18F-FDG PET/CT imaging has become an integral part in oncological imaging. Albeit an unspecific tracer for glucose metabolism, the increased uptake of 18F-FDG in tumor cells due to their increased glucose metabolism, known as the “warburg effect”, makes 18F-FDG a highly useful tracer for the detection of metastatic disease in many cancer types. However, the true potential of this imaging method can only be unleashed if a precise anatomic allocation of a focus exhibiting an increased glucose metabolism can be achieved. Hence, only the combination of PET and CT imaging in one single modality, PET/CT, has led to the widespread use of 18F-FDG PET imaging in clinical practice and the introduction into several guidelines, most notably in lung and head and neck cancer.
Despite its advantages, the low soft tissue contrast of CT makes the evaluation of several body regions difficult. In local tumor assessment, especially the precise prediction of local tumor infiltration can be problematic, most notably in head and neck cancer or soft tissue sarcoma. In the evaluation of distant metastases, especially small metastases can be difficult to detect on PET images in tissues with a high background uptake such as the liver or the brain. Unfortunately, even contrast enhanced CT imaging does not increase the diagnostic accuracy in this regard.
Therefore, the idea of combining PET and MRI in one single scanner was applauded by radiologists and nuclear medicine physicians alike and the introduction of integrated PET/MRI scanners into clinical practice was accompanied by a tremendous hype. The most recent publications show, however, that the differences between the two hybrid modalities are smaller than initially expected. This is most likely caused by the high sensitivity and specificity of PET for detection of distant metastases. To unleash the full potential of PET/MRI, it is therefore necessary to combine a fast whole-body protocol comprising only few selected sequences with high resolution MR imaging of selected regions to assess local tumor extent and to detect metastases in frequently affected regions such as the brain in lung cancer or the liver in colorectal cancer, to perform true “one stop shop” examinations. Another advantage of PET/MRI is the simultaneous acquisition of functional MRI and PET data, allowing true multiparametric tumor evaluation. Especially diffusion weighted imaging (DWI) as a marker of cellular density and perfusion imaging are promising techniques.
Therefore, the advantages, disadvantages and potential pitfalls in oncological 18F-FDG PET/MRI imaging concerning local tumor evaluation, lymph node and distant metastasis staging as well as restaging and therapy response assessment will be discussed and differences in comparison to 18F-FDG PET/CT will be highlighted.
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