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First obtuse marginal branch

The first obtuse marginal branch (OM1) is a major lateral branch of the left circumflex artery (LCx), which itself is a terminal branch of the left coronary artery (LCA). The OM1 usually arises from the mid-portion of the LCx and courses along the obtuse (lateral) margin of the left ventricle, supplying the lateral free wall.

The OM1, together with additional marginal branches (OM2, OM3), provides important perfusion to the anterolateral and posterolateral walls of the left ventricle. In some individuals, the OM1 may be large enough to significantly contribute to perfusion of the posterolateral left ventricle and, in left-dominant circulation, may anastomose with posterior branches of the right coronary artery.

Because of its location, OM1 is frequently involved in lateral wall ischemia and infarction, which may manifest as ST-segment changes in lateral ECG leads (I, aVL, V5–V6). It is a key vessel considered in percutaneous coronary interventions (PCI) and coronary artery bypass grafting (CABG), often grafted with a saphenous vein graft.

Synonyms

  • OM1 artery

  • First obtuse marginal artery

  • First marginal branch of the LCx

Function

  • Supplies oxygenated blood to the lateral free wall of the left ventricle

  • Contributes to perfusion of the anterolateral and posterolateral LV regions

  • Maintains contractility and pumping efficiency of the left ventricle

  • Plays a role in collateral circulation with branches of the RCA and LAD

Branches

  • Typically no major sub-branches, but may give off small lateral ventricular branches to the myocardium

  • Occasionally forms collateral connections with diagonal branches of the LAD or posterolateral branches of RCA

MRI Appearance

T1-weighted images:

  • OM1 lumen appears as a signal void (black) due to flowing blood

  • Epicardial fat enhances visualization against hypointense wall

T2-weighted images:

  • Lumen remains a signal void

  • Myocardial edema from ischemia or infarction in OM1 territory appears hyperintense

STIR (Short Tau Inversion Recovery):

  • Highlights edema and inflammation in the lateral LV myocardium supplied by OM1

  • Vessel itself remains dark (flow void)

T1 Post-Contrast (Gadolinium-enhanced):

  • OM1 lumen enhances brightly and homogeneously

  • Late gadolinium enhancement (LGE) of the lateral wall indicates myocardial infarction or scar

MRI Non-Contrast Cardiac-Gated 3D Coronary MRA:

  • Depicts OM1 as a bright, continuous epicardial vessel arising from the LCx

  • Shows course along the obtuse margin and myocardial supply region

  • Useful for detecting congenital anomalies or major stenosis without contrast

CT Appearance

Non-contrast CT (Calcium Scoring):

  • Calcified plaques in OM1 appear as hyperdense foci, quantified in coronary calcium scoring

CT Coronary Angiography (CCTA):

  • Best non-invasive modality for OM1 imaging

  • Demonstrates origin from the LCx, proximal/mid/distal course along the obtuse margin

  • Detects stenosis, occlusion, aneurysm, or anomalous course

  • Characterizes plaque morphology (calcified, non-calcified, or mixed)

  • Multiplanar reformats and 3D reconstructions are essential for PCI planning and CABG graft targeting

CT images

First obtuse marginal branch CT axial image -img-00000-00000

CT images

First obtuse marginal branch CT axial image -img-00000-00000_00001