Applied Radiology

After receiving an Associate degree from North Central Technical College in Mansfield, Ohio, Dr. Calendine worked as a radiologic technologist until graduating with a Bachelor degree from Ashland University in Ashland, Ohio. He attended medical school at Wright State University, School of Medicine in Dayton, Ohio. Dr. Calendine is currently in radiology residency, post-graduate year four, at Ohio State University.

Magnetic resonance imaging (MRI) of the heart has many potential applications in the noninvasive evaluation of cardiac disease, including congenital anomalies, cardiomyopathy, neoplasms, valve disease, coronary artery disease, and myocardial perfusion, function, and viability. ¹ Cardiac MRI is unsurpassed in anatomic detail, and superior in volumetric and motion analysis. Now more widely available thanks to advances in computer technology, MRI of the heart is beginning to take a more important role in the work-up of cardiac disease. ² The intent of this article is to serve as an overview of current cardiac MRI techniques and clinical applications.

The heart, an organ nearly abandoned by radiologists, has had a remarkable resurgence in popularity. This cardiac renaissance has resulted in an increased demand by consultant cardiologists, who often lack familiarity with MRI, rejuvenating a symbiotic relationship with the radiologist similar to that in nuclear cardiology. Currently, MRI is still often reserved for cases in which echocardiography (ECHO) has been nondiagnostic or confusing. ECHO, although less expensive ³ and more readily available and familiar to clinicians, is far inferior to cardiac MRI for anatomic structure detail and volumetric assessment of the atria and ventricles. ^4,5

Getting started: Technique

Perhaps the greatest obstacle in obtaining cardiac MR images is the continuous movement of the heart and adjacent great vessels. ¹ Since the early evaluation of electrocardiogram- (ECG) gating MR in 1984, ^6-8 intense work has continued in the field to acquire images with fewer artifacts. Achieving this requires more precise cardiac gating, reducing scan time, and increasing signal-to-noise ratio (SNR).

Cardiac gating --In ECG-gated imaging, the R wave is used for acquisition triggering in order to obtain each portion of an image at the same time in the cardiac cycle. The phase encoding steps occurring at the same time interval with respect to the R wave for multiple ECG waves are then used in creating an image of the heart at that single point in the cardiac cycle. For instance, when the R wave is detected, phase encoding then occurs x seconds afterward. At the next R wave, advancement of the phase encoding is triggered to occur again x seconds after the R wave, and so on (figure1). With the appropriate number of phase encoding steps (usually 256 or 512), an image is generated. When interference occurs with the ECG signal, altering the R wave, the detector switch responsible for initiation of phase encoding can be activated inappropriately. The result is randomization of acquisition giving images that are uninterpretable. ^1,6-9

For optimization of ECG signal, the skin must be cleaned and abraded to obtain good contact. Care must be taken for appropriate lead placement on the chest, typically anterior over the heart, all within a 10-cm radius (figure 2), and the lead wires should exit the bore over the shoulder parallel to the magnetic field to reduce ECG interference. ¹ A new approach to cardiac gating known as vectorcardiographic (VCG) gating could potentially negate the dependence on traditional ECG gating. ¹⁰ By depending on the direction of the heart movement as detected by VCG instead of the electrical conduction, more accurate gating can be achieved because of avoidance of intrinsic ECG noise from magnetohydrodynamic effect signal created by flow of blood within the patient in the magnetic field. In addition, gating for patients with arrhythmias may be performed with greater accuracy. ¹⁰

Coils

A number of different coils and coil combinations can be used. At our institution, two surface coils are used, one placed along the anterior chest and the other posterior (figure 3). Four coils can be used with the addition of coils along the lateral chest wall bilaterally. ¹¹

Sequences --Many cardiac se-quences are in use today, each with specific strengths and weaknesses. The most popular are the "black blood" and "white blood" sequences. Black blood refers to fast double or triple inversion recovery (IR) fast spin echo (FSE) techniques obtained in a single breath hold. ¹² Double IR is best at discriminating between structures and evaluation of morphology, as soft-tissue contrast is optimized. A low echo time (TE) (20 to 60 msec) is used for the double IR-FSE. The repetition time (TR) is dependent on the patient's heart rate and is usually equal to the R-R interval on the ECG (i.e., heart rate of 60 beats per minute will have an R-R interval of 1 second and the TR would be 1,000 msec). The blood in this case does not receive the full 90 and then 180 degrees radiofrequency (RF) pulses given in IR because it is moving, hence diminishing the signal.

White blood refers to ultrafast gradient echo cine images. White blood images are best at depicting wall motion, and can be used for volumetric analysis. ¹ They are acquired using FSPGR (fast spoiled gradient-recalled acquisition in the steady state) with TE minimized, a flip angle of 15 to 20 degrees, and a very short TR (i.e., 8 msec). Circulation during the sequence moves blood with unsaturated protons into the imaging volume with resulting high signal of the blood. For cine of a cardiac cycle, 16 to 30 images are required.

Echo-planar imaging (EPI) may also be instituted in cardiac MR because of the extraordinarily short acquisition time. ^13,14 Unlike conventional MR sequences, EPI can fill in the entire k-space of an MR image with one measurement, like a snapshot. ¹⁵ Images may be taken without the use of ECG gating and even without a breath hold. The major deficiency with single-shot EPI is decreased detail compared with IR-FSE techniques. Fat suppression is required for EPI sequences, and the heterogeneity of the heart can result in image degradation. Also, IR sequences reduce the artifact caused by blood flow. Multi-shot EPI demonstrates great promise, particularly for the evaluation of the coronary arteries. ¹⁶ Multi-shot EPI has the advantage of increased detail compared to single-shot EPI, but ECG gating is then needed to reduce motion artifact. ¹⁵

Another technique, developed in 1997, is termed SMASH (simultaneous acquisition of spatial harmonics) imaging. ¹⁷ The SMASH technique exploits the geometry of an RF coil array to encode multiple lines of MR image data at the same time, decreasing image acquisition time significantly. ^17,18 There is ongoing research in the evaluation, application and im-provement of these techniques.

Another useful technique developed about 10 years ago is called myocardial tagging. ^20,21 A grid of magnetic saturation is produced by applying a sequence of RF pulses, either with the magnetic field gradients switched on while they are applied ¹⁹ or separated by magnetic field gradients. ^20,21 This creates a crosshatch pattern made of alternating low (initial RF pulses) and high (conventional sequence) signal (figure 4). During the cardiac cycle, the boxes of the grid change shape, allowing for quantification of wall thickening and motion using mathematical analysis. Though shown to be quite accurate, it is labor intensive in post-processing time. ²¹

Two popular sequences for achieving myocardial tagging are used. One is termed CSPAMM (complementary spatial modulation of magnetization), described by Fischer et al ²² as improvement on the originally described SPAMM (spatial modulation of magnetization) sequence. ^20,23 The other sequence is called DANTE (delays alternating with nutations for tailored excitations). The biggest advantage of the DANTE tagging sequence is higher resolution of the tags and the ability to get more closely spaced tag lines. ^19,21 In DANTE, myocardial velocity mapping, also used for myocardial motion analysis, uses the phase shifts of the spins to encode the velocity into the MR signal. Once the myocardial contours have been segmented, the data can be automatically processed to obtain quantitative measurements. ²⁰

Newer techniques

Other areas of technique research, include 23Na MRI of the heart, ²⁴ which can be used to detect myocardial viability and diffusion imaging are in early investigation. ²¹ At the RSNA 2000 meeting, another new method called UNFOLD was reviewed. UNFOLD exploits the variable movement of the field of view (FOV) centrally compared with the relatively static periphery, and can result in significant reduction of image acquisition time. ²⁵

Prescribing planes

Radiologists feel comfortable with the classical anatomic planes: axial, sagittal, and coronal. From computed tomography (CT) images, we are accustomed to seeing the heart in an axial plane. Nothing is more confounding in image interpretation than becoming disoriented to the relation of the anatomic structures. For this reason, some radiologists in the field of cardiac MRI argue that looking at images in the three classical anatomic planes will suffice in many cases. ¹¹ However, having the long and short axes of the heart obliquely oriented across the classical planes can result in over estimation of wall thickness, and functional studies cannot be performed accurately. ¹ So it is important, if not imperative, to evaluate the heart with the views analogous to those used in ECHO. Indeed a tremendous advantage of MRI is the ability to image in any plane, and some of these views require double obliquity from the classical planes. Obtaining these "ECHO-equal" images requires knowledge of the planes and how to prescribe them.

At our institution, a gradient-recalled sagittal localizer is obtained first (figure 5A). From this, images are prescribed parallel to the long axis of the heart, from the middle of the mitral valve to the ventricular apex. Images then taken at a right angle (a coronal oblique) to the long axis images, but still along the long axis, will be the 4-chamber views (figure 5B). Using long-axis views, a short-axis FSE cine series can be prescribed, perpendicular to the long axis (figure 5C). Utilizing the short-axis images, an outflow tract view is prescribed (figure 5D). The result is an off-coronal plane, which parallels the aortic outflow tract.

Long-axis 4-chamber views are best for evaluating ventricular walls and chambers. Aortic and mitral valves may also be analyzed in this view. Short-axis views demonstrate concentric rings of myocardium from base to apex, and can be used for quantification of myocardial thickening, analogous to cardiac single photon emission computed tomography (SPECT) imaging.

Cardiac MRI applications

As cardiac MR has evolved, a wide array of applications have been proposed. Currently, MR is most often utilized when high anatomic detail is needed (as for the pericardium), for further characterization of abnormalities discovered on ECHO, for diagnosis and post-surgical follow-up of complex congenital heart malformations, and for ventricular morphological and functional evaluation. Other applications include coronary MR angiography, myocardial perfusion and viability, valvular heart disease, and cardiomyopathy.

Pericardial evaluation --A perplexing question not uncommonly raised is the differentiation of restrictive cardiomyopathy versus constrictive pericarditis. MRI offers far superior pericardial evaluation than ECHO, and can, at the same time, evaluate physiologic changes occurring with heart function. ^26,27 Axial and coronal planes can completely image the pericardium, whereas the pericardium is very difficult to evaluate with ECHO, particularly posteriorly (figure 6). ¹ MRI is also very sensitive for detection of pericardial effusion.

Tumor localization and characterization --ECHO is the initial modality of choice for the work-up of cardiac tumors. The spatial resolution and real-time evaluation are beyond that of other modalities. The greatest advantage with MRI, however, is the unsurpassed soft-tissue contrast it provides. Abnormalities seen on ECHO may be misleading as to where they are located or what they are. Ectopic or displaced normal structures may even mimic tumors. ¹ Paracardiac, pericardial, and cardiac tumors can be depicted on MRI, and may be characterized by their signal on different sequences (figures 7 and 8). For instance, angiosarcoma, the most common primary malignancy of the heart, often hemorrhages, resulting in high signal intensity on T1-weighted images. Cardiac lipoma or lipomatous hypertrophy of the atrial septum both show increased T-1 signal which decreases after fat saturation. ²⁸ Benign and malignant tumors about the heart increase morbidity and mortality, ^28,29 as lesions that are histologically benign may be malignant by location. Tumor extent when not well displayed on ECHO can be of particular importance in prognostication, treatment op-tions, and surgical planning. ²⁷

Congenital heart disease --MRI made a place for itself in studying patients with congenital heart disease (CHD) before and after surgical intervention (figure 9). ^30,31 Visualization of posterior mediastinal structures and su-pracardiac anatomy is possible with MR, both of which are blind spots on ECHO. Functional MRI can limit the need for repeated angiography to follow patients along their course. Because of these strengths, many clinicians now turn to MRI for evaluation of complex cardiovascular anomalies (figure 10). In addition to the anatomic depiction of CHD, MRI is capable of functional imaging, including measurement of intracardiac shunts, differential pulmonary blood flow, pressure gradients across valvular and vascular stenoses, and valvular regurgitant fraction. ³²

When used in children, these techniques often require conscious sedation with close nursing and anesthesiologist supervision. Sedated patients cannot hold their breath on demand, requiring gating with the respiratory cycle. A technique called respiratory-ordered phase encoding (ROPE) ³³ orders phase-encoding steps in such a way as to reduce respiration-induced artifacts by selecting a phase angle proportional to the current phase of the respiratory cycle. ^32,33 ROPE can greatly improve image quality and requires no additional acquisition time.

Evaluation of the cardiac images should be done using a segmental approach, which is beyond the scope of this article, as this approach has been established as a reliable way to accurately diagnose and describe CHD. ³⁴

Evaluation of ventricles and cardiomyopath y--Cardiomyopathies can be placed into five categories; restrictive, hypertrophic (figure 11), dilated, obliterative, and dysplastic. Dilated cardiomyopathy is the most common in the United States, usually secondary to coronary artery disease (CAD). Cine MRI or hybrid EPI is important for assessment of ventricular function in that it gives excellent visualization of the morphologic changes that occur. Blood flow and wall thickening can be seen, and parameters, including velocities and ejection fraction, can be calculated. ³⁵

Although the left ventricle can usually be well demonstrated on ECHO, the right ventricle is often not. This is due to the complex geometry of the right ventricle and its location behind the sternum. For this reason, right ventricular dysfunction often goes undiagnosed until late in disease, after it becomes clinically apparent. MRI can be useful in early detection of poor ventricular function, as in cor pulmonale, and thereby result in earlier medical management. ^36,37 MRI of the right ventricle has also proven beneficial in the diagnosis of arrhythmogenic right ventricular dysplasia (ARVD)--also called right ventricular cardiomyopathy--in which the degeneration of right ventricular myocytes is followed by infiltration of lipoproteinaceous or fibrous material. ^38,39 The first clinical sign of ARVD is often ventricular tachycardia (VT), frequently elicited by exercise, but cardiac arrest may be the presenting manifestation. ⁴⁰ In those who survive, conventional work-up with ECHO, angiography, and SPECT often reveal a normal heart. MRI may show increased signal within the right ventricular wall (in 50% to 60% of cases), right ventricular wall thinning or other nonspecific findings of systolic free wall bulging, right ventricular outflow tract bulging, or regional wall abnormalities. ^38,39

Two-dimensional (2D) volumetric analysis of ventricular volume and ejection fraction using geometric estimation by modified Simpson rule has been verified and widely applied. Three-dimensional (3D) volume rendering has proved laborious and not significantly more precise. ⁴¹ A new development has been made in the evaluation of left ventricular function building on the conventional 2D methods of ventricular wall contour mapping in the short axis ⁴² to make 3D volumetric assessment possible. Adding information of contour mapping from long axis views appears to increase accuracy of left ventricular volume by better defining the level of the mitral valve. ⁴³

Valvular heart disease

Although the role for MRI is currently limited in the realm of valve disease, it has been shown efficacious in the ability to diagnose and quantify valvular abnormalities. ^44-46 MRI of the heart valves can be used for identification of valvular stenosis with or without regurgitation (figure 12). Pressure gradients across a stenotic valve can be calculated. The role of MRI has primarily been investigational to date, however it often proves superior in the determination of ventricular mass and volumes and is effective in monitoring ventricular changes with disease progression. In the same way, MRI can be utilized for evaluation of treatment efficacy. Additionally, MRI using velocity encoded cine imaging is the only noninvasive modality for objective quantification of valve regurgitation even with multivalvular disease. ⁴⁴

Ischemic heart disease

Coronary artery disease is the number one cause of death in the United States today in both men and women. Because of the high prevalence of the disease and the catastrophic outcome, early detection and treatment are imperative. A tremendous amount of work has been done and is in progress in the application of MRI to the detection of myocardial ischemia, perfusion, and viability. ^47-52 Magnetic resonance angiography (MRA) is also under development, and new techniques for coronary MRA show promise in CAD detection. ⁵⁴

Drawing from methods developed for nuclear cardiology and ECHO, an MRI cardiac stress test has been developed. ⁵⁴ The MRI stress test has some advantages over both. Using the MRI stress tests allows higher spatial resolution than cardiac SPECT, and on MRI, the full ventricular wall thickness and individual regions can be analyzed, whereas ECHO is limited to evaluation of transmural thickness abnormalities alone. MRI stress test sensitivity and specificity have been shown to equal stress ECHO, but the detailed MR imaging greatly facilitates wall motion analysis and ejection fraction calculation, which can be determined objectively rather than subjectively.

Conclusions

Cardiac MRI in the twenty-first century will be an exciting and remarkably dynamic field, with applications encompassing all aspects of heart disease evaluation. MRI has become more practical as computer speed has increased, and innovative sequencing has been developed. Its usefulness can no longer be disputed in many cases and it is of particular benefit in evaluating the pericardium and right ventricle. Complex congenital heart anomalies are better depicted with MRI, significantly aiding both preoperative planning and postoperative follow-up. Valvular disease is often well depicted, and gradients across stenotic valves can be noninvasively and accurately measured. Stress cardiac MR imaging is being performed with excellent correlation to ECHO, although work is needed to make it more practical. Newer techniques including coronary angiography, spectroscopy, and 3-D 23Na imaging show promise for future applications. The role of the radiologist is now quite different than in the recent past. ⁵⁵ As demand for cardiac MRI increases and availability becomes widespread, we will again be called upon for our expertise.