Cardiovascular MRI in the Year 2020 (and Beyond): Changing How We Risk-Stratify Healthy Patients

Vincent L. Sorrell, MD; Gurpreet Baweja, MD

Disclosures

Cardiovasc Rev Rep. 2004;25(5) 

Imagine this. It's your 40th birthday. You're proud that you lead a healthy, active lifestyle. Your doctor states that your blood pressure, blood sugar, and cholesterol levels are normal for "someone without heart disease." You remember your father having a heart attack when he was young. You request a stress test, but your doctor informs you that even an abnormal result would be considered a false-positive finding. You inquire about a computed tomography scan. Your doctor quickly points out that even severe coronary artery obstructions may develop in the absence of significant calcium buildup. You are told to notify the physician if any heart symptoms should occur, and you leave remembering your father, who never had any symptoms before his heart attack.

Although entirely fictitious, this scenario is extremely common throughout the developed world, where we base our level of risk factor modification aggressiveness on a combination of findings: history, physical examination, blood results, and the accuracy of diagnostic tests. A perfectly safe, comprehensive, highly accurate, noninvasive diagnostic test would alter how we assess for coronary artery disease.

Now imagine this. On your 40th birthday, knowing your family history, your doctor recommends cardiac magnetic resonance imaging (MRI). You arrive for testing and are told this test is as safe as ultrasound and lasts just about as long. While resting comfortably, you receive headphones, goggles, and a choice of movies to watch. You select The Year in Review, 2020. A number of electrodes are placed on your chest and a light jacket on top of that. You are then placed within a large "magnetic donut," at which point the movie begins and you remain for the next 60 minutes. While lying comfortably and breathing normally, you are unaware that detailed images and critical physiologic information is being obtained on the state of your cardiovascular system. Suddenly, the table automatically moves, and you wonder why it had to end before you completed your movie. As you leave, you are handed a report for your personal records that includes astounding details: left ventricle (LV) and right ventricle (RV) volumes and function; cardiac valve physiology; comments on the pericardium; coronary artery anatomy with images of the lumen as well as the walls; myocardial thickness, perfusion, and viability. From the information obtained, you have been categorized as "class II—mild increased risk for cardiac events." Based on this information, your doctor prescribes the "heart pill," (which you remember learning about from The Year in Review, 2020 movie). This pill includes an aspirin, a statin, an angiotensin-converting enzyme inhibitor, and the recently discovered, coronary artery-specific, anti-inflammatory agent "Deplaquerisk." You thank your doctor and leave while thinking back to your father.

Given the current clinical and research capabilities of MRI, it is our belief that this fictitious scenario will become a reality in the not-so-distant future. In this review, we provide an overview of the current state-of-the art cardiac MRI capabilities and delve into the future directions in the development of clinical and research applications that will form the basis for noninvasive cardiac imaging for the remainder of this century.

The ground-breaking invention of MRI has saved or transformed the lives of millions of people over the last 20 years, but only recently has it become possible to capture a beating heart. With sophisticated computer technology, high-strength magnets, rapid acquisition software, and novel designer contrast agents, we can now routinely obtain detailed three-dimensional images of the heart that are unlike any other imaging modality. Cardiac magnetic resonance (CMR) has a distinct advantage over echocardiography and nuclear scintigraphy in terms of fewer artifacts and unlimited acquisition windows. It is noninvasive, lacks ionizing radiation, and uses benign paramagnetic contrast agents without nephrotoxicity (unlike computed tomography). It has positioned itself to become a "one-stop diagnostic shop," offering extensive results during a single examination currently only available through a combination of multiple, separate, diagnostic procedures.

Computer software sophistication has developed to allow extremely rapid cardiac acquisitions. Even real-time imaging (acquisition of a single image within milliseconds) is now possible.[1,2] Images can be scouted faster, resulting in less total time necessary for comprehensive cardiac examinations and provides a more robust environment for dobutamine stress MRI. Advancements in respiratory compensation techniques lessen the current need for patient cooperation and breath holding. This will further expand the utilization of CMR to include patients currently unable to cooperate because of age or underlying cardiopulmonary illness. Also, by eliminating breath holding, current time restraints can be removed and will allow even higher spatial resolution. Soon, the need for peripheral contrast administration will be removed entirely. Normal and pathologic tissue characteristics, as well as blood flow assessments, will be determined by their natural tissue variations in water content. This eliminates the need for IV administration and creates even faster examinations.

Due to extraordinary myocardial wall definition, dobutamine stress MRI has proved to be highly accurate in detection of ischemia-induced minor reductions in wall thickening. Patients scheduled for echo stress testing with poor ultrasound windows are excellent candidates for dobutamine stress MRI examinations.

The spatial resolution of CMR (2-3 mm) is superior to positron emission tomography and single photon emission CT (4-8 mm), providing differentiating capabilities of partial myocardial thickness perfusion abnormalities. This has shown great promise in "syndrome X" patients where, for the first time, a diagnostic test could demonstrate an abnormal perfusion map (limited to the subendocardium), despite "normal" epicardial coronary arteries.[3]Currently, myocardial stress perfusion is performed by measuring first-pass kinetics of gadolinium-based contrast agents.[4] It is extremely likely that image acquisition will evolve to allow the measurement of myocardial perfusion without the need for additional contrast—taking advantage of our "natural contrast." Furthermore, knowing that coronary disease alters the autonomic regulatory tone of coronary vessels, one could theoretically detect this in the resting state—without the need for any additional stress agents.

Myocardial viability is determined by CMR using dobutamine stress MRI (identical to low-dose dobutamine echo methods), metabolic imaging (magnetic resonance [MR] spectroscopy) or most commonly, quantifying delayed enhancement after contrast injection. Similarly, the prerequisite for injected contrast will likely surrender to natural contrast methods as MR progresses. This application of CMR may eventually become the diagnostic test of choice to detect heart transplant rejection,[5] predict the likelihood of malignant cardiac dysrhythmias and potential benefit of implantable cardioverter-defibrillator implantation, and risk-stratify patients with hypertrophic and other cardiomyopathies.[6,7,8,9,10]

Coronary MR angiography has already proven its value in the detection of anomalous coronary origins, the evaluation of coronary artery bypass graft obstruction, and the assessment of the most critical, proximal, and mid portions of native coronary arteries.[11,12,13] Details of the arterial plaque, thick (stable) vs. thin (vulnerable) fibrous caps, have been imaged in the larger aortic and carotid arteries. Although coronary artery plaques are much smaller, more tortuous, and highly mobile, these have also been imaged with success.[14] As motion correction techniques advance, and designer contrast agents targeted to vulnerable plaques are developed, CMR will be able to accurately measure the plaque volume and composition, making this the tool of choice to serially follow the progression/regression of atherosclerosis ("noninvasive cellular imaging"). In time, these tools will combine to provide a robust and uncomplicated coronary angiographic technique, delivering information on coronary artery anatomy (obstruction and plaque vulnerability), as well as myocardial blood flow and microvascular integrity in a single examination.

Due to excellent endocardial border delineation and no need for geometric assumptions of LV or RV shape, CMR is now regarded as the gold standard for the reproducible assessment of ventricular mass, volumes, and global ejection fraction.[15] To detect a 10-gram change in LV mass (power 90%, error 0.05), a conventional echo study would need to enroll 505 patients, but the same CMR study would only need to recruit 14 patients.[16] Similar, but less striking results are found with LV and RV functional assessment, creating a highly attractive research tool.[17] Regional cardiac function can be assessed by myocardial deformation measured with cine MR tagging and myocardial strain and strain rate using harmonic phase analysis and velocity-encoded techniques, respectively.[18]

Abnormal flow in the heart can be detected as a signal void on cine MRI (visually similar to color-flow Doppler). Unlike single-plane spectral Doppler techniques, multidirectional blood flow velocity can be calculated in any given plane with velocity-encoded MRI (phase velocity mapping). By combining the most accurate LV and RV volume assessment with this robust tool for arterial flow mapping, CMR should be considered the gold standard for the assessment of complex flow physiology as seen in complex congenital heart diseases and acquired multi-valve lesions.

MR angiography is currently being employed to create detailed roadmaps of the peripheral circulation to facilitate revascularization procedures.[19] The use of CMR-guided coronary interventional procedures eliminates the radiation exposure and contrast risks associated with conventional x-ray-guided angiographic techniques. As CMR-safe catheters are developed, the operator will combine complete three-dimensional data sets with coronary physiology and myocardial viability to immediately determine the success of the procedure. It has now become feasible to guide gene and stem cell transfer to the myocardium under MRI guidance.[20]

The development of new tissue or lesion-specific contrast agents will make molecular imaging with MRI a reality and allow us to diagnose various pathologies noninvasively.[21] Thrombus-specific agents based on small gadolinium-labeled peptides will soon become available, which will make CMR the gold standard for diagnosing thrombi.[22]

The use of phosphate (31P), carbon (13C), and proton (1H) MR spectroscopy for the assessment of myocardial metabolism in vivo is a powerful new development. A detailed understanding of energy metabolism in normal and abnormal myocardium will help improve preventive, diagnostic, and therapeutic modalities for heart failure, ischemic heart disease, genetic cardiomyopathy, and even valvular heart disease.[23]

Since cardiac MRI is rapidly becoming mainstream technology in the everyday clinical practice, the need for non-ferromagnetic devices is vital. New pacer leads with minor magnetic field interactions and relative lack of heating are on the horizon. As clinical research begins to utilize more CMR techniques to assess results of treatments and timing of interventions, patient management decisions will be based on the findings from CMR, rather than our current conventional noninvasive techniques.

Furthermore, as the amount of imaging data to review increases, it is imperative that a rapid, automated processing technique become uniform. Comparing this development to the early advances of nuclear scintigraphy is a relevant reminder of the common delay in processing capabilities compared to acquisition capabilities. It has only recently become possible to rapidly process the volume of imaging data routinely obtained with today's nuclear scanners. The "one-stop" cardiac patient examination demands an integrated, robust "one-stop" post-processing approach. Only then will CMR approach the clinical impact that nuclear and echo imaging enjoy.

Over the next 25 years, noninvasive cardiac imaging will experience an extraordinary transition! Patients will be imaged with a far greater spatial resolution. Coronary arteries in jeopardy will be identified. Currently utilized imaging modalities that have small but potential risks will be reduced, as will needless diagnostic interventional procedures and unnecessary surgeries. Our current clinical paradigms will be challenged by these data, and the evolution from disease management to disease prevention will be the physician's mantra.

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