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Background
Myocardial bridging (MB) is a congenital abnormality in which the coronary arteries take an intramural course through the myocardium, instead of the usual epicardial route. It is most commonly seen at the mid segment of the left anterior descending (LAD) artery. MB is frequently asymptomatic and discovered only at autopsy, with a mean prevalence of 25%. The high incidence of asymptomatic MB has been attributed to the diastolic flow period, which comprises the majority of coronary circulation time. Symptoms such as stable angina or acute coronary syndrome, can present when systolic myocardial pressure occludes the affected coronary arteries or with decrease diastolic filling time. Systolic compression has been reported in angiographic studies in up to 12% of the population.

Educational Goals / Teaching Points
Illustrate several manifestations of myocardial bridging on cardiac CT Angiography, optimization of cardiac techniques to better evaluate MB, comparison with other imaging modalities, review clinical presentation and treatment.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Myocardial bridging is most commonly seen at the mid segment of the LAD on coronary angiography, but bridging of the right coronary artery, left circumflex artery, diagonal and marginal branches have been seen at autopsy. Atherosclerotic plaques are most frequently seen proximal to the tunneled segment, although the effects on ischemic events is unclear. The bridged segment of the coronary artery is usually free of atherosclerosis, due to the presence of contractile smooth muscle cells that appear to be protective against atherosclerosis. In contrast, the segment of the artery proximal to the bridge experiences high shear and expresses vasoactive agents that lead to smooth muscle proliferation. CCTA technique: Cardiac CT angiography needs to be performed with retrospective gating to evaluate the dynamic compression. Conventional Angiography: Myocardial bridging is indirectly demonstrated on angiography through the milking effect, where compression of the coronary artery is seen during systole with decompression during diastole. Nuclear Medicine: Compared to catheter angiography the sensitivity and specificity for 99mTc-SPECT MPI, 201TI-SPECT MPI, and 99mTc-SPECT/CTA are 88% and 70%, 84% and 71%, and 100% and 80.8% respectively. Outcome: Asymptomatic patients can be treated with lifestyle modification and avoidance of coronary artery disease risk factors. In symptomatic patients, pharmacologic therapy with beta blockers and/or calcium channel blockers is the mainstay of treatment, with PCI, CABG and decompressive myotomy reserved for refractory cases. Importantly, restenosis rates are higher when compared to the same interventions for atherosclerotic disease.

Conclusion
Myocardial bridging of the coronary arteries is usually asymptomatic but should be considered as a differential diagnosis for chest pain, especially in young patients. Conventional angiography used to be the main diagnostic modality. Currently, cardiac CT angiography is a non-invasive tool used to provide excellent information on depth and location of the tunnel.