I think the big difference with diffusion is old from new or acute from subacute. We can see acute occipital infarcts in Figure 31 . If we look at the MRA, there is severe intracranial atherosclerotic disease. Here we can see that every vessel really looks bad. And the diffusion-weighted image is abnormal in the same areas.
Diffusion returns to normal within seven days after an ischemic event.
So diffusion is really only positive in the hyperacute and acute phases and this is a very important distinguishing factor. Figure 35 shows that one of these UBOs is positive on diffusion images. This is a small acute infarct in addition to multiple small focal areas of UBOs. This next case best illustrates the distinction between what is seen on various pulse sequences in a patient who continued to throw emboli and had a variety of different infarcts. The occipital posterior temporal infarct on the left is positive on diffusion and is acute.