UTE-T2∗ mapping detects sub-clinical meniscus injury after anterior cruciate ligament tear

Seven tesla knee MRI T2*-mapping detects intrasubstance meniscus degeneration in patients with posterior root tears

Background

Medial meniscus root tears often lead to knee osteoarthritis. The extent of meniscal tissue changes beyond the localized root tear is unknown.

Purpose

To evaluate if 7 Tesla 3D T2*-mapping can detect intrasubstance meniscal degeneration in patients with arthroscopically verified medial meniscus posterior root tears (MMPRTs), and assess if tissue changes extend beyond the immediate site of the posterior root tear detected on surface examination by arthroscopy.

Methods

In this prospective study we acquired 7 T knee MRIs from patients with MMPRTs and asymptomatic controls. Using a linear mixed model, we compared T2* values between patients and controls, and across different meniscal regions. Patients underwent arthroscopic assessment before MMPRT repair. Changes in pain levels before and after repair were calculated using Knee Injury & Osteoarthritis Outcome Score (KOOS). Pain changes and meniscal extrusion were correlated with T2* using Pearson correlation (r).

Results

Twenty patients (mean age 53 ± 8; 16 females) demonstrated significantly higher T2* values across the medial meniscus (anterior horn, posterior body and posterior horn: all P < .001; anterior body: P = .007), and lateral meniscus anterior (P = .024) and posterior (P < .001) horns when compared to the corresponding regions in ten matched controls (mean age 53 ± 12; 8 females). Elevated T2* values were inversely correlated with the change in pain levels before and after repair. All patients had medial meniscal extrusion of ≥2 mm. Arthroscopy did not reveal surface abnormalities in 70% of patients (14 out of 20).

Conclusions

Elevated T2* values across both medial and lateral menisci indicate that degenerative changes in patients with MMPRTs extend beyond the immediate vicinity of the posterior root tear. This suggests more widespread meniscal degeneration, often undetected by surface examinations in arthroscopy.

Ultrashort Echo Time Quantitative Magnetic Resonance Imaging of the Cruciate Ligaments in Normal Beagles

OBJECTIVE

 The aim of this study was to provide normative ultrashort echo time magnetic resonance imaging (UTE MRI) data of the patellar ligament (PL), cranial cruciate ligament (CrCL) and caudal cruciate ligament (CdCL) in non-lame Beagles.

STUDY DESIGN

 Eight stifles from four subjects obtained immediately postmortem were imaged using UTE MRI in the true sagittal plane. Regions of interest were drawn manually and the total (T2*), short T2* (T2*S) and long T2* (T2*L) values of the signal decay were calculated to evaluate the bound and free water components of the tendon. The T2*S, T2*L and T2* values were compared between the PL, CrCL and CdCL RESULTS:  The mean and standard deviation of T2*S, T2*L and T2* were as follows: 0.54 ± 0.13, 4.65 ± 1.08 and 8.35 ± 0.82 ms for the PL; 0.46 ± 0.14, 5.99 ± 0.52 and 8.88 ± 0.4 ms for the CrCL and 0.41 ± 0.13, 7.06 ± 0.57 and 9.26 ± 0.18 ms for the CdCL. Significant differences were found between the T2*L component of the PL and each CrCL/CdCL and a smaller difference was noted between the T2*L of the CrCL and CdCL (p = 0.05). No difference of the T2*S value was found between any of the ligaments.

CONCLUSION

 Establishing normative UTE data of the canine stifle is valuable for comparison in future studies in which normal and damaged ligaments may be evaluated, particularly in those affected limbs in which no instability is identified on physical examination in which normal and damaged ligaments may be evaluated.

Quantitative evaluation of meniscus injury using synthetic magnetic resonance imaging

BACKGROUND

Magnetic resonance imaging (MRI) can diagnose meniscal lesions anatomically, while quantitative MRI can reflect the changes of meniscal histology and biochemical structure. Our study aims to explore the association between the measurement values obtained from synthetic magnetic resonance imaging (SyMRI) and Stoller grades. Additionally, we aim to assess the diagnostic accuracy of SyMRI in determining the extent of meniscus injury. This potential accuracy could contribute to minimizing unnecessary invasive examinations and providing guidance for clinical treatment.

METHODS

Total of 60 (n=60) patients requiring knee arthroscopic surgery and 20 (n=20) healthy subjects were collected from July 2022 to November 2022. All subjects underwent conventional MRI and SyMRI. Manual measurements of the T1, T2 and proton density (PD) values were conducted for both normal menisci and the most severely affected position of injured menisci. These measurements corresponded to the Stoller grade of meniscus injuries observed in the conventional MRI. All patients and healthy subjects were divided into normal group, degeneration group and torn group according to the Stoller grade on conventional MRI. One-way analysis of variance (ANOVA) was employed to compare the T1, T2 and PD values of the meniscus among 3 groups. The accuracy of SyMRI in diagnosing meniscus injury was assessed by comparing the findings with arthroscopic observations. The diagnostic efficiency of meniscus degeneration and tear between conventional MRI and SyMRI were analyzed using McNemar test. Furthermore, a receiver operating characteristic curve (ROC curve) was constructed and the area under the curve (AUC) was utilized for evaluation.

RESULTS

According to the measurements of SyMRI, there was no statistical difference of T1 value or PD value measured by SyMRI among the normal group, degeneration group and torn group, while the difference of T2 value was statistically significant among 3 groups (P=0.001). The arthroscopic findings showed that 11 patients were meniscal degeneration and 49 patients were meniscal tears. The arthroscopic findings were used as the gold standard, and the difference of T1 and PD values among the 3 groups was not statistically significant, while the difference of T2 values (32.81±2.51 of normal group, 44.85±3.98 of degeneration group and 54.42±3.82 of torn group) was statistically significant (P=0.001). When the threshold of T2 value was 51.67 (ms), the maximum Yoden index was 0.787 and the AUC value was 0.934.

CONCLUSIONS

The measurement values derived from SyMRI could reflect the Stoller grade, illustrating that SyMRI has good consistency with conventional MRI. Moreover, the notable consistency observed between SyMRI and arthroscopy suggests a potential role for SyMRI in guiding clinical diagnoses.

High angular resolution diffusion imaging (HARDI) of porcine menisci: a comparison of diffusion tensor imaging and generalized q-sampling imaging

Background

Diffusion magnetic resonance imaging (MRI) allows for the quantification of water diffusion properties in soft tissues. The goal of this study was to characterize the 3D collagen fiber network in the porcine meniscus using high angular resolution diffusion imaging (HARDI) acquisition with both diffusion tensor imaging (DTI) and generalized q-sampling imaging (GQI).

Methods

Porcine menisci (n=7) were scanned ex vivo using a three-dimensional (3D) HARDI spin-echo pulse sequence with an isotropic resolution of 500 µm at 7.0 Tesla. Both DTI and GQI reconstruction techniques were used to quantify the collagen fiber alignment and visualize the complex collagen network of the meniscus. The MRI findings were validated with conventional histology.

Results

DTI and GQI exhibited distinct fiber orientation maps in the meniscus using the same HARDI acquisition. We found that crossing fibers were only resolved with GQI, demonstrating the advantage of GQI over DTI to visualize the complex collagen fiber orientation in the meniscus. Furthermore, the MRI findings were consistent with conventional histology.

Conclusions

HARDI acquisition with GQI reconstruction more accurately resolves the complex 3D collagen architecture of the meniscus compared to DTI reconstruction. In the future, these technologies have the potential to nondestructively assess both normal and abnormal meniscal structure.

Deep-Learning-Aided Evaluation of Spondylolysis Imaged with Ultrashort Echo Time Magnetic Resonance Imaging

Isthmic spondylolysis results in fracture of pars interarticularis of the lumbar spine, found in as many as half of adolescent athletes with persistent low back pain. While computed tomography (CT) is the gold standard for the diagnosis of spondylolysis, the use of ionizing radiation near reproductive organs in young subjects is undesirable. While magnetic resonance imaging (MRI) is preferable, it has lowered sensitivity for detecting the condition. Recently, it has been shown that ultrashort echo time (UTE) MRI can provide markedly improved bone contrast compared to conventional MRI. To take UTE MRI further, we developed supervised deep learning tools to generate (1) CT-like images and (2) saliency maps of fracture probability from UTE MRI, using ex vivo preparation of cadaveric spines. We further compared quantitative metrics of the contrast-to-noise ratio (CNR), mean squared error (MSE), peak signal-to-noise ratio (PSNR), and structural similarity index (SSIM) between UTE MRI (inverted to make the appearance similar to CT) and CT and between CT-like images and CT. Qualitative results demonstrated the feasibility of successfully generating CT-like images from UTE MRI to provide easier interpretability for bone fractures thanks to improved image contrast and CNR. Quantitatively, the mean CNR of bone against defect-filled tissue was 35, 97, and 146 for UTE MRI, CT-like, and CT images, respectively, being significantly higher for CT-like than UTE MRI images. For the image similarity metrics using the CT image as the reference, CT-like images provided a significantly lower mean MSE (0.038 vs. 0.0528), higher mean PSNR (28.6 vs. 16.5), and higher SSIM (0.73 vs. 0.68) compared to UTE MRI images. Additionally, the saliency maps enabled quick detection of the location with probable pars fracture by providing visual cues to the reader. This proof-of-concept study is limited to the data from ex vivo samples, and additional work in human subjects with spondylolysis would be necessary to refine the models for clinical use. Nonetheless, this study shows that the utilization of UTE MRI and deep learning tools could be highly useful for the evaluation of isthmic spondylolysis.

Early MRI-based quantitative outcomes are associated with a positive functional performance trajectory from 6 to 24 months post-ACL surgery

PURPOSE

Quantitative magnetic resonance imaging (qMRI) has been used to determine the failure properties of ACL grafts and native ACL repairs and/or restorations. How these properties relate to future clinical, functional, and patient-reported outcomes remain unknown. The study objective was to investigate the relationship between non-contemporaneous qMRI measures and traditional outcome measures following Bridge-Enhanced ACL Restoration (BEAR). It was hypothesized that qMRI parameters at 6 months would be associated with clinical, functional, and/or patient-reported outcomes at 6 months, 24 months, and changes from 6 to 24 months post-surgery.

METHODS

Data of BEAR patients (n = 65) from a randomized control trial of BEAR versus ACL reconstruction (BEAR II Trial; NCT02664545) were utilized retrospectively for the present analysis. Images were acquired using the Constructive Interference in Steady State (CISS) sequence at 6 months post-surgery. Single-leg hop test ratios, arthrometric knee laxity values, and International Knee Documentation Committee (IKDC) subjective scores were determined at 6 and 24 months post-surgery. The associations between traditional outcomes and MRI measures of normalized signal intensity, mean cross-sectional area (CSA), volume, and estimated failure load of the healing ACL were evaluated based on bivariate correlations and multivariable regression analyses, which considered the potential effects of age, sex, and body mass index.

RESULTS

CSA (r = 0.44, p = 0.01), volume (r = 0.44, p = 0.01), and estimated failure load (r = 0.48, p = 0.01) at 6 months were predictive of the change in single-leg hop ratio from 6 to 24 months in bivariate analysis. CSA (βstandardized = 0.42, p = 0.01), volume (βstandardized = 0.42, p = 0.01), and estimated failure load (βstandardized = 0.48, p = 0.01) remained significant predictors when considering the demographic variables. No significant associations were observed between MRI variables and either knee laxity or IKDC when adjusting for demographic variables. Signal intensity was also not significant at any timepoint.

CONCLUSION

The qMRI-based measures of CSA, volume, and estimated failure load were predictive of a positive functional outcome trajectory from 6 to 24 months post-surgery. These variables measured using qMRI at 6 months post-surgery could serve as prospective markers of the functional outcome trajectory from 6 to 24 months post-surgery, aiding in rehabilitation programming and return-to-sport decisions to improve surgical outcomes and reduce the risk of reinjury.

LEVEL OF EVIDENCE

Level II.

Automated segmentation of the healed anterior cruciate ligament from T2 * relaxometry MRI scans

Collagen organization of the anterior cruciate ligament (ACL) can be evaluated using T2 * relaxometry. However, T2 * mapping requires manual image segmentation, which is a time-consuming process and prone to inter- and intra- segmenter variability. Automating segmentation would address these challenges. A model previously trained using Constructive Interference in Steady State (CISS) scans was applied to T2 * segmentation via transfer learning. It was hypothesized that there would be no significant differences in the model's segmentation performance between T2 * and CISS, structural measures versus ground truth manual segmentation, and reliability versus independent and retest manual segmentation. Transfer learning was conducted using 54 T2 * scans of the ACL. Segmentation performance was assessed with Dice coefficient, precision, and sensitivity, and structurally with T2 * value, volume, subvolume proportions, and cross-sectional area. Model performance relative to independent manual segmentation and repeated segmentation by the ground truth segmenter (retest) were evaluated on a random subset. Segmentation performance was analyzed with Mann-Whitney U tests, structural measures with Wilcoxon signed-rank tests, and performance relative to manual segmentation with repeated-measures analysis of variance/Tukey tests (α = 0.05). T2 * segmentation performance was not significantly different from CISS on all measures (p > 0.35). No significant differences were detected in structural measures (p > 0.50). Automatic segmentation performed as well as the retest on all segmentation measures, whereas independent segmentations were lower than retest and/or automatic segmentation (p < 0.023). Structural measures were not significantly different between segmenters. The automatic segmentation model performed as well on the T2 * sequence as on CISS and outperformed independent manual segmentation while performing as well as retest segmentation.

Histological Findings and T2 Relaxation Time in Canine Menisci of Elderly Dogs—An Ex Vivo Study in Stifle Joints

Osteoarthritis is a chronic disease that often affects the canine stifle joint. Due to their biomechanical function, the menisci in the canine stifle play an important role in osteoarthritis. They compensate for the incongruence in the joint and distribute and minimize compressive loads, protecting the hyaline articular cartilage from damage. Meniscal degeneration favors the development and progression of stifle joint osteoarthritis. Qualitative magnetic resonance imaging (MRI) is the current golden standard for detecting meniscal changes, but it has limitations in detecting early signs of meniscal degeneration. A quantitative MRI offers new options for detecting early structural changes. T2 mapping can especially visualize structural changes such as altered collagen structures and water content, as well as deviations in proteoglycan content. This study evaluated T2 mapping and performed a histological scoring of menisci in elderly dogs that had no or only low radiographic osteoarthritis grades. A total of 16 stifles from 8 older dogs of different sex and breed underwent ex vivo magnet resonance imaging, including a T2 mapping pulse sequence with multiple echoes. A histological analysis of corresponding menisci was performed using a modified scoring system. The mean T2 relaxation time was 18.2 ms and the mean histological score was 4.25. Descriptive statistics did not reveal a correlation between T2 relaxation time and histological score. Ex vivo T2 mapping of canine menisci did not demonstrate histological changes, suggesting that early meniscal degeneration can be present in the absence of radiological signs of osteoarthritis, including no significant changes in T2 relaxation time.

Ultrashort echo time magnetization transfer imaging of knee cartilage and meniscus after long-distance running

OBJECTIVE

To assess the detection of changes in knee cartilage and meniscus of amateur marathon runners before and after long-distance running using a 3D ultrashort echo time MRI sequence with magnetization transfer preparation (UTE-MT).

METHODS

We recruited 23 amateur marathon runners (46 knees) in this prospective cohort study. MRI scans using UTE-MT and UTE-T2* sequences were performed pre-race, 2 days post-race, and 4 weeks post-race. UTE-MT ratio (UTE-MTR) and UTE-T2* were measured for knee cartilage (eight subregions) and meniscus (four subregions). The sequence reproducibility and inter-rater reliability were also investigated.

RESULTS

Both the UTE-MTR and UTE-T2* measurements showed good reproducibility and inter-rater reliability. For most subregions of cartilage and meniscus, the UTE-MTR values decreased 2 days post-race and increased after 4 weeks of rest. Conversely, the UTE-T2* values increased 2 days post-race and decreased after 4 weeks. The UTE-MTR values in lateral tibial plateau, central medial femoral condyle, and medial tibial plateau showed a significant decrease at 2 days post-race compared to the other two time points (p < 0.05). By comparison, no significant UTE-T2* changes were found for any cartilage subregions. For meniscus, the UTE-MTR values in medial posterior horn and lateral posterior horn regions at 2 days post-race were significantly lower than those at pre-race and 4 weeks post-race (p < 0.05). By comparison, only the UTE-T2* values in medial posterior horn showed a significant difference.

CONCLUSIONS

UTE-MTR is a promising method for the detection of dynamic changes in knee cartilage and meniscus after long-distance running.

KEY POINTS

• Long-distance running causes changes in the knee cartilage and meniscus. • UTE-MT monitors dynamic changes of knee cartilage and meniscal non-invasively. • UTE-MT is superior to UTE-T2* in monitoring dynamic changes in knee cartilage and meniscus.

Detection of iron oxide nanoparticle (IONP)-labeled stem cells using quantitative ultrashort echo time imaging: a feasibility study

Background

In this study, we investigated the feasibility of quantitative ultrashort echo time (qUTE) magnetic resonance (MR) imaging techniques in the detection and quantification of iron oxide nanoparticle (IONP)-labeled stem cells.

Methods

A stem cell phantom containing multiple layers of unlabeled or labeled stem cells with different densities was prepared. The phantom was imaged with quantitative UTE (qUTE) MR techniques [i.e., UTE-T1 mapping, UTE-T2* mapping, and UTE-based quantitative susceptibility mapping (UTE-QSM)] as well as with a clinical T2 mapping sequence on a 3T clinical MR system. For T1 mapping, a variable flip angle (VFA) method based on actual flip angle imaging (AFI) technique was utilized. For T2* mapping and UTE-QSM, multiple images with variable, interleaved echo times including UTE images and gradient recalled echo (GRE) images were used. For UTE-QSM, the phase information from the multi-echo images was utilized and processed using a QSM framework based on the morphology-enabled dipole inversion (MEDI) algorithm. The qUTE techniques were also evaluated in an ex vivo experiment with a mouse injected with IONP-labeled stem cells.

Results

In the phantom experiment, the parameters estimated with qUTE techniques showed high linearity with respect to the density of IONP-labeled stem cells (R2>0.99), while the clinical T2 parameter showed impaired linearity (R2=0.87). In the ex vivo mouse experiment, UTE-T2* mapping and UTE-QSM showed feasibility in the detection of injected stem cells with high contrast, whereas UTE-T1 and UTE-T2* showed limited detection. Overall, UTE-QSM demonstrated the best contrast of all, with other methods being subjected more to a confounding factor due to different magnetic susceptibilities of various types of neighboring tissues, which creates inhomogeneous contrast that behaves similar to IONP.

Conclusions

In this study, we evaluated the feasibility of a series of qUTE imaging techniques as well as conventional T2 mapping for the detection of IONP-labeled stem cells in vitro and ex vivo. UTE-QSM performed superior amongst other qUTE techniques as well as conventional T2 mapping in detecting stem cells with high contrast.

Qualitative and Quantitative Evaluation of Morpho-Metabolic Changes in Bone Cartilage Complex of Knee Joint in Osteoarthritis Using Simultaneous 18F-NaF PET/MRI—A Pilot Study

Background Articular cartilage (AC) loss and deterioration, as well as bone remodeling, are all symptoms of osteoarthritis (OA). As a result, an ideal imaging technique for researching OA is required, which must be sensitive to both soft tissue and bone health.

Objective The aim of this study was to assess the potential of simultaneous 18F sodium fluoride (18F-NaF) positron emission tomography/magnetic resonance imaging (PET/MRI) to identify as well as classify osseous metabolic abnormalities in knee OA and to see if degenerative changes in the cartilage and bone on MRI might be correlated with subchondral 18F-NaF uptake on PET.

Methods Sixteen (32 knees) volunteers with no past history of knee injury, with or without pain, were enrolled for the research from January to July 2021. The images of both knees were taken utilizing an molecular magnetic resonance (mMR) body matrix coil on a simultaneous PET/MRI biograph mMR. The acquisition was conducted after 45 minutes of intravenous infusion of 18F-NaF 185–370 MBq (5–10 mCi) over one PET bed for 40 minutes, while MRI sequences were performed simultaneously.

Results All pathologies showed significantly higher maximum standardized uptake value (SUVmax) than the background. Thirty-four subchondral magic spots were identified on 18F-NaF PET without any structural alteration on MRI. Bone marrow lesions (BMLs) and osteophytes with higher MRI osteoarthritis knee score (MOAKS) score showed higher 18F-NaF uptake (grade1˂grade2˂grade3). BMLs had corresponding AC degeneration. There was discordance between grade 1 osteophytes (86.6%), sclerosis (53.7%) and grade 1 BML in cruciate ligament insertion site (91.66%); they did not have high uptake of 18F-NaF. In case of cartilage, there was significant difference between AC grades and average subchondral SUVmax and T2* relaxometry (grade0˂grade1˂grade2˂grade3˂grade4). BMLs are much more metabolically active than other pathologies, while sclerosis is the least. We also found that the subchondral uptake was statistically increased in the areas of pathology:

Conclusion 18F-NaF PET/MRI was able to detect knee abnormalities unseen on MRI alone and simultaneously assessed metabolic and structural markers of knee OA across multiple tissues in the joint. Thus, it is a promising tool for detection of early metabolic changes in OA.

Reproducibility and postacquisition correction methods for quantitative magnetic resonance imaging of the anterior cruciate ligament (ACL)

Quantitative magnetic resonance imaging has been used to evaluate the structural integrity of knee joint structures. However, variations in acquisition parameters between scanners pose significant challenges. Understanding the effect of small differences in acquisition parameters for quantitative sequences is vital to the validity of cross-institutional studies, and for the harmonization of large, heterogeneous datasets to train machine learning models. The study objective was to assess the reproducibility of T2 * relaxometry and the constructive interference in steady-state sequence (CISS) across scanners, with minimal hardware-necessitated changes to acquisition parameters. It was hypothesized that there would be no significant differences between scanners in anterior cruciate ligament T2 * relaxation times and CISS signal intensities (SI). Secondarily, it was hypothesized that differences could be corrected by rescaling the SI distribution to harmonize between scanners. Seven volunteers were scanned on 3T Prisma and Tim Trio scanners (Siemens). Three correction methods were evaluated for T2 *: inverse echo time scaling, z-scoring, and Nyúl histogram matching. For CISS, scans were normalized to cortical bone, scaled by the background noise ratio, and log-transformed. Before correction, significant mean differences of 6.0 ± 3.2 ms (71.8%; p = 0.02) and 0.49 ± 0.15 units (40.7%; p = 0.02) for T2 * and CISS across scanners were observed, respectively. After rescaling, T2 * differences decreased to 2.6 ± 2.7 ms (23.9%; p = 0.03), 1.3 ± 2.5 ms (10.9%; p = 0.13), and 1.27 ± 3.0 ms (19.6%; p = 0.40) for inverse echo time, z-scoring, and Nyúl, respectively, while CISS decreased to 0.01 ± 0.11 units (4.0%; p = 0.87). These findings suggest that small acquisition parameter differences may lead to large changes in T2 * and SI values that must be reconciled to compare data across magnets.

Making the invisible visible-ultrashort echo time magnetic resonance imaging: Technical developments and applications

Magnetic resonance imaging (MRI) uses a large magnetic field and radio waves to generate images of tissues in the body. Conventional MRI techniques have been developed to image and quantify tissues and fluids with long transverse relaxation times (T2s), such as muscle, cartilage, liver, white matter, gray matter, spinal cord, and cerebrospinal fluid. However, the body also contains many tissues and tissue components such as the osteochondral junction, menisci, ligaments, tendons, bone, lung parenchyma, and myelin, which have short or ultrashort T2s. After radio frequency excitation, their transverse magnetizations typically decay to zero or near zero before the receiving mode is enabled for spatial encoding with conventional MR imaging. As a result, these tissues appear dark, and their MR properties are inaccessible. However, when ultrashort echo times (UTEs) are used, signals can be detected from these tissues before they decay to zero. This review summarizes recent technical developments in UTE MRI of tissues with short and ultrashort T2 relaxation times. A series of UTE MRI techniques for high-resolution morphological and quantitative imaging of these short-T2 tissues are discussed. Applications of UTE imaging in the musculoskeletal, nervous, respiratory, gastrointestinal, and cardiovascular systems of the body are included.

UTE-T2* versus conventional T2* mapping to assess posterior cruciate ligament ultrastructure and integrity-an in-situ study

Background

Clinical-standard morphologic magnetic resonance imaging (MRI) is limited in the refined diagnosis of posterior cruciate ligament (PCL) injuries. Quantitative MRI sequences such as ultrashort echo-time (UTE)-T2* mapping or conventional T2* mapping have been theorized to quantify ligament (ultra-) structure and integrity beyond morphology. This study evaluates their diagnostic potential in identifying and differentiating partial and complete PCL injuries in a standardized graded injury model.

Methods

Ten human cadaveric knee joint specimens were imaged on a clinical 3.0 T MRI scanner using morphologic, conventional T2* mapping, and UTE-T2* mapping sequences before and after standardized arthroscopic partial and complete PCL transection. Following manual segmentation, quantitative T2* and underlying texture features (i.e., energy, homogeneity, and variance) were analyzed for each specimen and PCL condition, both for the entire PCL and its subregions. For statistical analysis, Friedman’s test followed by Dunn’s multiple comparison test was used against the level of significance of P≤0.01.

Results

For the entire PCL, T2* was significantly increased as a function of injury when acquired with the UTE-T2* sequence [entire PCL: 11.1±3.1 ms (intact); 10.9±4.6 ms (partial); 14.3±4.9 ms (complete); P<0.001], but not when acquired with the conventional T2* sequence [entire PCL: 10.0±3.2 ms (intact); 11.4±6.2 ms (partial); 15.5±7.8 ms (complete); P=0.046]. The PCL subregions and texture variables showed variable changes indicative of injury-associated disorganization.

Conclusions

In contrast to the conventional T2* mapping, UTE-T2* mapping is more receptive in the detection of structural damage of the PCL and allows quantitative assessment of ligament (ultra-)structure and integrity that may help to improve diagnostic differentiation of distinct injury states. Once further substantiated beyond the in-situ setting, UTE-T2* mapping may refine diagnostic evaluation of PCL injuries and -possibly- monitor ligament healing, ageing, degeneration, and inflammation.

Ligamentization of the reconstructed ACL differs between the intraarticular and intraosseous regions: A quantitative assessment using UTE-T2* mapping

Background

The purpose of this study was to prospectively observe the trends of ultrashort echo time (UTE)-T2* values for the intraarticular and intraosseous regions of reconstructed anterior cruciate ligaments from 6 to 12 months after anterior cruciate ligament reconstruction by using UTE-T2* mapping, and to investigate the changes and differences over time in each region.

Methods

Ten patients underwent UTE-T2* mapping of the operated knee at 6, 9, and 12 months after anterior cruciate ligament reconstruction. The UTE-T2* values of intraarticular and intraosseous regions of reconstructed anterior cruciate ligaments at 6, 9, and 12 months postoperatively were statistically compared.

Results

The UTE-T2* values of the intraarticular region at 6 months postoperatively were significantly higher than those at 9 and 12 months. There were no significant differences in the UTE-T2* values at 6, 9, and 12 months postoperatively in the intraosseous region. At 6 months postoperatively, the UTE-T2* values of the intraarticular region were significantly higher than those of the intraosseous region. The UTE-T2* values of the intraosseous region at the tibia were significantly lower than those of the other sites at any postoperative time point.

Conclusions

According to UTE-T2*mapping-based findings, histological maturation of reconstructed ACLs is faster in the intraosseous region than in the intraarticular region. In particular, the intraarticular region is still undergoing rapid histologic changes at 6 months postoperatively, and its tissue structure is less substantial than normal. The findings of this study may provide clues to determine the optimal timing for safe return to sports in terms of ligamentaization of reconstructed ACLs.

Radial T2* mapping reveals early meniscal abnormalities in patients with knee osteoarthritis

OBJECTIVE

We aimed to validate a 2D radial T2* mapping method and its ability to reveal subtle alterations in the menisci of patients with knee osteoarthritis (OA).

METHODS

Of 40 enrolled participants, 20 were diagnosed with OA, and 20 were age- and sex-matched asymptomatic controls. Data from the right knee of each participant were collected using a 1.5-T MRI equipped with a single-channel knee coil. T2* values were acquired using a conventional T2* mapping protocol and a radial T2* mapping method. Mean T2* values in the meniscal white zones, meniscal red zones, and total menisci were calculated. Numerical simulation was performed for validation.

RESULTS

Both simulation and clinical data confirmed that 2D radial T2* mapping provided better discrimination than the conventional method. Compared to controls, the OA group showed significantly greater mean (standard deviation) T2* values in the white zones (9.33 [2.29] ms vs. 6.04 [1.05] ms), red zones (9.18 [2.03] ms vs. 6.81 [1.28] ms), and total menisci (9.26 [2.06] ms vs. 6.34 [1.14] ms). Correlations were found between the Lequesne index and the meniscal T2* values in all three regions (r = 0.528, p = 0.017; r = 0.635, p = 0.003; and r = 0.556, p = 0.011, respectively).

CONCLUSION

These findings indicate that in early OA, radial T2* mapping is an alternative means of assessing meniscal degeneration and can be used to monitor its progression.

KEY POINTS

• Radial T2* mapping outperforms Cartesian T2* mapping. • Radial T2* measurements are useful in assessing meniscal degeneration. • Meniscal T2* values correlate well with disease severity.

Can we afford to ignore the biology of joint healing and graft incorporation after ACL reconstruction?

Anterior cruciate ligament (ACL) reconstruction is successful at restoring stability to return ACL injured patients to high-demand work, sports, and recreational activities. The development of posttraumatic osteoarthritis (OA) in roughly half of patients just 10-15 years after ACLR highlight the need to improve clinical care pathways. Graft failure and reinjury rates, which further increase OA risk, also remain high for younger and more active patients. The biological components of joint recovery and graft incorporation, therefore, impact short- and long-term clinical outcomes. Biochemical and magnetic resonance imaging (MRI) data show substantial compromise of articular cartilage metabolism and matrix composition after ACL injury and reconstructive surgery suggesting a potential need for activity modulation in early recovery. Furthermore, joint recovery is variable with compositional MRI studies showing progressive cartilage degeneration 1 and 2 years after ACLR. Biopsy and MRI studies also show high variability in ACL graft characteristics within the 1st year after ACLR followed by continued graft maturation into the 2nd year and beyond. To improve the care of ACL injured patients, there is a critical need for clinical attention and scientific inquiry into timing the reintroduction of higher load activities in relationship to neuromuscular recovery, joint biology, and graft maturation. In addition to symptomatic and mechanical recovery, development and validation of biological markers for joint and cartilage homeostasis as well as ACL graft healing are needed for personalized decision making on rehabilitation needs, reduction of OA risk, and resumption of athletic, recreational, and vocational activities.

A transfer learning approach for automatic segmentation of the surgically treated anterior cruciate ligament

Quantitative magnetic resonance imaging enables quantitative assessment of the healing anterior cruciate ligament or graft post-surgery, but its use is constrained by the need for time consuming manual image segmentation. The goal of this study was to validate a deep learning model for automatic segmentation of repaired and reconstructed anterior cruciate ligaments. We hypothesized that (1) a deep learning model would segment repaired ligaments and grafts with comparable anatomical similarity to intact ligaments, and (2) automatically derived quantitative features (i.e., signal intensity and volume) would not be significantly different from those obtained by manual segmentation. Constructive Interference in Steady State sequences were acquired of ACL repairs (n = 238) and grafts (n = 120). A previously validated model for intact ACLs was retrained on both surgical groups using transfer learning. Anatomical performance was measured with Dice coefficient, sensitivity, and precision. Quantitative features were compared to ground truth manual segmentation. Automatic segmentation of both surgical groups resulted in decreased anatomical performance compared to intact ACL automatic segmentation (repairs/grafts: Dice coefficient = .80/.78, precision = .79/.78, sensitivity = .82/.80), but neither decrease was statistically significant (Kruskal-Wallis: Dice coefficient p = .02, precision p = .09, sensitivity p = .17; Dunn post-hoc test for Dice coefficient: repairs/grafts p = .054/.051). There were no significant differences in quantitative features between the ground truth and automatic segmentation of repairs/grafts (0.82/2.7% signal intensity difference, p = .57/.26; 1.7/2.7% volume difference, p = .68/.72). The anatomical similarity performance and statistical similarities of quantitative features supports the use of this automated segmentation model in quantitative magnetic resonance imaging pipelines, which will accelerate research and provide a step towards clinical applicability.

Ultrashort Echo Time Magnetic Resonance Imaging Techniques: Met and Unmet Needs in Musculoskeletal Imaging

This review article summarizes recent technical developments in ultrashort echo time (UTE) magnetic resonance imaging of musculoskeletal (MSK) tissues with short-T2 relaxation times. A series of contrast mechanisms are discussed for high-contrast morphological imaging of short-T2 MSK tissues including the osteochondral junction, menisci, ligaments, tendons, and bone. Quantitative UTE mapping of T1, T2*, T1ρ, adiabatic T1ρ, magnetization transfer ratio, MT modeling of macromolecular proton fraction, quantitative susceptibility mapping, and water content is also introduced. Met and unmet needs in MSK imaging are discussed. EVIDENCE LEVEL: 1 TECHNICAL EFFICACY: Stage 3.

Emerging role of integrated PET-MRI in osteoarthritis

Osteoarthritis (OA) is a common degenerative disorder of the articular cartilage, which is associated with hypertrophic changes in the bone, synovial inflammation, subchondral sclerosis, and joint space narrowing (JSN). Radiography remains the first line of imaging till now. Due to the lack of soft-tissue depiction in radiography, researchers are exploring various imaging techniques to detect OA at an early stage and understand its pathophysiology to restrict its progression and discover disease-modifying agents in OA. As the OA relates to the degradation of articular cartilage and remodeling of the underlying bone, an optimal imaging tool must be sensitive to the bone and soft tissue health. In that line, many non-invasive imaging and minimally invasive techniques have been explored. Out of these, the non-invasive compositional magnetic resonance imaging (MRI) for evaluation of the integrity of articular cartilage and positron emission tomography (PET) scan with fluorodeoxyglucose (FDG) and more specific bone-seeking tracer like sodium fluoride (18F-NaF) for bone cartilage interface are some of the leading areas of ongoing work. Integrated PET-MRI system, a new hybrid modality that combines the virtues of the above two individual modalities, allows detailed imaging of the entire joint, including soft tissue cartilage and bone, and holds great potential to research complex disease processes of OA. This narrative review attempts to signify individual characteristics of MRI, PET, the fusion of these characteristics in PET-MRI, and the ongoing research on PET-MRI as a potential tool to understand the pathophysiology of OA.

Disturbances in Metabolic Pathways and the Identification of a Potential Biomarker Panel for Early Cartilage Degeneration in a Rabbit Anterior Cruciate Ligament Transection Model

OBJECTIVE

This study aimed to assess the association between synovial fluid (SF) metabolites and magnetic resonance imaging (MRI) measurements of cartilage biochemical composition to identify potential SF biomarkers for detecting the early onset of cartilage degeneration in a rabbit model.

METHODS

Both knees of 12 New Zealand White rabbits were used. The anterior cruciate ligament transection (ACLT) model was performed on right knees, and the sham surgery on left knees. MRI UTE-T2* scanning and SF sample collection were performed on ACLT knees at 4 and 8 weeks postsurgery and on sham surgery knees at 4 weeks postsurgery. Ultra-performance liquid chromatography-mass spectrometry and multivariate statistical analysis were used to distinguish samples in three groups. Pathway and receiver operating characteristic analyses were utilized to identify potential metabolite biomarkers.

RESULTS

There were 12 knees in sham surgery models, 11 in ACLT models at 4 weeks postsurgery, and 10 in ACLT models at 8 weeks postsurgery. UTE-T2* values for the lateral tibia cartilage showed significant decreases over the study period. Levels of 103 identified metabolites in SF were markedly different among three groups. Furthermore, 24 metabolites were inversely correlated with UTE-T2* values of the lateral tibia cartilage, while hippuric acid was positively correlated with UTE-T2* values of the lateral tibia cartilage. Among 25 potential markers, N1-acetylspermidine, 2-amino-1,3,4-octadecanetriol, l-phenylalanine, 5-hydroxy-l-tryptophan, and l-tryptophan were identified as potential biomarkers with high area under the curve values and Pearson correlation coefficients.

CONCLUSION

Five differential metabolites in SF were found as potential biomarkers for the early detection of cartilage degeneration in the rabbit ACLT model.

Evaluation of Meniscal Tissue after Meniscal Repair Using Ultrahigh Field MRI

The study evaluates the meniscal tissue after primary meniscal suturing using 7-Tesla (T) magnetic resonance imaging with T₂* mapping at 6 and 12 months after surgery to investigate the differences between repaired meniscal tissue and healthy meniscal tissue in the medial and lateral compartment. This prospective study included 11 patients (9m/2f) with a mean age of 30.6 years (standard deviation 9.0). Patients with a meniscal tear that was treated arthroscopically with meniscus suturing, using an all-inside technique, were included. All patients and seven healthy volunteers were imaged on a 7-T whole-body system. T2* mapping of the meniscus was applied on sagittal slices. Regions-of-interest were defined manually in the red and white zone of each medial and lateral meniscus to measure T2*-values. In the medial posterior and medial anterior horn similar T₂*-values were measured in the red and white zone at 6- and 12-month follow-up. Compared with the control group higher T₂*-values were found in the repaired medial meniscus. After 12-months T₂*-values decreased to normal values in the anterior horn and remained elevated in the posterior horn. In the red zone of the lateral posterior horn a significant decrease in the T₂*-values (from 8.2 milliseconds to 5.9 milliseconds) (p = 0.04), indicates successful repair; a tendency toward a decrease in the white zone between the 6 and 12 months follow-up was observed. In the red zone of the lateral anterior horn the T₂*-values decreased significantly during follow-up and in the white zone of the lateral anterior horn T₂*-values were comparable. In comparison to the control group higher T₂*-values were measured at 6-months; however, the T₂*-values showed comparable values in the repaired lateral meniscus after 12 months. The T2* mapping results of the current study indicated a better healing response of the red zone of the lateral posterior horn compared with the medial posterior horn.

3D MRI Models of the Musculoskeletal System

Computed tomography (CT) is most commonly used to produce three-dimensional (3D) models for evaluating bone and joint morphology in clinical practice. However, 3D models created from magnetic resonance imaging (MRI) data can be equally effective for comprehensive and accurate assessment of osseous and soft tissue structure morphology and pathology. The quality of 3D MRI models has steadily increased over time, with growing potential to replace 3D CT models in various musculoskeletal (MSK) applications. In practice, a single MRI examination for two-dimensional and 3D assessments can increase the value of MRI and simplify the pre- and postoperative imaging work-up. Multiple studies have shown excellent performance of 3D MRI models in shoulder injuries, in the hip in the setting of femoroacetabular impingement, and in the knee for the creation of bone surface models. Therefore, the utility of 3D MRI postprocessed models is expected to continue to rise and broaden in applications. Computer-based and artificial intelligence-assisted postprocessing techniques have tremendous potential to improve the efficiency of 3D model creation, opening many research avenues to validate the applicability of 3D MRI and establish 3D-specific quantitative assessment criteria. We provide a practice-focused overview of 3D MRI acquisition strategies, postprocessing techniques for 3D model creation, MSK applications of 3D MRI models, and an illustration of cases from our daily clinical practice.

Common Biochemical and Magnetic Resonance Imaging Biomarkers of Early Knee Osteoarthritis and of Exercise/Training in Athletes: A Narrative Review

Knee osteoarthritis (OA) is the most common joint disease of the world population. Although considered a disease of old age, OA also affects young individuals and, more specifically among them, those practicing knee-joint-loading sports. Predicting OA at an early stage is crucial but remains a challenge. Biomarkers that can predict early OA development will help in the design of specific therapeutic strategies for individuals and, for athletes, to avoid adverse outcomes due to exercising/training regimens. This review summarizes and compares the current knowledge of fluid and magnetic resonance imaging (MRI) biomarkers common to early knee OA and exercise/training in athletes. A variety of fluid biochemical markers have been proposed to detect knee OA at an early stage; however, few have shown similar behavior between the two studied groups. Moreover, in endurance athletes, they are often contingent on the sport involved. MRI has also demonstrated its ability for early detection of joint structural alterations in both groups. It is currently suggested that for optimal forecasting of early knee structural alterations, both fluid and MRI biomarkers should be analyzed as a panel and/or combined, rather than individually.

Visualizing pre-osteoarthritis: Integrating MRI UTE-T2* with mechanics and biology to combat osteoarthritis-The 2019 Elizabeth Winston Lanier Kappa Delta Award

Osteoarthritis (OA) is a leading cause of pain and disability for which disease-modifying treatments remain lacking. This is because the symptoms and radiographic changes of OA occur after the onset of likely irreversible changes. Defining and treating earlier disease states are therefore needed to delay or to halt OA progression. Taking this concept a step further, studying OA pathogenesis before disease onset by characterizing potentially reversible markers of increased OA risk to identify a state of "pre-osteoarthritis (pre-OA)" shifts the paradigm towards OA prevention. The purpose of this review is to summarize the 42 studies comprising the 2019 Kappa Delta Elizabeth Lanier Award where conceptualization of a systems-based definition for "pre-osteoarthritis (pre-OA)" was followed by demonstration of potentially reversible markers of heightened OA risk in patients after anterior cruciate ligament (ACL) injury and reconstruction. In the process, these efforts contributed a new magnetic resonance imaging method of ultrashort echo time (UTE) enhanced T2* mapping to visualize joint tissue damage before the development of irreversible changes. The studies presented here support a transformative approach to OA that accounts for interactions between mechanical, biological, and structural markers of OA risk to develop and evaluate new treatment strategies that can delay or prevent the onset of clinical disease. This body of work was inspired by and performed for patients. Shifting the paradigm from attempting to modify symptomatic radiographic OA towards monitoring and reversing markers of "pre-OA" opens the door for transforming the clinical approach to OA from palliation to prevention.

A pilot study to assess the healing of meniscal tears in young adult goats

Meniscal tears are a common orthopedic injury, yet their healing is difficult to assess post-operatively. This impedes clinical decisions as the healing status of the meniscus cannot be accurately determined non-invasively. Thus, the objectives of this study were to explore the utility of a goat model and to use quantitative magnetic resonance imaging (MRI) techniques, histology, and biomechanical testing to assess the healing status of surgically induced meniscal tears. Adiabatic T1ρ, T2, and T2* relaxation times were quantified for both operated and control menisci ex vivo. Histology was used to assign healing status, assess compositional elements, and associate healing status with compositional elements. Biomechanical testing determined the failure load of healing lesions. Adiabatic T1ρ, T2, and T2* were able to quantitatively identify different healing states. Histology showed evidence of diminished proteoglycans and increased vascularity in both healed and non-healed menisci with surgically induced tears. Biomechanical results revealed that increased healing (as assessed histologically and on MRI) was associated with greater failure load. Our findings indicate increased healing is associated with greater meniscal strength and decreased signal differences (relative to contralateral controls) on MRI. This indicates that quantitative MRI may be a viable method to assess meniscal tears post-operatively.

Graft healing after anterior cruciate ligament reconstruction (ACLR)

Anterior cruciate ligament reconstruction (ACLR) is a commonly performed procedure in Orthopaedic sports medicine. With advances in surgical techniques providing better positioning and fixation of the graft, subsequent graft failure to certain extent should be accounted by poor graft healing. Although different biological modulations for enhancement of graft healing have been tried in different clinical and animal studies, complete graft incorporation into bone tunnels and the “ligamentization” of the intra-articular part have not been fully achieved yet. Based on the understanding of graft healing process and its failure mechanism, the purpose of this review is to combine both the known basic science & clinical evidence, to provide a much clearer picture of the obstacle encountered in graft healing, so as to facilitate researchers on subsequent work on the enhancement of ACL graft healing.

Quantitative 3D Ultrashort Echo Time Magnetization Transfer Imaging for Evaluation of Knee Cartilage Degeneration In Vivo

BACKGROUND

Recent studies suggest that macromolecular fraction (MMF) derived from three-dimensional ultrashort echo time magnetization transfer (UTE-MT) imaging is insensitive to the magic angle effect. However, its clinical use in osteoarthritis (OA) remains to be investigated.

PURPOSE

To investigate the feasibility of 3D UTE-MT-derived MMF in differentiating normal from degenerated cartilage.

STUDY TYPE

Prospective.

SUBJECTS

Sixty-two participants (54.8 ± 16.7 years, 30 females) with and without OA, plus two healthy volunteers (mean age 35.0 years) for reproducibility test.

FIELD STRENGTH/SEQUENCE

3 T/UTE-MT sequence.

ASSESSMENT

A 3D UTE-MT sequence was employed to calculate MMF based on a two-pool model. Kellgren-Lawrence (KL) grade and Whole-Organ Magnetic Resonance Imaging Score (WORMS) were evaluated by three experienced musculoskeletal radiologists. KL grade was condensed into three groups: KL0, KL1-2, and KL3-4. WORMS was regrouped based on extent of lesion (extent group) and depth of lesion (depth group), respectively. The performance of MMF at evaluating the degeneration of cartilage was assessed via Spearman's correlation coefficient and the area under the curve (AUC) calculated according to the receiver-operating characteristic curve.

STATISTICAL TESTS

After normality check, one-way analysis of variance was used to evaluate the performance. Tukey-Kramer test was performed for post hoc testing.

RESULTS

MMF showed significant negative correlations with KL grade (r = -0.53, P < 0.05) and WORMS (r = -0.49, P < 0.05). Significantly lower MMFs were found in subjects with greater KL grade (11.8 ± 0.8% for KL0; 10.9 ± 0.9% for KL1-2; 10.6 ± 1.1% for KL3-4; P < 0.05) and in cartilage with greater extent (12.1 ± 1.6% for normal cartilage; 10.9 ± 1.6% for regional lesions; 9.6 ± 1.7% for diffuse lesions; P < 0.05) and depth (12.1 ± 1.6% for normal cartilage; 10.6 ± 1.6% for partial-thickness lesions; 8.8 ± 1.7% for full-thickness lesions; P < 0.05) of lesions. AUC values of MMF for doubtful-minimal OA (KL1-2) and mild cartilage degradation (WORMS1-2) were 0.8 and 0.7, respectively.

DATA CONCLUSION

This study highlights the clinical potential of MMF in the detection of early OA.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 2.

Automated magnetic resonance image segmentation of the anterior cruciate ligament

The objective of this study was to develop an automated segmentation method for the anterior cruciate ligament that is capable of facilitating quantitative assessments of the ligament in clinical and research settings. A modified U-Net fully convolutional network model was trained, validated, and tested on 246 Constructive Interference in Steady State magnetic resonance images of intact anterior cruciate ligaments. Overall model performance was assessed on the image set relative to an experienced (>5 years) "ground truth" segmenter in two domains: anatomical similarity and the accuracy of quantitative measurements (i.e., signal intensity and volume) obtained from the automated segmentation. To establish model reliability relative to manual segmentation, a subset of the imaging data was resegmented by the ground truth segmenter and two additional segmenters (A, 6 months and B, 2 years of experience), with their performance evaluated relative to the ground truth. The final model scored well on anatomical performance metrics (Dice coefficient = 0.84, precision = 0.82, and sensitivity = 0.85). The median signal intensities and volumes of the automated segmentations were not significantly different from ground truth (0.3% difference, p = .9; 2.3% difference, p = .08, respectively). When the model results were compared with the independent segmenters, the model predictions demonstrated greater median Dice coefficient (A = 0.73, p = .001; B = 0.77, p = NS) and sensitivity (A = 0.68, p = .001; B = 0.72, p = .003). The model performed equivalently well to retest segmentation by the ground truth segmenter on all measures. The quantitative measures extracted from the automated segmentation model did not differ from those of manual segmentation, enabling their use in quantitative magnetic resonance imaging pipelines to evaluate the anterior cruciate ligament.

Differences in the magnetic resonance imaging parameter T2* may be identified during the course of canine patellar tendon healing: a pilot study

Background

Previous studies have utilized ultrashort echo (UTE) magnetic resonance imaging (MRI), and derived T2* maps, to evaluate structures with highly ordered collagen structures such as tendon. T2* maps may provide a noninvasive means to assess tendon damage and healing. This pilot study evaluated the longitudinal relationship of an induced mechanical strain on the patellar tendon with corresponding UTE T2* metrics, histologic and biomechanical evaluation at two post-operative time points.

Methods

A total of 27 patellar tendons in male Beagles were surgically subjected to stretching by a small diameter (SmD) or a large diameter (LgD) diameter rod to induce damage due to strain, and evaluated at 4- and 8-week intervals using quantitative MRI (qMRI), biomechanical testing, and histology. A separate set of 16 limbs were used as controls.

Results

The tendons experienced a 67% and 17% prolongation of short T2* values as compared to controls at 4 and 8 weeks post-operatively, respectively. Histologic analysis displayed a trend of increased collagen disruption at 4 weeks followed by presence of greater organization at 8 weeks. Biomechanical evaluation found a reduction of tendon modulus and failure strain at both time points, and an increase in cross-sectional area at 4 weeks as compared to controls.

Conclusions

These findings display tendon healing in response to an imposed strain and present the utility of qMRI to evaluate longitudinal differences of patellar tendon T2* values in a model of induced subclinical tendon damage. The qMRI technique of UTE provides a means to non-invasively evaluate the healing process of a mechanically damaged tendon.

Evaluation of the degenerative pattern of PCL in osteoarthritis patients using UTE-T2 mapping

Background

The posterior cruciate ligament (PCL) is one of the essential stabilizers of the knee joint and it was demonstrated that its degenerative change related to the knee osteoarthritis (OA). We aimed to evaluate signal of the PCL in OA patients in comparison with healthy young and elderly volunteers using the ultra-short echo timeenhanced (UTE)-T2∗ mapping, and to validate these findings with histology.

Methods

Thirty asymptomatic volunteers, 13 young people (younger group) and 17 elderly people (elder group), and 27 patients who had undergone total knee arthroplasty (OA group) were enrolled in this study. UTE-T2∗ maps of PCL were obtained from all participants. The PCL was divided into proximal, middle, and distal parts and the UTET2∗ values obtained from each part were compared among the groups. In OA group, the sacrificed PCLs were evaluated histologically in each part corresponding to the part of UTE-T2∗ maps and compared.

Results

The UTE-T2∗ values in OA group were significantly higher than those in other groups except in distal part. In elder group, the UTE-T2∗ values were significantly higher than those in younger group only in the proximal part. Moreover, in OA group, the UTE-T2∗ values in proximal and middle parts were significantly higher than those in distal part. There was a moderate correlation between the UTE-T2∗ values and histological scores.

Conclusions

The specific signal intensity pattern of the PCL in patients with OA was demonstrated using UTE-T2∗ mapping, and these findings were related to histological degenerated status of the PCL.

Quantitative magnetic resonance imaging of meniscal pathology ex vivo

OBJECTIVE

To determine the ability of conventional spin echo (SE) T2 and ultrashort echo time (UTE) T2* relaxation times to characterize pathology in cadaveric meniscus samples.

MATERIALS AND METHODS

From 10 human donors, 54 triangular (radially cut) meniscus samples were harvested. Meniscal pathology was classified as normal (n = 17), intrasubstance degenerated (n = 33), or torn (n = 4) using a modified arthroscopic grading system. Using a 3-T MR system, SE T2 and UTE T2* values of the menisci were determined, followed by histopathology. Effect of meniscal pathology on relaxation times and histology scores were determined, along with correlation between relaxation times and histology scores.

RESULTS

Mean ± standard deviation UTE T2* values for normal, degenerated, and torn menisci were 3.6 ± 1.3 ms, 7.4 ± 2.5 ms, and 9.8 ± 5.7 ms, respectively, being significantly higher in degenerated (p < 0.0001) and torn (p = 0.0002) menisci compared to that in normal. In contrast, the respective mean SE T2 values were 27.7 ± 9.5 ms, 25.9 ± 7.0 ms, and 35.7 ± 10.4 ms, without significant differences between groups (all p > 0.14). In terms of histology, we found significant group-wise differences (each p < 0.05) in fiber organization and inner-tip surface integrity sub-scores, as well as the total score. Finally, we found a significant weak correlation between UTE T2* and histology total score (p = 0.007, R_s² = 0.19), unlike the correlation between SE T2 and histology (p = 0.09, R_s² = 0.05).

CONCLUSION

UTE T2* values were found to distinguish normal from both degenerated and torn menisci and correlated significantly with histopathology.

Compositional MRI of the anterior cruciate ligament of professional alpine ski racers: preliminary report on seasonal changes and load sensitivity

The purpose of this study was to investigate potential changes in the anterior cruciate ligament (ACL) structure of alpine ski racers over the course of an entire season using quantitative magnetic resonance imaging (T2* mapping). The dominant legs of three alpine ski racers were examined on a 3-T MR scanner four times at 3-month intervals. Multi-echo sequences for T2* maps, which were coregistered with high-resolution morphological sequences for reproducible definition of ACL regions of interest, were acquired. Means and standard deviations of T2* values from the central and femoral portion of the ACL were extracted and presented in a descriptive manner. T2* values were subject to seasonal changes, which were most pronounced in the ligament central region. Substantial increases (+ 41%) occurred between the measurements taken in January and April. A partial recovery of T2* (-19%) was observed in the July follow-up. The increased T2* times may reflect decreased stress tolerance and increased susceptibility for fatigue tears at the end of the competitive season. Further research in larger samples is required. The likeliness of ACL tears may depend on the precedent history of mechanical loading and vary in professional athletes over the course of the competitive season.

Rapid Knee MRI Acquisition and Analysis Techniques for Imaging Osteoarthritis

Osteoarthritis (OA) of the knee is a major source of disability that has no known treatment or cure. Morphological and compositional MRI is commonly used for assessing the bone and soft tissues in the knee to enhance the understanding of OA pathophysiology. However, it is challenging to extend these imaging methods and their subsequent analysis techniques to study large population cohorts due to slow and inefficient imaging acquisition and postprocessing tools. This can create a bottleneck in assessing early OA changes and evaluating the responses of novel therapeutics. The purpose of this review article is to highlight recent developments in tools for enhancing the efficiency of knee MRI methods useful to study OA. Advances in efficient MRI data acquisition and reconstruction tools for morphological and compositional imaging, efficient automated image analysis tools, and hardware improvements to further drive efficient imaging are discussed in this review. For each topic, we discuss the current challenges as well as potential future opportunities to alleviate these challenges. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY STAGE: 3.

Rapid single scan ramped hybrid-encoding for bicomponent T2* mapping in a human knee joint: A feasibility study

The purpose of this study is to determine the feasibility of using a single scan ramped hybrid-encoding (RHE) method for rapid bicomponent T2* analysis of the human knee joint. The proposed method utilizes RHE to acquire ultrashort echo time (UTE) and subsequent gradient echo images at 16 different echo times ranging between 40 μs and 30 ms in a single scan. In the proposed RHE technique, UTE imaging was followed by acquisition of 14 gradient recalled echo images, where an additional UTE image was obtained within the first readout by oversampling single point imaging (SPI) encoding. The single scan RHE method with a 9-minute scan time was performed on human cadaveric knee joints from six donors and in vivo knee joints from four healthy volunteers at 3 T. A bicomponent signal model was used to characterize the short T2* and long T2* water components. Mean bicomponent T2* parameters for patellar tendon, anterior cruciate ligament (ACL), posterior cruciate ligament (PCL) and meniscus were calculated. In the experimental results, the RHE technique provided bicomponent T2* parameter estimations of tendon, ACL, PCL and meniscus, which were similar to previously reported values in the literature. In conclusion, the proposed single scan RHE technique provides rapid bicomponent T2* analysis of the human knee joint with a total scan time of less than 9 minutes.

Quantitative Assessment of In Vivo Human Anterior Cruciate Ligament Autograft Remodeling: A 3-Dimensional UTE-T2* Imaging Study

BACKGROUND

The timing of return to play after anterior cruciate ligament (ACL) reconstruction is still controversial due to uncertainty of true ACL graft state at the time of RTP. Recent work utilizing ultra-short echo T2* (UTE-T2*) magnetic resonance imaging (MRI) as a scanner-independent method to objectively and non-invasively assess the status of in vivo ACL graft remodeling has produced promising results.

PURPOSE/HYPOTHESIS

The purpose of this study was to prospectively and noninvasively investigate longitudinal changes in T2* within ACL autografts at incremental time points up to 12 months after primary ACL reconstruction in human patients. We hypothesized that (1) T2* would increase from baseline and initially exceed that of the intact contralateral ACL, followed by a gradual decline as the graft undergoes remodeling, and (2) remodeling would occur in a region-dependent manner.

STUDY DESIGN

Case series; Level of evidence, 4.

METHODS

Twelve patients (age range, 14-45 years) who underwent primary ACL reconstruction with semitendinosus tendon or bone-patellar tendon-bone autograft (with or without meniscal repair) were enrolled. Patients with a history of previous injury or surgery to either knee were excluded. Patients returned for UTE MRI at 1, 3, 6, 9, and 12 months after ACL reconstruction. Imaging at 1 month included the contralateral knee. MRI pulse sequences included high-resolution 3-dimensional gradient echo sequence and a 4-echo T2-UTE sequence (slice thickness, 1 mm; repetition time, 20 ms; echo time, 0.3, 3.3, 6.3, and 9.3 ms). All slices containing the intra-articular ACL were segmented from high-resolution sequences to generate volumetric regions of interest (ROIs). ROIs were divided into proximal/distal and core/peripheral sub-ROIs using standardized methods, followed by voxel-to-voxel registration to generate T2* maps at each time point. This process was repeated by a second reviewer for interobserver reliability. Statistical differences in mean T2* values and mean ratios of T2*_inj/T2*_intact (ie, injured knee to intact knee) among the ROIs and sub-ROIs were assessed using repeated measures and one-way analyses of variance. P < .05 represented statistical significance.

RESULTS

Twelve patients enrolled in this prospective study, 2 withdrew, and ultimately 10 patients were included in the analysis (n = 7, semitendinosus tendon; n = 3, bone-patellar tendon-bone). Interobserver reliability for T2* values was good to excellent (intraclass correlation coefficient, 0.84; 95% CI, 0.59-0.94; P < .001). T2* values increased from 5.5 ± 2.1 ms (mean ± SD) at 1 month to 10.0 ± 2.9 ms at 6 months (P = .001), followed by a decline to 8.1 ± 2.0 ms at 12 months (P = .129, vs 1 month; P = .094, vs 6 months). Similarly, mean T2*_inj/T2*_intact ratios increased from 62.8% ± 22.9% at 1 month to 111.1% ± 23.9% at 6 months (P = .001), followed by a decline to 92.8% ± 29.8% at 12 months (P = .110, vs 1 month; P = .086, vs 6 months). Sub-ROIs exhibited similar increases in T2* until reaching a peak at 6 months, followed by a gradual decline until the 12-month time point. There were no statistically significant differences among the sub-ROIs (P > .05).

CONCLUSION

In this preliminary study, T2* values for ACL autografts exhibited a statistically significant increase of 82% between 1 and 6 months, followed by an approximate 19% decline in T2* values between 6 and 12 months. In the future, UTE-T2* MRI may provide unique insights into the condition of remodeling ACL grafts and may improve our ability to noninvasively assess graft maturity before return to play.

Quantitative Magnetic Resonance Imaging UTE-T2* Mapping of Tendon Healing After Arthroscopic Rotator Cuff Repair: A Longitudinal Study

BACKGROUND

Quantitative ultrashort echo time-T2* (UTE-T2*) mapping shows promise for the detection of potential tendon biochemical conditions, while validation against established clinical studies in the shoulder is needed.

PURPOSE

To evaluate and characterize the healing process of the repaired rotator cuff based on longitudinal changes in UTE-T2* values, clinical outcomes, and repair status in patients after arthroscopic rotator cuff repair (ARCR).

STUDY DESIGN

Cohort study; Level of evidence, 2.

METHODS

Patients with ARCR (n = 25) underwent quantitative MRI and clinical examinations at serial follow-up time points: 3, 6, 12, and 24 months postoperatively. Age-matched healthy controls (n = 15) were evaluated at 3 and 12 months after enrollment. Clinical scores included the Constant, American Shoulder and Elbow Surgeons, and Fudan University Shoulder score, and visual analog scale for pain. The MRI examination included UTE-T2*mapping. UTE-T2* maps were generated for T2* values at the healing site. Sugaya classification was adopted to evaluate the repair status. Longitudinal analyses of clinical outcomes, UTE-T2* changes, and Sugaya classification were conducted.

RESULTS

The overall retear rate was 8% (2/25, all Sugaya type IV). All patients (including the ones with retear) achieved satisfactory outcomes at 12 months that lasted to 24 months on the basis of clinical scores. The mean UTE-T2* values at the healing site showed an increase from 3 to 6 months (P = .03) and then decreased to a level similar to that observed in age-matched healthy tendons at 12 months (P = .1). No significant differences were found between UTE-T2* values at 12 and 24 months (P = .6). UTE-T2* values at the healing site significantly varied with the repair status according to Sugaya classification (P < .05). Moreover, significant correlations were noted between clinical scores and UTE-T2* values at 6 months (r = -0.6 to -0.3; all P < .05) and 12 months (r = -0.6 to -0.2; all P < .05).

CONCLUSION

This study indicated a healing-related relationship between clinical outcomes and quantitative UTE-T2* values, which highlights the potential of using UTE-T2* mapping to track the tendon-healing process noninvasively. Moreover, the repaired tendon was comparable to age-matched healthy controls at 12-month follow-up based on UTE-T2* values.

Convincing evidence for magic angle less-sensitive quantitative T1ρ imaging of articular cartilage using the 3D ultrashort echo time cones adiabatic T1ρ (3D UTE cones-AdiabT1ρ ) sequence

PURPOSE

To investigate the magic angle effect in three-dimensional ultrashort echo time Cones Adiabatic T_1ρ (3D UTE Cones-AdiabT_1ρ ) imaging of articular cartilage at 3T.

METHODS

The magic angle effect was investigated by repeated 3D UTE Cones-AdiabT_1ρ imaging of eight human patellar samples at five angular orientations ranging from 0° to 90° relative to the B₀ field. Cones continuous wave T_1ρ (Cones-CW-T_1ρ ) and Cones- T 2 ∗ sequences were also applied for comparison. Cones-AdiabT_1ρ , Cones-CW-T_1ρ and Cones- T 2 ∗ values were measured for four regions of interest (ROIs) (10% superficial layer, 60% transitional layer, 30% radial layer, and a global ROI) for each sample at each orientation to evaluate their angular dependence.

RESULTS

3D UTE Cones-AdiabT_1ρ values increased from the radial layer to the superficial layer for all angular orientations. The superficial layer showed the least angular dependence (around 4.4%), while the radial layer showed the strongest angular dependence (around 34.4%). Cones-AdiabT_1ρ values showed much reduced magic angle effect compared to Cones-CW-T_1ρ and Cones- T 2 ∗ values for all four ROIs. On average over eight patellae, Cones-AdiabT_1ρ values increased by 27.2% (4.4% for superficial, 23.8% for transitional, and 34.4% for radial layers), Cones-CW-T_1ρ values increased by 76.9% (11.3% for superficial, 59.1% for transitional, and 117.8% for radial layers), and Cones- T 2 ∗ values increased by 237.5% (87.9% for superficial, 262.9% for transitional, and 327.3% for radial layers) near the magic angle.

CONCLUSIONS

The 3D UTE Cones-AdiabT_1ρ sequence is less sensitive to the magic angle effect in the evaluation of articular cartilage compared to Cones- T 2 ∗ and Cones-CW-T_1ρ .

Relating MR relaxation times of ex vivo meniscus to tissue degeneration through comparison with histopathology

Background

Quantitative magnetic resonance imaging (MRI), e.g. relaxation parameter mapping, may be sensitive to structural and compositional tissue changes, and could potentially be used to non-invasively detect and monitor early meniscus degeneration related to knee osteoarthritis.

Objective

To investigate MR relaxation times as potential biomarkers for meniscus degeneration through comparisons with histopathology.

Methods

We measured MR relaxation parameters in the posterior horn of 40 menisci (medial and lateral) at a wide range of degenerative stages. T1, T2 and T2* were mapped using standard and ultrashort echo time sequences at 9.4 T and compared to gold standard histology using Pauli's histopathological scoring system, including assessment of surface integrity, collagen organization, cellularity and Safranin-O staining.

Results

All three relaxation times increased with total Pauli score (mean difference per score (95% CI) for T2*: 0.62 (0.37, 0.86), T2: 0.83 (0.53, 1.1) and T1: 24.7 (16.5, 32.8) ms/score). Clear associations were seen with scores of surface integrity (mean difference per score for T2*: 3.0 (1.8, 4.2), T2: 4.0 (2.5, 5.5) and T1: 116 (75.6, 156) ms/score) and collagen organization (mean difference between highest and lowest score for T2*: 5.3 (1.6, 8.9), T2: 6.1 (1.7, 11) and T1: 204 (75.9, 332) ms). The results were less clear for the remaining histopathological measures.

Conclusions

MR relaxation times T1, T2 and T2* of ex vivo human menisci are associated with histologically verified degenerative processes, in particular related to surface integrity and collagen organization. If confirmed in vivo, MR relaxation times may thus be potential biomarkers for meniscus degeneration.

Clinical Utility of Advanced Imaging of the Knee

Advanced imaging modalities, including computed tomography, magnetic resonance imaging (MRI), and dynamic fluoroscopic imaging, allow for a comprehensive evaluation of the knee joint. Compositional sequences for MRI can allow for an evaluation of the biochemical properties of cartilage, meniscus, and ligament that offer further insight into pathology that may not be apparent on conventional clinical imaging. Advances in image processing, shape modeling, and dynamic studies also offer a novel way to evaluate common conditions and to monitor patients after treatment. The purpose of this article is to review advanced imaging modalities of the knee and their current and anticipated future applications to clinical practice. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:473-482, 2020.

Ultrashort Time to Echo Magnetic Resonance Evaluation of Calcium Pyrophosphate Crystal Deposition in Human Menisci

OBJECTIVES

In human menisci, we aimed to investigate whether calcium pyrophosphate crystal deposition (CPPD) affects biomechanical and quantitative MR properties, and their zonal distribution.

MATERIALS AND METHODS

From 9 cadaveric knees, sectioned triangular meniscus pieces were harvested. Samples were classified into "normal" or "CPPD" groups based upon visual inspection. Micro computed tomography scan verified CPPD. Using magnetic resonance imaging, ultrashort echo time (UTE) T2* and spin echo (SE) T2, quantitative values in 3 zones (red, red-white, and white) were determined. Using biomechanical test, indentation forces in the same zones were determined. Effects of CPPD and meniscal zone on indentation force and quantitative MR values were compared.

RESULTS

On UTE MRI scans, CPPD-affected menisci exhibited punctate dark regions, found mostly (92%) in avascular white and red-white zones. Indentation forces were significantly higher for CPPD samples in the red-white (all P < 0.02) and white (all P < 0.004) zones but not in the vascular red zone (all P > 0.2). Similarly, UTE T2* red zone values were similar between both groups (~6.6 milliseconds, P = 0.8), whereas in the red-white and white zones, CPPD samples had significantly lower values (~5.1 milliseconds, P = 0.005 to 0.007). In contrast, SE T2 values showed no difference with CPPD (P = 0.12 to 0.16). UTE T2*, but not SE T2, correlated significantly with indentation force (R = -0.29, P = 0.009).

CONCLUSIONS

Dark CPP deposits were detectable on UTE images featuring high signal intensity from surrounding meniscal tissue. Preliminary results indicate that CPP deposits were almost exclusively found in the avascular zones. Compared with normal, CPPD menisci featured higher indentation stiffness and lower UTE T2* values in the affected zones.

Quantitative three-dimensional ultrashort echo time cones imaging of the knee joint with motion correction

Knee degeneration involves all the major tissues in the joint. However, conventional MRI sequences can only detect signals from long T2 tissues such as the superficial cartilage, with little signal from the deep cartilage, menisci, ligaments, tendons and bone. It is highly desirable to develop new sequences that can detect signal from all major tissues in the knee. We aimed to develop a comprehensive quantitative three-dimensional ultrashort echo time (3D UTE) cones imaging protocol for a truly "whole joint" evaluation of knee degeneration. The protocol included 3D UTE cones actual flip angle imaging (3D UTE-Cones-AFI) for T₁ mapping, multiecho UTE-Cones with fat suppression for T₂ * mapping, UTE-Cones with adiabatic T_1ρ (AdiabT_1ρ ) preparation for AdiabT_1ρ mapping, and UTE-Cones magnetization transfer (UTE-Cones-MT) for MT ratio (MTR) and modeling of macromolecular proton fraction (f). An elastix registration technique was used to compensate for motion during scans. Quantitative data analyses were performed on the registered data. Three knee specimens and 15 volunteers were evaluated at 3 T. The elastix motion correction algorithm worked well in correcting motion artifacts associated with relatively long scan times. Much improved curve fitting was achieved for all UTE-Cones biomarkers with greatly reduced root mean square errors. The averaged T₁ , T₂ *, AdiabT_1ρ , MTR and f for knee joint tissues of 15 healthy volunteers were reported. The 3D UTE-Cones quantitative imaging techniques (ie, T₁ , T₂ *, AdiabT_1ρ , MTR and MT modeling) together with elastix motion correction provide robust volumetric measurement of relaxation times, MTR and f of both short and long T₂ tissues in the knee joint.

Magnetic resonance imaging (MRI) studies of knee joint under mechanical loading: Review

Osteoarthritis (OA) is a very common disease that affects the human knee joint, particularly the articular cartilage and meniscus components which are regularly under compressive mechanical loads. Early-stage OA diagnosis is essential as it allows for timely intervention. The primary non-invasive approaches currently available for OA diagnosis include magnetic resonance imaging (MRI), which provides excellent soft tissue contrast at high spatial resolution. MRI-based knee investigation is usually performed on joints at rest or in a non-weight-bearing condition that does not mimic the actual physiological condition of the joint. This discrepancy may lead to missed detections of early-stage OA or of minor lesions. The mechanical properties of degenerated musculoskeletal (MSK) tissues may vary markedly before any significant morphological or structural changes detectable by MRI. Recognizing distinct deformation characteristics of these tissues under known mechanical loads may reveal crucial joint lesions or mechanical malfunctions which result from early-stage OA. This review article summarizes the large number of MRI-based investigations on knee joints under mechanical loading which have been reported in the literature including the corresponding MRI measures, the MRI-compatible devices employed, and potential challenges due to the limitations of clinical MRI sequences.

Cartilage Damage Is Related to ACL Stiffness in a Porcine Model of ACL Repair

Inferior anterior cruciate ligament (ACL) structural properties may inadequately restrain tibiofemoral joint motion following surgery, contributing to the increased risk of post-traumatic osteoarthritis. Using both a direct measure of ACL linear stiffness and an in vivo magnetic resonance imaging (MRI) T₂ *-based prediction model, we hypothesized that cartilage damage and ACL stiffness would increase over time, and that an inverse relationship between cartilage damage and ACL stiffness would emerge at a later stage of healing. After either 6, 12, or 24 weeks (w) of healing after ACL repair, ACL linear stiffness was determined from the force-displacement relationship during tensile testing ex vivo and predicted in vivo from the MRI T₂ *-based multiple linear regression model in 24 Yucatan minipigs. Tibiofemoral cartilage was graded postmortem. There was no relationship between cartilage damage and ACL stiffness at 6 w (R²  = 0.04; p = 0.65), 12 w (R²  = 0.02; p = 0.77), or when the data from all animals were pooled (R²  = 0.02; p = 0.47). A significant inverse relationship between cartilage damage and ACL stiffness based on both ex vivo measurement (R²  = 0.90; p < 0.001) and in vivo MRI prediction (R²  = 0.78; p = 0.004) of ACL stiffness emerged at 24 w. This result suggests that 90% of the variability in gross cartilage changes is associated with the repaired ACL linear stiffness at 6 months of healing. Clinical Significance: Techniques that provide a higher stiffness to the repaired ACL may be required to mitigate the post-traumatic osteoarthritis commonly seen after ACL injury, and MRI T₂ * can be used as a noninvasive estimation of ligament stiffness. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2249-2257, 2019.

3D-T1ρ prepared zero echo time-based PETRA sequence for in vivo biexponential relaxation mapping of semisolid short-T2 tissues at 3 T

BACKGROUND

In addition to the articular cartilage, osteoarthritis (OA) affects several other tissues such as tendons, ligaments, and subchondral bone. T1ρ relaxation study of these short T2 tissues may provide a more comprehensive evaluation of OA.

PURPOSE

To develop a 3D spin-lattice relaxation in the rotating frame (T1ρ ) prepared zero echo time (ZTE)-based pointwise encoding time reduction with radial acquisition (3D-T1ρ -PETRA) sequence for relaxation mapping of semisolid short-T2 tissues on a clinical 3 T scanner.

STUDY TYPE

Prospective.

POPULATION

Phantom, two bovine whole knee joint and Achilles tendon specimens, 10 healthy volunteers with no known inflammation, trauma or pain in the knee or ankle.

FIELD STRENGTH/SEQUENCE

A customized PETRA sequence to acquire fat-suppressed 3D T1ρ -weighted images tissues with semisolid short T2 / T 2 * relaxation times in the knee and ankle joints at 3 T.

ASSESSMENT

Mono- and biexponential T1ρ relaxation components were assessed in the patellar tendon (PT), anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), and Achilles tendon (AT).

STATISTICAL TESTS

Kruskal-Wallis with post-hoc Dunn's test for multiple pairwise comparisons.

RESULTS

Phantom and ex vivo studies showed the feasibility of T1ρ relaxation mapping using the proposed 3D-T1ρ -PETRA sequence. The in vivo study demonstrated an averaged mono-T1ρ relaxation of (median [IQR]) 15.9 [14.5] msec, 23.6 [9.4] msec, 17.4 [7.4] msec, and 5.8 [10.2] msec in the PT, ACL, PCL, and AT, respectively. The bicomponent analysis showed the short and long components (with their relative fractions) of 0.65 [1.0] msec (46.9 [15.3]%) and 37.3 [18.4] msec (53.1 [15.3]%) for PT, 1.7 [2.1] msec (42.5 [12.5]%) and 43.7 [17.8] msec (57.5 [12.5]%) for ACL, and 1.2 [1.9] msec (42.6 [14.0]%) and 27.7 [14.7] msec (57.3 [14.0]%) for PCL and 0.4 [0.02] msec (58.8 [13.3]%/) and 31.3 [10.8] msec (41.2 [13.3]%) for AT. Statistically significant (P ≤ 0.05) differences were observed in the mono- and biexponential relaxation between several regions.

DATA CONCLUSION

The 3D-T1ρ -PETRA sequence allows volumetric, isotropic (0.78 × 0.78 × 0.78 mm), biexponential T1ρ assessment with corresponding fractions of the tissues with semisolid short T2 / T 2 * .

LEVEL OF EVIDENCE

2 Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2019;50:1207-1218.

A multi-purpose force-controlled loading device for cartilage and meniscus functionality assessment using advanced MRI techniques

Response to loading of soft tissues as assessed by advanced magnetic resonance imaging (MRI) techniques is a promising approach to evaluate tissue functionality beyond (statically obtained) structural and compositional features. As cartilage and meniscus pathologies are closely intertwined in osteoarthritis (OA) and beyond, both tissues should ideally be studied to elucidate further the underlying mechanisms involved in load transmission and its failure leading to OA. Hence, we devised, constructed and validated a dedicated MRI-compatible pneumatic force-controlled loading device to study cartilage and meniscus functionality in a standardized and reproducible manner and in reference to alternative tissue evaluation methods. Mechanical reference measurements using digital force sensors confirmed the reproducible application of forces in the range of 0-76N. To demonstrate the device's utility in a basic research context, MRI measurements of human articular cartilage (obtained from the lateral femoral condyle, n = 5) and meniscus (obtained from lateral meniscus body, n = 5) were performed in the unloaded (δ₀) and loaded configurations (δ₁: [cartilage] 0.75 bar corresponding to 15.1 N, [meniscus] 2 bar corresponding to 37.1 N; δ₂: [cartilage] 1.5 bar corresponding to 28.6 N, [meniscus] 4 bar corresponding to 69.1 N). Cartilage samples were directly indented, while meniscus samples were subject to torque-induced compression using a dedicated lever compression device. Morphological MR Imaging using Proton Density-weighted sequences and quantitative MR Imaging using T2 and T1ρ mapping were performed serially and at high resolution. For reference, samples underwent subsequent biomechanical and histological reference evaluation. In conclusion, the force-controlled loading device has been validated for the non-invasive response-to-loading assessment of human cartilage and meniscus samples by advanced MRI techniques. Hereby, both tissues may be functionally evaluated in combination, beyond mere static analysis and in reference to histological and biomechanical measures.