The enduring role of M-mode in veterinary ultrasonography

The enduring role of M-mode in veterinary ultrasonography
Catherine

Once upon a time, M-mode was cutting edge technology. It represented a huge leap forward from a single A-mode line, which showed only the distance of the reflector from the transducer and its amplitude, as displayed against an x and y-axis, respectively. M-mode was therefore a lot more user-friendly, with amplitude of returned echos being depicted by different amplitudes on the image (such that strong reflectors were shown in bright white, weaker reflectors in shades of grey, and non-reflectors in black, as we are very familiar with in modern B-mode systems), and movement of structures being easy to observe as the image was plotted against time. For cardiology in particular, this was groundbreaking.

Below is a one-beat clip of the left ventricle in a normal human male (reproduced with permission), followed by an M-mode image of the same ventricle. Modern M-mode will always show a miniature B-mode preview above, so that the user can align their M-mode line correctly. This is really important when performing M-mode traces of fetal hearts in pregnant dogs, for example, as the dimensions are so much smaller that it can be easy to miss the heart altogether.


M-mode is still commonly used in echocardiography labs across the United Kingdom and United States, but is gradually falling out of favour as a new generation of sonographers enter the workplace. M-mode measurements of the left ventricle, for example, are all very well in young, echogenic subjects for whom an on-axis image is easily obtainable, but in some patients, it is virtually impossible to bring the walls of the heart perpendicular to the ultrasound beam. This means that an M-mode line will always be cutting through the structures at an angle, resulting in measurement inaccuracies. This can be dangerous if it makes a patient fall just inside or outside of normal values, labeling them with a potentially misleading diagnosis.

For qualitative as opposed to quantitative work, however, M-mode still has many important uses. To say that M-mode has better temporal resolution than B-mode is not quite true in modern day ultrasound machines, however, it can certainly display temporal information in a way that makes it quicker and easier for us to appreciate than a moving image. It is particularly useful for looking at the duration of flow reversal in cases of aortic regurgitation by imaging the descending aorta (and even the abdominal aorta in cases of severe regurgitation) or across the left ventricular outflow tract. It can also be used to look for systolic anterior motion (SAM) of the mitral valve in hypertrophic obstructive cardiomyopathy (HOCM), and some people find it useful to look for the fluttering of leaflet tips in regurgitant lesions or even endocarditis.

In the veterinary world, M-mode still appears to be the gold standard in terms of left ventricular measurements in the UK, presumably because it is advocated by those teaching it at universities as well as the majority of veterinary ultrasound textbooks. It is also a fact that very few veterinarians specialise in echocardiography and, as yet, there is no such role as a lay veterinary echocardiographer, so upholding standards nationally and keeping them up-to-date can be a real challenge.

 

If you are interested in echocardiography, you can download the full study here (may take about a minute to download as it contains a number of video clips). This scan followed published BSE guidelines.

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