Medical imaging
Because CT and MRI are sensitive to different tissue properties, the appearances of the images obtained with the two techniques differ markedly. In CT, X-rays must be blocked by some form of dense tissue to create an image, so the image quality when looking at soft tissues will be poor. In MRI, while any nucleus with a net nuclear spin can be used, the proton of the hydrogen atom remains the most widely used, especially in the clinical setting, because it is so ubiquitous and returns a large signal. This nucleus, present in water molecules, allows the excellent soft-tissue contrast achievable with MRI.
Low back pain
Imaging is indicated when there are red flags, ongoing neurological symptoms that do not resolve, or ongoing or worsening pain. In particular, early use of imaging (either MRI or CT) is recommended for suspected cancer, infection, or cauda equina syndrome. MRI is slightly better than CT for identifying disc disease; the two technologies are equally useful for diagnosing spinal stenosis. Only a few physical diagnostic tests are helpful. The straight leg raise test is almost always positive in those with disc herniation. Lumbar provocative discography may be useful to identify a specific disc causing pain in those with chronic high levels of low back pain. Similarly, therapeutic procedures such as nerve blocks can be used to determine a specific source of pain. Some evidence supports the use of facet joint injections, transforminal epidural injections and sacroilliac injections as diagnostic tests. Most other physical tests, such as evaluating for scoliosis, muscle weakness or wasting, and impaired reflexes, are of little use.