CARDIAC MAGNETIC RESONANCE IMAGING
CMR imaging has continued to advance as a robust cardiac noninvasive imaging technique. Through electromagnetic manipulation of biological hydrogen protons, CMR provides assessment of cardiac structure, function, perfusion, tissue characterization, blood flow velocity, cardiac masses, valvular heart disease, pericardial disease, and vascular disease. Continued improvements in hardware and pulse sequence design have allowed for improved image quality, speed of data acquisition, and reliability, further increasing the usefulness of CMR for clinical applications. CMR is similar to echocardiography in that neither uses ionizing radiation to acquire high-resolution images, which avoids the exposures inherent in invasive coronary angiography and SPECT imaging. CMR offers viewing cardiac motion in any view. In addition, the versatility of CMR permits imaging of a large field of view in nearly any plane, which allows for the assessment of both cardiac and noncardiac pathologies.
Technology of Cardiac Magnetic Resonance
MRI (including CMR) is based upon the electromagnetic manipulation of biological hydrogen protons. Hydrogen is the most abundant element present within the human body; it is present in all tissues, whether in water, adipose tissue, or soft tissue. Each water molecule contains two hydrogen nuclei with a single proton, and they behave like tiny magnets. Proton spins can be aligned by application of a powerful magnetic field in the β(0) direction, because of the appropriate frequency via the Larmor equation (f = γβ, where f is the precessional frequency; β is the magnet field strength; and γ is the gyromagnetic ratio). A second radio-frequency electromagnetic field can then be briefly applied and then rapidly discontinued. As protons return to their original alignment after the electromagnetic field is turned off (“relaxation”), they generate a net magnetization that decays to its former position with energy loss in the form of a radio signal that can be detected with a radiofrequency antenna and quantified. Image tissue contrast depends on differences in the decay of net magnetization in the longitudinal plane (T1) and transverse plane (T2). Through the application of additional electro magnetic fields (gradient fields), radio waves coming from the body can be spatially encoded, which allows localization within an imaging plane.
Data Acquisition Sequences and Techniques
CMR uses two basic imaging sequences: spin echo (“dark blood”) and gradient echo (“bright blood”). Spin-echo sequences are commonly used for multislice anatomic imaging, providing clear delineation of the mediastinum, cardiac chambers, and great vessels. Alternatively, gradient echo sequences are used more often for physiological assessment of function through cine acquisitions. Because of higher possible imaging speeds, gradient echo is more appropriately used for ventricular function and myocardial perfusion assessment, as well as valvular assessment. Phase contrast imaging (PCI) allows quantitative flow velocity and volume flow assessment. All cardiac and most vascular CMR sequences require cardiac ECG gating. Through data acquisition of segments at different phases of the cardiac cycle, a cine image loop can be created to track cardiac motion. Perfusion imaging, through the use of intravenous contrast agents, permits assessment of tissue vascularity. In the case of vasodilator stress perfusion imaging, assessment of myocardial ischemia is possible (Fig. 11.5). Inotropic stress imaging, typically with intravenous dobutamine, allows assessment of new regional wall motion abnormalities. Gadolinium-based contrast agents, chelated to other nontoxic molecules for clinical use, are commonly used for imaging the cardiovascular system.
FIG 11.5 Cardiac magnetic resonance stress perfusion imaging, demonstrating inferolateral ischemia.
CMR is highly accurate and reproducible, providing clinically useful measurements of cardiac wall thickness, chamber volumes, and systolic contractile function (Fig. 11.6). CMR is recognized as the gold standard for assessment of left and right ventricular function. Left ventricular ejection fraction, left ventricular end-diastolic volume, left ventricular end-systolic volume, stroke volume, cardiac output, and left ventricular mass can all be reliably quantified. Left ventricular diastolic function can also be reliably interrogated using PCI.
FIG 11.6 MRI can generate images of the heart in multiple user-defined orientation. LVOT, Left ventricular outflow tract.
CMR has rapidly evolved into a clinically reliable, reproducible modality to evaluate the aorta and its primary branch vessels. Gadolinium-enhanced, three-dimensional CMR angiography is an extremely rapid technique that can accurately depict aortic pathology. Serial monitoring of chronic aortopathy can be monitored safely, without continued radiation exposure, with CMR angiography.
CMR is the most sensitive cardiac imaging modality for assessment of myocardial viability and the extent of myocardial infarction. It is the imaging modality of choice for patients in whom there is a question about whether the myocardial tissue in the distribution of a planned revascularization is viable (Fig. 11.7). For this application, compared with nuclear imaging, CMR is much more sensitive in detecting subendocardial viability (and lack of viability), and obviously, CMR does not require radiation exposure for patients. Gadolinium is excluded from intact myocardial cell membranes and thus is useful in defining areas of infarction. Correlation with anatomic specimens suggests a sensitivity and specificity of >95%. Delayed hyperenhancement (DHE) protocols, which most often use phase sensitive inversion recovery imaging, are based on the high-signal intensity (bright) that results from T1 time shortening due to gadolinium contrast localization within scar tissue. Alternatively, first-pass perfusion images that appear hypointense are probably a combination of ischemic and infarcted tissues. The highest likelihood of recovery of contractility impairment exists when the transmural infarction extent, as assessed by DHE, is <50% transmural.
|FIG 11.7 Cardiac magnetic resonance imaging: transmural and nontransmural scars.|
CMR is an important tool in the evaluation of dilated cardiomyopathy, hypertrophic cardiomyopathy, and infiltrative disorders. It provides accurate assessment of ventricular function in patients with dilated cardiomyopathies. DHE CMR has a niche role in helping to differentiate heart failure related to dilated cardiomyopathy from CAD, although the distinction is not perfect. More than 10% of patients with dilated nonischemic cardiomyopathy have gadolinium enhancement that is identical in appearance to that seen in patients with CAD.
In hypertrophic cardiomyopathy, CMR can accurately localize hypertrophy, particularly when echocardiographic data are equivocal. Cine images can also demonstrate systolic anterior motion of the anterior mitral valve leaflet and dynamic outflow tract obstruction, which are useful measures in selecting an optimal therapeutic approach in this patient population. More recent data indicate that increased DHE scar burden in patients with hypertrophic cardiomyopathy is correlated with increased risk of arrhythmia or sudden cardiac death. CMR also has a role in the evaluation of patients with suspected infiltrative cardiomyopathies. Sarcoidosis is an infiltrative granulomatous disease pathologically known to nonuniformly involve the myocardium. This patchy distribution tends to result in a moderate to high number of false-negative cardiac biopsy results. When an initial biopsy result is negative in patients with suspected cardiac sarcoidosis, the benefits of repeated biopsy procedures must be considered because of the risks inherent in this procedure. CMR DHE imaging can depict areas of interstitial changes and granulomatous disease (Fig. 11.8). In patients with a high pretest probability for cardiac sarcoid, CMR can potentially serve as a reliable screening tool, obviating the need for biopsy, particularly if the diagnosis of sarcoidosis has been confirmed by biopsy of noncardiac tissue. Amyloid infiltration in the myocardium may show diffusely increased signal intensity with DHE imaging sequences. In addition, the combination of ventricular hypertrophy without ECG concordance, atrial wall thickening, valve thickening, pericardial and pleural effusion, and restrictive diastolic filling pattern can collectively raise the clinical suspicion for infiltrative cardiac amyloidosis. CMR is also capable of confirming the diagnosis of arrhythmogenic right ventricular dysplasia, a diagnosis that historically is based on meeting several major and minor criteria. Use of contrast agents and DHE imaging may permit detection of fibro-fatty right ventricular free wall infiltration, regional right ventricular wall motion abnormalities, and assessment of indexed right ventricular volume, which are observations that increase specificity for this otherwise difficult diagnosis.
FIG 11.8 Sarcoidosis: CMR phase-sensitive inversion recovery.
CMR permits assessment of pericardial effusion, constrictive pericarditis, pericardial cysts, and congenital absence of the pericardium. Normal pericardium thickness on CMR is 1 to 4 mm. Functional and structural abnormalities of the pericardium can be reliably assessed using CMR imaging. Pericardial DHE imaging has been demonstrated to correlate with active pericardial inflammation and neovascularization. In addition, free breathing cine imaging can demonstrate increased ventricular interdependence suggestive of constrictive pericarditis. Failure to see slippage between the visceral and parietal pericardial layers suggests fibrosis, scarring, or connections between these two normally separate tissue layers. CMR has also proven useful in the evaluation of pericardial cysts.
Valvular Heart Disease
CMR has become a valuable complementary technique for evaluating the severity of valvular heart disease. Through a combination of steady-state free precession and PCI, CMR can provide a comprehensive valvular assessment. Although echocardiography is capable of superior temporal resolution, is more accessible, and is less labor-intensive, CMR is capable of imaging flow in three dimensions (x, y, and z planes), which is more accurate for measuring absolute flow volumes and feasible in patients whose body habitus precludes obtaining optimal echocardiographic images. In valvular regurgitant lesions, PCI can provide exact quantifications of regurgitant volume and fraction. In patients with aortic stenosis, planimetry of the aortic valve provides accurate measurements rather than geometric estimations available via echocardiography and catheterization techniques. In addition, CMR provides accurate measurement of peak transstenotic jet velocities that are orthogonal to the valve, not merely across it.
CMR is the imaging modality of choice for evaluation of cardiac masses because of its ability to perform tissue characterization. Spin-echo imaging provides excellent images for evaluation of the presence, extent, attachment site, and secondary effects of cardiac mass lesions. CMR has a proven role in the identification of intracardiac thrombi, primary and secondary cardiac tumors, and pericardial cysts (Fig. 11.9).
FIG 11.9 Cardiac MRI showing left atrial myxoma.
Congenital Heart Disease
CMR is an ideal imaging modality for the assessment of congenital heart disease by providing superior anatomic imaging coupled with functional interrogation and reproducibility. In the evaluation of great vessel abnormalities, CMR is the gold standard, particularly for conditions such as aortic coarctation. Through velocity mapping of the coarctation jet, a pressure gradient across the area of narrowing can be determined. Tetralogy of Fallot, including overriding aorta, membranous ventricular septal defect, right ventricular hypertrophy, and infundibular or pulmonary stenosis, can be completely characterized before and after correction. In addition, as is often the case with patients who required surgical tetralogy repair, CMR is an excellent tool for monitoring patients for progressive pulmonary valvular regurgitation and right ventricular dilation. CMR is also capable of reliably depicting anomalous coronary artery origins and their relation to other cardiac structures and the great vessels.
Coronary Artery Bypass Graft Imaging
Although coronary angiography remains the gold standard for evaluating coronary atherosclerotic disease, CMR may be used in the future for noninvasive assessment of the coronary arteries. The main limitations to CMR coronary angiography include limited spatial resolution, respiratory motion, rapid coronary motion (up to 20 cm/s in certain phases), and an inability to easily assess distal runoff. Quantification (and sometimes even detection) of coronary luminal stenosis remains challenging. Currently, this is an area of significant ongoing research. Coronary flow velocities can be estimated by CMR, and some centers are now using adenosine infusion with CMR to measure coronary flow as a diagnostic test for functionally important CAD. Anomalous coronary arteries can be identified using CMR. In particular, CMR is well suited to demonstrate the relationship of anomalous coronary arteries with other vascular structures (the aorta and main pulmonary artery), and thus, to make decisions on the need and timing of surgery.