Diagnosis

Osteomyelitis

Case Discussion

Osteomyelitis denotes infection of bone/bone marrow. Various organisms can produce osteomyelitis, however pyogenic bacteria (staphylococcal 80-90 percent of the time) and less commonly mycobacteria (usually Mycobacterium tuberculosis) are the usual culprits. E. coli, pseudomonas and Klebsiella osteomyelitis may occur as a result of bacteremia in IV drug users and in those with genitourinary tract infections. Salmonella osteomyelitis has been described in patients with sickle cell disease.

The usual pyogenic osteomyelitis results from: 1) hematogenous spread, 2) contiguous spread from an adjacent site, or 3) direct implantation/direct inoculation.

Osteomyelitis may be subdivided into acute, subacute or chronic depending on the duration of disease, each with specific imaging appearances.

MRI is considered the imaging modality of choice for osteomyelitis given its high degree of accuracy, sensitivity, and specificity compared with conventional radiography.

One of the most sensitive and specific MRI findings for osteomyelitis includes destruction/loss of the normal thin linear hypointense cortex on T1-weighted imaging, in conjunction with high signal surrounding osteoedema (best appreciated on PD fat saturated, T2 fat saturated and STIR/SPIR fluid sensitive sequences).

Loss of T1 cortical and medullary signal or bone architecture is known as the MR bone “erasure sign.”

Other MRI findings include subperiosteal abscess formation and intraosseous abscess (Brodie abscess) with subacute or chronic osteomyelitis. Sequestrum, involucrum and cloaca are other signs of chronic osteomyelitis. A sequestrum is a necrotic fragment of bone within an area of osteomyelitis seen as signal low intensity on all sequences. An involucrum is a thickened shell surrounding the sequestrum. A cloaca is an opening in an involucrum, which allows drainage of purulent and necrotic material out of the dead bone. It is hypointense on T1-weighted imaging and hyperintense on T2-weighted imaging. Contrast enhancement of bone marrow, periosteum and adjacent soft tissue is often present. Abscesses enhance peripherally.

Key Images

Osteomyelitis of the right foot in an 18-year-old woman with a chronic wound s/p Lisfranc ligament graft repair, now complaining of increased drainage and pain.

A. Sagittal T1 FSE image of the right foot shows hypointense intraosseous abscess/collection (red arrow) surrounding the hypointense graft material (yellow arrow).
B. Axial STIR image shows a hyperintense sinus tract (red arrow) extending dorsally from the infected hypointense graft (yellow arrow) in the medial cuneiform.
C. Axial STIR image shows a high signal sinus tract (red arrow) extending medially from the medial cuneiform.
D. Coronal T2 FSE image shows a high signal intraosseous phlegmon/collection (red arrow) surrounding the hypointense graft material (yellow arrow).
E. Coronal T1 FSE image of the right foot shows intermediate signal intraosseous abscess/collection (red arrow) surrounding hypointense graft material (yellow arrow).

References

  1. Lee YJ, Sadigh S, Mankad K, Kapse N, Rajeswaran G. The imaging of osteomyelitis. Quant Imaging Med Surg 2016; 6(2):184-198
  2. Pineda C, Espinosa R, Pena A. Radiographic imaging in osteomyelitis: The role of plain radiography, computed tomography, ultrasonography, magnetic resonance imaging, and scintigraphy. Semin Plast Surg 2009; 23(2):80-89