I heard something troubling today. Two physicians were talking about 3-D imaging (rotational angiograpny), and one said to the other, “You can get subtraction on 3-D without doing two runs. The software subtracts it for you.” My first thought was, “How wrong can he be?” Then I thought about it. While it is technically incorrect, that is the effect. When you perform a 3-D spin, the software automatically enhances the contrast material in the vessels while adjusting the window and level settings (contrast and brightness) so that the vessels appear free of bone and other structures. It looks like a 3 dimensional subtracted image. However, if the window and level settings are adjusted, the other structures show up, proving that it is not a true subtraction. I choose to call this “pseudo-subtraction”.
True subtraction only occurs when two images are acquired: a positive, with contrast, and a negative, without contrast. The two are overlaid so that only the difference between the two shows: the contrast-filled vessels. A subtracted 3-D image must include two runs; a mask run without contrast and an image run, with contrast. Each 3-D run consists of multiple images and each image is overlaid over its corresponding image in the other run to create the subtracted image. In a true subtraction, adjusting the window and level will not show any new bony structures. They are removed, or “subtracted out” of the image.
If you choose to think of the 3-D reconstruction as a subtracted image, please remember that it is only “pseudo-subtracted”. A true subtraction MUST include 2 separate runs…even a 3-D spin!