F.L. Chan, M.D. responds:

MRI is the preferred technique to image infective spondylodiscitis as it offers multiplanar imaging capability, clear delineation of perivertebral soft tissue and intraspinal involvement, high sensitivity for early diagnosis, and reasonable imaging signs for monitoring responses to therapy. 
Compared with MRI, CT in general provides less information and may only be valuable at a later stage in the course of disease, although it can also visualize the disc narrowing, end-plate lesions, paravertebral involvement, and epidural abscesses, especially with contrast enhancement.  CT defines the bony trabecular destruction, paravertebral calcification, and intraosseous or intradiscal gas associated with spinal infection more clearly than MRI.

CT is applicable for cross-sectional imaging in a patient with infective spondylodiscitis under the following scenario:

1. When MRI is either inaccessible or contraindicated.
2. When precise anatomical display of the bone destruction, especially with 2-D or 3-D reformatted images, is warranted for surgery.
3. When imaging guidance is required for interventional procedures like
   transpedicular biopsy/drainage of disc or percutaneous drainage of psoas
   abscess.