Cardiovascular Magnetic Resonance Imaging
I.Introduction. Magnetic resonance (MR) imaging (MRI) has become widely available as a diagnostic technique for cardiovascular imaging, and its clinical indications continue to expand. Advantages of cardiovascular MR (CMR) include its ability to produce high-resolution and three-dimensional (3D) images of the cardiac chambers and thoracic vessels without ionizing radiation (unlike nuclear imaging and cardiac computed tomography) and its ability to do tissue characterization. In contrast to echocardiography, MRI is less operator dependent and is not limited by interferences from adjacent bone or air. Clinical applications of CMR include imaging of myocardia including ischemic heart disease; valvular, pericardial, aortic, and peripheral artery; congenital heart disease; and intracardiac masses.
Common indications and components of the CMR evaluation are listed in Table 50.1.
TABLE 50.1 Cardiovascular Magnetic Resonance Imaging Indications and Applications |
|
Indication(s) |
Applications |
Aortic disease |
Aortic aneurysm morphology and size; acute aortic pathology (dissection, intramural hematoma, penetrating ulcer); coarctation of the aorta; branch vessel disease; evidence of vasculitis; postoperative graft complication including infection or leak; assessment for aortic regurgitation or other associated pathologies |
Ischemic heart disease |
Ventricular volumes and function; myocardial scar and viability; quantification of mitral regurgitation; assessment for LV aneurysm, thrombus, VSD, and other complications |
Nonischemic cardiomyopathies |
Ventricular volumes and function; myocardial wall thickness; LV outflow tract obstruction in hypertrophic cardiomyopathy; presence and patterns of myocardial scar/fibrosis; assessment for myocardial iron deposition in suspected hemochromatosis; quantification of mitral regurgitation; evaluation for ARVD in patients with ventricular arrhythmias or syncope |
Pericardial disease |
Pericardial effusion; pericardial thickening with or without calcification; pericardial tethering; signs of constrictive physiology including conical/tubular deformity of the ventricles, ventricular interdependence, diastolic septal bounce, and early cessation of diastolic filling |
Congenital heart disease |
Anatomic definition; ventricular volume and function; valve morphology and function; shunt calculation; assessment for anomalous origin of the coronary arteries; anomalies of the aorta, pulmonary arteries, and systemic and pulmonary veins |
Valvular heart disease |
Valve morphology; regurgitation and/or stenosis etiology and severity; ventricular size and function |
Cardiac masses |
Size and extent of mass; tissue characterization |
Pulmonary veins |
Pulmonary vein anatomy and stenosis; cardiac anatomy and function |
ARVD, arrhythmogenic right ventricular dysplasia; LV, left ventricular; VSD, ventricular septal defect.
II.Contraindications. Contraindications to CMR imaging (CMRI) are listed in Table 50.2.
TABLE 50.2 Contraindications to Cardiovascular Magnetic Resonance Imaging |
|
Specific Devices |
Special Issues |
Cerebral aneurysm clips |
Certain cerebral aneurysm clips pose a danger because of the potential for displacement when exposed to a magnetic field. Aneurysm clips classified as “nonferromagnetic” or “weakly ferromagnetic” are safe |
Cardiac pacemakers and ICDs |
The presence of a pacemaker/defibrillator is usually a contraindication to MRI owing to several potential problems, including (1) movement, (2) malfunction, (3) heating induced in the leads, and (4) current induced in the leads. In addition, artifact from the leads will often cause significant image degradation. However, certain new devices are labeled “MRI safe” and patients with these devices can undergo CMR |
Cardiovascular catheters |
Catheters with conductive metallic components (e.g., pulmonary artery catheters) have the potential for excessive heating. Hence patients with such devices should not undergo CMR |
Cochlear implants and hearing aids |
Most types of implants employ a strong magnet or are electronically activated. Consequently, MRI is contraindicated because of potential injury or damage to the function of these implants. External hearing aids should be removed before the MRI procedure |
Intravascular coils, stents, and filters |
These devices typically become incorporated securely into the vessel wall within 6–8 wk after implantation; hence, most are considered MRI safe. However, specific information on the type of device should be obtained before MRI is planned (mrisafety.com). Intracoronary stents have been shown to be safe during MRI, even when performed on the day of implantation, although many stent manufacturers recommend waiting 6–8 wk |
ECG electrodes |
MR-safe ECG electrodes are strongly recommended to ensure patient safety and proper ECG recording |
Foley catheters |
Certain Foley catheters with temperature sensors have the potential for excessive heating. They are generally safe if positioned properly and disconnected from the temperature monitor during MRI |
Heart valve prostheses |
The majority of prosthetic heart valves and annuloplasty rings have been labeled as “MR safe” and should not be considered a contraindication to MR examination at ≤3 T any time after implantation |
Metallic foreign bodies |
All patients with a history of injury with metallic foreign bodies such as a bullet or shrapnel should be thoroughly evaluated, as serious injury may result from movement or dislodgement of the foreign body |
Metallic cardiac occluders (e.g., management of PDA, ASD, or VSD) |
MRI is safe for nonferromagnetic devices immediately after implant. Weakly ferromagnetic devices are generally safe approximately 6–8 wk after placement |
Retained epicardial pacing wires |
MRI in patients with retained epicardial pacing wires after cardiac surgery is safe. However, retained transvenous pacing wires are usually a contraindication to CMR |
ASD, atrial septal defect; CMR, cardiac magnetic resonance; ECG, electrocardiogram; ICDs, implantable cardioverter–defibrillator; MR, magnetic resonance; MRI, magnetic resonance imaging; PDA, patent ductus arteriosus; VSD, ventricular septal defect.
III.Basics of Cardiac MRI
A.MRI physics. Hydrogen is the most abundant atom in the body, and it is the excitation of the hydrogen nuclei, often referred to as protons, that forms the basis for clinical MRI. The nucleus has an inherent property called “spin” giving it a small magnetic moment. When these spins are placed in a magnetic field, they align parallel to the magnetic field and precess at a certain frequency (resonant or precessional frequency) but with different phase, creating a longitudinal magnetization. Application of a radiofrequency (RF) pulse with the same precessional frequency will cause excitation or resonance of the nucleus, temporarily changing its alignment within the magnetic field (transverse magnetization). However, this is an unstable state of higher energy. As the RF pulse is switched off, the spins quickly return to their resting state, that is, aligned with the field, because this is energetically the most favorable situation. The newly established transverse magnetization starts to disappear (a process called transversal relaxation), and the longitudinal magnetization grows back to its original size (a process called longitudinal relaxation). During this process, an RF signal is generated, which can be captured by the receiving coil and readily measured. This process constitutes the underlying principle of MRI.
The signal generated by an excited proton is dependent on its molecular environment, such that the MR signal from a hydrogen atom in blood can be discriminated from the MR signal from a hydrogen atom in fat or other tissue types. An MRI machine, therefore, includes a strong magnet that creates a continuous magnetic field and RF coils for transmitting the excitation pulses and receiving the radio signals generated by the excited protons. Application of predictable variations or “gradients” in the magnetic field, using gradient coils within the magnetic bore, allows 3D spatial localization of each signal. The raw data are initially mapped in “k-space”; then a Fourier transformation is performed to generate the final MRI image.
B.T1, T2, and image contrast. The rate of relaxation of an excited proton along the longitudinal axis (i.e., the direction of the external magnetic field) is described by its T1 time, whereas the transverse axis is described by its T2 time. T1 and T2 times depend on the molecular environment of the protons (intrinsic to the tissue characteristics) and the magnetic field strength. T1 and T2 relaxation times of different tissues are important determinants of image contrast and, although not measured directly, images can be either T1 or T2 “weighted” to facilitate tissue characterization.
C.Issues specific to CMR. Cardiac and respiratory motion poses significant challenges to CMR. In contrast to echocardiography, which is based on real-time imaging, CMRI sequences usually acquire a single image over several heart beats to optimize the spatial and temporal resolution. It is, therefore, necessary to gate images to the cardiac cycle with either an electrocardiographic or pulse signal. Respiratory motion is typically countered by performing breath-holds during the examination. In patients who are unable to maintain a breath-hold, averaging multiple MR signals may help decrease the noise created by respiratory motion, at the expense of increasing the examination time by a factor of the number of signals averaged. Respiratory navigator sequences that coordinate imaging with a particular phase of diaphragmatic and hence respiratory motion are also effective, and they are typically used for pulse sequences that are too long for a single breath-hold, such as free-breathing whole-heart 3D coronary magnetic resonance angiography (MRA) sequences. Finally, real-time imaging using newer ultra-fast pulse sequences can be used in the absence of electrocardiographic or respiratory gating, at the expense of a significant decrease in temporal and spatial resolution.
D.CMR pulse sequences and applications
1.Spin echo. Spin-echo sequences are characterized by a refocusing RF pulse after delivery of the initial excitation pulse. Rapidly flowing blood appears dark, hence they are also known as “black-blood” sequences. Spin-echo sequences provide still images, which are typically used for anatomic delineation of the heart and great vessels owing to their excellent tissue contrast and high signal-to-noise ratio (SNR). They are relatively insensitive to magnetic field inhomogeneities and artifacts related to ferromagnetic objects such as sternal wires and prosthetic heart valves. The main disadvantage of spin-echo sequences is the relatively longer time it takes to acquire an image, making them more susceptible to motion artifacts and unsuitable for cine imaging. Turbo spin echo is a newer technique that provides faster acquisition times than does standard spin echo.
2.Gradient echo. Gradient echo sequences are characterized by the use of refocusing gradients after the delivery of the initial excitation pulse. Rapidly flowing blood appears bright, hence they are also known as “bright blood” sequences. Gradient echo is a fast imaging technique that is relatively insensitive to motion artifacts, making it ideal for cine imaging. However, it has less tissue contrast and increased susceptibility to magnetic field inhomogeneities and ferromagnetic-related artifacts. A variety of gradient echo sequences are widely used in CMR for cine imaging, myocardial perfusion and scar assessment, coronary imaging, and MRA.
3.Cine imaging. The most widely used pulse sequence for cine imaging is a gradient echo sequence called balanced steady-state free precession (B-SSFP), which is characterized by high SNR, high image contrast between blood and myocardium, and low sensitivity to motion artifact. However, B-SSFP is relatively insensitive to blood flow and, therefore, can be suboptimal for imaging of valve dysfunction or intracardiac shunts, which can usually be better illustrated using other gradient echo pulse sequences, such as echo planar imaging or phase velocity mapping. In addition, B-SSFP is also more susceptible to magnetic field inhomogeneities which can be problematic in patients with mechanical valves or other cardiac implants.
4.Myocardial tagging. RF pulses can be applied before the excitation pulse to generate dark saturation lines or grids on cine images, which are then tagged to the myocardium and further used to assess myocardial deformation. The tags can be used to help qualitatively assess myocardial motion and pericardial tethering or to quantitatively measure myocardial strain.
5.Perfusion imaging. Very fast gradient echo sequences are used for dynamic imaging of left ventricular (LV) myocardial perfusion during the first pass of a gadolinium contrast agent during rest and stress states. Fast gradient echo techniques are commonly used, such as fast low-angle shot or B-SSFP with a prepulse to null or darken the myocardium. Normally perfused myocardium shows an increase in signal intensity because of gadolinium contrast, whereas abnormally perfused areas remain dark, reflecting hypoperfusion.
6.Delayed imaging. Delayed hyperenhancement imaging for myocardial scar or fibrosis is performed 10 to 30 minutes after injection of gadolinium contrast using gradient echo sequences with an inversion recovery prepulse to null signal from the myocardium. Areas of myocardial scar or fibrosis have a larger extracellular space with a greater accumulation and slower washout of gadolinium and, therefore, appear bright compared with dark, normal myocardium on delayed imaging.
7.Phase-contrast velocity mapping. The phase difference in the spin of protons in moving blood compared with nonmoving protons within a magnetic gradient is called the “spin phase shift” and is proportional to the velocity of the moving protons. A phase-encoded image is constructed, with the gray level of each pixel coded for velocity. Phase-contrast velocity mapping could be considered analogous to pulse wave Doppler echocardiography. It can be used to measure blood velocity and hence quantify cardiac output, shunts, and valve dysfunction. There are, however, limitations, given that the accuracy of this method is highly dependent on factors such as flow pattern, flow velocity, size, and tortuosity of the vessel. Flow-related signal loss can be a result of loss of phase coherence that can occur in cases of significant flow acceleration and even in higher orders of motion present in complex flow patterns.
8.Magnetic resonance angiography. MRA of the great vessels typically involves a 3D fast gradient echo acquisition after injection of gadolinium contrast. The image resolution is typically 2 × 2 × 3 mm, making MRA an excellent option for imaging of large to intermediate size arteries, but less optimal for imaging of smaller vessels.
9.Parallel imaging. A number of parallel imaging techniques make use of multiple receiving body coils to fasten the acquisition times. This improves temporal resolution, but at the cost of a small decrease in the SNR.
E.Contrast agents. A number of gadolinium chelates are used as contrast agents in clinical MRI. Gadolinium significantly shortens the relaxation time of nearby protons, thereby increasing their signal intensity. These contrast agents are safe, with a low side-effect profile. Prevalence of adverse reactions is approximately 2% and includes transient headache, nausea, vomiting, local burning or cool sensation, and hives. Anaphylactic reactions are extremely rare. However, gadolinium has been linked to a severe and rapidly progressive form of systemic sclerosis called nephrogenic systemic fibrosis, which appears to be related to extracellular accumulation of gadolinium after its administration in patients with end-stage renal disease. The U.S. Food & Drug Administration (FDA) has advised that gadolinium contrast agents should not be administered to patients with a glomerular filtration rate of 15 mL/min. Caution should be exercised in patients with moderate or severe renal impairment.
IV.Practical considerations
A.Safety
1.Magnetic force. Cardiac MRI scanners typically utilize powerful magnets of 1.5 to 3.0 T, several tens of thousands of times stronger than the earth’s magnetic field (0.00005 T). Large or small ferromagnetic objects in the vicinity of the MRI magnet bore can become fast moving projectiles, which may cause severe injury to patients and/or damage the MRI scanner. Health-care professionals working in the vicinity of an MRI scanner require MRI safety training and should be vigilant to risk posed by patients and health-care professionals not familiar with the danger.
2.Magnetic field gradients. Switching magnetic field gradients during a CMR study produces high acoustic noise levels (up to 115 dB) and can also lead to peripheral nerve stimulation. The FDA has determined limits to the power of magnetic field gradients and noise exposure. Headphones and earplugs are recommended to prevent discomfort and hearing loss to patients and MRI staff in the vicinity of the scanner.
3.Bioeffects of RF energy. The majority of RF energy to the patient is dissipated as heat and is recorded as the specific absorption rate (SAR). One SAR equals 1 J of RF energy per second per kilogram of body weight (i.e., watts per kilogram). The recommended SAR limit for the whole body is 4 W/kg.
B.Patient preparation
1.Screening. All patients should be screened for contraindications to MRI before the procedure (Table 50.2).
2.Patient size. Although the maximum table load weight limit is fairly generous (~250 kg or 550 lb), because of the fixed internal diameter of the magnet bore, very large patients may not fit within the MRI magnet. Typically, patients with a torso circumference of >60 cm cannot be imaged. Discussion with the MRI technologist before scanning is recommended for specific recommendations related to your unit.
3.Claustrophobia. The enclosed space of the magnet poses problems for many patients, even those who do not have a history of claustrophobia. The study can usually be successfully completed with the help of clear communication with the patient before and during the procedure and/or with light oral sedation (e.g., lorazepam 0.25 to 0.5 mg) 30 to 60 minutes before the procedure.
4.Attire. Patients should wear a cotton hospital gown with no metal snaps. All metal items, jewelry, and nylon undergarments should be removed for reasons of safety and possible image degradation.
5.Body coil. Phased array body coils are placed on the patient’s torso over the imaging area of interest. These use several smaller coils to acquire RF signals simultaneously and to facilitate parallel imaging. Some of these coils, for example, enable performance of 3D cardiac cine examinations with full-ventricle coverage in a single breath-hold. The net result is not only better image quality but also reduced exam time for the patient.
6.Electrocardiogram (ECG) monitoring. A good electrocardiographic tracing is essential for CMR. Although three or four MRI-safe, nonmetallic electrodes are placed on the patient’s chest and a single lead signal is used to trigger or gate the MRI images, the magnetic field affects the ECG tracing by inducing a voltage created by ions flowing within blood vessels (magnetohydrodynamic effect). This voltage artifact is commonly superimposed on the ST-segment (during the ejection of blood in systole), increasing its amplitude and causing false QRS detection in certain algorithms. Use of vector cardiogram allows the R–R interval to be registered as a 3D spatial vector that varies in magnitude and direction throughout the cardiac cycle. Furthermore, the use of fiber optic cables (instead of carbon leads) has also decreased the potential ECG interference of RF pulses and/or gradient field switches.
7.Emergencies. CMR is not appropriate in patients who are clinically unstable because of difficulties monitoring and treating patients within the magnet bore. Although MRI-safe equipment is available, it is safer to prescreen the patient’s clinical status and determine the need of CMR study before initiating the scanning.
8.Pregnancy. There is insufficient evidence regarding the safety of MRI in pregnant patients. Current guidelines state that MRI may be used in pregnant patients where other forms of nonionizing imaging are inadequate or if the examination provides important information that would otherwise require exposure to ionizing radiation.
9.Children. CMRI may be necessary in pediatric patients with congenital and acquired cardiovascular disease. Typically, children younger than 8 years will require general anesthesia.
V.Clinical applications
A.Diseases of the aorta
1.Aortic aneurysm. MRI can clearly visualize both the aortic vessel wall and the lumen. It is a reliable method for the identification, characterization, and follow-up of thoracic and abdominal aortic aneurysms, with accuracy comparable to that of computed tomography (CT). A combination of spin-echo sequences for characterization of the vessel wall, gradient echo cine sequences for dynamic imaging of the aorta and aortic valve, and contrast-enhanced magnetic resonance angiography (CE-MRA) for aortic and branch vessel luminography is typically used. The aorta may be highly tortuous and should be imaged in multiple planes, with double-oblique measurements performed from true short-axis cuts using reconstructed images.
2.Aortic dissection. MRI is a highly sensitive and specific technique for the detection of aortic dissection (sensitivity 98% to 100%, specificity 98% to 100%). Spin echo, B-SSFP, and CE-MRA are used to identify the intimal flap, true and false lumens, and involvement of aortic branch vessels, including the coronary arteries. Administration of contrast is not critical to the examination, making CMR particularly helpful in patients with significant renal impairment. In addition, potential complications of aortic dissection (e.g., pleural effusion, pericardial tamponade, and aortic regurgitation) are easily evaluated. However, the longer study acquisition time with MRI compared with CT and its unsuitability for imaging of unstable patients limit its application in the acute setting. However, MRI is well suited for follow-up of both surgically and medically treated aortic dissections.
3.Intramural hematoma and penetrating aortic ulcer. Intramural hematoma can be considered as the forme fruste of aortic dissection because of the spontaneous rupture of vasa vasorum within the media of the aortic wall. It occurs in up to 30% of all acute aortic syndromes and appears as a smooth crescentic area of thickened aortic wall without evidence of blood flow in the false channel on either B-SSFP or spin-echo sequences. Because of the short T1-relaxation time of fresh blood, differentiation from the adjacent mediastinal fat may be difficult. Intramural blood can be best detected on fat-saturated T1-weighted gradient echo or black-blood techniques. Furthermore, on spin-echo (“black-blood”) imaging, the intramural hematoma may be isointense (acute) or hyperintense (subacute) relative to skeletal muscle. Penetrating aortic ulcers appear as deep ulcerations of an aortic atheroma that extend through the intima to disrupt the underlying media and cause bulging of the outer aortic contour. They commonly appear at the isthmus beyond the left subclavian artery and in the distal descending thoracic aorta near the diaphragm. If acute, there may be evidence of intramural bleeding in the rim adjacent to the ulcer.
4.Atherosclerotic disease. MRI can clearly show irregular thickening of the aorta in atherosclerotic disease. CE-MRA has good accuracy for detecting significant peripheral stenoses and occlusions.
5.Aortic trauma. MRI can detect chronic or missed aortic tears, usually related to a previous motor vehicle accident. Tears are usually found in the area of the ligamentum arteriosum and diaphragmatic hiatus and are characterized by a localized saccular aneurysm, with or without associated periaortic hematoma.
6.Aortitis. In patients with inflammatory disorders affecting the aorta such as Takayasu disease (which tends to also involve the arch branch vessels) or giant cell arteritis, MRI can accurately detect diffuse wall thickening of the thoracic and abdominal aorta (especially after gadolinium administration in T1-weighted images) as well as stenosis and occlusion of the aortic branch vessels (CE-MRA sequences). Special imaging sequences using T2-weighting and short-tau inversion recovery for fat suppression allow assessment of wall edema and wall thickening/inflammation, respectively.
7.Aortic stents and stent grafts. Aortic stents and stent grafts can be safely imaged using MRI; however, both cine sequences (gradient echo and B-SSFP) are prone to ferromagnetic artifacts. Spin-echo imaging can be used successfully to evaluate stent graft morphology. Artifacts may limit assessment for endoleak using CE-MRA.
B.Assessment of ventricular function and coronary artery disease (CAD)
1.Assessment of global ventricular function. CMR is the gold standard for the assessment of ventricular mass, volumes, and systolic function. A significant advantage of CMR is its reproducibility and accuracy compared with 2D planar or projection techniques that depend on geometric assumptions in order to define mass and volume determinations. As a result, small changes in myocardial mass and/or volume can be detected over time or because of therapy. A typical approach is to perform a short-axis stack of B-SSFP cine sequences through the left and right ventricles. Manual or semiautomated tracing of the endocardial borders at end diastole and end systole is later performed off-line, and ventricular volumes and ejection fraction are calculated.
2.Assessment of regional ventricular function. As mentioned before, development of multichannel phase-array coils has enabled parallel imaging and significant improvements in temporal resolution and scan times. This has led CMR to become superior to echocardiography for precise assessment of regional wall motion. B-SSFP cine sequences provide excellent blood-myocardial contrast that permits clear definition of the endocardial border. Furthermore, myocardial tagging methods have also improved the assessment of regional myocardial function.
3.Myocardial ischemia. CMR stress testing can detect myocardial ischemia with either wall motion or perfusion analysis. The use of dobutamine stress CMRI for wall motion analysis is relatively more established than stress perfusion imaging with adenosine or dipyridamole. Studies of stress CMR with dobutamine have revealed good sensitivity (83% to 92%) and specificity (86%) for the detection of significant CAD on a per-patient level. Furthermore, CMR tagging may further improve the accuracy of dobutamine CMR for ischemia detection. Stress perfusion imaging by CMR has been shown to have slightly higher accuracy than that of thallium single-photon emission computed tomography (SPECT), with a sensitivity of 91% and specificity of 81% for the detection of significant CAD. The absence of ionizing radiation with MRI is an important consideration, particularly in younger patients.
4.Myocardial infarction (MI) and viability. T2-weighted spin-echo sequences with fat suppression may show areas of increased signal intensity consistent with tissue edema in the acute or subacute phase of an MI. This has been gaining interest as a target for clinical trials in acute coronary syndrome. The concept is that the myocardium at risk would correspond to the edematous minus the scarred area (seen on delayed enhancement). However, this technique has potential imaging artifacts (cardiac/respiratory motion, low SNR, slow flow, coil intensity profile, etc.) which could hinder the reproducibility of results seen in single-center studies. The current state-of-the-art technique for myocardial scar detection remains delayed enhancement imaging with an inversion recovery gradient echo sequence 10 to 30 minutes after injection of a gadolinium contrast agent. This method shows areas of myocardial scarring as bright and normal myocardium as dark and has shown excellent correlation with the location and extent of scar on histopathologic analysis. The superior spatial resolution of CMR makes it more sensitive for the detection of myocardial scar, and in particular subendocardial scar, than SPECT or positron emission tomography. In addition, detection of areas of microvascular obstruction, despite adequate epicardial vessel perfusion, can also be identified with CMRI and appear to be associated with worse outcomes. The transmural extent of scar is associated with myocardial viability. Transmural or near-transmural scar (>50%) suggests nonviable myocardium, whereas the absence of myocardial scar suggests that functional recovery is likely post revascularization.
5.LV thrombus. CMR is more sensitive than echocardiography for the detection of LV thrombus. Because of its high spatial resolution and tissue characterization capabilities, CMR can be quite advantageous in establishing or ruling out the diagnosis of intracardiac thrombus. The typical signal characteristics would include lack of contrast perfusion on first pass of gadolinium and low signal intensity on postcontrast delayed imaging with long inversion time (dark filling defect on the endocardial surface of the left ventricle).
C.Nonischemic cardiomyopathies. There is increasing recognition of myocardial fibrosis occurring in a variety of conditions in the absence of ischemia, including hypertensive and diabetic heart disease, hypertrophic cardiomyopathy (HCM), and idiopathic dilated cardiomyopathy (DCM).
Several new CMR techniques have been developed for the quantification of nonischemic myocardial fibrosis, with contrast-enhanced T1 mapping using a modified Look–Locker inversion recovery sequence being the most commonly used.
1.Dilated cardiomyopathy. CMR is useful for precise assessment of cardiac morphology and function in patients with DCM. In addition, delayed enhancement imaging will typically show enhancement in a mid-myocardial distribution in case of idiopathic cardiomyopathy.
2.Hypertrophic cardiomyopathy. MRI is accurate for the evaluation of the pattern and extent of hypertrophy, systolic anterior motion of the mitral valve, resting left ventricular outflow tract (LVOT) obstruction, and secondary mitral valve pathology and regurgitation. Because of the precise anatomic definition provided by CMRI, it is particularly helpful in planning for surgical myectomy or alcohol septal ablation. CMR can also help identify abnormal chordal or papillary muscle attachments, which may contribute to LVOT obstruction and which have been reported in up to 20% of patients with HCM. Delayed enhancement is frequently seen in patients with HCM and corresponds to areas of interstitial fibrosis. It is typically seen in areas of increased wall thickness as well as right ventricular (RV) insertion points in the interventricular septum. The extent of delayed enhancement in patients with HCM has been linked to sudden cardiac death and worse outcomes.
3.Infiltrative cardiomyopathy. Infiltrative cardiomyopathy is typically characterized by normal ventricular size and systolic function, increased LV and/or RV wall thickness, severe diastolic dysfunction, and biatrial enlargement. CMR can clearly visualize the typical findings of infiltrative cardiomyopathy and help distinguish it from constrictive pericarditis, the main differential diagnosis. In addition, specific causes may be identified by CMR, particularly with the use of gadolinium enhancement imaging and based on the particular pattern of enhancement. For example, diffuse subendocardial enhancement on delayed imaging is characteristic of cardiac amyloidosis. Also, specific CMR sequences can be used for the assessment of particular disease states. For instance, hemochromatosis is characterized by extensive signal loss on T2-weighted images, because of iron deposition in the myocardium. Measurement of the T2 relaxation time of the myocardium (T2* technique) allows precise detection of the amount of iron overload. In addition, this T2* technique has been shown to be prognostically important, identifying patients with thalassemia at high risk for heart failure and arrhythmia.
4.Arrhythmogenic right ventricular dysplasia (ARVD). CMR is frequently the imaging modality of choice in patients with suspected ARVD to assess for the presence of RV dilation, global RV dysfunction, and/or regional hypokinesia. Of note, fibrofatty replacement of the RV myocardium on CMR is not a diagnostic criterion.
The CMR examination for ARVD includes, in addition to careful assessment of RV size and function, delayed enhancement imaging for identification of myocardial fibrosis.
D.Diseases of the pericardium. The normal pericardium appears on CMR as a thin (≤2 mm) curvilinear line between the epicardial and pericardial fat. The normal pericardium is of low intensity on both T1- and T2-weighted imaging sequences.
1.Pericardial effusions are typically of low intensity on T1-weighted spin-echo images and of high intensity on gradient echo images. The exception is hemorrhagic effusion, which is of high intensity on T1-weighted spin-echo images and of low intensity on gradient echo images.
2.Pericarditis and constriction. MRI can readily define the presence and extent of pericardial thickening (≥4 mm). In inflammatory pericarditis, the pericardium will typically have increased signal intensity on delayed enhancement imaging, reflecting neovascularization in the inflamed pericardium. CMR has become the imaging technique of choice for the diagnosis and management of constrictive pericarditis. Typical features include pericardial thickening and tethering, associated with conical and tubular deformity of the right and left ventricles, respectively. Secondary changes include atrial enlargement, systemic and pulmonary vein dilation, hepatomegaly, ascites, and pleural effusions. Cine sequences can demonstrate features of constrictive physiology, including diastolic septal bounce and abrupt cessation of diastolic filling. Furthermore, real-time cine sequences with free breathing can demonstrate the interventricular dependence with exaggerated septal shift toward the left ventricle during inspiration. However, it is important to note that CMR is of limited value compared with CT in the evaluation of pericardial calcification because of its inability to visualize calcium (because of its lack of hydrogen ions).
3.Congenital absence of the pericardium. This is often left sided and can be complete or partial. It can be relatively easily demonstrated on CMR as it is typically associated with a leftward orientation and “teardrop” appearance of the heart. Insinuation of lung tissue between the aorta and pulmonary artery and between the inferior surface of the heart and left hemidiaphragm is also characteristically seen.
4.Pericardial cysts. These are benign developmental lesions formed when a portion of the pericardium is pinched off during embryogenesis. Pericardial cysts are classically seen at the right cardiophrenic angle. They typically contain fluid and are well marginated. Spin-echo images demonstrate round or ovoid lesions that are often contiguous with the normal pericardium. Simple cysts demonstrate low signal intensity on T1-weighted and high signal intensity on T2-weighted images. Hemorrhagic or proteinaceous filled cysts show high signal intensity on T1-weighted images.
E.Congenital heart disease. CMR has become a crucial tool in the management and follow-up of patients with congenital heart disease, particularly in cases with complex defects. Scans can be performed safely and reliably from infancy through adulthood. CMR provides excellent anatomic definition of simple and complex heart defects and precise, noninvasive quantification of valvular function, cardiac function (both left and right ventricle), and shunts. Common applications of CMR in adult congenital heart disease include noninvasive quantification of intracardiac shunts; evaluation of pulmonary regurgitation severity, ventricular volumes and function, and pulmonary artery branch vessel stenosis in patients post tetralogy of Fallot repair; identification of RV outflow tract or branch pulmonary artery obstruction in patients who are post arterial switch for dextro transposition of the great arteries (d-TGA); evaluation of baffle stenosis or leak and RV dysfunction in patients post Mustard or Senning procedure for d-TGA; and assessment for dysfunction of the systemic ventricle in patients with congenitally corrected or levo transposition of the great arteries (l-TGA).
F.Valvular heart disease. Although echocardiography remains the primary imaging modality for the diagnosis and management of valvular heart disease, CMR can provide additional important information in select cases. Particular strengths of CMR in the evaluation of valve dysfunction include an often clearer visualization of valve morphology, valve planimetry, precise quantification of regurgitant volumes, accurate and reproducible measurement of ventricular volumes and function, and assessment of associated abnormalities (e.g., bicuspid aortic valve and ascending -aortic dilation).
G.Cardiac masses. CMR plays a major role in the evaluation of cardiac masses, mainly because of its ability to provide, in addition to excellent anatomic detail, tissue characterization. Thrombus is the most common intracardiac mass. Fresh thrombus has higher signal intensity than myocardium on T1-weighted images. Older thrombi may have increased signal intensity on T1-weighted and decreased signal intensity on T2-weighted images. Thrombi usually have low signal intensity on delayed enhancement imaging and do not demonstrate delayed enhancement even with long inversion time. Myxomas are the most common intracardiac tumor and, in addition to a heterogeneous and irregular appearance, typically have higher signal intensity than myocardium on T2-weighted spin-echo imaging. Lipomas have a distinctive short T1 and, therefore, high signal intensity on T1-weighted images. Fat saturation sequences that null lipomatous tissue would confirm the diagnosis. Fibromas are an uncommon cardiac tumor and are typically seen within the ventricular myocardium in pediatric or young adult patients. They have decreased signal intensity relative to myocardium on T2-weighted images and show rim enhancement on delayed imaging.
Primary malignant tumors of the heart are rare. Imaging findings suggestive of a malignant cardiac tumor include a right atrial location, invasiveness without regard to the anatomical borders (i.e., involvement of >1 cardiac chamber, extension into the mediastinum or great vessels), associated hemorrhagic pericardial effusion, moderate or high contrast uptake on perfusion imaging (reflecting increased vascularity), and heterogeneous delayed enhancement. The most common is angiosarcoma followed by rhabdomyosarcoma. Angiosarcomas are most commonly seen in the right atrium and have a heterogeneous appearance with hyperintense areas on T1-weighted images. Delayed hyperenhancement shows heterogeneous enhancement, most marked in the periphery of the tumor. Metastatic heart disease is more common than primary cardiac tumors and typically involves the myocardium or pericardium. One limitation of CMR, as stated before, is its reduced sensitivity for the detection of calcification in cardiac masses.
H.Pulmonary veins. Imaging of the pulmonary veins is being increasingly performed prior to and after pulmonary vein ablation, to assess pulmonary venous anatomy and patency and look for complications, particularly pulmonary vein stenosis.
VI.Future applications
A.Coronary artery assessment. Coronary imaging with CMR is usually performed with gradient echo sequences, with either fat saturation or T2 prepulses to enhance the signal difference between the coronary lumen and the surrounding myocardium, as well as to decrease the venous signal. Three-dimensional acquisition with navigator-corrected (free-breathing) data set has higher SNR when compared with 2D sequences and has become the established approach to contrast-enhanced MR coronary angiography.Although CMR can be used reliably for the detection of coronary artery anomalies, it has not yet fulfilled its early promise for noninvasive imaging of coronary atherosclerotic disease. The coronary arteries provide significant challenges to imaging by MRI because of cardiac and respiratory motion, their small size and tortuosity, normal cyclic variations in coronary flow, and competing signal from neighboring blood pools.
B.Molecular imaging. MRI shows significant promise for the selective imaging of target cells using novel molecular contrast agents. Magnetically labeled mesenchymal stem cells have been successfully tracked by MRI in pig models used for stem cell therapy in myocardial injury. Supermagnetic nanoparticles have also been used to detect atherosclerotic plaque in both animal and human studies.
C.Interventional CMR. The use of CMR in interventional procedures is appealing because it may allow for radiation-free catheterization procedures. However, its main limitation is the availability of devices that are safe to use in an MR environment.
ACKNOWLEDGMENTS: The author thanks Drs. João L. Cavalcante and Ronan Curtin for contributions to earlier editions of this chapter.
Landmark Articles
Constantine G, Shan K, Flamm SD, et al. Role of MRI in clinical cardiology. Lancet. 2004;363(9427):2162–2171.
Fratz S, Chung T, Greil GF, et al. Guidelines and protocols for cardiovascular magnetic resonance in children and adults with congenital heart disease: SCMR expert consensus group on congenital heart disease. J Cardiovasc Magn Reson. 2013;15(1):51.
Grizzard JD, Ang GB. Magnetic resonance imaging of pericardial disease and cardiac masses. Cardiol Clin. 2007;25(1):111–140, vi.
Hundley WG, Bluemke DA, Finn JP, et al. ACCF/ACR/AHA/NASCI/SCMR 2010 expert consensus document on cardiovascular magnetic resonance: a report of the American College of Cardiology Foundation Task Force on Expert Consensus Documents. J Am Coll Cardiol. 2010;55(23):2614–2662.
Kramer CM, Barkhausen J, Flamm SD, et al. Standardized cardiovascular magnetic resonance (CMR) protocols 2013 update. J Cardiovasc Magn Reson. 2013;15(1):91.
Maceira AM, Prasad SK, Hawkins PN, et al. Cardiovascular magnetic resonance and prognosis in cardiac amyloidosis. J Cardiovasc Magn Reson. 2008;10:54.
Marcus FI, McKenna WJ, Sherrill D, et al. Diagnosis of arrhythmogenic right ventricular cardiomyopathy/dysplasia: proposed modification of the task force criteria. Circulation. 2010;121(13):1533–1541.
Mewton N, Liu CY, Croisille P, et al. Assessment of myocardial fibrosis with cardiovascular magnetic resonance. J Am Coll Cardiol. 2011;57(8):891–903.
Nandalur KR, Dwamena BA, Choudhri AF, et al. Diagnostic performance of stress cardiac magnetic resonance imaging in the detection of coronary artery disease: a meta-analysis. J Am Coll Cardiol. 2007;50(14):1343–1353.
Rickers C, Wilke NM, Jerosch-Herold M, et al. Utility of cardiac magnetic resonance imaging in the diagnosis of hypertrophic cardiomyopathy. Circulation. 2005;112(6):855–861.
Ridgway JP. Cardiovascular magnetic resonance physics for clinicians: part I. J Cardiovasc Magn Reson. 2010;12:71.
Sakuma H, Ichikawa Y, Chino S, et al. Detection of coronary artery stenosis with whole-heart coronary magnetic resonance angiography. J Am Coll Cardiol. 2006;48(10):1946–1950.
Schulz-Menger J, Bluemke DA, Bremerich J, et al. Standardized image interpretation and post processing in cardiovascular magnetic resonance: Society for Cardiovascular Magnetic Resonance (SCMR) Board of Trustees Task Force on Standardized Post Processing. J Cardiovasc Magn Reson. 2013;15(1):35.
Thomson LE, Kim RJ, Judd RM. Magnetic resonance imaging for the assessment of myocardial viability. J Magn Reson Imaging. 2004;19(6):771–788.
Weber OM, Higgins CB. MR evaluation of cardiovascular physiology in congenital heart disease: flow and function. J Cardiovasc Magn Reson. 2006;8(4):607–617.
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