New developments in PET instrumentation: quo vadis PET?

The growth of nuclear medicine at the end of the 20th century and the beginning of the 21st has been caused mostly by recognition of the clinical value of coincidence imaging, or PET. In the mid-1990s, hybrid SPECT–PET systems were developed along with a new generation of clinically oriented dedicated PET systems. More recently, new PET systems using lutetium oxyorthosilicate (LSO) and Ge 2SiO5 (GSO) crystals have been developed. Furthermore, PET systems have now been combined with CT to provide CT transmission scans for attenuation correction and superior coregistration of PET and CT data. These developments were underscored by the 2001 annual meeting of the Society of Nuclear Medicine, in Toronto, where practically all vendors offered one or even several PET scanners, ranging from lowto very high-end. These new scanners tend to use more 3-dimensional (3D) imaging, and some work in 3D mode only. The interesting article by Adam et al. (1) in this issue ofThe Journal of Nuclear Medicine describes the performance characteristics of such a clinically oriented PET scanner, C-PET, which works in 3D mode only, or without collimation. Up to now, most PET systems have had both options, that is, imaging in 2-dimensional (2D) or 3D mode. The article also gives insight into the National Electrical Manufacturers Association (NEMA) 2000 standard, which, although not yet official, describes tests, phantoms, and evaluation procedures for 2D and 3D PET systems. The principal advantage of 3D mode is significantly higher sensitivity, which can significantly reduce the tracer activity needed or shorten the acquisition time. 3D PET has been used mostly for brain imaging. An increase in sensitivity can also make cardiac gated studies possible, because in 2D mode, image quality is severely limited by the number of counts. However, compared with 2D mode, 3D imaging still has some disadvantages. Recently, the quality of 3D cardiac images was shown to be lower than ( 2) or, at best, comparable with ( 3) that of 2D PET images. Several reasons account for this finding. First, compared with 2D mode, 3D mode requires significantly more computer memory and significantly more time for reconstruction of images. Second, slice sensitivity along the axial field of view (FOV) in 2D mode is relatively uniform, except for the very edge of the FOV. In 3D mode, the same slice sensitivity varies greatly and is triangular ( 4). Therefore, C-PET, with an axial FOV of 25.6 cm, needs 8 bed movements to cover a 1-m length; that is, the scanner has a 50% overlap between 2 neighboring bed positions ( 1). Third and most important, 3D mode suffers from a high rate of scatter and random events. To fully understand the differences between 2D and 3D PET imaging, one needs to know how PET data are acquired. PET systems acquire 2 channels, usually called the total prompt channel and the total delayed channel. The total prompt channel collects true, scatter, and random events that occur within the coincidence time window. The total delayed channel estimates random events, which are then subtracted from the total prompt channel to give true and scatter events. Each event is placed in a line of response in the sinogram. From the sinograms, reconstructed images are obtained. The various available systems treat random and scatter events at different stages and in different ways. Therefore, in PET imaging, we cannot talk simply about counts. We have to specify whether we are talking about true, scatter, or random counts. This represents a significant difference between SPECT and PET. Therefore, when we speak of an increasing sensitivity of 3D PET, we should ask ourselves whether we are dealing with an increase in true counts, in scatter and random counts, or in a combination of all. The parameter that characterizes the relationship between true, scatter, and random events is the noise equivalent count (NEC) rate ( 4,5). Usually, the NEC rate is plotted as a function of activity concentration, which is expressed in kBq/mL ( Ci/mL). PET systems, like gamma cameras, can be paralyzed. In other words, the NEC rate will reach a peak and then start declining as the system saturates at higher activity concentrations. DeGrado et al. ( 5), using a leading bismuth germanate (BGO) PET system, showed that the peak NEC rate is greater in 2D than in 3D imaging for a uniform phantom. The higher number of counts in 3D imaging does not necessarily mean a better image quality, because the increase in counts is mostly an increase in scatter and random events. Resolution properties also differ between 2D and 3D PET imaging. Because of septa removal and exposure of more crystals, axial resolution is lightly worse in 3D imaging ( 4). However, this difference in axial resoReceived Jul. 17, 2001; revision accepted Jul. 31, 2001. For correspondence or reprints contact: Karin Knes̆aurek, PhD, Division of Nuclear Medicine, Mount Sinai Medical Center, Box 1141, One Gustave L. Levy Place, New York, NY 10029.