Page 5 - The Assertive Short Axis (part 2 - lessons from MRI tagging)
In MRI tagging, a grid pattern is deposited on the first image (on the left), at end diastole. The myocardium and the blood are both labeled by this grid, and retain the label in subsequent images. However, the blood flows out of the image and is replaced by unlabelled blood, so the images after the first are of the myocardium alone. The distortion of the grid pattern reflects the change in shape and position of these blocks of myocardium during the cardiac cycle. In the images above, the central pair are at end systole and the pair on the right are the last images in the sequence, just before end diastole. Note how the grid distortion returns to normal: this helps convince us that the grid distortion is a reasonable representation of what is happening in systole.
The short axis images, on the upper row, show how the myocardium has to distort to reshape the wall to its systolic position. I like to call this distortion "repacking". Note that the distortion is greatest in the subendocardial region.
I find the lower, long axis images just as interesting. On the last image the number of grid lines that correspond to the long axis dimension can be counted and their position in the diastolic and end systolic images determined, yielding the change in long axis dimension. It can be readily appreciated that the muscle in the wall, which is being reshaped by the long axis contraction, shows a very regular concertina effect. Each segment becomes a thinner and longer rectangle. Its outer margin cannot not bulge out as the wall is constricting in the short axis, as we can see from the short axis images. This must mean that the inner cells are being propelled toward the lumen at greater speed and distance than the outer. This fits with the repacking in the short axis, where some of the squares are projecting as wedges towards the lumen. This propulsive effect has a profound effect on the end-systolic cavity diameter, and is what was alluded to on page 2. Ingels called it the "hydraulic effect", and I cannot think of a better word for this difficult concept.
These images infer that a model of the LV wall can be constructed out of the changes expressed in just two planes, the short axis plane plus any of the long axis planes. In the next few pages, the regularity of the long axis repacking will assist greatly in the formation of a geometric model.