Time to move to PET-MR for cardiovascular imaging

Abstract

In cardiovascular imaging, both positron emission tomography (PET) and magnetic resonance imaging (MRI) have emerged separately as essential components of evaluating disease. The high sensitivity of PET and its ability to target many biological processes such as cellular metabolism make PET well suited for cardiovascular perfusion and tissue viability assessment. In comparison, the subtle soft-tissue contrast, high temporal and spatial resolution, and sensitivity to perfusion contrast agents of MRI make it well suited for many cardiovascular applications, including morphological and functional cine imaging and imaging of scar tissue. The potential for similar complementary information in many other applications has lead to the development of hybrid PET-MR systems by all major vendors in either sequential or simultaneous configurations. There is now a clear motivation for hybrid PET-MR in cardiovascular imaging.Despite the potential of hybrid systems, important technical issues must be addressed for PET-MR to become widely used in the clinic.1 Of primary concern is the required attenuation correction for PET imaging. In PET-CT imaging, the attenuation of x-rays in the CT image can be directly converted to attenuation values for the 511 keV photons in PET imaging. In PET-MR imaging, the MRI-signal is proportional to proton density and not electron density as in CT imaging, and therefore attenuation correction is a challenging proposition. Currently, an attenuation map is produced by image-based segmentation of an MRI image into four tissue classes (background air, lung, fat, and soft tissue), neglecting bone, 2 followed by assignment of attenuation values measured separately.Further issues for PET-MR include the attenuation of MRI receiver coils (both integrated body-transmit coils and spine coils embedded in the scan table, as well as flexible array-coils), the ability to estimate the attenuation map in large patients who exceed the useful field of view of the MRI system and the implementation of techniques to estimate activity in these areas (the MLAA technique referred to by Oldan et al 3 ), and the availability of time-of-flight (TOF) PET imaging in hybrid systems (although by use of the silicon-based photo-multipliers in the latest generation of systems, TOF is becoming feasible in simultaneous hybrid PET-MR systems).These issues, as well as possible differences in the detectors, detector geometries, and reconstruction algorithms used in PET-CT and PET-MR systems make it challenging to compare directly the quantitative activity and SUV values measured by different systems.In the current issue of the Journal, Oldan et al evaluated whether myocardial 18F-fluorodeoxyglucose (FDG)-PET-MR and PET-CT provide comparable images.3 The authors retrospectively analyzed myocardial FDG images of 27 patients undergoing PET-CT imaging followed by PET-MR imaging as part of a prior prospective study comparing the two modalities in oncology patients. They measured the mean standardized uptake value (SUV) for each of 17 standar...

Hybrid PET- and MR-driven attenuation correction for enhanced 18F-NaF and 18F-FDG quantification in cardiovascular PET/MR imaging

“…Simultaneous PET and Magnetic Resonance (MR) systems have recently enabled the co-registration of PET images of highly specific molecular activity with multi-parametric MR images of superior soft-tissue resolution (7)(8)(9)). Yet MR-based AC (MR-AC) may not be as accurate as CT-AC, due to MR's inherent limitations in ACFs quantification, especially for highly attenuating tissues, such as cortical bones (10)(11)(12).…”