Cath Lab Digest

When Combination Imaging Can Be Helpful
For Chronic Diabetic Ulcers And Septic Joints
When a chronic, non-healing diabetic ulceration exists, combination imaging is vital to determine if an underlying infection is present as a wound will not heal over infected soft tissue or bone. In these cases, it is most important to discriminate between an infected ulceration and an underlying osteomyelitis.^14,49,63,64 When performing a radionuclide leukocyte scan in the face of ulceration, keep in mind that drainage due to wound exudates into dressing materials will contain isotope-labeled leukocytes.⁶⁴ This will distort the area of interest as the radioactivity present in wound drainage can amplify the degree of uptake one sees in the region of interest.
Clinicians can avoid this technical error by having the patient perform a dressing change immediately prior to imaging. This will minimize the accumulation of radionuclide within dressing materials and prevent misinterpretation of the image data. The negative impact that a draining ulcer has on such imaging has been documented and previously reported in the literature.⁶⁴
In the case of a septic joint, one should obtain serial NMLI approximately two weeks after the completion of antibiotic therapy. Doing so helps confirm the absence of residual infectious activity. Keep in mind that a bone scan performed after treatment of a septic joint will be negative as there is no residual bone remodeling or hyperemia as opposed to what you would see in a bone scan after treatment for bone infection. After treating osteomyelitis, one can see bone remodeling and hyperemia on a bone scan for greater than one year’s time in many cases. Therefore, a negative ^99mTc-MDP scan after treatment of a septic joint will rule out the presence of an indolent inflammatory process.

Key Steps For Preventing Imaging Errors
Technical errors in performing radionuclide leukocyte labeling procedures may occur at every step of the process so it is important to understand the methods and statistical strategies involved in such work. In general, a radiolabeled leukocyte compound is viable for use when a 90 percent tag or greater is confirmed by the nuclear medicine pharmacy. This is then logged on the patients’ prescription for the isotope.

An insufficient label is apt to provide a poor quality exam as the target to background ratio is severely hindered by an increased background radiation or low percentage label. This increase in background radiation is due to the large amount of unbound isotope circulating free within the vascular compartment. When one encounters a negative study, it is prudent to check the prescription log to ensure the test was performed properly (i.e. >90 percent labeling efficiency of isotopes and leukocytes for imaging).
Another source of error in imaging is infiltration of the isotope at the sight of injection. Since extremely small concentrations of isotope are used, even a partial infiltration of an injected dose will compromise the data set. Performing imaging of the injection site routinely can help rule out false negative studies.