Cardiac stress-perfusion MRI: ready for primetime?

Can you practice cardiology in 2021 without access to cardiac magnetic resonance imaging (CMR)? A silly question, perhaps, but there are authorities who would answer in the affirmative¹. Comparison with echocardiography (ECHO, Table 1) is most natural, because the imaging planes are the same and at least some of the images look very similar. Is CMR just an expensive „ECHO plus”?

A major attraction of CMR is its excellent reproducibility, better than that of ECHO, which has been extensively proven and which can massively reduce calculated sample sizes necessary to demonstrate clinically meaningful changes in LV dimensions and ejection fraction by CMR vs. ECHO². Due to its excellent signal-to-noise ratio, CMR has rapidly become the gold standard for cardiac chamber and ejection fraction measurement³, and CMR is also clearly superior to ECHO for the assessment of RV structure and function^4,5. A truly unique feature of CMR is its ability to perform in-vivo myocardial tissue characterisation and provide, effectively, a non-invasive myocardial biopsy, and thus allow assessment of viability, replacement fibrosis, iron overload, myocardial oedema/inflammation or tumours⁶. Flow imaging allows valve assessment in a manner similar to, but more reproducible than, ECHO⁷. Parametric techniques such as T1 mapping⁸ hold promise for imaging of interstitial fibrosis and for the detection of an expanded extracellular myocardial compartment, while tensor vector imaging⁹ visualises the microstructure of the myocardium, with potentially fundamental clinical implications, yet to be fulfilled.

CMR has an excellent track record for the detection of inducible myocardial ischaemia. Most centres use a 4 to 6-minute adenosine intravenous infusion and image the myocardial distribution of gadolinium at maximum coronary artery vasodilation¹⁰. Dobutamine stress CMR, where inducible ischaemia is inferred from transient wall motion abnormalities at peak stress, is also available, but, being technically more demanding, is not widely used¹¹. Also the safety profile of vasodilator stress is superior to that of dobutamine, particularly considering the logistics of the patient being stressed inside the MRI scanner.

There is an unresolved „tension” between advocates of perfusion vs. anatomical imaging for the assessment of coronary artery disease. Over several decades, in the USA, SPECT has been the leading method for ischaemia detection, due to its robustness, wide availability¹². However, there is increasing concern about radiation exposure associated with nuclear cardiac techniques¹³, relatively low spatial resolution, as well as a shift towards anatomical techniques, as evidenced by the recommendation from NICE to use CTCA as the first test in patients with chest pain and low to moderate pre-test probability of CAD¹⁴.

In multiple direct comparisons and meta-analyses, stress perfusion CMR consistently comes top of the list for sensitivity and specificity in the detection of myocardial ischaemia^15,16. It is radiation-free, non-invasive, repeatable, offers extensive anatomical and functional assessment of the heart beyond perfusion assessment, and has better spatial resolution than SPECT¹⁷. Why then is it the least-adopted¹⁸ ischaemia test?

Undoubtedly, cost, limited availability and relative scarcity of training opportunities have a major part to play, although limited data suggest cost-effectiveness¹⁹. Whilst the significance of inducible myocardial ischaemia itself is now being questioned²⁰, there is increased recognition that ischemia imaging will continue to play a major role in cardiovascular medicine²¹.

In this issue of the „Review” Onciul et al. present their pioneering experience with CMR and stress-perfusion CMR in a large academic centre in Romania²². They are to be commended for taking the time and trouble to document their practice in comprehensive detail, in a context where CMR is still in its infancy. A simple Google search²³ reveals that out of 10 MRI imaging centres in Bucharest only 2 offer CMR. This ratio is not specific to Bucharest – in Wales, out of 15 major hospitals with cardiology departments, only 5 offer CMR²⁴ – CMR is still a minority pursuit.

In the era of personalised, quantitative medicine, with its emphasis on genetic markers of disease and on big data, as illustrated by the UK Biobank project for instance²⁵, CMR is an essential piece of the complex puzzle of progress and discovery in cardiology²⁶, and the data presented by Onciul et al. represent an important contribution to the wider adoption of a still underused imaging modality.