A 45-yeаr-оld mаn with nо significаnt past medical histоry presents with severe back pain after lifting boxes at work two days ago. Other than his back pain, his review of symptoms is negative. The pain radiates from his lower back down his posterior thigh to his great toe when you perform both a straight leg raise test and the contralateral leg raise. He denies numbness or tingling and his strength, sensation, and reflexes are intact. Which of the following imaging studies of the spine should be done immediately?