Cath Lab Digest

   Similar to ¹¹¹In-labeled leukocyte imaging, Tc-HMPAO is a timely process delivered to the patient in vitro, typically 24 hours prior to imaging.²⁷ Emerging alternatives to in vitro delivery have occurred by way of using monoclonal antibodies in conjunction with technetium, both of which are delivered in vivo.²⁶ The ⁹⁹mTC-labeled murine IgM antigranulocyte monoclonal antibody (MoAb) binds to the human polymorphonuclear leukocyte CD15 antigen, which is present in high concentrations in areas of suspected infection or inflammation.²⁸

   Preliminary studies utilizing this technique confirm that MoAb achieves comparable sensitivity and better specificity than ¹¹¹In-labeled leukocyte imaging in the diagnosis of skeletal osteomyelitis.²⁸ MoAb studies are unique in that they possess the potential to differentiate between Charcot and osteomyelitis. In theory, the destructive changes occurring in Charcot do not elicit the same accumulation of MoAb as osteomyelitis upon imaging.²³ Further research is necessary to explore the capability of this imaging modality.

   Single positron emission computed tomography (SPECT) is a nuclear medicine imaging technique that produces a multi-axial image based upon the distribution of gamma-emitting radionuclides within the human body.²⁹ This modality affords better representation and detail of lower extremity anatomy than traditional bone scintigraphy by providing sagittal, coronal and axial views. In addition, SPECT is often constructed to operate with a traditional CT scanner, resulting in what is known as a SPECT/CT.²⁹ The SPECT/CT combines multiphase images visible with bone scintigraphy superimposed over the detailed, high resolution anatomic structures visualized with conventional CT for improved diagnostic capabilities.

   The specificity of diagnosing osteomyelitis in the lower extremity with the use of SPECT alone versus SPECT/CT improves from 50 to 86 percent respectively.³⁰ Previously, a major disadvantage of using nuclear medicine imaging was a lack of specificity of diagnosing osteomyelitis due to poor imaging quality, requiring further modalities to confirm the diagnosis. However, by improving specificity with the combined use of CT imaging, nuclear medicine is emerging as a suitable alternative to MRI.

   The ¹⁸F-fluorodeoxyglucose (FDG) is a radiolabeled glucose analogue most commonly paired with positron emission tomography (PET). The FDG-PET is an imaging modality that oncologists traditionally employ in the monitoring and diagnosis of malignancy and metastases.³¹ Recently, this imaging technique has shown diagnostic capabilities beyond that of cancer, particularly showing promise in the realm of diagnosing inflammation and infection.³¹ Activated inflammatory cells within the human body possess cell surface glucose transporters. Glucose transporters amplify due to cytokine cell stimulation, resulting in the increased uptake of FDG by macrophages, lymphocytes and other inflammatory cells.³² The usefulness of ¹⁸F-FDG in regard to detecting increased glucose metabolic activity, commonly occurring in osteomyelitis, makes this a promising modality to aid in diagnosing osteomyelitis in the future.³³

   A meta-analysis by Termaat and co-workers concluded that FDG-PET maintained the highest accuracy in properly diagnosing osteomyelitis in comparison to all other imaging modalities (plain radiographs, MRI, CT, bone scintigraphy).²² Most recently, Nawaz and colleagues compared sensitivity, specificity, positive and negative predictor values, as well as the accuracy of various imaging modalities in diagnosing osteomyelitis in the diabetic foot. The study demonstrated FDG-PET scans to be more specific and accurate in the diagnosis of osteomyelitis in comparison to MRI (see “Comparing The Efficacy Of FDG-PET Scans To Other Modalities” at left).³⁴