Infective Inflammatory Bone Disease

Infectious inflammatory bone disease can develop via three routes: Hematogenous seeding, contiguous spread from an adjacent soft tissue or joint infection and direct implantation of microorganisms. A chronological view on osteomyelitis distinguishes acute, subacute and chronic osteomyelitis. The MR diagnosis of bone infection has to focus on (a) bone marrow inflammation, (low signal in T1, high signal in intermediate and T2-weighted TSE sequences, as well as in T1 with contrast), (b) intraosseous abscesses (similar to bone marrow inflammation but with a characteristic high signal rim in T1 with contrast), (c) sequestra (low signal in T1, low signal in intermediate and T2 and a peripheral enhancement in T1 with contrast), (d) cortical destruction (higher signal than bone in all sequences) and (e) sinus tracts into the peripheral soft tissues. In chronic post-traumatic osteomyelitis the same rules for diagnosis have to be applied, although extensive remodelling processes which include sclerosis, fibrosis and cortical thickening have to be taken into account. Sequestra and sinus tracts are regularly seen in active chronic osteomyelitis. Infections of the spine are divided into spondylitis, discitis, spondylodiscitis and spondylarthritis (septic arthritis of the facet joints) and mostly occur in the elderly. In the acute stage of the disease, spondylitis is characterised by increased signal intensity within the vertebral body in the water sensitive sequences and on the contrast-enhanced examination as well as decreased signal intensity on the T1-weighted spin echo sequence. Contrast-enhanced MRI is most sensitive in the identification of early vertebral infection. The infection typically begins at the antero-lateral aspect of the vertebral body and adjacent to the vertebral endplates. T1-weighted spin echo sequences depict the destruction of the vertebral endplates relatively well because the normal cortical signal void of the endplate changes to an intermediate signal. Disc involvement is seen early in spondylitis. T2-weighted spin echo sequences demonstrate increased signal intensity within the disc due to infection of the disc. The diagnosis of an abscess within the disc requires identification of signal intensity similar to fluid on T2-weighted spin echo sequences and lack of enhancement of this area on contrast-enhanced MRI. Osteomyelitis of the diabetic foot encounters special features based on the nature of the disease. In water sensitive sequences and after contrast an increased marrow signal is often seen in these patients and represents an unspecific finding. The diagnosis of osteomyelitis in diabetic foot has to rely mainly on the identification of an intraosseous abscess. Sinus tracts extending to bone, sequestrum formation and cortical destruction significantly help to build up a correct diagnosis.