Magnetic Resonance in Medicine最新文献

Purpose: Echo-Planar Imaging (EPI) is central to fMRI, diffusion MRI, and many dynamic clinical applications, yet rapid gradient switching induces strong mechanical vibrations, generates acoustic noise, and contributes to ghosting artifacts-effects that intensify at ultra-high fields. This study aims to predict how subtle timing changes in multi-train EPI can modulate acoustic energy and reduce ghosting without altering reconstruction or acquisition design.

Methods: We derived an analytic model describing the interference between short gradient trains and combined it with the system's acoustic transfer function to predict the acoustic energy of a scan. The model predicts cyclic variations in acoustic power resulting from small timing adjustments of slices and echoes. Validation was performed on a clinical 7 T and an investigational 10.5 T MRI systems by measuring acoustic output and ghosting-artifact levels while systematically sweeping timing conditions in single and multi-echo EPI protocols.

Results: Sub-millisecond timing modifications produced large, predictable changes in the acoustic power; changes ranging from 2-fold to 47-fold. Intriguingly, under certain conditions, doubling the acquisitions per unit-time reduced the minimal acoustic energy two-fold. In addition, navigator time shifting reduced the ghosting-artifact intensity, up to 5-fold, exhibiting cyclic dependence corresponding to the dominant acoustic resonance frequency. These trends were consistent across both the 7 T and the 10.5 T systems.

Conclusion: Minimal timing adjustments provide an effective, reconstruction-agnostic strategy to reduce acoustic noise, to mitigate ghosting artifacts, and to expand feasible EPI parameters space. The proposed model paves the way towards quieter, higher-quality EPI scans, particularly valuable for ultra-high-field and high-speed imaging.

Purpose: To evaluate the feasibility of measuring gas exchange between the alveolar-capillary membrane and red blood cells (RBCs) using hyperpolarized ¹²⁹Xe magnetic resonance, and to assess its potential for detecting disease-related changes in an animal model.

Methods: Experiments were performed on eight rats with bleomycin-induced pulmonary fibrosis and eight healthy controls. RBCs chemical shift saturation recovery (rCSSR) and equivalent chemical shift saturation recovery (eCSSR) sequences were developed to estimate the gas exchange time constants from alveoli to RBCs (T_G-R) and from membrane to RBCs (T_M-R). Group comparisons were performed, and correlations between rCSSR-derived parameters and pulmonary function tests (PFTs) and quantitative histology were also assessed. Statistical significance was defined as p < 0.05.

Results: T_M-R and T_G-R measured with rCSSR (denoted as T_M-R-R and T_G-R-R, respectively) were higher in the fibrosis group (8.74 ± 1.26 and 17.80 ± 3.08 ms, respectively) compared to controls (7.02 ± 0.58 and 13.89 ± 1.58 ms; p < 0.01). For the T_M-R and T_G-R derived from eCSSR (denoted as T_M-R-E and T_G-R-E, respectively), only T_G-R-E showed a significant difference. Additionally, T_M-R-R demonstrated strong correlations with forced vital capacity, quasi-static compliance from PFTs, and alveolar septal thickness measured by histology.

Conclusion: We proposed a ¹²⁹Xe MR-based approach for quantifying gas exchange from the alveolar-capillary membrane to RBCs. This technique shows promise as a sensitive, non-invasive tool for detecting pulmonary gas exchange impairment.

Purpose: To quantitatively assess the bias in the intravoxel incoherent motions (IVIM)-derived pseudo-diffusion volume fraction (f) caused by the differences in relaxation times between the tissue and fluid compartments, and to develop a two-dimensional (b-value-TE) fitting approach for simultaneous T2 and IVIM parameter estimation along with an optimal acquisition protocol for the relaxation-compensated T2-IVIM imaging in the liver.

Methods: Simulations were conducted to investigate the TR- and TE-dependent bias in f when using the IVIM model, and to evaluate the applicability of the 2D T2-IVIM model for reducing this bias. The numerical findings were then validated using the in vivo IVIM data from four healthy volunteers on a 3-Tesla MRI scanner. Finally, a numerical framework for optimizing the T2-IVIM protocol for relaxation-compensated f parameter estimation was proposed and tested using the in vivo data.

Results: In vivo, the traditional IVIM model showed a trend toward higher f with increasing TE in the liver (R = 0.46, p = 0.023), but not in the kidney cortex (R = -0.067, p = 0.76) or medulla (R = 0.039, p = 0.86). In both simulations and in vivo, 2D T2-IVIM modeling yielded lower f values and reduced variability in the liver. Our results further suggest that a b-TE protocol with six b-values and three TEs (50, 60, and 100 ms) may be optimal for liver T2-IVIM.

Conclusion: The extended 2D T2-IVIM model effectively minimizes the TE-dependent bias in f and allows simultaneous estimation of the IVIM parameter and compartmental T2 values in abdominal organs.

Purpose: Dynamic contrast-enhanced MRI (DCE-MRI) can assess tumor perfusion using pharmacokinetic models. However, poor DCE-MRI reproducibility from reliance on conventional T₁-weighted MRI has limited clinical translation. We evaluated whether Dynamic Contrast Enhanced-Magnetic Resonance Fingerprinting (DCE-MRF), which directly generates quantitative T₁ relaxation time constant maps, provides improved precision and statistical power for detecting treatment-induced vascular changes compared to conventional DCE-MRI.

Methods: Twenty female mice bearing orthotopic 4 T1 breast tumors were randomly assigned to DCE-MRF (n = 12) or conventional DCE-MRI (n = 8) cohorts. Both methods acquired matched spatial and temporal resolution (23-s) T₁ measurements at baseline, 3 h, and 48 h post-treatment with combretastatin A4 phosphate (120 mg/kg), a known vascular disrupting agent. Perfusion assessments were obtained with a pharmacokinetic Linear Reference Region Model (RK^trans and k_ep,T) and model-independent initial area under the curve assessments and compared using Wilcoxon signed-rank tests and Hedges' g effect sizes.

Results: DCE-MRF demonstrated 1.3-to-5.1-fold larger effect sizes for RK^trans across all regions and timepoint comparisons, and 1.3-to-1.9-fold larger effect sizes for tumor rim k_ep,T compared to conventional DCE-MRI. RK^trans and k_ep,T at 3 h post-treatment in all regions, detected significant vascular recovery for whole tumor RK^trans and both whole tuomor and tumor rim for k_ep,T at 48 h, whereas DCE-MRI only detected significant changes at 3 h.

Conclusions: DCE-MRF's improved measurement precision and increased effect size translates directly to enhanced statistical power for detecting treatment-induced vascular changes, positioning it as a more reproducible tumor perfusion assessment for animal models and eventually cancer patients.

Purpose: The aim was to estimate T1 relaxivity of gadobutrol and gadoteric acid in cerebrospinal fluid (CSF) at 3T, to support research on CSF-flow and the glymphatic system in humans utilizing T1 mapping after intrathecal injection.

Methods: Using a phantom, relaxivity was estimated for gadobutrol and gadoteric acid in lumbar CSF and an isotonic solution. All samples were scanned simultaneously using the variable flip angle method with B1 correction, repeated six times on one 3T scanner, and once on a second 3T scanner. Difference in relaxivity between CSF and the isotonic solution were evaluated from the repeated measurements.

Results: There was a significant difference in relaxivity between CSF and the isotonic solution for both gadobutrol and gadoteric acid. The relaxivity for gadobutrol for the respective scanners was estimated to 3.02 ± 0.09 vs. 3.63 L mmol^-1 s^-1 in CSF and 2.35 ± 0.05 vs. 2.74 L mmol^-1 s^-1 in isotonic solution. For gadoteric acid, corresponding results were 2.47 ± 0.02 vs. 2.91 L mmol^-1 s^-1 in CSF and 2.37 ± 0.03 vs. 2.8 L mmol^-1 s^-1 in isotonic solution. Between the scanners, there was a high correlation (R² 0.998) but an 18% scaling difference in the T1 relaxation rates and corresponding relaxivities.

Conclusions: The relaxivity was higher in CSF than in the isotonic solution, particularly for gadobutrol. Systematic differences in relaxivity between scanners may potentially be corrected using a scaling factor derived from the T1 time of baseline CSF. For CSF studies using T1 mapping with a gadolinium-based contrast agent, we recommend using a CSF-specific relaxivity constant.

Purpose: To evaluate the correspondence between myelin water fraction (MWF) estimates derived from multi-echo spin echo (MESE) and multi-echo gradient echo (MGRE) imaging in fixed mouse brain tissue, using a panel of myelin basic protein (Mbp) enhancer-edited mouse lines exhibiting graded hypomyelination.

Methods: Fifteen mouse brains from five genetically modified mouse lines were imaged using ex vivo 7 T MRI with high-resolution 3D MESE and MGRE protocols. MWF maps were computed from the MESE approach using regularized non-negative least squares (NNLS) decomposition, and from the MGRE approach using robust principal component analysis (rPCA). MWF values were then parcellated across major white matter tracts using an atlas-based pipeline and validated against biological markers, including Mbp gene expression and FluoroMyelin fluorescence intensity.

Results: Both MESE- and MGRE-derived MWF maps exhibited high sensitivity to myelin content and resolved mouse line-dependent differences across white matter tracts. Region-specific MWF estimates were highly correlated across contrasts (r = 0.96), with MGRE yielding consistently higher MWF values, particularly in smaller tracts. MESE derived MWF values showed subtle underestimation of myelin content in hypomyelinated white matter regions. Both MWF measures showed strong correlations with Mbp messenger ribonucleic acid (mRNA) (r = 0.72-0.97) and FluoroMyelin staining (r = 0.50-0.92), with stronger histological correlations for MGRE-derived values.

Conclusion: Both MESE and MGRE sequences provide biologically meaningful estimates of myelin content in fixed tissue but exhibit contrast-specific sensitivities and biases. MGRE combined with rPCA offers time efficient imaging and robustness in fine white matter structures, supporting its utility for high-resolution preclinical myelin mapping.

Purpose: Image quality and resolution in MRI are fundamentally constrained by the performance of radiofrequency (RF) coils used to excite the spins and receive the signal. Electromagnetic (EM) simulations are essential for optimizing coil performance. Existing tools are often slow, memory-intensive, and require expensive licenses. To address these limitations, we introduce a novel, comprehensive, open-source EM simulation tool for RF coil design in MRI.

Theory and methods: Our toolbox consists of four complementary software components: (1) a full-wave 3D EM solver based on the wire-surface-volume integral equation (WSVIE), where tensor decompositions reduce memory usage and accelerate simulations for fine body model resolutions; (2) a reduced-order model technique enabling patient-specific coil simulations in minutes; (3) a fully automatic circuit co-simulator for coil tuning, matching, decoupling, preamplifier decoupling, and detuning; and (4) a numerical EM basis generator that can be used to compute ultimate performance metrics. We demonstrated the toolbox with a series of simulations for different applications at 7 T MRI.

Results: Our proposed reduced-order model WSVIE solver was 70 times faster compared to a commercial solver for the simulation of a 31-channel 7 T head coil. Our co-simulator tuned, matched, and decoupled the array in 30 min, compared to a few days time of manual interactions needed with the commercial package. The average difference between the signal-to-noise ratio maps was less than 15%.

Conclusion: The proposed open-source simulation framework enables fast, memory-friendly, accurate, and anatomy-specific RF coil array design, making it a powerful tool for RF coil engineering.

Purpose: To achieve high resolution (≤ 1 mm isotropic) whole-brain perfusion imaging at 7 T with next generation ASL pulse sequence, reconstruction algorithm, and MRI hardware.

Methods: We capitalized on three major innovations: (1) FLASH-based pseudo-Continuous ASL (pCASL) sequence with rotated golden-angle stack-of-spirals (rGA-SoS) sampling; (2) dynamic compressed sensing (CS) reconstruction with high spatiotemporal resolution and motion-resolved self-navigation; and (3) high density array coil and high-performance Impulse gradient of the NexGen 7 T scanner. Whole-brain laminar perfusion imaging was validated by correlation with histological data of microvascular and cell body density, as well as through finger-tapping (FT) and working memory (WM) fMRI tasks.

Results: The proposed rGA-SoS sequence achieved a 3.3-fold SNR and 2-fold higher intraclass correlation coefficient (ICC) compared to matched Cartesian sampling at 7 T, enabling up to 0.8 mm isotropic spatial resolution and/or a temporal resolution of 14 s at 1 mm isotropic. Resting-state perfusion showed strong correlations with microvascular and cell body density. Laminar perfusion fMRI revealed a two-peak activation in the primary motor cortex induced by FT, and distinct laminar profiles for task-positive and task-negative networks during WM task.

Conclusion: This method offers a noninvasive imaging tool to bridge the gap between mesoscopic MRI with microscopic cellular imaging, as well as to investigate neural excitation and inhibition underlying positive and negative fMRI activations.

Purpose: To improve accuracy of apparent diffusion coefficient (ADC) measurement across different bone-marrow (BM) sites for myelofibrosis (MF) patients.

Methods: Vendor-provided ADC gradient nonlinearity correction (GNC) was implemented for 41 MF study subjects on a 3T clinical scanner. The degree of bias correction was assessed on b-maps across the BM regions-of-interest defined for femoral trochanter, posterior ilium and spine vertebrae of all subjects. Bias variability from subject repositioning was evaluated for five subjects with longitudinal scans using Bland-Altman analysis. BM ADC with and without GNC were compared in 22 subjects with MF grade > 0 (iliac fat fraction < 40%). The GNC effect on ADC heterogeneity trends across bone sites was assessed using paired t-test and correlation to MF-grade.

Results: The observed bias was substantial across BM sites ranging from -10% for edge vertebrae to +8.4% for trochanter across all subjects with a 13.7% intra-subject median range. The effect of irreproducible positioning in longitudinal scans was ±4.2% bias (95% limits-of-agreement) with range of ±8.5% (highest for L1 and L5). By removing bias, GNC improved ADC accuracy and longitudinal reproducibility and emphasized biological heterogeneity for femoral trochanter versus ilium and edge vertebrae (T11-L1, L5, and S1). GNC revealed significant ADC differences between trochanters and edge vertebrae (p = 0.007), while edge vertebrae and ilium ADC became more uniform (p = 0.07), with heterogeneity and values correlated to MF-grade.

Conclusion: On-scanner correction of gradient nonlinearity bias in bone marrow ADC reduces technical nonuniformity and variability and allows accurate and reproducible characterization of heterogeneous disease for myelofibrosis patients.