Question: Why does the breast need to be compressed for a mammogram?
Answer: So one of the reasons for compressing the breast is to hold it away from the chest wall. Another reason is that the thinner you can make anything you're X-raying the lower the dose that you'll need to use, so by thinning the breast we can use a lower X-ray dose. In addition, just as with any picture you take, if the breast moves during the study, then blurring will make it impossible to see the small details we need to see. So that the compression holds the breast still so there is no motion.
Another reason for compressing the breast is that if you do an X-ray picture, and an X-ray picture is really just like shining a light through the breast, if you shine the light through the thick back of the breast then the picture will be under-exposed and if you expose it so that the back of the breast is, as we say, well-penetrated, in other words we can see the details, then the front of the breast will be over-exposed. There will be too much light or too much X-ray that gets through. So the compression is also used to reduce or equalize the thickness, reducing the back of the breast thickness and making it more uniform so that the image is more easily interpreted.
Another reason for compressing the breast is that the structures in the breast are very irregular and they overlap one another. It's like a series of trees in a forest. By compressing the breast, we gently and safely are able to spread the structures apart so that it makes it easier to see the individual trees, if you will, in the forest as opposed to just all the different overlapping trees.
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