DOI:10.2214/AJR.06.0417
AJR 2007; 188:1074-1080
© American Roentgen Ray Society
Myocardial Bridging on MDCT
Tuncay Hazirolan1,
Murat Canyigit1,
Musturay Karcaaltincaba1,
Merve Gulbiz Dagoglu1,
Deniz Akata1,
Kudret Aytemir2 and
Aytekin Besim1
1 Department of Radiology, Hacettepe University Hospitals and Faculty of
Medicine, Sihhiye, Ankara 06100, Turkey.
2 Department of Cardiology, Hacettepe University Hospitals and Faculty of
Medicine, Ankara 06100, Turkey.
Received March 22, 2006;
accepted after revision August 1, 2006.
Address correspondence to M. Canyigit
(mcanyigit{at}yahoo.com).
Abstract
OBJECTIVE. The aim of this study is to show the usefulness of MDCT
in the diagnosis of myocardial bridging. Although most of the time myocardial
bridging is a benign condition, it may be associated with myocardial ischemia
and secondary complications. Therefore, it is important to be able diagnose
the presence of myocardial bridging.
CONCLUSION. MDCT is an effective noninvasive method for the
diagnosis of myocardial bridging because MDCT can show the length and the
depth of the tunneled artery and the diameter and percentage of stenosis in
the segments showing myocardial bridging in the systolic and diastolic phases.
Moreover, MDCT is efficient in showing the presence of other coronary artery,
myocardial, epicardial, and neighboring thoracic abnormalities.
Keywords: cardiovascular imaging • conventional angiography • CT angiography • CT imaging • MDCT • myocardial bridging
Introduction
Myocardial bridging is a congenital anomaly characterized by
myocardial encasement of a coronary artery segment, which normally courses
epicardially (Figs. 1A and
2B). This is also called
tunneled artery. Despite the fact that it is a congenital anomaly, symptoms
usually do not develop before the third decade of life. Although myocardial
bridging is considered a benign variation, it is clinically important because
of its association with myocardial ischemia and secondary complications
[1]. MDCT is evolving rapidly
as a noninvasive method in the diagnosis of myocardial bridging and the
evaluation of associated intracoronary hemodynamics.
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Fig. 1A —45-year-old man with hypercholesterolemia. Curved multiplanar
reconstruction (A) and short-axis (B) MDCT images show normal
epicardial route of left anterior descending artery (arrow,
B).
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Fig. 2B —59-year-old man with hypertension and hypercholesterolemia. Curved
multiplanar reconstruction (A) and short-axis (B) MDCT images
show myocardial bridging over mid segment of left anterior descending artery
(arrows).
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Incidence
Myocardial bridging was first defined at autopsy by Reyman
[2] in 1737. Later, Portmann
and Iwig [3] angiographically
described temporary occlusion in a segment of the left anterior descending
artery (LAD) during the systolic phase. In 1976, Noble et al.
[4] detected this temporary
occlusion in selective coronary angiography in 27 (0.5%) of 5,250 patients and
named it the "milking effect." In their studies of autopsies,
Ferreira et al. [5]
distinguished between two types of bridging: superficial bridges (75% of
cases) and deep bridges (25% of cases). However, there are no certain depth
criteria set to classify myocardial bridges, and they are classified according
to the routes that muscle bundles forming the myocardial bridges follow. Other
than these routes, arterial segments may also be located in a deep
interventricular gorge. The surface is not fully covered by myocardial fibers
but rather by a thin layer of connective tissue, nerves, and fatty tissue.
This kind of bridging, which is defined as incomplete (Fig.
3A,
3B), may also show compression
in the systolic phase [6].
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Fig. 3A —42-year-old man with hypertension and hypercholesterolemia. Curved
multiplanar reconstruction (A) and short-axis (B) MDCT images
show incomplete bridging on interventricular gorge formed by connective tissue
encasing distal segment of left anterior descending artery (arrows),
but no distinctive muscle fibers are seen over artery.
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Fig. 3B —42-year-old man with hypertension and hypercholesterolemia. Curved
multiplanar reconstruction (A) and short-axis (B) MDCT images
show incomplete bridging on interventricular gorge formed by connective tissue
encasing distal segment of left anterior descending artery (arrows),
but no distinctive muscle fibers are seen over artery.
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Although the rate of myocardial bridging is between 15% and 85% in autopsy
studies, it is only seen in 0.5-2.5% of angiographic studies
[1,
7]. On the other hand, the rate
rises to 40% with the provocation test used during conventional angiography
[6]. This gap between
angiography and autopsy series has been attributed to multiple factors
including the length and depth of the tunneled artery, with only deeply
located coronary artery segments within the ventricular myocardium appearing
to be sufficiently compressed during systole to be identified on angiography
[5]. In addition, the presence
of atherosclerotic plaques proximal to myocardial bridging may cause
underdiagnosis [6].
This anomaly is more frequently seen in patients with hypertrophic
cardiomyopathy; the prevalence rating is up to 30% on coronary angiography
[8]. The anomaly is also seen
with increased prevalence in patients who have undergone heart transplantation
[6].
Myocardial bridges are mostly seen on the LAD mid segment (Figs.
2A and
2B); however, there are various
cases in which myocardial bridging has been reported on other main arteries
and their branches [8] (Figs.
2A,
2B,
3A,
3B and
4). Multiple muscle bridges can
involve either the same vessel or different coronary arteries and their
branches [5] (Figs.
5A,
5B,
5C and
6A,
6B,
6C).
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Fig. 2A —59-year-old man with hypertension and hypercholesterolemia. Curved
multiplanar reconstruction (A) and short-axis (B) MDCT images
show myocardial bridging over mid segment of left anterior descending artery
(arrows).
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Fig. 4 —60-year-old man with hypercholesterolemia. Volume-rendered image
shows trifurcation of left main coronary artery into left anterior descending
artery (large arrowhead), intermediate artery, and circumflex artery
(small arrowhead). There is myocardial bridging on ramus intermedius
artery (arrow).
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Fig. 5A —49-year-old man with hypercholesterolemia. Curved multiplanar
reconstruction MDCT images show myocardial bridging on diagonal branch of left
anterior descending artery (arrowhead, B) and mid segment of
left anterior descending artery (arrows).
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Fig. 5B —49-year-old man with hypercholesterolemia. Curved multiplanar
reconstruction MDCT images show myocardial bridging on diagonal branch of left
anterior descending artery (arrowhead, B) and mid segment of
left anterior descending artery (arrows).
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Fig. 6A —69-year-old man who underwent MDCT for evaluation of patency of
coronary artery bypass graft. Curved multiplanar reconstruction image shows
short segment in middle part of left anterior descending artery
(arrow) and long segment in distal part of artery (small
arrowheads) encased by myocardium. Calcific atherosclerotic plaque
(large arrowhead) is seen just proximal to myocardial bridging on
middle part of artery.
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Fig. 6B —69-year-old man who underwent MDCT for evaluation of patency of
coronary artery bypass graft. Volume-rendered images show both bridges
(small arrow and small arrowheads) and proximally patent
(large arrow) and distally occluded (large arrowhead)
internal mammary artery-left anterior descending artery graft.
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Fig. 6C —69-year-old man who underwent MDCT for evaluation of patency of
coronary artery bypass graft. Volume-rendered images show both bridges
(small arrow and small arrowheads) and proximally patent
(large arrow) and distally occluded (large arrowhead)
internal mammary artery-left anterior descending artery graft.
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Myocardial muscle bundles derived from atrial myocardium surround the
vessel for three quarters of the circumference and return to atrial
myocardium—called myocardial loops (Fig.
7A,
7B). Occasionally, a bridge
may involve a coronary vein. However, myocardial loops and venous bridges
appear to have no clinical relevance
[6].
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Fig. 7A —41-year-old woman with hypercholesterolemia. Axial (A) and
volume-rendered (B) MDCT images show myocardial loop on mid segment of
right coronary artery encasing three quarters of circumference of right
coronary artery (arrows).
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Fig. 7B —41-year-old woman with hypercholesterolemia. Axial (A) and
volume-rendered (B) MDCT images show myocardial loop on mid segment of
right coronary artery encasing three quarters of circumference of right
coronary artery (arrows).
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Mechanism of Ischemia and Clinical Importance
Several mechanisms have been postulated to explain ischemia resulting from
myocardial bridging of a coronary artery, including vasospasm and systolic
kinking of the artery that results in direct physical damage to the underlying
endothelial cells. Delayed diastolic relaxation, increased contractility,
compression of the artery, and increased flow velocity may cause ischemia
[1,
6,
7,
9]. Stress and exercise may
induce ischemia, leading to tachycardia and increasing the systolic-diastolic
ratio [6]. In addition, these
arteries entrapped by myocardium become more vulnerable to atherosclerotic
changes, increasing the risk of ischemia. Previous studies have also noted
that atherosclerotic changes are seen more often just proximal to the tunneled
artery (Figs. 6A and
8A,
8B,
8C) than in or distal to the
tunneled artery [9].
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Fig. 8A —56-year-old man with hypercholesterolemia. Curved multiplanar
reconstruction MDCT image shows long segment of myocardial bridging on left
anterior descending artery (arrowheads) and soft plaque just proximal
to bridging (arrow).
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Fig. 8B —56-year-old man with hypercholesterolemia. MDCT images show tunneled
artery circumscribed by myocardial fibers (arrow, B) and soft
atherosclerotic plaque (arrow, C) just proximal to artery.
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Fig. 8C —56-year-old man with hypercholesterolemia. Short-axis MDCT images
show tunneled artery circumscribed by myocardial fibers (arrow,
B) and soft atherosclerotic plaque (arrow, C) just
proximal to artery.
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Although myocardial bridging is considered a benign condition, there are
articles in the literature in which complications secondary to ischemia were
reported. These complications vary in a wide range between transient ischemia
and sudden death [1].
Myocardial bridging should be ruled out, especially in the young patient with
chest pain or arrhythmia in the absence of any risks for atherosclerosis.
Conventional Angiography, Intravascular Sonography, and Intracoronary Doppler Sonography
Conventional angiography, intravascular sonography, and intracoronary
Doppler sonography are invasive methods currently in use for the diagnosis of
myocardial bridging [6,
9]. Angiography shows a milking
effect and stepdown-step-up phenomenon resulting from systolic compression in
the tunneled artery (Fig. 9A,
9B,
9C), whereas limited
information is obtained about functional effects at the myocardial level
[6]. In the presence of
atherosclerotic plaque proximal to the tunneled artery, myocardial bridging
may be underdiagnosed angiographically and only seen after treatment of the
stenosis by placing a stent or by angioplasty
[6].
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Fig. 9A —34-year-old man with chest pain. Conventional angiography images
show milking effect due to systolic compression of tunneled artery
(arrow, A). Absence of compression and therefore normal
configuration of artery (arrow, B) is seen during diastolic
phase.
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Fig. 9B —34-year-old man with chest pain. Conventional angiography images
show milking effect due to systolic compression of tunneled artery
(arrow, A). Absence of compression and therefore normal
configuration of artery (arrow, B) is seen during diastolic
phase.
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Intravascular sonography and intracoronary Doppler sonography are effective
in evaluating morphologic and functional features of myocardial bridging. Ge
et al. [9] defined a highly
specific half-moon sign in intravascular sonography. In flow rate measurements
with intracoronary Doppler sonography, another characteristic sign for
myocardial bridging has been defined—the diastolic fingertip phenomenon.
Ge et al. concluded that the higher aortic pressure, higher wall tension, and
turbulence of the blood proximal to myocardial bridging makes that portion of
the coronary artery vulnerable to atherosclerosis formation
[9].
The advantage of angiography over the other methods is that it allows
balloon angioplasty or stent placement during the procedure.
MDCT
Technique
Imaging of the heart requires a higher level of technology than other
organs because imaging of the heart must be performed during a fast and
complex cyclical motion. Therefore, image acquisition requires a high temporal
resolution and high spatial resolution for visualization of the cardiac
anatomy, especially the anatomy of the small coronary arteries. Despite
improvements in temporal and spatial resolution, motion artifacts still remain
the most important challenge for coronary CT angiography, even with 4-, 16-,
and 64-MDCT. The temporal resolution of MDCT (4-MDCT, 250 milliseconds;
16-MDCT, 183-250 milliseconds; 64-MDCT, 165-210 milliseconds) is substantially
lower than that of conventional angiography (< 10 milliseconds). Multiple
studies have shown that the highest image quality of coronary CT angiography
for 16- and 64-MDCT scanners can be achieved at low heart rates (< 65 beats
per minute). Temporal resolution of less than 100 milliseconds at all heart
rates is desirable to completely eliminate the need for heart-rate control
[10,
11].
The latest-generation CT scanners, dualsource CT, provide a temporal
resolution of 83 milliseconds for a single segment and 60 milliseconds mean
temporal resolution for two-segment reconstruction. Therefore, they are
independent of the heart rate. Initial experiences with dual-source CT have
shown that high temporal resolution also makes functional evaluation of the
heart valves and ventricular myocardium and dynamic reconstruction of the
coronary arteries possible
[11]. Furthermore, because
dual-source CT does not need heart rate control, ß-blocker administration
is not required. Beta-blockers decrease the systolic-diastolic ratio and may
decrease the compression effect on coronary arteries during systole.
Therefore, the use of dual-source CT may make it possible to show the milking
effect, as in conventional angiography, by 4D reconstruction.
Coronary CT Angiography Protocol
The image quality of coronary CTA on 16- and 64-MDCT scanners is
substantially improved in patients with a heart rate lower than 65 beats per
minute. The most common approach in current clinical practice is the
administration of an oral ß-blocker (50-100 mg of oral metoprolol
administered 1 hour before the scan) or an IV ß-blocker (5-20 mg of IV
metoprolol administered immediately before the scan) with a short half-life
[12].
A CT volume data set for the coronary arteries is acquired; the data set
covers the entire heart from the proximal ascending aorta (approximately 1-2
cm below the carina) to the diaphragmatic surface of the heart. The scan is
acquired in a single breath-hold during inspiration and starts with the
injection of a nonionic contrast agent with a concentration of 300-400 mg I/mL
at a flow rate of 4-6 mL/s. The total volume of contrast agent depends on the
scan length, but typically 100-120 mL for 16-MDCT and 60-80 mL for 64-MDCT are
injected, followed by a saline bolus (40-70 mL at 4-6 mL/s)
[12].
Myocardial Bridging on MDCT
There have been several studies demonstrating myocardial bridging in 4- and
16-MDCT. Kantarci et al. [13]
showed myocardial bridging in 22 (3.5%) of 626 patients, and Javier et al.
[14] showed myocardial
bridging in 68 (17. 8%) of 380 patients. Gaspar et al. (presented at the 2004
annual meeting of the Radiological Society of North America) showed myocardial
bridging in 29 (19%) of 152 patients. Earls et al. (presented at the 2004
annual meeting of the RSNA) showed myocardial bridging in 30 (16.5%) of 182
patients and incomplete bridging in 40 (22%) of 182 patients—a total of
70 patients for a prevalence of 38.5%. Moreover, in a study performed by
Carrascosa et al. (presented at the 2004 annual meeting of the RSNA) the
diameter in the systolic and diastolic phases and the percentage and length of
stenosis due to myocardial bridging were evaluated on both MDCT and
conventional angiography, and the results were correlated and found to be
consistent. In that study, it was concluded that MDCT is an alternative fast,
noninvasive diagnostic technique that accurately allows the evaluation of
myocardial bridging.
Multiplanar reconstruction images assessed from MDCT angiography provide
information about the lumen and walls of the coronary arteries and the
myocardium in any plane requested. Therefore, myocardial bridging can be shown
on MDCT regardless of the thickness and direction of muscle bundles within the
myocardial bridging (Figs. 2B,
3B,
10A,
5A,
5B,
5C, and
8B). Furthermore, MDCT is also
used effectively in the evaluation of associated atherosclerotic lesions
(Figs. 6A,
6B,
6C and
8A,
8B,
8C), patency of stents and
bypass grafts (Figs. 11B,
6B, and
6C), coronary artery anomalies
(Fig. 12B), and myocardial and
epicardial abnormalities (Fig.
11A). Because the presence of atherosclerotic plaques and the
location, depth, and length of the myocardial bridging (Figs.
1A,
1B,
2A,
2B,
3A,
3B,
4,
5A,
5B,
5C,
6A,
6B,
6C,
7A,
7B,
8A,
8B,
8C,
10A,
10B, and
11A,
11B) can be evaluated by MDCT,
this method is also useful during treatment planning
[13,
14].
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Fig. 10A —78-year-old man who underwent MDCT for evaluation of patency of
right coronary artery stent. Volume-rendered images show tunneled artery,
which is segment of right coronary artery within right ventricular myocardium
(arrows).
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Fig. 11B —70-year-old man who underwent MDCT for evaluation of patency of
coronary artery bypass graft. Volume-rendered image shows myocardial bridging
on obtuse marginal branch of left circumflex artery (arrow) and
patency of left internal mammary artery-left anterior descending artery graft
(arrowheads).
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Fig. 12B —44-year-old man with hypercholesterolemia. Volume-rendered MDCT
image shows associated right coronary artery anomaly (arrow) in which
right coronary artery originates from left coronary sinus and courses between
aorta (a) and main pulmonary artery (p) in right atrioventricular groove.
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Fig. 11A —70-year-old man who underwent MDCT for evaluation of patency of
coronary artery bypass graft. Axial MDCT image shows obtuse marginal branch of
left circumflex artery in left ventricular myocardium (arrow). In
addition, subendocardial infarct is seen in apical area
(arrowheads).
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Fig. 1B —45-year-old man with hypercholesterolemia. Curved multiplanar
reconstruction (A) and short-axis (B) MDCT images show normal
epicardial route of left anterior descending artery (arrow,
B).
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Fig. 10B —78-year-old man who underwent MDCT for evaluation of patency of
right coronary artery stent. Volume-rendered images show tunneled artery,
which is segment of right coronary artery within right ventricular myocardium
(arrows).
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Treatment
There is no need for treatment in asymptomatic patients. In symptomatic
cases, the primary step is medical treatment. This includes ß-blockers,
calcium channel blockers, and antiplatelet agents
[1,
7]. Nitrates generally should
be avoided because they increase systolic compression and may lead to
worsening of the clinical symptoms
[1]. When patients are
unresponsive to medical treatment, stent implantation, surgical myotomy, or
coronary bypass grafting can be performed
[7].
Conclusion
MDCT is a currently evolving method for an alternative in the diagnosis of
myocardial bridging. The advantages of MDCT over angiography are that it is
noninvasive and can show the length and depth of the tunneled artery and the
diameters and percentage of stenosis in the segments presenting myocardial
bridging in systolic and diastolic phases. MDCT is also efficient in showing
the presence of other coronary artery, myocardial, epicardial, and neighboring
thoracic abnormalities, which are important in treatment planning.
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Abstract
Introduction
