Changes in T2 relaxation time after stroke reflect clearing processes

Background and purpose: CT and MR imaging techniques are frequently used for the diagnosis and progress monitoring of ischemic stroke in clinical practice and research. After stroke, both methods are characterized by a transient pseudo-normalized imaging signal, the so-called fogging phenomenon. This study evaluates potential pathophysiological changes associated with fogging, as well as its influence on the correct determination of the ischemic lesion in a rat stroke model. Methods: Male spontaneously hypertensive rats were subjected to permanent middle cerebral artery occlusion. Ischemic lesion volume, brain edema and gray scale value spread within the ischemic lesion were determined on T2-weighted MR sequences at days 1, 4, 8, 11 and 29 after stroke onset, and compared with immunohistochemistry for astrogliosis, microglia/macrophage infiltration and angiogenesis. Results: All animals showed MR fogging at days 4, 8 and 11 after stroke. The transient normalization of T 2 signals occurred independently from the development of infarct volumes, but coincided well with the spatio-temporal occurrence of necrosis, angiogenesis and microglia/macrophage infiltration. Conclusions: Our results suggest that the fogging effect reflects the clearance of necrotic tissue within the ischemic lesion and is thus not relevant for the determination of the lesion volume.