T1-weighted MRI Imaging Features of Pathologically Proven Non-pedal Osteomyelitis

Rationale and Objectives

The objective of our study was to determine if the T1-weighted magnetic resonance imaging (MRI) features associated with diabetic pedal osteomyelitis are present in histopathologically proven cases of non-pedal osteomyelitis.

Materials and Methods

Seventy-five patients with a histopathologically proven diagnosis of non-pedal osteomyelitis and a preoperative MRI were identified between 2000 and 2007. The MRIs were retrospectively reviewed for signal characterization of T1-weighted images, including the signal intensity compared with skeletal muscle, distribution of abnormal signal intensity, and pattern of abnormal signal intensity. A subsequent chart review was performed to identify potential clinical factors that were more associated with atypical T1 features of osteomyelitis. Fisher’s exact test was performed to determine if there was a statistically significant difference in the T1-weighted imaging features of the hematogenous and nonhematogenous mechanisms of infection.

Results

Seventy of 75 cases demonstrated T1-weighted imaging features typical of pedal osteomyelitis with a confluent region of decreased signal intensity, hypointense, or isointense relative to skeletal muscle in a geographic pattern with medullary distribution. Of the 5 cases that did not demonstrate the typical T1 features associated with pedal osteomyelitis, 4 were considered to have a hematologic mechanism of infection given the absence of surgery, skin ulceration, or a penetrating injury.

Conclusion

The majority of cases of non-pedal osteomyelitis in our study demonstrate the typical T1-weighted imaging features previously documented to correlate with the diagnosis of pedal osteomyelitis. The cases in our series that did not demonstrate the typical T1-weighted features were predominantly secondary to a hematologic mechanism of infection.

Magnetic resonance imaging (MRI) has been documented to be a useful imaging tool in the diagnostic evaluation of osteomyelitis , particularly for the evaluation of pedal osteomyelitis associated with diabetic foot ulcers . Previous studies by Collins et al and Johnson et al have demonstrated the diagnostic utility of T1-weighted imaging features for the accurate diagnosis of pedal osteomyelitis associated with diabetic foot ulcers that included a confluent pattern of decreased T1 marrow signal intensity in a geographic medullary distribution .

The use of contrasted enhanced T1-weighted MRI has been proposed as a useful tool in the diagnosis of osteomyelitis by some investigators ; however, the enhancement pattern is nonspecific and may also be seen with reactive edema. Even normal hematopoietic red marrow is known to enhance variably in healthy individuals and this changes with age . Increased T2-weighted signal in bone marrow is a nonspecific finding associated with edema, inflammation, malignancy, and infection. Reactive edema is often present in the medullary cavity of bone adjacent to soft-tissue infections. This is particularly challenging in the commonly encountered clinical scenario of “rule out osteomyelitis” with an overlying skin ulcer. Osteomyelitis, as with malignancy, is a cellular process that typically displaces the adipocyte rich yellow marrow in adults, leading to the T1-weighted features described previously . Reactive marrow edema does not displace the fat, but rather surrounds the adipocytes in the bone marrow resulting in an intermediate T1-weighted signal. Secondary signs including overlying soft-tissue ulceration, sinus tract, and cortical bone interruption have also been shown to be of some value in making the accurate diagnosis of pedal osteomyelitis .

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Figure 1

A 35-year-old male with quadriplegia and decubitus ulcers presents with a chronic draining soft-tissue ulcer of the left buttock despite antibiotic therapy. An axial T1-weighted image of the pelvis demonstrates the typical T1-weighted imaging features of osteomyelitis with a focal confluent, medullary region of decreased T1-weighted signal that is isointense to skeletal muscle ( white arrow ).

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Materials (of subjects) and methods

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Results

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Table 1

T1-weighted Features of the Study Population

T1-weighted Features T1 signal compared with adjacent muscle Hypo/isointense 72 Hyperintense 3 T1 signal distribution Medullary 73 Subcortical 2 T1 signal pattern Confluent 73 Hazy/reticulated 2 All T1 imaging features Typical imaging features 70 Atypical imaging features 5

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Table 2

Demographics Comparing the Acute and Chronic Infection Groups

Group Acute Nonacute_P_ Value Number of patients_n_ = 6n = 69 Mean age (SD) 44.8 (35.2) 49 (22.1) Male:female 4:2 39:30 Mean days from magnetic resonance scan to pathologic diagnosis 2.7 14.8 T1-weighted imaging features Typical 5 65 .35 Atypical 1 4

Table 3

Demographics Comparing the Hematogenous and Pelvic/Local Mechanism of Infection Groups

Group Hematogenous Pelvic/Local_P_ Value Number of patients_n_ = 21n = 54 Mean age (SD) 28.6 (19.7) 56.5 (19.5) Male:female 11:10 32:22 Acute 4 2 Nonacute 17 52 Mean days from magnetic resonance to pathologic diagnosis 13.8 15.6 T1-weighted imaging features Typical 17 53 .02 Atypical 4 1

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Discussion

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References

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T1-weighted MRI Imaging Features of Pathologically Proven Non-pedal Osteomyelitis

Rationale and Objectives

Materials and Methods

Results

Conclusion

Materials (of subjects) and methods

Results

Discussion

References

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