Current Insights On Imaging Techniques For Diagnosing Infection

Timely diagnosis of lower extremity infections is essential to providing effective treatment and preventing complications. Accordingly, these authors discuss the roles of various imaging modalities ranging from plain radiographs and nuclear imaging to computed tomography, magnetic resonance imaging and positron emission tomography.

The clinical presentation of infection can range from signs and symptoms that are obvious and easily confirmed to ones that are elusive and cause much consternation. Confronted with the challenge of diagnosing difficult infections, podiatrists must understand that time is of the essence.

   The ability to identify an infectious process and initiate early intervention will lead to preferred outcomes relative to patient care. The failure to identify infection may lead to further tissue destruction, the spread of infection and possible loss of limb and life. In our litigious society, the inability or delay to properly identify infection has also served as the basis of malpractice cases.

   The use of plain film radiography is the first line of imaging in virtually every clinical scenario for podiatrists. This modality is readily available in most offices and is inexpensive as well as efficient at providing much data in the assessment for infection.

   Physicians often question the timing of obtaining radiographs when it comes to assessing an infectious situation. One should always take radiographs initially if there is an index of suspicion that there is a gas producing organism or a deep infection where you suspect bone involvement. Physicians should consider radiographs in cases in which the patient is not responding to treatment or the situation is deteriorating clinically.
Baseline radiographs of diabetic patients with ulcerations may prove useful as they will confirm the absence of infection as well as provide comparisons should infection persist. However, it is not mandatory to take radiographs in cases of superficial infection. In these cases, the radiograph will provide little if any additional information beyond what you see clinically.

   The use of digital radiographs (CR and DR) can greatly assist your evaluation for the potential of an infectious process. The ability to magnify, use negative imaging mode and adjust for radiographic contrast are options that are not available with standard plain film radiography.

Radiographs And Osteomyelitis: Keys To Assessment

The engineering and aviation industries utilize checklists in dealing with complex issues to improve results and diminish the chance of injury or disaster. We believe the use of a radiology checklist can be beneficial to podiatric physicians. A checklist for assessment of infection should include the following:

Soft tissue:
Ulceration
Gas
Deep tissue involvement

Bone:
Periosteal
Cortical
Medullary

   Although soft tissue changes are subtle, they are often the initial signs of infectious processes.1 In assessing for soft tissue findings, it is paramount for the physician to be cognizant of the potential route of contamination into the body. Infection may occur secondary to direct inoculation from stepping on a foreign body, from a defect in the skin such as a laceration or an ulceration, or from hematogenous spread of an infectious organism.2 Organisms that have been introduced to the level of bone via a puncture wound are more likely to have early periosteal reaction while organisms spread through the bloodstream are more likely to present with early intramedullary destruction.3-5

   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.

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

   Consider, for example, a typical outpatient setting, where the prevalence of underlying osteomyelitis accompanying infected neurotrophic ulcers may only be 20 percent.1,3 In this scenario, a negative foot X-ray will be fairly supportive of establishing the absence of bone involvement. This would effectively lower the post-test probability for having osseous infection to just 0.15.12 However, in the inpatient setting, where the prevalence of osteomyelitis may be as high as 66 to 95 percent in infected neurotrophic ulcers, patients will still be more likely than not (post-test probability > 0.5) to have an underlying bone infection when their foot X-rays are negative or unremarkable.12

   What is abundantly clear is that highly sensitive tests (e.g., bone scan and MRI) will always go a long way to rule out underlying osteomyelitis in situations when there is low clinical suspicion (e.g., outpatient setting) and you get a negative test result. The converse is also true. Highly specific tests (e.g., MRI) will go a long way to rule in osteomyelitis in settings (e.g., inpatient) where there is high clinical suspicion provided you get a positive test result.

In Conclusion

Currently, there is no gold standard as it applies to diagnostic imaging of infection. The true diagnosis of infection is based on biopsy and culture of infecting organisms, and the reliability of these methods has even been called into question recently.1,7 Often, plain film imaging is all that is necessary for achieving a diagnosis of infection. We cannot stress enough the need for early radiographic assessment as well as follow-up evaluation (i.e., serial X-rays).

   We believe the use of the aforementioned radiographic checklist will be beneficial in maximizing the analysis of imaging films. Moving beyond plain films, MRI is generally most useful in further assessment for possible infection involving the foot and ankle. It is important for podiatric physicians to utilize imaging centers that are comfortable in the assessment of pedal images. It is also important for podiatric physicians to review these images. Implementing this action can greatly enhance the care you provide for your patients.

   Dr. Evans is a Professor in the Department of Podiatric Medicine and Radiology at the William M. Scholl College of Podiatric Medicine at the Rosalind Franklin University of Medicine and Science in Chicago. He is a Fellow of the American College of Foot and Ankle Orthopedics and Medicine, and a Fellow of the American College of Podiatric Radiologists.

   Dr. Fleischer is an Assistant Professor of Radiology and Medicine at the William M. Scholl College of Podiatric Medicine at the Rosalind Franklin University of Medicine and Science in Chicago. He is a Fellow of the American College of Foot and Ankle Surgeons.

   Dr. Skratsky is an Assistant Professor in the Department of Podiatric Medicine and Radiology at the William M. Scholl College of Podiatric Medicine at the Rosalind Franklin University of Medicine and Science in Chicago.

References

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