ACL graft metabolic activity assessed by ¹⁸FDG PET–MRI

Background To demonstrate the use of ¹⁸Fluorodeoxyglucose positron emission tomography (PET) and magnetic resonance imaging (MRI) in combination (¹⁸FDG-PET) to assess the metabolic activity of ACL graft tissue and evaluate the utility of this technique for ligament imaging. Methods Twenty-one knees with intact ACL grafts in 19 patients at multiple time points following ACL reconstruction were recruited to participate. PET–MRI imaging was performed using a custom device to place knees in the same position for both studies. Images were co-registered for quantification of ¹⁸FDG-PET standardized uptake value (SUV) for the proximal, middle, and distal ACL was quantified. Signal in extra-articular muscle tissue in the index knee was also recorded as a control. Signal from each location was compared based on how far post-operative each knee was from ACL reconstruction (< 6 months, six to 12 months, 12–24 months, or > 24 months). Results Significant differences in ¹⁸FDG PET SUV between the four time points were observed in the proximal (p = 0.02), middle (p = 0.004), and distal (p = 0.007) portions of the ACL graft. The greater than 24 months group was noted to be different from other groups in each case. No difference in PET ¹⁸FDG SUV was noted in the extra-articular muscle in the index knee in each time group (p = 0.61). Conclusions Metabolic activity was noted to be significantly lower in grafts imaged greater than two years post-reconstruction relative to those grafts that had been in place for shorter periods of time.