Diagnostic Tools: X-Ray, Bone Scan, MRI, CT Scan
Plain Radiographs (X-Rays)
X-rays are not "routinely" necessary for most episodes of acute low back pain and have generally been overused. The main purpose of plain x-ray is to detect serious underlying structural, pathologic conditions. Selective criteria can be used to improve the usefulness of plain x-ray. These studies are generally not recommended in the first month of symptoms in the absence of "red flags." Oblique views are rarely indicated and increase both the cost and radiation exposure. The exception would include a young patient with an acute injury or repetitive extension activities, which can result in fracture of the pars interarticularis.
Bone scans are rarely needed in the evaluation of acute low back pain. They can be helpful in cases where tumor, infection, or fracture (occult or traumatic) is suspected. They are limited by relatively poor spatial resolution of the pertinent anatomy of the spine. The use of SPECT scans provides superior anatomical resolution when compared to the more traditional uniplanar radionuclide images. The SPECT is especially useful in the detection of an isolated pars interarticularis fracture in spondylolysis. A positive bone scan finding should generally be followed by confirmatory imaging such as MRI or CT, which provide for greater pathoanatomic detail of the spine.
Magnetic Resonance Imaging (MRI)
MRI has demonstrated excellent sensitivity in the diagnosis of lumbar disc herniation and is considered the imaging study of choice for root impingement. This is tempered, however, by the prevalence of "abnormal" findings in asymptomatic (no symptoms) subjects. Its use should therefore be reserved for selected patients. Indications for immediate MR imaging of the spine may include: patients with progressive neurologic deficits or cauda equina syndrome, and patients with a suggestive presentation and known history or high risk for malignancy or inflammatory disease.
MRIs are not necessary in all patients with exam findings consistent with a radiculopathy, and in fact, should generally be reserved for those cases in which the imaging results are likely to guide treatment. MRI may be helpful in patients with signs and symptoms of neurogenic claudication due to suspected central or foraminal stenosis. They may also be useful in determining exact levels of pathology in the candidate for a selective nerve root block when physical examination and electrodiagnostic findings are otherwise not definitive. In the absence of red flags, many patients (even those with a classic radiculopathy) can and should be managed without an MRI, especially if they are not considered surgical candidates or are not interested in surgical treatment.
Some clinicians reserve MRI for those patients not responding to treatment as expected. The addition of gadolinium (contrast agent) is not necessary in the overwhelming majority of cases unless the patient has had a previous surgery or there is interest in the enhancing qualities of a previously observed lesion.
Computer Tomography (CT)
CT imaging of the lumbar spine provides superior anatomic imaging of the osseous (bony) structures of the spine and good resolution for disc herniation. Its sensitivity for detecting disc herniation when used without myelography however is inferior to MRI. CT with myelography is also not as useful in detecting far lateral or foraminal pathology. As with MRI, there can be a significant number of "positive" findings in the asymptomatic population. CT imaging is best used in the face of suspected fracture, but can be utilized in the detection of disc injury in patients who cannot undergo MRI scanning. In addition, for patients when more detailed imaging of the bony architecture is important, CT imaging is recommended.