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Topic

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Interatrial septum

The interatrial septum (IAS) is the thin muscular and fibrous partition that separates the right and left atria of the heart. It develops embryologically from the septum primum and septum secundum, leaving a potential opening known as the foramen ovale in fetal circulation. After birth, the foramen ovale normally closes as left atrial pressure rises, leaving the fossa ovalis as a depression in the septum.

Structurally, the interatrial septum contributes to the integrity of atrial separation, ensuring unidirectional blood flow between systemic and pulmonary circulations. Pathologically, defects such as atrial septal defect (ASD) or patent foramen ovale (PFO) allow abnormal shunting between the atria, predisposing to paradoxical embolism, hypoxemia, and right heart overload.

Synonyms

  • IAS

  • Atrial septum

  • Septum interatriale

Function

  • Separates the right and left atria

  • Maintains pressure gradient and prevents mixing of oxygenated and deoxygenated blood

  • Serves as a site for fossa ovalis and PFO, relevant in fetal and postnatal circulation

  • Acts as a landmark for interventional cardiology procedures (e.g., transseptal puncture for left atrial access)

MRI Appearance

T1-weighted images:

  • IAS appears as a thin hypointense line between the bright blood pools of left and right atria

  • Fossa ovalis appears slightly thinner and less prominent

T2 Cine (Cardiac-gated):

  • Provides dynamic visualization of atrial motion and septal excursion

  • Detects septal bowing in pressure/volume overload (e.g., pulmonary hypertension, left atrial hypertension)

  • ASDs/PFOs appear as abnormal flow jets crossing the septum

STIR (Short Tau Inversion Recovery):

  • Fat suppression delineates the IAS against atrial walls

  • Useful in detecting inflammatory or infiltrative involvement of the septum

T1 Post-Contrast (Gadolinium-enhanced MRI):

  • Septum enhances minimally; gadolinium delineates atrial cavities

  • Contrast-enhanced perfusion may highlight shunts or abnormal flow across septal defects

  • Late gadolinium enhancement (LGE): may show septal scarring or fibrosis (e.g., in atrial cardiomyopathy)

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

  • Clearly depicts the IAS as a thin wall between atria in multiplanar 3D reconstructions

  • Excellent for detecting PFO, ASD, and septal aneurysm without contrast

  • Used in pre-interventional planning for transseptal procedures

CT Appearance

CT Coronary Angiography (CCTA):

  • IAS appears as a thin soft tissue structure between right and left atria

  • Fossa ovalis seen as a thinner portion of septum

  • ASDs or PFO may appear as contrast jet crossing between atria

  • Multiplanar reformats and 3D reconstructions allow precise septal defect characterization

  • CTA aids in sizing and planning septal occluder device placement

CT image

Interatrial septum CT axial image -img-00000-00000

MRI image

Interatrial septum mri image