British Journal of Radiology (2007) 80, e196-e200
© 2007 British Institute of Radiology
doi: 10.1259/bjr/85712380
Multidetector-row CT coronary angiographic finding of myocardial bridging
S M Ko, MD
1 and
K S Kim, MD
2
1 Department of Radiology, 2 Department of Cardiology, Dongsan Medical Center, Keimyung University, Daegu, Republic of Korea
Correspondence: SungMin Ko, Department of Radiology, Dongsan Medical Center, Keimyung University, 194 Dongsan-dong, Jung-gu, Daegu 700-712, Republic of Korea. E-mail: ksm9723{at}yahoo.co.kr
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Abstract
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Myocardial bridging is caused by muscles overlying the intramyocardial course of an epicardial coronary artery. It is a congenital anomaly characterized by systolic compression of the tunnelled segment, commonly affecting the mid-portion of the left anterior descending coronary artery. The authors report two cases of myocardial bridging using electrocardiogram-gated multidetector-row CT coronary angiography as a reliable and non-invasive imaging technique.
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Introduction
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Myocardial bridging is a congenital coronary anomaly in which a segment of a coronary artery or its major branch travels through the myocardium instead of on the surface. The middle segment of the left anterior descending artery (LAD) is the most common site involved [1–3]. Diagnosis of myocardial bridging is made possible with the identification of the tunnelled segment and its systolic compression on the catheter angiogram, intravascular ultrasound (IVUS) and intracoronary Doppler ultrasound (ICD) [2–7]. However, these procedures are invasive and therefore non-invasive multidetector-row CT (MDCT) has been recently used in cardiac CT imaging. In particular, it has shown a capability to perform high-resolution MDCT coronary angiography [8, 9]. The authors report two cases of myocardial bridging using electrocardiogram (ECG)-gated MDCT coronary angiography as a reliable and non-invasive imaging technique.
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MDCT protocol
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Before the examination, each patient's heart rate (HR) was measured. Patients with a pre-scan HR 
65 beats per minute were given 100 mg of metoprolol per os 1 h before the scan.
ECG-gated MDCT angiography was performed using a Sensation 16 scanner (Siemens, Forchheim, Germany). The following parameters were employed: 16x0.75 mm collimation, 0.37 s rotation time, feed/rotation 6.8 mm, 120 kV and 620 mA, resulting in a total scan time of about 20 s to cover the entire heart, acquired during suspended breathing using retrospective ECG gating. 100 ml of contrast agent (Ultravist 370; Schering, Berlin, Germany) was injected intravenously at a rate of 3.5 ml s–1, followed by 50 ml of saline at the same rate. CT scanning began with real-time bolus tracking (CARE bolus; Siemens, Forchheim, Germany) using a region of interest in the ascending aorta for monitoring a threshold of +100 Hounsfield units (HU) above the baseline attenuation.
Data were retrospectively reconstructed at various phases of the R–R interval. All images were post-processed on a workstation (Syngo, Wizard; Siemens Medical Solutions).
Reconstruction parameters were a 220 mm field of view, 1 mm effective slice thickness, 0.5 mm increment and kernel B30. A series of maximum intensity projection (MIP) images in the short axis (SA) and vertical long axis of the heart, and curved multiplanar reformation (MPR) of the coronary arteries, were displayed for viewing tunnelled coronary segments. Whole-heart volume-rendered images of the heart were also reconstructed and were rotated to best display the myocardial bridging.
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Case reports
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Case 1
A 44-year-old man presented with resting-onset anterior chest pain of 3 days' duration. His risk factors for coronary artery disease were heavy smoking (30 packs a year) and diabetes mellitus. ECG revealed non-specific ST–T changes. The creatinine kinase muscle and brain (MB) fraction level was 0.2 ng ml–1. Both echocardiography and a stress test revealed normal findings. His physician recommended MDCT coronary angiography to rule out coronary stenosis. There was no evidence of atherosclerosis in the coronary arteries. The mid-portion of the LAD just past the first diagonal branch, approximately 19 mm in length, was surrounded by myocardium. A decrease in the calibre of the tunnelled segment was identified. This finding was easily demonstrated in the curved MPR images (Figure 1a
). In the SA MIP images, the diameter of the tunnelled segment was 2.2 mm at end-systole (25% of the R–R interval; Figure 1b) and 2.9 mm at end-diastole (90% of the R–R interval; Figure 1c), indicating systolic compression of the segment. Volume-rendered images provided an excellent depiction of myocardial bridging (Figure 1d). The patient underwent oesophagogastroduodenoscopy in which reflux oesophagitis appeared. These findings were reviewed by the cardiologist and gastroenterologist. It was decided to follow the patient closely and start medication for reflux oesophagitis. His clinical symptoms gradually subsided.
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Figure 1. A 44-year-old man with myocardial bridging. (a) A curved multiplanar reformation image demonstrates the course of the mid-left anterior descending coronary artery (LAD) (arrow) dipping into the myocardium, corresponding to myocardial bridging. (b,c) Short-axis maximum intensity projection images clearly depict a segment of the LAD surrounded by myocardium (arrow). The diameter of the tunnelled segment is 2.2 mm at end-systole (b) and 2.9 mm at end-diastole (c), indicating systolic compression of the segment. (d) A volume-rendered image provides an excellent demonstration of myocardial bridging (arrow).
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Case 2
A 65-year-old woman was admitted with increasing frequency of substernal chest pain of 3 days' duration. She had suffered from food poisoning for 5 days before admission. She had been diagnosed with myocardial bridging with significant stenosis (>50% luminal narrowing) affecting the mid-portion of the LAD 3 years previously using catheter angiography (Figure 2a,b), and had been treated with beta-blockers. Cardiac evaluation including cardiac enzymes, ECG and echocardiography was normal. Even though she had suffered from food poisoning and was being treated with beta-blockers for myocardial bridging, her physician did not exclude ischaemic heart disease associated with atherosclerotic coronary artery stenosis. MDCT coronary angiography was performed. There was no evidence of atherosclerosis within any of the coronary arteries. In the proximal two-thirds of the LAD, a segment (approximately 15 mm in length) was surrounded by myocardium. A marked decrease in the calibre of the LAD in the region where it tunnelled into the myocardium was shown on the curved MPR (Figure 2c). Volume-rendered images confirmed myocardial bridging (Figure 2d). A decision was made to closely follow the patient and maintain treatment with beta-blockers instead of undertaking coronary intervention.
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Figure 2. A 65-year-old woman with myocardial bridging. On conventional angiography, the lumen of the segment of the mid-left anterior descending coronary artery (LAD) (arrow) is compressed by myocardial contraction in the systolic phase (a) but recovers its normal diameter in the diastolic phase (b). (c) A curved multiplanar reformation image demonstrates the significantly stenotic tunnelled segment of the mid-LAD (arrow). (d) The volume-rendered image clearly reveals myocardial bridging (arrow) in the mid-LAD.
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Discussion
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Myocardial bridging occurs when a segment of major epicardial coronary artery travels intramurally through the myocardium beneath the muscle bridge. It is a congenital anomaly and is characterized by systolic compression of the tunnelled segment, commonly the mid-portion of the LAD. The prevalence on autopsy is much higher than on angiography although it varies substantially among studies [1–3]. Ferreira et al [10] divided bridging into two types: (1) superficial bridges (75% of cases) crossing the LAD transversely towards the apex of the heart at an acute angle or perpendicularly; and (2) deep bridges (25% of cases) arising from the right ventricular apical trabeculae that cross the LAD transversely, obliquely or helically before terminating in the interventricular septum. An atherosclerotic plaque is often located at the segment proximal to the bridge, although the tunnelled segment is typically spared [2, 11].
Although generally benign, myocardial bridges can cause angina, myocardial ischaemia, myocardial infarction, left ventricular dysfunction, myocardial stunning, paroxysmal atrioventricular blockade, exercise-induced ventricular tachycardia and sudden cardiac death. Because the majority of the myocardial blood flow is normally in diastole, systolic compression of the tunnelled segment alone cannot sufficiently explain the ischaemia and associated symptoms. The myocardial ischaemia and associated symptoms are related to the length or depth of the tunnelled segment, the degree of systolic dysfunction, stress- or exercise-induced sympathetic drive, increased local shear forces and local coronary endothelial dysfunction [2, 3, 10, 12].
Coronary catheter angiography is the current gold standard for diagnosing myocardial bridges. The pathognomonic findings of the "milking effect" and the "step down–step up" phenomena are induced by systolic compression of the tunnelled segment. However, the lower rate (<5%) of angiographic bridging may be explained by the thin bridges causing little systolic compression or the presence of an atherosclerotic stenosis proximal to the bridge. Provocation tests and the use of other imaging techniques may be warranted [2, 3, 6, 13].
Myocardial bridging can also be evaluated using IVUS and ICD. IVUS demonstrates the "half-moon phenomenon" of the tunnelled segment. ICD with pullback of the Doppler wire reveals a characteristic flow pattern, the "fingertip phenomenon" or "spike-and-dome pattern". IVUS is a sensitive method for the assessment of wall thickness and vessel size prior to stenting myocardial bridges to avoid coronary perforation.
However, although catheter angiography, IVUS and ICD can be used to identify the morphological and functional features of myocardial bridging, these procedures are invasive [2, 3, 7]. MDCT coronary angiography is a non-invasive imaging modality that provides precise information on cardiac morphology and the coronary arteries. The standard MDCT protocol for coronary evaluation is focused on the elimination of cardiac motion artefacts, which are a major factor affecting the image quality. Multiple diastolic phases are needed for evaluation of the coronary arteries but usually mid-diastolic phase is enough to freeze the beating heart for coronary evaluation when the HR is below 65 beats per minute. MDCT coronary angiography with high-quality two-dimensional and three-dimensional reformations enables the length, depth and precise location of the tunnelled coronary segment to be assessed [14, 15]. The phasic change in the luminal diameter of the intramyocardial segment can be assessed by comparing systolic and diastolic images [14].
In Case 1, systolic luminal narrowing of the intramyocardial segment was identified. However, the contours of the cross-sections of the vessel on end-systolic and end-diastolic phases were slightly blurred and so the assessment of the diameter of the end-on vessel could be limited. This is caused by insufficient spatial and temporal resolution of the 16-slice MDCT scanner, which has a gantry rotation time of 375 ms and a detector width of 0.75 mm. The recently introduced 64-slice MDCT scanner, which has a rotation time of 330 ms and an isotropic spatial resolution of 0.4 mm3 [16], may allow a clear assessment of phasic lumen narrowings of the intramyocardial segment as a pathognomonic finding of myocardial bridging. Also, if a myocardial bridging-associated ischaemic change occurs or the concomitant atherosclerosis proximal to the tunnelled segment is significantly stenotic, decreased myocardial attenuation suggestive of a reversible ischaemic perfusion defect can be imaged using adenosine triphosphate stress MDCT [17].
Treatment for symptomatic patients with myocardial bridging varies. Medical treatment as a first-line therapy includes nitrates, beta-blockers and calcium antagonists. In patients with severe angina and clinically relevant ischaemia, surgical treatment such as myotomy and coronary artery bypass grafting is considered. Coronary stent implantation may be the treatment of choice for patients whose diseased state is complicated by infarction or recalcitrant ischaemia, in spite of the frequent occurrence of restenosis and major periprocedural complications [2, 3, 6, 18].
In conclusion, ECG-gated MDCT coronary angiography offers much information on myocardial bridging including the location, depth, length and luminal change of the tunnelled artery and any concomitant atherosclerotic stenosis. Also, MDCT coronary angiography may be helpful to avoid further invasive procedures in asymptomatic patients.
Received for publication July 21, 2005.
Revision received March 2, 2006.
Accepted for publication March 27, 2006.
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Abstract
Introduction
