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Pectinate muscles

The pectinate muscles are parallel, comb-like ridges of myocardium found primarily in the right atrium and extending into the right atrial appendage. They are less prominent in the left atrium, where they are mostly confined to the left atrial appendage.

These muscular ridges run perpendicular to the crista terminalis in the right atrium, giving the endocardial surface a striated appearance. They are thought to contribute to atrial contractility, help reduce wall stress, and play a role in directing blood flow within the atrium. The relative smoothness of the atrial septal wall compared to the trabeculated appendages reflects embryological development, with the pectinate muscles originating from the primitive atrium.

Clinically, the atrial appendages (containing pectinate muscles) are of great significance, especially the left atrial appendage (LAA), which is the most common site of thrombus formation in atrial fibrillation, due to its trabeculated nature.

Synonyms

  • Atrial trabeculations

  • Auricular muscles

  • Musculi pectinati

Function

  • Increase atrial contractile force without markedly increasing atrial mass

  • Help in directing blood flow within the atrium

  • Contribute to structural reinforcement of atrial appendages

  • Clinically important as sites prone to thrombus formation, particularly in the left atrial appendage

MRI Appearance

T1-weighted images:

  • Pectinate muscles appear as intermediate signal intensity myocardial ridges projecting into the atrial lumen

  • Blood pool shows signal void in non-contrast imaging

T2-weighted images:

  • Muscles remain intermediate to low signal, distinct from hyperintense blood pool

  • Useful for assessing atrial anatomy and trabecular prominence

STIR (Short Tau Inversion Recovery):

  • Suppresses fat, improving contrast between myocardium and pericardial/mediastinal fat

  • Helps identify myocardial edema or inflammation involving atrial walls and appendages

T1 Post-Contrast (Gadolinium-enhanced MRI):

  • Myocardial ridges (pectinate muscles) show homogeneous enhancement similar to other atrial myocardium

  • Thrombi within atrial appendages appear as non-enhancing filling defects, distinguishable from enhancing trabeculations

Cine MRI (Cardiac-gated):

  • Demonstrates dynamic movement of pectinate muscles and atrial appendages throughout the cardiac cycle

  • Useful in evaluating atrial contractility, appendage filling, and thrombus detection

CT Appearance

Contrast-Enhanced CT (Cardiac CT / CCTA):

  • Pectinate muscles appear as fine linear or ridge-like trabeculations projecting into the atrial appendages

  • Distinguishable from thrombus by enhancement patterns (muscles enhance like myocardium; thrombus remains hypoattenuating)

  • High-resolution CT is particularly valuable in pre-ablation planning for atrial fibrillation and in thrombus evaluation

  • Multiplanar and 3D reconstructions clearly demonstrate atrial morphology and trabecular architecture

CT image

Pectinate muscles anatomy CT axial image -img-00000-00000

MRI image

Pectinate muscles mri image