Applications of Cardiovascular Magnetic Resonance and Computed Tomography in Cardiovascular Diagnosis

Imaging Principles and Approaches

CMR imaging is performed using static and dynamic magnetic fields and does not use any ionizing radiation. Images are generated from induced radiofrequency signals arising from water and fat protons in the body. Differences in proton density, magnetic relaxation times (longitudinal relaxation time [T ₁ ] or transverse relaxation time [T ₂ ]), blood flow, and other parameters produce intrinsic signal contrast among tissues. CMR approaches can be broadly classified into spin-echo (black blood) and gradient-echo (bright blood) and the derived balanced steady-state free-precession (b-SSFP) sequences for cine acquisitions, often modified with prepulses. Spin-echo imaging is particularly useful for defining anatomic structure and tissue characterization (e.g., fat replacement or iron deposition). Gradient-echo techniques can produce single-shot (displaying a single phase during the cardiac cycle) or cine (displaying multiple phases at one level during the cardiac cycle) images, with an improved signal-to-noise ratio (SNR) using b-SSFP sequences. Cine images demonstrate motion of structures (such as cardiac chambers and valves) during the cardiac cycle, permitting qualitative and quantitative assessment of motion. Both spin-echo and gradient-echo CMR techniques are flow-sensitive. Because of the inherent contrast between the blood pool and surrounding tissue, administration of an exogenous CMR contrast agent is not required for general evaluation of cardiac anatomy. Although the U.S. Food and Drug Administration approved it for angiography (e.g., contrast-enhanced magnetic resonance angiography [CE-MRA]), but not for cardiac applications, the administration of gadolinium chelates as extracellular magnetic resonance (MR)-specific intravenous contrast agent enables certain applications, such as first-pass assessment of myocardial perfusion and late gadolinium enhancement (LGE) for fibrosis and scar identification. Gadolinium induces T ₁ shortening, which is detected as an increased signal in T ₁ -weighted images, although the signal enhancement is not linearly related to contrast agent concentration. Diverse techniques are under development to quantify magnetization dynamics before and/or after gadolinium administration (T ₁ mapping), allowing improved cardiac tissue characterization. Flow-velocity encoding (also known as phase contrast ) is an additional CMR modality that enables volumetric quantitation of blood flow through arteries, veins, conduits, and valves. This method enables determination of regurgitant volumes and shunt flows.

CCT uses ionizing radiation to generate images based on the attenuation of the body's tissues. Today, CCT is typically performed using third-generation multislice scanners. These CCT units are arranged with the radiation tube opposite a series of detectors attached to a gantry that rapidly rotates as the patient is advanced through the scanner. The detector arrays typically obtain 64, dual-64, 256, or 320 axial slices during a single gantry rotation. Higher numbers of slices allow for greater coverage with each gantry rotation, allowing for shorter imaging times, less iodinated contrast, and potentially reduced radiation exposure. In the helical mode, data are acquired in a helical path as the patient is advanced through the scanner. The speed at which the patient is advanced through the scanner is called the pitch . A high pitch (faster speed) is associated with a lower radiation exposure, whereas a low pitch (slower speed) is associated with a higher spatial resolution, but also higher radiation exposure. Cardiac imaging is generally performed with a relatively low pitch. The radiation exposure can be reduced by electrocardiogram (ECG) dose modulation, which varies the intensity of radiation exposure during the cardiac cycle.

Challenges of Cardiac Imaging

CMR and CCT have faced similar challenges posed by cardiac and respiratory motion, and requirements for high temporal, spatial, and contrast resolution. The acquisition of most cardiac images requires electrocardiographic gating or triggering during a specific portion of the cardiac cycle with data acquired from successive heartbeats. Thus, imaging is usually best when performed in patients with a regular sinus rhythm. The duration of image acquisition with CCT is generally fewer than 10 to 15 seconds, allowing a single breath-hold for suppression of respiratory motion. Although breath-holding is used for many CMR acquisitions, free breathing, navigator gating is often used for longer image acquisitions or in situations in which patients cannot sustain a breath-hold. Navigators are a CMR technique that identifies a “signal interface” such as that between the lung and the diaphragm. Most commonly, the dome of the right hemidiaphragm is used for this purpose. Real-time navigator data about the position of the lung/diaphragm interface can then be used for respiratory gating.

High temporal resolution is needed to take advantage of these gating techniques. CMR images can be acquired with a variable temporal resolution at the expense of increased acquisition time. Typical cine temporal resolutions of less than 40 ms are acquired. The temporal resolution of CCT is limited by gantry rotation speed, with typical gantry rotation speeds of 330 to 400 ms per rotation. The use of half-cycle reconstruction allows for an image to be acquired in one half-rotation, with an effective temporal resolution of 165 to 200 ms. Intravenous or oral β-blockers are typically administered for coronary artery CCT to prolong the period of diastasis during which the coronary artery can best be imaged. The temporal resolution of CCT can be improved to approximately 83 ms with dual-source technology, in which two sets of radiation tubes and detectors are mounted on the gantry. Each set acquires data over one quarter-rotation and the data are combined to form a single image. Multicycle reconstruction can also be used to improve the temporal resolution of CCT to approximately 40 ms by acquiring data for a single image over multiple cardiac cycles, but this is infrequently used because of the requirement for a very low pitch and a resultant very high radiation exposure.

The development of high spatial resolution has also been important for CMR and CCT, particularly for coronary artery imaging. Spatial resolution of CMR is also variable, but again at the expense of increased acquisition time and loss of SNR. The latter can be somewhat mitigated by imaging at higher field strengths (e.g., 3 T). Typical in-plane spatial resolutions of 1 to 2 mm are used with 3- to 8-mm slice thickness, whereas the spatial resolution of CCT has improved with the development of smaller detectors. Routine CCT generally has a higher spatial resolution compared with CMR, with CCT isotropic spatial resolution of 0.5 to 0.6 mm. Image contrast for CMR is created using specific imaging sequences and prepulses. The use of an exogenous contrast agent (e.g., gadolinium) is required for specific applications, as noted before. CCT does not have the inherent contrast between tissues of CMR. Thus, iodinated contrast is required for the vast majority of CCT imaging. The timing of iodinated contrast administration relative to image acquisition is critical, with imaging typically performed during passage of iodinated contrast in the ascending aorta and coronary arteries. The correct timing is determined by a small timing bolus or by automated detection of contrast appearance in the ascending or descending aorta. A similar process is used for CE-MRA of the aorta and pulmonary veins. LGE imaging also uses specific timing parameters but is overall less time-sensitive.

Imaging Comparisons

The advantages and limitations of CMR and CCT complement those of other imaging techniques such as echocardiography, fluoroscopic radiographic angiography, and radionuclide imaging. Compared with echocardiography and radionuclide imaging, CMR and CCT offer superior anatomic scope and spatial resolution. CMR is the noninvasive, nonionizing reference standard for evaluation of volumetric left ventricular (LV) and right ventricular (RV) cavity size, systolic function, and LV mass, providing highly reproducible measures for noninvasive follow-up of disease processes. CCT measures of LV cavity size and systolic function compare favorably with CMR. In contrast to echocardiography and nuclear imaging, CMR permits unrestricted image acquisition orientation, which can be readily adjusted to particular patient and study requirements. Although CCT acquisitions are always in the axial plane, high isotropic spatial resolution facilitates post-processing reconstruction in any desired orientation. There is also relatively advanced post-processing software for CCT. In contrast, echocardiography offers the advantages of portability, lower cost, lack of ionizing radiation (like CMR), widespread availability, greater ease of patient monitoring, and greater sensitivity for structures with chaotic motion, such as vegetations. Further comparisons among CMR, CCT, and other techniques will be made in later sections dealing with specific types of examinations.

Imaging Precautions

Precautions generally applicable to body magnetic resonance imaging (MRI) are applicable to CMR. Before imaging, all patients must undergo detailed screening for any potential contraindications to CMR. In addition to general concerns of metallic implants and severe claustrophobia, patients should be screened for the presence of any incompatible material. Excluded devices include some that are relatively common among those with cardiovascular disease, such as pacemakers, unconnected or retained permanent pacing leads, and implantable cardioverter-defibrillators. Protocols for scanning of non–pacemaker-dependent patients with modern (implanted after 2000) pacemakers or implantable cardioverter-defibrillators have been reported. In addition, special MR conditional pacemakers are now available and should be considered for patients likely to need future MR studies. Bioprosthetic and mechanical heart valves, sternotomy wires, thoracic vascular clips, and intracoronary stents are generally considered CMR-safe at field strengths up to 3 T (see www.mrisafety.com ), although they may produce local artifacts that reduce image quality. Because of bulk cardiac motion during systole and diastole, most CMR protocols require ECG triggering with images composed from data collected during multiple successive cardiac cycles. Despite this, good functional image quality can usually be obtained among patients with atrial fibrillation, although image quality may be impaired among subjects with frequent premature beats. Among patients with irregular rhythms, non-ECG gated real-time imaging CMR (which permits real-time image acquisition analogous to two-dimensional echocardiography but at lower spatial and temporal resolutions than that attained with a gated CMR technique) can provide useful information. Arrhythmias and tachycardia will also lead to suboptimal CCT data image quality. Compared with CMR, where the sequences are adapted for the patient's heart rate, CCT is restricted by gantry rotational speed and often requires β-blockers to slow the heart rate to 60 beats/min or less for coronary studies. All subjects require appropriate monitoring during their imaging studies. Basic monitoring modalities include ECG monitoring for rate and rhythm (pulsatile blood in the magnetic field distorts the ST segment appearance, rendering the ST wave uninterpretable during CMR), intercom voice contact, and visualization (by direct view, camera, or both). For patients requiring a greater intensity of monitoring, automated cuff blood pressure monitoring and pulse oximetry can be added, especially for hemodynamically unstable patients and for stress CMR.

As noted earlier, CMR often does not require an exogenous contrast agent. When needed, however, gadolinium-containing CMR contrast agents have a much more favorable safety profile in regards to both nephrotoxicity and anaphylaxis compared with the iodinated agents used in CCT. Administration of gadolinium contrast to patients with severe renal dysfunction can result in nephrogenic systemic fibrosis, a rare but severe scleroderma-like disorder that can result in death. Patients at high risk of renal dysfunction are screened with a Choyke questionnaire to identify those at increased risk for renal dysfunction. For these patients, a determination of the estimated glomerular filtration rate (eGFR) is required. Patients with mild renal impairment (eGFR 30-60 mL/kg per 1.73m ² ) can be safely imaged with a reduced dose of contrast agent. Noncontrast CMR or alternative imaging modalities should be considered for patients with severe renal dysfunction (eGFR < 20 mL/kg per 1.73m ² ), especially those on dialysis.

Because of the general requirement for iodinated contrast, CCT must be performed with caution in patients with renal insufficiency, as in patients with diabetes and use of nonsteroidal antiinflammatory drugs—factors associated with an increased risk of contrast-induced nephropathy. Alternative noninvasive imaging methods are preferable in the setting of moderate or greater renal insufficiency, unless dialysis has already been instituted. Proper hydration with saline solution is recommended, and bicarbonate solution and N -acetylcysteine may reduce the incidence of acute renal failure because of iodinated contrast, despite significant heterogeneity among studies.

Radiation exposure is a significant consideration in the use of CCT, especially for younger patients who are more likely to have numerous CT scans during their lifetime and have increased susceptibility to the harmful effects of ionizing radiation. With the technology moving from four-slice to 320-detector scanners, a significant reduction in radiation has been achieved. Dose reduction technologies including lower tube voltage, prospective gating, and dose modulation permit further reduction in the effective dose from 12 to 20 mSv to 4 to 7 mSv without compromising diagnostic accuracy.

Clinical Applications

Diseases of the Thoracic Aorta

Both CMR and CCT are widely used clinically for the assessment of the thoracic aorta for aneurysms and dissection. With CMR, the structure of the aorta is delineated by a combination of the following protocol components in the transverse, coronal, sagittal, and oblique planes: (1) ECG-gated spin-echo imaging, which reveals the aortic wall with rapidly flowing blood appearing black and thrombuslike, and slowly moving blood appearing gray; (2) ECG-gated SSFP imaging with bright-blood images in single-shot and cine acquisitions; and (3) three-dimensional CE-MRA using a gradient echo acquisition. Temporally resolved CE-MRA is particularly useful to minimize motion artifacts that would otherwise result in nondiagnostic or false-positive results. With CCT, imaging of the aorta involves iodinated contrast with a larger image acquisition volume to include the entire thoracic aorta and can be performed with or without ECG gating. ECG gating is preferred for the CCT evaluation of dissection to avoid motion artifacts that can mimic a dissection flap.

Aortic Aneurysm

Cardiovascular magnetic resonance and CCT are both superior methods for identification of true and false thoracic aortic aneurysms. In true aneurysms, the aneurysmal aortic wall is composed of intima, media, and adventitia. False aneurysms represent a contained rupture of the intima and media, with only the adventitia and periadventitial connective tissue limiting the hemorrhage ( Fig. 53-1 ). False aneurysms generally have a narrow “neck” or communication with the main aortic lumen. True aneurysms are more commonly fusiform (bulge aligned along the long axis of the aorta; Fig. 53-2 ) than saccular (sacklike bulge extending from a side of the aortic wall). CE-MRA reveals the presence and extent of these lesions as well as any associated thrombus. CCT is recommended as the imaging modality of choice for most patients in the assessment of acute disease, whereas three-dimensional CE-MRA is preferred for most patients with chronic disease requiring long-term monitoring. As with aortic dissection, advantages of CMR and CCT assessment compared with radiographic angiography include the capability to evaluate for associated complications such as hemopericardium and LV dysfunction. CMR can also assess for the presence of associated aortic regurgitation. After composite graft replacement of the ascending aorta, CMR and CCT are useful for detection of postoperative complications, such as leakage or hematoma formation, with CMR offering the option for noncontrast methods.

Aortic Dissection

Cardiovascular magnetic resonance and CTT, along with transesophageal echocardiography (TEE), are the primary methods used to diagnose patients with acute aortic dissection. Because each of these imaging modalities has high diagnostic accuracy for dissection, the selection among these methods is generally governed by patient condition, institutional access, and local expertise. In a meta-analysis, the three modalities showed high specificities and sensitivities in a setting of high pretest probability (>40%), but the pooled positive likelihood ratio was highest for MRI. CMR and CCT provide information regarding involvement of major branch vessels and all segments of the aorta, unlike TEE, which is limited to the thoracic aorta and by the adequacy of acoustic windows (particularly for the segment of ascending aorta anterior to the trachea). All three methods provide useful information regarding pericardial involvement. CMR and TEE can also assess aortic valve integrity. The main disadvantages of CMR in the acute setting are potential obstacles to continuous monitoring and care of an unstable patient during transport and performance of the study and the requirement that the patient remain motionless during the examination. CCT is much faster and is frequently available in the emergency department. Thus, CCT is the most common initial imaging modality chosen to diagnose acute aortic dissection, whereas CMR is considered the imaging procedure of choice for serial monitoring of the medically or surgically treated patient with dissection according to the Recommendations of the Task Force on Aortic Dissection of the European Society of Cardiology (which have been endorsed by the American College of Cardiology) because of the lack of radiation exposure. Follow-up is recommended after hospital discharge at 1, 3, 6, and 12 months, and yearly thereafter, ideally using the same modality. Accurate interpretation of postoperative images requires knowledge of the surgical procedure and the expected range of routine postoperative sequelae including thickening around the graft and presence of thrombus outside the graft and within the native aortic wrap.

CCT aortic assessment is typically completed in less than 1 minute, and often in less than 30 seconds, whereas CMR aortic assessment may require up to 20 minutes to complete. Both can display the location and extent of dissection identified as an intimal flap separating true and false aortic lumina, along with sites of intraluminal communication, and can readily assess involvement of the aortic root, arch vessels, and renal arteries ( Fig. 53-3 ). CMR spin-echo images may identify relatively bright regions within the true or false lumen attributable to stagnant blood flow or thrombus. Cine SSFP images demonstrate flap motion and blood flow in the true and false lumen. Three-dimensional CE-MRA is highly sensitive for dissection and can be implemented with subsecond temporal resolution to obviate the need for a breath-hold ( Fig. 53-4 ). Alternatively, three-dimensional free-breathing SSFP imaging without exogenous contrast can be accomplished with accuracy similar to CE-MRA (see Fig. 53- 2 ). LV function and involvement of the proximal coronary arteries can be assessed by CCT with a gated acquisition or using CMR with coronary imaging. Importantly, CMR can also assess the presence of coexistent aortic valve involvement (using cine imaging of the LV outflow tract) and the severity of aortic regurgitation (using a phase velocity–encoding acquisition at the base of the aortic root).

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