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Topic

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Left atrium

The left atrium is one of the four cardiac chambers, located posteriorly within the heart. It serves as the receiving chamber for oxygenated blood returning from the lungs via the four pulmonary veins (two from each lung). The left atrium forms the posterior surface of the heart (the "base") and communicates with the left ventricle through the mitral valve. Its walls are generally smooth, except for the left atrial appendage, which contains pectinate muscles. The left atrium plays a key role in filling the left ventricle, contributing to cardiac output and systemic perfusion.

Synonyms

  • Atrium sinistrum

  • Left heart atrium

  • Posterior atrial chamber

Function

  • Receives oxygenated blood from pulmonary veins

  • Acts as a reservoir during ventricular systole

  • Functions as a conduit for blood during early ventricular diastole

  • Provides atrial contraction ("atrial kick"), contributing up to 20–30% of left ventricular filling

  • Plays a role in maintaining efficient cardiac output and systemic circulation

Arterial Supply

  • Primarily from branches of the left circumflex artery (LCX)

  • Additional supply from small branches of the right coronary artery (RCA)

Venous Drainage

  • Drains into the coronary sinus via small atrial veins

  • Some venous drainage occurs directly into the left atrium through the venae cordis minimae (Thebesian veins)

Nerve Supply

  • Parasympathetic innervation: via the vagus nerve (CN X), modulating atrial contractility and conduction

  • Sympathetic innervation: from the cardiac plexus (thoracic sympathetic trunk), increasing heart rate and contractility

  • Rich autonomic innervation contributes to arrhythmogenesis (notably atrial fibrillation)

MRI Appearance

T1-weighted images:

  • Blood within the atrial cavity appears low signal (black) due to flow voids

  • Atrial walls appear intermediate signal intensity

  • Surrounding epicardial fat is hyperintense providing contrast

T2-weighted images:

  • Blood pool appears as a signal void with flowing blood

  • Wall appears intermediate to low signal intensity

  • Pathology such as thrombus or edema may appear hyperintense relative to normal myocardium

STIR (Short Tau Inversion Recovery):

  • Suppresses fat, allowing clearer visualization of edema, myocarditis, or infiltrative processes

  • Normal atrial wall remains low to intermediate signal

  • Areas of acute inflammation or infiltration appear bright hyperintense

T1 Post-Contrast (Gadolinium-enhanced):

  • Left atrial cavity opacifies with contrast, showing bright enhancement of blood pool

  • Pathology such as scar, fibrosis, or thrombus shows delayed or absent enhancement, aiding in detection (important in atrial fibrillation workup)

CT Appearance

Non-contrast CT:

  • Left atrium is seen as a thin-walled chamber posterior to the left ventricle

  • Lumen appears hypodense without contrast; thrombus appears as a filling defect

Contrast-enhanced CT (Cardiac CTA):

  • Chamber opacifies brightly with contrast

  • Excellent for assessing pulmonary venous connections, atrial size, and thrombus detection

  • CTA is especially useful in atrial fibrillation ablation planning to map pulmonary veins and atrial anatomy

MRI images

Left atrium MRI coronal image -img-00000-00000

CT image

Left atrium anatomy CT axial image -img-00000-00000