Injury to the anterior cruciate ligament (ACL) commonly requires surgical placement of a graft in order to restore function to the knee¹. Graft healing is not well delineated by MRI alone². Therefore a combined MRI and molecular imaging approach may serve to better evaluate the process of healing and rehabilitation management, as well as the decision process to return to sports or athletic activities. Combined conventional PET/MR has proven feasible for assessment of ACL graft healing following surgery using conventional imaging systems. We aimed to further evaluate this process by employing next generation digital photon counting, time of flight PET (dPET) combined with digital coil MR for imaging and quantification of ACL-graft metabolic activity.

10 patients had same day MRI on a 3T Ingenia CX using an 8 channel knee coil and PET/CT on a next generation (pre-commercial release) Vereos digital PET system (Philips Healthcare, Cleveland, OH). An in-house fabricated mold of the MR knee coil was used during PET acquisitions to ensure identical positioning between image sets. A single bed position centered on the knees was acquired following a 111 MBq ¹⁸F-FDG injection. Listmode data were reconstructed into dynamic frames of 5 minutes from the time of injection as well as static frame taken from 60-75 minutes post-injection. Patients were grouped according to time since surgery and PET data were quantified using SUV_max measured in the proximal, middle, and distal portions of the graft, femoral and tibial tunnels, the posterior cruciate ligament (PCL), and quadriceps muscle for reference. Matched ROIs were drawn in the contralateral knee. Co-registration of PET and MR image sets was readily feasible for all patients. Patients with more recent surgery were found to have markedly higher metabolic uptake in the graft and bone tunnels than patients with surgery greater than two years prior to imaging. It was also seen that with increasing time since surgery the activity level through the graft as well as surrounding tissues decreased toward values measured in healthy knees. To a lesser extent, the uptake in the native ACL and surrounding tissue of patients with recent surgery was found to be slightly higher than the healthy knees of those patients with longer healing times. Quantitative PET imaging of small structures, such as ligaments of the knee, has traditionally been limited by severe partial volume effects in structures at or near the resolution limits of the system. The introduction of digital photon counting in PET improves the system’s spatial resolution as well a Time of Flight timing resolution which facilitates an improved co-registration with high resolution MRI. The improvements helps to greatly decrease the amount of partial volume effects which can be leveraged by smaller voxel volume reconstruction. Previous assessment of ACL graft viability by combined PET/MR has suffered from the lack of quantitative accuracy in conventional PET imaging. This next phase of the study is seeing improved quantitative precision, as validated in phantom data, and thus refined evaluation of ACL graft healing. The distinction between uptake levels in newer grafts versus those in place longer was more apparent with use of dPET, allowing greater potential for correlation with clinical markers of graft healing. Further assessment will also include review of MR image characteristics in comparison to PET metabolic activity in addition to efforts at further PET radiopharmaceutical dose reduction.Molecular imaging assessment of ACL graft viability is improved by use of digital photon counting in PET which makes it preferable for combination with high resolution MRI. Enhanced quantitative accuracy as well as target to background levels provide advanced insight into graft healing, with the potential for ultra-low dose acquisitions in PET and faster imaging times in MR, increasing the clinical feasibility of such a combined imaging approach for this and other sports medicine applications.