Introduction
Magnetic resonance (MR) imaging has been used to detect ischemic neuronal damage following cardiac arrest in the human brain. Although it has been reported that high signal intensity is observed in the striatum in vegetative patients on T1-weighted MR images at 1–2 weeks after ischemia insult, the cause of the increase in signal intensity in the striatal region after cardiac arrest has not been investigated. We hypothesized that phagocytic activity of microglia generates fatty degeneration in the striatum and that this results in an increase in the signal intensity in T1-weighted MR images. The present study was designed to determine the possible involvement of fatty degeneration of damaged neurons in the increase in signal intensity in the striatal region using global ischemia model in rats.
Methods
Male Wistar rats were anesthetized with 1% halothane and were subjected to 20 minutes of 4-vessel occlusion. Five animals were used as sham controls. On day 1 (n=7), day 3 (n=7) or day 7 (n=7) after 4-vessel occlusion, rats were anesthetized with 1% halothane and were placed in a specially designed acrylic stereotaxic frame to obtain coronal T1-weighted MR images at the level of the striatum. T1- weighted imaging was performed using a 1.5 tesla unit with a 600-mm bore magnet equipped with an 85-mm (I.D.) detection coil. Images of 2 mm in section thickness were generated by a spin-echo sequence (repetition time of 500 msec and echo time of 15.0 msec). After obtaining MR images, each rat was immediately perfuse-fixed for histological evaluation. The frozen sections at the level of the striatum were double-stained with lectin and sudan black B for the detection of fatty degeneration and microglia, respectively.
Results
On days 1 and 3, there were no apparent abnormalities in MR images or in histological findings in the striatal region. On day 7, however, T1-weighted MR images showed high signal intensity in the area corresponding to the striatum. T1 values in the striatal region were 480 +−35, 483 +−59, 492 +−51 and 373 +−32 msec in the sham control rats and in the rats on day 1, day 3 and day 7 after 4-vessel occlusion, respectively. In the same rats, histological observation revealed infiltration of microglia in which numerous fatty drops were observed in the cell body. There was a close relationship between the number of lectin-positive cells and the T1 value in the striatal region (R = 0.87, p < 0.01).
Conclusions
The present study suggests that fatty degeneration of damaged neurons due to the phagocytosis of microglia plays an important role in the cause of the increase in signal intensity in T1-weighted MR images in the striatal region after global ischemia.