a 37-year-old man presents with worsening back pain for 1 week. he states that he has had back pain for years, but over the last week, his pain has increased, and he now has weakness in his legs. physical examination reveals 4/5 strength in the left leg and 3/5 strength in the right leg with decreased sensation. he has numbness to his buttocks and a post-void residual of 150 ml. what imaging study should be obtained