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

To compensate for bulk cardiac motion, accurate electrocardiographic (ECG) synchronization with QRS detection is required, and vector ECG approaches are preferred [42]. Coronary motion has been characterized using both catheter based x-ray angiography [43, 44] and CMR [45–47] methods during the cardiac cycle. Both the proximal/mid right coronary artery (RCA) and the left anterior descending (LAD) coronary artery display a triphasic pattern (Fig. 17.2), with the magnitude of in-plane motion nearly twice as great for the RCA. Coronary motion is minimal during isovolumic relaxation, approximately 350–400 ms after the R wave, and again at mid-diastole (immediately prior to atrial systole). The LAD diastasis is longer than the RCA, and begins earlier in the cardiac cycle [12]. The duration of the mid-diastolic diastasis period is inversely related to the heart rate and dictates the coronary data acquisition interval.

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. Semi-automated tools to identify the optimal data acquisition window have also been proposed [48, 49]. For patients with a heart rate of 60–70/min, a coronary artery MRI acquisition duration of ~80 ms during each cardiac cycle results in improved image quality [14]. The duration must be further abbreviated (e.g., <50 ms) at higher heart rates, while 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 [50, 51]. Image degradation can be caused by sinus arrhythmia, leading to heart rate variability, which is common especially in younger adults [52]. An adaptive real-time arrhythmia rejection algorithm can correct for heart rate variability, and improves coronary artery MRI quality [49].

The second major challenge for coronary artery MRI is compensation for respiratory motion. With inspiration, the diaphragm may descend up to 30 mm and the chest wall expands – resulting in an inferior displacement and anterior rotation of the heart [53]. Several approaches have been proposed to minimize respiratory motion artifacts, including sustained end-expiratory breath-holding, chest wall bellows, respiratory navigators, fat navigators and self-gating methods.

Prolonged (15–20 s) end-expiratory breath holds were utilized to suppress respiratory motion in initial 2D coronary artery MRI methods [54]. Breath holding offers the advantage of relative ease of implementation in compliant subjects, but it limits the temporal acquisition window, image spatial resolution and anatomic coverage. Additionally, many patients are unable to adequately sustain a breath-hold. Furthermore, slice registration errors (due to variability in end-expiratory diaphragmatic position) are very common as is diaphragmatic drift during the breath hold [54–57] and may occur in up to half of patients [12]. Supplemental oxygen and hyperventilation (separately or in combination) can be utilized to prolong the breath-hold duration [56, 57], but these methods may not be appropriate for all patients, and both diaphragmatic drift and slice registration errors persist [57].

Diaphragmatic respiratory navigators, first proposed by Ehman [58] for abdominal MR imaging, enable free-breathing acquisitions without the stringent time constraints and patient cooperation requirements imposed by multiple breath holds, and thus offer superior spatial resolution opportunities. Although the specifics of navigator implementation varies among CMR vendors, in the ideal implementation, the navigator can be positioned at any interface that accurately reflects respiratory motion, including the dome of the right hemidiaphragm (Fig. 17.3) [59, 60], the left hemidiaphragm, the anterior chest wall, the anterior free wall of the left ventricle [60, 61], or even through the coronary artery of interest. The navigator should not cause an image artifact and should be temporally located immediately preceding the imaging portion of the sequence with data accepted (used for image reconstruction) only when the navigator indicates that the “interface” (e.g., diaphragm position) falls within a user-defined window. The dome of the right hemidiaphragm has become the preferred location [27, 38–40] due to the simplicity and ease in set-up, where the motion of the right hemidiaphgragm in the superior-inferior direction can be tracked. From CMR studies of cardiac border position during the respiratory cycle, Wang observed that at end-expiration, the ratio between cardiac and diaphragmatic displacement is ~0.6 for the RCA and ~0.7 for the left coronary artery [53] though there is variability among subjects [20, 42, 43] and position (e.g., supine vs. prone imaging) [23]. This rule-of-thumb offers the opportunity for prospective navigator gating with real-time tracking [60, 62], in which the position of the interface (diaphragm) is determined, and the slice position coordinates can then be shifted in real-time (before the data collection) to appropriately adjust spatial coordinates [63]. This technique allows for the use of wider gating windows and increased navigator efficiency, leading to shorter scan times. Real-time tracking implementation with a 5-mm diaphragmatic gating window is often used resulting in a navigator efficiency approaching 50 % [62]. Coronary artery MRI with real-time navigator tracking has been shown to minimize registration errors (as compared with breath holding) while maintaining or improving the image quality [12, 62]. It should also be noted that the quality of coronary artery MRI is improved by using consistent ECG timing as well as respiratory suppression methodology for both the coronary localizing/motion scout images and for the coronary artery MRI acquisitions [51].

A number of refinements to the navigator method have been proposed. While a “fixed” superior-inferior correction factor of 0.6 (with no left-right or anterior-posterior correction) [44, 45] is commonly used, significant individual variability has been observed [55]. A subject-specific tracking factor has been advocated and shown to improve the quality of coronary images when the subject-specific tracking factor differs from 0.6 [64]. The use of multiple navigator locations, use of leading and trailing navigators, and navigators that provide guidance for affine transformations, i.e. 3D translations and rotations, of the slice prescription for each heart-beat have been proposed [23, 65–67]. The affine transformation permits use of larger navigator windows, and hence higher navigator efficiency. It has also been proposed that the heart itself be tracked [20, 68–70]. For instance, methods that track the epicardial fat to detect the heart position have been proposed [71–74]. In addition to navigator gating, respiratory self-gating techniques have been investigated, a method that derive the respiratory position of the heart from the imaging data itself [19, 20, 28, 68, 69, 75, 76], thus avoiding certain issues with navigators such as subject-dependent tracking factor [77] and hysteresis effects [78]. Navigator with fixed scan efficiency has also been recently introduced which results in imaging at a fixed scan time [79]. Novel k-space trajectories and various image reconstruction based method such as cross-correlation of low resolution images have also been proposed for respiratory motion compensation [28, 80–82].

The coronary arteries are surrounded by epicardial fat and the myocardium. Thus contrast-to-noise ratio (CNR) can be improved by suppressing the fat and myocardium signal surrounding the coronary arteries. Frequency (spectrally) selective pre-pulses are applied to saturate signal from fat tissue, thereby allowing visualization of the underlying coronary arteries [54, 83]. To differentiate myocardium and the coronary lumen, endogenous contrast preparation techniques are commonly utilized [14, 83–86]. Two methods that can enhance the contrast between the coronary lumen and underlying myocardium are T₂ preparation pre-pulses [14, 84, 85, 87] and magnetization transfer contrast [83, 86]. The former is often used for coronary artery MRI as it also suppresses deoxygenated venous blood, while the latter is used for coronary vein MRI [57].

The limited signal-to-noise ratio (SNR) in coronary artery MRI, along with constraints on acquisition duration, restricts the spatial resolution in the acquisition. Spatial resolution requirements for clinical coronary artery MRI depend on whether the goal is to identify the origin and proximal course of the coronary artery (e.g., issues of anomalous coronary disease), or whether the goal is to identify focal stenoses in the proximal and middle segments.

The SNR of coronary MRI can be enhanced by higher B₀ field strength [88], larger 3D spatial coverage [29], vasodilator administration, and contrast agents based on gadolinium chelates. The intrinsically higher SNR associated with higher magnetic field strengths may be advantageous for non-contrast coronary MRI. However, additional considerations, such as higher B₁ and B₀ inhomogeneity and higher specific absorption rate, affect certain aspects of coronary MRI, such as the diminished utility of SSFP sequences at 3 T due to increased field inhomogeneity and high specific absorption rate. Hence, GRE sequences, which are less sensitive to field inhomogeneity, as well as localized shimming [89] and contrast preparation techniques that deal with B₁ inhomogeneities [87, 90] have been advocated.

The increased coverage of whole-heart coronary MRI can potentially improve the SNR but this also increases the scan time. Thus, the SNR gain is often counteracted by the need for accelerated imaging to reduce scan time, which carries an SNR penalty. Despite this penalty, excellent image quality of whole-heart coronary MRI has been shown in several studies [26, 29], and an example from a single-center study [26] is depicted in Fig. 17.4. Furthermore, whole-heart imaging suffers from saturation effects of the inflowing blood magnetization [39]. Another technique to improve SNR in coronary MRI is the administration of vasodilators, since the increased coronary blood flow secondary to vasodilatation reduces the inflow saturation effects [91, 92]. Figure 17.5 demonstrates impact of sublingual isosorbide dinitrate administration on 3D targeted coronary MRI up to 30 min after drug administration in terms of subjective image quality and objective SNR and vessel sharpness.

The administration of exogenous gadolinium contrast agents (both extracellular [16, 93, 94] and intravascular [95–100]) that shorten the T₁ relaxation time provides an alternative flow-independent approach to improve SNR and CNR. Since conventional extracellular contrast agents, e.g. gadopentetate dimeglumine (Gd-DTPA), diffuse rapidly into the interstitial space, early contrast-enhanced coronary MRI studies focused on breath-hold technique to take advantage of the first-passage of these agents [94]. However, both the breath-hold and first-pass aspects of such approaches limit the spatial resolution and is unsuitable for whole-heart coronary acquisitions [29]. Following the availability of a high relaxivity extracellular contrast agent, gadobenate dimeglumine (Gd-BOPTA; MultiHance; Bracco Imaging SpA, Milan, Italy), improved whole-heart coronary MRI at 3 T was shown to be feasible using a T₁-weighted inversion recovery (IR) GRE sequence with a slow infusion of this Gd-BOPTA [16]. An example of contrast-enhanced whole-heart coronary MR image from a CAD patient and the corresponding x-ray angiogram are shown in Fig. 17.6, demonstrating agreement between two modalities in detecting significant stenosis. In a single center trial, 3 T whole-heart coronary MRA with slow infusion of Gd-BOPTA had 93 % sensitivity, 89 % specificity and 90 % accuracy for detecting >50 % diameter stenosis on a per-vessel basis (and 94 %, 82 % and 89 % on a per-patient basis) when compared with x-ray angiography [36]. A bolus infusion of Gd-BOPTA for coronary MRI has also been reported [101], and an example depicted in Fig. 17.7 shows a clear visualization of the three major coronary vessels in the reformatted and 3D volume rendered images. Furthermore, the bolus contrast injection method is advantageous in multiple ways, since it simplifies the initiation time of coronary MRI acquisition compared to slow infusion, and it is compatible with late gadolinium enhancement imaging, which enables the assessment of coronary artery stenosis and myocardium viability using a single bolus contrast injection.