Letter from the Editors

Article Outline

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This issue of Seminars in Nuclear Medicine is a first among its 38 years of publication. During this time, we have had numerous distinguished guest editors and several of them have served in that capacity more than once, helping to enhance the value of the journal and provide a broader point-of-view to complement that of the editors. However, this issue is the first in which a guest editor has served on two consecutive issues, each of which dealing with a different subject. Pat Zanzonico was one of the guest editors on the special supplemental section that was sponsored by the International Atomic Energy Agency (IAEA) for the March issue on radiotherapy, highlighting some of the IAEA’s sponsored research projects. In this issue, he changes roles and with Sherman Heller serves as guest editor on a review of developments in instrumentation. This is a particularly important Seminars issue since progress in this area has been extraordinarily impressive with improvements in radionuclide imaging approaching the ultimate potential resolution of positron emission tomography (PET). At the same time, hybrid imaging has become so routine that the sales of standalone PET machines has fallen off and approaches zero, while PET/computed tomography (CT) continues to grow logarithmically. The rapid extension of single-photon emission computed tomography (SPECT)/CT into the clinical arena and the improvement of image interpretation has also offered us a remarkable combination of rapid sequential anatomic and function imaging. The strength of nuclear medicine remains its basis in molecular imaging, but no one can deny that the contribution of this modality has been greatly enhanced by our ability to correlate what we see with anatomic images, so that the functional anatomic correlate achieves greater specificity and accuracy.

Dr. David Townsend, who is a pioneer in the development of PET/CT, presents us with a review article of this modality from the point-of-view of the basic scientist. It seems almost unbelievable that, as stated in his article, the first PET/CT prototype was only about 6 years ago. This innovation has rapidly revolutionized our approach to PET imaging. Dr. Townsend puts into perspective for us the historic approach to the combination of anatomic imaging and functional imaging and the role of image fusion utilizing software methodologies. He makes it clear and explains why the hybrid PET/CT machine has very significant advantages over image fusion studies done on patients on machines in different locations at different times with different protocols. It is interesting to note that, as mentioned in his article, a dual-head scintillation camera combined with x-ray was available clinically in 1999. Although PET/CT became available commercially two years later, it has gained acceptance rapidly while SPECT/CT is just now beginning to see a resurgence in utilization. The in-depth discussion of the basic principles and problems associated with PET/CT by the man who introduced it into clinical medicine is an article that should be read by anyone working in the field or with an interest in understanding the basics behind the success of PET/CT.

One of the serious problems that remains with performing PET/CT is the respiratory motion artifact that results from the difference in timing between the acquisition of a CT scan and a PET scan. Drs. Nehmeh and Erdi address this problem specifically, since its importance is enough to justify a single article in this issue of Seminars. The correction of misalignment between the PET acquisition and the CT acquisition is important both from the point-of-view of localization of the lesions that are seen and because of the problems that arise with attenuation correction that may lead the observer into an erroneous interpretation. The complexity and depth of this problem is perhaps best illustrated by the fact that Drs. Nehmeh and Erdi find it necessary to review 7 different approaches to correct for respiratory motion in PET. The number of differing methodologies is, itself, evidence that no single truly successful and practical methodology has yet emerged. However, a great deal of attention is being devoted to this issue and hopefully within a short time we will see a practical resolution of the difficulties of motion that exist in fusion imaging. No one would deny that even in the face of significant motion problems and misalignment issues, the combination of PET and CT on a single gantry has advanced nuclear medicine imaging by a major increment.

Complementing the articles on PET/CT is the article by Dr. Seo, Mari and Hasegawa on advances in SPECT/CT imaging. The pioneering investigators review how SPECT/CT has evolved from a device where the CT portion was dedicated solely to attenuation correction, to the newer instruments with high powered rapid CTs that provide high-resolution CT imaging. This advance from a simple low-resolution CT machine to the high-resolution CT devices has been paralleled by an improvement in the capability of SPECT in terms of sensitivity and resolution. One advantage that SPECT/CT has over PET/CT at this time is the large number of radiopharmaceuticals that have been developed over the years for SPECT imaging. There is a considerable amount of effort being directed at the development of new radiopharmaceuticals for PET imaging that will provide the opportunity to catch up to SPECT. It is of interest that there are a number of new SPECT imaging devices that have been developed for the specific purpose of cardiac imaging. These dedicated SPECT/CT devices offer the possibility of improving myocardial imaging by reducing acquisition time and improving resolution. Of course, the use of a dedicated cardiac SPECT/CT machine would only be appropriate for a center performing a significant number of cardiac nuclear medicine studies. Although there are some physical differences that provide an advantage for PET/CT over SPECT/CT, these appear to be in the process of being overcome. It seems unlikely that stand alone SPECT will disappear and be replaced by SPECT/CT in the way that stand alone PET appears to have disappeared and been replaced by PET/CT.

Another remarkable thing about the area of hybrid imaging is the parallel development of small animal imaging systems with clinical systems. This evolution is discussed by Drs. Rowland and Cherry. As with clinical PET and clinical SPECT, small animal imaging systems utilizing PET have advanced more rapidly in acceptance than small animal imaging systems using SPECT. Unlike their clinical counterpart, both small animal SPECT and small animal PET machines as standalone devices without combined CT have been successful and although there is a great deal of interest in the hybrid devices for small animal imaging, they remain in the minority. Also, unlike the clinical situation, preclinical SPECT can achieve spatial resolution that competes with the resolution of preclinical PET. Their review points out quite clearly the many advantages that can be achieved in small animal imaging and longitudinal studies of various animal models. Just as improvements are occurring rapidly in clinical PET/CT and SPECT/CT areas, so too are these advances being applied to small animal imaging and are likely to have a very profound impact on the way in which basic animal investigation will be performed.

Finally, a true look at the future is offered us in the paper by Drs. Pichler, Wehrl, Kolb, and Judenhofer. They present to us the concept of combining a PET machine with magnetic resonance imaging (MRI) rather than CT. This is particularly intriguing since, unlike CT, MRI provides the potential for functional as well as structural information. Prototypes of PET/MRI scanners have already been introduced and in their article one can review PET/MR images of mice and a rat acquired with a commercially produced PET/MRI scanner. The design challenges of a PET/MRI system are considerable, but it appears that with the advances in detector crystals for PET and innovative design it is possible to combine these two modalities in a single machine. These problems and their approaches to their resolution, along with the potential for application of the combined PET/MRI, are discussed by Dr. Pichler and his colleagues both for small animal and clinical imaging.

This issue of Seminars clearly presents the cutting edge of nuclear medicine instrumentation. It appears that we are at a point where the two main components of nuclear medicine—innovative design in imaging devices and the development of new radiopharmaceuticals—are proceeding at a rapid pace and changing our preconceived notions of the field and its capabilities.