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©2013 UpToDate ® Print Email Apical two chamber view of the left ventricle and mitral valve Mitral annular calcification (MAC) is seen as a bright echodense structure along the posterior mitral valve annulus as calcium is a dense ultrasound reflector. In the lower portion of the image, there is absence of ultrasound penetration in the far field distal to the MAC due to acoustic shadowing. With severe acoustic shadowing, the left atrium may be obscured, making it difficult to identify and quantify mitral regurgitation. Courtesy of Rosario V Freeman, MD, MS.