Positron emission tomography with a large axial acceptance angle: signal-to-noise considerations

Fast fully 3-D image reconstruction in PET using planograms

We present a method of performing fast and accurate three-dimensional (3-D) backprojection using only Fourier transform operations for line-integral data acquired by planar detector arrays in positron emission tomography. This approach is a 3-D extension of the two-dimensional (2-D) linogram technique of Edholm. By using a special choice of parameters to index a line of response (LOR) for a pair of planar detectors, rather than the conventional parameters used to index a LOR for a circular tomograph, all the LORs passing through a point in the field of view (FOV) lie on a 2-D plane in the four-dimensional (4-D) data space. Thus, backprojection of all the LORs passing through a point in the FOV corresponds to integration of a 2-D plane through the 4-D "planogram." The key step is that the integration along a set of parallel 2-D planes through the planogram, that is, backprojection of a plane of points, can be replaced by a 2-D section through the origin of the 4-D Fourier transform of the data. Backprojection can be performed as a sequence of Fourier transform operations, for faster implementation. In addition, we derive the central-section theorem for planogram format data, and also derive a reconstruction filter for both backprojection-filtering and filtered-backprojection reconstruction algorithms. With software-based Fourier transform calculations we provide preliminary comparisons of planogram backprojection to standard 3-D backprojection and demonstrate a reduction in computation time by a factor of approximately 15.

Figures of merit for comparing reconstruction algorithms with a volume-imaging PET scanner

For volume-imaging PET scanners, no septa are used to maximize the sensitivity by collecting events oblique to the scanner axis. We answer two questions: (i) how does the performance of an image reconstruction algorithm for a volume-imaging PET scanner depend on its general dimensions? and (ii) at what point is a three-dimensional (3D) reconstruction algorithm needed for a volume-imaging scanner, as the axial extent is increased? A 3D reconstruction algorithm will accurately incorporate the oblique events in a reconstruction of the original source distribution. From simulations of an existing volume PET scanner with a maximum axial acceptance angle (+/-alpha) of alpha = 9 degrees, however, we show that the single-slice rebinning algorithm is a good compromise between sensitivity, speed, and accuracy when compared to standard two-dimensional reconstruction (alpha = 1 degrees), and a 3D reconstruction with alpha = 9 degrees. We also show with simulations that a new scanner with alpha = 27 degrees requires 3D reconstruction in order to achieve maximum sensitivity without unacceptable losses in accuracy. Measurements of scanner performance are based on a series of figures of merit that characterize image quality and quantitative accuracy measured from a set of simulated test phantoms.

The problem of scatter correction in positron volume imaging

Thin, dense interslice septa have long been used to reduce the scattered radiation collected in positron emission tomography (PET). These septa prevent the acquisition of coincident gamma rays that are oblique to the scanning planes. Several volume imaging systems that acquire oblique rays have been built. The reasons why the scatter fraction is intrinsically higher in these systems are discussed, and two different scatter compensation techniques are compared. The comparison uses Monte Carlo simulations to generate average profiles and spectra for typical phantoms and scanning geometries. Comparison results indicate that deconvolution of the total event profiles with a filter adjusted to the object size and good energy discrimination provide an excellent scatter estimate as long as the object's diameter is less than the detector's radius. The profiles derived from a second, lower, energy window are always much flatter than the scattered counts in a window covering the photopeak. They do not match the scattered event profile and are less reliable, being based on a lower number of counts.

Effect of increased axial field of view on the performance of a volume PET scanner

The performance of the PENN-PET 240H scanner from UGM Medical Systems is tested and compared to the prototype PENN-PET scanner built at the University of Pennsylvania. The UGM PENN-PET scanner consists of six continuous position-sensitive NaI(Tl) detectors, which results in a 50 cm transverse field-of-view and a 12.8 cm axial field-of-view. The fine spatial sampling in the axial direction allows the data to be sorted into as many as 64 transverse planes, each 2 mm thick. A large axial acceptance angle, without interplane septa, results in a high-sensitivity and low-randoms fraction, with a low-scatter fraction due to the use of a narrow photopeak energy window. This work emphasizes those performance measurements that illustrate the special characteristics of a volume imaging scanner and how they change as the axial length is increased.

Instrumentation for positron emission tomography: tomographs and data processing and display systems

The field of positron emission tomography (PET) has evolved over the past 10 to 15 years from a basic research endeavor to a field in which both research and clinical studies are performed at over 100 institutions and medical facilities throughout the world. Most centers now use tomographs supplied by commercial vendors and operate data analysis systems employing identical software packages. PET scanners have advanced from prototypes assembled more than 15 years ago that imaged a single slice with 32 detectors at a resolution of greater than 2.0 cm full-width-at-half-maximum (FWHM), to current state-of-the-art instruments that simultaneously image as many as 47 slices using nearly 10,000 detector crystals at resolutions of less than 5 mm FWHM. In addition, specialized research instruments have been developed that have in-plane image resolution of approximately 2.5 mm FWHM. Other PET instrumentation advances include the development of detector block designs with multiple crystals per photomultiplier tube, continuous large position-sensing detectors, and the use of new detector materials having properties particularly suited for PET imaging. Image display and analysis systems have also advanced considerably over this time period. Early scanners typically used analysis stations with dedicated displays of only approximately 256 x 256 pixels and minimal internal processing capabilities. Current systems now offer displays with more than a megabyte (1,024 x 1,024) of image memory and have considerable internal processing capability. Further improvements in the ability to efficiently handle the massive volumes of data generated by the latest generation of scanners in increasingly sophisticated manners are crucial to the continued advancement of the PET field.

Current and future technological trends in positron emission tomography

Current trends in positron emission tomography (PET) instrumentation are examined, with an emphasis on providing information suitable to the prospective PET user. Basic principles underlying PET are explained and information on performance measurements, techniques, and quantitation are given in order to allow the user to compare and contrast different types of PET scanners. These scanner designs are described. Specific examples are given and the combination of PET with other modalities is discussed.