Chronic and Subacute Osteomyelitis
Osteomyelitis lasting for more than 6 weeks is generally referred to as chronic osteomyelitis. Radiographs exhibit periosteal reactions, sclerosis, and osteolysis. Findings in active inflammation include sequestra and sinus tracts. T1-weighted MR images show extensive SI decrease of the bone marrow. T1-weighted images, however, do not allow differentiation of active infection from changes caused by previous infection and healing, such as sclerosis or marrow fibrosis. However, on STIR and T2-weighted images areas of high signal intensity are found in active processes. Again, contrast- enhanced T1-weighted fat-suppressed sequences are most useful since a central non-enhancing area with peripheral enhancement is almost always proof of active infection.
Sinus tracts can be identified on T2-weighted images as linear SI increases extending from the bone to the skin. On contrast-enhanced T1-weighted fat-suppressed images, prominent contrast enhancement can be noted. Sequestrations (bony fragments with increased sclerosis) show low SI on T1-weighted and T2-weighted images and therefore stand out relative to the surrounding inflamed tissue.
Subacute Types of Osteomyelitis
Subacute types are Brodie’s abscess as well as Garré ’s chronic sclerosing osteomyelitis. These types develop when the host has moderately high resistance to infection, or when the infecting organisms have a somewhat reduced virulence. Brodie’s abscesses appear as well-circumscribed areas of low signal intensity on T1-weighted images and of high signal intensity on T2-weighted images. A rim of low signal intensity is found in all sequences, resulting from bone sclerosis. Surrounding bone marrow edema may be seen on T2-weighted and STIR images. A double line on T2-weighted images may be produced by granulation tissue on the inner wall of the abscess. Delineation of Brodie’s abscess is facilitated by intravenous contrast material, which causes peripheral rim enhancement.
Posttraumatic and Postoperative
Osteitis and Osteomyelitis In the early posttraumatic/postoperative phase, changes caused by fracture healing or osteosynthesis can raise diagnostic problems. In bone scintigraphy, the increased bone turnover leads to a long-lasting, nonspecific increased activity. When patients undergo osteosynthesis, the diagnostic role of CT is limited to a greater degree than that of MRI. In areas of former screw holes and in zones of fracture healing a band-like signal intensity reduction is found. In recent fractures, an inhomogeneous pattern of signal intensities results from the simultaneous presence of hematoma, granulation tissue and fracture callus formation.
Again, contrast-enhanced T1-weighted fat-suppressed sequences are very valuable. Central regions of fluid-like signal with peripheral enhancement are indicative of an active inflammatory process, especially when surrounding edema is present.
Chronic Recurrent Multifocal Osteomyelitis
Chronic recurrent multifocal osteomyelitis (CRMO) predominantly affects children and adolescents. CRMO is characterized by a prolonged course (over several years), with multifocal lesions. CRMO is a chronic, systemic aseptic inflammation and constitutes approximately 2–5% of all osteomyelitis cases. The disease predominantly affects the metaphyses of the long bones adjacent to the growth plates, and the clavicle and spine. The etiology is still unknown. Females are more frequently affected (F:M = 5:1). The skeletal manifestations may be associated with skin lesions. The disease is self-limiting and has a good prognosis.