M-mode stands for motion mode and this uses a stationary transducer with a moving recording device.

M-mode detects all motion down a line drawn through the tissues. It is a useful modality to measure moving objects, classically the cardiac walls and valves.

Essentially, M-mode takes a single line of the B-mode image and displays changes within this over time. Movement of reflectors along this line of sight generates a pattern across the monitor or a moving paper strip chart recorder.

The image shows a B-mode view of the heart (top) with the M-mode view (below). B-mode data is collected from along a single line of sight (indicated by the dotted line) and displayed across a time base. Movement of the mitral valve leaflet and the left ventricular walls is demonstrated. Click the image to see a larger version.