Diagnostic Imaging
Limited strength evidence supports the use of the following to aid in the diagnosis of PJI:

• 18F-NaF PET/CT
• CT


18F-FDG PET/CT: There were two high strength studies, with conflicting results. In one study (Aksoy, 2014) only patients with positive 18F-FDG results were included in the investigation and only the positive predictive value, which was 28%, could be calculated. In the other study (Kumar 2016), patients with both positive and negative results were included and the positive predictive value was 88.2%. Furthermore, the Kumar investigation was limited to hip  arthroplasties. In view of the conflicting results, the appropriate strength of the recommendation should be limited. 

18F-NaF PET/CT: The one high quality (Kumar 2016) study was limited to hip arthroplasties, so it may not be possible to extrapolate the data to knee arthroplasties and any recommendation should be limited to hip arthroplasties.

One moderate quality hip study evaluated the diagnostic accuracy of computed tomography (CT) (Cyteval 2002). The study used several different measures, including joint distention, fluid-filled bursae, and fluid collection in muscles and perimuscular fat (for the complete list see table 118 of eAppendix 2 ). CT may be useful for ruling in infection, with positive likelihood ratios (LR’s) ranging from poor to strong (positive LR range=.29 to 45.69). Seven of the 11 CT measures had positive LR’s over 2 (see table 118 of eAppendix2 for specific measure results), indicating that CT might be useful as a rule in test. The four CT measures under two (indicating a poor rule in test) were: focal low attenuation, bone abnormalities, nonfocal low attenuation and asymmetric position of femoral head.

However, the study indicated that CT may not be as good of a rule out test (negative LR range=.04 to 1.28). Nine of 11 CT measures had negative LR’s over .5, indicating a very low decrease in probability of PJI with a negative test result (see table 118 of eAppendix2 for specific measure results). The only two CT measures without poor negative LR’s were soft tissue abnormalities (negative LR=.04, strong rule out test), and joint distention (negative LR=.17, moderate rule out test).

In their recent retrospective MR imaging study in 108 consecutive patients with TKAs who underwent MR imaging within 1 year prior to revision surgery, Li et al. (2016) found different lamellated and hyperintense appearance of the synovium in infected joints which can be differentiated from frondlike and hypertrophied synovium associated with particle-induced synovitis and from homogeneous effusion with the signal intensity of fluid associated with nonspecific synovitis. When compared with surgical and microbiology results as the reference standard, MRI had 65.2-78.3 sensitivity and 97.6-98.8  specificity for infection. The diagnostic accuracy was higher in the index TKA cohort than in the revision TKA cohort. However, the quality of this single article was not sufficient to issue a limited recommendation for or against MRI.

There may be a radiation dose associated with imaging of the site but it is small enough to pose no real risk to the patient. Some metal implants are not MRI safe which must be determined prior to imaging. Caution should be used with intravenous administration of iodinated and gadolinium based contrast agents to patients with impaired renal function (https://www.acr.org/Clinical-Resources/Contrast-Manual). Additionally, not all test options are available at each center which may have resource, access to care, and cost implications not fully delineated in these recommendations.

More high-quality evidence is needed to determine if ultrasound and MRI are useful in diagnosis of PJI, andhigher quality diagnostic evidence is needed in order to create stronger recommendations.