Functional Imaging of the Knee-A Comprehensive Review

Knee pain is a common presenting problem in the general population. Radiographs and magnetic resonance imaging (MRI) are the cornerstones of imaging in current clinical practice. With advancements in technology, there has been increasing utilization of other modalities to evaluate knee disorders. Dynamic assessment utilizing computed tomography and portable ultrasounds have demonstrated the capacity to accurately assess and reproducibly quantify kinematics of knee disorders. Cartilage physiology can be evaluated with MRI. Emerging research has even demonstrated novel musculoskeletal applications of positron emission tomography to evaluate anterior cruciate ligament graft metabolic activity following reconstruction. As technology continues to evolve and traditional ways are improved upon, future comparative studies will elucidate the distinct advantages of the various modalities. Although radiology is still primarily an anatomic specialty, there is immense potential for functional imaging to be the standard of care. This review focuses on the most common musculoskeletal applications of functional imaging as well as future utilization.

PET/MRI reveals ongoing metabolic activity in ACL grafts one year post-ACL reconstruction

Purpose

To use serial PET/MRI imaging to radiographically evaluate the metabolic activity of the ACL graft over the first post-operative year.

Methods

Six patients undergoing primary ACL reconstruction were recruited in this prospective study in an inpatient university hospital. All patients underwent femoral and tibial suspensory cortical fixation with quadrupled semitendinosus autograft hamstring ACL reconstruction by an orthopaedic surgeon. Simultaneous ¹⁸F-FDG PET and MRI of both the operative and non-operative knee was performed at three, six, and 12 months post-operatively. Quantification of the mean standardized uptake value (SUV) within the whole-knee, as well as tibial tunnel, femoral tunnel, and intra-articular graft regions of interest (ROIs).

Results

PET whole-knee activity was increased at all time-points post-operatively compared to the control, non-operative knee. Activity decreased over time, yet considerable generalized activity remained 1 year post-operatively, with relative intensity 34% percent higher than control. When the operative knee was divided into three whole-regions, there was greater activity in the tibia at three than 12 months, the femur at six than 12 months, and in the tibia compared to the intra-articular region at 3 months. When they were separated into sub-regions, results demonstrated greater activity closer to the joint surface.

Conclusions

PET/MRI evaluation of ACL graft reconstructions demonstrates evolving biologic activity within the graft and both tunnels. Focal areas of increased activity within the tunnels may indicate of ligamento-osseous morphologic changes. These data suggest that graft incorporation continues well beyond 1 year post-operatively.

Level of evidence

Level IV.

Comparison of F18-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography and Dynamic Contrast-Enhanced Magnetic Resonance Imaging as Markers of Graft Viability in Anterior Cruciate Ligament Reconstruction

BACKGROUND

F18-fluorodeoxyglucose positron emission tomography/computed tomography (F18-FDG PET/CT) can be used to assess changes in the metabolism of an anterior cruciate ligament (ACL) graft as it is undergoing "ligamentization." Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is the preferred modality for noninvasive assessment of graft structure and graft vascularity.

PURPOSE

To compare the use of F18-FDG PET/CT and DCE-MRI to assess ligamentization within the ACL graft and correlate the results with clinical tests.

STUDY DESIGN

Case series; Level of evidence, 4.

METHODS

Among 30 recruited patients, 27 patients (3 females and 24 males) completed 2 follow-up assessments at a mean of 125 ± 22 days and 259 ± 38 days after arthroscopic ACL reconstruction. At both assessments, anterior drawer test, Lachman test, and Lysholm scoring (LS) were conducted. Images from F18-FDG PET/CT and MRI were analyzed qualitatively and quantitatively (maximum standardized uptake value [SUVmax], SUVmax ratio to the contralateral side [SUVmax CL], normalized enhancement [NE]) in 3 zones: femoral, intra-articular (IA), and tibial. Of the 27 recruited patients, 1 patient had reinjury due to a fall. Therefore, 26 patients were considered for the final analysis.

RESULTS

A significant improvement ( P = .0001) was found in median LS, from 78.5 (range, 62-90) to 94.5 (range, 84-100), at the second follow-up. All grafts were found to be viable on PET/CT and vascularized on MRI. All grafts were seen as continuous on MRI, with exception of 1 graft at the second follow-up. Dynamic MRI identified single-vessel supply to all of the grafts at the first follow-up and multiple-vessel supply in 10 patients at the second follow-up. Reduction in the median SUVmax, SUVmax CL, and NE at second follow-up was seen in all 3 zones. Only SUVmax CL in the IA zone showed a significant reduction ( P = .032); patients with excellent LS at the second follow-up showed significantly higher reduction ( P = .005) than patients with good LS. NE in the IA zone was correlated (0.39; P = .048) with LS only at the first follow-up, whereas SUVmax CL (-0.52; P = .006) and SUVmax (-0.49, P = .010) in the IA zone negatively correlated with LS at the second follow-up only. No correlation was observed between PET/CT and MRI parameters.

CONCLUSION

Glucose metabolism and vascularity in the graft tissue can be used to assess ligamentization of ACL graft. A viable and vascularized graft at first follow-up is associated with good to excellent final outcome, regardless of LS at this stage. Since no correlation was observed between PET/CT and MRI parameters, they may be assessing different domains of the same process. Higher NE in the IA zone at the first follow-up and lower SUVmax CL in the same region at second follow-up are associated with better outcome.

Correlation of benign incidental findings seen on whole-body PET-CT with knee MRI: patterns of 18F-FDG avidity, intra-articular pathology, and bone marrow edema lesions

OBJECTIVES

To correlate patterns of ¹⁸F-FDG uptake on whole-body PET-CT with MR findings and compare the degree of FDG activity between symptomatic and asymptomatic knees.

MATERIALS AND METHODS

Retrospective database query was performed using codes for knee MRI as well as whole-body PET-CT. Patients with malignant disease involving the knee or hardware were excluded. Patients who had both studies performed within 1 year between 2012 and 2017 were included for analysis. Knee joint osteoarthrosis, meniscal and ligamentous integrity, presence of joint effusion, and synovitis were assessed and recorded. Bone marrow edema lesions (BMELs) were identified, segmented, and analyzed using volumetric analysis. SUVmax was assessed over the suprapatellar joint space, intercondylar notch and Hoffa's fat pad. Symptomatic and asymptomatic knees were compared in patients with unilateral symptoms.

RESULTS

Twenty-two cases (20 patients) with mean age 63.3 years (range, 36-91 years) were included. Two patients had bilateral pain. The most FDG avid regions in both symptomatic and asymptomatic knees were the intercondylar notch (SUVmax = 1.84 vs. 1.51), followed by suprapatellar pouch (SUVmax = 1.74 vs. 1.29) and Hoffa's fat pad (SUVmax = 1.01 vs. 0.87). SUVmax was significantly associated with cartilage loss (mean modified Outerbridge score) (r = 0.60, p = 0.003) and degree of synovitis (r = 0.48, p = 0023). Overall, mean SUVmax was significantly higher in the presence of a meniscal tear (1.83 ± 0.67 vs. 1.22 ± 0.40, p = 0.030). Nine patients had BMELs (volume: range = 0.6-27.8, mean = 7.79) however there was no significant association between BMEL volume and SUVmax.

CONCLUSIONS

Higher FDG activity correlates with intra-articular derangement and the intercondylar notch represents the most metabolically active region of the knee.

Role of Nuclear Medicine for Diagnosing Infection of Recently Implanted Lower Extremity Arthroplasties

Infection is an infrequent complication of lower extremity prosthetic joint surgery. Approximately one-third develop within 3 months (early), another third within 1 year (delayed), and the remainder more than 1 year (late) after surgery. Diagnosing prosthetic joint infection, especially in the early postoperative period during the first year, is challenging. Pain is almost always present. The presence of fever is variable, ranging from less than 5% to more than 40% of patients. Leukocytosis is a poor predictor of infection. After primary uncomplicated arthroplasty, the C-reactive protein remains elevated for up to 3 weeks. The erythrocyte sedimentation rate can remain elevated for up to 1 year. Although joint aspiration with culture, the definitive preoperative diagnostic procedure, is specific, its sensitivity is variable. Plain radiographs lack sensitivity and specificity. Radionuclide studies are useful for evaluating painful joint replacements, but data on their utility during the early postoperative period are limited. During the first year after arthroplasty insertion, the bone scan can exclude infection. It is a good "rule-out" test, but it is not reliable for "ruling in" infection. Gallium-67 accumulates in normally healing surgical incisions and in aseptic inflammation. With an accuracy of 60%-80% for diagnosing prosthetic joint infection, there is little role for this radiopharmaceutical for evaluating prosthetic joints, regardless of age. Although data about diagnosing prosthetic joint infection with ¹⁸F-FDG in the early postoperative period are lacking, uptake of this radiopharmaceutical in a variety of postoperative settings for variable time periods is well known. Furthermore, its utility for diagnosing prosthetic joint infection in general, after nearly 2 decades of investigation, remains to be established. Indium-111-labeled leukocytes do not accumulate in normally healing surgical wounds, and in combination with marrow imaging, the test is about 90% accurate for diagnosing prosthetic joint infection. Preliminary data indicate a comparable accuracy in the early postoperative period.