Clinical History
A 67-year-old man complained of severe back pain and difficulty in mobility. The referring clinician ordered a MRI of the lumbar spine. He had no history of cancer or radiotherapy.
Imaging Findings
Sagittal T1w image (Fig. 1a) and sagittal T2w image (Fig. 1b) shows a hyperintense area in the anterior portions of the L4-L5 and L5-S1 intervertebral disc spaces. These areas show signal drop out on fat suppressive sequence (Fig. 1c), suggestive of fat content within the intervertebral disc. The lumbar vertebral bodies show normal stature, alignment and marrow signal characteristics. All other intervertebral disc spaces show normal signal intensity. No evidence of significant spinal canal/ neural foraminal stenosis.
Discussion
Fat replacement in the disc space is considered an additional and rare sign of degenerative changes. This may also be the result of migration of epidural fat into the disc space through defects in the disc annulus and nucleus pulposus [2]. Its incidence has not been recorded in the literature.
The anatomy of the intervertebral disc is simple and is best imaged with MRI. The intervertebral disc has a centrally located semiliquid nucleus pulposus and a peripherally located connective tissue annulus fibrosus [1]. On MRI, the normal intervertebral disc is isointense to muscle on T1W and bright on T2W images. Sometimes, the intervertebral disc appears with high signal in T1W images. High signal discs are demonstrated in proliferative and infiltrative bone marrow lesions due to effects of haematopoiesis, iron deposition, AIDS, radiotherapy, fibrosis and tumoral involvement [1, 2].
As a rule, in T1W images, an increased signal may represent fat, haemorrhage or a melanoma. This is also called the hyperintense disc sign / disc reversal sign. Calcification within discs is usually seen as hypointense areas, however, high signal intensity on T1 may be seen due to milk of calcium [2, 3, 4]. Confusion can be avoided by using the fat suppressive sequence on MRI or a simple plain radiograph to look for intradiscal calcification [3, 4].
Important differential diagnosis to consider are disc haemorrhage and oedema. Haemorrhage is usually seen as hyperintense signal on T1, T2 and STIR. Oedema shows increased signal on T2, STIR and low signal on T1. Myelofibrosis should be considered if multiple levels are involved with bone marrow changes.
To conclude, it is important to identify disc signals on all MRI sequences and all T1 hyperintense areas need to be explained in correlation with the fat suppressive sequence.
Differential Diagnosis List
L4-L5 and L5-S1 intervertebral disc fat
Intervertebral disc haemorrhage
Myelofibrosis
Final Diagnosis
L4-L5 and L5-S1 intervertebral disc fat
