M-mode is always listed in the specifications of any ultrasound system, and for many veterinarians, it is considered essential. Nearly all of our cardiac reference values, for example, are M-mode values. M-mode displays all the pixels along a single scan line, plotted against time.
M-mode has been shown by a number of authors to overestimate dimensions when measuring through spherical structures, however. This is because if the axis through which you are measuring is not exactly perpendicular to the M-mode line, it will be cut obliquely.
Example: left ventricular dimensions
Look at the two pictures below. The top one is a left ventricle from the right parasternal view which is perfectly on-axis. The interventricular septum and posterior wall are perpendicular to the ultrasound beam. The bottom image shows the left ventricle slightly off-axis.
Imagine running an M-mode beam through each of these ventricles. In the top one, you would obtain measurements which should be comparable to if you took the measurement in B-mode. On the bottom image, however the M-mode beam would cross the ventricle obliquely, resulting in a significant overestimation of the left ventricular diameter.
In impatient and sometimes distressed animals, it is not always possible to achieve a perfectly on-axis view. Some animals are so breathless that they cannot even be laid down on their side. M-mode is not a suitable tool for measurement in such animals, but with B-mode, you can measure with respect to the axis of the heart – not the transducer.
As mentioned above, the big problem is reference values. Normal values were collected by enthusiastic researchers back when this was all new – and those were the times of M-mode. B-mode normal values for every breed of dog and cat simply do not exist. It is all very well knowing that B-mode measurements are now superior to M-mode, but then what do you compare your values to?
For now, my recommendation would be to take both, particularly in borderline cases. You may wish to index your B-mode measurements to BSA. Ultrasound is an interactive imaging modality and any interpretation needs to be made in context and with a strong dose of good judgement.
Good uses of M-mode
M-mode is excellent for other aspects of cardiology. Some examples include:
- Colour M-mode across the left ventricular outflow tract in cases of aortic regurgitation (measuring jet height to LVOT diameter)
- M-mode to demonstrate systolic anterior motion (SAM) of the mitral valve
- M-mode to demonstrate subtle leaflet fluttering – for example, the anterior mitral valve leaflet due to an impinging aortic regurgitant jet, or the fluttering of a strand or valvular vegetation.