PET/MRI molecular imaging in sports medicine: Assessment of ACL graft viability with combined  18F-FDG and 3T MR imaging
Katherine Binzel1, Robert Magnussen2, Christopher Kaeding2, David C Flanigan2, Wenbo Wei1, Melanie U Knopp3, and Michael V Knopp1

1Radiology, The Ohio State University, Columbus, OH, United States, 2Sports Medicine, The Ohio State University, Columbus, OH, United States, 3Sports Medicine, Pepperdine University, Malibu, CA, United States

Synopsis

Evaluation of ACL graft healing by combined PET/MR imaging is readily feasible but has limited quantitative accuracy with use of conventional PET systems. The application of next generation digital photon counting in PET improves the quantitative accuracy and precision of assessment of graft metabolic activity. This advanced imaging modality also holds potential for ultra-low dose PET imaging, enhancing the clinical utility of such a combined imaging approach for detailed assessment of ACL graft viability after reconstructive surgery.

Purpose

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.

Methods

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 SUVmax 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.

Results

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.

Discussion

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.

Conclusion

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.

Acknowledgements

This research was supported by the Ohio Third Frontier OSDA TECH 11-044 and TECH 13-060 grants.

References

1. De Carlo MS, McDivitt R. Rehabilitation of Patients Following Autogenic Bone-Patellar Tendon-Bone ACL Reconstruction: A 20-Year Perspective. N Am J Sports Phys Ther. Aug 2006;1(3):108-123. PMID: 21522223.

2. Bencardino JT1, Beltran J, Feldman MI, Rose DJ. MR imaging of complications of anterior cruciate ligament graft reconstruction. Radiographics. 2009 Nov;29(7):2115-26. doi: 10.1148/rg.297095036.

Figures

Figure 1: Sample image from a patient less than 6 months since reconstructive surgery. Excellent co-registration between PET and MR image sets is seen through the entire knee. Areas of increased FDG uptake are noted through the proximal and distal portions of the graft, with less uptake through the middle portion. Moderate uptake in the femoral bone tunnel is visible as well.

Figure 2: Sample images from a patient in the 6-12 month category. Here a different pattern of metabolic activity is present on the PET images. Again exceptional co-registration of the PET and MR images is achieved. Here the uptake is seen to be less apparent through the ACL graft, however more intense through the PCL. Diminished activity in the bone tunnels as compared to patients with more recent surgery is notable as well.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
4461