So with strain imaging, the thickness of the breast is not a factor. On strain imaging, as long as you can get a B-mode image, you can get an accurate strain elastogram. Usually between 4 to 4 1/2 centimeters we can get accurate measurements. However, usually we're not able to generate shear waves much deeper than 4 1/2 centimeters, and therefore we don't get any information beyond that depth. One of the problems with shear wave imaging is that it uses a push pulse to generate the shear wave, and it’s attenuated, like all other ultrasound beams, by tissue so it has a problem at depth. They’re complementary tests and they’re measuring the same thing, but each has its advantages and disadvantages. Strain vs Shear WaveSo the question that’s often asked of me is should I buy strain or should I buy shear wave imaging. I would tend to focus on looking at the B-mode image to make sure you’re scanning things appropriately, and not look at the elastogram, but to save a video clip and then go back and review the elastogram from the video clip. You can use the B-mode image to determine the amount of displacement of tissues. Cancers are almost always much different than normal breast tissue. If the lesion has very similar color on the elastogram as normal breast tissue, it’s probably benign because it has a very similar stiffness. If the lesion is a cancer, it will be the stiffest lesion, and color-coded black.Īnother technique is to always compare the stiffness to other tissues, so what we want to do is compare the stiffness of the lesion to normal breast tissues. If you can include pectoralis muscle, if the lesion is not a cancer, the muscle will be the stiffest tissue and color-coded as black. Using this technique, fat is going to be the softest tissue and will always be displayed as white. This technique will get a little bit easier to interrupt (interpret?) and images will appear more constant-from patient to patient, or even within the same patient. The reason is that we now have a wide range of tissue stiffnesses, and that will help keep our dynamic range of the scale very constant. We want to include fat, normal breast tissue and the lesion, and to include the pectoralis muscle if possible. Technique TipsTo get a consistently good Elastogram, it is important to keep the field of view large. This limits the use of this technique in screening. Fat in an entirely fatty breast will appear darker than in a very dense breast. Therefore a given tissue will have a different shade of gray in each elastogram. The Compression Elastogram generated displays the relative stiffness to the rest of the image. The dynamic range of the coloring, either in black or white or in color, is going to be based on what is the stiffest and what is the softest tissue in the field of view. The other three grayscale boxes are varying differences, and they would be colored a shade of gray based on the amount of stiffness they have. It has changed the most, so it’s therefore the softest lesion, and that would be color-coded white. If you look at the box at the bottom of the second column. If we use a grayscale map where black is the hardest, that box would be color-coded black. So if we look at the box, in the middle of the second column, it does not change shape at all and is therefore the hardest. The second column is where we applied the stress, and you can see the boxes change shape based on how stiff they are. That’s what the tissues looked like before we applied the stress. In the column on the left side, you can see that there are five boxes.
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