Visual assessment of IVC collapse using B-mode (grayscale imaging) is generally favored over M-mode, although in theory, M-mode can provide an accurate measurement of the collapsibility index. There are several reasons for this. A major limitation of IVC M-mode is that the vessel moves craniocaudally and sometimes mediolaterally during respiration, with collapse occurring off axis from the true vertical. This gives erroneous measurements. Moreover, Since we’re visualizing a 3D structure in just 2D, the ultrasound beam can sometimes slice through the edge of the vessel instead of the center, giving a falsely smaller diameter – this is called the cylinder effect [see figure below]. It usually happens when the probe moves a little while hitting the M-mode button, even if you had a good diameter lined up at first.

The image below shows how the M-mode cursor can shift with inspiration because of the up-and-down (craniocaudal) movement of the IVC. This means we’re not measuring collapse at the exact same spot every time; and in this case, it makes the IVC look like it’s collapsing way more than it actually is.

However, some ultrasound machines do offer a ‘smart IVC’ feature to offset this issue, where the M-mode cursor automatically tracks the longitudinal motion of the vessel. Here’s an example.

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

The example below shows that IVC collapse doesn’t always happen along the plane of the M-mode cursor (antero-posteriorly). In the long-axis view, the IVC appears large with minimal collapse (note: the movement you are seeing at the IVC-RA junction is pseudo-collapse due to diaphragmatic pull), while the transverse view shows side-to-side variation in diameter.