Simultaneous estimation of T(2) and apparent diffusion coefficient in human articular cartilage in vivo with a modified three-dimensional double echo steady state (DESS) sequence at 3 T

T₂ mapping and diffusion‐weighted imaging complement morphological imaging for assessing cartilage disease and injury. The double echo steady state sequence has been used for morphological imaging and generates two echoes with markedly different T₂ and diffusion weighting. Modifying the spoiler gradient area and flip angle of the double echo steady state sequence allows greater control of the diffusion weighting of both echoes. Data from two acquisitions with different spoiler gradient areas and flip angles are used to simultaneously estimate the T₂ and apparent diffusion coefficient of each voxel. This method is verified in phantoms and validated in vivo in the knee; estimates from different regions of interest in the phantoms and cartilage are compared to those obtained using standard spin‐echo methods. The Pearson correlations were 0.984 for T₂ (～2% relative difference between spin‐echo and double echo steady state estimates) and 0.997 for apparent diffusion coefficient (?1% relative difference between spin‐echo and double echo steady state estimates) for the phantom study and 0.989 for T₂ and 0.987 for apparent diffusion coefficient in regions of interest in the human knee in vivo. High accuracy for simultaneous three‐dimensional T₂ and apparent diffusion coefficient measurements are demonstrated, while also providing morphologic three‐dimensional images without blurring or distortion in reasonable scan times. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Morphological and biochemical T2 evaluation of cartilage repair tissue based on a hybrid double echo at steady state (DESS‐T2d) approach

Purpose:

To use a new approach which provides, based on the widely used three‐dimensional double‐echo steady‐state (DESS) sequence, in addition to the morphological information, the generation of biochemical T2 maps in one hybrid sequence.

Materials and Methods:

In 50 consecutive MRIs at 3.0 Tesla (T) after matrix‐associated autologous chondrocyte transplantation (MACT) of the knee, by the use this new DESS‐T2d approach, the morphological Magnetic resonance Observation of CArtilage Repair Tissue (MOCART) score, as well as biochemical T2d values were assessed. Furthermore, these results were correlated to standard morphological sequences as well as to standard multi‐echo spin‐echo T2 mapping.

Results:

The MOCART score correlated (Pearson:0.945; P < 0.001) significantly as assessed with standard morphological sequences (68.8 ± 13.2) and the morphological images of the DESS T2d sequence (68.7 ± 12.6). T2 and T2d relaxation times (ms) were comparable in between the control cartilage (T2: 52.5 ± 11.4; T2d: 46.6 ± 10.3) and the repair tissue (T2: 54.4 ± 11.4; T2d: 47.5 ± 13.0) (T2: P = 0.157; T2d: P = 0.589). As expected, T2d values were lower than the standard‐T2 values, however, both functional relaxation times correlated significantly (Pearson:0.429; P < 0.001).

Conclusion:

The presented hybrid approach provides the possibility to combine morphological and biochemical MRI in one fast 3D sequence, and thus, may attract for the clinical use of biochemical MRI. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Optimization of a dual echo in the steady state (DESS) free-precession sequence for imaging cartilage

Three-dimensional (3D) MR imaging of the knee is useful to detect cartilage abnormalities, although the tissue contrast in 3D gradient-recalled echo (GRE) sequences such as gradient-recalled acquisition in the steady state (GRASS) or fast low-angle shot (FLASH) is poor. T2 contrast can be added to a GRASS sequence by combining the signals from the first and second gradient echoes, which form immediately after and immediately before each radiofrequency (RF) pulse in a 3D GRE sequence. We have optimized a 3D dual echo in the steady state (DESS) sequence, which produces one averaged image from the two echoes, for use in the detection of articular cartilage abnormalities. In the optimization process, we examined the imaging parameters of flip angle (α), repetition time (TR), echo time (TE), and bandwidth to maximize the contrast between cartilage and joint fluid. A theoretical simulation of the sequence was confirmed with experiments conducted on phantoms with known T1 and T2. On the basis of theoretical predictions and experiments using healthy volunteers, we determined that an optimized sequence with a bandwidth of 98 Hz per pixel, a TR of 30 msec, a TE of 7.1 msec, and an α of 60° produced the highest contrast between cartilage and fluid within a defined acquisition time of 6 minutes. Additional contrast was obtained by filtering the second-echo image to eliminate noise before adding it to the first-echo image.     

Magnetization transfer contrast and T2 mapping in the evaluation of cartilage repair tissue with 3T MRI

PURPOSE: To use magnetization transfer (MT) imaging in the visualization of healthy articular cartilage and cartilage repair tissue after different cartilage repair procedures, and to assess global as well as zonal values and compare the results to T2-relaxation. MATERIALS AND METHODS: Thirty-four patients (17 after microfracture [MFX] and 17 after matrix-associated autologous cartilage transplantation [MACT]) were examined with 3T MRI. The MT ratio (MTR) was calculated from measurements with and without MT contrast. T2-values were evaluated using a multiecho, spin-echo approach. Global (full thickness of cartilage) and zonal (deep and superficial aspect) region-of-interest assessment of cartilage repair tissue and normal-appearing cartilage was performed. RESULTS: In patients after MFX and MACT, the global MTR of cartilage repair tissue was significantly lower compared to healthy cartilage. In contrast, using T2, cartilage repair tissue showed significantly lower T2 values only after MFX, whereas after MACT, global T2 values were comparable to healthy cartilage. For zonal evaluation, MTR and T2 showed a significant stratification within healthy cartilage, and T2 additionally within cartilage repair tissue after MACT. CONCLUSION: MT imaging is capable and sensitive in the detection of differences between healthy cartilage and areas of cartilage repair and might be an additional tool in biochemical cartilage imaging. For both MTR and T2 mapping, zonal assessment is desirable.     

T2 quantitation of articular cartilage at 1.5 T

PURPOSE: To evaluate sources of error when using a multiecho sequence for quantitative T2 mapping of articular cartilage at 1.5 T. MATERIALS AND METHODS: Phantom measurements were used to assess the contribution of stimulated echoes to inaccuracy of T2 measurements in cartilage using a multiecho sequence. Five volunteer studies compared in vivo single-echo spin echo results to multiecho, single-slice and multiecho, multislice acquisitions for assessment of both the stimulated echo and magnetization transfer contrast (MTC) contributions to T2 measurement inaccuracy. RESULTS: Phantom experiments demonstrated that substantial inaccuracy (10%-13% longer T2 values) is introduced from stimulated echoes with a multiecho sequence with slice-selective refocusing pulses. The in vivo volunteer studies also demonstrated increases in measured T2 by up to 48% with a multiecho sequence. Use of the multiecho sequence in the multislice mode resulted in T2 values closer to the single-echo standards for the volunteer studies. However, this apparent increased accuracy should be regarded as circumstantial, as it only occurs because the error due to MTC has the opposite sign compared to the error due to the stimulated echo contribution. CONCLUSION: Use of a multiecho, multislice sequence for cartilage T2 measurements should be undertaken with the caution that substantial inaccuracy is introduced from stimulated echoes and MTC.     

T2* measurement of the knee articular cartilage in osteoarthritis at 3T

Purpose:

To measure reproducibility, longitudinal and cross‐sectional differences in T2* maps at 3 Tesla (T) in the articular cartilage of the knee in subjects with osteoarthritis (OA) and healthy matched controls.

Materials and Methods:

MRI data and standing radiographs were acquired from 33 subjects with OA and 21 healthy controls matched for age and gender. Reproducibility was determined by two sessions in the same day, while longitudinal and cross‐sectional group differences used visits at baseline, 3 and 6 months. Each visit contained symptomological assessments and an MRI session consisting of high resolution three‐dimensional double‐echo‐steady‐state (DESS) and co‐registered T2* maps of the most diseased knee. A blinded reader delineated the articular cartilage on the DESS images and median T2* values were reported.

Results:

T2* values showed an intra‐visit reproducibility of 2.0% over the whole cartilage. No longitudinal effects were measured in either group over 6 months. T2* maps revealed a 5.8% longer T2* in the medial tibial cartilage and 7.6% and 6.5% shorter T2* in the patellar and lateral tibial cartilage, respectively, in OA subjects versus controls (P < 0.02).

Conclusion:

T2* mapping is a repeatable process that showed differences between the OA subject and control groups. J. Magn. Reson. Imaging 2012;35:1422–1429. © 2012 Wiley Periodicals Inc.     

Combined T1-T2 mapping of human femoro-tibial cartilage with turbo-mixed imaging at 1.5T

PURPOSE: To evaluate the influence of Gd-DTPA on cartilage T2 mapping using turbo-mixed (tMIX) imaging, and to show the possible usefulness of the tMIX technique for simultaneously acquiring T1 and T2 information in cartilage. MATERIALS AND METHODS: Twenty volunteers underwent MRI of the knee using the tMIX sequence before and after gadolinium administration. T1 and T2 maps were calculated. The mean T1 was determined on the pre- and postcontrast T1 maps. T2 relaxation values before and after gadolinium administration were statistically analyzed. RESULTS: The obtained relaxation values are in correspondence with previously published data. The mean T1 before gadolinium administration was 449 msec +/- 34.2 msec (SD), and after gadolinium administration it was 357 msec +/- 55.8 msec (SD). The postcontrast T1 relaxation range was 221.5-572.8 msec. The mean T2 of the precontrast T2 maps was 34.2 msec +/- 3.1 msec (SD), and the mean T2 of the postcontrast T2 maps was 32.5 msec +/- 3.1 msec (SD). These are statistically significant different values. A correction for the postcontrast T2 values, using a back-calculation algorithm, yielded a 98% correlation with the precontrast T2 values. CONCLUSION: The absolute difference of pre- and postcontrast T2 is very small and is ruled out using the back-calculation algorithm. Combined T1-T2 tMIX cartilage mapping is a valuable alternative for separate T1 and T2 cartilage mapping.     

Effect of multislice acquisition on T1 and T2 measurements of articular cartilage at 3T

PURPOSE: The aim of this study was to investigate the effect of magnetization transfer on multislice T1 and T2 measurements of articular cartilage. MATERIALS AND METHODS: A set of phantoms with different concentrations of collagen and contrast agent (Gd-DTPA2-) were used for the in vitro study. A total of 20 healthy knees were used for the in vivo study. T1 and T2 measurements were performed using fast-spin-echo inversion-recovery (FSE-IR) sequence and multi-spin-echo (MSE) sequence, respectively, in both in vitro and in vivo studies. We investigated the difference in T1 and T2 values between that measured by single-slice acquisition and that measured by multislice acquisition. RESULTS: Regarding T1 measurement, a large drop of T1 in all slices and also a large interslice variation in T1 were observed when multislice acquisition was used. Regarding T2 measurement, a substantial drop of T2 in all slices was observed; however, there was no apparent interslice variation when multislice acquisition was used. CONCLUSION: This study demonstrated that the adaptation of multislice acquisition technique for T1 measurement using FSE-IR methodology is difficult and its use for clinical evaluation is problematic. In contrast, multislice acquisition for T2 measurement using MSE was clinically applicable if inaccuracies caused by multislice acquisition were taken into account.     

3 T MRI关节软骨生化成分成像的研究进展

近年来,3 T MRI技术发展十分迅速,已广泛应用于软骨病变的诊断和疗效评价。各种软骨成像序列不断被开发和应用,目前已从传统的二维、三维形态成像向生化成分成像转变,以期能够对关节软骨形态学改变之前的软骨生化改变进行显示,为早期软骨损伤诊断和监测提供临床参考依据。目前,关节软骨生化成分成像技术包括T2mapping、T2＊mapping及T1ρ成像等。就3 T MR关节软骨生化成分成像的临床应用及研究进展予以综述。     

Comparison of a 28-channel receive array coil and quadrature volume coil for morphologic imaging and T2 mapping of knee cartilage at 7T

Purpose:

To compare a new birdcage‐transmit, 28‐channel receive array (28‐Ch) coil and a quadrature volume coil for 7T morphologic MRI and T2 mapping of knee cartilage.

Materials and Methods:

The right knees of 10 healthy subjects were imaged on a 7T whole body magnetic resonance (MR) scanner using both coils. 3D fast low‐angle shot (3D‐FLASH) and multiecho spin‐echo (MESE) sequences were implemented. Cartilage signal‐to‐noise ratio (SNR), contrast‐to‐noise ratio (CNR), thickness, and T2 values were assessed.

Results:

SNR/CNR was 17%–400% greater for the 28‐Ch compared to the quadrature coil (P ≤ 0.005). Bland–Altman plots show mean differences between measurements of tibial/femoral cartilage thickness and T2 values obtained with each coil to be small (?0.002 ± 0.009 cm / 0.003 ± 0.011 cm) and large (?6.8 ± 6.7 msec/?8.2 ± 9.7 msec), respectively. For the 28‐Ch coil, when parallel imaging with acceleration factors (AF) 2, 3, and 4 was performed SNR retained was: 62%–69%, 51%–55%, and 39%–45%.

Conclusion:

A 28‐Ch knee coil provides increased SNR/CNR for 7T cartilage morphologic imaging and T2 mapping. Coils should be switched with caution during clinical studies because T2 values may differ. The greater SNR of the 28‐Ch coil could be used to perform parallel imaging with AF2 and obtain similar SNR as the quadrature coil. J. Magn. Reson. Imaging 2012;441‐448. © 2011 Wiley Periodicals, Inc.

     

Parallel imaging of knee cartilage at 3 Tesla

PURPOSE: To evaluate the feasibility and reproducibility of quantitative cartilage imaging with parallel imaging at 3T and to determine the impact of the acceleration factor (AF) on morphological and relaxation measurements. MATERIALS AND METHODS: An eight-channel phased-array knee coil was employed for conventional and parallel imaging on a 3T scanner. The imaging protocol consisted of a T2-weighted fast spin echo (FSE), a 3D-spoiled gradient echo (SPGR), a custom 3D-SPGR T1rho, and a 3D-SPGR T2 sequence. Parallel imaging was performed with an array spatial sensitivity technique (ASSET). The left knees of six healthy volunteers were scanned with both conventional and parallel imaging (AF = 2). RESULTS: Morphological parameters and relaxation maps from parallel imaging methods (AF = 2) showed comparable results with conventional method. The intraclass correlation coefficient (ICC) of the two methods for cartilage volume, mean cartilage thickness, T1rho, and T2 were 0.999, 0.977, 0.964, and 0.969, respectively, while demonstrating excellent reproducibility. No significant measurement differences were found when AF reached 3 despite the low signal-to-noise ratio (SNR). CONCLUSION: The study demonstrated that parallel imaging can be applied to current knee cartilage quantification at AF = 2 without degrading measurement accuracy with good reproducibility while effectively reducing scan time. Shorter imaging times can be achieved with higher AF at the cost of SNR.     

Differentiating normal hyaline cartilage from post-surgical repair tissue using fast gradient echo imaging in delayed gadolinium-enhanced MRI (dGEMRIC) at 3 Tesla

The purpose was to evaluate the relative glycosaminoglycan (GAG) content of repair tissue in patients after microfracturing (MFX) and matrix-associated autologous chondrocyte transplantation (MACT) of the knee joint with a dGEMRIC technique based on a newly developed short 3D-GRE sequence with two flip angle excitation pulses. Twenty patients treated with MFX or MACT (ten in each group) were enrolled. For comparability, patients from each group were matched by age (MFX: 37.1 ± 16.3 years; MACT: 37.4 ± 8.2 years) and postoperative interval (MFX: 33.0 ± 17.3 months; MACT: 32.0 ± 17.2 months). The Δ relaxation rate (ΔR1) for repair tissue and normal hyaline cartilage and the relative ΔR1 were calculated, and mean values were compared between both groups using an analysis of variance. The mean ΔR1 for MFX was 1.07 ± 0.34 versus 0.32 ± 0.20 at the intact control site, and for MACT, 1.90 ± 0.49 compared to 0.87 ± 0.44, which resulted in a relative ΔR1 of 3.39 for MFX and 2.18 for MACT. The difference between the cartilage repair groups was statistically significant. The new dGEMRIC technique based on dual flip angle excitation pulses showed higher GAG content in patients after MACT compared to MFX at the same postoperative interval and allowed reducing the data acquisition time to 4 min.