Visual estimation of IVC collapse on B-mode (grey scale image) is generally preferred to M-mode, though in theory, M-mode measurement might be able to give accurate collapsibility index. There are several reasons for this. A major limitation of IVC M-mode is that the vessel moves mediolaterally and craniocaudally during respiration, with collapse occurring off axis from the true vertical. This gives erroneous measurements. Moreover, as we are measuring a 3-dimensional structure in 2 dimensions, it is possible that the ultrasound beam is slicing the vessel at its periphery leading to false measurement, = cylinder effect [see figure]. This typically happens when we are trying to hit the M-mode button on the machine and the probe position changes accidentally.

Below image demonstrates how the position of M-mode cursor can change with inspiration (= we are not measuring collapse at the same point). In this case, we tend to assume that IVC is collapsing lot more than it actually is.

Here is a nice example of cylinder effect as seen on B-mode. Images obtained from the same patient by 2 different providers a few minutes apart.

Here is another example illustrating that the IVC collapse does not necessarily occur in the plane of M-mode cursor (antero-posteriorly). Long axis image shows a big, minimally collapsing IVC (collapse should NOT be assessed close to IVC-RA junction, where there is pseudo-collapse due to diaphragmatic pull) while the transverse image shows side-to-side variation in diameter.