Acute osteomyelitis

In the acute phase of osteomyelitis, the edema and exudates within the medullary space produce an ill-defined low-signal intensity on the T1-weighted images and a high signal on T2-weighted and STIR or fat-suppressed sequences. Usually the surrounding soft tissues are also abnormal, with ill-defined planes. The cortical bone can be disrupted and can have abnormally increased signal intensity. The absence of cortex thickening in acute osteomyelitis helps to differentiate it from a chronic infection of bone. Bone marrow findings are nonspecific; other conditions, such as trauma with bone bruise, fracture, infarct, ischemia, and neoplastic processes, may have the same signal intensity alterations as seen in osteomyelitis.

Suppression of fat signal in MRI has proven to extend the dynamic range of tissue contrast, eliminating the strong interfering signal of fat on T1- and T2-weighted images, and post-intravenous contrast injection images (gadolinium). These advantages of fat suppression have been extensively used in clinical practice [100-112]. Because of the superior visualization of inflammatory edema, the most suitable sequences for screening acute osteomyelitis are the short tau inversion recovery (STIR) or the T2-weighted fat-suppressed fast spin-echo (SE) sequence [34]. On STIR or fat-suppressed sequences osteomyelitis looks hyperintense [84,113]. STIR images generally have a lower spatial resolution than conventional T1- and T2-weighted images and cannot be used to differentiate fluid collections, such as abscesses, from circumscribed soft tissue edema [34,114] (Fig. 16). Fat suppressed sequences can help in the visualization of lesions with relatively high water content, such as osteomyelitis, edema, and tumors [115-117]; these sequences must be supplemented by T1-weighted SE images, which provide excellent anatomic detail.

Acute Osteomyelitis

Fig. 16. This 43-year-old man developed pain in the thigh and a limp. Transaxial T2-weighted (TR/TE, 2000/80) SE image demonstrates high signal intensity within the soft tissues. Bone marrow abnormalities are manifested as focal areas of high signal intensity consistent with early stages of osteomyelitis. (Courtesy of Sergio Fernandez Tapia, MD, Tampico, Mexico.)

Fig. 16. This 43-year-old man developed pain in the thigh and a limp. Transaxial T2-weighted (TR/TE, 2000/80) SE image demonstrates high signal intensity within the soft tissues. Bone marrow abnormalities are manifested as focal areas of high signal intensity consistent with early stages of osteomyelitis. (Courtesy of Sergio Fernandez Tapia, MD, Tampico, Mexico.)

STIR and T2-weighted TSE images with fat suppression consistently show perosseous edema. Strong peripheral contrast enhancement confirms the presence of an abscess. Necrotic sequestra depict low-intensity regions without contrast enhancement on STIR and T2-weighted images, in contrast to the high signal intensity of an abscess [73].

Gadolinium may define areas of necrosis [73,84] and also is useful to demonstrate abscess on Tl-weighted images (Fig. 17) [2]. Sinus tracts can extend from the marrow and bone, through the soft tissues, and out the skin as high signal intensity areas on T2-weighted images [1].

In acute osteomyelitis intra- and extramedullary fat globules can be seen in MRI; this nonpathognomonic sign has been suggested to be secondary to an increased intramedullary pressure, which leads to septic necrosis, lipocyte death, and free fatty globule release [118].

In patients who have septic arthritis who fail to respond to appropriate antibiotic therapy within 48 hours, osteomyelitis must be suspected and MRI should be considered [84]. Subperiosteal fluid collections may be seen, with low signal intensity on the T1-weighted sequences and intermediate to high signal intensity on the T2 and fat-suppressed images.

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