Magnetic resonance imaging: comparative study of radiofrequency pulse techniques in the evaluation of focal cerebral ischemia.
The recently developed technique of magnetic resonance (MR) imaging utilizes radiofrequency (RF) radiation in the presence of a strong magnetic field to provide cross sectional displays of body anatomy similar to computed tomography. When utilizing MR, the operator alters tissue contrast electronically by changing RF pulse sequences. The three most frequently used RF pulse sequences are partial-saturation (PS), inversion-recovery (IR), and spin-echo (SE). We evaluated the sensitivity of these RF sequences to detect ischemic changes in our primate model. Serial MR scans were carried out using all three pulse formats 5 to 60 hours after middle cerebral artery occlusion (MCAO) in four animals. SE- and IR-sequenced proton MR images readily identified areas of evolving infarct 5 to 60 hours after MCAO, whereas PS scans that were performed during this acute period appeared normal. From 24 to 60 hours after MCAO, PS-sequenced scans showed focal areas of progressively decreasing signal intensity. However, SE and IR scans performed at the same intervals always demonstrated more extensive tissue changes. The basis of MR imaging, the effects of altering RF pulse sequences, and the resulting interpretation of changes observed in MR sections are presented.