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Figure 8b.   Abdominal aortic aneurysm. (a) Contrast-enhanced MR angiogram shows an abdominal aortic aneurysm (arrows), but bilateral iliac arteries are not demonstrated. (b) Dynamic subtraction MR angiogram clearly shows the aneurysm (arrows) and iliac arteries (arrowheads) owing to subtraction of background signal, such as that from fatty tissue.