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

Accelerated 3D multi-echo spin-echo sequence with a subspace constrained reconstruction for whole mouse brain T 2 mapping

PURPOSE

To accelerate whole-brain quantitativeT 2 $$ {\mathrm{T}}_2 $$ mapping in preclinical imaging setting.

METHODS

A three-dimensional (3D) multi-echo spin echo sequence was highly undersampled with a variable density Poisson distribution to reduce the acquisition time. Advanced iterative reconstruction based on linear subspace constraints was employed to recover high-quality raw images. Different subspaces, generated using exponential or extended-phase graph (EPG) simulations or from low-resolution calibration images, were compared. The subspace dimension was investigated in terms ofT 2 $$ {\mathrm{T}}_2 $$ precision. The method was validated on a phantom containing a wide range ofT 2 $$ {\mathrm{T}}_2 $$ and was then applied to monitor metastasis growth in the mouse brain at 4.7T. Image quality andT 2 $$ {\mathrm{T}}_2 $$ estimation were assessed for 3 acceleration factors (6/8/10).

RESULTS

The EPG-based dictionary gave robust estimations of a large range ofT 2 $$ {\mathrm{T}}_2 $$ . A subspace dimension of 6 was the best compromise betweenT 2 $$ {\mathrm{T}}_2 $$ precision and image quality. Combining the subspace constrained reconstruction with a highly undersampled dataset enabled the acquisition of whole-brainT 2 $$ {\mathrm{T}}_2 $$ maps, the detection and the monitoring of metastasis growth of less than 500μ m 3 $$ \mu {\mathrm{m}}^3 $$ .

CONCLUSION

Subspace-based reconstruction is suitable for 3DT 2 $$ {\mathrm{T}}_2 $$ mapping. This method can be used to reach an acceleration factor up to 8, corresponding to an acquisition time of 25 min for an isotropic 3D acquisition of 156μ $$ \mu $$ m on the mouse brain, used here for monitoring metastases growth.

Improved quantitative parameter estimation for prostate T2 relaxometry using convolutional neural networks

OBJECTIVE

Quantitative parameter mapping conventionally relies on curve fitting techniques to estimate parameters from magnetic resonance image series. This study compares conventional curve fitting techniques to methods using neural networks (NN) for measuring T2 in the prostate.

MATERIALS AND METHODS

Large physics-based synthetic datasets simulating T2 mapping acquisitions were generated for training NNs and for quantitative performance comparisons. Four combinations of different NN architectures and training corpora were implemented and compared with four different curve fitting strategies. All methods were compared quantitatively using synthetic data with known ground truth, and further compared on in vivo test data, with and without noise augmentation, to evaluate feasibility and noise robustness.

RESULTS

In the evaluation on synthetic data, a convolutional neural network (CNN), trained in a supervised fashion using synthetic data generated from naturalistic images, showed the highest overall accuracy and precision amongst the methods. On in vivo data, this best performing method produced low-noise T2 maps and showed the least deterioration with increasing input noise levels.

DISCUSSION

This study showed that a CNN, trained with synthetic data in a supervised manner, may provide superior T2 estimation performance compared to conventional curve fitting, especially in low signal-to-noise regions.

T2 Mapping of Patellar Cartilage After a Single First-Time Episode of Traumatic Lateral Patellar Dislocation

BACKGROUND

In most cases, lateral patellar dislocation (LPD) is accompanied by chondral injury and may initiate gradual degeneration of patellar cartilage, which might be detected with a T2 mapping, a well-established method for cartilage lesions assessment.

PURPOSE

To examine short-term consequences of single first-time LPD in teenagers by T2 mapping of the patellar-cartilage state.

STUDY TYPE

Prospective.

POPULATION

95 patients (mean age: 15.1 ± 2.3; male/female: 46/49) with first-time, complete, traumatic LPD and 51 healthy controls (mean age: 14.7 ± 2.2, male/female: 29/22).

FIELD STRENGTH/SEQUENCE

3.0 T; axial T2 mapping acquired using a 2D turbo spin-echo sequence.

ASSESSMENT

MRI examination was conducted 2-4 months after first LPD. T2 values were calculated in manually segmented cartilage area via averaging over three middle level slices in six cartilage regions: deep, intermediate, superficial layers, and medial lateral parts.

STATISTICAL TESTS

ANOVA analysis with Tukey's multiple comparison test, one-vs.-rest logistic regression analysis. The threshold of significance was set at P < 0.05.

RESULTS

In lateral patellar cartilage, a significant increase in T2 values was found in deep and intermediate layers in both patient groups with mild (deep: 34.7 vs. 31.3 msec, intermediate: 38.7 vs. 34.6 msec, effect size = 0.55) and severe (34.8 vs. 31.3 msec, 39.1 vs. 34.6 msec, 0.55) LPD consequences as compared to controls. In the medial facet, only severe cartilage damage showed significant prolongation of T2 times in the deep layer (34.3 vs. 30.7 msec, 0.55). No significant changes in T2 values were found in the lateral superficial layer (P = 0.99), whereas mild chondromalacia resulted in a significant decrease of T2 in the medial superficial layer (41.0 vs. 43.8 msec, 0.55).

DATA CONCLUSION

The study revealed substantial difference in T2 changes after LPD between medial and lateral areas of patellar cartilage.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY STAGE: 2.

Improved Quantitative Parameter Estimation for Prostate T2 Relaxometry using Convolutional Neural Networks

This work seeks to evaluate multiple methods for quantitative parameter estimation from standard T2 mapping acquisitions in the prostate. The T2 estimation performance of methods based on neural networks (NN) was quantitatively compared to that of conventional curve fitting techniques. Large physics-based synthetic datasets simulating T2 mapping acquisitions were generated for training NNs and for quantitative performance comparisons. Ten combinations of different NN architectures, training strategies, and training corpora were implemented and compared with four different curve fitting strategies. All methods were compared quantitatively using synthetic data with known ground truth, and further compared on in vivo test data, with and without noise augmentation, to evaluate feasibility and noise robustness. In the evaluation on synthetic data, a convolutional neural network (CNN), trained in a supervised fashion using synthetic data generated from naturalistic images, showed the highest overall accuracy and precision amongst all the methods. On in vivo data, this best-performing method produced low-noise T2 maps and showed the least deterioration with increasing input noise levels. This study showed that a CNN, trained with synthetic data in a supervised manner, may provide superior T2 estimation performance compared to conventional curve fitting, especially in low signal-to-noise regions.

MRI-based measurement of in vivo disc mechanics in a young population due to flexion, extension, and diurnal loading

Background

Intervertebral disc degeneration is often implicated in low back pain; however, discs with structural degeneration often do not cause pain. It may be that disc mechanics can provide better diagnosis and identification of the pain source. In cadaveric testing, the degenerated disc has altered mechanics, but in vivo, disc mechanics remain unknown. To measure in vivo disc mechanics, noninvasive methods must be developed to apply and measure physiological deformations.

Aim

Thus, this study aimed to develop methods to measure disc mechanical function via noninvasive MRI during flexion and extension and after diurnal loading in a young population. This data will serve as baseline disc mechanics to later compare across ages and in patients.

Materials & Methods

To accomplish this, subjects were imaged in the morning in a reference supine position, in flexion, in extension, and at the end of the day in a supine position. Disc deformations and vertebral motions were used to quantify disc axial strain, changes in wedge angle, and anterior-posterior (A-P) shear displacement. T2 weighted MRI was also used to evaluate disc degeneration via Pfirrmann grading and T2 time. All measures were then tested for effect of sex and disc level.

Results

We found that flexion and extension caused level-dependent strains in the anterior and posterior of the disc, changes in wedge angle, and A-P shear displacements. Flexion had higher magnitude changes overall. Diurnal loading did not cause level-dependent strains but did cause small level-dependent changes in wedge angle and A-P shear displacements.

Discussion

Correlations between disc degeneration and mechanics were largest in flexion, likely due to the smaller contribution of the facet joints in this condition.

Conclusion

In summary, this study established methods to measure in vivo disc mechanical function via noninvasive MRI and established a baseline in a young population that may be compared to older subjects and clinical disorders in the future.

T2* quantification using multi-echo gradient echo sequences: a comparative study of different readout gradients

To quantify T2*, multiple echoes are typically acquired with a multi-echo gradient echo sequence using either monopolar or bipolar readout gradients. The use of bipolar readout gradients achieves a shorter echo spacing time, enabling the acquisition of a larger number of echoes in the same scan time. However, despite their relative time efficiency and the potential for more accurate quantification, a comparative investigation of these readout gradients has not yet been addressed. This work aims to compare the performance of monopolar and bipolar readout gradients for T2* quantification. The differences in readout gradients were theoretically investigated with a Cramér-Rao lower bound and validated with computer simulations with respect to the various imaging parameters (e.g., flip angle, TR, TE, TE range, and BW). The readout gradients were then compared at 3 T using phantom and in vivo experiments. The bipolar readout gradients provided higher precision than monopolar readout gradients in both computer simulations and experimental results. The difference between the two readout gradients increased for a lower SNR and smaller TE range, consistent with the prediction made using Cramér-Rao lower bound. The use of bipolar readout gradients is advantageous for regions or situations where a lower SNR is expected or a shorter acquisition time is required.

A Weighted Stochastic Conjugate Direction Algorithm for Quantitative Magnetic Resonance Images—A Pattern in Ruptured Achilles Tendon T2-Mapping Assessment

This study presents an accurate biexponential weighted stochastic conjugate direction (WSCD) method for the quantitative T2-mapping reconstruction of magnetic resonance images (MRIs), and this approach was compared with the non-negative-least-squares Gauss–Newton (GN) numerical optimization method in terms of accuracy and goodness of fit of the reconstructed images from simulated data and ruptured Achilles tendon (AT) MRIs. Reconstructions with WSCD and GN were obtained from data simulating the signal intensity from biexponential decay and from 58 MR studies of postrupture, surgically repaired ATs. Both methods were assessed in terms of accuracy (closeness of the means of calculated and true simulated T2 values) and goodness of fit (magnitude of mean squared error (MSE)). The lack of significant deviation in correct T2 values for the WSCD method was demonstrated for SNR ≥ 20 and for GN–SNR ≥ 380. The MSEs for WSCD and GN were 287.52 ± 224.11 and 2553.91 ± 1932.31, respectively. The WSCD reconstruction method was better than the GN method in terms of accuracy and goodness of fit.

Fast, Accurate, and Robust T2 Mapping of Articular Cartilage by Neural Networks

For T2 mapping, the underlying mono-exponential signal decay is traditionally quantified by non-linear Least-Squares Estimation (LSE) curve fitting, which is prone to outliers and computationally expensive. This study aimed to validate a fully connected neural network (NN) to estimate T2 relaxation times and to assess its performance versus LSE fitting methods. To this end, the NN was trained and tested in silico on a synthetic dataset of 75 million signal decays. Its quantification error was comparatively evaluated against three LSE methods, i.e., traditional methods without any modification, with an offset, and one with noise correction. Following in-situ acquisition of T2 maps in seven human cadaveric knee joint specimens at high and low signal-to-noise ratios, the NN and LSE methods were used to estimate the T2 relaxation times of the manually segmented patellofemoral cartilage. In-silico modeling at low signal-to-noise ratio indicated significantly lower quantification error for the NN (by medians of 6−33%) than for the LSE methods (p < 0.001). These results were confirmed by the in-situ measurements (medians of 10−35%). T2 quantification by the NN took only 4 s, which was faster than the LSE methods (28−43 s). In conclusion, NNs provide fast, accurate, and robust quantification of T2 relaxation times.

New setup for multi-parametric MRI in young and old rat gastrocnemius at 4.7 and 7 T during muscle stimulation

T₁ and T₂ relaxation times combined with ³¹ P spectroscopy have been proven efficient for muscular disease characterization as well as for pre- and post-muscle stimulation measurements. Even though ³¹ P spectroscopy can already be performed during muscle exercise, no method for T₁ and T₂ measurement enables this possibility. In this project, a complete setup and protocol for multi-parametrical MRI of the rat gastrocnemius before, during and after muscle stimulation at 4.7 and 7 T is presented. The setup is fully MRI compatible and is composed of a cradle, an electro-stimulator and an electronic card in order to synchronize MRI sequences with muscle stimulation. A 2D triggered radial-encoded Look-Locker sequence was developed, and enabled T₁ measurements in less than 2 min on stimulated muscle. Also, a multi-slice multi-echo sequence was adapted and synchronized for T₂ measurements as well as ³¹ P spectroscopy acquisitions in less than 4 min in both cases on stimulated muscle. Methods were validated on young rats using different stimulation paradigms. Then they were applied on older rats to compare quantification results, using the different stimulation paradigms, and allowed observation of metabolic changes related to aging with good reproducibility. The robustness of the whole setup shows wide application opportunities.

Separating spin compartments in arterial spin labeling using delays alternating with nutation for tailored excitation (DANTE) pulse: A validation study using T2 -relaxometry and application to arterial cerebral blood volume imaging

PURPOSE

To clarify the type of spin compartment in arterial spin labeling (ASL) that is eliminated by delays alternating with nutation for tailored excitation (DANTE) pulse using T₂ -relaxometry, and to demonstrate the feasibility of arterial cerebral blood volume (CBV_a ) imaging using DANTE-ASL in combination with a simplified two-compartment model.

METHOD

The DANTE and T₂ -preparation modules were combined into a single ASL sequence. T₂ values under the application of DANTE were determined to evaluate changes in T₂ , along with the post-labeling delay (PLD) and the relationship between transit time without DANTE (TT_noVS ) and T₂ . The reference tissue T₂ (T_{2_ref} ) was also obtained. Subsequently, the DANTE module was embedded into the Hadamard-encoded ASL. Cerebral blood flow (CBF) and CBV_a were computed using two Hadamard-encoding datasets (with and without DANTE) in a rest and breath-holding (BH) task.

RESULTS

While T₂ without DANTE (T_{2_noVS} ) decreased as the PLD increased, T₂ with DANTE (T_{2_DANTE} ) was equivalent to T_{2_ref} and did not change with the PLD. Although there was a significant positive correlation between TT_noVS and T_{2_noVS} with short PLD, T_{2_DANTE} was not correlated with TT_noVS nor PLD. Baseline CBV_a values obtained at rest were 0.64 ± 0.12, 0.64 ± 0.11, and 0.58 ± 0.15 mL/100 g for anterior, middle, and posterior cerebral arteries, respectively. Significant CBF and CBV_a elevations were observed in the BH task.

CONCLUSION

Microvascular compartment signals were eliminated from the total ASL signals by DANTE. CBV_a can be measured using Hadamard-encoded DANTE-ASL in combination with a simplified two-compartment model.

Compositional MRI of the Hip: Reproducibility, Effect of Joint Unloading, and Comparison of T2 Relaxometry with Delayed Gadolinium-Enhanced Magnetic Resonance Imaging of Cartilage

OBJECTIVE

Our aim was to compare T2 with delayed gadolinium-enhanced magnetic resonance imaging of cartilage (dGEMRIC) in the hip and assess the reproducibility and effect of joint unloading on T2 mapping.

DESIGN

Ten individuals at high risk of developing hip osteoarthritis (SibKids) underwent contemporaneous T2 mapping and dGEMRIC in the hip (10 hips). Twelve healthy volunteers underwent T2 mapping of both hips (24 hips) at time points 25, 35, 45, and 55 minutes post offloading. Acetabular and femoral cartilage was manually segmented into regions of interest. The relationship between T2 and dGEMRIC values from anatomically corresponding regions of interests was quantified using Pearson's correlation. The reproducibility of image analysis for T2 and dGEMRIC, and reproducibility of image acquisition for T2, was quantified using the intraclass correlation coefficient (ICC), root mean square coefficient of variance (RMSCoV), smallest detectable difference (SDD), and Bland-Altman plots. The paired t test was used to determine if difference existed in T2 values at different unloading times.

RESULTS

T2 values correlated most strongly with dGEMRIC values in diseased cartilage (r = -0.61, P = <0.001). T2 image analysis (segmentation) reproducibility was ICC = 0.96 to 0.98, RMSCoV = 3.5% to 5.2%, and SDD = 2.2 to 3.5 ms. T2 values at 25 minutes unloading were not significantly different to longer unloading times (P = 0.132). SDD for T2 image acquisition reproducibility was 7.1 to 7.4 ms.

CONCLUSIONS

T2 values in the hip correlate well with dGEMRIC in areas of cartilage damage. T2 shows high reproducibility and values do not change beyond 25 minutes of joint unloading.

Quantitative analysis of repaired rabbit supraspinatus tendons (± channeling) using magnetic resonance imaging at 7 Tesla

Background

The quantitative assessment of supraspinatus tendons by conventional magnetic resonance is limited by low contrast-to-noise ratio (CNR). Magnetic resonance imaging (MRI) scanners operating at 7 Tesla offer high signal-to noise ratio (SNR), low CNR and high spatial resolution that are well-suited for rapidly relaxing tissues like tendons. Few studies have applied T2 and T2* mapping to musculoskeletal imaging and to the rotator cuff tendons. Our objective was to analyze the T2 and T2* relaxation times from surgically repaired supraspinatus tendons and the effect of bone channeling.

Methods

One supraspinatus tendon of 112 adult female New Zealand white rabbits was surgically detached and repaired one week later. Rabbits were randomly assigned to channeling (n=64) or control (n=48) groups and harvested at 0, 1, 2, and 4 weeks. A 7T magnet was used for signal acquisition. For T2 mapping, a sagittal multi slice 2D multi-echo spin-echo (MESE) CPMG sequence with fat saturation was applied and T2* mapping was performed using a 3D UTE sequence. Magnetic resonance images from supraspinatus tendons were analyzed by two raters. Three regions of interest were manually drawn on the first T2-weighted dataset. For T2 and T2*, different ROI masks were generated to obtain relaxation times.

Results

T2-weighted maps but not T2*-weighted maps generated reliable signals for relaxation time measurement. Torn supraspinatus tendons had lower T2 than controls at the time of repair (20.0±3.4 vs. 25.6±3.9 ms; P<0.05). T2 increased at 1, 2 and 4 postoperative weeks: 22.7±3.1, 23.3±3.9 and 24.0±5.1 ms, respectively, and values were significantly different from contralateral supraspinatus tendons (24.8±3.1; 26.8±4.3 and 26.5±3.6 ms; all P<0.05). Bone channeling did not affect T2 (P>0.05).

Conclusions

Supraspinatus tendons detached for 1 week had shorter T2 relaxation time compared to contralateral as measured with 7T MRI.

Utility of Quantitative T2-Mapping Compared to Conventional and Advanced Diffusion Weighted Imaging Techniques for Multiparametric Prostate MRI in Men with Hip Prosthesis

BACKGROUND

Diffusion weighted imaging (DWI) is fundamental for prostate cancer (PCa) detection with MRI; however, limited by susceptibility artifact from hip prosthesis.

PURPOSE

To evaluate image quality and ability to detect PCa with quantitative T2-mapping and DWI in men with hip prosthesis undergoing prostate MRI.

STUDY TYPE

Prospective, cross-sectional study.

POPULATION

Thirty consecutive men with hip replacement (18 unilateral, 12 bilateral) undergoing prostate MRI from 2019 to 2021.

FIELD STRENGTH/SEQUENCE

3-T; multiparametric MRI (T2W, DCE-MRI, echo-planar [EPI]-DWI), T2-mapping (Carr-Purcell-Meiboom-Gill), FOCUS-EPI-DWI, PROPELLER-DWI.

ASSESSMENT

Five blinded radiologists independently evaluated MRI image quality using a 5-point Likert scale. PI-RADS v2.1 scores were applied in four interpretation strategies: 1) T2W-FSE+DCE-MRI+EPI-DWI, 2) T2W-FSE+DCE-MRI+EPI-DWI+FOCUS-EPI-DWI, 3) T2W-FSE+DCE-MRI+EPI-DWI+PROPELLER-DWI, 4) T2W-FSE+DCE-MRI+EPI-DWI+T2-maps. Five-point confidence scores were recorded.

STATISTICAL ANALYSIS

ANOVA, Kruskal-Wallis with pair-wise comparisons by Wilcoxon sign-rank, and paired t-tests, P < 0.05 was considered significant. Cohen's Kappa (k) for PI-RADSv2.1 scoring and proportion of correctly classified lesions tabulated for pathology-confirmed cases with 95% confidence intervals (CIs).

RESULTS

For all radiologists, T2-map image quality was significantly higher than EPI-DWI, FOCUS-EPI-DWI, and PROPELLER-DWI and similar (P = 0.146-0.706) or significantly better (for two readers) than T2W-FSE and DCE-MRI. PI-RADS v2.1 agreement improved comparing strategy A (k = 0.46) to strategy B (k = 0.58) to strategy C (k = 0.58) and was highest with strategy D which included T2-maps (k = 1.00). Radiologists' confidence was significantly highest with strategy D. Strategies B and C had similar confidence (P = 0.051-0.063) both significantly outperforming strategy A. Twelve men with 17 lesions had pathology confirmed diagnoses (13 PCa, 4 benign). Strategy D had the highest proportion of correctly classified lesions (76.5-82.4%) with overlapping 95% confidence intervals.

DATA CONCLUSION

T2-mapping may be a valuable adjunct to prostate MRI in men with hip replacement resulting in improved image quality, higher reader confidence, interobserver agreement, and accuracy in PI-RADS scoring.

LEVEL OF EVIDENCE

1 TECHNICAL EFFICACY STAGE: 2.

Anterior cruciate ligament abnormalities are associated with accelerated progression of knee joint degeneration in knees with and without structural knee joint abnormalities: 96-month data from the Osteoarthritis Initiative

OBJECTIVE

To compare progression over 8 years in knee compositional cartilage degeneration and structural joint abnormalities in knees with different types of anterior cruciate ligament (ACL) abnormalities over 8 years.

METHOD

Baseline MR images of the right knees of 1899 individuals of the Osteoarthritis Initiative (OAI) with no evidence of or mild to moderate radiographic osteoarthritis were assessed for nontraumatic ACL abnormalities. The knees of 91 individuals showed nontraumatic ACL abnormalities (age 60.6 ± 9.8 y, 46 females; mucoid degeneration (MD), N = 37; complete tear (CT), N = 22; partial tear (PT), N = 32) and were frequency-matched to 91 individuals with normal ACL. MRIs were assessed for knee joint abnormalities using the Whole-Organ Magnetic Resonance Imaging Score (WORMS) and cartilage T2 mapping at baseline, 4- and 8-year follow-up.

RESULTS

Over 8 years, cartilage T2 values of the medial tibia showed a significantly greater increase in individuals with MD, PT or CT compared to those with normal ACL (adjusted rate of change/year [95% confidence interval], normal ACL: 0.06 [0.01, 0.23], MD: 0.34 [0.07, 0.73], PT, 0.21 [0.02, 0.33], CT, 0.51 [0.16, 0.78]), indicating an association of ACL abnormalities and an increased progression rate of cartilage degeneration in subjects with and without knee joint degeneration. This effect was also seen in cartilage T2 values averaged over all compartments (normal ACL: 0.08 [0.05, 0.20] vs abnormal ACL: 0.27 [0.06, 0.56]).

CONCLUSIONS

Over 8 years, higher progression rates of cartilage degeneration, especially in the medial tibia, were associated with ACL abnormalities compared to those with normal ACL, in subjects with and without knee joint abnormalities.

What Is the Correlation Among dGEMRIC, T1p, and T2* Quantitative MRI Cartilage Mapping Techniques in Developmental Hip Dysplasia?

BACKGROUND

Delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) is a validated technique for evaluating cartilage health in developmental dysplasia of the hip (DDH), which can be a helpful prognosticator for the response to surgical treatments. dGEMRIC requires intravenous injection of gadolinium contrast, however, which adds time, expense, and possible adverse reactions to the imaging procedure. Newer MRI cartilage mapping techniques such as T1 rho (ρ) and T2* have been performed in the hip without the need for any contrast, although it is unknown whether they are equivalent to dGEMRIC.

QUESTION/PURPOSE

In this study, our purpose was to determine the correlation between the relaxation values of three cartilage mapping techniques, dGEMRIC, T1ρ, and T2*, in patients with DDH.

METHODS

Fifteen patients with DDH (three male, 12 female; mean age 29 ± 9 years) scheduled for periacetabular osteotomy underwent preoperative dGEMRIC, T1ρ, and T2* MRI at 3T with quantitative cartilage mapping. The outcomes of dGEMRIC, T1ρ, and T2* mapping were calculated for three regions of interest (ROI) to analyze the weightbearing cartilage of the hip: global ROI, anterior and posterior ROI, and further subdivided into medial, intermediate, and lateral to generate six smaller ROIs. The correlation between the respective relaxation time values was evaluated using the Spearman correlation coefficient (rS) for each ROI, categorized as negligible, weak, moderate, strong, or very strong. The relaxation values within the subdivided ROIs were compared for each of the three cartilage mapping techniques using the Kruskal-Wallis test.

RESULTS

There was a moderate correlation of T1ρ and T2* relaxation values with dGEMRIC relaxation values. For the global ROI, there was a moderate correlation between dGEMRIC and T2* (moderate; rS = 0.63; p = 0.01). For the anterior ROI, a moderate or strong correlation was found between dGEMRIC and both T1ρ and T2*: dGEMRIC and T1ρ (strong; rS = -0.71; p = 0.003) and dGEMRIC and T2* (moderate; rS = 0.69; p = 0.004). There were no correlations for the posterior ROI. The mean dGEMRIC, T1ρ, and T2* relaxation values were not different between the anterior and posterior ROIs nor between the subdivided six ROIs.

CONCLUSION

Quantitative T1ρ and T2* cartilage mapping demonstrated a moderate correlation with dGEMRIC, anteriorly and globally, respectively. However, the clinical relevance of such a correlation remains unclear. Further research investigating the correlation of these two noncontrast techniques with clinical function and outcome scores is needed before broad implementation in the preoperative investigation of DDH.

LEVEL OF EVIDENCE

Level II, diagnostic study.

Weight Cycling and Knee Joint Degeneration in Individuals with Overweight or Obesity: Four-Year Magnetic Resonance Imaging Data from the Osteoarthritis Initiative

OBJECTIVE

The aim of this study was to investigate the associations between weight cycling and knee joint degeneration in individuals with overweight or obesity with different patterns of weight change over 4 years.

METHODS

A total of 2,271 individuals from the Osteoarthritis Initiative database were assessed (case-control study). Linear regression models using annual BMI measurements over 4 years were used to classify participants as weight cyclers or noncyclers. 3-T magnetic resonance imaging was used to quantify knee cartilage transverse relaxation time (T2) and cartilage thickness annually over 4 years in all subjects. Whole-Organ Magnetic Resonance Imaging Scores (WORMS) were obtained for cartilage, meniscus, and bone-marrow abnormalities in 958 subjects at baseline and at the 4-year follow-up. The longitudinal differences in cartilage T2 and thickness between weight cyclers and noncyclers were assessed using general estimating equations, whereas the differences in WORMS outcomes were compared using general linear models.

RESULTS

No significant differences in the rate of change of cartilage thickness or T2 were found between weight cyclers and noncyclers. However, increases in maximum cartilage WORMS (P = 0.0025) and bone-marrow abnormalities (P = 0.04) were significantly greater in weight cyclers than in noncyclers.

CONCLUSIONS

Although participants' intent for weight cycling in this study was unknown, weight cyclers had significantly greater increases in cartilage and bone-marrow abnormalities over 4 years than noncyclers, independent of weight gain and loss.

Longitudinal Femoral Cartilage T2 Relaxation Time and Thickness Changes with Fast Sequential Radiographic Progression of Medial Knee Osteoarthritis-Data from the Osteoarthritis Initiative (OAI)

This study tested for longitudinal changes in femoral cartilage T2 relaxation time and thickness in fast-progressing medial femorotibial osteoarthritis (OA). From the Osteoarthritis Initiative (OAI) database, nineteen knees fulfilled the inclusion criteria, which included medial femorotibial OA and sequential progression from Kellgren–Lawrence grade (KL) 1 to KL2 to KL3 within five years. Median T2 value and mean thickness were calculated for six condylar volumes of interest (VOIs; medial/lateral anterior, central, posterior) and six sub-VOIs (medial/lateral anterior external, central, internal). T2 value and thickness changes between severity timepoints were tested using repeated statistics. T2 values increased between KL1 and KL2 and between KL1 and KL3 in the medial compartment (p ≤ 0.02), whereas both increases and decreases were observed between the same timepoints in the lateral compartment (p ≤ 0.02). Cartilage thickness decreased in VOI/subVOIs of the medial compartment from KL1 to KL2 and KL3 (p ≤ 0.014). Cartilage T2 value and thickness changes varied spatially over the femoral condyles. While all T2 changes occurred in the early radiographic stages of OA, thickness changes occurred primarily in the later stages. These data therefore support the use of T2 relaxation time analyses in methods of detecting disease-related change during early OA, a valuable period for therapeutic interventions.

Quantitative T2 -mapping magnetic resonance imaging for assessment of muscle motor unit recruitment patterns

INTRODUCTION

In this study, we aimed to determine whether muscle transverse relaxation time (T₂ ) magnetic resonance (MR) mapping results correlate with motor unit loss, as defined by motor unit recruitment patterns on electromyography (EMG).

METHODS

EMG and 3-Tesla MRI exams were acquired no more than 31 days apart in subjects referred for peripheral nerve MRI. Two musculoskeletal radiologists qualitatively graded T₂ -weighted, fat-suppressed sequences for severity of muscle edema-like patterns and manually placed regions of interest within muscles to obtain T₂ values from T₂ -mapping sequences. Concordance was calculated between qualitative and quantitative MR grades and EMG recruitment categories (none, discrete, decreased) as well as interobserver agreement for both MR grades.

RESULTS

Thirty-four muscles (21 abnormal, 13 control) were assessed in 13 subjects (5 females and 8 males; mean age, 46 years) with 14 EMG-MRI pairs. T₂ -relaxation times were significantly (P < .001) increased in all EMG recruitment categories compared with control muscles. T₂ differences were not significant between EMG grades of motor unit recruitment (P = .151-.702). T₂ and EMG score concordance was acceptable (Harrell's concordance index [c index]: rater A, 0.71; 95% confidence interval [CI], 0.51-0.87; rater B, 0.77; 95% CI, 0.57-0.91). Qualitative MRI and EMG score concordance was poor to acceptable (c index: rater A, 0.60; 95% CI, 0.50-0.79; rater B, 0.72; 95% CI, 0.55-0.89). T₂ values had moderate-to-substantial ability to distinguish between absent vs incomplete (ie, decreased or discrete) motor unit recruitment (c index: rater A, 0.78; 95% CI, 0.50-1.00; rater B, 0.86; 95% CI, 0.57-1.00).

DISCUSSION

Quantitative T₂ MR muscle mapping is a promising tool for noninvasive evaluation of the degree of motor unit recruitment loss.

Transection of the medial meniscus anterior horn results in cartilage degeneration and meniscus remodeling in a large animal model

The meniscus plays a central load-bearing role in the knee joint. Unfortunately, meniscus injury is common and can lead to joint degeneration and osteoarthritis (OA). In small animal models, progressive degenerative changes occur with the unloading of the meniscus via destabilization of the medial meniscus (DMM). However, few large animal models of DMM exist and the joint-wide initiation of the disease has not yet been defined in these models. Thus, the goal of this study is to develop and validate a large animal model of surgically induced DMM and to use multimodal (mechanical, histological, and magnetic resonance imaging) and multiscale (joint to tissue level) quantitative measures to evaluate degeneration in both the meniscus and cartilage. DMM was achieved using an arthroscopic approach in 13 Yucatan minipigs. One month after DMM, joint contact area decreased and peak pressure increased, indicating altered load transmission as a result of meniscus destabilization. By 3 months, the joint had adapted to the injury and load transmission patterns were restored to baseline, likely due to the formation and maturation of a fibrovascular scar at the anterior aspect of the meniscus. Despite this, we found a decrease in the indentation modulus of the tibial cartilage and an increase in cartilage histopathology scores at 1 month compared to sham-operated animals; these deleterious changes persisted through 3 months. Over this same time course, meniscus remodeling was evident through decreased proteoglycan staining in DMM compared to sham menisci at both 1 and 3 months. These findings support that arthroscopic DMM results in joint degeneration in the Yucatan minipig and provide a new large animal testbed in which to evaluate therapeutics and interventions to treat post-traumatic OA that originates from a meniscal injury.

Multi-vendor multi-site T1ρ and T2 quantification of knee cartilage

OBJECTIVE

To develop 3D T_1ρ and T₂ imaging based on the same sequence structure on MR systems from multiple vendors, and to evaluate intra-site repeatability and inter-site inter-vendor reproducibility of T_1ρ and T₂ measurements of knee cartilage.

METHODS

3D magnetization-prepared angle-modulated partitioned k-space spoiled gradient echo snapshots (3D MAPSS) were implemented on MR systems from Siemens, GE and Philips. Phantom and human subject data were collected at four sites using 3T MR systems from the three vendors with harmonized protocols. Phantom data were collected by means of different positioning of the coil. Volunteers were scanned and rescanned after repositioning. Two traveling volunteers were scanned at all sites. Data were transferred to one site for centralized processing.

RESULTS

Intra-site average coefficient of variations (CVs) ranged from 1.09% to 3.05% for T_1ρ and 1.78-3.30% for T₂ in phantoms, and 1.60-3.93% for T_1ρ and 1.44-4.08% for T₂ in volunteers. Inter-site average CVs were 5.23% and 6.45% for MAPSS T_1ρ and T₂, respectively in phantoms, and 8.14% and 10.06% for MAPSS T_1ρ and T₂, respectively, In volunteers.

CONCLUSION

This study showed promising results of multi-site, multi-vendor reproducibility of T_1ρ and T₂ values in knee cartilage. These quantitative measures may be applied in large-scale multi-site, multi-vendor trials with controlled sequence structure and scan parameters and centralized data processing.

Rapid mono and biexponential 3D-T1ρ mapping of knee cartilage using variational networks

In this study we use undersampled MRI acquisition methods to obtain accelerated 3D mono and biexponential spin-lattice relaxation time in the rotating frame (T1ρ) mapping of knee cartilage, reducing the usual long scan time. We compare the accelerated T1ρ maps obtained by deep learning-based variational network (VN) and compressed sensing (CS). Both methods were compared with spatial (S) and spatio-temporal (ST) filters. Complex-valued fitting was used for T1ρ parameters estimation. We tested with seven in vivo and six synthetic datasets, with acceleration factors (AF) from 2 to 10. Median normalized absolute deviation (MNAD), analysis of variance (ANOVA), and coefficient of variation (CV) were used for analysis. The methods CS-ST, VN-S, and VN-ST performed well for accelerating monoexponential T1ρ mapping, with MNAD around 5% for AF = 2, which increases almost linearly with the AF to an MNAD of 13% for AF = 8, with all methods. For biexponential mapping, the VN-ST was the best method starting with MNAD of 7.4% for AF = 2 and reaching MNAD of 13.1% for AF = 8. The VN was able to produce 3D-T1ρ mapping of knee cartilage with lower error than CS. The best results were obtained by VN-ST, improving CS-ST method by nearly 7.5%.

Rapid Knee MRI Acquisition and Analysis Techniques for Imaging Osteoarthritis

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

Associations Between Vitamins C and D Intake and Cartilage Composition and Knee Joint Morphology Over 4 Years: Data From the Osteoarthritis Initiative

OBJECTIVE

To determine the cross-sectional and longitudinal associations of vitamin C and D intake with magnetic resonance imaging (MRI) measures of cartilage composition (T2) and joint structure (cartilage, meniscus, and bone marrow) using data from the Osteoarthritis Initiative (OAI) cohort.

METHODS

A total of 1,785 subjects with radiographic Kellgren/Lawrence knee grades 0-3 in the right knee were selected from the OAI database. Vitamins C and vitamin D intake (diet, supplements, and total) were assessed using the Block Brief 2000 Food Frequency Questionnaire at baseline. The MRI analysis protocol included 3T cartilage T2 quantification and semiquantitative joint morphology gradings (Whole-Organ Magnetic Resonance Imaging Score [WORMS]) at baseline and 4 years. Linear regression was used to assess the association between standardized baseline vitamin intake and both baseline WORMS scores and standardized cartilage T2 values.

RESULTS

Higher vitamin C intake was associated with lower average cartilage T2 values, medial tibia T2 values, and medial tibia WORMS scores (standardized coefficient range -0.07 to -0.05, P < 0.05). Higher vitamin D intake was associated with a lower cartilage WORMS sum score and medial femur WORMS score (standardized coefficient range -0.24 to -0.09, P < 0.05). Consistent use of vitamin D supplements of 400 IU at least once a week over 4 years was associated with significantly less worsening of cartilage, meniscus, and bone marrow abnormalities (odds ratio range 0.40-0.56, P < 0.05).

CONCLUSION

Supplementation with vitamin D over 4 years was associated with significantly less progression of knee joint abnormalities. Given the observational nature of this study, future longitudinal randomized controlled trials of vitamin D supplementation are warranted.

Association of blood pressure with knee cartilage composition and structural knee abnormalities: data from the osteoarthritis initiative

OBJECTIVE

To investigate the associations of systolic blood pressure (SBP) and diastolic blood pressure (DBP) with changes in knee cartilage composition and joint structure over 48 months, using magnetic resonance imaging (MRI) data from the Osteoarthritis Initiative (OAI).

MATERIALS AND METHODS

A total of 1126 participants with right knee Kellgren-Lawrence (KL) score 0-2 at baseline, no history of rheumatoid arthritis, blood pressure measurements at baseline, and cartilage T2 measurements at baseline and 48 months were selected from the OAI. Cartilage composition was assessed using MRI T2 measurements, including laminar and gray-level co-occurrence matrix texture analyses. Structural knee abnormalities were graded using the whole-organ magnetic resonance imaging score (WORMS). We performed linear regression, adjusting for age, sex, body mass index, physical activity, smoking status, alcohol use, KL score, number of anti-hypertensive medications, and number of nonsteroidal anti-inflammatory drugs.

RESULTS

Higher baseline DBP was associated with greater increases in global T2 (coefficient 0.22 (95% CI 0.09, 0.34), P = 0.004), global superficial layer T2 (coefficient 0.39 (95% CI 0.20, 0.58), P = 0.001), global contrast (coefficient 15.67 (95% CI 8.81, 22.53), P < 0.001), global entropy (coefficient 0.02 (95% CI 0.01, 0.03) P = 0.011), and global variance (coefficient 9.14 (95% CI 5.18, 13.09), P < 0.001). Compared with DBP, the associations of SBP with change in cartilage T2 parameters and WORMS subscores showed estimates of smaller magnitude.

CONCLUSION

Higher baseline DBP was associated with higher and more heterogenous cartilage T2 values over 48 months, indicating increased cartilage matrix degenerative changes.

Impact of pulse sequence, analysis method, and signal to noise ratio on the accuracy of intervertebral disc T 2 measurement

Noninvasive assessments of intervertebral disc health and degeneration are critical for addressing disc degeneration and low back pain. Magnetic resonance imaging (MRI) is exceptionally sensitive to tissue with high water content, and measurement of the MR transverse relaxation time, T 2, has been applied as a quantitative, continuous, and objective measure of disc degeneration that is linked to the water and matrix composition of the disc. However, T 2 measurement is susceptible to inaccuracies due to Rician noise, T 1 contamination, and stimulated echo effects. These error generators can all be controlled for with proper data collection and fitting methods. The objective of this study was to identify sequence parameters to appropriately acquire MR data and to establish curve fitting methods to accurately calculate disc T 2 in the presence of noise by correcting for Rician noise. To do so, we compared T 2 calculated from the typical monoexponential (MONO) fits and noise corrected exponential (NCEXP) fits. We examined how the selected sequence parameters altered the calculated T 2 in silico and in vivo. Typical MONO fits were frequently poor due to Rician noise, and NCEXP fits were more likely to provide accurate T 2 calculations. NCEXP is particularly less biased and less uncertain at low SNR. This study showed that the NCEXP using sequences with data from 20 echoes out to echo times of ~300 ms is the best method for calculating T 2 of discs. By acquiring signal data out to longer echo times and accounting for Rician noise, the curve fitting is more robust in calculating T 2 despite the noise in the data. This is particularly important when considering degenerate discs or AF tissue because the SNR of these regions is lower.

Identification of the most important features of knee osteoarthritis structural progressors using machine learning methods

Objectives:

The aim was to identify the most important features of structural knee osteoarthritis (OA) progressors and classification using machine learning methods.

Methods:

Participants, features and outcomes were from the Osteoarthritis Initiative. Features were from baseline (1107), including articular knee tissues (135) assessed by quantitative magnetic resonance imaging (MRI). OA progressors were ascertained by four outcomes: cartilage volume loss in medial plateau at 48 and 96 months (Prop_CV_48M, 96M), Kellgren–Lawrence (KL) grade ⩾ 2 and medial joint space narrowing (JSN) ⩾ 1 at 48 months. Six feature selection models were used to identify the common features in each outcome. Six classification methods were applied to measure the accuracy of the selected features in classifying the subjects into progressors and non-progressors. Classification of the best features was done using an automatic machine learning interface and the area under the curve (AUC). To prioritize the top five features, sparse partial least square (sPLS) method was used.

Results:

For the classification of the best common features in each outcome, Multi-Layer Perceptron (MLP) achieved the highest AUC in Prop_CV_96M, KL and JSN (0.80, 0.88, 0.95), and Gradient Boosting Machine for Prop_CV_48M (0.70). sPLS showed the baseline top five features to predict knee OA progressors are the joint space width, mean cartilage thickness of the medial tibial plateau and sub-regions and JSN.

Conclusion:

In this comprehensive study using a large number of features (n = 1107) and MRI outcomes in addition to radiological outcomes, we identified the best features and classification methods for knee OA structural progressors. Data revealed baseline X-ray and MRI-based features could predict early OA knee progressors and that MLP is the best classification method.

Cartilage degeneration post-meniscectomy performed for degenerative disease versus trauma: data from the Osteoarthritis Initiative

OBJECTIVE

To compare the extent of cartilage deterioration in knees with prior meniscal resection related to trauma versus knees with resection related to degenerative disease, and to compare cartilage deterioration in knees with meniscal surgery to knees without meniscal surgery, controlling for prior knee trauma.

MATERIALS AND METHODS

In this cross-sectional study, we assessed cartilage deterioration in right knees of Osteoarthritis Initiative participants: (i) with meniscal surgery due to injury (n = 79); (ii) matched control knees with a prior injury but without meniscal surgery (n = 79); (iii) with meniscal surgery but without preceding injury (n = 36); and (iv) matched control knees without meniscal surgery or prior knee injury (n = 36). Cartilage composition was measured using T2 measurements derived using semi-automatic cartilage segmentation of the right. Linear regression analysis was used to compare compartmental values of T2 between groups.

RESULTS

Comparing the mean T2 values in surgical cases with and without injury our results did not show significant differences (group i vs. iii, p > 0.05). However, knees with previous meniscal surgery showed significantly (p < 0.001) higher mean T2 values across all compartments (i.e., global T2) when compared to those without meniscal surgery for both knees with a history of trauma (group i vs. ii) and knees without prior trauma (group iii vs. iv). Similar results were obtained when analyzing the compartments separately.

CONCLUSIONS

Cartilage deterioration, assessed by T2, is similar in knees undergoing meniscal surgery after trauma and for degenerative conditions. Both groups demonstrated greater cartilage deterioration than nonsurgical knees, controlling for prior knee injury.

Accelerated mono- and biexponential 3D-T1ρ relaxation mapping of knee cartilage using golden angle radial acquisitions and compressed sensing

PURPOSE

To use golden-angle radial sampling and compressed sensing (CS) for accelerating mono- and biexponential 3D spin-lattice relaxation time in the rotating frame (T_1ρ ) mapping of knee cartilage.

METHODS

Golden-angle radial stack-of-stars and Cartesian 3D-T_1ρ -weighted knee cartilage datasets (n = 12) were retrospectively undersampled by acceleration factors (AFs) 2-10. CS-based reconstruction using 8 different sparsifying transforms were compared for mono- and biexponential T_1ρ -mapping of knee cartilage, including spatio-temporal finite differences, wavelets, dictionary from principal component analysis, and exponential decay models, and also low rank and low rank plus sparse models (L+S). Complex-valued fitting was used and Marchenko-Pastur principal component analysis filtering also tested.

RESULTS

Most CS methods performed well for an AF of 2, with relative median normalized absolute deviation below 10% for monoexponential and biexponential mapping. For monoexponential mapping, radial sampling obtained a median normalized absolute deviation below 10% up to AF of 10, while Cartesian obtained this level of error only up to AF of 4. Radial sampling was also better with biexponential T_1ρ mapping, with median normalized absolute deviation below 10% up to AF of 6.

CONCLUSION

Golden-angle radial acquisitions combined with CS outperformed Cartesian acquisitions for 3D-T_1ρ mapping of knee cartilage, being it is a good alternative to Cartesian sampling for reducing scan time and/or improving image and mapping quality. The methods exponential decay models, spatio-temporal finite differences, and low rank obtained the best results for radial sampling patterns.

Single- and Bicomponent Analyses of T2⁎ Relaxation in Knee Tendon and Ligament by Using 3D Ultrashort Echo Time Cones (UTE Cones) Magnetic Resonance Imaging

The collagen density is not detected in the patellar tendon (PT), posterior cruciate ligament (PCL), and anterior cruciate ligament (ACL) in clinic. We assess the technical feasibility of three-dimension multiecho fat saturated ultrashort echo time cones (3D FS-UTE-Cones) acquisitions for single- and bicomponent T2⁎ analysis of bound and free water pools in PT, PCL, and ACL in clinic. The knees of five healthy volunteers and six knee joint samples from cadavers were scanned via 3D multiecho FS-UTE-Cones acquisitions on a clinical scanner. Single-component fitting of T2⁎_M and bicomponent fitting of short T2⁎ (T2⁎_S), long T2⁎ (T2⁎_L), short T2⁎ fraction (Frac__S), and long T2⁎ fraction (Frac__L) were performed within tendons and ligaments. Our results showed that biexponential fitting was superior to single-exponential fitting in PT, PCL, and ACL. For knee joint samples, there was no statistical difference among all data in PT, PCL, and ACL. For volunteers, all parameters of bicomponent fitting were statistically different across PT, PCL, and ACL, except for T2⁎_S, T2⁎_L, and T2⁎_M resulting in flawed measurements due to the magic angle effect. 3D multiecho FS-UTE-Cones acquisition allows high resolution T2⁎ mapping in PT, PCL, and ACL of keen joint samples and PT and PCL of volunteers. The T2⁎ values and their fractions can be characterized by bicomponent T2⁎ analysis that is superior to single-component T2⁎ analysis, except for ACL of volunteers.

Multicenter validation of the magnetic resonance T2* technique for quantification of pancreatic iron

OBJECTIVES

To assess the transferability of the magnetic resonance imaging (MRI) multislice multiecho T2* technique for pancreatic iron overload assessment.

METHODS

Multiecho T2* sequences were installed on ten 1.5-T MRI scanners of the three main vendors. Five healthy subjects (n = 50) were scanned at each site. Five patients with thalassemia (n = 45) were scanned locally at each site and were rescanned at the reference site within 1 month. T2* images were analyzed using a previously validated software and the global pancreatic T2* value was calculated as the mean of T2* values over the head, body, and tail.

RESULTS

T2* values of healthy subjects were above 26 ms and showed inter-site homogeneity. The T2* values measured in the MRI sites were comparable to the correspondent values observed in the reference site (12.02 ± 10.20 ms vs 11.98 ± 10.47 ms; p = 0.808), and the correlation coefficient was 0.978 (p < 0.0001). Coefficients of variation (CoVs) ranged from 4.22 to 9.77%, and the CoV for all the T2* values independently from the sites was 8.55%. The intraclass correlation coefficient (ICC) for each MRI site was always excellent and the global ICC was 0.995, independently from the sites. The mean absolute difference in patients with pancreatic iron (n = 39) was -0.15 ± 1.38 ms.

CONCLUSION

The gradient-echo T2* MRI technique is an accurate and reproducible means for the quantification of pancreatic iron and may be transferred among MRI scanners by different vendors in several centers.

KEY POINTS

• The gradient-echo T2* MRI technique is an accurate and reproducible means for the quantification of pancreatic iron. • The gradient-echo T2* MRI technique for the quantification of pancreatic iron may be transferred among MRI scanners by different vendors in several centers. • Pancreatic iron might serve as an early predictor of cardiac siderosis and is the strongest overall predictor of glucose dysregulation.

Associations between molecular biomarkers and MR-based cartilage composition and knee joint morphology: data from the Osteoarthritis Initiative

OBJECTIVE

The purpose of this study was to assess the associations between serum/urine biomarkers for osteoarthritis and magnetic resonance (MR) imaging measures of cartilage composition and joint structure (cartilage, meniscus, and bone marrow), using MR imaging data from the Osteoarthritis Initiative (OAI).

DESIGN

141 subjects with Kellgren Lawrence (KL) grades 0-3 in the right knee and with available serum/urine biomarker assays were selected from the OAI. Cartilage magnetic resonance imaging (MRI) T2 measurements were performed in the medial femur, lateral femur, medial tibia, lateral tibia, and patella compartments. Compartment-specific knee morphologic grading [whole-organ magnetic resonance imaging score (WORMS)] in the cartilage, meniscus, and bone marrow was also performed. We focused on associations of serum hyaluronan (sHA), serum cartilage oligomeric matrix protein (sCOMP), serum matrix metalloproteinase-3 (sMMP3), and Urine Carboxy-Terminal Telepeptides of Type II Collagen (uCtX-II)) with MRI parameters (T2, WORMS), assessed using partial correlations adjusted for age, gender, body mass index (BMI), KL grade in both knees, and diabetes status.

RESULTS

Higher levels of sHA, sMMP3 and sCOMP were correlated (P < 0.05) with T2 of the lateral femur (r = 0.18 to 0.32) and lateral tibia (r = 0.17 to 0.23), and with average T2 of all knee regions (r = 0.23). uCTXII was correlated with patellar T2 (r = 0.19, P = 0.04). Among the morphologic measures, sHA and sMMP3 was positively correlated (r = 0.17 to 0.21, P < 0.05) with meniscal damage.

CONCLUSIONS

This study suggests weak, but statistically significant, correlations between serum biomarkers of OA (sHA, sCOMP, and sMMP3) and MRI T2 measures of cartilage extra-cellular matrix degeneration.

High-Resolution MR Imaging of Muscular Fat Fraction-Comparison of Three T2-Based Methods and Chemical Shift-Encoded Imaging

Chemical shift-encoded imaging (CSEI) is the most common magnetic resonance imaging fat–water separation method. However, when high spatial resolution fat fraction (FF) images are desired, CSEI might be challenging owing to the increased interecho spacing. Here, 3 T₂-based methods have been assessed as alternative methods for obtaining high-resolution FF images. Images from the calf of 10 healthy volunteers were acquired; FF maps were then estimated using 3 T₂-based methods (2- and 3-parameter nonlinear least squares fit and a Bayesian probability method) and CSEI for reference. In addition, simulations were conducted to characterize the performance of various methods. Here, all T₂-based methods resulted in qualitatively improved high-resolution FF images compared with high-resolution CSEI. The 2-parameter fit showed best quantitative agreement to low-resolution CSEI, even at low FF. The estimated T₂-values of fat and water, and the estimated muscle FF of the calf, agreed well with previously published data. In conclusion, T₂-based methods can provide improved high-resolution FF images of the calf compared with the CSEI method.

Type 2 diabetes patients have accelerated cartilage matrix degeneration compared to diabetes free controls: data from the Osteoarthritis Initiative

PURPOSE

Osteoarthritis (OA) and diabetes mellitus (DM) share common risk factors with a potential underlying relationship between both diseases. The purpose of this study was to investigate the longitudinal effects of DM on cartilage deterioration over 24-months with MR-based T2 relaxation time measurements.

METHODS

From the Osteoarthritis Initiative (OAI) cohort 196 diabetics were matched in small sets for age, sex, BMI and Kellgren-Lawrence score with 196 non-diabetic controls. Knee cartilage semi-automatic segmentation was performed on 2D multi-slice multi-echo spin-echo sequences. Texture of cartilage T2 maps was obtained via grey level co-occurrence matrix analysis. Linear regression analysis was used to compare cross-sectional and changes in T2 and texture parameters between the groups.

RESULTS

Both study groups were similar in age (63.3 vs 63.0 years, P = 0.70), BMI (30.9 vs 31.2 kg/m2, P = 0.52), sex (female 53.6% vs 54.1%, P = 0.92) and KL score distribution (P = 0.97). In diabetics, except for the patella, all compartments showed a significantly higher increase in mean T2 values when compared to non-diabetic controls. Global T2 values increased almost twice as much; 1.77ms vs 0.98ms (0.79ms [CI: 0.39,1.19]) (P < 0.001). Additionally, global T2 values showed a significantly higher increase in the bone layer (P = 0.006), and in a separate analysis of the texture parameters, diabetics also showed consistently higher texture values (P < 0.05), indicating a more disordered cartilage composition.

CONCLUSION

Cartilage T2 values in diabetics show a faster increase with a consistently more heterogeneous cartilage texture composition. DM seems to be a risk factor for developing early OA with an accelerated degeneration of the articular cartilage in the knee.

MRI T2 Mapping of Knee Articular Cartilage Using Different Acquisition Sequences and Calculation Methods at 1.5 Tesla

OBJECTIVE

This study aims to determine how magnetic resonance imaging (MRI) acquisition techniques and calculation methods affect T2 values of knee cartilage at 1.5 tesla and to identify sequences that can be used for high-resolution T2 mapping in short scanning times.

MATERIALS AND METHODS

This study was performed on phantom and 29 patients who underwent MRI of the knee joint at 1.5 tesla. The protocol includes T2 mapping sequences based on Single-Echo Spin Echo (SESE), Multi-Echo Spin Echo (MESE), Fast Spin Echo (FSE) and Turbo Gradient Spin Echo (TGSE). The T2 relaxation times were quantified and evaluated using three calculation methods (MapIt, Syngo Offline and mono-exponential fit). signal-to-noise ratios (SNR) were measured in all sequences. All statistical analyses were performed using the t-test.

RESULTS

The average T2 values in phantom were 41.7 ± 13.8 ms for SESE, 43.2 ± 14.4 ms for MESE, 42.4 ± 14.1 ms for FSE and 44 ± 14.5 ms for TGSE. In the patient study, the mean differences were 6.5 ± 8.2 ms, 7.8 ± 7.6 ms and 8.4 ± 14.2 ms for MESE, FSE and TGSE compared to SESE, respectively; these statistical results were not significantly different (p > 0.05). The comparison between the three calculation methods showed no significant difference (p > 0.05). The t-test showed no significant difference between SNR values for all sequences.

CONCLUSION

T2 values depend not only on the sequence type but also on the calculation method. None of the sequences revealed significant differences compared to the SESE reference sequence. TGSE with its short scanning time can be used for high-resolution T2 mapping.

Conservatively treated knee injury is associated with knee cartilage matrix degeneration measured with MRI-based T2 relaxation times: data from the osteoarthritis initiative

OBJECTIVE

To investigate the association of cartilage degeneration with previous knee injuries not undergoing surgery, determined by morphologic and quantitative 3-T magnetic resonance imaging (MRI).

MATERIALS AND METHODS

We performed a nested cross-sectional study of right knee MRIs from participants in the Osteoarthritis Initiative (OAI) aged 45-79 with baseline Kellgren-Lawrence score of 0-2. Cases were 142 right knees of patients with self-reported history of injury limiting the ability to walk for at least 2 days. Controls were 426 right knees without history of injury, frequency-matched to cases on age, BMI, gender, KL scores and race (1:3 ratio). Cases and controls were compared using covariate-adjusted linear regression analysis, with the outcomes of region-specific T2 mean, laminar analysis and heterogeneity measured by texture analysis to investigate early cartilage matrix abnormalities and the Whole-Organ Magnetic Resonance Imaging Score (WORMS) to investigate morphologic knee lesions.

RESULTS

Compared to control subjects, we found significantly higher mean T2 values in the injury [lateral tibia (28.10 ms vs. 29.11 ms, p = 0.001), medial tibia (29.70 ms vs. 30.40 ms, p = 0.014) and global knee cartilage (32.73 ms vs. 33.29 ms, p = 0.005)]. Injury subjects also had more heterogeneous cartilage as measured by GLCM texture contrast, variance and entropy (p < 0.05 in 14 out of 18 texture parameters). WORMS gradings were not significantly different between the two groups (p > 0.05).

CONCLUSION

A history of knee injury not treated surgically is associated with higher and more heterogeneous T2 values, but not with morphologic knee abnormalities. Our findings suggest that significant, conservatively treated knee injuries are associated with permanent cartilage matrix abnormalities.

Tool for osteoarthritis risk prediction (TOARP) over 8 years using baseline clinical data, X-ray, and MRI: Data from the osteoarthritis initiative

BACKGROUND

Osteoarthritis (OA), a multifactorial disease causing joint degeneration, often leads to severe disability. The rising rates of disability highlight the need for implementing preventative measures at early stages of the disease, which would especially benefit subjects at high risk for OA development.

PURPOSE

To develop a risk prediction tool for moderate-severe OA (TOARP) over 8 years based on subject characteristics, knee radiographs, and MRI data at baseline using data from the Osteoarthritis Initiative (OAI).

STUDY TYPE

Retrospective.

SUBJECTS

641 subjects with no/mild radiographic OA (Kellgren-Lawrence [KL] 0-2) and no clinically significant symptoms (Western Ontario and McMaster Universities Arthritis Index [WOMAC] 0-1) were selected from the OAI.

FIELD STRENGTH/SEQUENCE

MR images were obtained using 3.0T.

ASSESSMENT

Compartment-specific cartilage and meniscus morphology and cartilage T₂ were assessed. Baseline subject demographics, risk factors, KL score, cartilage WORMS score, presence of meniscus tear, and cartilage T₂ were used to predict the development of moderate/severe OA (KL = 3-4 or WOMAC pain ≥5 or total knee replacement [TKR]) over 8 years.

STATISTICAL TESTS

Best subsets variable selection followed by cross-validation were used to assess which combinations of variables best predict moderate/severe OA.

RESULTS

Model 1 included KL score, previous knee injury in the last 12 months, age, gender, and BMI. Model 2 included all variables in Model 1 plus presence of cartilage defects in the lateral femur and patella, and presence of a meniscal tear. Model 3 included all variables in Models 1 and 2, plus cartilage T₂ in the medial tibia and medial femur. Compared to Model 1 (cross-validated AUC = 0.67), Model 3 performed significantly better (AUC = 0.72, P = 0.04), while Model 2 showed a statistical trend (AUC = 0.71, P = 0.08).

DATA CONCLUSION

We established a risk calculator for the development of moderate/severe knee OA over 8 years that includes radiographic and MRI data. The inclusion of MRI-based morphological abnormalities and cartilage T₂ significantly improved model performance.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2018;47:1517-1526.

Fisher information and Cramér-Rao lower bound for experimental design in parallel imaging

PURPOSE

The Cramér-Rao lower bound (CRLB) is widely used in the design of magnetic resonance (MR) experiments for parameter estimation. Previous work has considered only Gaussian or Rician noise distributions in this calculation. However, the noise distribution for multi-coil acquisitions, such as in parallel imaging, obeys the noncentral χ-distribution under many circumstances. The purpose of this paper is to present the CRLB calculation for parameter estimation from multi-coil acquisitions.

THEORY AND METHODS

We perform explicit calculations of Fisher matrix elements and the associated CRLB for noise distributions following the noncentral χ-distribution. The special case of diffusion kurtosis is examined as an important example. For comparison with analytic results, Monte Carlo (MC) simulations were conducted to evaluate experimental minimum standard deviations (SDs) in the estimation of diffusion kurtosis model parameters. Results were obtained for a range of signal-to-noise ratios (SNRs), and for both the conventional case of Gaussian noise distribution and noncentral χ-distribution with different numbers of coils, m.

RESULTS

At low-to-moderate SNR, the noncentral χ-distribution deviates substantially from the Gaussian distribution. Our results indicate that this departure is more pronounced for larger values of m. As expected, the minimum SDs (i.e., CRLB) in derived diffusion kurtosis model parameters assuming a noncentral χ-distribution provided a closer match to the MC simulations as compared to the Gaussian results.

CONCLUSION

Estimates of minimum variance for parameter estimation and experimental design provided by the CRLB must account for the noncentral χ-distribution of noise in multi-coil acquisitions, especially in the low-to-moderate SNR regime. Magn Reson Med 79:3249-3255, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Predicting early symptomatic osteoarthritis in the human knee using machine learning classification of magnetic resonance images from the osteoarthritis initiative

The purpose of this study is to evaluate the ability of a machine learning algorithm to classify in vivo magnetic resonance images (MRI) of human articular cartilage for development of osteoarthritis (OA). Sixty-eight subjects were selected from the osteoarthritis initiative (OAI) control and incidence cohorts. Progression to clinical OA was defined by the development of symptoms as quantified by the Western Ontario and McMaster Universities Arthritis (WOMAC) questionnaire 3 years after baseline evaluation. Multi-slice T2 -weighted knee images, obtained through the OAI, of these subjects were registered using a nonlinear image registration algorithm. T2 maps of cartilage from the central weight bearing slices of the medial femoral condyle were derived from the registered images using the multiple available echo times and were classified for "progression to symptomatic OA" using the machine learning tool, weighted neighbor distance using compound hierarchy of algorithms representing morphology (WND-CHRM). WND-CHRM classified the isolated T2 maps for the progression to symptomatic OA with 75% accuracy.

CLINICAL SIGNIFICANCE

Machine learning algorithms applied to T2 maps have the potential to provide important prognostic information for the development of OA. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2243-2250, 2017.

Improved Liver R2* Mapping by Averaging Decay Curves

Liver R2* mapping is often degraded by the low signal-to-noise ratio (SNR) especially in the presence of severe iron. This study aims to improve liver R2* mapping at low SNRs by averaging decay curves before the process of curve-fitting. Independently filtering echo images by nonlocal means (NLM) demonstrated improved quality of R2* mapping, but may introduce new errors due to the nonlinear nature of the NLM filter, during which the averaging weights may vary with different image contents at multiple echo times. In addition, the image denoising effect of the NLM may decline when no sufficient similar patches are available. To overcome these drawbacks, we proposed to filter decay curves instead of images. In this novel scheme, decay curves were averaged in a local window, each with a weight assigned according to the curve-similarity measured by the distance between one of the neighboring curves and the targeted one. The proposed method was tested on simulated, phantom and patient data. The results demonstrate that the proposed method can provide more accurate R2* mapping compared with the NLM algorithm, and hence has the potential to improve diagnosis and therapy in patients with liver iron.

Association of diabetes mellitus and biochemical knee cartilage composition assessed by T2 relaxation time measurements: Data from the osteoarthritis initiative

PURPOSE

To investigate the association of the presence and severity of diabetes mellitus (DM) with articular cartilage composition, using magnetic resonance imaging (MRI)-based T₂ relaxation time measurements, and structural knee abnormalities.

MATERIALS AND METHODS

In the Osteoarthritis Initiative 208, participants with DM (age 63.0 ± 8.9 years; 111 females) and risk factors for osteoarthritis (OA) or mild radiographic tibiofemoral OA (Kellgren-Lawrence [KL] grade ≤2) were identified and group-matched with 208 controls without DM (age 63.3 ± 9.1 years; 111 females). Subjects with diabetes-related renal or ophthalmological complications or insulin treatment at baseline (n = 50) were defined as severe DM. 3T MR images of the right knee were assessed for articular cartilage T₂ , including texture and laminar analyses derived from the patella, medial, and lateral femur and tibia and for structural abnormalities using the modified whole-organ magnetic resonance imaging score (WORMS). Clustered linear regression analyses were used to assess associations of DM with MRI findings.

RESULTS

DM subjects had significantly higher cartilage T₂ in the patella (mean difference 0.92 msec [95% confidence interval (CI) 0.79, 1.06]; P = 0.001) and medial femur (mean difference 0.36 msec [95% CI 0.27, 0.81]; P = 0.006) compared to controls. Averaged over all compartments, DM subjects showed significantly higher texture parameters (variance, P = 0.001; contrast, P = 0.002; entropy, P < 0.001). Subjects with severe DM additionally showed higher T₂ in the medial tibial deep and superficial layers (P = 0.011 and P = 0.041) compared to controls. No significant differences in cartilage, meniscus, and overall WORMS were found between the groups (P > 0.05).

CONCLUSION

In comparison to nondiabetic controls, cartilage in DM subjects showed higher and more heterogeneous cartilage T₂ values, indicating increased articular cartilage degeneration. This affected even more compartments in subjects with severe DM.

LEVEL OF EVIDENCE

2 Technical Efficacy: 5 J. Magn. Reson. Imaging 2018;47:380-390.

Bicomponent ultrashort echo time T2* analysis for assessment of patients with patellar tendinopathy

PURPOSE

To compare bicomponent ultrashort echo time (UTE) T2* parameters of patellar tendon between healthy volunteers and patients with patellar tendinopathy.

MATERIALS AND METHODS

This study was performed with Institutional Review Board approval and with all subjects signing informed consent. A UTE- T2* mapping sequence was performed at 3.0T on the knees of 10 healthy volunteers and in 11 patients with patellar tendinopathy. The UTE- T2* relaxation times of the fast relaxing macromolecular bound water component ( T2*F) and the slow relaxing bulk water component ( T2*S) and the fraction of the fast relaxing macromolecular bound water component (F_F ) of patellar tendon were measured in all subjects. Wilcoxon rank-sum tests were used to compare UTE- T2* parameters between healthy volunteers and patients with patellar tendinopathy.

RESULTS

Mean T2*F, T2*S, and F_F of the patellar tendon was 1.5 msec, 23.1 msec, and 79.5%, respectively, for healthy volunteers and 1.9 msec, 22.3 msec, and 75.5%, respectively, for patients with patellar tendinopathy. There were statistically significant differences between groups of subjects for T2*F (P = 0.01) and F_F (P = 0.007) but not T2*S (P = 0.10) of the patellar tendon.

CONCLUSION

Patients with patellar tendinopathy had significantly higher T2*F and significantly lower F_F of patellar tendon than healthy volunteers, which suggests that bicomponent UTE- T2* parameters can detect changes in the composition and microstructure of degenerative tendon.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2017;46:1441-1447.

Medial femur T2 Z-scores predict the probability of knee structural worsening over 4-8 years: Data from the osteoarthritis initiative

OBJECTIVE

The purpose of this study was to determine the probability of structural worsening of knee cartilage and whole joint degeneration over 4-8 years based on cartilage T₂ Z-scores at baseline.

DESIGN

Right knees with Kellgren-Lawrence (KL) grades of 0-2 in 587 participants from the Osteoarthritis Initiative were studied. 3T MR images were used to perform baseline cartilage T₂ quantification and assess 4-year changes in cartilage morphology (WORMS scoring) in 5 regions. Changes in joint space narrowing (JSN) and KL were assessed over 8 years. T₂ Z-scores were based on a reference database of knees without morphologic cartilage degeneration at baseline. Odds ratios for, and predicted probabilities of any worsening in WORMS cartilage, JSN and KL grade were obtained from logistic regression models.

RESULTS

A one-unit increase in the baseline medial femur T₂ Z-score was associated with cartilage worsening in the same region (OR = 1.59; P < 0.0001) and in any region (OR = 1.37; P < 0.0001), and with worsening JSN (OR = 1.82; P < 0.0001) and KL grades (OR = 1.69; P < 0.0001). Predicted probabilities of worsening in knees with a medial femur T₂ Z-score from 2-4 were 38% for medial femur cartilage WORMS, 70% for any cartilage region, 28% for increasing JSN and 31% for increasing KL grade.

CONCLUSION

Knees with elevated cartilage T₂ (especially in the medial femur and those that are 2 to 4 SDs above the mean reference values) are significantly more likely to have structural worsening over 4 to 8 years. Knowing cartilage T₂ Z-scores may aid in targeting prevention efforts at early stages of osteoarthritis.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2017;46:1128-1136.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Assessment of different fitting methods for in-vivo bi-component T2* analysis of human patellar tendon in magnetic resonance imaging

PURPOSE

To investigate the robustness of four fitting methods for bi-component effective spin-spin T2 (T2*) relaxation time analysis of human patellar tendon.

METHODS

A three-dimensional (3D) cone ultra-short echo-time (UTE) sequence was performed on the knees of ten healthy volunteers at 3.0T. Four fitting methods incorporating either Gaussian or Rician noise distribution were used for voxel-by-voxel bi-component T2* analysis of the patellar tendon. The T2* for the short relaxing (T**,s ) and long relaxing (T*2,l ) water components and the fraction of the short relaxing water component (fs ) were measured, and different fitting methods were compared using Friedman's and Wilcoxon signed rank tests. A numerical simulation study was also performed to predict the accuracy and precision of bi-component T2* parameter estimation in tendon at different signal-to-noise ratios (SNR) levels.

RESULTS

The average T*2,s , T*2,l , fs of human patellar tendon were 1.5ms, 30ms, and 80% respectively. Incorporating different noise models and fitting methods influenced the measured bi-component T2* parameters. Fitting methods incorporating Rician noise were superior to traditional fitting methods for bi-component T2* analysis especially at lower SNR. fs and T*2,s were less sensitive than T*2,1 to noise at even moderate and low SNR. The result of the in-vivo bi-component T2* analysis of tendon agreed well with numerical simulations.

CONCLUSION

Our study demonstrated the use of a 3D cone UTE sequence to perform in vivo voxel-by-voxel bi-component T2* analysis of human patellar tendon. Incorporating Rician noise was useful for improving bi-component T2* analysis especially at lower SNR.

LEVEL OF EVIDENCE

IV.

Quantitative magnetic resonance imaging of the lumbar intervertebral discs

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

High-resolution three-dimensional macromolecular proton fraction mapping for quantitative neuroanatomical imaging of the rodent brain in ultra-high magnetic fields

A well-known problem in ultra-high-field MRI is generation of high-resolution three-dimensional images for detailed characterization of white and gray matter anatomical structures. T₁-weighted imaging traditionally used for this purpose suffers from the loss of contrast between white and gray matter with an increase of magnetic field strength. Macromolecular proton fraction (MPF) mapping is a new method potentially capable to mitigate this problem due to strong myelin-based contrast and independence of this parameter of field strength. MPF is a key parameter determining the magnetization transfer effect in tissues and defined within the two-pool model as a relative amount of macromolecular protons involved into magnetization exchange with water protons. The objectives of this study were to characterize the two-pool model parameters in brain tissues in ultra-high magnetic fields and introduce fast high-field 3D MPF mapping as both anatomical and quantitative neuroimaging modality for small animal applications. In vivo imaging data were obtained from four adult male rats using an 11.7T animal MRI scanner. Comprehensive comparison of brain tissue contrast was performed for standard R₁ and T₂ maps and reconstructed from Z-spectroscopic images two-pool model parameter maps including MPF, cross-relaxation rate constant, and T₂ of pools. Additionally, high-resolution whole-brain 3D MPF maps were obtained with isotropic 170µm voxel size using the single-point synthetic-reference method. MPF maps showed 3-6-fold increase in contrast between white and gray matter compared to other parameters. MPF measurements by the single-point synthetic reference method were in excellent agreement with the Z-spectroscopic method. MPF values in rat brain structures at 11.7T were similar to those at lower field strengths, thus confirming field independence of MPF. 3D MPF mapping provides a useful tool for neuroimaging in ultra-high magnetic fields enabling both quantitative tissue characterization based on the myelin content and high-resolution neuroanatomical visualization with high contrast between white and gray matter.

Changes in the T2 value of cartilage after meniscus transplantation over 1 year

OBJECTIVES

To evaluate the changes in the mean T2 values of articular cartilage on serial follow-up images up to 1 year in patients who underwent lateral meniscus allograft transplantation (MAT).

METHODS

Fifty-two patients who underwent lateral MAT surgery at our hospital were evaluated preoperatively and at 2 days, 6 weeks, 3 months, 6 months, and 1 year after MAT using 3.0-T magnetic resonance imaging (MRI) that included T2 mapping. T2 value changes according to the arthroscopic grading of chondromalacia were evaluated in the lateral and medial compartment. Lysholm scores were obtained pre- and postoperatively.

RESULTS

The T2 values of cartilage were significantly increased 2 days after operation, and then gradually reduced to the baseline level after 1 year in both compartments. In morphologic assessment performed after 1 year, most areas (92.9 %) showed no interval change of chondromalacia grade. Lyshom knee scores increased significantly from the mean preoperative value of 62.5 (range, 23-95) to 89.7 (range, 64-100) at 1 year (p < 0.001).

CONCLUSION

Mean T2 values of cartilage following MAT exhibited a return to baseline level after 1 year. T2 measurement can be a useful tool for quantitative evaluation of postoperative cartilage changes compared to conventional MRI.

KEY POINTS

• T2 mapping provides objective data for longitudinal monitoring following surgery. • Increased cartilage T2 values post-MAT returned to baseline in one year. • Further studies are required to predict the chondroprotective effect of MAT.