MRI semiotics of acute vertebral osteomyelitis
Magnetic resonance imaging allows the detection of infectious processes at early stages and accurately differentiates them from herniations or malignant neoplasms. The typical imaging pattern of pyogenic (suppurative) spondylodiscitis involves one intervertebral disc and two adjacent vertebral bodies.
Key MRI findings:
- Bone marrow: edema develops, presenting with low signal on T1-weighted images (T1WI) and high signal on T2-weighted images (T2WI), as well as on STIR sequences. The endplates undergo destruction.
- Intervertebral disc: the intranuclear cleft disappears. The disc appears hypointense on T1WI and hyperintense on T2WI.
- Soft tissues: to evaluate paravertebral and epidural fluid collections (abscesses), gadolinium-enhanced T1WI with fat saturation (fat saturation) is used. Purulent collections appear as areas of high signal on T2WI.
Important clinical rule: when infection is suspected, the entire spine must be scanned. Epidural abscesses can extend caudally at a significant distance from the primary disc lesion, causing spinal cord compression.
Capabilities of X-ray and CT
Standard plain radiography has very low sensitivity in the acute phase of the disease. In early stages, only minimal irregularity of the endplate contours may be noticeable. With established osteomyelitis, the radiograph will show marked bone destruction, paravertebral soft tissue edema, and the development of kyphoscoliosis.
Computed tomography (CT) with multiplanar reconstructions is used if MRI is contraindicated. However, the main task of CT is to provide precise navigation during coaxial percutaneous biopsy of the affected bone or disc. The obtained samples are sent for histology and culture. Culture yield is up to 77%, but it drops critically if antibiotic therapy has already been initiated prior to the biopsy.
Pyogenic and tuberculous infections: what is the difference?
The causative agent of purulent osteomyelitis is most frequently Staphylococcus aureus, which reaches the vascular-rich endplate zone hematogenously (from skin foci, in endocarditis, septic arthritis). On MRI, such lesions demonstrate diffuse and contrast enhancement, while developing disc abscesses show typical ring-like contrast enhancement.
Tuberculous spondylitis has several distinct features:
- The infection often spreads beneath the ligaments, involving three or more vertebral segments simultaneously.
- Paraspinal abscesses in tuberculosis are typically large, well-defined, with a thin enhancing rim.
- The primary pyogenic process always starts in the bone (or rarely in the facet joints), whereas tuberculosis tends to form massive cold abscesses.
Differential diagnosis
Spinal infection is easily confused with pronounced degenerative processes. The main source of error is Modic type 1 disc degeneration.
In Modic 1 changes, bone marrow edema is also present on MRI. The main differentiating criterion is the evaluation of the paravertebral and epidural adipose tissue. In degeneration, the fat planes around the disc remain preserved (normal), whereas in infection, they are completely erased and replaced by inflammatory infiltrates. Furthermore, a vacuum phenomenon is frequently observed in degeneration, and the disc itself does not show increased signal on T2WI.
Another condition with a similar picture is neuropathic arthropathy of the spine. It is characterized by a vacuum phenomenon, marked osteosclerosis, involvement of facet joints in destruction, and the formation of large osteophytes (pseudarthrosis), reflecting a reaction to chronic microtrauma against the background of sensory loss.
Isolated and postoperative infections
In addition to classic spondylodiscitis, isolated forms are encountered in clinical practice:
- Infectious sacroiliitis: involvement of the sacroiliac joint. It presents strictly asymmetrically (unilaterally), unlike ankylosing spondylitis. MRI reveals extensive muscle edema, joint capsule bulging, and periosteal edema.
- Postoperative infections: develop in 0.7–12% of surgical interventions. It is important to remember that deep infection with a sinus tract may be localized not in the spine itself, but in the bone graft donor site (e.g., the posterior superior iliac spine). CT fistulography is optimal for visualizing them.
Briefly: answers to topical questions
What are the pathways of infection spread in pyogenic spondylodiscitis?
Infection in pyogenic vertebral osteomyelitis and discitis spreads through several main pathways. Hematogenous seeding of the pathogen is the most common. Direct spread of the infectious process from adjacent infected soft tissues is also possible. Direct inoculation of microorganisms during surgery is less frequent. The developmental mechanism usually begins as osteomyelitis of the vertebral endplates, from where the infection spreads to the adjacent disc space and subsequently to neighboring vertebrae.
What bacteria, besides Staphylococcus aureus, cause pyogenic spondylodiscitis?
In addition to Staphylococcus aureus, causative agents of vertebral osteomyelitis and non-specific spondylitis may include:
- Coagulase-negative staphylococci, including Staphylococcus epidermidis — especially in the presence of spinal implants;
- Escherichia coli (Escherichia coli);
- Proteus (Proteus spp.);
- Pseudomonas aeruginosa (Pseudomonas aeruginosa);
- Streptococci.
To differentiate osteomyelitis from Modic 1 degeneration, evaluate the fat around the disc: in infection, the fat planes are "eaten away" (erased); in degeneration, they are intact.
Class and exam questions
Why is radiography unsuitable for early diagnosis?
In the early stages of acute vertebral osteomyelitis, bone destruction is not yet prominent, which is why radiography has low sensitivity. Changes become visible only upon progression to the subacute or chronic phase.
Why is CT used during bone biopsy?
Computed tomography allows precise planning of the biopsy needle trajectory into the affected vertebra while avoiding damage to blood vessels and neural structures.
How to differentiate tuberculous spondylitis from pyogenic spondylitis on MRI?
Tuberculosis more frequently forms large, well-defined paraspinal abscesses with a thin enhancing rim, and spreads beneath the ligaments across 3 or more segments.
When does the yield of a biopsy culture decrease?
The rate of positive cultures is extremely low if the patient has already been started on antibacterial therapy prior to the biopsy.
What else to study on this topic
- Features of the imaging pattern of septic arthritis of the zygapophyseal joints
- Specifics of fungal spinal infections (e.g., Candida)
- Application of PET/CT and gallium scintigraphy in skeletal infections
- Management tactics for patients with infected spinal implants
- Imaging signs of epidural phlegmons without concomitant osteomyelitis
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