What does the m. r. i. do?

核磁共振到底是干嘛的？

When you fell out of bed this morning, you hit your temple, hard. The c. t. didn't catch it, but the m. r. i. did.

今天早晨你从床上摔下来的时候撞伤了额头，蛮严重的。CT没有看到，但磁共振看到了。