PET/MRI: Paving the Way for the Next Generation of Clinical Multimodality Imaging Applications

Abstract

Multimodality imaging and, more specifically, the combination of PET and CT has matured into an important diagnostic tool. During the same period, concepts for PET scanners integrated into an MR tomograph have emerged. The excellent soft-tissue contrast of MRI and the multifunctional imaging options it offers, such as spectroscopy, functional MRI, and arterial spin labeling, complement the molecular information of PET. The development of a fully integrated PET/MRI system is technologically challenging. It requires not only significant modifications of the PET detector to make it compact and insensitive to magnetic fields but also a major redesign of the MRI hardware.Key Words: clinical multimodality imaging; PET; CT; PET/CT; arterial spin labeling (ASL); fMRI; spectroscopy; magnetic resonance tomograph (MRT); PET/MRI; positron emission tomography; MR/PET J Nucl Med 2010; 51: 333-336 DOI: 10.2967/jnumed.109.061853 Over the last 20 years, PET and MRI systems have evolved slowly but steadily. The photomultiplier-based PET detectors have remained more or less unchanged, ensuring stable operation and good signal performance despite bulk and high sensitivity to even the smallest magnetic fields. Improvements in PET technology were achieved with faster and low-noise electronics, faster and brighter scintillation crystals, optimized light-sharing schemes for the scintillation crystal arrangements, and smaller crystals (1). These adaptations led to PET scanners with whole-body scan times as short as 10 min, yielding-together with improved reconstructions, attenuation-and scatter-correction algorithms-low-noise PET images. Advances paved the way to implementing the idea of time-of-flight PET in clinical scanners (2). Without a doubt, the most important step toward the establishment of PET as a clinically viable tool was the introduction of combined PET/ CT in 1998 by David Townsend and Ronald Nutt (3-5). Nevertheless, many physicians of that time remained skeptical about the advantages of this dual-modality imaging system over stand-alone PET and CT.Clinical MRI evolved toward higher fields, faster imaging sequences, and whole-body imaging capabilities. Especially for brain imaging, 3-T MRI is now the standard. Novel coil concepts combined with parallel acquisition techniques helped shorten examination times while maintaining high imaging quality (6). PET/MRI: TECHNICAL EVOLUTIONThe idea to combine PET and MRI arose as early as the mid 1990s, even before PET/CT was introduced. Simon Cherry and Paul Marsden saw the need for PET/MRI in small-animal imaging studies to add anatomic landmarks with high softtissue contrast to the molecular information delivered by PET (7). Preclinical PET/MRI work was followed by immediate commercial interest in combining PET and MRI, probably driven by the limited sensitivity of MRI to trace biomarkers or to reveal metabolites.The PET/MRI combination requires 3 risky technologic steps that modify state-of-the-art PET and MRI. First, the photomultiplier technology must be replaced with magn...

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