T1 assessment of hip joint cartilage following intra‐articular gadolinium injection: A pilot study

This pilot study defines the feasibility of cartilage assessment in symptomatic femoroacetabular impingement patients using intra‐articular delayed gadolinium‐enhanced MRI of cartilage (ia‐dGEMRIC). Nine patients were scanned preliminary to study the contrast infiltration process into hip joint cartilage. Twenty‐seven patients with symptomatic femoroacetabular impingement were subsequently scanned with intra‐articular delayed gadolinium‐enhanced MRI of cartilage. These T₁ findings were correlated to morphological findings. Zonal variations were studied. This pilot study demonstrates a significant difference between the pre‐ and postcontrast T₁ values (P < 0.001) remaining constant for 45 min. We noted higher mean T₁ values in morphologically normal‐appearing cartilage than in damaged cartilage, which was statistically significant for all zones except the anterior‐superior zone. Intraobserver (0.972) and interobserver correlation coefficients (0.933) were statistically significant. This study outlines the feasibility of intra‐articular delayed gadolinium‐enhanced MRI of cartilage for assessment of cartilage changes in patients with femoroacetabular impingement. It can also define the topographic extent and differing severities of cartilage damage. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Variable flip angle‐based fast three‐dimensional T1 mapping for delayed gadolinium‐enhanced MRI of cartilage of the knee: Need for B1 correction

Delayed gadolinium‐enhanced MRI of cartilage is a technique, which involves T₁ mapping to identify changes in the structural integrity of cartilage associated with osteoarthritis. Currently, the gold standard is 2D inversion recovery turbo spin echo, which suffers from long acquisition times and limited coverage. Three‐dimensional variable flip angle (VFA) is an alternate technique, which has been shown to be accurate when an estimate of T₁ is available a priori. This study validates the variable flip angle method for delayed gadolinium‐enhanced MRI of cartilage of the femoro‐tibial knee cartilage. When amplitude of (excitation) radiofrequency field inhomogeneities were minimized using nonselective pulses and amplitude of (excitation) radiofrequency field correction using an additional acquisition of a amplitude of (excitation) radiofrequency field map, the accuracy of T₁ measurements were improved, and slice‐to‐slice variations over the 3D volume were minimized. In conclusion, fast 3D T₁ mapping using the variable flip angle method with amplitude of (excitation) radiofrequency field correction appears to be an efficient and accurate method for delayed gadolinium‐enhanced MRI of cartilage of the knee. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

A B1‐insensitive high resolution 2D T1 mapping pulse sequence for dGEMRIC of the HIP at 3 Tesla

Early detection of cartilage degeneration in the hip may help prevent onset and progression of osteoarthritis in young patients with femoroacetabular impingement. Delayed gadolinium‐enhanced MRI of cartilage is sensitive to cartilage glycosaminoglycan loss and could serve as a diagnostic tool for early cartilage degeneration. We propose a new high resolution 2D T₁ mapping saturation–recovery pulse sequence with fast spin echo readout for delayed gadolinium‐enhanced magnetic resonance imaging of cartilage of the hip at 3 T. The proposed sequence was validated in a phantom and in 10 hips, using radial imaging planes, against a rigorous multipoint saturation–recovery pulse sequence with fast spin echo readout. T₁ measurements by the two pulse sequences were strongly correlated (R² > 0.95) and in excellent agreement (mean difference = ?8.7 ms; upper and lower 95% limits of agreement = 64.5 and ?81.9 ms, respectively). T₁ measurements were insensitive to B₁₊ variation as large as 20%, making the proposed T₁ mapping technique suitable for 3 T. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Delayed contrast‐enhanced MRI of cartilage: Comparison of nonionic and ionic contrast agents

The objective of this study was to evaluate if cartilage fixed charge density is the only factor determining the distribution of the measured delayed gadolinium‐enhanced magnetic resonance imaging of cartilage index, T₁(Gd‐DTPA^2?), across cartilage in the clinical delayed gadolinium‐enhanced magnetic resonance imaging of cartilage protocol. Nineteen subjects with osteoarthritis and 14 controls were included. Cartilage T₁(Gd) was measured following administration of 0.2 mmol kg^?1 of nonionic (Gd‐DTPA‐BMA) and, at a different date, anionic (Gd‐DTPA^2?). T₁(Gd‐DTPA‐BMA) was plotted against T₁(Gd‐DTPA^2?); a slope of 0 would indicate domination by charge effects; a nonzero slope would suggest that other factors influence T₁(Gd‐DTPA‐BMA), and hence potentially T₁(Gd‐DTPA^2?). The low slope of the curve found in osteoarthritis subjects (0.31) indicates that Gd‐DTPA‐BMA penetrated most osteoarthritis cartilage to the same extent, and T₁(Gd‐DTPA‐BMA) did not differentiate cartilages, which were differentiated by T₁(Gd‐DTPA^2?). The higher slopes in control subjects (0.88) are possibly due to inhibited transport of contrast agent into healthier cartilage, potentially exaggerated by the fast body clearance of the nonionic contrast agent. Overall, the use of anionic Gd‐DTPA^2? for delayed gadolinium‐enhanced magnetic resonance imaging of cartilage is indicated for better discrimination of the health status of cartilage. Future studies could be designed to use contrast‐enhanced dynamics to understand the transport properties of tissues in the joint and to evaluate the concentration of tissue constituents. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Further studies on the anisotropic distribution of collagen in articular cartilage by μMRI

To further study the anisotropic distribution of the collagen matrix in articular cartilage, microscopic magnetic resonance imaging experiments were carried out on articular cartilages from the central load‐bearing area of three canine humeral heads at 13 μm resolution across the depth of tissue. Quantitative T₂ images were acquired when the tissue blocks were rotated, relative to B₀, along two orthogonal directions, both perpendicular to the normal axis of the articular surface. The T₂ relaxation rate (R₂) was modeled, by three fibril structural configurations (solid cone, funnel, and fan), to represent the anisotropy of the collagen fibrils in cartilage from the articular surface to the cartilage/bone interface. A set of complex and depth‐dependent characteristics of collagen distribution was found in articular cartilage. In particular, there were two anisotropic components in the superficial zone and an asymmetrical component in the radial zone of cartilage. A complex model of the three‐dimensional fibril architecture in articular cartilage is proposed, which has a leaf‐like or layer‐like structure in the radial zone, arises in a radial manner from the subchondral bone, spreads and arches passing the isotropic transitional zone, and exhibits two distinct anisotropic components (vertical and transverse) in the surface portion of the tissue. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

In vivo transport of Gd-DTPA(2-) in human knee cartilage assessed by depth-wise dGEMRIC analysis

Purpose:

To investigate the transport of Gd‐DTPA^2? in different layers of femoral knee cartilage in vivo.

Materials and Methods:

T₁ measurements (1.5 Tesla) were performed in femoral knee cartilage of 23 healthy volunteers. The weight‐bearing central cartilage was analyzed before contrast and at eight time points after an intravenous injection of Gd‐DTPA^2?: 12–60 min (4 volunteers) and 1–4 h (19 volunteers). Three regions of interest were segmented manually: deep, middle, and superficial.

Results:

Before contrast injection, a depth‐wise variation of T₁ was observed with 50% higher values in the superficial region compared with the deep region. In the deep region, the uptake of Gd‐DTPA^2? was not detected until 36 min and the concentration increased until 240 min, whereas in the superficial region, the uptake was seen already at 12 min and the concentration decreased after 180 min (P < 0.01). There was a difference between medial and lateral compartment regarding bulk, but not superficial Gd‐DTPA^2? concentration. The bulk gadolinium concentration was negatively related to the cartilage thickness (r = ?0.68; P < 0.01).

Conclusion:

The depth‐wise and thickness dependent variations in Gd‐DTPA² transport influence the interpretation of bulk dGEMRIC analysis in vivo. In thick cartilage, incomplete penetration of Gd‐DTPA² will yield a falsely too long T₁. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.

     

Quantitative cartilage degeneration associated with spontaneous osteoarthritis in a guinea pig model

Purpose:

To determine (i) the feasibility and intra‐ and inter‐scan reproducibility of T_1ρ MRI in assessing cartilage degeneration in a guinea pig model with naturally occurring joint disease that closely mimics human osteoarthritis (OA), (ii) demonstrate the sensitivity of T_1ρ MRI in assessing the age dependent cartilage degeneration in OA progression as compared to histopathological changes.

Materials and Methods:

Duncan‐Hartley guinea pigs were obtained at various ages and maintained under an IACUC approved protocol. The left hind stifle joint was imaged using T_1ρ MRI on a 9.4 Tesla Varian horizontal 20 cm bore scanner using a custom surface coil. Reproducibility of T_1ρ MRI was assessed using 4‐month‐old guinea pigs (N = 3). Three age cohorts; 3 month (N = 8), 5 month (N = 6), and 9 month (N = 5), were used to determine the age‐dependent osteoarthritic changes as measured by T_1ρ MRI. Validation of age‐dependent cartilage degeneration was confirmed by histology and Safranin‐O staining.

Results:

T_1ρ values obtained in the cartilage of the stifle joint in guinea pigs were highly reproducible with an inter‐scan mean coefficient of variation (CV) of 6.57% and a maximum intra‐scan CV of 9.29%. Mean cartilage T_1ρ values in animals with late stage cartilage degeneration were 56.3–56.9 ms (5–9 month cohorts) were both significantly (P < 0.01) higher than that obtained from 3‐month‐old cohort (44 ms) demonstrating an age‐dependent variation. T_1ρ was shown to be significantly greater than T₂. T_1ρ dispersion was observed in this animal model for the first time showing an increase of 45% between 500 Hz and 1500 Hz spin‐locking frequency. Cartilage thickness measurements were calculated from single mid‐coronal histology sections from same animals used for T_1ρ MRI. Thickness calculations showed insignificant differences between 3‐ and 5‐month cohorts and was significantly decreased by 9 months of age (P < 0.01). A moderate correlation (R² = 0.45) existed between T_1ρ values and signal intensity of Safranin‐O stain.

Conclusion:

The data presented demonstrate that T_1ρ MRI is highly reproducible in this spontaneous model of OA and may serve as a noninvasive tool to characterize joint cartilage degeneration during OA. Age‐dependent changes, verified with histological measurements of proteoglycan loss, correlated with T_1ρ across different age groups. T_1ρ has adequate dynamic range and is sensitive to detect and track the progression of cartilage degeneration in the guinea pig model before gross anatomical changes such as cartilage thinning has occurred. This study presents a technological advancement that would permit longitudinal studies of evaluating disease‐modifying therapies useful for treating human OA. J. Magn. Reson. Imaging 2012;35:891–898. © 2011 Wiley Periodicals, Inc.     

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. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Longitudinal analysis of MRI T2 knee cartilage laminar organization in a subset of patients from the osteoarthritis initiative: A texture approach

Cartilage magnetic resonance imaging T₂ relaxation time is sensitive to hydration, collagen content, and tissue anisotropy, and a potential imaging‐based biomarker for knee osteoarthritis. This longitudinal pilot study presents an improved cartilage flattening technique that facilitates texture analysis using gray‐level co‐occurrence matrices parallel and perpendicular to the cartilage layers, and the application of this technique to the knee cartilage of 13 subjects of the osteoarthritis initiative at baseline, 1‐year follow‐up, and 2‐year follow‐up. Cartilage flattening showed minimum distortion (～ 0.5 ms) of mean T₂ values between nonflattened and flattened T₂ maps. Gray‐level co‐occurrence matrices texture analysis of flattened T₂ maps detected a cartilage laminar organization at baseline, 1‐year follow‐up, and 2‐year follow‐up by yielding significant (P < 0.05) differences between texture parameters perpendicular and parallel to the cartilage layers. Tendencies showed higher contrast, dissimilarity, angular second moment, and energy perpendicular to the cartilage layers; and higher homogeneity, entropy, variance, and correlation parallel to them. Significant (P < 0.05) longitudinal texture changes were also detected reflecting subtle signs of a laminar disruption. Tendencies showed decreasing contrast, dissimilarity, and entropy; and increasing homogeneity, energy, and correlation. Results of this study warrant further investigation to complete the assessment of the usefulness of the presented methodology in the study of knee osteoarthritis. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Delayed gadolinium‐enhanced magnetic resonance imaging (dGEMRIC) of hip joint cartilage in femoroacetabular impingement (FAI): Are pre‐ and postcontrast imaging both necessary?

The purpose of this study was to assess if delayed gadolinium MRI of cartilage using postcontrast T₁ (T_1Gd) is sufficient for evaluating cartilage damage in femoroacetabular impingement without using noncontrast values (T₁₀). T_1Gd and ΔR₁ (1/T_1Gd ? 1/T₁₀) that include noncontrast T₁ measurements were studied in two grades of osteoarthritis and in a control group of asymptomatic young‐adult volunteers. Differences between T_1Gd and ΔR₁ values for femoroacetabular impingement patients and volunteers were compared. There was a very high correlation between T_1Gd and ΔR₁ in all study groups. In the study cohort with Tonnis grade 0, correlation (r) was ?0.95 and ?0.89 with Tonnis grade 1 and ?0.88 in asymptomatic volunteers, being statistically significant (P < 0.001) for all groups. For both T_1Gd and ΔR₁, a statistically significant difference was noted between patients and control group. Significant difference was also noted for both T_1Gd and ΔR₁ between the patients with Tonnis grade 0 osteoarthritis and those with grade 1 changes. Our results prove a linear correlation between T_1Gd and ΔR₁, suggesting that T_1Gd assessment is sufficient for the clinical utility of delayed gadolinium MRI of cartilage in this setting and additional time‐consuming T₁₀ evaluation may not be needed. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Influence of knee positions on T2, T*2, and dGEMRIC mapping in porcine knee cartilage

We examined the influence of flexed knee positions on cartilage MR assessments. Sagittal T₂, T*₂, and delayed gadolinium‐enhanced MRI of cartilage (dGEMRIC) maps of the femoral cartilage were obtained in eight 6‐month‐old porcine femorotibial joints in the extended knee position (position A: flexion 0° and femoral shaft in parallel with the amplitude of static field), flexed knee position (position B: flexion 40° and femoral shaft oriented at 40° to the amplitude of static field), and oblique‐placed knee‐extended position (position C: flexion 0° and femoral shaft oriented at 40° to the amplitude of static field). Comparison of the MR parameters between positions A and C showed isolated influence of the magic‐angle effect, and comparison between positions A and B showed effects of knee flexion. Proteoglycan and hydroxyproline content in cartilage specimen at each region of interest had no significant correlation with T₂, T*₂, and dGEMRIC values. At the central zone, located on a weight‐bearing area and parallel to the amplitude of static field, T₂/T*₂/dGEMRIC values increased by 6.8/11/0.8% at position C and by 24/44/31% at position B compared with position A. There was a significant increase in T₂ and T*₂ values at position B compared with those at position A. The substantial changes in T₂, T*₂, and dGEMRIC were shown in response to knee flexion, presumably due to the compounding influence of the magic‐angle effect and change in the intra‐articular biomechanical condition. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

T2 measurement in articular cartilage: Impact of the fitting method on accuracy and precision at low SNR

T₂ relaxation time is a promising MRI parameter for the detection of cartilage degeneration in osteoarthritis. However, the accuracy and precision of the measured T₂ may be substantially impaired by the low signal‐to‐noise ratio of images available from clinical examinations. The purpose of this work was to assess the accuracy and precision of the traditional fit methods (linear least‐squares regression and nonlinear fit to an exponential) and two new noise‐corrected fit methods: fit to a noise‐corrected exponential and fit of the noise‐corrected squared signal intensity to an exponential. Accuracy and precision have been analyzed in simulations, in phantom measurements, and in seven repetitive acquisitions of the patellar cartilage in six healthy volunteers. Traditional fit methods lead to a poor accuracy for low T₂, with overestimations of the exact T₂ up to 500%. The noise‐corrected fit methods demonstrate a very good accuracy for all T₂ values and signal‐to‐noise ratio. Even more, the fit to a noise‐corrected exponential results in precisions comparable to the best achievable precisions (Cramér‐Rao lower bound). For in vivo images, the traditional fit methods considerably overestimate T₂ near the bone‐cartilage interface. Therefore, using an adequate fit method may substantially improve the sensitivity of T₂ to detect pathology in cartilage and change in T₂ follow‐up examinations. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Longitudinal analysis of MRI T2 knee cartilage laminar organization in a subset of patients from the osteoarthritis initiative

The purpose of this pilot study was to longitudinally quantify the T₂ laminar integrity of knee cartilage in a subset of subjects with osteoarthritis from the Osteoarthritis Initiative at baseline, 1‐year follow‐up, and 2‐year follow‐up. Cartilage from 13 subjects was divided into six compartments and subdivided into deep and superficial layers. At each time point, mean T₂ values in superficial and deep layers were compared. Longitudinal analysis included full‐thickness mean T₂, mean deep T₂, mean superficial T₂, mean T₂ laminar difference, mean percentage T₂ laminar difference, and two‐dimensional measures of cartilage thickness. More compartments showed significantly higher superficial T₂ than deep T₂ values at baseline and 1‐year follow‐up compared to 2‐year follow‐up. No significant longitudinal changes of full‐thickness mean T₂ and superficial T₂ values were observed. Significant longitudinal changes were observed in the deep T₂ values, T₂ laminar difference, and percentage T₂ laminar difference. Cartilage thickness had no influence on T₂ analysis. Results of this study suggest that laminar analysis may improve the sensitivity to detect longitudinal T₂ changes and that disruption of the T₂ laminar organization of knee cartilage may be present in knee osteoarthritis progressors. Further investigation is warranted to evaluate the potential of the presented methodology to better characterize evolution and pathophysiology of osteoarthritis. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Reduction of residual dipolar interaction in cartilage by spin‐lock technique

The influence of radiofrequency (RF) spin‐lock pulse on the laminar appearance of articular cartilage in MR images was investigated. Spin‐lock MRI experiments were performed on bovine cartilage plugs on a 4.7 Tesla small‐bore MRI scanner, and on human knee cartilage in vivo on a 1.5 Tesla clinical scanner. When the normal to the surface of cartilage was parallel to B₀, a typical laminar appearence was exhibited in T₂‐weighted images of cartilage plugs, but was absent in T_1ρ‐weighted images of the same plugs. At the “magic angle” orientation (when the normal to the surface of cartilage was 54.7° with respect to B₀), neither the T₂ nor the T_1ρ images demonstrated laminae. At the same time, T_1ρ values were greater than T₂ at both orientations throughout the cartilage. T_1ρ dispersion (i.e., the dependence of the relaxation rate on the spin‐lock frequency ω₁) was observed, which reached a steady‐state value of close to 2 kHz in both parallel and magic‐angle orientations. These results suggest that residual dipolar interaction from motionally‐restricted water and relaxation processes, such as chemical exchange, contribute to T_1ρ dispersion in cartilage. Further, one can reduce the laminar appearance in human articular cartilage by applying spin‐lock RF pulses, which may lead to a more accurate diagnosis of degenerative changes in cartilage. Magn Reson Med 52:1103–1109, 2004. © 2004 Wiley‐Liss, Inc.     

Sensitivity and specificity of univariate MRI analysis of experimentally degraded cartilage

MRI is increasingly used to evaluate cartilage in tissue constructs, explants, and animal and patient studies. However, while mean values of MR parameters, including T₁, T₂, magnetization transfer rate k_m, apparent diffusion coefficient (ADC), and the dGEMRIC‐derived fixed charge density, correlate with tissue status, the ability to classify tissue according to these parameters has not been explored. Therefore, the sensitivity and specificity with which each of these parameters was able to distinguish between normal and trypsin‐degraded, and between normal and collagenase‐degraded, cartilage explants were determined. Initial analysis was performed using a training set to determine simple group means to which parameters obtained from a validation set were compared. T₁ and apparent diffusion coefficient showed the greatest ability to discriminate between normal and degraded cartilage. Further analysis with k‐means clustering, which eliminates the need for a priori identification of sample status, generally performed comparably. Use of fuzzy c‐means (FCM) clustering to define centroids likewise did not result in improvement in discrimination. Finally, an FCM clustering approach in which validation samples were assigned in a probabilistic fashion to control and degraded groups was implemented, reflecting the range of tissue characteristics seen with cartilage degradation. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Feasibility of T mapping for the evaluation of hip joint cartilage at 1.5T using a three‐dimensional (3D), gradient‐echo (GRE) sequence: A prospective study

This study defines the feasibility of utilizing three‐dimensional (3D) gradient‐echo (GRE) MRI at 1.5T for T mapping to assess hip joint cartilage degenerative changes using standard morphological MR grading while comparing it to delayed gadolinium‐enhanced MRI of cartilage (dGEMRIC). MRI was obtained from 10 asymptomatic young adult volunteers and 33 patients with symptomatic femoroacetabular impingement (FAI). The protocol included T mapping without gadolinium‐enhancement utilizing a 3D‐GRE sequence with six echoes, and after gadolinium injection, routine hip sequences, and a dual‐flip‐angle 3D‐GRE sequence for dGEMRIC T₁ mapping. Cartilage was classified as normal, with mild changes, or with severe degenerative changes based on morphological MRI. T₁ and T findings were subsequently correlated. There were significant differences between volunteers and patients in normally‐rated cartilage only for T₁ values. Both T₁ and T values decreased significantly with the various grades of cartilage damage. There was a statistically significant correlation between standard MRI and T (T₁) (P < 0.05). High intraclass correlation was noted for both T₁ and T. Correlation factor was 0.860 to 0.954 (T‐T₁ intraobserver) and 0.826 to 0.867 (T‐T₁ interobserver). It is feasible to gather further information about cartilage status within the hip joint using GRE T mapping at 1.5T. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

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

Disorganization of collagen fibers is a sign of early‐stage cartilage degeneration in osteoarthritic knees. Water molecules trapped within well‐organized collagen fibrils would be sensitive to collagen alterations. Multicomponent effective transverse relaxation (T₂*) mapping with ultrashort echo time acquisitions is here proposed to probe short T₂ relaxations in those trapped water molecules. Six human tibial plateau explants were scanned on a 3T MRI scanner using a home‐developed ultrashort echo time sequence with echo times optimized via Monte Carlo simulations. Time constants and component intensities of T₂* decays were calculated at individual pixels, using the nonnegative least squares algorithm. Four T₂*‐decay types were found: 99% of cartilage pixels having mono‐, bi‐, or nonexponential decay, and 1% showing triexponential decay. Short T₂* was mainly in 1‐6 ms, while long T₂* was ～22 ms. A map of decay types presented spatial distribution of these T₂* decays. These results showed the technical feasibility of multicomponent T₂* mapping on human knee cartilage explants. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Quantitative mapping of T2 using partial spoiling

Fast quantitative MRI has become an important tool for biochemical characterization of tissue beyond conventional T₁, T₂, and T₂*‐weighted imaging. As a result, steady‐state free precession (SSFP) techniques have attracted increased interest, and several methods have been developed for rapid quantification of relaxation times using steady‐state free precession. In this work, a new and fast approach for T₂ mapping is introduced based on partial RF spoiling of nonbalanced steady‐state free precession. The new T₂ mapping technique is evaluated and optimized from simulations, and in vivo results are presented for human brain at 1.5 T and for human articular cartilage at 3.0 T. The range of T₂ for gray and white matter was from 60 msec (for the corpus callosum) to 100 msec (for cortical gray matter). For cartilage, spatial variation in T₂ was observed between deep (34 msec) and superficial (48 msec) layers, as well as between tibial (33 msec), femoral, (54 msec) and patellar (43 msec) cartilage. Excellent correspondence between T₂ values derived from partially spoiled SSFP scans and the ones found with a reference multicontrast spin‐echo technique is observed, corroborating the accuracy of the new method for proper T₂ mapping. Finally, the feasibility of a fast high‐resolution quantitative partially spoiled SSFP T₂ scan is demonstrated at 7.0 T for human patellar cartilage. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Spatial distribution and relationship of T1ρ and T2 relaxation times in knee cartilage with osteoarthritis

T_1ρ and T₂ relaxation time constants have been proposed to probe biochemical changes in osteoarthritic cartilage. This study aimed to evaluate the spatial correlation and distribution of T_1ρ and T₂ values in osteoarthritic cartilage. Ten patients with osteoarthritis (OA) and 10 controls were studied at 3T. The spatial correlation of T_1ρ and T₂ values was investigated using Z‐scores. The spatial variation of T_1ρ and T₂ values in patellar cartilage was studied in different cartilage layers. The distribution of these relaxation time constants was measured using texture analysis parameters based on gray‐level co‐occurrence matrices (GLCM). The mean Z‐scores for T_1ρ and T₂ values were significantly higher in OA patients vs. controls (P < 0.05). Regional correlation coefficients of T_1ρ and T₂ Z‐scores showed a large range in both controls and OA patients (0.2–0.7). OA patients had significantly greater GLCM contrast and entropy of T_1ρ values than controls (P < 0.05). In summary, T_1ρ and T₂ values are not only increased but are also more heterogeneous in osteoarthritic cartilage. T_1ρ and T₂ values show different spatial distributions and may provide complementary information regarding cartilage degeneration in OA. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

T1ρ MRI quantification of arthroscopically confirmed cartilage degeneration

Nine asymptomatic subjects and six patients underwent T₁ρ MRI to determine whether Outerbridge grade 1 or 2 cartilage degeneration observed during arthroscopy could be detected noninvasively. MRI was performed 2‐3 months postarthroscopy, using sagittal T₁‐weighted and axial and coronal T₁ρ MRI, from which spatial T₁ρ relaxation maps were calculated from segmented T₁‐weighted images. Median T₁ρ relaxation times of patients with arthroscopically documented cartilage degeneration and asymptomatic subjects were significantly different (P < 0.001), and median T₁ρ exceeded asymptomatic articular cartilage median T₁ρ by 2.5 to 9.2 ms. In eight observations of mild cartilage degeneration at arthroscopy (Outerbridge grades 1 and 2), mean compartment T₁ρ was elevated in five, but in all observations, large foci of increased T₁ρ were observed. It was determined that T₁ρ could detect some, but not all, Outerbridge grade 1 and 2 cartilage degeneration but that a larger patient population is needed to determine the sensitivity to these changes. Magn Reson Med 63:1376–1382, 2010. © 2010 Wiley‐Liss, Inc.