Cardiac MRI: The Physician-Scientist’s View
David Sosnovik, MD
Synopsis: Cardiovascular magnetic resonance (CMR) is playing an increasingly important role in clinical care and extremely promising techniques are being developed in the research community. However, a large gap in quality remains between scans obtained in controlled research settings and the routine imaging of patients with cardiac disease. New hardware, and acquisition and reconstruction techniques will need to be developed to narrow this gap.
Summary of main findings: The breadth of contrast mechanisms available in CMR is extremely useful in phenotyping the heart. However, CMR in routine practice is still hindered by acquisition challenges and artifacts, which will require innovative technical solutions.
Abstract: Most patients with cardiovascular disease will have a transthoracic echocardiogram (TTE) before consideration of a cardiac MRI. TTE is cheap and portable and does an excellent job of answering many clinical questions. However, those questions not addressable with TTE are frequently of high significance and inform decisions to refer patients for invasive procedures and surgery, or start/stop life-saving drugs. The findings on CMR can be subtle and, given the clinical context of many CMR exams, require accurate and artifact free techniques.
The vast majority of current CMR exams include 3 elements: 1) Bright blood cine imaging of the heart, usually with the bSSFP sequence, 2) T1/T2 mapping and/or T2W imaging to look for edema and/or fibrosis and 3) the use gadolinium to detect ischemia and myocardial injury (infarction, scar, inflammation, infiltration).¹ In many cases these sequences can be acquired without artifacts and provide excellent information. However, B0 and B1 inhomogeneity in the thorax and motion-induced artifacts remains a major problem. Additionally, many patients referred for CMR exams have frequent PVCs (premature ventricular contractions) and other complex cardiac rhythms, which can result in major artifacts. Finally, many patients referred for CMR exams now have pacemakers and implantable defibrillators, which can also produce major artifacts. There is, therefore, a major need for better approaches/solutions to deal with artifacts in clinical CMR studies.²
The challenge of artifacts in CMR often leads to a debate about the optimal field strength to use. Despite significant advantages with some techniques, such as late gadolinium enhancement (LGE) at 3T, some in the community have continued to embrace 1.5T because of fewer artifacts with the bSSFP cine sequence. At present, however, most academic sites perform CMR at 3T. Several research groups are investigating CMR at 7T and some niche clinical applications may emerge from this. Low field (~0.5T) CMR systems have recently been introduced,³ and could have significant advantages in clinical CMR. The integration of low-field MRI with angiography systems may be easier and this would be of significant appeal in interventional cardiology and electrophysiology. However, techniques will need to be developed to maintain the image quality and speed currently obtainable at 1.5-3T. Point-of-care portable MRI systems are being developed to image the brain but in the cardiovascular context there is likely to be little need for this due to the availability of TTE.
As with all of medical imaging, there are major pressures on clinical services to reduce CMR scan times. Until a few years ago most of CMR was performed with individual breath-holds for each 2D slice. Techniques have now been developed to obtain 3D cine and 3D LGE images and/or image the entire heart in one breath-hold. However, several CMR techniques, such as T1 mapping, are still routinely performed on a slice-by-slice basis. Techniques will need to be developed to address this. A wealth of new techniques are being developed to characterize the myocardium with CMR including T1-rho, MR-elastography, CEST, hyperpolarized ¹³C and diffusion-weighted CMR. However, the addition of these techniques to routine CMR exams will require the major issues discussed above, namely artifact reduction and speed, to be aggressively addressed. There is significant appeal in the development of a contrast (gadolinium) free CMR exam, but this will require major improvements in the spatial resolution and accuracy of emerging non-contrast techniques. Early studies in controlled research settings have been encouraging,⁴ but much more experience and development will be needed, particularly for the detection of subtle yet important clinical findings. While we are likely to see steady progress in this domain, gadolinium-enhanced CMR will remain the clinical standard for the foreseeable future. CMR studies on integrated PET-MR systems have been reported but the case for the routine clinical use of such systems has yet to be made. Likewise, while interventional and hybrid MR systems remain of significant theoretical appeal, the bar for replacing conventional angiography systems is extremely high.
In conclusion, CMR is now a critical component of routine cardiac care. However, significant opportunities to improve the CMR exist in artifact reduction, speed, and improved accuracy of endogenous-contrast techniques. For the foreseeable future 3T systems are likely to remain the clinical platform of choice but with appropriate advances low field systems could become extremely useful in clinical CMR.
References
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References

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