Proton spin-lock ratio imaging for quantitation of glycosaminoglycans in articular cartilage

Quantitative MRI methods for the assessment of structure, composition, and function of musculoskeletal tissues in basic research and preclinical applications

Osteoarthritis (OA) is a disabling chronic disease involving the gradual degradation of joint structures causing pain and dysfunction. Magnetic resonance imaging (MRI) has been widely used as a non-invasive tool for assessing OA-related changes. While anatomical MRI is limited to the morphological assessment of the joint structures, quantitative MRI (qMRI) allows for the measurement of biophysical properties of the tissues at the molecular level. Quantitative MRI techniques have been employed to characterize tissues' structural integrity, biochemical content, and mechanical properties. Their applications extend to studying degenerative alterations, early OA detection, and evaluating therapeutic intervention. This article is a review of qMRI techniques for musculoskeletal tissue evaluation, with a particular emphasis on articular cartilage. The goal is to describe the underlying mechanism and primary limitations of the qMRI parameters, their association with the tissue physiological properties and their potential in detecting tissue degeneration leading to the development of OA with a primary focus on basic and preclinical research studies. Additionally, the review highlights some clinical applications of qMRI, discussing the role of texture-based radiomics and machine learning in advancing OA research.

Difference in quantitative MRI measurements of cartilage between Wiberg type III patella and stable patella based on a 3.0-T synthetic MRI sequence

Purpose

The purpose of this study was to investigate the difference between the quantitative MRI values of Wiberg type III and stable patellar cartilage, and to improve the accuracy of MRI quantification in early patellar cartilage damage.

Methods

The knee joints of 94 healthy volunteers were scanned by a GE Signa Pioneer 3.0-T synthetic MRI machine. According to the Wiberg classification, the patella was divided into types I-III. Types I-II made up the stable patella group, and type III made up the unstable patella group. Two radiologists independently measured patellar cartilage thickness and quantitative synthetic MRI values (T1, T2, PD) in both groups. Interobserver agreement for quantitative variables was assessed using the Bland-Altman method. A third radiologist assessed differences in measurements.

Results

The medial T2 and T1 value of Wiberg III patella did not show a normal distribution (all P > 0.05). Compared with the stable group, the Wiberg type III group had thinner cartilage of the medial surface of the patella (P < 0.05), lower cartilage T2 and PD values (P < 0.05), but a similar cartilage T1 value (P > 0.05). There was no significant difference in the cartilage thickness, T1, T2, or PD value of the lateral patella between the Wiberg type III and the stable group (P > 0.05).

Conclusion

There were certain differences in the cartilage thickness of the medial surface of the patella and the quantitative value of synthetic MRI in Wiberg type III patellas. Quantitative studies of patellar cartilage MRI measurements need to consider the influence of patellar morphology.

Three-dimensional multi-parameter brain mapping using MR fingerprinting

The purpose of this study was to develop and test a 3D multi-parameter MR fingerprinting (MRF) method for brain imaging applications. The subject cohort included 5 healthy volunteers, repeatability tests done on 2 healthy volunteers and tested on two multiple sclerosis (MS) patients. A 3D-MRF imaging technique capable of quantifying T1, T2 and T1ρ was used. The imaging sequence was tested in standardized phantoms and 3D-MRF brain imaging with multiple shots (1, 2 and 4) in healthy human volunteers and MS patients. Quantitative parametric maps for T1, T2, T1ρ, were generated. Mean gray matter (GM) and white matter (WM) ROIs were compared for each mapping technique, Bland-Altman plots and intra-class correlation coefficient (ICC) were used to assess repeatability and Student T-tests were used to compare results in MS patients. Standardized phantom studies demonstrated excellent agreement with reference T1/T2/T1ρ mapping techniques. This study demonstrates that the 3D-MRF technique is able to simultaneously quantify T1, T2 and T1ρ for tissue property characterization in a clinically feasible scan time. This multi-parametric approach offers increased potential to detect and differentiate brain lesions and to better test imaging biomarker hypotheses for several neurological diseases, including MS.

Arthroscopic surgery for symptomatic discoid lateral meniscus improves meniscal status assessed by magnetic resonance imaging T2 mapping

INTRODUCTION

Discoid lateral meniscus (DLM) is an anatomic knee variant associated with increased tears and degeneration. This study aimed to quantify meniscal status with magnetic resonance imaging (MRI) T2 mapping before and after arthroscopic reshaping surgery for DLM.

MATERIALS AND METHODS

We retrospectively reviewed the records of patients undergoing arthroscopic reshaping surgery for symptomatic DLM with ≥ 2-year follow-up. MRI T2 mapping was performed preoperatively and at 12 and 24 months postoperatively. T2 relaxation times of the anterior and posterior horns of both menisci and of the adjacent cartilage were assessed.

RESULTS

Thirty-six knees from 32 patients were included. The mean age at surgery was 13.7 years (range 7-24), and the mean follow-up duration was 31.0 months. Saucerization alone was performed on five knees and saucerization with repair on 31 knees. Preoperatively, the T2 relaxation time of the anterior horn of the lateral meniscus was significantly longer than that of the medial meniscus (P < 0.01). T2 relaxation time significantly decreased at 12 and 24 months postoperatively (P < 0.01). Assessments of the posterior horn were comparable. The T2 relaxation time was significantly longer in the tear versus non-tear side at each time point (P < 0.01). There were significant correlations between the T2 relaxation time of the meniscus and that of the corresponding area of the lateral femoral condyle cartilage (anterior horn: r = 0.504, P = 0.002; posterior horn: r = 0.365, P = 0.029).

CONCLUSIONS

The T2 relaxation time of symptomatic DLM was significantly longer than that of the medial meniscus preoperatively, and it decreased 24 months after arthroscopic reshaping surgery. The meniscal T2 relaxation time of the tear side was significantly longer than that of the non-tear side. There were significant correlations between the cartilage and meniscal T2 relaxation times at 24 months after surgery.

New methods for robust continuous wave T1ρ relaxation preparation

Measurement of the longitudinal relaxation time in the rotating frame of reference (T_1ρ ) is sensitive to the fidelity of the main imaging magnetic field (B₀ ) and that of the RF pulse (B₁ ). The purpose of this study was to introduce methods for producing continuous wave (CW) T_1ρ contrast with improved robustness against field inhomogeneities and to compare the sensitivities of several existing and the novel T_1ρ contrast generation methods with the B₀ and B₁ field inhomogeneities. Four hard-pulse and four adiabatic CW-T_1ρ magnetization preparations were investigated. Bloch simulations and experimental measurements at different spin-lock amplitudes under ideal and non-ideal conditions, as well as theoretical analysis of the hard-pulse preparations, were conducted to assess the sensitivity of the methods to field inhomogeneities, at low (ω₁  << ΔB₀ ) and high (ω₁ >> ΔB₀ ) spin-locking field strengths. In simulations, previously reported single-refocus and new triple-refocus hard-pulse and double-refocus adiabatic preparation schemes were found to be the most robust. The mean normalized absolute deviation between the experimentally measured relaxation times under ideal and non-ideal conditions was found to be smallest for the refocused preparation schemes and broadly in agreement with the sensitivities observed in simulations. Experimentally, all refocused preparations performed better than those that were non-refocused. The findings promote the use of the previously reported hard-pulse single-refocus ΔB₀ and B₁ insensitive T_1ρ as a robust method with minimal RF energy deposition. The double-refocus adiabatic B₁ insensitive rotation-4 CW-T_1ρ preparation offers further improved insensitivity to field variations, but because of the extra RF deposition, may be preferred for ex vivo applications.

Quantitative assessment of articular cartilage degeneration using 3D ultrashort echo time cones adiabatic T1ρ (3D UTE-Cones-AdiabT1ρ) imaging

OBJECTIVES

To evaluate articular cartilage degeneration using quantitative three-dimensional ultrashort-echo-time cones adiabatic-T1ρ (3D UTE-Cones-AdiabT1ρ) imaging.

METHODS

Sixty-six human subjects were recruited for this study. Kellgren-Lawrence (KL) grade and Whole-Organ Magnetic-Resonance-Imaging Score (WORMS) were evaluated by two musculoskeletal radiologists. The human subjects were categorized into three groups, namely normal controls (KL0), doubtful-minimal osteoarthritis (OA) (KL1-2), and moderate-severe OA (KL3-4). WORMS were regrouped to encompass the extent of lesions and the depth of lesions. The UTE-Cones-AdiabT1ρ values were obtained using 3D UTE-Cones data acquisitions preceded by seven paired adiabatic full passage pulses that corresponded to seven spin-locking times (TSLs) of 0, 12, 24, 36, 48, 72, and 96 ms. The performance of the UTE-Cones-AdiabT1ρ technique in evaluating the degeneration of knee cartilage was assessed via the ANOVA comparisons with subregional analysis and Spearman's correlation coefficient as well as the receiver-operating-characteristic (ROC) curve.

RESULTS

UTE-Cones-AdiabT1ρ showed significant positive correlations with KL grade (r = 0.15, p < 0.05) and WORMS (r = 0.57, p < 0.05). Higher UTE-Cones-AdiabT1ρ values were observed in both larger and deeper lesions in the cartilage. The differences in UTE-Cones-AdiabT1ρ values among different extent and depth groups of cartilage lesions were all statistically significant (p < 0.05). Subregional analyses showed that the correlations between UTE-Cones-AdiabT1ρ and WORMS varied with the location of cartilage. The AUC value of UTE-Cones-AdiabT1ρ for mild cartilage degeneration (WORMS=1) was 0.8. The diagnostic threshold value of UTE-Cones-AdiabT1ρ for mild cartilage degeneration was 39.4 ms with 80.8% sensitivity.

CONCLUSIONS

The 3D UTE-Cones-AdiabT1ρ sequence can be useful in quantitative evaluation of articular cartilage degeneration.

KEY POINTS

• The 3D UTE-Cones-AdiabT1ρ sequence can distinguish mild cartilage degeneration from normal cartilage with a diagnostic threshold value of 39.4 ms for mild cartilage degeneration with 80.8% sensitivity. • Higher UTE-Cones-AdiabT1ρ values were observed in both larger and deeper lesions in the articular cartilage. • UTE-Cones-AdiabT1ρ is a promising biomarker for quantitative evaluation of early cartilage degeneration.

Effects of Angular Resolution and b Value on Diffusion Tensor Imaging in Knee Joint

OBJECTIVE

To investigate the influences of the diffusion gradient directions (angular resolution) and the strength of the diffusion gradient (b value) on diffusion tensor imaging (DTI) metrics and tractography of various connective tissues in knee joint.

DESIGN

Two rat knee joints were scanned on a preclinical 9.4-T system using a 3-dimensional diffusion-weighted spin echo pulse sequence. One protocol with b value of 500, 1500, and 2500 s/mm² were acquired separately using 43 diffusion gradient directions. The other protocol with b value of 1000 s/mm² was performed using 147 diffusion gradient directions. The in-plane resolution was 45 µm isotropic. Fractional anisotropy (FA) and mean diffusivity (MD) were compared at different angular resolution. Tractography was quantitatively evaluated at different b values and angular resolutions in cartilage, ligament, meniscus, and growth plate.

RESULTS

The ligament showed higher FA value compared with growth plate and cartilage. The FA values were largely overestimated at the angular resolution of 6. Compared with FA, MD showed less sensitivity to the angular resolution. The fiber tracking was failed at low angular resolution (6 diffusion gradient directions) or high b value (2500 s/mm²). The quantitative measurements of tract length and track volume were strongly dependent on angular resolution and b value.

CONCLUSIONS

To obtain consistent DTI outputs and tractography in knee joint, the scan may require a proper b value (ranging from 500 to 1500 s/mm²) and sufficient angular resolution (>14) with signal-to-noise ratio >10.

Magnetization-prepared spoiled gradient-echo snapshot imaging for efficient measurement of R2 -R1ρ in knee cartilage

PURPOSE

To validate the potential of quantifying R₂ -R_1ρ using one pair of signals with T_1ρ preparation and T₂ preparation incorporated to magnetization-prepared angle-modulated partitioned k-space spoiled gradient-echo snapshots (MAPSS) acquisition and to find an optimal preparation time (T_prep ) for in vivo knee MRI.

METHODS

Bloch equation simulations were first performed to assess the accuracy of quantifying R₂ -R_1ρ using T_1ρ - and T₂ -prepared signals with an equivalent T_prep . For validation of this technique in comparison to the conventional approach that calculates R₂ -R_1ρ after estimating both T₂ and T_1ρ , phantom experiments and in vivo validation with five healthy subjects and five osteoarthritis patients were performed at a clinical 3T scanner.

RESULTS

Bloch equation simulations demonstrated that the accuracy of this efficient R₂ -R_1ρ quantification method and the optimal T_prep can be affected by image signal-to-noise ratio (SNR) and tissue relaxation times, but quantification can be closest to the reference with an around 25 ms T_prep for knee cartilage. Phantom experiments demonstrated that the proposed method can depict R₂ -R_1ρ changes with agarose gel concentration. With in vivo data, significant correlation was observed between cartilage R₂ -R_1ρ measured from the conventional and the proposed methods, and a T_prep of 25.6 ms provided the most agreement by Bland-Altman analysis. R₂ -R_1ρ was significantly lower in patients than in healthy subjects for most cartilage compartments.

CONCLUSION

As a potential biomarker to indicate cartilage degeneration, R₂ -R_1ρ can be efficiently measured using one pair of T_1ρ -prepared and T₂ -prepared signals with an optimal T_prep considering cartilage relaxation times and image SNR.

Zone- and layer-specific differences in proteoglycan content in patellofemoral pain syndrome are detectable on T1ρ MRI

OBJECTIVE

Determine if differences in T1ρ would be detected in specific regions or layers of patellofemoral cartilage between patients with symptomatic patellofemoral pain syndrome and asymptomatic control subjects.

MATERIALS AND METHODS

Ten subjects diagnosed with patellofemoral pain syndrome were compared with ten age-, gender-, and BMI-matched control subjects with no knee pain or prior trauma. Conventional turbo (fast) spin echo sequences and T1ρ-weighted imaging were performed on the symptomatic knee in each of the ten subjects. At the patella and distal femur, cartilage regions of interest were divided into medial and lateral sub-regions, each then further sub-divided by layer (superficial, middle, or deep). Two-tailed t test and chi-squared tests were used to analyze demographic data. A mixed effect model was run for each sub-region of T1ρ imaging. Statistical significance was determined using the likelihood ratio test against reduced models without patellofemoral pain syndrome symptomatic status as a fixed effect.

RESULTS

There was no difference in age, sex, or BMI between symptomatic and control patients. T1ρ values were significantly higher among patellofemoral pain syndrome patients when compared with controls in the superficial zone of the lateral patella (58.43 vs. 50.83, p = 0.03) and the middle zone of the lateral patella (52.67 vs. 43.60, p = 0.03). T1ρ was also higher in the superficial zone of the medial femur (50.94 vs. 46.70, p = 0.09) with a value approaching statistical significance.

CONCLUSION

We report statistically significant differences in the T1ρ value in the superficial and middle zones of the lateral patella in patients with patellofemoral pain syndrome who had no abnormalities seen on conventional MRI sequences, suggesting an alteration the macromolecular structure of the cartilage in this population.

Assessment of Meniscal Healing Status by Magnetic Resonance Imaging T2 Mapping After Meniscal Repair

BACKGROUND

Although the sensitivity and specificity of magnetic resonance imaging (MRI) for the diagnosis of primary meniscal tears are high, these values are lower for the assessment of healing status of repaired menisci.

PURPOSE

To compare the accuracy of MRI T2 mapping and conventional MRI in assessing meniscal healing after repair.

STUDY DESIGN

Cohort study (diagnosis); Level of evidence, 2.

METHODS

Patients who underwent meniscal repair with concurrent anterior cruciate ligament reconstruction between 2012 and 2016 and had a follow-up second-look arthroscopy were enrolled. The patients were divided into healed and incompletely/not healed groups based on the second-look arthroscopy findings. For the repaired menisci, the following were compared between the groups, (1) Stoller and Crues classification on conventional MRI with a proton density-weighted fat-saturated sequence and (2) the remaining colored meniscal tear line on T2 mapping coincident with the high signal line showing the primary tear on conventional MRI were compared. The change of T2 relaxation time (ΔT2) of the colored meniscal tear line pre- to postoperatively was compared between the groups. The mean T2 relaxation time of the whole area of the postoperative meniscus at each slice was also compared with that of control menisci to assess the whole quality of the repaired meniscus.

RESULTS

A total of 26 menisci from 24 knees were assessed (16 healed menisci, 10 incompletely/not healed menisci). According to the Crues classification on conventional MRI, 8 of 16 healed menisci and 3 of 10 incompletely/not healed menisci improved from grade 3 to 2, with there being no significant difference between the groups (P = .43). However, the colored meniscal tear line remained in only 3 of the 16 healed menisci as compared with 9 of the 10 incompletely/not healed menisci, and the presence of this colored line allowed differentiation between healed menisci and incompletely/not healed menisci (sensitivity, 81.3%; specificity, 90.0%; odds ratio, 39.0; P = .001). The mean (SD) ΔT2 was -31.1 ± 3.2 and -19.9 ± 4.4 ms in the healed and incompletely/not healed groups, respectively (P < .001). Receiver operating characteristic curve analysis showed a cutoff ΔT2 value of -22.3 ms for separation of meniscal healing (P < .001). The T2 relaxation times of the whole area of the repaired menisci were 31.7 ± 3.4 and 32.8 ± 3.8 ms in the healed and incompletely/unhealed groups, respectively (P = .69), with these values being significantly longer than the 26.9 ± 2.2 ms in the controls (P < .001).

CONCLUSION

MRI T2 mapping allowed the differentiation of healing status after meniscal repair, with high sensitivity and specificity as compared with conventional MRI.

Brain T1ρ mapping for grading and IDH1 gene mutation detection of gliomas: a preliminary study

INTRODUCTION

The longitudinal relaxation time in the rotating frame (T1ρ) has proved to be sensitive to metabolism and useful in application to neurodegenerative diseases. However, few literature exists on its utility in gliomas. Thus, this study was conducted to explore the performance of T1ρ mapping in tumor grading and characterization of isocitrate dehydrogenase 1 (IDH1) gene mutation status of gliomas.

METHODS

Fifty-seven patients with gliomas underwent brain MRI and quantitative measurements of T1ρ and apparent diffusion coefficient (ADC) were recorded. Parameters were compared between high-grade gliomas (HGG) and low-grade gliomas (LGG) and between IDH1 mutant and wildtype groups.

RESULTS

HGG showed significantly higher T1ρ values in both the solid and peritumoral edema areas compared with LGG (P < 0.001 and P = 0.005, respectively), whereas no significant differences in the two areas were found for ADC (both P > 0.05). Receiver operating characteristic (ROC) curve analysis showed that T1ρ value in the solid area achieved the highest area under the ROC curve (AUC, 0.841) in grading with a sensitivity of 80.6% and a specificity of 81.0%. In the grade II/III glioma group, multivariate logistic regression showed that both tumor frontal lobe location (odds ratio [OR] 526.608; P = 0.045) and T1ρ value of the peritumoral edema area (OR 0.863; P = 0.037) were significant predictors of IDH1 mutation. Using the combination, the diagnostic sensitivity and specificity for IDH1 mutated gliomas were 93.3% and 88.9%, respectively.

CONCLUSIONS

Our study shows the feasibility of applying T1ρ mapping in assessing the histologic grade and IDH1 mutation status of gliomas.

Dynamics of magnetization under stimulus-induced rotary saturation sequence

We studied stimulus-induced rotary-saturation preparation (which enables measurement of oscillating magnetic fields using MRI) and derived an analytical solution of the Bloch equation to understand magnetization dynamics mathematically and comprehensively and to conduct simulations without sequential-calculation techniques such as the Runge-Kutta method. We formulated the dynamics using the Bloch equation, introducing an additional rotating frame and some approximations to make it into a homogeneous differential equation. Moreover, we found that there are two modes depending on the target oscillating magnetic field. To confirm the validity of the solution, we experimentally investigated its characteristics and performed curve fitting using the analytical model. Considering the constraints on the frame, the analytical solution was found to agree with experimental data. The experimental data indicate that it is necessary to design robust sequences compensating B₀ or B_1lock spatial inhomogeneity to improve measurements. Therefore, experimenters should consider the dynamics of magnetization with RF pulses to rewind the spin phase for accurate measurements.

Cluster analysis of quantitative MRI T2 and T1ρ relaxation times of cartilage identifies differences between healthy and ACL-injured individuals at 3T

PURPOSE

To identify focal lesions of elevated MRI T₂ and T_1ρ relaxation times in articular cartilage of an ACL-injured group using a novel cluster analysis technique.

MATERIALS AND METHODS

Eighteen ACL-injured patients underwent 3T MRI T₂ and T_1ρ relaxometry at baseline, 6 months and 1 year and six healthy volunteers at baseline, 1 day and 1 year. Clusters of contiguous pixels above or below T₂ and T_1ρ intensity and area thresholds were identified on a projection map of the 3D femoral cartilage surface. The total area of femoral cartilage plate covered by clusters (%CA) was split into areas above (%CA+) and below (%CA-) the thresholds and the differences in %CA(+ or -) over time in the ACL-injured group were determined using the Wilcoxon signed rank test.

RESULTS

%CA+ was greater in the ACL-injured patients than the healthy volunteers at 6 months and 1 year with average %CA+ of 5.2 ± 4.0% (p = 0.0054) and 6.6 ± 3.7% (p = 0.0041) for T₂ and 6.2 ± 7.1% (p = 0.063) and 8.2 ± 6.9% (p = 0.042) for T_1ρ, respectively. %CA- at 6 months and 1 year was 3.0 ± 1.8% (p > 0.1) and 5.9 ± 5.0% (p > 0.1) for T₂ and 4.4 ± 4.9% (p > 0.1) and 4.5 ± 4.6% (p > 0.1) for T_1ρ, respectively.

CONCLUSION

With the proposed cluster analysis technique, we have quantified cartilage lesion coverage and demonstrated that the ACL-injured group had greater areas of elevated T₂ and T_1ρ relaxation times as compared to healthy volunteers.

Potential of PET-MRI for imaging of non-oncologic musculoskeletal disease

Early detection of musculoskeletal disease leads to improved therapies and patient outcomes, and would benefit greatly from imaging at the cellular and molecular level. As it becomes clear that assessment of multiple tissues and functional processes are often necessary to study the complex pathogenesis of musculoskeletal disorders, the role of multi-modality molecular imaging becomes increasingly important. New positron emission tomography-magnetic resonance imaging (PET-MRI) systems offer to combine high-resolution MRI with simultaneous molecular information from PET to study the multifaceted processes involved in numerous musculoskeletal disorders. In this article, we aim to outline the potential clinical utility of hybrid PET-MRI to these non-oncologic musculoskeletal diseases. We summarize current applications of PET molecular imaging in osteoarthritis (OA), rheumatoid arthritis (RA), metabolic bone diseases and neuropathic peripheral pain. Advanced MRI approaches that reveal biochemical and functional information offer complementary assessment in soft tissues. Additionally, we discuss technical considerations for hybrid PET-MR imaging including MR attenuation correction, workflow, radiation dose, and quantification.

Quantification of oxygen metabolic rates in Human brain with dynamic 17 O MRI: Profile likelihood analysis

PURPOSE

Parameter identifiability and confidence intervals were determined using a profile likelihood (PL) analysis method in a quantification model of the cerebral metabolic rate of oxygen consumption (CMRO₂ ) with direct ¹⁷ O MRI.

METHODS

Three-dimensional dynamic ¹⁷ O MRI datasets of the human brain were acquired after inhalation of ¹⁷ O₂ gas with the help of a rebreathing system, and CMRO₂ was quantified with a pharmacokinetic model. To analyze the influence of the different model parameters on the identifiability of CMRO₂ , PLs were calculated for different settings of the model parameters. In particular, the ¹⁷ O enrichment fraction of the inhaled ¹⁷ O₂ gas, α, was investigated assuming a constant and a linearly varying model. Identifiability was analyzed for white and gray matter, and the dependency on different priors was studied.

RESULTS

Prior knowledge about only one α-related parameter was sufficient to resolve the CMRO₂ nonidentifiability, and CMRO₂ rates (0.72-0.99 µmol/g_tissue /min in white matter, 1.02-1.78 µmol/g_tissue /min in gray matter) are in a good agreement with the results of ¹⁵ O positron emission tomography studies. Nonconstant α values significantly improved model fitting.

CONCLUSION

The profile likelihood analysis shows that CMRO₂ can be measured reliably in ¹⁷ O gas MRI experiment if the ¹⁷ O enrichment fraction is used as prior information for the model calculations. Magn Reson Med 78:1157-1167, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Validation and optimization of adiabatic T1ρ and T2ρ for quantitative imaging of articular cartilage at 3 T

PURPOSE

The aim of the present work was to validate and optimize adiabatic T_1ρ and T_2ρ mapping for in vivo measurements of articular cartilage at 3 Tesla (T).

METHODS

Phantom and in vivo experiments were systematically performed on a 3T clinical system to evaluate the sequences using hyperbolic secant HS1 and HS4 pulses. R_1ρ and R_2ρ relaxation rates were studied as a function of agarose and chondroitin sulfate concentration and pulse duration. Optimal in vivo protocol was determined by imaging the articular cartilage of two volunteers and varying the sequence parameters, and successively applied in eight additional subjects. Reproducibility was assessed in phantoms and in vivo.

RESULTS

Relaxation rates depended on agarose and chondroitin sulfate concentration. The sequences were able to generate relaxation time maps with pulse lengths of 8 and 6 ms for HS1 and HS4, respectively. In vivo findings were in good agreement with the phantoms. The implemented adiabatic T_1ρ and T_2ρ sequences demonstrated regional variation in relaxation time maps of femorotibial cartilage. Reproducibility in phantoms and in vivo was good to excellent for both adiabatic T_1ρ and T_2ρ .

CONCLUSIONS

The findings indicate that sequences are suitable for quantitative in vivo assessment of articular cartilage at 3 T. Magn Reson Med 77:1265-1275, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Is T1ρ Mapping an Alternative to Delayed Gadolinium-enhanced MR Imaging of Cartilage in the Assessment of Sulphated Glycosaminoglycan Content in Human Osteoarthritic Knees? An in Vivo Validation Study

PURPOSE

To determine if T1ρ mapping can be used as an alternative to delayed gadolinium-enhanced magnetic resonance imaging of cartilage (dGEMRIC) in the quantification of cartilage biochemical composition in vivo in human knees with osteoarthritis.

MATERIALS AND METHODS

This study was approved by the institutional review board. Written informed consent was obtained from all participants. Twelve patients with knee osteoarthritis underwent dGEMRIC and T1ρ mapping at 3.0 T before undergoing total knee replacement. Outcomes of dGEMRIC and T1ρ mapping were calculated in six cartilage regions of interest. Femoral and tibial cartilages were harvested during total knee replacement. Cartilage sulphated glycosaminoglycan (sGAG) and collagen content were assessed with dimethylmethylene blue and hydroxyproline assays, respectively. A four-dimensional multivariate mixed-effects model was used to simultaneously assess the correlation between outcomes of dGEMRIC and T1ρ mapping and the sGAG and collagen content of the articular cartilage.

RESULTS

T1 relaxation times at dGEMRIC showed strong correlation with cartilage sGAG content (r = 0.73; 95% credibility interval [CI] = 0.60, 0.83) and weak correlation with cartilage collagen content (r = 0.40; 95% CI: 0.18, 0.58). T1ρ relaxation times did not correlate with cartilage sGAG content (r = 0.04; 95% CI: -0.21, 0.28) or collagen content (r = -0.05; 95% CI = -0.31, 0.20).

CONCLUSION

dGEMRIC can help accurately measure cartilage sGAG content in vivo in patients with knee osteoarthritis, whereas T1ρ mapping does not appear suitable for this purpose. Although the technique is not completely sGAG specific and requires a contrast agent, dGEMRIC is a validated and robust method for quantifying cartilage sGAG content in human osteoarthritis subjects in clinical research.

Errors in quantitative T1rho imaging and the correction methods

The spin-lattice relaxation time constant in rotating frame (T1rho) is useful for assessment of the properties of macromolecular environment inside tissue. Quantification of T1rho is found promising in various clinical applications. However, T1rho imaging is prone to image artifacts and quantification errors, which remains one of the greatest challenges to adopt this technique in routine clinical practice. The conventional continuous wave spin-lock is susceptible to B1 radiofrequency (RF) and B0 field inhomogeneity, which appears as banding artifacts in acquired images. A number of methods have been reported to modify T1rho prep RF pulse cluster to mitigate this effect. Adiabatic RF pulse can also be used for spin-lock with insensitivity to both B1 RF and B0 field inhomogeneity. Another source of quantification error in T1rho imaging is signal evolution during imaging data acquisition. Care is needed to affirm such error does not take place when specific pulse sequence is used for imaging data acquisition. Another source of T1rho quantification error is insufficient signal-to-noise ratio (SNR), which is common among various quantitative imaging approaches. Measurement of T1rho within an ROI can mitigate this issue, but at the cost of reduced resolution. Noise-corrected methods are reported to address this issue in pixel-wise quantification. For certain tissue type, T1rho quantification can be confounded by magic angle effect and the presence of multiple tissue components. Review of these confounding factors from inherent tissue properties is not included in this article.

MRI of rod cell compartment-specific function in disease and treatment in vivo

Rod cell oxidative stress is a major pathogenic factor in retinal disease, such as diabetic retinopathy (DR) and retinitis pigmentosa (RP). Personalized, non-destructive, and targeted treatment for these diseases remains elusive since current imaging methods cannot analytically measure treatment efficacy against rod cell compartment-specific oxidative stress in vivo. Over the last decade, novel MRI-based approaches that address this technology gap have been developed. This review summarizes progress in the development of MRI since 2006 that enables earlier evaluation of the impact of disease on rod cell compartment-specific function and the efficacy of anti-oxidant treatment than is currently possible with other methods. Most of the new assays of rod cell compartment-specific function are based on endogenous contrast mechanisms, and this is expected to facilitate their translation into patients with DR and RP, and other oxidative stress-based retinal diseases.

Using 7.0T MRI T2 mapping to detect early changes of the cartilage matrix caused by immobilization in a rabbit model of immobilization-induced osteoarthritis

OBJECTIVE

The goal of this study was to detect early changes in the cartilage matrix caused by immobilization in a rabbit model of immobilization-induced osteoarthritis (OA) by T2 mapping with 7.0T MRI.

MATERIALS AND METHODS

Left knee joints of 28 mature rabbits were immobilized at 180° of extension with orthopedic casting tape for 1, 2, or 3weeks (n=7 rabbits each). No immobilization was performed in the control group (n=7 rabbits). T2 mapping was performed after 1, 2, and 3weeks. Osteochondral specimens harvested from the trochlea groove (TG) and medial femoral condyle (MFC) were subjected to histologic, immunohistochemical, and microscopic evaluation, followed by biochemical assays for water, glycosaminoglycan (GAG), and collagen. The ability of T2 mapping to reveal changes in the cartilage matrix was further assessed.

RESULTS

Rabbits demonstrated elevated T2 values (9.9% in TG, 10.6% in MFC), a dulled cartilage surface, reduced Safranin-O staining, and decreased GAG content (14.2% in TG and MFC) after 2weeks, with cartilage surface softening, irregularity, and markedly reduced GAG content by 3weeks. T2 values were correlated positively with water (r=0.836 in TG, r=0.821 in MFC) and negatively with GAG content (r=-0.945 in TG, r=-0.957 in MFC), but had no discernible relationship with collagen content (r=-0.196 in TG, r=-0.213 in MFC).

CONCLUSIONS

7.0T MRI T2 mapping can be used to detect early changes of the cartilage matrix caused by immobilization in an immobilization-induced OA model.

MRI methods for the evaluation of high intensity focused ultrasound tumor treatment: Current status and future needs

Thermal ablation with high intensity focused ultrasound (HIFU) is an emerging noninvasive technique for the treatment of solid tumors. HIFU treatment of malignant tumors requires accurate treatment planning, monitoring and evaluation, which can be facilitated by performing the procedure in an MR-guided HIFU system. The MR-based evaluation of HIFU treatment is most often restricted to contrast-enhanced T1 -weighted imaging, while it has been shown that the non-perfused volume may not reflect the extent of nonviable tumor tissue after HIFU treatment. There are multiple studies in which more advanced MRI methods were assessed for their suitability for the evaluation of HIFU treatment. While several of these methods seem promising regarding their sensitivity to HIFU-induced tissue changes, there is still ample room for improvement of MRI protocols for HIFU treatment evaluation. In this review article, we describe the major acute and delayed effects of HIFU treatment. For each effect, the MRI methods that have been-or could be-used to detect the associated tissue changes are described. In addition, the potential value of multiparametric MRI for the evaluation of HIFU treatment is discussed. The review ends with a discussion on future directions for the MRI-based evaluation of HIFU treatment.

Imaging strategies for assessing cartilage composition in osteoarthritis

Efforts to reduce the ever-increasing rates of osteoarthritis (OA) in the developed world require the ability to non-invasively detect the degradation of joint tissues before advanced damage has occurred. This is particularly relevant for damage to articular cartilage because this soft tissue lacks the capacity to repair itself following major damage and is essential to proper joint function. While conventional magnetic resonance imaging (MRI) provides sufficient contrast to visualize articular cartilage morphology, more advanced imaging strategies are necessary for understanding the underlying biochemical composition of cartilage that begins to break down in the earliest stages of OA. This review discusses the biochemical basis and the advantages and disadvantages associated with each of these techniques. Recent implementations for these techniques are touched upon, and future considerations for improving the research and clinical power of these imaging technologies are also discussed.

Is the T1ρ MRI profile of hyaline cartilage in the normal hip uniform?

BACKGROUND

T1ρ MRI is an imaging technique sensitive to proteoglycan (PG) content of hyaline cartilage. However, normative T1ρ values have not been established for the weightbearing cartilage of the hip, and it is not known whether it is uniform or whether there is topographic variation. Knowledge of the T1ρ profile of hyaline cartilage in the normal hip is important for establishing a baseline against which comparisons can be made to experimental and clinical arthritic subjects.

QUESTIONS/PURPOSES

In this diagnostic study, we determined (1) the T1ρ MRI values of hyaline cartilage of the normal hip; and (2) whether the T1ρ MRI profile of the normal hip hyaline cartilage is uniform.

METHODS

Fourteen asymptomatic volunteers (11 men, three women; mean age, 35 years) prospectively underwent 1.5-T T1ρ MRI of a single hip. The weightbearing hyaline cartilage bilayer of the acetabulum and femoral head was evaluated on sagittal images and segmented into four zones: (1) anterior; (2) anterosuperior; (3) posterosuperior; and (4) and posterior. For the full region of interest and within each zone and each sagittal slice, we calculated the mean T1ρ relaxation value, a parameter that indirectly quantifies PG content, where T1ρ is inversely related to PG concentration.

RESULTS

There was variation in the T1ρ relaxation values depending on zone (anterior to posterior) and slice (medial to lateral). When combining the most anterior quadrants (Zones 1 and 2), the T1ρ relaxation values were lower than those in the combined posterior quadrants (Zones 3 and 4) (30.4 msec versus 32.2 msec, respectively; p = 0.002), reflecting higher PG concentration. There was a difference between the T1ρ relaxation values of the sagittal slices (p = 0.038), most pronounced anteriorly in Zone 1 (26.6 msec, p = 0.001). With a selective combination of zones and slices, there were lower mean T1ρ values in the anterolateral-most region compared with the remainder of the weightbearing portion of the hip (28.6 msec versus 32.2 msec, respectively; p = 0.001).

CONCLUSIONS

The T1ρ profile of normal hyaline cartilage of the hip is not uniform with the topographic differences identified suggesting regional variations in PG concentration. This study, through determination of lower T1ρ relaxation values, suggests inherently greater PG concentrations in the more anterolateral region of the normal hip hyaline cartilage. Furthermore, it demonstrates that T1ρ MRI has the ability to detect even subtle, microscopic local differences in hyaline cartilage composition. This technique has the potential to facilitate basic science and clinical research by serving as a noninvasive surrogate or biomarker of cartilage health and thus may be added to the growing repertoire of advanced, biochemical MRI techniques for evaluating hyaline cartilage.

Spin-lock MR enhances the detection sensitivity of superparamagnetic iron oxide particles

PURPOSE

To evaluate spin-lock MR for detecting superparamagnetic iron oxides and compare the detection sensitivity of quantitative T1ρ with T2 imaging.

METHODS

In vitro experiments were performed to investigate the influence of iron oxide particle size and composition on T1ρ . These comprise T1ρ and T2 measurements (B0 = 1.41T) of agar (2%) with concentration ranges of three different iron oxide nanoparticles (IONs) (Sinerem, Resovist, and ION-Micelle) and microparticles of iron oxide (MPIO). T1ρ dispersion was measured for a range of spin-lock amplitudes (γB1 = 6.5-91 kHz). Under relevant in vivo conditions (B0 = 9.4T; γB1 = 100-1500 Hz), T1ρ and T2 mapping of the liver was performed in seven mice pre- and 24 h postinjection of Sinerem.

RESULTS

Addition of iron oxide nanoparticles decreased T1ρ as well as the native T1ρ dispersion of agar, leading to increased contrast at high spin-lock amplitudes. Changes of T1ρ were highly linear with iron concentration and much larger than T2 changes. MPIO did not show this effect. In vivo, a decrease of T1ρ was observed with no clear influence on T1ρ dispersion.

CONCLUSION

By suppression of T1ρ dispersion, iron oxide nanoparticles cause enhanced T1ρ contrast compared to T2 . The underlying mechanism appears to be loss of lock. Spin-lock MR is therefore a promising technique for sensitive detection of iron oxide contrast agents.

Development of an MRI biomarker sensitive to tetrameric visual arrestin 1 and its reduction via light-evoked translocation in vivo

Rod tetrameric arrestin 1 (tet-ARR1), stored in the outer nuclear layer/inner segments in the dark, modulates photoreceptor synaptic activity; light exposure stimulates a reduction via translocation to the outer segments for terminating G-protein coupled phototransduction signaling. Here, we test the hypothesis that intraretinal spin-lattice relaxation rate in the rotating frame (1/T1ρ), an endogenous MRI contrast mechanism, has high potential for evaluating rod tet-ARR1 and its reduction via translocation. Dark- and light-exposed mice (null for the ARR1 gene, overexpressing ARR1, diabetic, or wild type with or without treatment with Mn2+, a calcium channel probe) were studied using 1/T1ρ MRI. Immunohistochemistry and single-cell recordings of the retinas were also performed. In wild-type mice with or without treatment with Mn2+, 1/T1ρ of avascular outer retina (64% to 72% depth) was significantly (P < 0.05) greater in the dark than in the light; a significant (P < 0.05) but opposite pattern was noted in the inner retina (<50% depth). Light-evoked outer retina Δ1/T1ρ was absent in ARR1-null mice and supernormal in overexpressing mice. In diabetic mice, the outer retinal Δ1/T1ρ pattern suggested normal dark-to-light tet-ARR1 translocation and chromophore content, conclusions confirmed ex vivo. Light-stimulated Δ1/T1ρ in inner retina was linked to changes in blood volume. Our data support 1/T1ρ MRI for noninvasively assessing rod tet-ARR1 and its reduction via protein translocation, which can be combined with other metrics of retinal function in vivo.

T₁ρ MRI of human musculoskeletal system

Magnetic resonance imaging (MRI) offers the direct visualization of the human musculoskeletal (MSK) system, especially all diarthrodial tissues including cartilage, bone, menisci, ligaments, tendon, hip, synovium, etc. Conventional MRI techniques based on T1 - and T2 -weighted, proton density (PD) contrast are inconclusive in quantifying early biochemically degenerative changes in MSK system in general and articular cartilage in particular. In recent years, quantitative MR parameter mapping techniques have been used to quantify the biochemical changes in articular cartilage, with a special emphasis on evaluating joint injury, cartilage degeneration, and soft tissue repair. In this article we focus on cartilage biochemical composition, basic principles of T1ρ MRI, implementation of T1ρ pulse sequences, biochemical validation, and summarize the potential applications of the T1ρ MRI technique in MSK diseases including osteoarthritis (OA), anterior cruciate ligament (ACL) injury, and knee joint repair. Finally, we also review the potential advantages, challenges, and future prospects of T1ρ MRI for widespread clinical translation.

Cartilage repair surgery: outcome evaluation by using noninvasive cartilage biomarkers based on quantitative MRI techniques?

BACKGROUND

New quantitative magnetic resonance imaging (MRI) techniques are increasingly applied as outcome measures after cartilage repair.

OBJECTIVE

To review the current literature on the use of quantitative MRI biomarkers for evaluation of cartilage repair at the knee and ankle.

METHODS

Using PubMed literature research, studies on biochemical, quantitative MR imaging of cartilage repair were identified and reviewed.

RESULTS

Quantitative MR biomarkers detect early degeneration of articular cartilage, mainly represented by an increasing water content, collagen disruption, and proteoglycan loss. Recently, feasibility of biochemical MR imaging of cartilage repair tissue and surrounding cartilage was demonstrated. Ultrastructural properties of the tissue after different repair procedures resulted in differences in imaging characteristics. T2 mapping, T1rho mapping, delayed gadolinium-enhanced MRI of cartilage (dGEMRIC), and diffusion weighted imaging (DWI) are applicable on most clinical 1.5 T and 3 T MR scanners. Currently, a standard of reference is difficult to define and knowledge is limited concerning correlation of clinical and MR findings. The lack of histological correlations complicates the identification of the exact tissue composition.

CONCLUSIONS

A multimodal approach combining several quantitative MRI techniques in addition to morphological and clinical evaluation might be promising. Further investigations are required to demonstrate the potential for outcome evaluation after cartilage repair.

Quantitative mapping of human cartilage at 3.0T: parallel changes in T₂, T₁ρ, and dGEMRIC

RATIONALE AND OBJECTIVES

The objectives of this study were to measure the parallel changes of transverse relaxation times (T₂), spin-lattice relaxation time in the rotating frame (T₁ρ), and the delayed gadolinium-enhanced magnetic resonance imaging of cartilage (dGEMRIC)-T1 mapping of human knee cartilage in detecting cartilage degeneration at 3.0T.

MATERIALS AND METHODS

Healthy volunteers (n = 10, mean age 35.6 years) and patients (n = 10, mean age 65 years) with early knee osteoarthritis (OA) were scanned at 3.0T MR using an 8-channel phased array knee coil (transmit-receive). Quantitative assessment of T₂, T₁ρ, and dGEMRIC-T₁ values (global and regional) were correlated between asymptomatic subjects and patients with OA.

RESULTS

The average T₂ (39 ± 2 milliseconds [mean ± standard deviation] vs. 47 ± 6 milliseconds, P < .0007) and T₁ρ (48 ± 3 vs. 62 ± 8 milliseconds, P < .0002) values were all markedly increased in all patients with OA when compared to healthy volunteers. The average dGEMRIC-T₁ (1244 ± 134 vs. 643 ± 227 milliseconds, P < .000002) value was sharply decreased after intravenous administration of gadolinium contrast agent in all patients with OA.

CONCLUSIONS

The research results showed that all the T₂, T₁ρ, and dGEMRIC-T₁ relaxation times varied with the cartilage degeneration. The dGEMRIC-T₁ and T₁ρ relaxation times seem to be more sensitive than T₂ in detecting early cartilage degeneration. The preliminary study demonstrated that the early biochemical changes in knee osteoarthritic patients could be detected noninvasively in in vivo using T₁ρ and dGEMRIC-T₁ mapping.

T1rho MRI of menisci in patients with osteoarthritis at 3 Tesla: a preliminary study

PURPOSE

To compare and assess subregional, compartmental, and whole T1rho values of menisci in patients with doubtful-minimal (Kellgren-Lawrence [KL] grade 1-2) as compared to moderate-severe (KL3-4) osteoarthritis (OA) and healthy controls at 3 Tesla (T).

MATERIALS AND METHODS

Forty-six subjects were included in the study and subdivided into three subgroups: 16 healthy controls (4 females, 12 males; mean age = 34.4 ± 10.2 years; age range, 24-63 years), 20 patients with doubtful-minimal (KL1-2) OA (9 females, 11 males; mean age = 61.9 ± 10.8 years; age range, 40-80 years), and 10 patients with moderate-severe (KL3-4) OA (4 females, 6 males; mean age = 71.1 ± 9.6 years; age range, 58-89 years). All subjects were evaluated on a 3T MR scanner using a spin-lock-based three-dimensional GRE sequence for T1rho mapping. Clinical proton density (PD)-weighted fast spin echoes (FSE) images in the sagittal (without fat saturation), axial, and coronal (fat-saturated) planes were acquired for cartilage Whole-Organ MR Imaging Score (WORMS) grading. Analysis of covariance was performed to determine whether there were any statistically significant differences between subregional, compartmental, and whole T1rho values of meniscus among healthy controls, OA patients with KL1-2 and with KL3-4.

RESULTS

Lateral anterior (median ± interquartile range: 26 ± 3 ms) and medial posterior (29 ± 6 ms) meniscus subregions in healthy controls had significantly lower T1rho values (P < 0.05) than the corresponding meniscus subregions in both KL1-2 (29 ± 7 ms and 35 ± 8 ms, respectively) and KL3-4 (30 ± 12 ms and 40 ± 13 ms, respectively) OA subjects. Significantly lower meniscus T1rho values (P < 0.05) were also identified in the medial compartment in healthy controls (28 ± 5 ms) relative to both KL1-2 OA subjects and KL3-4 OA subjects (32 ± 7 ms and 37 ± 7 ms, respectively). The entire meniscus T1rho values in healthy controls (28 ± 4 ms) were significantly lower than those of both KL1-2 and KL3-4 OA subjects (33 ± 6 ms and 34 ± 6 ms, respectively).

CONCLUSION

Significant elevations of T1rho values in specific regions of menisci in both KL1-2 and KL3-4 OA patients indicate that T1rho mapping may be sensitive to meniscus degeneration. The preliminary results suggest that damage in the medial posterior subregion and medial compartment of menisci may possibly be associated with osteoarthritis.

Cartilage and meniscal T2 relaxation time as non-invasive biomarker for knee osteoarthritis and cartilage repair procedures

OBJECTIVE

The purpose of this work was to review the current literature on cartilage and meniscal T2 relaxation time.

METHODS

Electronic searches in PubMed were performed to identify relevant studies about T2 relaxation time measurements as non-invasive biomarker for knee osteoarthritis (OA) and cartilage repair procedures.

RESULTS

Initial osteoarthritic changes include proteoglycan loss, deterioration of the collagen network, and increased water content within the articular cartilage and menisci. T2 relaxation time measurements are affected by these pathophysiological processes. It was demonstrated that cartilage and meniscal T2 relaxation time values were significantly increased in subjects with compared to those without radiographic OA and focal knee lesions, respectively. Subjects with OA risk factors such as overweight/obesity showed significantly greater cartilage T2 values than normal controls. Elevated cartilage and meniscal T2 relaxation times were found in subjects with vs without knee pain. Increased cartilage T2 at baseline predicted morphologic degeneration in the cartilage, meniscus, and bone marrow over 3 years. Furthermore, cartilage repair tissue could be non-invasively assessed by using T2 mapping. Reproducibility errors for T2 measurements were reported to be smaller than the T2 differences in healthy and diseased cartilage indicating that T2 relaxation time may be a reliable discriminatory biomarker.

CONCLUSIONS

Cartilage and meniscal T2 mapping may be suitable as non-invasive biomarker to diagnose early stages of knee OA and to monitor therapy of OA.

Quantitative MRI techniques of cartilage composition

Due to aging populations and increasing rates of obesity in the developed world, the prevalence of osteoarthritis (OA) is continually increasing. Decreasing the societal and patient burden of this disease motivates research in prevention, early detection of OA, and novel treatment strategies against OA. One key facet of this effort is the need to track the degradation of tissues within joints, especially cartilage. Currently, conventional imaging techniques provide accurate means to detect morphological deterioration of cartilage in the later stages of OA, but these methods are not sensitive to the subtle biochemical changes during early disease stages. Novel quantitative techniques with magnetic resonance imaging (MRI) provide direct and indirect assessments of cartilage composition, and thus allow for earlier detection and tracking of OA. This review describes the most prominent quantitative MRI techniques to date-dGEMRIC, T2 mapping, T1rho mapping, and sodium imaging. Other, less-validated methods for quantifying cartilage composition are also described-Ultrashort echo time (UTE), gagCEST, and diffusion-weighted imaging (DWI). For each technique, this article discusses the proposed biochemical correlates, as well its advantages and limitations for clinical and research use. The article concludes with a detailed discussion of how the field of quantitative MRI has progressed to provide information regarding two specific patient populations through clinical research-patients with anterior cruciate ligament rupture and patients with impingement in the hip. While quantitative imaging techniques continue to rapidly evolve, specific challenges for each technique as well as challenges to clinical applications remain.

T1ρ mapping of pediatric epiphyseal and articular cartilage in the knee

PURPOSE

To evaluate the feasibility of measuring T1ρ values in epiphyseal cartilage in children, we have conducted a novel study of spin locking techniques. Adult articular cartilage has been widely studied with spin locking techniques by magnetic resonance imaging. However, no results are available for in vivo T1ρ imaging of developing cartilage.

MATERIALS AND METHODS

Ten volunteers of age 6 ± 3 years were recruited to have T1ρ mapping performed on the knee at the conclusion of their clinical study. T1ρ maps were generated using a spin-lock cluster followed by a fast spin-echo imaging sequence. Regions of interest (ROIs) were placed in non-load-bearing (NLB), load-bearing (LB), and articular cartilage.

RESULTS

Student's t-tests were performed to compare means among the ROIs. Mean T1ρ for epiphyseal and articular cartilage was 49.8 ± 9 and 76.6 ± 7 ms, respectively. LB and NLB T1ρ vales were 47.1 ± 9.5 and 52.5 ± 9 ms, respectively. Significant differences were found in T1ρ values between epiphyseal and articular cartilage layers (P = 0.0001). No difference in T1ρ was observed between NLB and LB layers. A modest trend was also noted for epiphyseal and articular cartilage regions with age.

CONCLUSION

It is feasible to quantify differences in epiphyseal and articular cartilage layers with SL techniques. T1ρ holds promise as a noninvasive method of studying normal and abnormal developmental states of cartilage in children.

In vivo MRI of fresh stored osteochondral allograft transplantation with delayed gadolinium-enhanced MRI of cartilage: protocol considerations and recommendations

The protocol for delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) was adapted for the evaluation of transplanted osteochondral allograft cartilage. Eight patients with focal grade 4 cartilage defects of the femoral condyle were treated with single cylindrical osteochondral allografts. At 1 and 2 years, dGEMRIC image sequences were acquired and regions of interest (ROIs) were drawn in repair and native control cartilage. Mean T1 values of region of interest were used to calculate established dGEMRIC metrics. The correlation was measured between the ΔR1 and R1 -Post metrics for repair and native cartilage. T1 times were measured in deep and superficial zones of cartilage. A strong correlation was identified between full-thickness, deep, and superficial ΔR1 and R1 -Post values for native cartilage and repair cartilage for all years (range: 0.893-1.0). The mean T1 times and ΔR1 rate between deep and superficial regions of articular cartilage were statistically different for all regions of the distal femora analyzed at 1 year and 2 years after osteochondral allograft transplantation (P<0.05). The dGEMRIC pre-Gadolinium scan is unnecessary when evaluating transplanted osteochondral allograft cartilage. The observation of stratified T1 and ΔR1 values indicates a need to re-evaluate the methodology behind the placement of region of interest in dGEMRIC.

Contributions of chemical and diffusive exchange to T1ρ dispersion

Variations in local magnetic susceptibility may induce magnetic field gradients that affect the signals acquired for MR imaging. Under appropriate diffusion conditions, such fields produce effects similar to slow chemical exchange. These effects may also be found in combination with other chemical exchange processes at multiple time scales. We investigate these effects with simulations and measurements to determine their contributions to rotating frame (R1ρ ) relaxation in model systems. Simulations of diffusive and chemical exchange effects on R1ρ dispersion were performed using the Bloch equations. Additionally, R1ρ dispersion was measured in suspensions of Sephadex and latex beads with varying spin locking fields at 9.4 T. A novel analysis method was used to iteratively fit for apparent chemical and diffusive exchange rates with a model by Chopra et al. Single- and double-inflection points in R1ρ dispersion profiles were observed, respectively, in simulations of slow diffusive exchange alone and when combined with rapid chemical exchange. These simulations were consistent with measurements of R1ρ in latex bead suspensions and small-diameter Sephadex beads that showed single- and double-inflection points, respectively. These observations, along with measurements following changes in temperature and pH, are consistent with the combined effects of slow diffusion and rapid -OH exchange processes.

Observation of bi-exponential T(1ρ) relaxation of in-vivo rat muscles at 3T

BACKGROUND

Spin-lattice relaxation in the rotating frame, or T(1ρ) relaxation, is normally described by a mono-exponential decay model. However, compartmentation of tissues and microscopic molecular exchange may lead to bi-exponential or multi-exponential T(1ρ) relaxation behavior in certain tissues under the application of spin lock pulse field strength.

PURPOSE

To investigate the presence of bi-exponential T(1ρ) relaxations in in-vivo rat head tissues of brain and muscle.

MATERIAL AND METHODS

Five Sprague-Dawley rats underwent T(1ρ) imaging at 3T. A B(1)-insensitive rotary echo spin lock pulse was used for T(1ρ) preparation with a spin lock frequency of 500Hz. Twenty-five T(1ρ)-weighted images with spin lock times ranging from 1 to 60 ms were acquired using a 3D spoiled gradient echo sequence. Image intensities over different spin lock times were fitted using mono-exponential as well as bi-exponential models both on region-of-interest basis and pixel-by-pixel basis. F-test with a significance level P value of 0.01 was used to evaluate whether bi-exponential model gave a better fitting than mono-exponential model.

RESULTS

In rat brains, only mono-exponential but no apparent bi-exponential T(1ρ) relaxation (~70-71 ms) was found. In contrast, bi-exponential T(1ρ) relaxation was observed in muscles of all five rats (P < 10(-4)). A longer and a shorter T(1ρ) relaxation component were extracted to be ~37- ~41 ms (a fraction of ~80- ~88%) and ~9- ~11 ms (~12-20%), compared to the normal single T(1ρ) relaxation of ~30- ~33 ms.

CONCLUSION

Bi-exponential relaxation components were detected in rat muscles. The long and the short T(1ρ) relaxation component are thought to correspond to the restricted intracellular water population and the hydrogen exchange between amine and hydroxyl groups, respectively.

Use magnetic resonance imaging to assess articular cartilage

Magnetic resonance imaging (MRI) enables a noninvasive, three-dimensional assessment of the entire joint, simultaneously allowing the direct visualization of articular cartilage. Thus, MRI has become the imaging modality of choice in both clinical and research settings of musculoskeletal diseases, particular for osteoarthritis (OA). Although radiography, the current gold standard for the assessment of OA, has had recent significant technical advances, radiographic methods have significant limitations when used to measure disease progression. MRI allows accurate and reliable assessment of articular cartilage which is sensitive to change, providing the opportunity to better examine and understand preclinical and very subtle early abnormalities in articular cartilage, prior to the onset of radiographic disease. MRI enables quantitative (cartilage volume and thickness) and semiquantitative assessment of articular cartilage morphology, and quantitative assessment of cartilage matrix composition. Cartilage volume and defects have demonstrated adequate validity, accuracy, reliability and sensitivity to change. They are correlated to radiographic changes and clinical outcomes such as pain and joint replacement. Measures of cartilage matrix composition show promise as they seem to relate to cartilage morphology and symptoms. MRI-derived cartilage measurements provide a useful tool for exploring the effect of modifiable factors on articular cartilage prior to clinical disease and identifying the potential preventive strategies. MRI represents a useful approach to monitoring the natural history of OA and evaluating the effect of therapeutic agents. MRI assessment of articular cartilage has tremendous potential for large-scale epidemiological studies of OA progression, and for clinical trials of treatment response to disease-modifying OA drugs.

Image-based tissue engineering of a total intervertebral disc implant for restoration of function to the rat lumbar spine

Nonbiological total disc replacement is currently being used for the treatment of intervertebral disc (IVD) disease and injury, but these implants are prone to mechanical wear, tear and possible dislodgement. Recently, tissue-engineered total disc replacement (TE-TDR) has been investigated as a possible alternative to more fully replicate the native IVD properties. However, the performance of TE-TDRs has not been studied in the native disc space. In this study, MRI and microcomputed tomography imaging of the rat spine were used to design a collagen (annulus fibrosus)/alginate (nucleus pulposus) TE-TDR to a high degree of geometric accuracy, with less than 10% difference between TE-TDR and the native disc dimensions. Image-based TE-TDR implants were then inserted into the L4/L5 disc space of athymic rats (n = 5) and maintained for 16 weeks. The disc space was fully or partially maintained in three of five animals and proteoglycan and collagen histology staining was similar in composition to the native disc. In addition, good integration was observed between TE-TDR and the vertebral bodies, as well as remnant native IVD tissue. Overall, this study provides evidence that TE-TDR strategies may yield a clinically viable treatment for diseased or injured IVD.

MR imaging of articular cartilage physiology

The newer magnetic resonance (MR) imaging methods can give insights into the initiation, progression, and eventual treatment of osteoarthritis. Sodium imaging is specific for changes in proteoglycan (PG) content without the need for an exogenous contrast agent. T1ρ imaging is sensitive to early PG depletion. Delayed gadolinium-enhanced MR imaging has high resolution and sensitivity. T2 mapping is straightforward and is sensitive to changes in collagen and water content. Ultrashort echo time MR imaging examines the osteochondral junction. Magnetization transfer provides improved contrast between cartilage and fluid. Diffusion-weighted imaging may be a valuable tool in postoperative imaging.

Exchange-mediated contrast agents for spin-lock imaging

Measurements of relaxation rates in the rotating frame with spin-locking techniques are sensitive to substances with exchanging protons with appropriate chemical shifts. The authors develop a novel approach to exchange-rate selective imaging based on measured T(1ρ) dispersion with applied locking field strength, and demonstrate the method on samples containing the X-ray contrast agent Iohexol with and without cross-linked bovine serum albumin. T(1ρ) dispersion of water in the phantoms was measured with a Varian 9.4-T magnet by an on-resonance spin-locking pulse with fast spin-echo readout, and the results used to estimate exchange rates. The Iohexol phantom alone gave a fitted exchange rate of ~1 kHz, bovine serum albumin alone was ~11 kHz, and in combination gave rates in between. By using these estimated rates, we demonstrate how a novel spin-locking imaging method may be used to enhance contrast due to the presence of a contrast agent whose protons have specific exchange rates.

T1rho MRI of menisci and cartilage in patients with osteoarthritis at 3T

OBJECTIVE

To assess and compare subregional and whole T1rho values (median±interquartile range) of femorotibial cartilage and menisci in patients with doubtful (Kellgren-Lawrence (KL) grade 1) to severe (KL4) osteoarthritis (OA) at 3T.

MATERIALS AND METHODS

30 subjects with varying degrees of OA (KL1-4, 13 females, 17 males, mean age±SD=63.9±13.1 years) were evaluated on a 3T MR scanner using a spin-lock-based 3D GRE sequence for T1rho mapping. Clinical proton density (PD)-weighted fast spin echo (FSE) images in sagittal (without fat saturation), axial, and coronal (fat-saturated) planes were acquired for cartilage and meniscus Whole-organ MR imaging score (WORMS) grading. Wilcoxon rank sum test was performed to determine whether there were any statistically significant differences between subregional and whole T1rho values of femorotibial cartilage and menisci in subjects with doubtful to severe OA.

RESULTS

Lateral (72±10 ms, median±interquartile range) and medial (65±10 ms) femoral anterior cartilage subregions in moderate-severe OA subjects had significantly higher T1rho values (P<0.05) than cartilage subregions and whole femorotibial cartilage in doubtful-minimal OA subjects. There were statistically significant differences in meniscus T1rho values of the medial posterior subregion of subjects with moderate-severe OA and T1rho values of all subregions and the whole meniscus in subjects with doubtful-minimal OA. When evaluated based on WORMS, statistically significant differences were identified in T1rho values between the lateral femoral anterior cartilage subregion in patients with WORMS5-6 (advanced degeneration) and whole femorotibial cartilage and all cartilage subregions in patients with WORMS0-1 (normal).

CONCLUSION

T1rho values are higher in specific meniscus and femorotibial cartilage subregions. These findings suggest that regional damage of both femorotibial hyaline cartilage and menisci may be associated with osteoarthritis.

Contributions of chemical exchange to T1ρ dispersion in a tissue model

Variations in T(1ρ) with locking-field strength (T(1ρ) dispersion) may be used to estimate proton exchange rates. We developed a novel approach utilizing the second derivative of the dispersion curve to measure exchange in a model system of cross-linked polyacrylamide gels. These gels were varied in relative composition of comonomers, increasing stiffness, and in pH, modifying exchange rates. Magnetic resonance images were recorded with a spin-locking sequence as described by Sepponen et al. These measurements were fit to a mono-exponential decay function yielding values for T(1ρ) at each locking-field measured. These values were then fit to a model by Chopra et al. for estimating exchange rates. For low stiffness gels, the calculated exchange values increased by a factor of 4 as pH increased, consistent with chemical exchange being the dominant contributor to T(1ρ) dispersion. Interestingly, calculated chemical exchange rates also increased with stiffness, likely due to modified side-chain exchange kinetics as the composition varied. This article demonstrates a new method to assess the structural and chemical effects on T(1ρ) relaxation dispersion with a suitable model. These phenomena may be exploited in an imaging context to emphasize the presence of nuclei of specific exchange rates, rather than chemical shifts.

Cross-relaxation imaging of human articular cartilage

In this article, cross-relaxation imaging is applied to human ex vivo knee cartilage, and correlations of the cross-relaxation imaging parameters with macromolecular content in articular cartilage are reported. We show that, unlike the more commonly used magnetization transfer ratio, the bound pool fraction, the cross-relaxation rate (k) and the longitudinal relaxation time (T(1)) vary with depth and can therefore provide insight into the differences between the top and bottom layers of articular cartilage. Our cross-relaxation imaging model is more sensitive to macromolecular content in the top layers of cartilage, with bound pool fraction showing moderate correlations with proteoglycan content, and k and T(1) exhibiting moderate correlations with collagen.

Prediction of glycosaminoglycan content in human cartilage by age, T1ρ and T2 MRI

OBJECTIVE

A relationship between T1ρ relaxation time and glycosaminoglycan (GAG) content has been demonstrated in chemically degraded bovine cartilage, but has not been demonstrated with quantitative biochemistry in human cartilage. A relationship has also been established between T2 relaxation time in cartilage and osteoarthritis (OA) severity. We hypothesized that T1ρ relaxation time would be associated with GAG content in human cartilage with normal T2 relaxation times.

METHODS

T2 relaxation time, T1ρ relaxation time, and glycosaminoglycan as a percentage of wet weight (sGAG) were measured for top and bottom regions at 7 anatomical locations in 21 human cadaver patellae. For our analysis, T2 relaxation time was classified as normal or elevated based on a threshold defined by the mean plus one standard deviation of the T2 relaxation time for all samples.

RESULTS

In the normal T2 relaxation time subset, T1ρ relaxation time correlated with sGAG content in the full-thickness and bottom regions, but only marginally in the top region alone. sGAG content decreased significantly with age in all regions.

CONCLUSION

In the subset of cartilage specimens with normal T2 relaxation time, T1ρ relaxation time was inversely associated with sGAG content, as hypothesized. A predictive model, which accounts for T2 relaxation time and the effects of age, might be able to determine longitudinal trends in GAG content in the same person based on T1ρ relaxation time maps.