is performed to approximately align features. Bony structures are then segmented from the CT data using a semi-automated process, and bounding boxes for each vertebra are established. Each CT subvolume is then individually registered to the MRI data using a piece-wise rigid registration algorithm and a mutual information image similarity measure. The resulting set of rigid transformations allows for accurate registration of the parts of the CT and MRI data representing the vertebrae, but not the adjacent soft tissue. To align the soft tissue, a smoothly-varying deformation is computed using a thin plate spline (TPS) algorithm. The TPS technique requires a sparse set of landmarks that are to be brought into correspondence. These landmarks are automatically obtained from the segmented data using simple edge-detection techniques and random sampling from the edge candidates. A smoothness parameter is also included in the TPS formulation for characterization of the stiffness of the soft tissue. Estimation of an appropriate stiffness factor is obtained iteratively by using the mutual information cost function on the result of the global deformable transformation.
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