Fibromuscular elements of the right atrium: pseudomass at MR imaging.

Results of 20 random cardiac spin-echo and cine gradient-echo magnetic resonance (MR) imaging examinations were reviewed for nodular and/or linear soft-tissue structures projecting from the right atrial wall. A nodular soft-tissue structure (mean diameter, 6 mm) isointense with myocardium was observed along the posterior atrial wall in 18 of 20 patients (90%). The structure extended between the orifices of the superior and inferior venae cavae. Linear strand-like projections originating from the structure coursed across the atrial chamber in 5 of 20 patients (25%). Similar findings were present in one patient who underwent cardiac surgery for a suspected atrial mass. The region of nodular thickening was shown histologically to represent myocyte hypertrophy and fibrosis. The nodular and linear structures correlated anatomically with the crista terminalis muscle bundle and Chiari network, respectively. These structures are frequently visualized on cardiac MR images, are of variable prominence, and may closely simulate the appearance of tumor or other cardiac masses in some patients. Awareness of the location and MR imaging features of these structures will help prevent misdiagnosis.