Magnetic Resonance in Medicine最新文献

Purpose: Redox homeostasis plays a key role in regulating the overall health and development of organisms. This study aimed to develop a compact and mobile continuous-wave (CW) electron paramagnetic resonance (EPR) imager to facilitate stable, highly sensitive fast three-dimensional (3D) whole-body imaging of nitroxide-infused mice.

Methods: A multiturn loop gap resonator with a diameter of 30 mm and length of 35 mm was designed for whole-body EPR imaging. A compact and mobile CW-EPR imager operating at 750 MHz was developed using this resonator. The automatic matching and tuning control systems were also adjusted to compensate for perturbations caused by the movement of the mice.

Results: When the mice were inserted into the resonator, the resonant frequency was easily determined for all parts of the mouse, from the head to the lower abdomen. 3D EPR images of the mouse body from the thoracic region to the lower abdomen were obtained following infusion of a nitroxide, 3-carboxy-2,2,5,5-tetramethylpyrrolidine-1-oxyl (CxP). The EPR images clearly visualized the CxP distribution in various organs at different concentrations. Time-dependent EPR images also revealed that the signal intensities of the CxP decayed over time, and the decay rates for the heart, liver, and kidneys were evaluated.

Conclusion: A compact and mobile EPR imager that enables 3D whole-body EPR image of nitroxide in mice was developed. The EPR imager exhibited long-term stability against motion effects caused by respiratory motion and heartbeats in mice. The EPR images clearly visualized the in vivo distribution, clearance, and metabolism of the nitroxide in organs.

Purpose: This work aims to raise a novel design for navigator-free multiband (MB) multishot uniform-density spiral (UDS) acquisition and reconstruction, and to demonstrate its utility for high-efficiency, high-resolution diffusion imaging.

Theory and methods: Our design focuses on the acquisition and reconstruction of navigator-free MB multishot UDS diffusion imaging. For acquisition, radiofrequency-pulse encoding was used to achieve controlled aliasing in parallel imaging in MB imaging. For reconstruction, a new algorithm named slice-projection onto convex sets-enhanced inherent correction of phase errors (slice-POCS-ICE) was proposed to simultaneously estimate diffusion-weighted images and intershot phase variations for each slice. The efficacy of the proposed methods was evaluated in both numerical simulation and in vivo experiments.

Results: In both numerical simulation and in vivo experiments, slice-POCS-ICE estimated phase variations more precisely and provided results with better image quality than other methods. The intershot phase variations and MB slice aliasing artifacts were simultaneously resolved using the proposed slice-POCS-ICE algorithm.

Conclusion: The proposed navigator-free MB multishot UDS acquisition and reconstruction method is an effective solution for high-efficiency, high-resolution diffusion imaging.

Purpose: Pulmonary MRI faces challenges due to low proton density, rapid transverse magnetization decay, and cardiac and respiratory motion. The fermat-looped orthogonally encoded trajectories (FLORET) sequence addresses these issues with high sampling efficiency, strong signal, and motion robustness, but has not yet been applied to phase-resolved functional lung (PREFUL) MRI-a contrast-free method for assessing pulmonary ventilation during free breathing. This study aims to develop a reconstruction pipeline for FLORET UTE, enhancing spatial resolution for three-dimensional (3D) PREFUL ventilation analysis.

Methods: The FLORET sequence was used to continuously acquire data over 7 ± 2 min in 36 participants, including healthy subjects (N = 7) and patients with various pulmonary conditions (N = 29). Data were reconstructed into respiratory images using motion-compensated low-rank reconstruction, and a 3D PREFUL algorithm was adapted to quantify static and dynamic ventilation surrogates. Image sharpness and signal-to-noise ratio were evaluated across different motion states. PREFUL ventilation metrics were compared with static ¹²⁹Xe ventilation MRI.

Results: Optimal image sharpness and accurate ventilation dynamics were achieved using 24 respiratory bins, leading to their use in the study. A strong correlation was found between 3D PREFUL FLORET UTE ventilation defect percentages (VDPs) and ¹²⁹Xe VDPs (r ≥ 0.61, p < 0.0001), although PREFUL FLORET static VDPs were significantly higher (mean bias = -10.1%, p < 0.0001). In diseased patients, dynamic ventilation parameters showed greater heterogeneity and better alignment with ¹²⁹Xe VDPs.

Conclusion: The proposed reconstruction pipeline for FLORET UTE MRI offers improved spatial resolution and strong correlation with ¹²⁹Xe MRI, enabling dynamic ventilation quantification that may reveal airflow abnormalities in lung disease.

Purpose: The aim of the work is to develop a cascaded diffusion-based super-resolution model for low-resolution (LR) MR tagging acquisitions, which is integrated with parallel imaging to achieve highly accelerated MR tagging while enhancing the tag grid quality of low-resolution images.

Methods: We introduced TagGen, a diffusion-based conditional generative model that uses low-resolution MR tagging images as guidance to generate corresponding high-resolution tagging images. The model was developed on 50 patients with long-axis-view, high-resolution tagging acquisitions. During training, we retrospectively synthesized LR tagging images using an undersampling rate (R) of 3.3 with truncated outer phase-encoding lines. During inference, we evaluated the performance of TagGen and compared it with REGAIN, a generative adversarial network-based super-resolution model that was previously applied to MR tagging. In addition, we prospectively acquired data from 6 subjects with three heartbeats per slice using 10-fold acceleration achieved by combining low-resolution R = 3.3 with GRAPPA-3 (generalized autocalibrating partially parallel acquisitions 3).

Results: For synthetic data (R = 3.3), TagGen outperformed REGAIN in terms of normalized root mean square error, peak signal-to-noise ratio, and structural similarity index (p < 0.05 for all). For prospectively 10-fold accelerated data, TagGen provided better tag grid quality, signal-to-noise ratio, and overall image quality than REGAIN, as scored by two (blinded) radiologists (p < 0.05 for all).

Conclusions: We developed a diffusion-based generative super-resolution model for MR tagging images and demonstrated its potential to integrate with parallel imaging to reconstruct highly accelerated cine MR tagging images acquired in three heartbeats with enhanced tag grid quality.

Purpose: This study proposes a novel, contrast-free Magnetic Resonance Fingerprinting (MRF) method using balanced Steady-State Free Precession (bSSFP) sequences for the quantification of cerebral blood volume (CBV), vessel radius (R), and relaxometry parameters (T ${\hspace{0.17em}}_1$ , T ${\hspace{0.17em}}_2$ , T ${\hspace{0.17em}}_2$ *) in the brain.

Methods: The technique leverages the sensitivity of bSSFP sequences to intra-voxel frequency distributions in both transient and steady-state regimes. A dictionary-matching process is employed, using simulations of realistic mouse microvascular networks to generate the MRF dictionary. The method is validated through in silico and in vivo experiments on six healthy subjects, comparing results with standard MRF methods and literature values.

Results: The proposed method shows strong correlation and agreement with standard MRF methods for T ${\hspace{0.17em}}_1$ and T ${\hspace{0.17em}}_2$ values. High-resolution maps provide detailed visualizations of CBV and microvascular structures, highlighting differences in white matter (WM) and gray matter (GM) regions. The measured GM/WM ratio for CBV is 1.91, consistent with literature values.

Conclusion: This contrast-free bSSFP-based MRF method offers an new approach for quantifying CBV, vessel radius, and relaxometry parameters. Further validation against DSC imaging and clinical studies in pathological conditions is warranted to confirm its clinical utility.

Purpose: To provide a fast quantitative imaging approach for a 0.55T scanner, where signal-to-noise ratio is limited by the field strength and k-space sampling speed is limited by a lower specification gradient system.

Methods: We adapted the three-dimensional spiral projection imaging MR fingerprinting approach to 0.55T scanners, with additional features incorporated to improve the image quality of quantitative brain and musculoskeletal imaging, including (i) improved k-space sampling efficiency, (ii) Cramér-Rao lower bound optimized flip-angle pattern for specified T₁ and T₂ at 0.55 T, (iii) gradient trajectory correction, (iv) attention-based denoising, and (v) motion estimation and correction.

Results: The proposed MRF acquisition and reconstruction framework can provide high-quality 1.2-mm isotropic whole-brain quantitative maps and 1-mm isotropic knee quantitative maps, each acquired in 4.5 min. The proposed method was validated in both phantom and in vivo brain and knee studies.

Conclusion: By proposing novel methods and integrating advanced techniques, we achieved high-isotropic-resolution MRF on a 0.55T scanner, demonstrating enhanced efficiency, motion resilience, and quantitative accuracy.

Purpose: Two-shot γ-aminobutyric acid (GABA) difference editing techniques have been used widely to detect the GABA H4 resonance at 3.01 ppm. Here, we introduce a single-shot method for detecting the full GABA H2 resonance signal, which avoids contamination from the coedited M_3.00 macromolecules.

Methods: Density matrix simulation was conducted to optimize the pulse-sequence timing, aiming to reduce the interfering glutamate H4 signal and minimize the correlation between glutamate and GABA arising from spectral overlap. The optimized sequence was used to acquire MR spectroscopy data from a 14-mL voxel in the anterior cingulate cortex of 6 healthy participants. ¹H-MRS experiments following the oral administration of [U-¹³C]glucose were also conducted.

Results: The GABA H2 peak was consistently observed in all participants. The GABA/creatine ratios in the participants were determined to be 0.07 ± 0.01 with Cramer-Rao lower bounds of 8.0% ± 2.2%. Spectra acquired following [U-¹³C]glucose intake demonstrated the feasibility of using GABA H2 as a highly sensitive reporter for GABA C2.

Conclusion: The proposed single-shot GABA editing method effectively minimizes interference from the glutamate H4 signal in the detection of the full GABA H2 signal, which resonates at a spectral region with much reduced macromolecule contamination.

Purpose: To correct maternal breathing and fetal bulk motion during fetal 4D flow MRI.

Methods: A Doppler-ultrasound fetal cardiac-gated free-running 4D flow acquisition was corrected post hoc for maternal respiratory and fetal bulk motion in separate automated steps, with optional manual intervention to assess and limit fetal motion artifacts. Compressed-sensing reconstruction with a data outlier rejection algorithm was adapted from previous work. Pre- and post-motion correction comparison included qualitative visibility of vasculature on phase-contrast MR angiograms (five-point Likert scale), conservation of mass of the aortic isthmus, ductus arteriosus, and descending aorta, and coefficient of variation of flow along the descending aorta.

Results: Twenty-nine third trimester acquisitions were performed for 15 healthy fetuses and two patients with postnatally confirmed aortic coarctation during a single examination for each participant. Only 15/27 (56%) of all volunteers and 1/2 (50%) of all patient precorrection acquisitions were suitable for flow analysis. Motion correction recovered eight "failed" acquisitions, including one patient, with 24/29 (83%) suitable for flow analysis. In the 15 viable uncorrected volunteer acquisitions, motion correction improved phase-contrast MR angiograms visibility significantly in the ductus arteriosus (from 4.0 to 4.3, p = 0.04) and aortic arch (3.7 to 4.0, p = 0.03). Motion correction improved conservation of mass to a median (interquartile range) percent difference of 5% (9%) from 14% (24%) with improvement shown in 14/15 acquisitions (p = 0.002), whereas coefficient of variation changes were not significantly different (uncorrected: 0.15 (0.09), corrected: 0.11 (0.09), p = 0.3).

Conclusions: Motion correction compensated for maternal and fetal motion in fetal 4D flow MRI data, improving image quality and conservation of mass.

Purpose: Brain temperature is tightly regulated and reflects a balance between cerebral metabolic heat production and heat transfer between the brain, blood, and external environment. Blood temperature and flow are critical to the regulation of brain temperature. Current methods for measuring in vivo brain and blood temperature are invasive and impractical for use in small animals. This work presents a methodology to measure both brain and arterial blood temperature in anesthetized mice by MRI using a paramagnetic lanthanide complex: thulium tetramethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (TmDOTMA^-).

Methods: A phase-based imaging approach using a multi-TE gradient echo sequence was used to measure the temperature-dependent chemical shift difference between thulium tetramethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid methyl protons and water, and from this calculate absolute temperature using calibration data.

Results: In a series of mice in which core body temperature was held stable but at different values within the range of 33° to 37°C, brain temperature away from the midline was independent of carotid artery blood temperature. In contrast, midline voxels correlated with carotid artery blood temperature, likely reflecting the preponderance of larger arteries and veins in this region.

Conclusion: These results are consistent with brain temperature being actively regulated. A limitation of the present implementation is that the spatial resolution in the brain is coarse relative to the size of the mouse brain, and further optimization is required for this method to be applied for finer spatial scale mapping or to characterize focal pathology.

Purpose: In brain tumors, disruption of the blood-brain barrier (BBB) indicates malignancy. Clinical assessment is qualitative; quantitative evaluation is feasible using the K₂ leakage parameter from dynamic susceptibility contrast MRI. However, contrast agent-based techniques are limited in patients with renal dysfunction and insensitive to subtle impairments. Assessing water transport times across the BBB (T_ex) by multi-echo arterial spin labeling promises to detect BBB impairments noninvasively and potentially more sensitively. We hypothesized that reduced T_ex indicates impaired BBB. Furthermore, we assumed higher sensitivity for T_ex than dynamic susceptibility contrast-based K₂, because arterial spin labeling uses water as a freely diffusible tracer.

Methods: We acquired 3T MRI data from 28 patients with intraparenchymal brain tumors (World Health Organization Grade 3 & 4 gliomas [n = 17] or metastases [n = 11]) and 17 age-matched healthy controls. The protocol included multi-echo and single-echo Hadamard-encoded arterial spin labeling, dynamic susceptibility contrast, and conventional clinical imaging. T_ex was calculated using a T₂-dependent multi-compartment model. Areas of contrast-enhancing tissue, edema, and normal-appearing tissue were automatically segmented, and parameter values were compared across volumes of interest and between patients and healthy controls.

Results: T_ex was significantly reduced (-20.3%) in contrast-enhancing tissue compared with normal-appearing gray matter and correlated well with |K₂| (r = -0.347). Compared with healthy controls, T_ex was significantly lower in tumor patients' normal-appearing gray matter (T_ex,tumor = 0.141 ± 0.032 s vs. T_ex,HC = 0.172 ± 0.036 s) and normal-appearing white matter (T_ex,tumor = 0.116 ± 0.015 vs. T_ex,HC = 0.127 ± 0.017 s), whereas |K₂| did not differ significantly. Receiver operating characteristic analysis showed a larger area under the curve for T_ex (0.784) than K₂ (0.604).

Conclusion: T_ex is sensitive to pathophysiologically impaired BBB. It agrees with contrast agent-based K₂ in contrast-enhancing tissue and indicates sensitivity to subtle leakage.