Cardiac Positron Emission Tomography/Magnetic Resonance

Positron Emission Tomography/Magnetic Resonance Scanners and Instrumentation

To generate fused PET/MR images, several approaches exist. In the past the only practicable solution was to use software to register and fuse separately acquired PET and MR data. Although this method works relatively well for body areas with little deformability such as the head, imaging the heart poses problems with respect to coregistration as a result of patient breathing and cardiac motion, as well as patient positioning. Therefore sequential PET/MR systems provide improvement of coregistration if the patient undergoes both scans in a row on a mobile table system without repositioning. Compared with that, integrated PET/MR systems allow for a completely simultaneous data acquisition and in vivo observation of physiologic processes. Moreover, this technique minimizes the likelihood of movement-related misregistration and leads to a significant reduction of the scan time.

However, for PET/MR the technical problem of system integration is a major challenge attributed to the presence of magnetic fields. In PET/MR the real goal has been to fully integrate both systems without reducing the performance of the PET and the MR components. Uniform magnetic fields are of utmost importance in MR imaging. Thus any additional electronic circuits, which can distort the magnetic field, could potentially deteriorate the accuracy and quality of MR images. Besides the interference with the magnetic field, conventional PET photo-multiplier tubes (PMT) were not designed to be used inside strong electromagnetic fields and do not function properly in or near these fields. Consequently, the main challenge in combining PET and MR into one integrated system has been the development of MR-compatible PET detector technology. Current integrated PET/MR systems are either based on avalanche photodiodes (APD; Siemens mMR Biograph) or silicon photon multipliers (SiPM; GE Signa), where cross-interference with the MR is minimized.

Attenuation Correction in Positron Emission Tomography/Magnetic Resonance

For the attenuation correction of acquired PET data, modern PET and PET/CT systems use attenuation maps (µ-maps) that contain the radiodensity of each body volume element for 511-keV photons. These are typically calculated using transmission scans with external radionuclide sources or coregistered computed tomography (CT) data, which needs an additional transformation to convert to the radiodensity for 511-keV photons. However, for integrated PET/MR systems without a CT or external radionuclide source, new techniques for the creation of attenuation maps are needed. One approach is based on tissue segmentation using specialized computer algorithms to segment MR data into a fixed number of tissue types with a priori assigned coefficients of radiodensity. The currently most common approach uses a multipoint Dixon sequence for the segmentation into lung, fat, soft tissue, and background. However, one limitation of this method is that bones and calcifications are not assigned into a separate class but classified as soft tissue. Consequently, standardized uptake values of tissue close to bone might be significantly underestimated, which could particularly apply to the retrosternal parts of the heart. However, underestimation seems to be rather small in cardiac imaging.

To overcome this limitation, modified segmentation methods based on ultrashort echo time (UTE) sequences can be used, segmenting tissue with very short T2* (such as bone) into a separate class. However, due to a rather small field of view, this technique is not yet usable in cardiac PET/MR imaging.

Because of the fact that all objects between the patient and the PET detector can potentially attenuate and thus compromise the acquired PET data, all instrumentation used in the PET field of view during PET data acquisition has to be optimized for PET transparency. This has particular significance for radiofrequency surface coils, due to their unfavorable attenuation profiles. However, dedicated PET/MR surface coils with minimal attenuation for gamma quanta are commercially available. Alternatively, technical methods exist which allow for integration of attenuation coefficients of standard coil systems into the attenuation map after these have been previously measured in a CT scanner; however, one limitation of this approach is that the expansion of the attenuation map presumes knowledge of the exact coil position in the acquired image area.

Motion Correction

Image Postprocessing, Visualization, and Quantification

To achieve a wide acceptance of complex cardiac imaging scans in clinical practice, a semiautomated or even automated processing of cardiac imaging data is required. Meanwhile, several software products are available from commercial and academic sources. However, most of these software products focus on either CMR or PET, but a few software packages analyzing both cardiac PET and MR data are available as well (e.g., Munich Heart or syngo.via). For analysis of clinical cardiac PET/MR scans, the respective software solutions should include tools for the assessment of ventricular function, viability, fibrosis and scar, perfusion, flow quantification, and tissue composition (e.g., T1, T2, T2* mapping) as well as dedicated postprocessing of cardiac PET data, including creation of bull's eye plots and comparison of myocardial tracer uptake to normal databases.