Understanding The Role Of Advanced Imaging Techniques In Diagnosing Infection

Unfortunately, many physicians rely on various imaging modalities to deliver a diagnosis of infection, particularly osteomyelitis. I have seen a multitude of imaging modalities worthlessly ordered and performed with the physician desperately trying to get a definitive diagnosis to no avail.

There are many imaging modalities available and they include plain radiographs, various scintigraphy techniques, CT scans and MRI. Physicians are also using technically advanced imaging techniques including: in vivo labeled leukocyte scans; radiolabeled polyclonal iGg scans; radiolabeled antibiotic (ciprofloxacin) scans; fluorodeoxyglucose-positron emission tomography (FDG-PET) scans; and positron emission tomography/computed tomography (PET/CT) scans. How do we know which ones to utilize?

Accordingly, let us take a closer look at the advantages and disadvantages of various imaging modalities. I will also provide my opinion on which imaging modality to use and when to use it. However, it is vitally important to remember that the ideal imaging modality should aid in the diagnosis and help devise a treatment plan. Physicians should never rely solely upon an imaging modality for the actual diagnosis. The combination of the clinical exam and the imaging study should give you a working diagnosis.

Radiographs are typically the first imaging study we obtain when evaluating an infection. Even if the physician does not suspect osseous involvement, one should obtain radiographs to serve as a baseline so he or she can compare it to a subsequent X-ray if the infection progresses.

While we typically use radiographs to evaluate osseous structures, they offer limited soft tissue evaluation. However, soft tissue edema will usually be apparent if it exists. Possible osseous changes that physicians can identify with infection include osteolysis or destruction, cortical breaking and erosion, and periosteal reaction. Osteomyelitis, however, may take up to four weeks in some cases to become visible on radiographs. For this reason, radiographs are not a reliable modality for the diagnosis of osteomyelitis.

A Closer Look At Scintigraphy

Scintigraphy is very popular in the medical community for the evaluation of infection. There are many different tests available and they include but are not limited to 3-phase bone scans, gallium scans and leukocyte labeled scans such as indium, hexylmethylpropylene amineoxine (HMPAO) and monoclonal antigranulocyte antibody (MoAb).

The important thing to remember about these scans is that they are a marker of metabolic activity so these are studies of inflammation, not infection. The one major advantage to scintigraphy is that it is highly sensitive. However, these scans are very non-specific. They can also be technically cumbersome. Some techniques require withdraw of the patient’s blood, labeling of the blood and cells and re-injection of the blood back into the patient. The scans are done at various timing intervals and may take up to 24 to 48 hours for a complete study. Due to the poor specificity, false positives for osteomyelitis commonly result. False negatives can also result, especially in patients with chronic infection and/or poor blood flow.

Technetium 99m-labeled MoAb is a white blood cell labeling technique that is done in vivo. In a 2003 study, Palestro and co-workers compared the 99m-Moab study with Indium-111 and technetium 99 for the diagnosis of osteomyelitis.1

The Moab study showed 90 percent sensitivity and 67 percent specificity. The indium study was 80 percent sensitive and 67 percent specific. However, the technetium scan was only 27 percent specific. Out of 20 patients, they had eight true positives, eight true negatives, three false positives, and one false negative. Although this modality was statistically more specific than the technetium 99 scan, it is still generally non-specific for diagnosing osteomyelitis.

Physicians may use CT scans to aid in the evaluation of infection. However, I feel there are much better modalities available and they have a very limited use in my practice for this purpose. CT scans are good to evaluate cortical bone. However, while CT scans show any existing bone erosion, they do not show inflammation. They also have very limited use for soft tissue evaluation in comparison to other modalities such as an MRI. The patient also obtains a radiation dose when undergoing a CT scan. If metal is present in the area of concern, a starburst effect will be produced and may obscure visibility of the area.

Emphasizing The Value Of Magnetic Resonance Imaging

Magnetic resonance imaging is my technique of choice when it comes to evaluating for infection. It offers many advantages including good anatomic appreciation of all structures involved including soft tissue and bone. It is also the most specific modality for the evaluation of infection and can identify subtle bone marrow changes. While there is no radiation dose to the patient, one must be aware of any implanted metal.

Even if a physician knows that osteomyelitis is present, MRI may still be helpful at times to help aid in pre-operative planning. One should ensure full visualization of all structures so the physician can identify the extent of the infection and determine the necessary level of debridement. Magnetic resonance imaging can also aid in identifying other areas of infection, such as a deep abscess, if a patient is not responding to current therapies.

A 2007 meta-analysis, published in the Archives of Internal Medicine, compared imaging modalities for the diagnosis of osteomyelitis. Researchers compared MRIs with plain radiographs, bone scans and white blood cell labeled studies.2 They found that MRIs were markedly superior, noting that MRI is a strong test to aid in both confirming and excluding osteomyelitis of the foot. The authors of the meta-analysis concluded that a positive MRI results in an 84 percent chance of having the diagnosis of osteomyelitis. Other exam findings, such as substantial depth of the ulcer or probing to bone, “cinches” the diagnosis.

They also state that the use of technetium bone scanning in the diagnosis of osteomyelitis of the foot should be diminished since the lack of adequate specificity creates many false positive results.

In 2004, authors of an article in Clinical Infectious Diseases stated that the Infectious Disease Society of America (IDSA) recognizes MRI as the preferred advanced imaging test for suspected osteomyelitis but recommends performing serial plain radiography before ordering MRI.3

What About The Potential Of The FDG-PET Scan?

An imaging modality that holds great promise for diagnosing osteomyelitis is the FDG-PET scan. Historically, physicians have utilized FDG-PET scans to evaluate malignancies but this modality has recently become valuable for evaluating chronic osteomyelitis, infected prosthetics, sarcoidosis, fever of unknown origin and AIDS. This modality seems to offer many potential advantages such as diagnostic results in 1.5 to 2 hours. It offers excellent spatial resolution with accurate anatomical localization of abnormal sites. Researchers have also reported that the FDG-PET scan detects or excludes osteomyelitis with an accuracy of 92.3 percent.4

Despite all these available imaging modalities and their great technological advances, the unmet challenge of imaging when trying to diagnose infection still remains because inflammation does not equal infection. All of these modalities identify inflammation. It is up to the physician to know the limitations of the study and apply this understanding clinically. It is also important to remember that the imaging modality, no matter how specific, can never truly diagnose osteomyelitis. The gold standard for diagnosing osteomyelitis remains bone biopsy.

In Conclusion

In summary, imaging modalities should aid in the diagnosis of infection and help one devise an effective treatment plan. Physicians should never rely upon them solely to make the actual diagnosis.

When it comes to suspected infections, physicians should obtain plain radiographs, which can serve as a baseline study. Bone scans, even the white blood cell labeled studies, are sensitive but non-specific. They have limited use in my hands for evaluation of infection. I do feel other modalities are superior. Magnetic resonance imaging is my imaging modality of choice because it is the most specific modality readily available for the diagnosis of infection. It allows me to evaluate both soft tissue and bone, and offers excellent anatomic visualization to guide surgical planning. The FDG-PET scan offers great hope for evaluating osteomyelitis and may become the imaging modality of choice in the future as it becomes more readily available.

References

1. Palestro CJ, Caprioli R, Love C, et al. Rapid diagnosis of pedal osteomyelitis in diabetics with a technetium-99m-labeled monoclonal antigranulocyte antibody. J Foot Ankle Surg 2003 Jan-Feb;42(1):2-8.

2. Kapoor A, Page S, Lavalley M, et al. Magnetic resonance imaging for diagnosing foot osteomyelitis: a meta-analysis. Arch Intern Med 2007:167(2):125-32.

3. Jeffcoate WJ, Lipsky BA. Controversies in diagnosing and managing osteomyelitis of the foot in diabetes. Clinical Infectious Diseases 2004;39:S115-S122.

4. Nawaz A, Torigian DA, Siegelman ES, et al. Diagnostic performance of FDG-PET, MRI and Pplain film radiography (PFR) for the diagnosis of osteomyelitis in the diabetic foot. Mol Imaging Biol 2009 Oct 9 (Epub ahead of print).