Cardiac PET/MR: In pursuit of the killer app

 
 
 
 - myocardial perfusion on PET-MR
Simultaneous acquisition of myocardial perfusion using a fully integrated PET/MR system. 13NH3 PET perfusion (top), gadolinium-DTPA MR perfusion (bottom), and coregistration of both modalities (middle) at mid-ventricular slice.
Source: J. of Nuclear Med. (jnumed.112.105353)
 

Hybrid PET/MR has demonstrated its potential in cardiac imaging thanks to its superior tissue contrast and ability to deliver multimodal quantitative imaging parameters; however, technical and financial hurdles remain, according to a review article published in the March issue of the Journal of Nuclear Medicine .

Providing a state-of-the-technology review for PET/MR, Christoph Rischpler, MD, of the Technical University of Munich, and colleagues focused on cardiac imaging applications.

“Both MR and PET have independently gained wide acceptance for the assessment of perfusion and tissue viability in patients with CAD,” wrote the authors. “However, there are no obvious clinical indications or killer applications yet defined for the use of the integrated PET/MR instrumentation.”

Advantages of MR over CT as an adjunct to PET include the fact that MR provides higher soft-tissue contrast without exposing the patient to radiation, explained Rischpler and colleagues. MR imaging represents the gold standard for the assessment of left ventricular function. MR also offers features such as spectroscopy and diffusion spectrum MRI tractography.

“Continuous motion correction provided by MR imaging may improve substantially the quantification of regional PET tracer uptake,” wrote the authors. “Access to coregistered, almost simultaneous physiologic and biologic measurements have already made PET/MR the most sophisticated quantitative imaging modality in cardiology, although these applications are in their infancy.”

On the downside, MR increases workflow complexity compared with CT, according to the authors. CT is fast and easy to perform, while MR protocols are time-consuming and require more patient compliance. The electromagnetic fields needed for MR also restrict its use on patients with pacemakers, implantable cardioverter defibrillators and mechanical heart valves, which is a major disadvantage in cardiac imaging.

The authors noted that it remains to be proven whether the higher acquisition costs for PET/MR will pay off in the long-term. “PET/MR is entering routine clinical practice at present, but it is still not clear whether PET/MR will be able to offer added value or generate an additional demand for imaging studies that PET/CT cannot satisfy.”

Rischpler and colleagues added that PET/MR is an attractive research platform that can provide comprehensive phenotyping of cardiovascular diseases. They expect the combination of molecular imaging and excellent functional characterization of cardiac performance will aid in the staging of heart failure and the development of predictive parameters for tissue recovery and therapy response. “The results of such sophisticated research applications will define new clinical indications, which need to be validated as superior and cost-effective replacements for existing imaging approaches in cardiology.”