A magnetic resonance imaging (MRI) scan is not always necessary to diagnose sciatica, but it can be used to assess the anatomy of the spine. An MRI scan provides detailed 3-dimensional views of spinal bones and tissues, and is particularly good at picking up problems where the nerves are being pinched. However, no single imaging technique is 100% accurate and each has its limitations. An MRI is done with the patient lying on their back (supine). In this position, the spine is not loaded in the same way as when it is upright. Some nerve impingements only occur in the upright position, and the MRI scan might miss those.