Is PET always an advantage versus planar and SPECT imaging?

The article by Abass Alavi and Sandip Basu in this issue of the European Journal of Nuclear Medicine and Molecular Imaging vividly embodies, in a scenario that seems to be easy to reach, the expectations that imaging through positron emission tomography (PET) has raised in the nuclear medicine community at large. In Alavi’s view, this scenario resolutely points to soon-to-come wide availability of a complete armamentarium of PET radiopharmaceuticals capable of substituting virtually all of the single photon agents currently employed. Therefore, the most likely horizon that the author depicts for the future of diagnostic nuclear medicine is PET imaging, with dismal chances for both planar imaging and single photon computed tomography (SPECT) to even survive in the daily clinical routine [1]. For envisaging the inescapable evolution from single photon to PET imaging, Alavi presents an exciting compendium of the many routes followed to achieve the long-pursued “dream come true” for nuclear medicine, i.e., functional imaging combined with the highest possible anatomic definition, thus directly translating molecular/ metabolic information into an immediate clinical impact for the widest possibile range of disease states. Two examples are especially poignant to illustrate Alavi’s view on what he considers to be an irreversibile trend for nuclear medicine imaging: (1) agents other than 18F-fluoro-deoxy-glucose ([18F]FDG) based on fluorine-18, and (2) peptides labeled with gallium-68. To the former group belong 18F-fluoride for exploring bone turnover in place of bone scintigraphy with 99mTc-phosphonates (possibly the diagnostic procedure most commonly performed in clinical nuclear medicine) and 18F-Fluoro-DOPA for exploring neuroendocrine tumors [2]. To Alavi’s list we would also add PET with 18F-Fluoro-DOPA [3, 4] in place of SPECT with either 123I-FP-CIT or 99mTc-Trodat-1 for investigating movement disorders (one of the fastest growing applications in conventional nuclear medicine imaging). The advantages of PET with 68Ga-labeled somatostatin analogs over SPECT with 111In-peptides for localizing neuroendocrine tumors are also obvious [5], considering the possibility of supplying nuclear medicine centers with 68Ge/68Ga generators at a relatively low cost. Nevertheless, although we certainly share many of the issues and expectations raised by Alavi in support of his view on the future prospects of radionuclide imaging, we do not totally share his optimistic view on the soon-to-come large-scale availability of several PET radiopharmaceuticals and his conclusion on the impending obsolescence of single photon planar and tomographic imaging. Our position is based on several considerations dealing with a wide range of issues including physical quality of the emissions of radionuclides, continuing clinical impact of imaging that can only be based on single-photon emission, regulation of approval for new agents, and finally local socioeconomic factors worldwide.