A 59 year old man presents with worsening back pain and persistent low grade fevers. He has a history of spinal stenosis and underwent L4-L5 laminectomy with posterior fusion 4 months ago.
Radiographs performed several weeks earlier showed intact hardware, as well as vascular calcification and stents in the right renal, inferior mesenteric, and right iliac arteries:
He underwent further evaluation with lumbar spine MRI:
T1 (left), T2 (middle), and STIR (right) midline sagittal images of the lumbar spin demonstrate multiple findings:
Post-surgical changes of L4 and L5 laminectomy, with partial L3 laminectomy
T1 and T2 hypointensity of the L3 and L4 vertebral bodies, suggesting Modic type III (sclerotic) changes to related to degenerative disc disease
Mild loss of height in the L3-L4 intervertebral disc, with fluid signal appearing within the disc which was new compared to several months previously
Though nonspecific, these findings raised the possibility of discitis given the patient’s presentation. The patient went on to further nuclear medicine imaging…
This is a patient with Staphylococcus aureus discitis and osteomyelitis.
The Indium WBC scan demonstrates decreased uptake at the surgical site, and no uptake in the L3-L4 disc greater than background marrow activity. The Gallium-67 scan shows focal intense activity at the L3-L4 disc and inferior L3 vertebral body. Both SPECT/CT acquisitions show erosive endplate changes at L3-L4, which developed over several weeks when compared to initial radiographs above.
MRI is the first line imaging modality for spinal infection, and in the case of discitis will demonstrate abnormal fluid signal (often more pronounced than the early findings in this case). There may be associated marrow edema and/or enhancement, as well as epidural or paravertebral fluid collections.
In most circumstances, labelled white blood cell imaging (with either Tc-99m or In-111) is preferable to Gallium-67 for evaluation of musculoskeletal infection. One notable exception is the case of suspected discitis / osteomyelitis. A high proportion (approximately 50%) of spinal osteomyelitis cases show nonspecific decreased radiotracer uptake with labelled WBCs, especially with symptoms for > 1 month. Palestro CJ, et al. J Nucl Med 1991;32:1861-5. This was likely not known by the recommending radiologist in this case.
Gallium-67 has higher sensitivity and specificity than WBC imaging or bone scan for spinal infection (approximate accuracy 80%), but has generally been supplanted by FDG PET/CT imaging for this purpose in many centers (accuracy 90%). Fuster D, et al. Clin Nucl Med 2012;37:827-32. Seifen L, et al. Nuklearmedizin 2012;1:194-200.
