Clinical Radiology (1991) 44, 1 2

Editorial The Current Status of Myelography

Myelography continues to be a commonly performed examination in the United Kingdom, in large measure because of nationwide under-provision of CT scanners as well as the more striking deficiency in MR! facilities. Experience, particularly from North America, indicates that MRI can significantly reduce the demand for myelography, and there is certainly a strong case for avoiding the latter. Myelography can be an unpleasant and painful procedure, particularly for patients with limited mobility and those with a spinal 'block'. Compli- cations may also develop which are due to the inherent neurotoxicity of the contrast media used and to the lumbar puncture itself. On occasion a myelogram can cause deterioration in the patient's condition and even the adoption of the standard position for cervical myelogra- phy, with the patient prone and the neck extended, has been reported to narrow further an already compromised spinal canal sufficiently to produce symptomatic cord compression (Robertson and Smith, 1990). The nature of the myelographic examination is such that the presence of spinal pathology or neural compression is inferred by distortions of the contrast column rather than by the 'direct' visualization possible with CT or MRI.

Myelography is, on the other hand, a familiar tech- nique, especially to our clinical colleagues. The study can embrace the whole length of the spinal canal and is, to some extent, dynamic, enabling the effects of spinal instability on the thecal sac to be demonstrated. The definition of a 'spinal subarachnoid block' is based on the myelographic appearances alone.

The introduction of CT or MRI in place of myelogra- phy requires referring clinicians to pursue a learning curve, and there may be some resistance to this. The radiologist must then shoulder twin burdens of responsi- bility: first interpretation, in what may be to him also a novel technique; and second persuading clinicians to consider a non-invasive option to spinal imaging where appropriate.

The quality of the image varies between different CT scanners and in MRI, which is still undergoing rapid technological advances. None of the statements made here should therefore be considered 'engraved in stone' but equally it is counterproductive to claim complete mastery for MRI in all aspects of spinal imaging.

Myelography supplemented by CT has gained wide acceptance, combining the advantages of two techniques, and is the 'gold standard' with which MRI and plain CT must be compared.

The relative accessibility ofmyelography, CT and MRI is an important determinant of their usage but their merits are best discussed assuming uniform availability and interpretative skills and the provision of optimal quality images. The importance of adequate patient co-operation to a successful MR examination should not be underesti- mated. In the literature the comparative value of myelo- graphy, CT and MRI is often, quite reasonably, described m terms of individual pathologies, but a number of diagnostic possibilities mayexist following clinical assess- ment whose anatomical location is perhaps not entirely clear. Axial CT is quite inappropriate for an Overview of

the spine and MRI, with its ability to produce longitudi- nal images of the spine, has a much better claim in this respect. Nevertheless if gadolinium is to be used, the MR examination can become very time consuming unless there is careful supervision of the examination and tailoring of protocols to address the clinical problem.

To what extent and under what circumstances can non- invasive imaging replace myelography?

In cases of suspected prolapse of a lumbar interverte- bral disc it is surely time for myelography to be aban- doned as a first investigation in favour of CT or MRI. MRI has the advantage over CT that the unsuspected conus region tumour can be visualized in standard sagittal views which routinely embrace the lower thoracic segments, Myelography is known to be insensitive to a lateral disc protrusion, well shown by axial CT or MRI (Novetsky et al., 1982). The sagittal MR image is valuable to assess vertebral alignment, disc height and water content, but it is in the axial plane that neural compres- sion is best sought. Anecdotally one is aware of superior spatial resolution of CT in this plane, particularly when comparing 'high resolution' CT myelography using the normal wide windows with its MR equivalent - the T2- weighted axial image. Additionally, the contrast between bone and disc is greater and bone detail is easier to appreciate with CT because of the heterogeneous signal from bone on MRI depending on its mineralization and marrow content. These considerations are also important in the assessment of contributions of bone and soft tissue to lumbar canal stenosis.

Compressive spondylotic cervical myelopathy can be shown well using MRI, which should now also be regarded as the investigation of choice in rheumatoid disease of the craniovertebral junction. The patient with suspected compressive cervical radiculopathy should continue to have a myelogram supplemented by axial thin section CT at the levels indicated by the clinical or the myelographic findings. The distinction between neural compression due to osteophyte and that due to disc may have surgical implications and is usually shown quite clearly with CT. Plain axial CT, with intravenous contrast medium to enhance the epidural vessels, can demonstrate a cervical disc prolapse. The degree of neural compression encountered may however be rather subtle and demand intrathecal contrast medium for clarification. In addition, the number of fine axial sections required to demonstrate the neural foramina renders an examination of the entire cervical spine quite impractical. Rarely is clinical confi- dence so great as to authorize scanning only a single cervical vertebral level.

Elegant demonstration of intradural extra-medullary mass lesions is possible using MRI, especially with gadolinium enhancement. This would seem a clear way forward and should be adopted as the method of choice in combination with CT, if necessary, to gauge any bone erosion. I f there is to be greater reliance on MRI then plain films of the spine are important to assess bone mineralization and to identify vertebral levels. It is possible to mark the level of any spinal lesion discovered with MRI using fish oil capsules taped to the skin surface

2 CLINICAL RADIOLOGY

(Thomson, 1988). MRI offers obvious advantages to patients with central neurofibromatosis and indeed to any patient for whom repeated spinal examinations may be required. MRI is also superior to all other techniques in the demonstration of intramedullary pathology. Very early on its value in demonstrating a syrinx cavity even in those cases where the spinal cord is of a normal or near normal calibre became evident.

Myelography and CT myelography rarely demonstrate anything other than a contour abnormality in tumours of the spinal cord, and where this is subtle or absent, as in some intramedullary metastases, MRI with gadolinium enhancement has no equal.

The situation regarding extradural metastatic disease is rather more problematic since for MRI to be a viable option the facility must not only exist near to a neurosur- gical centre but also operate an 'out-of-hours' service. In progressive spinal cord compression due to metastatic disease, prompt assessment and treatment are essential. To this end myelography can be accomplished fairly rapidly and the site of obstruction located. With further injection of contrast medium or isotonic saline it is usually possible to circumvent the 'block' and, using CT if necessary, to identify any further lesions which may affect management. CT is, in any case, valuable to show the extent of extraspinal involvement and the pattern of bone destruction, which may determine the surgical approach.

A combination of MRI with contrast enhancement and supplementary CT is an attractive alternative to myelo- graphy with its attendant morbidity in cases of spinal subarachnoid 'block' (Hollis et al., 1986) but if radio- therapy alone is contemplated then CT is less of an imperative. Nevertheless for practical purposes the mye- logram will be with us in the UK for many years to come in the diagnosis of metastatic thecal compression.

The argument for MRI with supplementary CT can be extended to the diagnosis of congenital spinal anomalies where MRI is the examination of choice (Jaspan et al., 1988) and this must be a particular kindness to affected children.

In the diagnosis of suspected spinal vascular malforma- tions there seems little reason to abandon myelography followed by spinal angiography as the definitive diagnos-

tic pathway, since MRI cannot currently be relied upon as a screening test for these often subtle lesions. MR! can however be valuable in the detection of intramedullary angiomas.

Spinal trauma is a relatively rare indication for myelo- graphy but MRI confers advantages in showing directly traumatic cord damage and haematoma. Plain CT is used regularly in the author's own unit to elucidate spinal fractures.

In conclusion, the continued popularity of myelogra- phy in the UK results not only from the absence of any realistic alternative in many hospitals but also because of a degree of clinical conservatism. The case in terms of economic justification and clinical utility for the installa- tion of MR imagers in National Health hospitals has been made persuasively by du Boulay et al. (1990). Neverthe- less even when MRI is available, myelography has a useful part to play in the investigation of some spinal disorders but, for many - perhaps most - patients, not in the first instance. Rather it should be called upon if non- invasive methods fail to provide the necessary informa- tion.

REFERENCES

Du Boulay, GH, Hawkes, S, Lee, C-C, Teather, BA & Teather, D (1990). Comparing the cost of spinal MR with conventional myelography and radiculography. Neuroradiology, 32, 124 136.

Hollis, PH, Malis, LI & Zapulla, RA (1986). Neurological deterioration after lumbar puncture below complete spinal subarachnoid block. Journal of Neurosurgery, 64, 253-256.

Jaspan, T, Worthington, BS & Holland, IM (1988). A comparative study of magnetic resonance imaging and computed tomography - assisted myelography in spinal dysraphism. British Journal of Radiology, 61, 445-453.

Novetsky, GJ, Berlin, L, Epstein, AJ et al. (1982). The extraforaminal herniated disc: detection by computed tomography. American Journal of Neuroradiology, 3, 653-655.

Robertson, HJ & Smith, RD (1990). Cervical myelography: survey of modes of practice and major complications. Radiology, 174, 79-83.

Thomson, JLG (1988). A simple skin marker for magnetic resonance imaging. British Journal of Radiology, 61,638.

PAUL BUTLER Department o f Neuroradiology,

The Royal London Hospital