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

Weighing The Pros And Cons Of CT, MRI and PET Scans

Computed tomography (CT) is of benefit due to its ability to provide cross-sectional analysis. This is of particular benefit in the midfoot and rearfoot areas. Computed tomography is also the best advanced imaging modality for the assessment of cortical bone, making it particularly well suited for the detailed evaluation of chronic osteomyelitis. Computed tomography is also the best advanced imaging modality for the assessment of cortical bone, making it particularly well suited for the detailed evaluation of chronic osteomyelitis, in which cloacae, sequestrum and involucrum frequently occur.

   However, CT is limited in its ability to assess for subtle marrow involvement, making it less helpful in the evaluation of acute osteomyelitis. When it comes to acute osteomyelitis, MRI is more advantageous and will also provide better identification of soft tissue abscess and/or sinus tract formation.

   Like CT, MRI has the advantage of giving cross-sectional analysis. It also has an advantage over traditional CT in that it can provide multi-planar cross sections without the need for reformatting of the image. This allows for greater analysis of detail. Magnetic resonance imaging is excellent at identifying ulceration as well as abscess formation. Although it is difficult to assess loss of cortical bone, the ability to assess bone marrow is highly augmented. Generally, a diminished signal intensity on T1-weighted images in combination with an increased signal intensity on T2-weighted images is highly suggestive of osteomyelitis.7,8

   Positron emission tomography (PET) scans are nuclear imaging exams that produce images of functional processes in the body. The nuclear isotope is attached to glucose (fluorodeoxyglucose – FDG). Then inject FDG into the body and a scan identifies emission of positrons as they undergo decay.

   One of the difficulties with this exam is its poor spatial resolution. Recently, PET images have been integrated with either CT or MRI images, allowing for greater orientation and correlation. The exposure to ionizing radiation one encounters with these exams is relatively high at 23-26 mSv (millisievert). A typical commercial airline flight crew member would be exposed to 4-9 mSv per year.9,10

   At this time, the Centers for Medicare and Medicaid Services (CMS) only provides coverage for the utilization of PET scans for tumor analysis. The use of PET is starting to move from the research realm to the diagnostic/clinical sphere. In a recent article evaluating the use of single-photon emission computed tomography/computed tomography (SPECT/CT) in directing management of patients with diabetic foot infections, researchers noted that 94 percent of scans assisted in directing the management of the patient toward either conservative therapy or limb salvage procedures.11

Why The Clinical Setting Is A Key Factor In Interpreting Radiology Studies

Consideration of the clinical context in which your patients present is paramount to interpreting the results of any of the aforementioned radiology studies. In recent years, it has become increasingly clear just how important osteomyelitis prevalence (a.k.a. pre-test probability) is when interpreting test results in patients with diabetic foot infections.12,13

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