Magnetic Resonance Imaging of Coronary Arteries: Technique

Despite significant efforts in prevention and treatment, coronary artery disease (CAD) remains the leading cause of death in the United States, accounting for one in every seven deaths. Each year nearly 700,000 Americans are estimated to have a new myocardial infarction (MI), and nearly 325,000 to have a recurrent infarction. Furthermore, an additional estimated 165,000 will have their first silent MI. The current clinical “gold standard” for the diagnosis of significant (≥50% diameter stenosis) CAD is catheter-based invasive x-ray angiography. More than a million catheter-based x-ray coronary angiograms are performed annually in the United States, with a higher volume in Europe. However, these invasive tests have a relatively low yield, with less than 40% of patients referred for x-ray coronary angiography having obstructive CAD, unnecessarily exposing these patients to the potential risks and complications of an invasive test that includes ionizing radiation and iodinated contrast. To relieve symptoms or decrease pharmaceutical use, percutaneous coronary intervention in single vessel disease is commonly performed, but the greatest impact on mortality occurs with mechanical intervention among patients with left main (LM) and multivessel CAD. Thus alternative noninvasive imaging modalities, which allow direct visualization of the proximal and mid native coronary vessels for the accurate identification/exclusion of LM/multivessel CAD, are desirable.

Cardiovascular magnetic resonance (CMR) imaging is a very promising noninvasive tool for comprehensive early risk assessment, guidance of therapy, and treatment monitoring of CAD. CMR is considered the gold standard for the assessment of cardiac anatomy, biventricular systolic function, myocardial viability (late gadolinium enhancement [LGE]) and rest/stress myocardial perfusion. Clinical research studies also have demonstrated its usefulness for quantitative myocardial tissue characterization (T ₁ and T ₂ relaxation time mapping), and its ability to differentiate between healthy and diseased tissue. Coronary magnetic resonance imaging (MRI) is a noninvasive diagnosis alternative to catheter-based x-ray angiography among patients with suspected anomalous coronary artery disease and coronary artery aneurysms. Although coronary multi-detector computed tomography (MDCT) offers superior isotropic spatial resolution and more rapid imaging, coronary MRI has advantages over MDCT in several respects, including the absence of ionizing radiation or iodinated contrast, which facilitates follow-up scanning, as well as smaller artifacts related to epicardial calcium. Because of the advantages of coronary MRI and its diagnostic accuracy, coronary MRI is recommended and deemed appropriate in patients suspected of anomalous coronary artery disease by both the American College of Cardiology and American Heart Association. However, CMR assessment of coronary lumen integrity and plaque burden/activity remains challenging. Nevertheless, CMR has shown great potential for coronary lumen, plaque (with and without contrast agents) and thrombus/hemorrhage visualization. The combination of these techniques could add invaluable prognostic information for patients at risk or with known CAD. Thus technical developments are being investigated to allow coronary MRI to achieve similar diagnostic lumen accuracy as the current clinical gold standard, as well as plaque characterization. Main technical challenges include suboptimal spatial resolution (as a result of long acquisition times required), and coronary motion suppression with unpredictable scan times (depending largely on the breathing pattern of the subject). In this chapter, we will review the technical imaging strategies for MRI of coronary arteries, coronary vessel walls, and coronary veins. The clinical results of coronary MRI are addressed in Chapter 24 .

Coronary Magnetic Resonance Imaging

The early approaches to coronary artery MRI were based on two-dimensional (2D) breath-hold electrocardiogram (ECG)-triggered segmented sequences. Subsequently, three-dimensional (3D) free-breathing approaches have replaced 2D breath-hold approaches, enabling greater anatomical coverage and higher signal-to-noise ratio (SNR). Three-dimensional coronary MRI can be acquired using a targeted or whole-heart coverage of the coronary anatomies. In the targeted technique, a double-oblique 3D volume aligned along the major axis of the left or right coronary artery (RCA) is acquired. For the visualization of the RCA, the imaging plane passing through the proximal, mid, and distal coordinates of the RCA is identified and the targeted 3D coronary sequence is acquired in this orientation, typically with a 30-mm slab with 20 slices, using a segmented acquisition. For the imaging of the left main (LM), left anterior descending (LAD), and left circumflex (LCX) coronary arteries, a 3D volume is interactively prescribed in the axial plane centered about the LM coronary artery ( Fig. 23.1 ). In the whole-heart coronary MRI technique, an axial (or coronal) 3D volume encompassing the entire heart is sampled in a single acquisition, in a manner analogous to coronary MDCT. This facilitates the imaging setup via simpler slab prescription, and provides a more complete anatomical coverage, positioned ~1 cm above the LM and extending to the inferior cardiac border. However, based on single-center trials to date, it has not been shown to be superior to the targeted approach for CAD assessment ( Table 23.1 ).

TABLE 23.1

Single-Center Echocardiogram-Triggered, Free-Breathing, Targeted Three-Dimensional and Whole-Heart Coronary Magnetic Resonance Imaging With and Without Contrast Agents

Study	Single-Center/Multicenter	No. of Patients	Sensitivity	Specificity
Noncontrast 3D Targeted Coronary MRI
Kim	Multicenter	109	88%–98%	32%–52%
Bunce	Single-center	46	50%–89%	72%–100%
Sommer	Single-center	107	74%–88%	63%–91%
Bogaert	Single-center	21	85%–92%	50%–83%
Noncontrast 3D Whole-Heart Coronary MRI
Jahnke	Single-center	21	79%	91%
Sakuma	Single-center	39	82%	91%
Sakuma	Single-center	131	82%	90%
Pouleur	Single-center	77	100%	72%
Kato	Multicenter	138	88%	72%
Contrast Enhanced 3D Whole-Heart Coronary MRI
Yang	Single-center	62	94%	82%
Yang	Multicenter	272	91%	80%

3D , Three-dimensional; MRI , magnetic resonance imaging.

Targeted thin-slab 3D acquisitions have been acquired using both gradient recalled echo (GRE) and balanced steady-state free precession (bSSFP) sequences. A thin-slab 3D targeted acquisition with a GRE sequence results in more homogenous blood pool signal, but is heavily dependent on the inflow of unsaturated protons. Saturation effects will cause a local signal loss if coronary artery flow is slow or stagnant. This signal loss is often relatively exaggerated, as compared with the lumen stenosis. Compared with GRE sequences, bSSFP provides intrinsically higher SNR because of its balanced gradients and improved blood-myocardium contrast attributed to its T ₁ /T ₂ weighting, with reduced sensitivity to inflow effects. Both GRE and bSSFP have been used for targeted 3D coronary MRI, where both have shown similar diagnostic accuracy for CAD. For whole-heart noncontrast coronary MRI at 1.5 T, SSFP appears to be the sequence of choice as a result of its higher blood–myocardium contrast and superior inflow properties.

Even with these technical advances, clinical acceptance of coronary MRI remains challenging because of coronary artery motion, long scan times, limited spatial resolution, suboptimal SNR and blood–myocardium contrast-to-noise-ratio (CNR). The technical challenges in coronary artery MRI are different from other CMR acquisitions as a result of unique issues including the coronary artery: (1) small caliber (3–6 mm diameter), (2) high level of tortuosity, (3) near-constant motion during both the respiratory and the cardiac cycles, and (4) surrounding signal from adjacent epicardial fat and myocardium.

Cardiac-Induced Motion

Bulk epicardial coronary artery motion is a major impediment to coronary CMR, and it can be separated into motion related to direct cardiac contraction/relaxation during the cardiac cycle and motion attributed to superimposed diaphragmatic and chest wall movement during respiration. The magnitude of motion from each component may greatly exceed the coronary artery diameter, leading to blurring artifacts in the absence of motion–suppressive methods.

To compensate for bulk cardiac motion, accurate external ECG synchronization with QRS detection is required, and vector ECG approaches are preferred. Coronary artery motion during the cardiac cycle has been characterized using both catheter based x-ray angiography and CMR. Both the proximal/mid RCA and the LAD display a triphasic pattern, with the magnitude of in-plane motion nearly twice as great for the RCA. Coronary artery motion is minimal during isovolumic relaxation, approximately 350 to 400 ms after the R wave, and again at mid diastole (immediately before atrial systole). The duration of LAD diastasis is longer than that of the RCA, and it begins earlier in the cardiac cycle. The duration of the mid diastolic diastasis period is inversely related to the heart rate and dictates the preferred coronary artery data acquisition interval.

As compared with MDCT in which the acquisition is constrained by gantry rotation, for coronary artery MRI the acquisition interval is adapted to the heart rate/diastasis interval using a patient-specific diastasis period. This can be readily identified by the acquisition of high temporal resolution cine dataset orthogonal to the long axis of the proximal/mid RCA and of the LAD. Semiautomated tools to identify the optimal data acquisition window have also been proposed. For patients with a heart rate of 60 to 70 beats per minute, a coronary artery MRI acquisition duration of ~80 ms during each cardiac cycle results in improved image quality. The duration must be further abbreviated (e.g., <50 ms) at higher heart rates, whereas with bradycardia, the acquisition interval can be expanded to 120 ms or longer. The use of patient-specific acquisition windows serves to reduce overall scan time. Image degradation can be caused by sinus arrhythmia, leading to heart rate variability, which is common especially in younger adults. An adaptive real-time arrhythmia rejection algorithm can correct for heart rate variability and improves coronary artery MRI quality.

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