Ordering an X-ray or an MRI can sometimes become reflexive. A patient has lower back pain so an X-ray is ordered. Why is it ordered? What does it hope to detect or rule out? Let us pause for a moment to consider what imaging studies tell us, what they don’t tell us, and when they should be ordered for disorders of the lumbosacral spine.

Let’s start with the basics. Imaging studies give us a picture of the anatomy of the spine, but they don’t tell us if the pain is coming from that structure. Facet joint arthropathy, degenerative disc disease, and herniated discs that are evident on imaging studies may be causing a person’s pain, but they may also be incidental findings [1]. Sometimes, the best looking segment on an MRI can be causing the person’s pain. As will be discussed in greater detail in subsequent chapters, even in the case of radiographic findings of osteoporotic compression fractures, the pain may be coming from a completely different structure. Imaging studies are important as the best way to visualize the anatomy short of direct visualization during surgery, but their findings must be taken in the context of their inherent limitations, namely, that they show anatomy and not pain.

As discussed in Chap. 3, imaging studies are not indicated in acute lower back pain in the absence of red flag signs or symptoms [1]. If neurologic signs and symptoms are present, then an X-ray is not likely to be useful but an MRI may be indicated. MRI is the best noninvasive way to visualize the spine, including the soft tissues, discs, and nerves [2]. If the patient has a history of cancer or if the patient has a history of spinal surgery at the spinal level in question, then MRI with and without contrast is indicated. CT scans can also show detailed anatomy of the lumbosacral spine but recall that a single CT scan uses significantly more radiation than an X-ray so limiting their use is preferred if possible [3, 4]. A CT myelogram may show better surgical anatomy [5], but the use of CT myelogram, which involves intrathecal injection of contrast and is often painful, is generally limited to presurgical decision-making.

If a stress fracture is being considered as part of the diagnosis, then X-rays can be obtained. If a spondylolysis is suspected, then it is important to order oblique X-rays. X-rays may miss acute stress fractures or very mild fractures. Therefore, the absence of a fracture on X-ray does not conclusively rule out a fracture. CT scan and MRI offer better evaluation of the spine for that purpose [6, 7].