MRI texture features are generally considered to be sensitive to variations in signal-to-noise ratio and spatial resolution, which represents an obstacle for the widespread clinical application of texture-based pattern discrimination with MRI. This study investigates the sensitivity of texture features of different categories (co-occurrence matrix, run-length matrix, absolute gradient, autoregressive model, and wavelet transform) to variations in the number of acquisitions (NAs), repetition time (TR), echo time (TE), and sampling bandwidth (SBW) at different spatial resolutions. Special emphasis was placed on the influence of MRI protocol heterogeneity and implications for the results of pattern discrimination. Experiments were performed using two polystyrene spheres and agar gel phantoms with different nodular patterns. T2-weighted multislice multiecho images were obtained using a 3.0 T scanner equipped with a microimaging gradient insert coil. Linear discriminant analysis and k nearest neighbor classification were used for texture-based pattern discrimination. Results show that texture features of all categories are increasingly sensitive to acquisition parameter variations with increasing spatial resolution. Nevertheless, as long as the spatial resolution is sufficiently high, variations in NA, TR, TE, and SBW have little effect on the results of pattern discrimination. Texture features derived from the co-occurrence matrix are superior to features of other categories because they enable discrimination of different patterns close to the resolution limits for the smallest structures of physical texture even for datasets that are heterogeneous with regard to different acquisition parameters, including spatial resolution.

Purpose:To use a 3D gradient-echo (GRE) sequence with two flip angles for delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) to evaluate relative glycosaminoglycan content of repair tissue after matrix-associated autologous chondrocyte transplantation (MACT). Materials and Methods:In a phantom study, T1-mapping based on a 3D-GRE sequence with different flip angle combinations was compared with a standard inversion recovery (IR) sequence at 3.0T. Fifteen patients were examined after MACT in the knee at "3-13 months" (group I) and "19 -42 months" (group II). The ␦ relaxation rate (␦R1) calculated for repair tissue and normal hyaline cartilage was measured and mean values were compared in different postoperative intervals using analysis of variance. Results:The flip angle combination 35/10°provided the best agreement with IR sequence for short and long T1 values. The mean ␦R1 for repair tissue was 2.49 versus 1.04 at the intact control site in group I and 1.90 compared with 0.81 in group II. Differences from repair tissue to control sites showed statistically significance for both groups; no significant difference was found between groups. Conclusion:The 3D dual flip angle dGEMRIC technique optimized for cartilage imaging is comparable to standard T1 IR technique for T1 mapping. Furthermore, the preliminary in vivo study demonstrates the feasibility of the technique in the evaluation of MACT patients.

ObjectiveTo compare mono- and bi-exponential T2* analysis in healthy and degenerated Achilles tendons using a recently introduced magnetic resonance variable-echo-time sequence (vTE) for T2* mapping.MethodsTen volunteers and ten patients were included in the study. A variable-echo-time sequence was used with 20 echo times. Images were post-processed with both techniques, mono- and bi-exponential [T2*m, short T2* component (T2*s) and long T2* component (T2*l)]. The number of mono- and bi-exponentially decaying pixels in each region of interest was expressed as a ratio (B/M). Patients were clinically assessed with the Achilles Tendon Rupture Score (ATRS), and these values were correlated with the T2* values.ResultsThe means for both T2*m and T2*s were statistically significantly different between patients and volunteers; however, for T2*s, the P value was lower. In patients, the Pearson correlation coefficient between ATRS and T2*s was −0.816 (P = 0.007).ConclusionThe proposed variable-echo-time sequence can be successfully used as an alternative method to UTE sequences with some added benefits, such as a short imaging time along with relatively high resolution and minimised blurring artefacts, and minimised susceptibility artefacts and chemical shift artefacts. Bi-exponential T2* calculation is superior to mono-exponential in terms of statistical significance for the diagnosis of Achilles tendinopathy.Key Points• Magnetic resonance imaging offers new insight into healthy and diseased Achilles tendons• Bi-exponential T2* calculation in Achilles tendons is more beneficial than mono-exponential• A short T2* component correlates strongly with clinical score• Variable echo time sequences successfully used instead of ultrashort echo time sequences

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T2 mapping allows visualization of cartilage repair tissue maturation. Global T2 repair tissue values approach that of control sites after more than 1.5 years, similar behavior is seen in the zonal organization.

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The aim of this study was to investigate T₂* in the Achilles tendon (AT), in vivo, using a three-dimensional ultrashort time echo (3D-UTE) sequence, to compare field strength differences (3 and 7 T) and to evaluate a regional variation of T₂* in healthy and pathologic tendon. Ten volunteers with no history of pain in the AT and five patients with chronic Achilles tendinopathy were recruited. 3D-UTE images were measured with the following echo times, at echo time = [0.07, 0.2, 0.33, 0.46, 0.59, 0.74, 1.0, 1.5, 2.0, 4.0, 6.0, and 9.0 ms]. T₂* values in the AT were calculated by fitting the signal decay to biexponential function. Comparing volunteers between 3 and 7 T, short component T(2s)* was 0.71 ± 0.17 ms and 0.34 ± 0.09 ms (P < 0.05); bulk long component T(2l)* was 12.85 ± 1.87 ms and 10.28 ± 2.28 ms (P < 0.05). In patients at 7 T, bulk T(2s)* was 0.53 ± 0.17 ms (P = 0.045, compared to volunteers), T(2l)* was 11.49 ± 4.28 ms (P = 0.99, compared to volunteers). The results of this study suggest that the regional variability of AT can be quantified by T₂* in in vivo conditions. Advanced quantitative imaging of the human AT using a 3D-UTE sequence may provide additional information to standard clinical imaging. Finally, as the preliminary patient data suggest, T(2s)* may be a promising marker for the diagnosis of pathological changes in the AT.