Current Insights On Imaging Techniques For Diagnosing Infection

Author(s): 
Daniel P. Evans, DPM, FACFAOM, FACPR, Adam Fleischer, DPM, MPH, FACFAS, and Melissa Skratsky, DPM

   Soft tissue analysis should always start at the level of the integument. One should assess for expansion of the contour or for a defect in the soft tissue envelope. Should you identify an ulceration, it is important to assess the structures deep to the ulceration in a fan shaped direction. Pay close attention to subtle changes in areas deep to an ulceration.

   The presence of gas or air in the soft tissue may be indicative of a potential limb- or life-threatening infectious process. Gas will tend to accumulate in pockets and follow tissue planes or the course of tendons. In regard to the lucency of emphysema, one should not confuse this with the presence of air, which may be introduced from a puncture wound or retained following surgical closure of tissue planes.

   When it comes to deep soft tissue involvement, one will often identify this at the periosteal soft tissue interface and this may be the earliest of radiographic changes clinicians encounter.

   Periosteal response to infection may occur relatively early in situations in which a puncture wound causes a perforation of the periosteum.3 One may also encounter exuberant periosteal response in a postoperative infection when an organism has been introduced to the level of bone. Keep in mind that the periosteum of digits is extremely thin and osteomyelitis of digits may occur without evidence of any periosteal involvement.

   The integrity of the cortex of bone in the region of infection is invaluable in the assessment of infection. Subtle changes in the thickness of the cortex or slight cortical defects may be indicative of bone destruction. Cortical destruction will generally precede medullary involvement in cases of the spread of infection from more superficial locations.

   Medullary involvement will generally cause lysis and loss of trabecular patterns on the image. In cases of hematogenously borne organisms, internal destruction may be relatively severe before one identifies any other signs.

What You Should Know About Nuclear Imaging

The utilization of radionuclide imaging in the assessment of pedal infections has waxed and waned in the podiatric community. Clinicians have used numerous isotopes to assess for the presence of infection. Although isotopes are sensitive to bony turnover, they often have a low specificity. However, tagging an isotope such as Technetium99 in vitro to the patient’s white blood cells (Ceretec®) does increase specificity for bone infection. The sensitivity of nuclear imaging may also be limited in an ischemic foot.6

   The use of this imaging modality is usually reserved for cases where minimal radiographic changes are evident or when magnetic resonance imaging (MRI) findings are inconclusive. However, Ceretec scans can be valuable in differentiating Charcot arthropathy from conditions such as acute and chronic osteomyelitis.

   Ceretec scans are also valuable for assessing osteomyelitis around implanted hardware following open reduction internal fixation (ORIF) and arthrodesis procedures in the foot and ankle. In these situations, the utilization of MRI is limited by the ferromagnetic artifact and bone marrow edema that one usually encounters postoperatively.

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