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  • Look Ahead: The Future of Cardiac MRI

    Cardiovascular imaging expert James Carr, MD, discusses the progress and potential of cardiac MRI as a diagnostic modality. By James Carr, MD


    February 1, 2018

    It has now been over a decade and a half since cardiac MRI (CMR) made its advent onto the medical diagnostic scene with much promise and fanfare about finally achieving the elusive goal of becoming the comprehensive cardiac exam. Certainly early on it looked like CMR would be able to provide functional and structural information together with direct noninvasive visualization of the coronary arteries with coronary MR angiography (MRA) in a single study. As with most things in medicine, the world did not sit still as another modality, CT angiography, quickly replaced the more complex, time-consuming coronary MRA acquisition. Despite this minor setback, CMR continued to progress with leaps and bounds and has now become a routine test for evaluating cardiac disease, particularly for common indications such as ischemic heart disease and heart failure. So, where does this maturing diagnostic modality go from here? There are many advances on the horizon, which will make CMR a more clinically accepted tool, while at the same time helping us better understand the mechanisms that cause cardiac disease in a noninvasive manner.

    MRI Acceleration Strategies Will Help Simplify, Shorten Routine CMR Exam

    The current CMR exam is considered by most to be absolutely not “routine,” due to its complexity and time-consuming acquisition. The standard protocol consists of cine MRI in multiple cardiac orientations followed by delayed enhanced imaging after an injection of gadolinium contrast. Each image is acquired as a single slice during breath holding, making for a long exam, which is best suited for cooperative patients. Additionally, ECG gating is required to overcome the effects of cardiac motion, therefore ECG leads have to be attached at the beginning of the study (which prolongs the patient preparation time) and it is preferable that patients be in sinus rhythm in order to avoid significant degradation caused by motion artifact. Adding to that, CMR is not easy, requiring knowledge about cardiac anatomy and cardiac planes as well as familiarity with complex pulse sequences. Several strategies will become available over the coming years that will help simplify the current CMR protocol and speed it up significantly.

    First, compressed sensing, which has been applied in other modalities such as CT, can speed up the MRI acquisition four- to five-fold. With CMR, this may allow the entire heart to be imaged in a single breath hold, or alternatively may facilitate rapid, free-breathing real-time cine MRI of the heart with comparable image quality to the routine “slower” segmented approach (Figure 1). Second, self-gating strategies for respiratory and cardiac gating, where motion is recorded in line during the MRI acquisition, will permit so called “leadless” cardiac imaging during free breathing, thereby markedly simplifying and shortening the entire CMR protocol. Third, automated computer algorithms are currently being integrated into the user interface software, where cardiac imaging planes are set up automatically, thereby simplifying the exam for the technologist. Finally, post-processing tools will become fully automated in line and will use deep learning strategies to improve analysis over time. Before long we will attain the holy grail for the routine CMR exam, leadless rapid 3-D cine and delayed enhanced imaging with automated slice reconstruction and inline calculation of cardiac functional parameters, all in less than 15 minutes.

    MRI Reveals More About Microstructural Properties of the Heart

    The routine CMR protocol of cine and delayed enhanced imaging provides about 80 percent of what we need and want to know about cardiac disease. Once we can shorten this to less than 15 minutes, we can spend more time focusing on what is going on at the microscopic level using mapping techniques. As we know, MR images are created from differences in T1 and T2 relaxation times between different tissues providing qualitative depictions of anatomy.

    With recently developed mapping techniques, T1 and T2 can be measured quantitatively in different tissues and can be used to more accurately characterize pathology and normal tissue structures. T2 mapping to detect abnormally high T2 values, which are used as a surrogate for edema, can be used to image inflammatory conditions such as myocarditis and transplant rejection. With T1 mapping, abnormally high T1 values are seen with myocardial fibrosis and short T1 values are seen in iron deposition.

    A relatively novel parameter called the extracellular volume fraction (ECV) is calculated from T1 values pre- and post-contrast and the patient’s hematocrit. ECV values are very sensitive to myocardial fibrosis and are high in any condition that causes myocardial fibrosis, such as hypertrophic cardiomyopathy and in infiltrative disorders such as cardiac amyloidosis. Using these techniques, CMR is rapidly becoming a quantitative imaging tool, in which imaging biomarkers are routinely calculated and becoming part of the imaging report. Acceleration techniques will also shorten these acquisitions, perhaps with 3-D imaging, and automated algorithms will calculate values in line. It is only a matter of time before deep learning strategies will integrate MRI-derived imaging biomarkers with clinical parameters to predict outcomes and potential response to therapies.

    3-D Hemodynamic Flow Imaging Demonstrates the Heart and Vessels in Exquisite Detail

    Phase contrast MRI (PC-MRI) is the basic technique for measuring velocity in the heart and vascular system and flow can be calculated from that data. 2-D PC-MRI is limited in that it is only able to encode velocity in one direction. For instance, if the jet is off axis to the imaging place, the velocity will be underestimated because only a vector of the true velocity is being measured. Additionally, conventional PC-MRI is a 2-D acquisition and is therefore dependent on accurate placement of the imaging slice at the region of interest.

    With 4-D flow MRI, the velocity is encoded in the x, y and z directions during the same acquisition, permitting full representation of velocity fields in multiple directions and allowing visualization of vortical and turbulent flow. Additionally, the data is acquired spatially in three dimensions allowing larger anatomic regions to be imaged. This technique has proven to be useful clinically in regions and pathologies where the cardiovascular anatomy is complex, such as congenital heart disease (Figure 2) or thoracic aortic aneurysms. Novel imaging biomarkers, such as wall shear stress and energy loss, can be derived from 4-D flow MRI data and may help us better understand mechanisms of certain diseases such as aneurysm formation associated with bicuspid aortic valve disease. A disadvantage of 4-D flow MRI is the long acquisition time, sometimes over 12 to 15 minutes, which is highly disruptive to an already complex and lengthy CMR protocol. Acceleration strategies based on parallel imaging and compressed sensing may be particularly beneficial to 4-D flow MRI and have already been shown to reduce acquisition times in the aorta to less than two minutes. Similarly, post processing is onerous and complicated, however, newer easy-to-use tools have become more widely available and promise to make this technique more applicable and practical in the clinical setting.

    Future Expectations

    As CMR becomes simpler, quicker and easier to use, it will become more widely adapted in routine clinical practice. CMR has the advantage of identifying disease to the microstructural level, which gives it a distinct advantage over other modalities such as echocardiography. Newer hybrid systems, such as PET-MR, will facilitate the combination of structure-function imaging with MRI and molecular imaging with PET, opening the gates to true diagnostic precision medicine.




    Carr
    James Carr, MD, is director of cardiovascular imaging and vice chair for research in the Department of Radiology at Northwestern University Feinberg School of Medicine. Dr. Carr is the Knight Family Professor of Cardiac Imaging and professor of radiology, internal medicine and biomedical engineering. He is an immediate past president of the Society for Magnetic Resonance Angiography and is a member of the executive board of the Society for Cardiovascular MR. He is also on the program committees of RSNA and the International Society of Magnetic Resonance in Medicine, as well as co-chair of the RSNA Vice Chairs of Research Group.

    Fig 1
    Top row: Segmented cine MRI in a patient with atrial fibrillation has marked motion artifact due to arrhythmia. Each slice is a seven-second breath hold. Bottom row: Compressed sensing real-time cine MRI in the same patient has no motion artifact despite arrhythmia. The entire heart is covered free breathing in 24 seconds.

    Fig 2
    Patent ductus arteriosus. A. MRA of thoracic aorta shows a patent ductus arteriosus (PDA) between descending thoracic aorta and main plumonary trunk. B. 4-D flow MRI also shows PDA with flow from aorta to pulmonary circuation. The shunt can be calculated directly from 4-D flow MRI.




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