The mitral annulus is a ring that is attached to the mitral valve leaflets. Unlike prosthetic valves, it is neither circular nor continuous. The mitral annulus is saddle shaped and it is dynamic in shape throughout the cardiac cycle.The annulus contracts and reduces its surface area during systole to help provide complete closure of the leaflets. Annular dilatation can result in poor leaflet apposition, leading to functional mitral regurgitation. 
The normal diameter of the mitral annulus is 3.1 ± 0.4 cm, and the circumference is 8 to 9 cm. There is no histologic evidence of an annular structure anteriorly, where the mitral valve leaflet is contiguous with the posterior aortic root. 
Rarely there can be a severe form known as caseous calcification of the mitral valve that can be mistaken for intracardiac mass or thrombus.
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- Perloff, JK; Roberts, WC (1972). "The mitral apparatus. Functional anatomy of mitral regurgitation". Circulation 46 (2): 227–39. doi:10.1161/01.cir.46.2.227.
- Otto, Catherine M.; Robert Bonow (September 2009). Valvular Heart Disease: A Companion to Braunwald's Heart Disease: Expert Consult - Online and Print. Elsevier Health Sciences. ISBN 1-4160-5892-3.
- Kosior, Dariusz A. (2014). "Mitral annulus caseous calcification mimicking cardiac mass in asymptomatic patient – multimodality imaging approach to incidental echocardiographic finding". Polish Journal of Radiology 79: 88–90. doi:10.12659/PJR.889830.
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