MRI of the Feet

Source: Morrison WB, et al. Radiology 1998;207:625-632.

While MRI imaging has become increasingly useful in the evaluation of osteomyelitis, the interpretation of marrow changes in the small bones of the feet, especially in those with soft tissue abnormalities, is difficult. Changes in marrow signal intensity can occur in patients with articular infection, biomechanical stress, or the result of recent surgery (such change may persist for weeks to months). On the other hand, T2-weighted images may fail to manifest classical marrow changes in chronic osteomyelitis instead showing abnormally low signal intensity. Also, technical difficulties with volume averaging can lead to falsely-positive or negative results in the small bones of the feet.

Morrison et al examined multiple factors in the interpretation of bone abnormalities seen on MRI of 73 feet in 62 patients, all of whom had at least six weeks of follow-up. Four different MRI images were independently reviewed by two radiologists, including assessment of fast spin-echo short inversion time (fast SE STIR), T1- and gadolinium-enhanced fat-suppressed T1-weighted images, and fat-suppressed T2-weighted images. Signs of soft tissue infection were also assessed.

Of the 73 feet (85% were in diabetics), 43 (59%) had osteomyelitis; only one patient had documented neuropathic bone changes. The two readers found discordance between the different MRI images in 8% and 21% of cases. The fast SE STIR images had the highest sensitivity (96%) and the best negative predictive value (94%), whereas both the T1- and gadolinium-enhanced T1-weighted images were highly specific (88% and 86%, respectively). T2-weighted images were the least helpful and resulted in a higher number of false-positive and -negative readings. Cortical disruption was the most reliable secondary sign associated with the presence of osteomyelitis (sensitivity 88%, specificity 77%). Visualization of adjacent sinus tract formation, soft tissue abscess, and ulceration was somewhat less helpful, but cellulitis and soft tissue mass were poorly associated with the presence of osteomyelitis.