Subacute osteomyelitis

Brodie's abscess, single or multiple radiolucent abscesses, can be evident during subacute or chronic stages of osteomyelitis. It is best diagnosed by the combination of conventional radiography and MRI [73]. The central abscess cavity is of low signal intensity on T1-weighted images and high signal

Subacute Osteomyelitis Mri

Fig. 17. (A) Sagittal T1-weighted MRI (TR/TE, 500/12) shows a soft tissue abscess in the posterior aspect of the distal tibia. (B) Gadolinium enhanced T1-imaging demonstrates the extent of the infectious process without associated bony abnormalities. (Courtesy of Sergio Fernandez Tapia, MD, Tampico, Mexico.)

Fig. 17. (A) Sagittal T1-weighted MRI (TR/TE, 500/12) shows a soft tissue abscess in the posterior aspect of the distal tibia. (B) Gadolinium enhanced T1-imaging demonstrates the extent of the infectious process without associated bony abnormalities. (Courtesy of Sergio Fernandez Tapia, MD, Tampico, Mexico.)

intensity on STIR and T2-weighted images. The bone marrow surrounding the Brodie's abscess often demonstrates reactive hyperemia. High signal intensity on T2-weighted images around the area of an abscess reflects the hy-peremic bone marrow. The granulation tissue lining the inner wall of the abscess has low signal intensity on T1. The high signal intensity of the granulation tissue surrounded by the low signal intensity band of bone sclerosis creates a "double-line effect,'' with peripheral ring enhancement with gadolinium administration and high T2 signal intensity [114,119,120]. This central abscess with the surrounding granulation tissue, outer ring of fibrotic reaction [121], and a peripheral rim of endosteal reaction produces a target appearance with four distinct layers that are more evident after gadolinium injection [122]. The rim sign (peripheral low signal intensity) in subacute and chronic osteomyelitis has been reported in 93% of patients who have chronic osteomyelitis and in less than 1% of patients who have acute infection [116].

It has been demonstrated that MRI is as sensitive as 99Tc methylene di-phosphonate bone scintigraphy in demonstrating osteomyelitis and because of its spatial resolution it is more specific and more sensitive than other techniques in demonstrating soft tissue infections [123]. One study compared the results achieved with plain radiography, CT, and indium-111 labeled leukocyte scanning with MRI in patients who had suspected osteomyelitis or soft tissue infections; results suggested that MRI is more accurate for detection of this kind of infection than plain radiography and CT. Patients who had nonmagnetic devices were better evaluated with MRI than CT [121]. In 50% of cases, subacute osteomyelitis is confused with a tumor; bone erosions and periosteal reaction with reactive bone formation also can be confused with osteosarcoma or Ewing sarcoma [32].

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