MRI in acute cerebral ischaemia: perfusion imaging with superparamagnetic iron oxide in a rat model.

An imaging technique capable of detecting ischaemic cerebral injury at an early stage could improve diagnosis in acute or transient cerebral ischaemia. We compared the ability of superparamagnetically contrast-enhanced MRI and conventional T2-weighted MRI to detect ischaemic injury early after unilateral occlusion of the middle cerebral artery in 12 male Wistar rats. Permanent vessel occlusion was achieved by a transvascular approach, which has the advantage of not requiring a craniectomy. At 45-60 min after the procedure, the animals had conventional T2-weighted MRI before and after administration of a superparamagnetic contrast agent (iron oxide particles). Unenhanced images were normal in all animals. After administration of iron oxide particles, the presumed ischaemic area was clearly visible, as relatively increased signal, in all animals; this high signal area corresponded to the area of ischaemic brain infarction seen on histological studies. Magnetic susceptibility effects of iron particles cause low signal in normally perfused cerebral tissue, whereas tissue with reduced or absent blood flow continues to give relatively high signal. Our results suggest that superparamagnetic iron particles may significantly reduce the interval between an ischaemic insult and the appearance of parenchymal changes on MRI.