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Interventricular septal branch S1

The first interventricular septal branch (S1), commonly known as the first septal perforator, is the most proximal and usually the largest septal branch of the left anterior descending artery (LAD). It arises from the proximal LAD near its origin and descends vertically into the interventricular septum.

S1 supplies the anterior two-thirds of the interventricular septum, including the proximal portions of the right and left bundle branches of the conduction system. Its perfusion territory makes it critically important for maintaining normal cardiac conduction and ventricular synchrony. Occlusion of S1, either by atherosclerotic plaque or iatrogenic injury during PCI, can result in conduction block, arrhythmias, or septal infarction.

Anatomical variations are common; in some patients, S1 may be small, duplicated, or arise at a slightly different LAD level. Its precise mapping is essential during coronary interventions and CABG planning.

Synonyms

  • First septal perforator

  • First interventricular septal branch

  • S1 branch of LAD

Function

  • Provides arterial supply to the anterior interventricular septum

  • Perfuses the proximal conduction system (right and left bundle branches, part of His bundle)

  • Contributes to ventricular synchrony and conduction integrity

  • Critical artery in ischemia-related arrhythmias

MRI Appearance

T1-weighted images (non-contrast):

  • Vessel lumen shows signal void (black)

  • Delineated by surrounding septal myocardium

T2-weighted images:

  • Vessel lumen remains a signal void

  • Septal edema or infarction in S1 territory appears hyperintense

STIR (Short Tau Inversion Recovery):

  • Suppresses fat and enhances detection of myocardial edema

  • Septal myocardium supplied by S1 appears bright hyperintense in acute ischemia or myocarditis

T1 Post-Contrast (Gadolinium-enhanced):

  • Enhances lumen brightly

  • Myocardial late gadolinium enhancement (LGE) highlights septal infarction in the S1 territory

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

  • Depicts S1 as a small bright enhancing branch from proximal LAD

  • Identifies origin, vertical septal course, and relation to conduction pathways

  • Useful for congenital coronary mapping or patients unsuitable for contrast

CT Appearance

Non-contrast CT (Calcium Scoring):

  • Calcification at the LAD origin may extend into S1, but vessel itself not well visualized without contrast

CT Coronary Angiography (CCTA):

  • Clearly demonstrates S1 origin from proximal LAD and vertical course into septum

  • High-resolution imaging allows detection of stenosis, occlusion, or anomalous branching

  • Essential for evaluating ischemia-related septal infarcts, conduction disturbances, or surgical/interventional planning

  • Multiplanar and 3D reconstructions enhance visualization of small septal branches

CT image

first interventricular septal branch (S1) anantomy CT axial image -img-00000-00000

MRI image

Interventricular septal branch S1