The hybrid modality of PET/MRI combines real-time soft-tissue visualization capabilities and the good functional detail of MRI with the metabolic visualization capabilities of PET. At the recent European Congress of Radiology (ECR 2015) in Austria, a special focus session on new frontiers in brain imaging included a presentation on the value of PET/MRI for brain tumour diagnosis.

Francesco Fraioli, a senior clinical lecturer at the Institute of Nuclear Medicine at University College London Hospital, has been conducting clinical research using PET/MRI since 2011, using the first commercial system in the UK available for whole-body simultaneous imaging. He believes that this represents cutting-edge advanced imaging for brain tumours, because new PET tracers are designed to target specific metabolic processes, and MRI enables microstructural appraisal, functional assessment and tissue characterization. For selected brain tumours, this combination can help improve selection of biopsies and potentially separate tumour tissue better from scarring and necrosis.

Three scenarios

"Are we ready for multiparametric imaging?" asked Fraioli. "It is important to know before purchasing a PET/MRI system that this modality requires a multidisciplinary approach by both radiologists and nuclear medicine specialists, each with different skills." He explained that there are three models of PET/MRI scanning protocols.

The first is PET with anatomical MRI to localize the PET signal. The structural MRI is performed for attenuation correction and image localization of the PET signal, and the PET is used for characterization. The integration of two modalities allows radiologists to identify and localize lesions, such as obtaining a differential diagnosis of lymphoma and glioblastoma multiforme. Multiple tracers may be used.

A second clinical scenario is PET with diagnostic MRI – a "one-stop shop" exam that is the most commonly used and preferred today, because it provides higher accuracy for diagnosis and follow-up. It also avoids the use of two scanners and potentially two sedations, which is particularly important in frail patients, such as the paediatric population.

Fraioli said that a study to describe 18F-FDG uptake across a spectrum of paediatric brain tumours in 203 children showed that PET with diagnostic MRI could help diagnosis and follow-up of different tumour types. The study, published in 2014, retrospectively analysed children with a spectrum of newly diagnosed or recurrent/refractory brain tumours who were enrolled in phase I/II clinical trials through the Pediatric Brain Tumor Consortium from August 2000 to June 2010. The study determined that the 18F-FDG uptake pattern and MRI contrast enhancement varied by tumour type, aiding diagnosis and follow up (J. Nucl. Med 55 1473).

A further interesting application of PET/MRI is the evaluation of meningiomas either for baseline diagnosis or treatment planning. 68Ga-DOTATATE is a tracer that is selective for the somatostatin receptors that are rich in tumours such as neuroendocrine, meningioma and paraganglioma.

Radiation therapy of meningiomas may experience a new era with integrated PET/MR imaging. "'In a preliminary evaluation that my colleagues and I conducted, we found that PET/MRI allowed us to change treatment planning in up to 73% of cases by providing more diagnostic information and better definition of safe margins. This also helped us calibrate residual tumour volume and spare important vital organs," Fraioli said.

Advanced approach

The third type of PET/MRI protocol – multiparametric MRI coupled with dynamic PET acquisition for advanced tissue characterization – is currently less of a clinical and more of a research application, Fraioli explained. It represents the real added value of a PET/MRI scanner, because when a patient is injected with tracer, the dynamic data of the PET signal are acquired during simultaneous acquisition of functional sequences. With dynamic PET, the gradient of the highlighted area of the tracer acquisition curve can be mapped back to each pixel to indicate the rate of accumulation/production of metabolism. This provides information regarding the distribution in the viable tissue and analysis of wash-in and wash-out curves within the lesion.

"With the multiparametric imaging of PET/MRI, it is possible to image the relationship between hypoxia and perfusion," said Fraioli. "Imaging hypoxia is particularly difficult, as hypoxia and the vascular network change rapidly over time. In fact, it is not surprising that even during the same respiration, there can be differences in oxygenation within the tumour. With PET/MRI multiparametric imaging, we can acquire at the same time, structural, metabolic and functional information, gaining a clear understanding of biological effects." Fraioli also emphasized the importance of post-processing reconstruction analysis in providing information about perfusion, kinetic parameters of perfusion and texture."

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