Myocardial perfusion in 3 dimensions.

Once in a while a study is published that could mark an important step forward but, at the same time, is long overdue. The study by Schäfers et al. (1) in this issue ofThe Journal of Nuclear Medicine, describing the measurement of myocardial blood flow (MBF) using H2O and a 3-dimen sional (3D) acquisition protocol, is such a study. After preliminary efforts, successful development of the first quantitative PET scanner (2,3) was based on several key design features. One of these features was protection of (single slice) crystals from out-of-field scatter using side shielding. In the second generation of (multislice) scanners, interplane septa were used to reduce detection of scattered events. In fact, it was believed that scatter was the main enemy of quantification. A change of thought came about in the late 1980s ( 4,5), when it was realized that another enemy was lack of sensitivity, especially within the context of repeated measurements on healthy subjects (brain activation studies) and increasing awareness of radiation doses. Scanners with retractable interplane septa were developed (6), and it became common practice to perform brain studies in the 3D mode (i.e., with the interplane septa retracted). This practice was based on the significant improvement in sensitivity (factor of 8 in the center of the field of view) coupled with the development of satisfactory methods to correct for scatter (7,8). However, these 3D studies were limited to the brain. Over the years, 3D studies of the thorax were reported in only a few investigations, but most of these investigations were concerned with image quality in diagnostic (qualitative) studies. In general, it was thought that there was little or no quantitative gain of 3D compared with 2-dimensional (2D) myocardial studies. The study by Schäfers et al. (1) is the first attempt to quantify MBF using H2O and a 3D acquisition protocol. This could mark an important step forward because the number of scanners that can acquire data only in the 3D mode (i.e., scanners without interplane septa) is increasing ( 9–12). Therefore, 3D data acquisition and analysis strategies need to be developed to guarantee that those scanners can also be used for quantitative myocardial studies. The study by Schäfers et al. is a nice example of that need because it was performed on a 3D-only scanner. Nevertheless, the study is long overdue. Three-dimensional PET has been commercially available since the early 1990s, but the number of 3D myocardial PET studies reported is very limited. This is true not only for the PET community at large but also for the Hammersmith group itself, because the scanner used by Schäfers et al. was installed some 6 y ago. The time gap between the introduction of 3D PET and its use in quantitative myocardial studies is probably the best illustration of how complex the issues are that need to be addressed in studies outside the brain. There are several potential advantages of 3D PET. First, for a given dose and based on the higher efficiency, scan time could be reduced. The shorter scan time would provide higher patient throughput and, therefore, lower scanning costs (in addition, the absence of interplane septa also reduces scanner costs). This, however, applies only to qualitative (diagnostic) scans. In general, for quantification, dynamic scanning protocols are required and study duration is not determined by the number of counts but, rather, by the tracer being used and the physiologic process being studied. Because study duration is fixed, higher sensitivity would allow better statistics or a lower injected dose. For a countlimiting technique, in which radiation dose to the patient is always important, both options are attractive. There is no doubt that if reduction of injected dose and improved statistics were the only issues involved, 3D PET would have been the accepted standard for myocardial studies. Unfortunately, also in this case, there is no free lunch. Three-dimensional PET comes with several problems, especially for studies of the thorax. First, normalization of 3D-only scanners is not trivial and there is still scope for improvement, especially when the crystal size is reduced. Second, in contrast to 2D/3D scanners where transmission scans are obtained in the 2D mode, in 3D-only scanners the transmission scan has to be acquired in the 3D mode. At present, the accepted method is to use a singles point source together with egmentation of the acquired data. Again, this is not trivial and more studies are required to validate the method. In particular, there is no experience in situations in which the range of attenuation coefficients might be larger than usual (e.g., patients with pacemakers). Third, the scatter fraction is increased significantly in 3D PET compared with 2D PET. This means that a validated method to correct for scattered events needs to be implemented. Although good methods have been developed, these methods need to be validated for each scanner design (i.e., the method used by Schäfers et al. (1) is not necessarily valid for other scanners). An important issue is the contribution of out-of-field scatter. For example, for myocardial studies this could involve Received Mar. 13, 2002; revision accepted Apr. 4, 2002. For correspondence or reprints contact: Adriaan A. Lammertsma, PhD, Clinical PET Centre, VU University Medical Centre, P.O. Box 7057, Amsterdam, 1007 MB, The Netherlands. E-mail: aa.lammertsma@vumc.nl