Magnetic Resonance in Medicine最新文献

Purpose: The purpose of this study is to develop a method for selecting uniform wave vectors for double diffusion encoding (DDE) to improve the accuracy and reliability of diffusion measurements.

Methods: The method relies on identifying orthogonal wave vectors with rotations, and representing these rotations as points on a three-dimensional sphere in four dimensions using quaternions. This enables an electrostatic repulsion algorithm to achieve a uniform distribution of these points. The optimal points are then converted back into orthogonal wave vectors (or rotations).

Results: The method was validated by comparing the distribution of directions to those generated by uniform sampling and by evaluating the error in the powder-averaged signal for various models. Our results demonstrate that the electrostatic repulsion approach effectively achieves a uniform distribution of wave vectors.

Conclusion: The proposed method provides a systematic way to generate uniform diffusion directions suitable, for example, for DDE, enhancing the precision of diffusion measurements and reducing potential bias in experimental results. The method is also capable of generating uniform sets of B-tensors, and is thus applicable for general free waveform encoding.

Purpose: To quantify T₂ relaxation in the brain at 3 T and 7 T to study its field dependence and correlation with iron content, and to investigate whether iron can be separated from other sources of T₂ relaxation based on this field dependence.

Methods: Nine subjects were scanned at both field strengths with the same acquisition technique, which used multiple gradient-echo sampling of a spin echo. This allowed for separation of T₂ relaxation from static dephasing by B₀ field inhomogeneities and the effects of radiofrequency refocusing imperfections. The average relaxation rates (R₂ = 1/T₂) in multiple regions of interest in the brain were fitted with a model linear in B₀ and correlated with literature iron values.

Results: The relationship between the R₂ values at the two field strengths appeared to be linear over all regions of interest. The R₂ values (in s^-1) in the regions of interest for which both an iron and a lipid mass fraction have been documented in the literature were fitted as $R_2=9+\left(0.9+2·{10}^4\left[\mathrm{Fe}\right]+5.7\left[\text{lipid}\right]\right)·B_0$ , where $\left[\mathrm{Fe}\right]$ and $\left[\text{lipid}\right]$ indicate the putative mass fractions of iron and lipid.

Conclusion: The R₂ relaxation rate is well described by a constant plus a term linear in B₀, with both iron and lipid content contributing to the slope. This indicates that the contributions of lipid and iron to R₂ cannot be separated based solely on the field dependence of R₂ in the field range of 3-7 T.

Purpose: To determine the expected range of NMR relaxation times (T₁ and T₂) in the neonatal brain at 7 T.

Methods: Data were acquired in a total of 40 examinations on infants in natural sleep. The cohort included 34 unique subjects with postmenstrual age range between 33 and 52 weeks and contained a mix of healthy individuals and those with clinical concerns. Single-slice T₁ and T₂ mapping protocols were used to provide measurements in white matter, cortex, cerebellum, and deep gray matter. Automatic image segmentation of a separate T₂-weighted brain volume was used to define regions of interest for analysis.

Results: Linear regression was used to estimate relaxation times at term equivalent age (40 weeks postmenstrual age). $T_1^{40\mathrm{wk}}$ with 95% confidence intervals was measured to be 2933 [2893, 2972] ms in white matter; 2653 [2604, 2701] ms in cerebellum; and 2486 [2439, 2532] ms in basal ganglia. $T_2^{40\mathrm{wk}}$ was estimated as 119 [116, 121] ms in white matter, 99 [96, 102] ms in cerebellum, and 90 [89, 92] ms in basal ganglia. Most tissue-relaxation times showed a significant negative correlation with postmenstrual age, with the strongest correlation seen in cerebellum.

Conclusions: We describe neonatal brain tissue and age-specific T₁ and T₂ relaxation values at 7 T. The presented values differ substantially from both adult values at 7 T and neonate values measured at lower field strengths, and will be essential for pulse-sequence optimization for neonatal studies.

Purpose: To implement a flexible framework, named HydrOptiFrame, for the design and optimization of time-efficient water-excitation (WE) RF pulses using B-spline interpolation, and to characterize their lipid suppression performance.

Methods: An evolutionary optimization algorithm was used to design WE RF pulses. The algorithm minimizes a composite loss function that quantifies the fat-water contrast using Bloch equation simulations. In a first study, B-spline interpolated optimized (BSIO) pulses designed with HydrOptiFrame with durations of 1 and 0.76 ms were generated for 3 T and characterized in healthy volunteers' knees. The femoral bone marrow SNR was compared to that obtained with to 1-1 WE and lipid insensitive binomial off resonant excitation (LIBRE) pulses. In a second study, in the heart at 1.5 T, the water-fat contrast ratio and coronary artery vessel length obtained with a 2.56 ms BSIO pulse was compared to 1-1 WE and LIBRE pulses in free-running cardiovascular MR.

Results: The 1 ms BSIO pulse resulted in higher fat suppression and lower contrast ratio (CR) in the bone marrow than the state-of-the-art pulses (4.1 ± 0.2 vs. 4.7 ± 0.4 and 4.4 ± 0.3 for the BSIO, the 1-1 WE and LIBRE respectively, p < 0.05 vs. both) at 3 T. At 1.5 T, the BSIO pulse resulted in a higher blood-epicardial fat CR (3.8 ± 1.3 vs. 1.6 ± 0.6 and 2.4 ± 1.1 for the BSIO, 1-1 WE and LIBRE, respectively, p < 0.05 vs. both) and longer traceable left coronary artery vessel length (8.7 ± 1.4 cm vs. 7.0 ± 1.0 cm [p = 0.04] and 7.5 ± 1.2 cm [p = 0.09]).

Conclusion: The HydrOptiFrame framework offers a new opportunity to design WE RF pulses that are robust to B₀ inhomogeneity at multiple magnetic field strengths and for variable RF pulse durations.

Purpose: To develop and validate a 3D turbo spin-echo (TSE)-compatible approach to enhancing black-blood (BB) effects while preserving T₁ weighting and overall SNR.

Methods: Following the excitation RF pulse, a 180° RF pulse sandwiched by a pair of flow-sensitive dephasing (FSD) gradient pulses in the phase- (y) and partition-encoding (z) directions, respectively, is added. The polarity of FSD gradients in z direction is toggled every TR, achieving an interleaved FSD (iFSD) configuration in y-z plane. The technique was optimized and evaluated in 18 healthy volunteers and 32 patients with neurovascular disease or brain metastases. Comparisons were made among TSE with and without one of BB preparations: iFSD, delay alternating with nutation for tailored excitation, and motion-sensitized driven equilibrium.

Results: iFSD-TSE achieved the best blood flow suppression indicated by venous sinus SNR and parenchyma-to-sinus contrast-to-noise ratio (CNR). iFSD-TSE yielded slightly lower white matter SNR (106.6 ± 32.9) and white-to-gray matter CNR (27.3 ± 8.1) compared to TSE (111.4 ± 31.5 and 28.6 ± 8.8), which were significantly higher than those of delay alternating with nutation for tailored excitation-prepared TSE (84.3 ± 25.0 and 16.8 ± 4.8) and motion-sensitized driven equilibrium-prepared TSE (77.3 ± 26.6 and 15.9 ± 5.3). At the neurovascular wall lesions, iFSD-TSE yielded the highest wall-to-lumen CNR among the three sequences with a BB preparation, all of which significantly outperformed TSE. iFSD-TSE effectively suppressed slow-flow artifacts that otherwise mimicked an atherosclerotic lesion or strongly contrast-enhancing vessel wall. In diagnosing brain metastases, iFSD allowed for highest inter-reader agreement (κ 0.75) and shortest reading time.

Conclusion: iFSD is a promising approach compatible with 3D TSE for robust blood flow suppression and preserved T₁ weighting and overall SNR.

Purpose: To characterize the diffusion time (Δ_eff) dependence of apparent diffusion coefficient (ADC) and intravoxel incoherent motion-related parameters in the human kidney at 3 T.

Methods: Sixteen healthy volunteers underwent an MRI examination at 3 T including diffusion-weighted imaging at different Δ_eff ranging from 24.1 to 104.1 ms. The extended mono-exponential ADC and intravoxel incoherent motion models were fitted to the data for each Δ_eff and the medullary and cortical ADC, (pseudo-)diffusion coefficients (D* and D) and flow-related signal fraction (f) were calculated.

Results: When all the data were used for fitting, a significant trend toward higher ADC with increasing Δ_eff was observed between 24.1 and 104.1 ms (median and interquartile range: 2.38 [2.19, 2.47] to 2.84 [2.36, 2.90] × 10^-3 mm²/s for cortex, and 2.28 [2.18, 2.37] to 2.82 [2.58, 3.11] × 10^-3 mm²/s for medulla). In contrast, no significant differences in ADC were found when only the data acquired at b-values higher than 200 s/mm² were used for fitting. When the intravoxel incoherent motion model was applied, cortical and medullary f increased significantly (cortex: 0.21 [0.15 0.27] to 0.37 [0.32, 0.49] × 10^-3 mm²/s; medulla: 0.15 [0.13 0.29] to 0.41 [0.36 0.51] × 10^-3 mm²/s). No significant changes in cortical and medullary D and D* were observed as diffusion time increased.

Conclusion: Renal perfusion and tubular flow substantially contribute to the observed increase in ADC over a wide range of Δ_eff between 24 and 104 ms.

Purpose: To demonstrate the feasibility of measuring pulmonary hematocrit (Hct) in blood in vivo using oscillation of hyperpolarized ¹²⁹Xe MR signals and its potential for disease assessment in animal models.

Methods: Hyperpolarized ¹²⁹Xe dynamic MR spectroscopy was performed on 10 anemia model rats and 10 control rats. A concise model based on hyperpolarized ¹²⁹Xe MR signal oscillations was built for calculating pulmonary Hct. Blood tests and chemical shift saturation recovery were conducted on each rat to obtain Hct. Correlations of Hct obtained from different methods were analyzed using SPSS 22.0.

Results: Hct measurements were strongly correlated with blood test (Spearman correlation coefficient r = 0.871, p < 0.001) and chemical shift saturation recovery measurements (r = 0.956, p < 0.001). Hct was 0.198 ± 0.054 for the anemic cohort and 0.457 ± 0.039 for the control group (p < 0.001).

Conclusion: We developed an approach that provided a way to quantify changes in pulmonary Hct using oscillations of hyperpolarized ¹²⁹Xe signals. This method shows promise for noninvasive pulmonary Hct assessment and disease evaluation.

Purpose: To develop a deep learning-based method for robust and rapid estimation of the fatty acid composition (FAC) in mammary adipose tissue.

Methods: A physics-based unsupervised deep learning network for estimation of fatty acid composition-network (FAC-Net) is proposed to estimate the number of double bonds and number of methylene-interrupted double bonds from multi-echo bipolar gradient-echo data, which are subsequently converted to saturated, mono-unsaturated, and poly-unsaturated fatty acids. The loss function was based on a 10 fat peak signal model. The proposed network was tested with a phantom containing eight oils with different FAC and on post-menopausal women scanned using a whole-body 3T MRI system between February 2022 and January 2024. The post-menopausal women included a control group (n = 8) with average risk for breast cancer and a cancer group (n = 7) with biopsy-proven breast cancer.

Results: The FAC values of eight oils in the phantom showed strong correlations between the measured and reference values (R² > 0.9 except chain length). The FAC values measured from scan and rescan data of the control group showed no significant difference between the two scans. The FAC measurements of the cancer group conducted before contrast and after contrast showed a significant difference in saturated fatty acid and mono-unsaturated fatty acid. The cancer group has higher saturated fatty acid than the control group, although not statistically significant.

Conclusion: The results in this study suggest that the proposed FAC-Net can be used to measure the FAC of mammary adipose tissue from gradient-echo MRI data of the breast.

Purpose: To develop a breath-hold cardiac quantitative susceptibility mapping (QSM) sequence for noninvasive measurement of differential cardiac chamber blood oxygen saturation (ΔSO₂).

Methods: A non-gated three-dimensional stack-of-spirals QSM sequence was implemented to continuously sample the data throughout the cardiac cycle. Measurements of ΔSO₂ between the right and left heart chamber obtained by the proposed sequence and a previously validated navigator Cartesian QSM sequence were compared in three cohorts consisting of healthy volunteers, coronavirus disease 2019 survivors, and patients with pulmonary hypertension. In the pulmonary-hypertension cohort, Bland-Altman plots were used to assess the agreement of ΔSO₂ values obtained by QSM and those obtained by invasive right heart catheterization (RHC).

Results: Compared with navigator QSM (average acquisition time 419 ± 158 s), spiral QSM reduced the scan time on average by over 20-fold to a 20-s breath-hold. In all three cohorts, spiral QSM and navigator QSM yielded similar ΔSO₂. Among healthy volunteers and coronavirus disease 2019 survivors, ΔSO₂ was 17.41 ± 4.35% versus 17.67 ± 4.09% for spiral and navigator QSM, respectively. In pulmonary-hypertension patients, spiral QSM showed a slightly smaller ΔSO₂ bias and narrower 95% limits of agreement than that obtained by navigator QSM (1.09% ± 6.47% vs. 2.79% ± 6.99%) when compared with right heart catheterization.

Conclusion: Breath-hold three-dimensional spiral cardiac QSM for measuring differential cardiac chamber blood oxygenation is feasible and provides values in good agreement with navigator cardiac QSM and with reference right heart catheterization.