Is radionuclide transmission scanning obsolete for dual-modality PET/CT systems?

With the introduction of dual-modality PET/CT imaging, the nuclear medicine community is witnessing a revolution in its daily clinical practice. The hope is that this technology will alleviate the complexity of the clinical decision-making process and improve patient management [1]. Even though combined PET/CT units have been accepted commercially, the clinical benefits of and the need for these systems remain controversial [2] and are still being debated [3]. For example, PET alone provides enough information to resolve clinically relevant metabolic problems for many malignant diseases, offering a sensitivity and specificity in excess of 90%; some argue that an incremental improvement in specificity or sensitivity beyond that point probably cannot justify the cost of performing image fusion systematically for all patients on a routine basis. The marketing strategy of vendors (supported by many scientists) aiming to achieve wider diffusion of hybrid PET/CT technology in clinical practice is that the added value of combined units is well established and represents the ultimate solution for image co-registration, allowing appropriate combination of imaging technologies to yield useful fusion of functional and anatomical images [1]. It appears that more than 90% of last year’s PET sales were PET/CT; this is leading almost all scanner manufacturers to entirely replace PET-only scanners by combined PET/CT, a questionable choice according to some active researchers in the field [2]. Whereas combined PET/CT has many interesting features and offers many advantages compared with software approaches to image co-registration for patient diagnosis and image-guided radiation therapy, it is often argued that combined PET/ CT is not the ultimate solution for image co-registration and will most likely not be considered a major breakthrough that revolutionised the paradigm of medical imaging [3]. It is the role of medical physicists providing physics support to clinical PET facilities and involved in today’s biomedical imaging research enterprise to debate important issues related to design aspects of this technology and optimal data acquisition and processing protocols with the aim of improving image quality and obtaining accurate quantitative measures. Among many other issues, the important role of attenuation correction (AC) in PET has been discussed in a number of review papers [4–6] and debated in editorials [7, 8] and point/counterpoint papers [9–11].