Introduction

Coronary MRA is technically demanding since the coronary arteries are small, tortuous vessels embedded in epicardial fat that move with cardiac and respiratory motion.

Coronary MRA techniques

Available techniques are free breathing or breath-hold. These can provide either 2D or 3D data sets using SE or GE sequences. For clinical imaging, we recommend using 3D free breathing or breath-hold SSFP GE techniques. In both methods, scans are ECG-gated to commence at mid to late diastole when cardiac motion is minimal. Optimal fat suppression is mandatory. Gadolinium contrast agent is not routinely given.

Free breathing combined with 3D imaging provides good signal-to-noise ratio (SNR) and anatomical coverage. GE sequences image blood within the coronaries, and SE sequences image the vessel wall. Subjects breathe normally throughout the scan and respiratory artefact is minimized by the prior addition of respiratory navigators. These provide a way of monitoring respiratory motion during data collection and correcting that data for the motion. Methods of motion assessment differ, and one commonly used is detection of the superior–inferior movement of the dome of the right hemidiaphragm (Figure 9.1). Following placement and navigator initiation, the user automatically accepts data when the diaphragm–lung interface falls within a pre-defined window and rejects data that fall outside this narrow limit. The acceptance window is positioned at the end expiratory pause period of the respiratory cycle and extends 2–3 mm either side of a patient-specific diaphragm–lung interface value.

Figure 9.1 3D free breathing GE coronary MRA sequence in an oblique transaxial plane showing characteristic signal loss from the respiratory navigator on the dome of the right hemidiaphragm (solid white arrow) and a RCA stent (dashed white arrow).

Free breathing sequences acquire data over several minutes and provide higher resolution than breath-hold techniques. Breath-hold 3D GE coronary MRA requires subjects to suspend their respiration for 20–25 s depending on their heart rate. Since navigators are avoided, breath-holding requires less expertise and circumvents problems arising from suboptimal respiratory patterns. The resolution and coverage sacrificed with the breath-hold option becomes relevant at the limits of coronary MRA indications, such as evaluation of coronary artery stenoses, and is not of significance for delineation of proximal coronary artery origin and early course. With free breathing SSFP GE coronary MRA a typical spatial resolution is 1.1 ×0.7 ×1.5 mm³ with 30 to 48 mm through-plane coverage, as compared to 1.3 ×1.1 ×1.6 mm³ with 16 to 19.2 mm through-plane coverage using the breath-hold sequence. Breath-holding permits several volumes of interest to be imaged in the same time it takes to perform one free breathing scan. Patients do vary in their response to the two methods with consequent compromise to image quality and diagnostic accuracy, and both may need to be attempted (Figure 9.2). Maximal and optimal resolution with either technique is not equivalent. Maximizing resolution will assist accuracy but will reduce SNR and lead to grainy images, while optimal resolution may be a little lower but produce better quality scans. Post-processing curved MPR is an excellent way of presenting findings in coronary MRA reports (Figure 9.3).

Figure 9.2 Raw data MRA of the left coronary system from a normal 32-year-old male subject in an oblique transaxial plane demonstrating equivalent image quality using (a) free breathing and (b) breath-hold techniques.

With kind permission from Varghese, Keegan and Pennell, Coronary Artery Dis, P. 2005; 16: 355–364.

Figure 9.3 Curved MPR showing the rare, interarterial left main stem (LMS) anomaly originating from the right sinus of Valsalva (SoV; white arrow). This variant is considered the highest risk for ischaemia and sudden death.

Coronary MRA protocol

Coronary MRA protocols for evaluation of proximal coronary artery origins and course take approximately 15 to 20 min to perform:

Figure 9.4 Two examples of pilot coronal FSE imaging showing approximate coronary artery origins from the aortic root (white arrows). Subsequent high-resolution 3D coronary MRA data acquisition is targeted to a volume of interest centred in an oblique transaxial plane encompassing these origins (white rectangle).

Anomalous coronary arteries

Anomalous coronary arteries are rare and usually asymptomatic. Prevalence ranges from 1% in patients with otherwise normal cardiac anatomy to 36% in individuals with ACHD such as TOF. Their importance lies in the association of specific variants with myocardial ischaemia or MI, syncope and sudden cardiac death, particularly amongst young adults. This premature morbidity and mortality typically occurs during strenuous physical exercise with coronary arteries that traverse between the aorta and PA (Figure 9.3). Three postulated mechanisms are: mechanical embarrassment from aortic and PA dilatation at a time of increased coronary flow demand, kinking of an ostial or proximal segment due to a sharp turn or bend at the origin, and a congenitally slit-like ostium which is inadequate to meet extra requirements.

Variations of anomalies include:

Figure 9.5 Raw data coronary MRA in an oblique sagittal plane from a 63-year-old male patient with hypertension and a positive exercise tolerance test.

X-ray coronary angiography revealed a non-dominant RCA, patent LAD and aberrant LCx arising from the right SoV. Coronary MRA was requested in order to evaluate the course of the LCx, which is seen to be the common retroaortic variant (white arrows).

Coronary MRA is less helpful in the direct visualization of fistulas and coronary origination outside the sinuses of Valsalva, such as from the PAs.

Referrals for coronary MRA are often made following X-ray coronary angiography where there is failure to engage one of the coronary ostia, or when an anomaly is angiographically demonstrated and delineation of the proximal course is needed. Coronary MRA is important in patients with ACHD and as part of the work-up in young patients with unexplained syncope or chest pain. These latter groups invariably undergo more comprehensive CMR evaluation than coronary MRA alone. A potential preparticipation screening role in competitive athletes has also been suggested, again with a CMR protocol encompassing more than isolated exclusion of anomalous coronary arteries.

Clinical reports comment on:

Coronary artery aneurysms and Kawasaki disease

Coronary artery aneurysms are rare. Causes are congenital or acquired secondary to atherosclerosis, coronary interventions and connective tissue or inflammatory disorders such as Kawasaki disease (Figure 9.6). Kawasaki disease is a vasculitis affecting children. Cardiovascular complications are the leading cause of mortality and morbidity and include myocarditis and valvular regurgitation. Coronary artery aneurysms can lead to MI, sudden death or IHD and sites for aneurysm formation in reducing order of frequency are:

Figure 9.6 3D breath-hold GE coronary MRA in a 14-year-old boy showing a giant left coronary artery aneurysm (white arrows) secondary to previous Kawasaki disease.

(a, b) Raw data in oblique transaxial and coronal planes. The aneurysm involved the LMS 5 mm from its ostium with extension into the proximal LAD and dimensions of 33 ×16 mm². (c) Curved MPR from repeat CMR evaluation performed 8 months later suggested no change in aneurysm size.

The commonest sites are therefore accessible for coronary MRA, which is usually requested following TTE diagnosis in childhood. Current management guidelines for patients with a solitary, small to medium sized aneurysm (between 3 and 6 mm) in one or more coronary arteries are annual ECG and TTE, and stress testing with myocardial perfusion imaging every 2 years after the age of 10. Patients with at least one large coronary artery aneurysm (≥6 mm), or segmented aneurysms without evidence of coronary artery obstruction, and those with evidence of coronary artery obstruction at X-ray angiography, are suggested to have biannual ECG and TTE and annual stress tests with myocardial perfusion evaluation. Additionally, peripheral and abdominal angiography may be required prior to the first X-ray coronary angiography procedure since aneurysms can also occur outside the coronary arterial system. Common sites include the subclavian, femoral and iliac arteries, with less frequent involvement at the abdominal aorta and renal arteries. CMR is therefore an ideal modality for follow-up in this condition.

Clinical reports on cardiac findings comprise:

Indications

Coronary MRA can:

Coronary MRA cannot: