PET/MRI of the Heart

https://doi.org/10.1053/j.semnuclmed.2014.12.004Get rights and content

Hybrid imaging devices including PET/CT and SPECT/CT have seen a great success in clinical routine, especially in the field of oncology. With the recent advent of PET/MRI scanners, expectations that PET/MRI would replicate that success were accordingly high. The combination of molecular imaging with a variety of very specific PET tracers and the high spatial resolution of MRI are expected to result in increased diagnostic accuracy or even in the creation of additional demands for hybrid imaging. However, as these systems have entered the market just recently, experience in the field of nuclear cardiology is limited and some applications still need to be validated. Owing to the profound technical differences between CT and MRI, which influences not only the estimation of the photon attenuation but also causes marked differences in the workflow, particularly in cardiovascular studies (such as the need for special personnel training and interaction between nuclear medicine specialists, radiologists, and physicists), the ā€œfamiliarization phaseā€ with this new technique also seems to be extended. However, the approach to study various conditions such as perfusion, viability, and atherosclerosis in a single imaging examination session using PET and MRI offers advantages. Although MRI allows for a detailed morphologic characterization of the studied tissue, PET adds the information on functional biological markers that are not (or at least not fully) measurable by MRI. Thus, this combined imaging approach will prove valuable in distinct cardiac diseases (such as myocarditis and cardiac sarcoidosis) and will offer vast research opportunities.

Introduction

Before the wide distribution of PET/CT systems, which was mainly owing to their great success in oncology, nuclear cardiology was restricted to SPECT systems. With the broader availability of PET/CT systems, more imaging strategies using different PET tracers such as 18F-FDG for viability or 13N-ammonia (13N-NH3) for perfusion imaging became feasible. Besides the inherent advantages of PET over SPECT such as higher spatial and temporal resolution and higher sensitivity, PET/CT systems with 64-slice (or more) CT components have allowed the combined assessment of metabolic imaging with CT-coronary angiography, which also has resulted in an increased interest in hybrid imaging strategies.1 However, a common ā€œside effectā€ of hybrid imaging systems is an increased complexity of the workflow, which may result in a higher susceptibility to errors. Owing to the disproportionately higher complexity of MRI when compared with CT, it became clear that this problem might be aggravated for integrated PET/MRI systems. On the contrary, with the advantages of MRI, such as higher soft tissue contrast resolution, the lack of ionizing radiation, and the better tolerable contrast agents, it was generally expected that the positive aspects would outweigh the constraints.

In this review, we aim to discuss cardiac-specific questions. We have given a brief overview on the technical aspects that are relevant for cardiac PET/MRI (eg, implantable devices and workflow). Furthermore, we discuss ā€œtraditionalā€ cardiovascular applications that we think may profit most from hybrid PET/MRI acquisition (such as myocardial perfusion imaging [MPI] and myocardial tissue characterization) and we refer to current trends and literature on this. Last, we have reviewed certain cardiac diseases and future applications in which PET/MRI may prove a particular benefit.

Section snippets

Technical Aspects

Building an integrated PET/MRI system was hardly considered feasible for a long time, as the high static magnetic field, the rapidly shifting gradient fields, and radiofrequency signals from the MR component disturb the proper operation of conventional photomultiplier tubes and the associated electronics of PET detectors. On the contrary, PET detectors may cause inhomogeneities in magnetic fields and electromagnetic interferences, resulting in a degradation of the MR image quality. To overcome

MPI, T1 Mapping, and Extracellular Volume Quantification

With respect to absolute quantification, one of the major limitations of MR imaging is the lack of clinically relevant information in the measured signal intensities rather than the detection of morphologic contours. MR signal intensities are a function of imaging hardware, pulse sequences, and many other factors, which can produce widely variable results between vendors, field strength, and scanner models. Although in morphologic functional imaging (such as the delineation of dimensions,

Other Potential Applications in Cardiac Molecular Imaging

In the following section, we give an overview of our first experience in distinct disease entities and potential applications for simultaneous cardiac PET/MRI.

Conclusion

PET and MRI are used to represent competing imaging modalities and integration of the two seemed practically impossible for a long time. It is not surprising that the recent advent of PET/MRI made the impression of ā€œa marriage of contrasts.ā€ Studies with the focus on cardiac imaging evaluating this novel technique are still scarce. The main advantages are reduction of ionizing radiation to the patient, increased patient comfort, possibly a higher patient throughput compared with sequential

Acknowledgments

The preparation of this manuscript would not have been possible without the efforts of many of our staff members. We would like to especially thank Sebastian FĆ¼rst for valuable input and Sylvia Schachoff and Brigitte Dzewas for their technical assistance during PET/MRI acquisitions.

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