There are a variety of ways to image the spine, many of which are expensive. The role of the radiologist is to ensure that diagnostic algorithms are selected for diagnostic accuracy, clinical relevance and cost-effectiveness. Each diagnostic imaging procedure has a different degree of sensitivity and specificity when applied to a particular diagnostic problem. A combination of imaging techniques can be used in a complementary way to enhance diagnostic accuracy. The appropriate use of the available methods of investigating the spine is essential, requiring a sensible sequence and timing of the procedures to ensure cost-effectiveness, maximal diagnostic accuracy and clinical effectiveness with minimum discomfort to the patient.

The philosophy underlying the management of low back pain and sciatica encompasses the following fundamental points:

The need for radiological investigation of the lumbosacral spine is based on the results of a thorough clinical examination. A useful and basic preliminary step, which will avoid unnecessary investigations, is to determine whether the predominant symptom is back pain or leg pain. Leg pain extending to the foot is indicative of nerve root compression and imaging needs to be directed towards the demonstration of a compressive lesion, typically disc prolapse. This is most commonly seen at the L4/5 or L5/S1 levels (90–95%), and MRI should be employed as the primary mode of imaging. If the predominant symptom is back pain, with or without proximal lower limb radiation, then invasive techniques may be required, including discography and facet joint arthrography. The presence of degenerative disc and facet disease demonstrated by plain films, CT or MRI has no direct correlation with the incidence of clinical symptomatology. The annulus fibrosus of the intervertebral disc and the facet joints are richly innervated, and only direct injection can assess them as a potential pain source. However, unless there are therapeutic implications, there is no indication to go to these lengths, as many patients can be managed by physiotherapy and mild analgesics.

Routine radiographic evaluation at the initial assessment of a patient with acute low back pain does not usually provide clinically useful information. Eighty-five per cent of such patients will return to work within 2 months having received only conservative therapy, indicating the potential for non-contributory imaging. Despite the known limitations of radiographs, it is often helpful to obtain routine radiographs of the lumbar spine before another investigation is requested. The role of conventional radiographs can be summarized in the following points:

CT and MRI have replaced myelography as the primary method for investigating suspected disc prolapse. High-quality axial imaging by CT is an accurate means of demonstrating disc herniation but, in practice, many studies are less than optimal due to obesity, scoliosis and beam-hardening effects due to dense bone sclerosis. For these reasons, and because of better contrast resolution, MRI is the preferred technique and CT is only employed when MRI cannot be used. MRI alone has the capacity to show the morphology of the intervertebral disc, and can show ageing changes, typically dehydration, in the nucleus pulposus. It provides sagittal sections, which have major advantages for the demonstration of the spinal cord and cauda equina, vertebral alignment, stenosis of the spinal canal, and for showing the neural foramina. Far lateral disc herniation cannot be shown by myelography, but is readily demonstrated by CT or MRI. CT may be preferred to MRI where there is a suspected spinal injury, in the assessment of primary spinal tumours of bony origin, and in the study of spondylolysis and Paget’s disease. MRI in spinal stenosis provides all the required information showing all the relevant levels on a single image, the degree of narrowing at each level and the secondary effects such as the distension of the vertebral venous plexus. The relative contributions of bone, osteophyte, ligament or disc, while better evaluated by CT, are relatively unimportant in the management decisions. Furthermore, MRI will show conditions which may mimic spinal stenosis such as prolapsed thoracic disc, ependymoma of the conus medullaris and dural arteriovenous fistula.

In addition to the diagnosis of prolapsed intervertebral disc, CT and MRI differentiate the contained disc, where the herniated portion remains in continuity with the main body of the disc, from the sequestrated disc, where there is a free migratory disc fragment. This distinction may be crucial in the choice of conservative or surgical therapy, and of percutaneous rather than open surgical techniques. MRI studies have shown that even massive extruded disc lesions can resolve naturally with time, without intervention. Despite the presence of nerve root compression, a disc prolapse can be entirely asymptomatic. Gadolinium enhancement of compressed lumbar nerve roots is seen in symptomatic disc prolapse with a specificity of 95.9%.¹

Finally, in the decision as to whether to choose CT or MRI it should be remembered that lumbar spine CT delivers a substantial radiation dose, which is important, particularly in younger patients.

The main remaining uses of myelography are in patients with claustrophobia or who are otherwise not suitable for MRI. There are advocates for the use of CT myelography in the investigation of MRI-negative cervical radiculopathy. Myelography also allows a dynamic assessment of the spinal canal in instances of spinal stenosis and instability. The use of a special MR-compatible spinal harness that provides axial loading, and the availability of open and upright MR scanners also provide non-invasive dynamic MR imaging capability.

The problems of the ‘post-laminectomy’ patient or ‘failed back surgery syndrome’ are well known. Accurate pre-operative assessment should limit the number of cases resulting from inappropriate surgery and surgery at the wrong level. The investigation of the postoperative lumbar spine is difficult and re-operation has a poor outcome in many cases. Although the investigation of the postoperative lumbar spine is difficult, it is vital to make the distinction between residual or recurrent disc prolapse at the operated level and epidural fibrosis in order to minimize the risk of a negative re-exploration. The best available technique is gadolinium-enhanced MRI.

Arachnoiditis is a cause of postoperative symptoms and its features are shown on myelography, CT myelography and MRI. In the past, many cases were caused by the use of myodil (Pantopaque) as a myelographic contrast medium. It is likely that the use of myodil as an intrathecal contrast agent caused arachnoiditis in most cases, but this became symptomatic in only a minority. The potentiating effects of blood in the CSF, particularly as a result of surgery, have been evident in many cases. New cases of arachnoiditis are now rarely seen, but there is residue of chronic disease still presenting from time to time.