Cerebral Hemodynamics on MR Perfusion Images before and after Bypass Surgery in Patients with Giant Intracranial Aneurysms

Patient with a giant intracranial aneurysm of the left internal carotid artery (patient 2).

A, Maps of rCBV (top left), CBFi (top right), MTT (bottom left), and TTP (bottom right) before surgery. The MTT and TTP maps show a large hyperintense area on the left hemisphere compared with the contralateral (mean asymmetry index ± SD, 1.46 ±.0.34), indicating slower transit time in the side of the aneurysm. The CBFi maps show reduced flow in the same region (asymmetry index, 0.73 ± 0.09), whereas the rCBV maps show a minimal increase (asymmetry index, 1.13 ± 0.11). This hemodynamic pattern, characterized by a mismatch between flow and volume, can be explained as a vasodilatory response of the brain to a decrease in perfusion pressure.

B, Maps of rCBV (top left), CBFi (top right), MTT (bottom left), and TTP (bottom right) after extra-intracranial bypass surgery. Temporal parameter maps (MTT and TTP) in the left hemisphere are only slightly elevated compared with the contralateral side (asymmetry index 1.23 ± 0.44), especially in the posterior region. The CBFi and rCBV asymmetry indices are only minimally altered (asymmetry index of CBFi, 0.91 ± 0.08; of rCBV, 1.11 ± 0.32).

C, Postoperative T2-weighted images show the large aneurysmal sac, which is hyperintense, suggesting absence of flow due to exclusion from circulation.