Cardiac Magnetic Resonance Imaging: An Evolving Tool in Clinical Practice

Terry Duggan-Jahns, RT(R)(CT)(MR)(M)

    ^*Manager, Outpatient Diagnostic Imaging, St. Joseph Medical Center, Tacoma, Washington.
   Address correspondence to: Terry Duggan-Jahns, RT(R)(CT)(MR)(M), Manager, Outpatient Diagnostic Imaging, St. Joseph Medical Center, 1717 South J Street, Tacoma, WA 98405. E-mail: tdugganjahns@mac.com.

Disclosure Statement: Ms Duggan-Jahns reports having no significant financial or advisory relationships with corporate organizations related to this activity.

ABSTRACT

Cardiac magnetic resonance imaging (MRI) is a rapidly evolving modality that offers new excitement and challenges to the radiology profession. Although echocardiography is still the most widely used imaging modality in cardiovascular medicine, cardiac MRI is a noninvasive, well-established, powerful, and complementary technique to echocardiography. It is now considered the gold standard for the assessment of regional and global systolic function, myocardial infarction and viability, and congenital heart disease. This modality is particularly useful for evaluating cardiac wall motion; the size and thickness of cardiac chambers; ejection fraction; and cardiac function, perfusion, and myocardial viability. In addition, cardiac MRI provides useful information to help the cardiologist develop a treatment plan for cardiovascular problems and to monitor a patient's progress or recovery following treatment. This article will review MRI techniques used to evaluate cardiac morphology and function, perfusion, viability, and flow velocity quantification, with an emphasis on the specialized training and commitment required by radiology staff responsible for overseeing this advanced diagnostic modality.

Introduction
Heart disease continues to be a major source of morbidity and mortality in the United States. It has been estimated that approximately 770 000 will suffer from an ischemic coronary attack in 2008, and 430 000 will suffer from a recurrent attack.¹ In addition, heart failure continues to affect many Americans. Although mortality rates due to heart failure have improved substantially in recent decades, the rate of hospital discharges attributed to heart failure increased from 400 000 in 1979 to 1 084 000 in 2005.¹ Both of these manifestations of cardiovascular disease, as well as a range of other cardiac conditions, often require advanced imaging modalities to provide effective diagnosis and patient management. As a result, cardiac magnetic resonance imaging (MRI) and other imaging techniques have been introduced to supplement the echocardiography that has been traditionally performed in patients suspected to be suffering from cardiac dysfunction.

Cardiac MRI is an advanced modality that continues to offer challenges to radiology professionals. Cardiac wall motion, the morphology of cardiac chambers, ejection fraction, cardiac perfusion, and myocardial viability are all effectively imaged with the use of cardiac MRI. However, due to the rapid evolution and complexities of cardiac MRI, this specialized modality requires a team of highly trained and dedicated cardiologists, radiologists, and MRI technologists. This review will provide an introduction to cardiac MRI and will address specific techniques, safety considerations, and indications suitable for the use of cardiac MRI.

The Cardiac MRI Technology
In general, MRI is a diagnostic imaging technique that uses a strong magnetic field and radio waves to create images.² Although the MRI modality had been initially difficult to translate to diagnostic applications in cardiology, due to the fact that the heart is constantly beating and artifacts related to this movement would be difficult to control for, new technologies and MRI techniques are emerging that reduce or eliminate the concerns posed with imaging a moving organ.

Because there is no ionizing radiation (X rays) involved in producing an MRI image, the modality does not pose a danger to the patient in terms of radiation exposure, unlike other radiology studies. However, standardized safety requirements require that patients undergoing any MRI procedure must be carefully screened for contraindications that would prevent them from having these procedures.

MRI Safety: Operational Issues and Contrast Agent Administration Precautions
The American College of Radiology (ACR) has developed a guidance document that addresses important MRI safety concerns. For instance, the ACR emphasizes the importance of creating 4 separate zones of increasingly limited access to prevent the entry of hazardous objects, including any metal object that is subject to attraction by a magnetic field, from entering the MRI operational suite. These precautions were spelled out as a result of reports of metallic objects being brought into the MRI suite and becoming deadly projectile objects when entering the magnetic field.³ The ACR guidance document also warns that patients should be properly evaluated to ensure that they have not received any implanted objects that could likewise be subject to the magnetic field, which could disrupt the study or cause considerable patient harm due to burns or other complications.³

The administration of an intravenous (IV) contrast agent is another important safety consideration in MRI studies. Individuals can potentially suffer from adverse reactions to gadolinium-based contrast agents (GBCA), and those who have experienced a reaction to another agent may also react to GBCA. The ACR recommends that those who have suffered from previous reactions can be treated with corticosteroids, and in some cases antihistamines, prior to the administration of the GBCA. Likewise, although the use of MRI is generally accepted as safe during pregnancy, the ACR recommends that studies with contrast agents should be avoided, if possible, during pregnancy.³

Although rare, cases of nephrogenic systemic fibrosis have been reported in individuals with previous kidney disease who had undergone MRI studies with gadodiamide, a GBCA. As a result, the US Food and Drug Administration (FDA) recommends that clinicians should refrain from conducting imaging studies requiring the administration of GBCA in patients with advanced kidney disease (glomerular filtration rate [GFR] <60 mL/min/m²).⁴ The ACR also recommends that patients with a history of renal disease (including solitary kidney, renal transplant, or renal tumor); age greater than 60 years; a history of hypertension; a history of diabetes; or a history of severe hepatic disease, liver transplant, or pending liver transplant, should receive an updated GFR assessment within 6 weeks of undergoing an imaging study requiring the administration of GBCA.⁵

Although these precautions should be strictly followed in administering GBCA, the contrast agents used in MRI studies, including cardiac MRI, are generally safe and do not pose excessive hazards to most patients.

Indications for Cardiac MRI
Cardiac MRI is a useful tool in a range of cardiac disorders that require an accurate view of the morphology and function of the heart. Appropriate indications for cardiac MRI include:

• Congenital heart disease evaluation for arrhythmogenic right ventricular dysplasia, atrial septal defect, or ventricular septal defect;
• Hypertrophic cardiomyopathy;
• Valvular heart disease;
• Pericardial disease and cardiomyopathy;
• Cardiac masses, including myxomas, lipomas, and fibromas;
• Cardiac primary tumors, including angiosarcomas and rhabdomyosarcomas;
• Thoracic aorta and great vessel evaluation (to detect coarctation of the aorta or to evaluate a dissecting aortic aneurysm);
• Ischemic heart disease; and
• Cardiac thrombus.

The American Medical Association current procedural terminology (CPT) codes that are commonly applied to cardiac MRI are listed in the Table.⁶ However, clinicians and MRI technologists should note that the Centers for Medicare and Medicaid Services, which oversees the Medicare and Medicaid programs, will currently only reimburse for cardiac MRI studies that fall under 2 CPT codes. These include cardiac MRI for morphology and function without contrast materials (CPT code 75557) and cardiac MRI for morphology and function without contrast materials, followed by contrast materials and further sequences (CPT code 75561). Meanwhile, studies that include flow/velocity quantification or stress imaging are not currently covered under Medicare or Medicaid. When considering patients who could potentially benefit from cardiac MRI due to the possible or established presence of 1 of the cardiac conditions listed above, it may be necessary to restrict the use of certain studies if the patient is a Medicare and/or Medicaid beneficiary and reimbursement is a concern.

Cardiac MRI Techniques
Imaging Planes in Cardiac MRI
Routine cardiac MRI studies are performed in a variety of imaging planes, depending on the desired diagnostic application. Transverse or axial planes show the relationship of the 4 cardiac chambers. Sagittal images show the connection between the ventricles and the great vessels. Coronal images are most useful for evaluation of the left ventricle outflow tract, left atrium, and the pulmonary veins. The optimal planes also depend on the global positioning of the heart in the thorax. The heart is more vertical in younger patients and more diaphragmatic in elderly patients.⁷ This changing morphology makes cardiac MRI more challenging than imaging other areas of the body.

A variety of views can be performed with cardiac MRI to obtain the proper anatomical views according to each patient's body habitats or heart position. These views include 2-chamber long axis views, 3-chamber views, 4-chamber views, and short-axis (SA) views. Cardiac MRI can also be used to view the right ventricular outflow tract or left ventricle outflow tract to detect obstructions or other abnormalities. Depending on the patient's condition, techniques such as black blood imaging, bright blood cine, phase contrast, or viability and perfusion techniques can be used to maximize the image contrast and diagnostic value.⁸

Imaging Techniques
Magnetic resonance imaging is the optimal imaging method for evaluating pericardial masses because it allows evaluation of mediastinal, pericardial, and myocardial involvement in a single study. A variety of MRI imaging techniques and imaging planes are necessary in cardiac imaging to obtain proper tissue differentiation. These techniques help determine solid masses versus cystic masses; masses with hypervascular versus hypovascular involvement; viable tissue versus necrotic tissue; edematous versus normal physiology; calcification versus non-calcified tissue; fibrotic versus non-fibrotic tissue; and fatty infiltration versus normal soft tissue.⁹ The specific techniques used in cardiac MRI applications are described below.⁹

Black Blood Techniques
Black blood techniques are used to produce static images for evaluations of cardiac anatomy and myocardial viability. The sequences used in these studies are usually spin-echo or turbo/fast spin-echo. These studies also frequently use double or triple inversion-recovery sequences for delayed imaging following the administration of IV GBCA. The signal from moving blood is very low and appears as black on the images on these sequences (Figures 1 and 2).

Bright Blood Imaging Techniques
Bright blood imaging techniques uses gradient-echo and steady-state free procession (SSFP) sequences. These sequences show signal intensity of moving blood as high or bright (Figure 3).

Cine Techniques
Cine techniques result in moving images that depict cardiac morphology as the heart is beating. Fast low-angle shot MRI and SSFP MRI both achieve an effective shortening of the MRI measuring time required, and can be used in cine applications to obtain detailed images of the beating heart, even in patients who are unable to comply with breath-holding techniques (Figures 4 and 5). In particular, cine techniques routinely incorporate bright blood imaging techniques that provide sufficient contrast to evaluate cardiac wall motion, cardiac valve morphology, and ejection fraction. These measurements depend on the changes in cavity volume over the cardiac cycle, or differences in stroke volume between the 2 ventricles. Ejection fraction is the ratio of stroke volume to end diastole volume, and is reported as a percentage. Cine MRI techniques must be obtained with high temporal resolution of the cardiac cycle, of less than or equal to 50 msecs.¹⁰

Because cine MRI images are obtained with a high temporal resolution, these studies can be performed at rest or during the IV administration of a pharmacologic agent that induces cardiac stress, such as dobutamine. Dobutamine administration causes an increase in heart rate and heart contractility. Although stress studies are usually performed using echocardiography, image quality can be suboptimal with this modality. On the other hand, cardiac MRI offers reliable image quality with higher spatial and temporal resolution and is considered a more ideal technique to assess the myocardial contractile reserve and to visualize latent myocardial ischemia in stress testing.⁴ Importantly, technologists performing MRI cardiac stress studies need to be aware of the patient's medical history and strictly observe the known contraindications to this modality, because physical access to the patient is more limited than in echocardiography. In general, absolute contraindications to cardiac stress testing include recent myocardial infarction (MI), unmanaged unstable angina, uncontrolled arrhythmia, and pericarditis.^4,8 Technologists performing cardiac MRI stress testing should also take precautions to ensure rapid extraction of the patient to enable immediate clinical assessment and treatment if an adverse event does occur. Resuscitation equipment and medication should be immediately available and should be administered by experienced staff in the event of a clinical emergency. During the administration of any pharmacologic stress agent, a patient's hemodynamics, including blood pressure, heart rate, oxygen saturation, and rhythm assessment, should be continuously monitored. In addition, a 12-lead electrocardiogram (ECG) should be obtained before and after the examination. These ECG findings should be compared for differences suggestive of induced ischemia or infarction during the stress test.

Contrast-Enhanced Cardiac MRI
Contrast-enhanced cardiac MRI requires the administration of IV GBCA. As discussed previously, clinicians considering a study that requires the administration of GBCA need to carefully weight the potential risks and benefits in certain patient groups, including those who are pregnant, those who have exhibited a previous sensitivity to contrast agents, and those with advanced kidney disease.³ Contrast-enhanced MRI is particularly useful for obtaining high-resolution 3-dimensional (3D) MR angiography studies to evaluate the great vessels, cardiac perfusion imaging studies, and cardiac viability sequences. Technologists should note that the FDA has approved IV GBCA for contrast studies at doses of 0.1 to .0.2 mmol/kg (20–40 mL).

Perfusion cardiac MRI is a particular application of contrast-enhanced techniques that can overcome limitations of traditional imaging studies. Although nuclear medicine scans with thallium or sestamibi imaging agents are the most routine techniques to evaluate patients for suspected myocardial ischemia, there are important limitations to these scans. The clinical value of nuclear medicine scans can be constrained by limited spatial resolution, difficulties in imaging certain areas of the cardiac anatomy, and a lack of scan penetration in obese patients. Meanwhile, perfusion studies with cardiac MRI are not limited by these factors and can be used in patients who present with challenges to effective imaging.¹¹

First pass or near-real time perfusion imaging, with the rapid administration of MRI contrast agent, can be used to effectively evaluate the adequacy of blood delivery to the myocardial tissue based on patterns of tissue enhancement. This technique requires a 6- to 10-mL bolus injection of MRI contrast, at a rate of 3 to 5 mL per second. Studies demonstrating a low perfusion of myocardial tissue are consistent with a myocardial infarct resulting in ischemic or necrotic tissue.

Meanwhile, delayed imaging or viability techniques are also useful for the detection of necrotic myocardial tissue. This technique is particularly useful to differentiate between ischemic tissue versus non-viable myocardium. Delayed imaging is obtained between 15 and 30 minutes after the injection of IV GBCA (to incorporate a washout phase), using black blood spin-echo/turbo spin-echo or double inversion-recovery techniques. Time for inversion (TI) is used to suppress the appearance of normal myocardium. To obtain high TI tissue contrast, effective TI times used are between 175 to 250 msec during the washout phase. The normal myocardium signal is suppressed or nulled by choosing the correct TI time. Necrotic or infarcted myocardium will appear white on these delayed images, whereas normal myocardium will appear dark.⁷

Contrast-enhanced cardiac MRI perfusion studies are particularly important in clinical practice, because having accurate viability information allows the cardiologist to determine whether a patient who has suffered from an MI is likely to benefit from a therapeutic revascularization procedure.

Phase-Contrast and Flow/Velocity Studies
Phase-contrast, flow/velocity studies, and sensitivity imaging techniques are continuing to evolve in cardiac MRI applications. Phase-contrast techniques do not require the administration of IV GBCA, and may expand diagnostic options in patients for whom GBCA is contraindicated. Velocity-encoded, phase-contrast cine MRI measures phase-shift due to blood flow, allowing direct noninvasive derivation of blood velocity volume flow and flow direction. Flow rates can be calculated by integrating this velocity over the measured cross-sectional area. This is beneficial in the morphologic and functional evaluation of cardiac valves in patients with suspected valvular insufficiencies or regurgitation (Figure 6). These studies can also be used to demonstrate directional flow, retrograde or antegrade flow, or stenoses of major arterial and venous structures.

Myocardial Tagging
Myocardial tagging is another emerging technique for cardiac functional assessment to characterize myocardial contraction. A grid of tag lines or radiofrequency bands is laid over the heart magnetically in diastole; these tag lines are then tracked as they distort with myocardial contraction through systole. The motion of the bands or tags can be visually observed to assess focal areas of wall motion abnormalities.⁹ Myocardial tagging has been particularly useful in assessing areas of contractile dysfunction after MI, and has been used to more accurately characterize segmental function in cardiomyopathy, especially hypertrophic cardiomyopathy.¹²

Patient Preparation for Cardiac MRI
Patients undergoing cardiac MRI should be properly screened for any contraindications to MRI procedures, and should be inspected for any removable metal objects that could pose a safety hazard or interfere with the effectiveness of the study. The MRI technologist should then consider important aspects of patient positioning, patient instructions, and equipment set-up prior to the initiation of the examination.

Contraindications to MRI
Any patient with a range of non-removable, implanted devices should not, under most circumstances, be imaged with MRI. These devices include any of the following:

• Cardiac pacemakers;
• Automatic implanted cardiac defibrillators;
• Non-MRI-compatible brain aneurysm clips;
• Carotid artery vascular clips;
• Neurostimulators;
• Insulin or infusion pumps;
• Implanted drug infusion devices;
• Bone growth or bone fusion stimulators; and
• Cochlear, otologic, or ear implants.

As previously discussed, some patients should not receive GBCA and should therefore not undergo contrast MRI studies, including women who are pregnant or lactating; patients with certain hemoglobinpathies; and patients with severe renal disease.^3,7

Patient Positioning
Proper patient positioning begins with a thorough explanation to the patient of what will happen during the examination. The technologist should not assume that the patient has already received a detailed explanation of the procedure from the referring physician. This explanation should be given prior to the placement of the cardiac coil, the placement of electrodes, and, most importantly, before the patient enters the magnet. The patient should be given detailed breath-holding instructions, which should be practiced and explained in detail. In addition, all patients should be provided with hearing protection, as well as a squeeze ball with which the technologist can be alerted if the patient experiences any problems during the examination.

In addition, studies requiring the IV administration of contrast agent need to begin with an assurance of proper IV access during the study. Ideally, needles of 22-gauge or larger should be accommodated for cardiac MRI contrast studies.

Breath-Holding Techniques
Breath-holding techniques in cardiac MRI and acquisition times to capture images requiring the patient to hold their breath are always limited by patient comfort, capability, and physical function. In general, breath-holding periods should be limited to 16 to 20 seconds in duration for most patients. The patient should be instructed to hold his breath at the end of expiration. This technique allows for a greater reproducibility of views and reduces the likelihood of partial volume effects due to slice misregistration. In studies of patients who are unable to hold their breath, technologists can modify the study by using free-breathing or navigator sequences. In navigator sequences, a navigator pulse is placed at the level between the diaphragm and the liver. The patient should be instructed to remain motionless while breathing as regularly as possible. In addition, the patient should be instructed to not take any deep breaths, and should likewise avoid falling asleep during the examination.

Equipment Set-Up
In cardiac MRI studies, the spatial resolution and signal-to-noise field of view (FOV) should be reduced to maintain an adequate spatial resolution. The signal-to-noise ratio (SNR) should also be a consideration in developing cardiac MRI protocols. Spatial resolution is a function of slice thickness, FOV, and matrix size, whereas SNR is a function of voxel size, acquisitions, field strength, coil, and bandwidth. Cardiac MRI requires a high spatial and temporal resolution of 1cm maximum. This resolution can be achieved by obtaining 8-mm slices with a 2-mm gap between slices.

Those performing cardiac MRI studies should recognize that although a higher spatial and temporal resolution is required than that used in other modalities, this needs to be balanced against acquisition time. Acquisition times in cardiac MRI are usually defined by the number of heartbeats that occur during the active period of image acquisition. For sequences performed to assess cardiac function, the ideal temporal resolution should be 50 to 60 msecs.

Image contrast is determined by several factors in cardiac MRI. Technologists can adjust repetition time, echo time, flip angle, the administration of the IV contrast medium, and the imaging sequence used to obtain images of the desired contrast. Although some of these techniques are well established, this process requires a well-trained, dedicated MRI technologist who is routinely challenged by common hurdles to effective imaging.

Surface Coil Placement
In preparation for the MRI study, the patient should be instructed to lie supine on the MRI table, or should be placed supine on the table if the patient experiences limited physical function. Cardiac MRI requires the use of specific cardiac or torso multi-element phase array coils. These specialized coils have both anterior and posterior elements. With the use of these coils, proper patient positioning is critical to obtain adequate signal reception. As with all MRI coils, the technologist should run a scout sequence to ensure correct placement before performing the study. This is especially helpful when performing parallel imaging techniques, which are useful to increase spatial resolution or reduce the imaging time for breath-hold sequences.

Electrocardiogram Synchronization
In performing cardiac MRI, the accurate peak detection in the ECG is critical to obtaining good image quality during the study. ECG electrodes should be placed on the patient in a cross-shaped or triangular pattern. Prior to the application of these electrodes, the skin should be cleaned with an alcohol pad or an abrasive gel to ensure effective surface contact. If necessary, the patient's chest may be shaved to achieve better surface contact. Depending on the desired study, images can be obtained by using either prospective or retrospective ECG data. Retrospective ECG gating allows reconstruction of images throughout the cardiac cycle, whereas prospective gating typically excludes the late diastole of the cardiac cycle.

Magnetic resonance imaging cine gradient-echo acquisition studies rely on robust and consistent ECG gating during the examination.¹³ ECG gating involves a start pulse from the ECG that triggers the beginning of the image acquisition. Therefore, before proceeding with the examination, the MRI technologist should check ECG gating by ensuring a high, constant-amplitude R wave and a low T wave. The time between consecutive R waves on the ECG, or the R-R interval, is used to coordinate ECG gating. The R-R interval is the duration of 1 heartbeat, and is typically expressed in milliseconds. If a high, consistent R wave and low T wave cannot be effectively established, then the technologist should reposition the ECG electrodes. When performing prospective triggered techniques, it is necessary to consider the trigger window, which is the short interval between the end of data sampling and the next expected R wave. ECG-triggered sequences also require a consideration of trigger delay, which refers to the delay between the detection of the R wave and the initiation of imaging.

Newer MRI systems that offer advanced triggering modules based on vectorcardiography (VCG) are available to improve R-wave detection and streamline cardiac MRI set-up. The VCG technology synchronizes the MRI image acquisition with cardiac motion by using temporal and spatial information about the cardiac electrical activity to better differentiate between true signals and signal artifacts that result from a magnetohydrodynamic effect and other noise stemming from physiologic processes.¹³

Cardiac MRI: Personnel Qualifications and Equipment Requirements
Standardized equipment requirements and personnel qualifications have been developed by the ACR that specifically address facilities that provide cardiac MRI imaging services.⁹ The ACR recommends that technologists responsible for performing cardiac MRI should be certified by the American Registry of Radiologic Technologists or the Canadian Association of Medical Radiation Technologists. Also, the ACR recommends that technologists performing cardiac MRI should have advanced MR certification, and should have supervised experience in the performance of cardiac MRI and in the IV administration of imaging contrast agents. To ensure patient safety, the ACR recommends that technologists responsible for cardiac MRI should also have training in basic life support and the use of an automatic defibrillator.⁹

The ACR has also identified appropriate qualifications for the supervising and interpreting physician responsible for cardiac MRI. These professionals should have prior qualifications in MRI and should either have received cardiac MRI training approved by the Accreditation Council for Graduate Medical Education (ACGME) or the American Osteopathic Association, according to the ACR guidelines. In addition, interpreting physicians should have completed at least 30 hours of category 1 continuing medical education (CME) in cardiac imaging. Meanwhile, physicians without prior qualifications in general MRI should demonstrate having completed an ACGME-approved training program in specialty practices, plus 200 hours of category 1 CME in MRI. Overall, physicians performing cardiac MRI examinations should demonstrate evidence of continuing competencies in MRI interpretation and examination reporting, according to the ACR.⁹

The ACR has outlined a series of suggested equipment requirements for facilities providing cardiac MRI. These include the requirement that the MRI scanner have a field strength of 1 Tesla (T) or greater, as well as a slew rate of at least 70 mT/meters/sec. The ACE guidelines cites 1.5T as the accepted clinical standard scanner strength for cardiac MRI, but adds that 3T scanner technology is evolving and may offer additional benefits due to increased SNR and expanded spatial resolution capabilities.⁹

Magnetic resonance imaging coil requirements suggested by the ACR guidelines are defined as cardiac or body torso coils with a multichannel radiofrequency surface coil configuration. The ECG capabilities for cardiac MRI include prospective triggering, retrospective gating, and triggered retrospective gating. The guidelines add that systems with VCG gating are desirable, but not required for the effective delivery of cardiac MRI services.⁹

Facilities that perform myocardial perfusion imaging or any MR angiography imaging must have an MRI-compatible power injector. In addition, scanners that are to be used for cardiac MRI should also be capable of fast 3D gradient-echo imaging, SSFP cine, phase contrast flow quantification, fast or dynamic multislice myocardial perfusion imaging, and delayed contrast-enhanced myocardial imaging. Ideally, systems should also be capable of parallel imaging and half-Fourier capabilities so that experienced technologists can adjust for shortened breath-hold requirements in their patients. Finally, the ACR emphasizes that qualified facilities should have FDA-approved software to perform image data processing, determine diagnostic criteria such as ejection fraction, and manipulate angiographic data. The guidelines stress that post-processing of MRI data should only be performed or supervised by a cardiac MRI physician.⁹

MRI Scanner Strength
Although most cardiac MRI studies have been performed on scanners with a strength of 1.5T, scanners with a greater strength of 3T are now increasingly available for cardiac MRI applications. Cardiac MRI studies require an appropriate combination of temporal and spatial resolution. Cardiac imaging with a strength of 3T offers improved speed, the ability to obtain higher spatial and temporal resolution, an increase in SNR, decreased imaging time, reduced background noise, and better image contrast as compared to 1.5T imaging. With the increase in SNR with 3T, heart perfusion images provide better visual delineation of perfusion abnormalities and cardiac ischemic evaluation.

An increased SNR with 3T MRI also reduces the loss of image quality due to patient movement, which is an important consideration in cardiac MRI due to heart motion. Furthermore, cardiac patients are often very sick and may suffer from irregular heart rates, and could therefore experience difficulty holding their breath during the procedure. New cardiac MRI protocols and technologic advancements have improved imaging speed, resulting in the ability to perform free breathing techniques for patients unable to hold their breath during the procedure. Likewise, frequency artifacts or flow artifacts prevalent during cine imaging (bright blood imaging) in earlier versions of 3T cardiac imaging have been resolved with improvements in software frequency correction techniques. This software allows the MRI technologist to run a frequency scout in any of the cardiac imaging planes. A set of 6 images can be obtained at 1 location with 6 different frequencies. The technologist reviews the images and simply selects the image with the least amount of frequency artifacts and inputs the correct frequency into the frequency selection card for the desired sequence.

Future Applications
Although clinical applications for cardiac MRI in coronary artery disease are limited at this time, the technology is currently advancing in the hopes of providing greater diagnostic applications in this disease state that will have a profound impact on public health in the developed world. Cardiac MRI can be used today to evaluate the patency coronary artery bypass grafts, and can indirectly detect stenoses in these grafts. The coronary arteries curve around the epicardial surface of the heart, which often makes slice positioning difficult. With anticipated improved 3D thin slab techniques, improved respiratory and cardiac motion suppression techniques, improvements in IV contrast and fast imaging techniques, there is a great potential for MRI coronary imaging in the future.¹³

Conclusions
Cardiac MRI is an effective problem-solving tool and complementary technique to echocardiography. Cardiac MRI is particularly valuable in assessing patients with cardiomyopathy or ischemic heart disease, and can also be used to assess patients for the presence of cardiac masses or thrombosis. Noninvasive 3D, high-resolution real-time imaging of the cardiovascular system continues to evolve. Likewise, advances in contrast media used in cardiac MRI are offering expanded capabilities and applications in effectively diagnosing and managing a wide range of cardiac diseases. Although cardiac MRI currently offers a range of functionality to diagnose and manage serious heart conditions, this modality will continue to evolve and offer new applications with anticipated improvements in software and hardware capabilities.

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