Multicomponent T2* mapping of knee cartilage: technical feasibility ex vivo

Performance of MR learned pulse sequences for 3D bi-exponential, stretched-exponential, and mono-exponential T2 and T1ρ mapping of knee cartilage

PURPOSE

To compare the performance of a learned magnetization-prepared gradient echo (L-MPGRE) sequence against a commonly used sequence for 3D T2 and T1ρ mapping of the knee joint, the magnetization-prepared angle-modulated partitioned k-space spoiled gradient echo snapshots (MAPSS), on bi-exponential (BE), stretched-exponential (SE), and mono-exponential (ME) relaxation models.

METHODS

We used a combined differentiable and non-differentiable optimization to learn pulse sequence structure and its parameters for 3D T2 and T1ρ mapping of the knee joint using ME, SE, and BE models. The learned pulse sequence framework was used to improve quantitative accuracy and SNR and to reduce filtering effects. We compare the measured multi-compartment values between the two sequences (n = 8), and their repeatability (n = 4) in healthy volunteers (n = 12 total).

RESULTS

The voxel-wise median absolute percentage difference (MAPD) between the T2 and T1ρ maps obtained with each sequence was 18.6% and 19.9%, respectively. The T2 and T1ρ repeatability tests showed a MAPD of 18.5% and 19.1% for MAPSS, and 16.8% and 15.5% for L-MPGRE. Bland-Altman region of interest (ROI)-wise analysis shows that bias is small, close to -1.5%, and the coefficient of variation is around 5.5% when comparing ROIs from both sequences.

CONCLUSION

The L-MPGRE sequences can be used as a replacement for MAPSS for T2 and T1ρ mapping in the knee cartilage with advantages, achieving similar accuracy and 15% better repeatability in only half of its scan time.

Cartilage compositional MRI-a narrative review of technical development and clinical applications over the past three decades

Articular cartilage damage and degeneration are among hallmark manifestations of joint injuries and arthritis, classically osteoarthritis. Cartilage compositional MRI (Cart-C MRI), a quantitative technique, which aims to detect early-stage cartilage matrix changes that precede macroscopic alterations, began development in the 1990s. However, despite the significant advancements over the past three decades, Cart-C MRI remains predominantly a research tool, hindered by various technical and clinical hurdles. This paper will review the technical evolution of Cart-C MRI, delve into its clinical applications, and conclude by identifying the existing gaps and challenges that need to be addressed to enable even broader clinical application of Cart-C MRI.

A feasibility study of in vivo quantitative ultra-short echo time-MRI for detecting early cartilage degeneration

OBJECTIVES

To explore the feasibility of Ultra-short echo time (UTE) - MRI quantitative imaging in detecting early cartilage degeneration in vivo and underlying pathological and biochemical basis.

METHODS

Twenty volunteers with osteoarthritis (OA) planning for total knee arthroplasty (TKA) were prospectively recruited. UTE-MRI sequences and conventional sequences were performed preoperatively. Regions of interests (ROIs) were manually drawn on the tibial plateau and lateral femoral condyle images to calculate MRI values. Cartilage samples were collected during TKA according to the preset positions corresponding to MR images. Pathological and biochemical components of the corresponding ROI, including histological grading, glycosaminoglycan (GAG) content, collagen integrity, and water content were obtained.

RESULTS

91 ROIs from volunteers of 7 males (age range: 68 to 78 years; 74 ± 3 years) and 13 females (age range: 57 to 79 years; 67 ± 6 years) were evaluated. UTE-MTR (r = -0.619, p < 0.001), UTE-AdiabT1ρ (r = 0.568, p < 0.001), and UTE-T2* values (r = -0.495, p < 0.001) showed higher correlation with Mankin scores than T2 (r = 0.287, p = 0.006) and T1ρ (r = 0.435, p < 0.001) values. Of them, UTE-MTR had the highest diagnostic performance (AUC = 0.824, p < 0.001). UTE-MTR, UTE-AdiabT1ρ and UTE-T2* value was mainly related to collagen structural integrity, PG content and water content, respectively (r = 0.536, -0.652, -0.518, p < 0.001, respectively).

CONCLUSION

UTE-MRI have shown greater in vivo diagnostic value for early cartilage degeneration compared to conventional T2 and T1ρ values. Of them, UTE-MTR has the highest diagnostic efficiency. UTE-MTR, UTE-AdiabT1ρ, and UTE-T2* value mainly reflect different aspects of cartilage degeneration--integrity of collagen structure, PG content, and water content, respectively.

CRITICAL RELEVANCE STATEMENT

Ultra-short echo time (UTE)-MRI has the potential to be a novel image biomarkers for detecting early cartilage degeneration in vivo and was correlated with biochemical changes of early cartilage degeneration.

KEY POINTS

Conventional MR may miss some early cartilage changes due to relatively long echo times. Ultra-short echo time (UTE)-MRI showed the ability in identifying early cartilage degeneration in vivo. UTE-MT, UTE-AdiabT1ρ, and UTE-T2* mapping mainly reflect different aspects of cartilage degeneration.

MRI-derived Articular Cartilage Strains Predict Patient-Reported Outcomes Six Months Post Anterior Cruciate Ligament Reconstruction

Key terms

Multicontrast and Multiparametric, Magnetic Resonance Imaging, Osteoarthritis, Functional Biomechanical Imaging, Knee Joint Degeneration What is known about the subject: dualMRI has been used to quantify strains in a healthy human population in vivo and in cartilage explant models. Previously, OA severity, as determined by histology, has been positively correlated to increased shear and transverse strains in cartilage explants. What this study adds to existing knowledge: This is the first in vivo use of dualMRI in a participant demographic post-ACL reconstruction and at risk for developing osteoarthritis. This study shows that dualMRI-derived strains are more significantly correlated with patient-reported outcomes than any MRI relaxometry metric.

Background

Anterior cruciate ligament (ACL) injuries lead to an increased risk of osteoarthritis, characterized by altered cartilage tissue structure and function. Displacements under applied loading by magnetic resonance imaging (dualMRI) is a novel MRI technique that can be used to quantify mechanical strain in cartilage while undergoing a physiological load.

Purpose

To determine if strains derived by dualMRI and relaxometry measures correlate with patient-reported outcomes at six months post unilateral ACL reconstruction.

Study Design

Cohort study.

Methods

Quantitative MRI (T2, T2*, T1ρ) measurements and transverse, axial, and shear strains were quantified in the medial articular tibiofemoral cartilage of 35 participants at six-months post unilateral ACL reconstruction. The relationships between patient-reported outcomes (WOMAC, KOOS, MARS) and all qMRI relaxation times were quantified using general linear mixed-effects models. A combined best-fit multicontrast MRI model was then developed using backwards regression to determine the patient features and MRI metrics that are most predictive of patient-reported outcome scores.

Results

Higher femoral strains were significantly correlated with worse patient-reported functional outcomes. Femoral shear and transverse strains were positively correlated with six-month KOOS and WOMAC scores, after controlling for covariates. No relaxometry measures were correlated with patient-reported outcome scores. We identified the best-fit model for predicting WOMAC score using multiple MRI measures and patient-specific information, including sex, age, graft type, femoral transverse strain, femoral axial strain, and femoral shear strain. The best-fit model significantly predicted WOMAC score (p<0.001) better than any one individual MRI metric alone. When we regressed the model-predicted WOMAC scores against the patient-reported WOMAC scores, we found that our model achieved a goodness of fit exceeding 0.52.

Conclusions

This work presents the first use of dualMRI in vivo in a cohort of participants at risk for developing osteoarthritis. Our results indicate that both shear and transverse strains are highly correlated with patient-reported outcome severity could serve as novel imaging biomarkers to predict the development of osteoarthritis.

Quantification of Synovial Fluid Using Magnetic Resonance Fingerprinting Multicomponent Imaging in the Articular Cartilage of the Knee

RATIONALE AND OBJECTIVES

The purpose of this study was to verify the feasibility of magnetic resonance fingerprinting (MRF)-derived synovial fluid fraction (SFF) mapping for quantifying subvoxel-sized cartilage defects.

MATERIALS AND METHODS

MRF was performed on a 3-Tesla scanner and used to derive T2 and SFF maps. An ex vivo experiment was performed using bovine bone; different numbers of holes (4, 6, 8, 10, and 12) were drilled separately on the articular surface, and SFF values were compared among the drilled areas. In a clinical study, 16 osteoarthritis patients underwent sagittal 3D fast spinecho (FSE) and MRF scanning, and knee cartilage segmentation was performed on each image. For morphologic analysis, fluid-excluded images of the SFF (FEISFF) and T2 maps (FEIT2) were generated using the cartilage segmentations, and the whole-organ magnetic resonance imaging score (WORMS) of each FEI and 3D FSE image were compared using the kappa coefficient. For quantitative analysis, intact cartilage volumes in the SFF (VSFF) and T2 maps (VT2) were calculated, and their correlations with reference to the actual cartilage volume on 3D FSE images (V3D) were evaluated.

RESULTS

In the ex vivo experiment, the SFF value increased as the number of holes increased. The kappa coefficients of the WORMS were 0.80 and 0.64 in the SFF and T2 maps, respectively, and substantial to almost perfect agreement was observed in the medial tibiofemoral joint. The V3D-VSFF and V3D-VT2 correlation coefficients differed by 0.03 or more in the medial tibiofemoral joint.

CONCLUSION

The MRF-derived SFF map can feasibly evaluate small, invisible cartilage defects and quantify cartilage volumes.

Updates on Compositional MRI Mapping of the Cartilage: Emerging Techniques and Applications

Osteoarthritis (OA) is a widely occurring degenerative joint disease that is severely debilitating and causes significant socioeconomic burdens to society. Magnetic resonance imaging (MRI) is the preferred imaging modality for the morphological evaluation of cartilage due to its excellent soft tissue contrast and high spatial resolution. However, its utilization typically involves subjective qualitative assessment of cartilage. Compositional MRI, which refers to the quantitative characterization of cartilage using a variety of MRI methods, can provide important information regarding underlying compositional and ultrastructural changes that occur during early OA. Cartilage compositional MRI could serve as early imaging biomarkers for the objective evaluation of cartilage and help drive diagnostics, disease characterization, and response to novel therapies. This review will summarize current and ongoing state-of-the-art cartilage compositional MRI techniques and highlight emerging methods for cartilage compositional MRI including MR fingerprinting, compressed sensing, multiexponential relaxometry, improved and robust radio-frequency pulse sequences, and deep learning-based acquisition, reconstruction, and segmentation. The review will also briefly discuss the current challenges and future directions for adopting these emerging cartilage compositional MRI techniques for use in clinical practice and translational OA research studies. EVIDENCE LEVEL: 2 TECHNICAL EFFICACY: Stage 2.

High frame rate deformation analysis of knee cartilage by spiral dualMRI and relaxation mapping

PURPOSE

Daily activities including walking impose high-frequency cyclic forces on cartilage and repetitive compressive deformation. Analyzing cartilage deformation during walking would provide spatial maps of displacement and strain and enable viscoelastic characterization, which may serve as imaging biomarkers for early cartilage degeneration when the damage is still reversible. However, the time-dependent biomechanics of cartilage is not well described, and how defects in the joint impact the viscoelastic response is unclear.

METHODS

We used spiral acquisition with displacement-encoding MRI to quantify displacement and strain maps at a high frame rate (25 frames/s) in tibiofemoral joints. We also employed relaxometry methods (T1 , T1ρ , T2 , T2 *) on the cartilage.

RESULTS

Normal and shear strains were concentrated on the bovine tibiofemoral contact area during loading, and the defected joint exhibited larger compressive strains. We also determined a positive correlation between the change of T1ρ in cartilage after cyclic loading and increased compressive strain on the defected joint. Viscoelastic behavior was quantified by the time-dependent displacement, where the damaged joint showed increased creep behavior compared to the intact joint. This technique was also successfully demonstrated on an in vivo human knee showing the gradual change of displacement during varus load.

CONCLUSION

Our results indicate that spiral scanning with displacement encoding can quantitatively differentiate the damaged from intact joint using the strain and creep response. The viscoelastic response identified with this methodology could serve as biomarkers to detect defects in joints in vivo and facilitate the early diagnosis of joint diseases such as osteoarthritis.

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.

Can Bone Erosion in Axial Spondyloarthropathy be Detected by Ultrashort Echo Time Imaging? A Comparison With Computed Tomography in the Sacroiliac Joint

BACKGROUND

Structural lesion evaluation in axial spondyloarthropathy (SpA) can improve accuracy of diagnosis. However, structural lesions (bone erosions) are difficult to be assessed using conventional MRI compared to computed tomography (CT).

PURPOSE

To evaluate the diagnostic performance of ultrashort echo time (UTE) for detecting bone erosion in axial SpA compared to T1WI and three-dimensional double-echo steady-state (3D DESS) imaging using CT as the reference standard.

STUDY TYPE

Retrospective.

POPULATION

Fourteen patients (eight females, 57.1%) and 14 healthy controls (seven females, 50.0%) who underwent sacroiliac (SI) joint MRI and CT.

FIELD STRENGTH/SEQUENCE

3 T; TSE T1WI, 3D DESS, 2D UTE.

ASSESSMENT

The bilateral SI joints were assessed for bone erosion. Three observers scored bone erosion for all three sequences of MRI. CT was used as the gold standard. Diagnostic confidence in axial SpA was measured based on a four-point confidence score.

STATISTICAL TESTS

Correlation of erosion scores between CT and MRI were evaluated using Spearman's correlation test. Sensitivity, specificity, and positive-negative predictive values were calculated. Confidence scores were compared using the Wilcoxon sum rank test. Statistical significance was set at P < 0.05.

RESULTS

Compared with erosion scores of CT, the correlation coefficients for each MRI sequence showed significant low-to-high positive correlations (0.39-0.72). UTE imaging showed the highest correlation coefficients for all observers (0.70, 0.72, and 0.67, respectively). The specificity of UTE imaging was equal or higher than those of T1WI and 3D DESS for all observers (0.86 vs. 0.71 vs. 0.57; 0.93 vs. 0.71 vs. 0.57; 0.79 vs. 0.79 vs. 0.43). All observers had the highest confidence in interpreting UTE imaging for detecting bone erosion among the three sequences (3.5, 3.4, and 3.3 for UTE; 3.1, 3.0, and 2.6 for T1WI; and 3.2, 2.7, and 2.4 for DESS).

DATA CONCLUSION

UTE imaging can detect bone erosions in patients with axial SpA and show higher specificity than conventional T1WI and 3D DESS.

EVIDENCE LEVEL

4 TECHNICAL EFFICACY: Stage 2.

Magic angle effect on diffusion tensor imaging in ligament and brain

PURPOSE

To evaluate the magic angle effect on diffusion tensor imaging (DTI) measurements in rat ligaments and mouse brains.

METHODS

Three rat knee joints and three mouse brains were scanned at 9.4 T using a modified 3D diffusion-weighted spin echo pulse sequence with the isotropic spatial resolution of 45 μm. The b value was 1000 s/mm2 for rat knee and 4000 s/mm2 for mouse brain. DTI model was used to investigate the quantitative metrics at different orientations with respect to the main magnetic field. The collagen fiber structure of the ligament was validated with polarized light microscopy (PLM) imaging.

RESULTS

The signal intensity, signal-to-noise ratio (SNR), and DTI metrics in the ligament were strongly dependent on the collagen fiber orientation with respect to the main magnetic field from both simulation and actual MRI scans. The variation of fractional anisotropy (FA) was about ~32%, and the variation of mean diffusivity (MD) was ~11%. These findings were further validated with the numerical simulation at different SNRs (~10.0 to 86.0). Compared to the ligament, the DTI metrics showed little orientation dependence in mouse brains.

CONCLUSION

Magic angle effect plays an important role in DTI measurements in the highly ordered collagen-rich tissues, while MD showed less orientation dependence than FA.

Articular Cartilage Assessment Using Ultrashort Echo Time MRI: A Review

Articular cartilage is a major component of the human knee joint which may be affected by a variety of degenerative mechanisms associated with joint pathologies and/or the aging process. Ultrashort echo time (UTE) sequences with a TE less than 100 µs are capable of detecting signals from both fast- and slow-relaxing water protons in cartilage. This allows comprehensive evaluation of all the cartilage layers, especially for the short T₂ layers which include the deep and calcified zones. Several ultrashort echo time (UTE) techniques have recently been developed for both morphological imaging and quantitative cartilage assessment. This review article summarizes the current catalog techniques based on UTE Magnetic Resonance Imaging (MRI) that have been utilized for such purposes in the human knee joint, such as T₁, T2∗ , T_1ρ, magnetization transfer (MT), double echo steady state (DESS), quantitative susceptibility mapping (QSM) and inversion recovery (IR). The contrast mechanisms as well as the advantages and disadvantages of these techniques are discussed.

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.

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.

Fast multicomponent 3D-T1ρ relaxometry

NMR relaxometry can provide information about the relaxation of the magnetization in different tissues, increasing our understanding of molecular dynamics and biochemical composition in biological systems. In general, tissues have complex and heterogeneous structures composed of multiple pools. As a result, bulk magnetization returns to its original state with different relaxation times, in a multicomponent relaxation. Recovering the distribution of relaxation times in each voxel is a difficult inverse problem; it is usually unstable and requires long acquisition time, especially on clinical scanners. MRI can also be viewed as an inverse problem, especially when compressed sensing (CS) is used. The solution of these two inverse problems, CS and relaxometry, can be obtained very efficiently in a synergistically combined manner, leading to a more stable multicomponent relaxometry obtained with short scan times. In this paper, we will discuss the details of this technique from the viewpoint of inverse problems.

Assessment of mechanical properties of articular cartilage with quantitative three-dimensional ultrashort echo time (UTE) cones magnetic resonance imaging

Conventional magnetic resonance imaging (MRI) is not capable of detecting signal from the deep cartilage due to its short transverse relaxation time (T2). Moreover, several quantitative MRI techniques are significantly influenced by the magic angle effect. The combinations of ultrashort echo time (UTE) MRI with magnetization transfer (UTE-MT) and Adiabatic T_1ρ (UTE-AdiabT_1ρ) imaging allow magic angle-insensitive assessments of all regions of articular cartilage. The purpose of this study was to investigate the correlations between quantitative three-dimensional UTE MRI biomarkers and mechanical properties of human tibiofemoral cartilage specimens. In total, 40 human tibiofemoral cartilage specimens were harvested from three male and four female donors (64 ± 18 years old). Cartilage samples were scanned using a series of quantitative 3D UTE Cones T₂* (UTE-T₂*), T₁ (UTE-T₁), UTE-AdiabT_1ρ, and UTE-MT sequences in a standard knee coil on a clinical 3T scanner. UTE-MT data were acquired with a series of MT powers and frequency offsets to calculate magnetization transfer ratio (MTR), as well as macromolecular fraction (MMF) and macromolecular T₂ (T₂mm) through modeling. Cartilage stiffness and Hayes elastic modulus were measured using indentation tests. Correlations of 3D UTE Cones MRI measurements in the superficial layer, deep layer, and global regions of interest (ROIs) with mechanical properties were investigated. Cartilage mechanical properties demonstrated highest correlations with UTE measures of the superficial layer of cartilage. AdiabT_1ρ, MTR, and MMF in superficial layer ROIs showed significant correlations with Hayes elastic modulus (p < 0.05, R = -0.54, 0.49, and 0.66, respectively). These UTE measures in global ROIs showed significant, though slightly lower, correlations with Hayes elastic modulus (p < 0.05, R = -0.37, 0.52, and 0.60, respectively). Correlations between other UTE MRI measurements (T₂*, T₁, and T₂mm) and mechanical properties were non-significant. The 3D UTE-AdiabT_1ρ and UTE-MT sequences were highlighted as promising surrogates for non-invasive assessment of cartilage mechanical properties. MMF from UTE-MT modeling showed the highest correlations with cartilage mechanics.

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.

Susceptibility Perturbation MRI Maps Tumor Infiltration into Mesorectal Lymph Nodes

Noninvasive characterization of lymph node involvement in cancer is an enduring onerous challenge. In rectal cancer, pathologic lymph node status constitutes the most important determinant of local recurrence and overall survival, and patients with involved lymph nodes may benefit from preoperative chemo and/or radiotherapy. However, knowledge of lymph node status before surgery is currently hampered by limited imaging accuracy. Here, we introduce Susceptibility-Perturbation MRI (SPI) as a novel source of contrast to map malignant infiltration into mesorectal lymph nodes. SPI involves multigradient echo (MGE) signal decays presenting a nonmonoexponential nature, which we show is sensitive to the underlying microstructure via susceptibility perturbations. Using numerical simulations, we predicted that the large cell morphology and the high cellularity of tumor within affected mesorectal lymph nodes would induce signature SPI decays. We validated this prediction in mesorectal lymph nodes excised from total mesorectal excision specimens of patients with rectal cancer using ultrahigh field (16.4 T) MRI. SPI signals distinguished benign from malignant nodal tissue, both qualitatively and quantitatively, and our histologic analyses confirmed cellularity and cell size were the likely underlying sources for the differences observed. SPI was then adapted to a clinical 1.5 T scanner, added to patients' staging protocol, and compared with conventional assessment by two expert radiologists. Nonmonoexponential decays, similar to those observed in the ex vivo study, were demonstrated, and SPI classified lymph nodes more accurately than standard high-resolution T₂-weighted imaging assessment. These findings suggest this simple, yet highly informative, method can improve rectal cancer patient selection for neoadjuvant therapy. SIGNIFICANCE: These findings introduce an MRI methodology tailored to detect magnetic susceptibility perturbations induced by subtle alterations in tissue microstructure.

Quantitative measurement of T2, T1ρ and T1 relaxation times in articular cartilage and cartilage-bone interface by SE and UTE imaging at microscopic resolution

Both spin-echo (SE) and ultra-short echo (UTE) based MRI sequences were used on a 7 T µMRI system to quantify T2, T1ρ and T1 relaxation times from articular cartilage to the cartilage-bone interface on canine humeral specimens at 19.5 µm pixel resolution. A series of five relaxation-weighted images were acquired to calculate one relaxation map (T2, T1ρ or T1), from which the depth-dependent profiles were examined between the SE method and the UTE method, over the entire non-calcified cartilage and within the cartilage-bone interface. SE-based methods enabled the quantification of relaxation profiles over the noncalcified cartilage, from 0 µm (articular surface) to approximately 460 µm in depth (near the end of radial zone). Most of the cartilage-bone interface was imaged by the UTE-based methods, to a tissue depth of about 810 µm. Pixel-by-pixel calculation of the relaxation times between the independent SE and UTE methods correlated well with each other. A better understanding of the tissue properties reliably over the cartilage-bone interface region by a non-invasive MRI approach could contribute to the clinical diagnostics of trauma-induced osteoarthritis.

Rapid compositional mapping of knee cartilage with compressed sensing MRI

More than a decade after the introduction of compressed sensing (CS) in MRI, researchers are still working on ways to translate it into different research and clinical applications. The greatest advantage of CS in MRI is the reduced amount of k-space data needed to reconstruct images, which can be exploited to reduce scan time or to improve spatial resolution and volumetric coverage. Efficient data acquisition using CS is extremely important for compositional mapping of the musculoskeletal system in general and knee cartilage mapping techniques in particular. High-resolution quantitative information about tissue biochemical composition could be obtained in just a few minutes using CS MRI. However, in order to make this goal a reality, some issues still need to be addressed. In this article we review the current state of the art of CS methods for rapid compositional mapping of knee cartilage. Specifically, data acquisition strategies, image reconstruction algorithms, and data fitting models are discussed. Different CS studies for T2 and T1ρ mapping of knee cartilage are reviewed, with illustrative results. Future directions, opportunities, and challenges of rapid compositional mapping techniques are also discussed. Level of Evidence: 4 Technical Efficacy: Stage 6 J. Magn. Reson. Imaging 2018;47:1185-1198.

The comparison of the performance of 3 T and 7 T T2 mapping for untreated low-grade cartilage lesions

OBJECTIVE

To investigate T₂ mapping as a possible marker for low-grade human articular cartilage lesions during a one-year follow-up, possible changes during the follow-up and compare the reliability and sensitivity of these measurements on high-field (3 T) and ultra-high-field (7 T) MRI scanners.

DESIGN

Twenty-one patients with femoral, tibial and patellar cartilage defect in the knee joint participated in the study. The MRI protocol consisted of morphological, as well as three-dimensional triple-echo steady-state (3D-TESS) T₂ mapping sequences with similar parameters at 3T and 7T. Patients were scanned at five time-points up to 12 months. T₂ values were evaluated in the lesion and healthy-appearing regions for superficial and deep cartilage zone. The repeated ANOVA was used to determine differences in T₂ values at various time points.

RESULTS

A significant decrease in T₂ values was observed between baseline and six months in the superficial layer of the lesion in patients at 3 T (decrease from 41.89 ± 9.3 ms to 31.21 ± 7.2 ms, which is a difference of -5.67 ± 2.2 ms (p = 0.031)), and at 12 months in the superficial layer of the lesion in patients at 3 T (decrease from 41.89 ± 9.3 ms to 35.28 ± 4.9 ms, which is a difference of -6.60 ± 4.4 ms (p = 0.044). No significant differences were recorded at 7 T.

CONCLUSION

The change in T₂ values acquired with 3 T 3D-TESS appears to be reflecting subtle changes of cartilage composition in the course of low-grade lesion development. 7 T T₂ mapping does not reflect these changes probably due to completely decayed short T₂ component.

T2* Mapping of the Hip in Asymptomatic Volunteers with Normal Cartilage Morphology: An Analysis of Regional and Age-Dependent Distribution

Objective To assess age-dependent and regional differences in T2* relaxation measurements in hip joint cartilage of asymptomatic volunteers at 3 T. Design Three age cohorts (cohort 1: age 20-30 years, 15 individuals; cohort 2: age 30-40 years, 17 individuals; cohort 3: age 40-50 years, 15 individuals) were enrolled. T2* values were obtained in the central and peripheral cartilage of the acetabulum and the femoral head in 7 regions (anterior to superior and posterior). Results T2* did not differ among age cohorts in acetabular cartilage (cohort 1: 24.65 ± 6.56 ms, cohort 2: 24.70 ± 4.83 ms, cohort 3: 25.81 ± 5.10 ms, P = 0.10) and femoral head cartilage (cohort 1: 27.08 ± 8.24 ms, cohort 2: 25.90 ± 7.82 ms, cohort 3: 26.50 ± 5.61 ms, P = 0.34). Analysis of the regional T2* distribution pattern indicates increased T2* values in the anterior, anterior-superior, superior-anterior, and the posterior-superior aspects of acetabular and femoral head cartilage. For acetabular cartilage, higher values were observed in the central region (25.90 ± 4.80 ms vs. 24.21 ± 4.05 ms, P < 0.0001) whereas femoral head cartilage did not reveal such differences (26.62 ± 5.74 ms vs. 26.37 ± 5.89 ms, P = 0.44). Conclusions The T2* analysis of presumably healthy hip joint cartilage does not seem to be stratified according to age in this population. Regional T2* variation throughout hip joint cartilage is apparent in this modality.

Magnetic Resonance Imaging for Patellofemoral Chondromalacia: Is There a Role for T2 Mapping?

Background:

Patellofemoral pain is common, and treatment is guided by the presence and grade of chondromalacia.

Purpose:

To evaluate and compare the sensitivity and specificity in detecting and grading chondral abnormalities of the patella between proton density fat suppression (PDFS) and T2 mapping magnetic resonance imaging (MRI).

Study Design:

Cohort study; Level of evidence, 2.

Methods:

A total of 25 patients who underwent MRI of the knee with both a PDFS sequence and T2 mapping and subsequently underwent arthroscopic knee surgery were included. The cartilage surface of the patella was graded on both MRI sequences by 2 independent, blinded radiologists. Cartilage was then graded during arthroscopic surgery by a sports medicine fellowship–trained orthopaedic surgeon. Reliability, sensitivity, specificity, and accuracy were determined for both MRI methods. The findings during arthroscopic surgery were considered the gold standard.

Results:

Intraobserver and interobserver agreement for both PDFS (98.5% and 89.4%, respectively) and T2 mapping (99.4% and 91.3%, respectively) MRI were excellent. For T2 mapping, the sensitivity (61%) and specificity (64%) were comparable, whereas for PDFS there was a lower sensitivity (37%) but higher specificity (81%) in identifying cartilage abnormalities. This resulted in a similar accuracy for PDFS (59%) and T2 mapping (62%).

Conclusion:

Both PDFS and T2 mapping MRI were reliable but only moderately accurate in predicting patellar chondromalacia found during knee arthroscopic surgery.

Bi-component T1ρ and T2 Relaxation Mapping of Skeletal Muscle In-Vivo

The goal of this paper was to evaluate the possibility of bi-component T1ρ and T2 relaxation mapping of human skeletal muscle at 3 T in clinically feasible scan times. T1ρ- and T2-weighted images of calf muscle were acquired using a modified 3D-SPGR sequence on a standard 3 T clinical MRI scanner. The mono- and biexponential models were fitted pixel-wise to the series of T1ρ and T2 weighted images. The biexponential decay of T1ρ and T2 relaxations was detected in ~30% and ~40% of the pixels across all volunteers, respectively. Monoexponential and bi-exponential short and long T1ρ relaxation times were estimated to be 26.9 ms, 4.6 ms (fraction 22%) and 33.2 ms (fraction: 78%), respectively. Similarly, the mono- and bi-exponential short and long T2 relaxation times were 24.7 ms, 4.2 ms (fraction 15%) and 30.4 ms (fraction 85%) respectively. The experiments had good repeatability with RMSCV < 15% and ICC > 60%. This approach could potentially be used in exercise intervention studies or in studies of inflammatory myopathies or muscle fibrosis, permitting greater sensitivity and specificity via measurement of different water compartments and their fractions.

Biexponential T1ρ relaxation mapping of human knee cartilage in vivo at 3 T

The purpose of this study was to demonstrate the feasibility of biexponential T_1ρ relaxation mapping of human knee cartilage in vivo. A three-dimensional, customized, turbo-flash sequence was used to acquire T_1ρ -weighted images from healthy volunteers employing a standard 3-T MRI clinical scanner. A series of T_1ρ -weighted images was fitted using monoexponential and biexponential models with two- and four-parametric non-linear approaches, respectively. Non-parametric Kruskal-Wallis and Mann-Whitney U-statistical tests were used to evaluate the regional relaxation and gender differences, respectively, with a level of significance of P = 0.05. Biexponential relaxations were detected in the cartilage of all volunteers. The short and long relaxation components of T_1ρ were estimated to be 6.9 and 51.0 ms, respectively. Similarly, the fractions of short and long T_1ρ were 37.6% and 62.4%, respectively. The monoexponential relaxation of T_1ρ was 32.6 ms. The experiments showed good repeatability with a coefficient of variation (CV) of less than 20%. A biexponential relaxation model showed a better fit than a monoexponential model to the T_1ρ relaxation decay in knee cartilage. Biexponential T_1ρ components could potentially be used to increase the specificity to detect early osteoarthritis by the measurement of different water compartments and their fractions.

Three-dimensional ultrashort echo time cones T1ρ (3D UTE-cones-T1ρ ) imaging

We report a novel three-dimensional (3D) ultrashort echo time (UTE) sequence employing Cones trajectory and T_1ρ preparation (UTE-Cones-T_1ρ ) for quantitative T_1ρ assessment of short T₂ tissues in the musculoskeletal system. A basic 3D UTE-Cones sequence was combined with a spin-locking preparation pulse for T_1ρ contrast. A relatively short TR was used to decrease the scan time, which required T₁ measurement and compensation using 3D UTE-Cones data acquisitions with variable TRs. Another strategy to reduce the total scan time was to acquire multiple Cones spokes (N_sp ) after each T_1ρ preparation and fat saturation. Four spin-locking times (TSL = 0-20 ms) were acquired over 12 min, plus another 7 min for T₁ measurement. The 3D UTE-Cones-T_1ρ sequence was compared with a two-dimensional (2D) spiral-T_1ρ sequence for the imaging of a spherical CuSO₄ phantom and ex vivo meniscus and tendon specimens, as well as the knee and ankle joints of healthy volunteers, using a clinical 3-T scanner. The CuSO₄ phantom showed a T_1ρ value of 76.5 ± 1.6 ms with the 2D spiral-T_1ρ sequence, as well as 85.7 ± 3.6 and 89.2 ± 1.4 ms for the 3D UTE-Cones-T_1ρ sequences with N_sp of 1 and 5, respectively. The 3D UTE-Cones-T_1ρ sequence provided shorter T_1ρ values for the bovine meniscus sample relative to the 2D spiral-T_1ρ sequence (10-12 ms versus 16 ms, respectively). The cadaveric human Achilles tendon sample could only be imaged with the 3D UTE-Cones-T_1ρ sequence (T_1ρ  = 4.0 ± 0.9 ms), with the 2D spiral-T_1ρ sequence demonstrating near-zero signal intensity. Human studies yielded T_1ρ values of 36.1 ± 2.9, 18.3 ± 3.9 and 3.1 ± 0.4 ms for articular cartilage, meniscus and the Achilles tendon, respectively. The 3D UTE-Cones-T_1ρ sequence allows volumetric T_1ρ measurement of short T₂ tissues in vivo.

Cartilage MRI T2∗ relaxation time and perfusion changes of the knee in a 5/6 nephrectomy rat model of chronic kidney disease

OBJECTIVE

Chronic kidney disease (CKD) is characterized by metabolic disturbances in calcium and phosphorus homeostasis as kidney function declines. Alterations in blood perfusion in bone resulting from arteriosclerosis of bone vessels may relate to the progression of CKD. Herein, change in dynamic contrast enhanced (DCE) MRI parameters (A: amplitude, k_el: elimination constant, and k_ep: permeability rate constant) and MRI T2^∗ relaxation time of the knee cartilage were measured in a rodent nephrectomy model in order to (1) examine the relationship of peripheral blood perfusion to CKD and (2) demonstrate the feasibility of using DCE-MRI parameters and MRI T2^∗ as imaging biomarkers to monitor disease progression.

DESIGN

Two groups of male Sprague-Dawley rats received either (1) no intervention or (2) 5/6 nephrectomy.

RESULTS

We found that the CKD group (compared with the control group) had lower A and k_el values and similar k_ep value in the lateral and medial articular cartilages beginning at 12 weeks (P < 0.05); statistically significantly higher T2^∗ values in the lateral and medial articular cartilages beginning at 18 weeks (P < 0.05); statistically significantly decreased inner luminal diameter of the popliteal artery, and altered structure of the lateral and medial articular cartilages (P < 0.05).

CONCLUSION

Perfusion deficiency and CKD may be related. DCE parameters and MRI T2^∗ could serve as imaging biomarkers of cartilage degeneration in CKD progression.

Biexponential T2 relaxation estimation of human knee cartilage in vivo at 3T

PURPOSE

To evaluate biexponential T₂ relaxation mapping of human knee cartilage in vivo in clinically feasible scan times.

MATERIALS AND METHODS

T₂ -weighted magnetic resonance (MR) images were acquired from eight healthy volunteers using a standard 3T clinical scanner. A 3D Turbo-Flash sequence was modified to enable T₂ -weighted imaging with different echo times. Series of T₂ -weighted images were fitted using mono- and biexponential models with two- and four-parametric nonlinear approaches, respectively.

RESULTS

Biexponential relaxation of T₂ was detected in the knee cartilage in five regions of interest in all eight healthy volunteers. Short/long relaxation components of T₂ were estimated to be 8.27 ± 0.68 / 45.35 ± 3.79 msec with corresponding fractions of 41.3 ± 1.1% / 58.6 ± 4.6%, respectively. The monoexponential relaxation of T₂ was measured to be 26.9 ± 2.27 msec. The experiments showed good repeatability with coefficient of variation root mean square (CV_rms ) < 18% in all regions. The only difference in gender was observed in medial tibial cartilage, where the biexponential T₂ in female volunteers was significantly higher compared to male volunteers (P = 0.014). Significant differences were observed in T₂ relaxation between different regions on interest.

CONCLUSION

Biexponential relaxation of T₂ was observed in the human knee cartilage in vivo. The short and long components are thought to be related to the tightly bound and loosely bound macromolecular water compartments. These preliminary results of biexponential T₂ analysis could potentially be used to increase the specificity for detection of early osteoarthritis by measuring different water compartments and their fractions.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:809-819.

Morphologic and quantitative magnetic resonance imaging of knee articular cartilage for the assessment of post-traumatic osteoarthritis

Orthopedic trauma, such as anterior cruciate ligament (ACL) disruption, is a common source of osteoarthritis in the knee. Magnetic resonance imaging (MRI) is a non-invasive multi-planar imaging modality commonly used to evaluate hard and soft tissues of diarthrodial joints following traumatic injury. The contrast provided by generated images enables the evaluation of bone marrow lesions as well as delamination and degeneration of articular cartilage. We will provide background information about MRI signal generation and decay (T₁ and T₂ values), the utility of morphologic MRI, and the quantitative MRI techniques of T_1ρ , T₂ , and T₂ * mapping, to evaluate subjects with traumatic knee injuries, such as ACL rupture. Additionally, we will provide information regarding the dGEMRIC, sodium, and gagCEST imaging techniques. Finally, the description and utility of newer post hoc analysis techniques, such as texture analysis, will be given. Continued development and refinement of these advanced MRI techniques will facilitate their clinical translation. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:412-423, 2017.

MR measurement of luminal water in prostate gland: Quantitative correlation between MRI and histology

PURPOSE

To determine the relationship between parameters measured from luminal water imaging (LWI), a new magnetic resonance imaging (MRI) T₂ mapping technique, and the corresponding tissue composition in prostate.

MATERIALS AND METHODS

In all, 17 patients with prostate cancer were examined with a 3D multiecho spin echo sequence at 3T prior to undergoing radical prostatectomy. Maps of seven MR parameters, called N, T_2-short , T_2-long , A_short , A_long , geometric mean T₂ time (gmT₂ ), and luminal water fraction (LWF), were generated using nonnegative least squares (NNLS) analysis of the T₂ decay curves. MR parametric maps were correlated to digitized whole-mount histology sections. Percentage area of tissue components, including luminal space, nuclei, and cytoplasm plus stroma, was measured on the histology sections by using color-based image segmentation. Spearman's rank correlation test was used to evaluate the correlation between MR parameters and the corresponding tissue components, with particular attention paid to the correlation between LWF and percentage area of luminal space.

RESULTS

N, T_2-short , A_long , gmT₂ , and LWF showed significant correlation (P < 0.05) with percentage area of luminal space and stroma plus cytoplasm. T_2-short and gmT₂ also showed significant correlation (P < 0.05) with percentage area of nuclei. Overall, the strongest correlation was observed between LWF and luminal space (Spearman's coefficient of rank correlation = 0.75, P < 0.001).

CONCLUSION

Results of this study show that LWF measured with MRI is strongly correlated with the fractional amount of luminal space in prostatic tissue. This result suggests that LWI can potentially be applied for evaluation of prostatic diseases in which the extent of luminal space differs between normal and abnormal tissues.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 1 J. MAGN. RESON. IMAGING 2017;46:861-869.

Imaging and T2 relaxometry of short-T2 connective tissues in the knee using ultrashort echo-time double-echo steady-state (UTEDESS)

PURPOSE

To develop a radial, double-echo steady-state (DESS) sequence with ultra-short echo-time (UTE) capabilities for T₂ measurement of short-T₂ tissues along with simultaneous rapid, signal-to-noise ratio (SNR)-efficient, and high-isotropic-resolution morphological knee imaging.

METHODS

THe 3D radial UTE readouts were incorporated into DESS, termed UTEDESS. Multiple-echo-time UTEDESS was used for performing T₂ relaxometry for short-T₂ tendons, ligaments, and menisci; and for Dixon water-fat imaging. In vivo T₂ estimate repeatability and SNR efficiency for UTEDESS and Cartesian DESS were compared. The impact of coil combination methods on short-T₂ measurements was evaluated by means of simulations. UTEDESS T₂ measurements were compared with T₂ measurements from Cartesian DESS, multi-echo spin-echo (MESE), and fast spin-echo (FSE).

RESULTS

UTEDESS produced isotropic resolution images with high SNR efficiency in all short-T₂ tissues. Simulations and experiments demonstrated that sum-of-squares coil combinations overestimated short-T₂ measurements. UTEDESS measurements of meniscal T₂ were comparable to DESS, MESE, and FSE measurements while the tendon and ligament measurements were less biased than those from Cartesian DESS. Average UTEDESS T₂ repeatability variation was under 10% in all tissues.

CONCLUSION

The T₂ measurements of short-T₂ tissues and high-resolution morphological imaging provided by UTEDESS makes it promising for studying the whole knee, both in routine clinical examinations and longitudinal studies. Magn Reson Med 78:2136-2148, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Quantitative magnetic resonance imaging of the lumbar intervertebral discs

Human lumbar spine is composed of multiple tissue components that serve to provide structural stability and proper nutrition. Conventional magnetic resonance (MR) imaging techniques have been useful for evaluation of IVD, but inadequate at imaging the discovertebral junction and ligamentous tissues due primarily to their short T2 nature. Ultrashort time to echo (UTE) MR techniques acquire sufficient MR signal from these short T2 tissues, thereby allowing direct and quantitative evaluation. This article discusses the anatomy of the lumbar spine, MR techniques available for morphologic and quantitative MR evaluation of long and short T2 tissues of the lumbar spine, considerations for T2 relaxation modeling and fitting, and existing and new techniques for spine image post-processing, focusing on segmentation. This article will be of interest to radiologic and orthopaedic researchers performing lumbar spine imaging.

Quantitative sodium magnetic resonance imaging of cartilage, muscle, and tendon

Sodium magnetic resonance imaging (MRI), or imaging of the 23Na nucleus, has been under exploration for several decades, and holds promise for potentially revealing additional biochemical information about the health of tissues that cannot currently be obtained from conventional hydrogen (or proton) MRI. This additional information could serve as an important complement to conventional MRI for many applications. However, despite these exciting possibilities, sodium MRI is not yet used routinely in clinical practice, and will likely remain strictly in the domain of exploratory research for the coming decade. This paper begins with a technical overview of sodium MRI, including the nuclear magnetic resonance (NMR) signal characteristics of the sodium nucleus, the challenges associated with sodium MRI, and the specialized pulse sequences, hardware, and reconstruction techniques required. Various applications of sodium MRI for quantitative analysis of the musculoskeletal system are then reviewed, including the non-invasive assessment of cartilage degeneration in vivo, imaging of tendinopathy, applications in the assessment of various muscular pathologies, and assessment of muscle response to exercise.

Ultrashort time to echo magnetic resonance techniques for the musculoskeletal system

Magnetic resonance (MR) imaging has been widely implemented as a non-invasive modality to investigate musculoskeletal (MSK) tissue disease, injury, and pathology. Advancements in MR sequences provide not only enhanced morphologic contrast for soft tissues, but also quantitative biochemical evaluation. Ultrashort time to echo (UTE) sequence, in particular, enables novel morphologic and quantitative evaluation of previously unseen MSK tissues. By using short minimum echo times (TE) below 1 msec, the UTE sequence can unveil short T2 properties of tissues including the deepest layers of the articular cartilage, cartilaginous endplate at the discovertebral junction, the meniscus, and the cortical bone. This article will discuss the application of UTE to evaluate these MSK tissues, starting with tissue structure, MR imaging appearance on standard versus short and ultrashort TE sequences, and provide the range of quantitative MR values found in literature.

Can ultrashort-TE (UTE) MRI sequences on a 3-T clinical scanner detect signal directly from collagen protons: freeze-dry and D2 O exchange studies of cortical bone and Achilles tendon specimens

Ultrashort-TE (UTE) sequences can obtain signal directly from short-T2 , collagen-rich tissues. It is generally accepted that bound and free water can be detected with UTE techniques, but the ability to detect protons directly on the collagen molecule remains controversial. In this study, we investigated the potential of UTE sequences on a 3-T clinical scanner to detect collagen protons via freeze-drying and D2 O-H2 O exchange studies. Experiments were performed on bovine cortical bone and human Achilles tendon specimens, which were either subject to freeze-drying for over 66 h or D2 O-H2 O exchange for 6 days. Specimens were imaged using two- and three-dimensional UTE with Cones trajectory techniques with a minimum TE of 8 μs at 3 T. UTE images before treatment showed high signal from all specimens with bi-component T2 * behavior. Bovine cortical bone showed a shorter T2 * component of 0.36 ms and a longer T2 * component of 2.30 ms with fractions of 78.2% and 21.8% by volume, respectively. Achilles tendon showed a shorter T2 * component of 1.22 ms and a longer T2 * component of 15.1 ms with fractions of 81.1% and 18.9% by volume, respectively. Imaging after freeze-drying or D2 O-H2 O exchange resulted in either the absence or near-absence of signal. These results indicate that bound and free water are the sole sources of UTE signal in bovine cortical bone and human Achilles tendon samples on a clinical 3-T scanner. Protons on the native collagen molecule are not directly visible when imaged using UTE sequences. Copyright © 2016 John Wiley & Sons, Ltd.

Quantitative OCT and MRI biomarkers for the differentiation of cartilage degeneration

OBJECTIVE

To evaluate the usefulness of quantitative parameters obtained by optical coherence tomography (OCT) and magnetic resonance imaging (MRI) in the comprehensive assessment of human articular cartilage degeneration.

MATERIALS AND METHODS

Human osteochondral samples of variable degeneration (n = 45) were obtained from total knee replacements and assessed by MRI sequences measuring T1, T1ρ, T2 and T2* relaxivity and by OCT-based quantification of irregularity (OII, optical irregularity index), homogeneity (OHI, optical homogeneity index]) and attenuation (OAI, optical attenuation index]). Samples were also assessed macroscopically (Outerbridge classification) and histologically (Mankin classification) as grade-0 (Mankin scores 0-4)/grade-I (scores 5-8)/grade-II (scores 9-10)/grade-III (score 11-14). After data normalisation, differences between Mankin grades and correlations between imaging parameters were assessed using ANOVA and Tukey's post-hoc test and Spearman's correlation coefficients, respectively. Sensitivities and specificities in the detection of Mankin grade-0 were calculated.

RESULTS

Significant degeneration-related increases were found for T2 and OII and decreases for OAI, while T1, T1ρ, T2* or OHI did not reveal significant changes in relation to degeneration. A number of significant correlations between imaging parameters and histological (sub)scores were found, in particular for T2 and OII. Sensitivities and specificities in the detection of Mankin grade-0 were highest for OHI/T1 and OII/T1ρ, respectively.

CONCLUSION

Quantitative OCT and MRI techniques seem to complement each other in the comprehensive assessment of cartilage degeneration. Sufficiently large structural and compositional changes in the extracellular matrix may thus be parameterized and quantified, while the detection of early degeneration remains challenging.

UTE bi-component analysis of T2* relaxation in articular cartilage

OBJECTIVES

To determine T2* relaxation in articular cartilage using ultrashort echo time (UTE) imaging and bi-component analysis, with an emphasis on the deep radial and calcified cartilage.

METHODS

Ten patellar samples were imaged using two-dimensional (2D) UTE and Car-Purcell-Meiboom-Gill (CPMG) sequences. UTE images were fitted with a bi-component model to calculate T2* and relative fractions. CPMG images were fitted with a single-component model to calculate T2. The high signal line above the subchondral bone was regarded as the deep radial and calcified cartilage. Depth and orientation dependence of T2*, fraction and T2 were analyzed with histopathology and polarized light microscopy (PLM), confirming normal regions of articular cartilage. An interleaved multi-echo UTE acquisition scheme was proposed for in vivo applications (n = 5).

RESULTS

The short T2* values remained relatively constant across the cartilage depth while the long T2* values and long T2* fractions tended to increase from subchondral bone to the superficial cartilage. Long T2*s and T2s showed significant magic angle effect for all layers of cartilage from the medial to lateral facets, while the short T2* values and T2* fractions are insensitive to the magic angle effect. The deep radial and calcified cartilage showed a mean short T2* of 0.80 ± 0.05 ms and short T2* fraction of 39.93 ± 3.05% in vitro, and a mean short T2* of 0.93 ± 0.58 ms and short T2* fraction of 35.03 ± 4.09% in vivo.

CONCLUSION

UTE bi-component analysis can characterize the short and long T2* values and fractions across the cartilage depth, including the deep radial and calcified cartilage. The short T2* values and T2* fractions are magic angle insensitive.

Characterization of the ultrashort-TE (UTE) MR collagen signal

Although current cardiovascular MR (CMR) techniques for the detection of myocardial fibrosis have shown promise, they nevertheless depend on gadolinium-based contrast agents and are not specific to collagen. In particular, the diagnosis of diffuse myocardial fibrosis, a precursor of heart failure, would benefit from a non-invasive imaging technique that can detect collagen directly. Such a method could potentially replace the need for endomyocardial biopsy, the gold standard for the diagnosis of the disease. The objective of this study was to measure the MR properties of collagen using ultrashort TE (UTE), a technique that can detect short T2* species. Experiments were performed in collagen solutions. Via a model of bi-exponential T2* with oscillation, a linear relationship (slope = 0.40 ± 0.01, R(2) = 0.99696) was determined between the UTE collagen signal fraction associated with these properties and the measured collagen concentration in solution. The UTE signal of protons in the collagen molecule was characterized as having a mean T2* of 0.75 ± 0.05 ms and a mean chemical shift of -3.56 ± 0.01 ppm relative to water at 7 T. The results indicated that collagen can be detected and quantified using UTE. A knowledge of the collagen signal properties could potentially be beneficial for the endogenous detection of myocardial fibrosis.

Compositional MRI techniques for evaluation of cartilage degeneration in osteoarthritis

Osteoarthritis (OA), a leading cause of disability, affects 27 million people in the United States and its prevalence is rising along with the rise in obesity. So far, biomechanical or behavioral interventions as well as attempts to develop disease-modifying OA drugs have been unsuccessful. This may be partly due to antiquated imaging outcome measures such as radiography, which are still endorsed by regulatory agencies such as the United States Food and Drug Administration (FDA) for use in clinical trials. Morphological magnetic resonance imaging (MRI) allows unparalleled multi-feature assessment of the OA joint. Furthermore, advanced MRI techniques also enable evaluation of the biochemical or ultrastructural composition of articular cartilage relevant to OA research. These compositional MRI techniques have the potential to supplement clinical MRI sequences in identifying cartilage degeneration at an earlier stage than is possible today using morphologic sequences only. The purpose of this narrative review is to describe compositional MRI techniques for cartilage evaluation, which include T2 mapping, T2* Mapping, T1 rho, dGEMRIC, gagCEST, sodium imaging and diffusion weighted imaging (DWI). We also reviewed relevant clinical studies that have utilized these techniques for the study of OA. The different techniques are complementary. Some focus on isotropy or the collagen network (e.g., T2 mapping) and others are more specific in regard to tissue composition, e.g., gagCEST or dGEMRIC that convey information on the GAG concentration. The application and feasibility of these techniques is also discussed, as they will play an important role in implementation in larger clinical trials and eventually clinical practice.

Change in T2* relaxation time of Hoffa fat pad correlates with histologic change in a rat anterior cruciate ligament transection model

The Hoffa fat pad (infrapatellar fat pad) is a source of post-traumatic anterior knee pain, and Hoffa disease is a syndrome leading to chronic inflammation of the fat pad. Herein, change in T2* relaxation time of the fat pad was measured in a rodent anterior cruciate ligament transection (ACLX) model in order to (i) examine the causal relationship of anterior cruciate ligament (ACL) deficiency and Hoffa disease and (ii) demonstrate the feasibility of using T2* as an imaging biomarker to monitor disease progression. Three groups of male Sprague Dawley rats (n = 6 each group), received either (i) no intervention; (ii) sham surgery at the right knee; or (iii) right ACLX. T2* relaxation time was measured and histology was examined in the Hoffa fat pad after surgery. At 13 and 18 weeks after surgery, T2* values were significantly higher in the right fat pad than the left (p < 0.001) and significantly higher in the ACLX group than the control and sham groups (p < 0.001). Histology showed fibrosis and degeneration of adipocytes in the right knees of the ACLX group. We conclude that ACL deficiency and Hoffa disease are causally related and that MRI T2* value can serve as an imaging biomarker of Hoffa disease progression.

Articular Cartilage of the Human Knee Joint: In Vivo Multicomponent T2 Analysis at 3.0 T

PURPOSE

To compare multicomponent T2 parameters of the articular cartilage of the knee joint measured by using multicomponent driven equilibrium single-shot observation of T1 and T2 (mcDESPOT) in asymptomatic volunteers and patients with osteoarthritis.

MATERIALS AND METHODS

This prospective study was performed with institutional review board approval and with written informed consent from all subjects. The mcDESPOT sequence was performed in the knee joint of 13 asymptomatic volunteers and 14 patients with osteoarthritis of the knee. Single-component T2 (T2(Single)), T2 of the fast-relaxing water component (T2F) and of the slow-relaxing water component (T2S), and the fraction of the fast-relaxing water component (F(F)) of cartilage were measured. Wilcoxon rank-sum tests and multivariate linear regression models were used to compare mcDESPOT parameters between volunteers and patients with osteoarthritis. Receiver operating characteristic analysis was used to assess diagnostic performance with mcDESPOT parameters for distinguishing morphologically normal cartilage from morphologically degenerative cartilage identified at magnetic resonance imaging in eight cartilage subsections of the knee joint.

RESULTS

Higher cartilage T2(Single) (P < .001), lower cartilage F(F) (P < .001), and similar cartilage T2F (P = .079) and T2S (P = .124) values were seen in patients with osteoarthritis compared with those in asymptomatic volunteers. Differences in T2(Single) and F(F) remained significant (P < .05) after consideration of age differences between groups of subjects. Diagnostic performance was higher with F(F) than with T2(Single) for distinguishing between normal and degenerative cartilage (P < .05), with greater areas under the curve at receiver operating characteristic analysis.

CONCLUSION

Patients with osteoarthritis of the knee had significantly higher cartilage T2(Single) and significantly lower cartilage F(F) than did asymptomatic volunteers, and receiver operating characteristic analysis results suggested that F(F) may allow greater diagnostic performance than that with T2(Single) for distinguishing between normal and degenerative cartilage.

Effects of inversion time on inversion recovery prepared ultrashort echo time (IR-UTE) imaging of bound and pore water in cortical bone

Water is present in cortical bone in different binding states. In this study we aimed to investigate the effects of inversion time (TI) on the signal from bound and pore water in cortical bone using an adiabatic inversion recovery prepared ultrashort echo time (IR-UTE) sequence on a clinical 3 T scanner. In total ten bovine midshaft samples and four human tibial midshaft samples were harvested for this study. Each cortical sample was imaged with the UTE and IR-UTE sequences with a TR of 300 ms and a series of TI values ranging from 10 to 240 ms. Five healthy volunteers were also imaged with the same sequence. Single- and bi-component models were utilized to calculate the T2 * and relative fractions of short and long T2 * components. Bi-component behavior of the signal from cortical bone was seen with the IR-UTE sequence, except with a TI of around 80 ms, where the short T2 * component alone were seen and a mono-exponential decay pattern was observed. In vivo imaging with the IR-UTE sequence provided high contrast-to-noise images with direct visualization of bound water and reduced signal from long T2 muscle and fat. Our preliminary results demonstrate that selective nulling of the pore water component can be achieved with the IR-UTE sequence with an appropriate TI, allowing selective imaging of the bound water component in cortical bone in vivo using clinical MR scanners.

Quantitative magnetic resonance imaging of the articular cartilage of the knee joint

Osteoarthritis is characterized by a decrease in the proteoglycan content and disruption of the highly organized collagen fiber network of articular cartilage. Various quantitative magnetic resonance imaging techniques have been developed for noninvasive assessment of the proteoglycan and collagen components of cartilage. These techniques have been extensively used in clinical practice to detect early cartilage degeneration and in osteoarthritis research studies to monitor disease-related and treatment-related changes in cartilage over time. This article reviews the role of quantitative magnetic resonance imaging in evaluating the composition and ultrastructure of the articular cartilage of the knee joint.

Quantitative T2* (T2 star) relaxation times predict site specific proteoglycan content and residual mechanics of the intervertebral disc throughout degeneration

Degeneration alters the biochemical composition of the disc, affecting the mechanical integrity leading to spinal instability. Quantitative T2* MRI probes water mobility within the macromolecular network, a potentially more sensitive assessment of disc health. We determined the relationship between T2* relaxation time and proteoglycan content, collagen content, and compressive mechanics throughout the degenerative spectrum. Eighteen human cadaveric lumbar (L4-L5) discs were imaged using T2* MRI. The T2* relaxation time at five locations (nucleous pulposus or NP, anterior annulus fibrosis or AF, posterior AF, inner AF, and outer AF) was correlated with sulfated-glycosaminoglycan (s-GAG) content, hydroxyproline content, and residual stress and strain at each location. T2* relaxation times were significantly correlated with s-GAG contents in all test locations and were particularly strong in the NP (r = 0.944; p < 0.001) and inner AF (r = 0.782; p < 0.001). T2* relaxation times were also significantly correlated with both residual stresses and excised strains in the NP (r = 0.857; p < 0.001: r = 0.816; p < 0.001), inner AF (r = 0.535; p = 0.022: r = 0.516; p = 0.028), and outer AF (r = 0.668; p = 0.002: r = 0.458; p = 0.041). These strong correlations highlight T2* MRI's ability to predict the biochemical and mechanical health of the disc. T2* MRI assessment of disc health is a clinically viable tool showing promise as a biomarker for distinguishing degenerative changes.

UTE imaging in the musculoskeletal system

Tissues, such as bone, tendon, and ligaments, contain a high fraction of components with "short" and "ultrashort" transverse relaxation times and therefore have short mean transverse relaxation times. With conventional magnetic resonance imaging (MRI) sequences that employ relatively long echo times (TEs), there is no opportunity to encode the decaying signal of short and ultrashort T2 /T2 * tissues before it has reached zero or near zero. The clinically compatible ultrashort TE (UTE) sequence has been increasingly used to study the musculoskeletal system. This article reviews the UTE sequence as well as various modifications that have been implemented since its introduction. These modifications have been used to improve efficiency or contrast as well as provide quantitative analysis. This article reviews several clinical musculoskeletal applications of UTE.

GRE T2∗-weighted MRI: principles and clinical applications

The sequence of a multiecho gradient recalled echo (GRE) T2*-weighted imaging (T2*WI) is a relatively new magnetic resonance imaging (MRI) technique. In contrast to T2 relaxation, which acquires a spin echo signal, T2* relaxation acquires a gradient echo signal. The sequence of a GRE T2*WI requires high uniformity of the magnetic field. GRE T2*WI can detect the smallest changes in uniformity in the magnetic field and can improve the rate of small lesion detection. In addition, the T2* value can indirectly reflect changes in tissue biochemical components. Moreover, it can be used for the early diagnosis and quantitative diagnosis of some diseases. This paper reviews the principles and clinical applications as well as the advantages and disadvantages of GRE T2*WI.

Prediction of cartilage compressive modulus using multiexponential analysis of T(2) relaxation data and support vector regression

Evaluation of mechanical characteristics of cartilage by magnetic resonance imaging would provide a noninvasive measure of tissue quality both for tissue engineering and when monitoring clinical response to therapeutic interventions for cartilage degradation. We use results from multiexponential transverse relaxation analysis to predict equilibrium and dynamic stiffness of control and degraded bovine nasal cartilage, a biochemical model for articular cartilage. Sulfated glycosaminoglycan concentration/wet weight (ww) and equilibrium and dynamic stiffness decreased with degradation from 103.6 ± 37.0 µg/mg ww, 1.71 ± 1.10 MPa and 15.3 ± 6.7 MPa in controls to 8.25 ± 2.4 µg/mg ww, 0.015 ± 0.006 MPa and 0.89 ± 0.25MPa, respectively, in severely degraded explants. Magnetic resonance measurements were performed on cartilage explants at 4 °C in a 9.4 T wide-bore NMR spectrometer using a Carr-Purcell-Meiboom-Gill sequence. Multiexponential T2 analysis revealed four water compartments with T2 values of approximately 0.14, 3, 40 and 150 ms, with corresponding weight fractions of approximately 3, 2, 4 and 91%. Correlations between weight fractions and stiffness based on conventional univariate and multiple linear regressions exhibited a maximum r(2) of 0.65, while those based on support vector regression (SVR) had a maximum r(2) value of 0.90. These results indicate that (i) compartment weight fractions derived from multiexponential analysis reflect cartilage stiffness and (ii) SVR-based multivariate regression exhibits greatly improved accuracy in predicting mechanical properties as compared with conventional regression.

Incorporation of Rician noise in the analysis of biexponential transverse relaxation in cartilage using a multiple gradient echo sequence at 3 and 7 Tesla

PURPOSE

Previous work has evaluated the quality of different analytic methods for extracting relaxation times from magnitude imaging data exhibiting Rician noise. However, biexponential analysis of relaxation in tissue, including cartilage, and materials is of increasing interest. We, therefore, analyzed biexponential transverse relaxation decay in the presence of Rician noise and assessed the accuracy and precision of several approaches to determining component fractions and apparent transverse relaxation times.

THEORY AND METHODS

Comparisons of four different voxel-by-voxel fitting methods were performed using Monte Carlo simulations, and phantom and ex vivo bovine nasal cartilage (BNC) experiments. In each case, preclinical and clinical imaging field strengths of 7 Tesla (T) and 3T, respectively, and parameters, were investigated across a range of signal-to-noise ratios (SNR). Results were compared with Cramér-Rao lower bound calculations.

RESULTS

As expected, at high SNR, all methods performed well. At lower SNR, fits explicitly incorporating the analytic form of the Rician noise maintained performance. The much more efficient correction scheme of Gudbjartsson and Patz performed almost as well in many cases. Ex vivo experiments on phantoms and BNC were consistent with simulation results.

CONCLUSION

Explicit incorporation of Rician noise greatly improves accuracy and precision in the analysis of biexponential transverse decay data.