Magnetic Resonance in Medicine最新文献

Purpose: To enable robust, motion- and distortion-corrected T2-IVIM parameter estimation within clinically feasible scan times.

Methods: A single-shot, multi-echo spin-echo EPI sequence was used to acquire abdominal diffusion-weighted MRI with time-efficient sampling of b-value and TE pairs. The multi-echo acquisition enabled distortion correction using reverse phase-encoding between echoes. Motion and distortion correction were applied before fitting a joint T2-IVIM model across the b-value and TE dimensions to obtain TE-independent IVIM parameters and compartment-specific T2 estimates. For comparison, a previously established single-echo T2-IVIM protocol with longer scan times and a single-echo protocol matched to the multi-echo parameters were acquired. Uncertainty was evaluated with wild bootstrap error analysis.

Results: The multi-echo approach enabled motion- and distortion-corrected T2-IVIM mapping in under 5 min, compared with 11-13 min for the prior minimal single-echo protocol or nearly 19 min when acquired as separate shots. The liver was selected as the target organ due to its marked sensitivity to $T_2$ effects in standard IVIM. Error analysis showed comparable per-voxel uncertainty between the multi-echo method and the minimal single-echo protocol.

Conclusion: The combination of multi-echo sequence design and artifact correction enabled stable fitting of the extended T2-IVIM model with improved liver coverage and less than half the scan time of prior protocols. These advances support broader clinical applicability of T2-IVIM imaging by reducing acquisition burden while enhancing artifact correction and parameter robustness.

Purpose: High-resolution diffusion-weighted imaging (DWI) is clinically demanding. The purpose of this work is to develop an efficient self-supervised algorithm unrolling technique for submillimeter-resolution DWI.

Methods: We developed submillimeter DWI acquisition utilizing multi-band multi-shot EPI with diffusion shift encoding. We unrolled the alternating direction method of multipliers (ADMM) to perform scan-specific self-gated self-supervised DeepDWI learning for multi-shot echo planar imaging with diffusion shift encoding on a clinical 7 T scanner.

Results: We demonstrate that (1) ADMM unrolling is generalizable across slices, (2) ADMM unrolling outperforms multiplexed sensitivity-encoding (MUSE) and compressed sensing with locally-low rank (LLR) regularization in terms of image sharpness, tissue continuity, and motion robustness, and (3) ADMM unrolling enables clinically feasible inference time.

Conclusion: Our proposed ADMM unrolling enables whole brain DWI of 21 diffusion volumes at 0.7 mm isotropic resolution and 10 min scan, and shows higher signal-to-noise ratio (SNR), clearer tissue delineation, and improved motion robustness, which makes it plausible for clinical translation.

Purpose: Magnetic resonance imaging (MRI) is a sensitive method for assessing silicone implant integrity, with $T_2$ -weighted imaging being essential for detecting abnormalities in surrounding tissue. Silicone breast imaging protocols often require multiple tailored sequences for species suppression and diagnostic contrast. We propose a single sequence suitable for patients with or without implants that enables $T_2$ -weighted, high-quality imaging and three-species separation within a clinically feasible scan time.

Methods: Our approach uses a 2D fast spin echo (FSE) sequence with seven bipolar multi-echo gradient echo readouts, enabling field mapping and water-fat-silicone separation. Incoherent $k_y$ - $TE$ undersampling combined with joint multi-echo reconstruction leverages temporal correlations and applies compressed sensing regularization directly to the separated species.

Results: We achieve high-resolution, artifact-free water, fat, and silicone (WFS) images across three planes from one sequence, regardless of shim quality, and for different breast implant types. Compared to independent echo reconstruction and separation, joint multi-echo reconstruction with incoherent $k_y$ - $TE$ sampling allows acceleration of $R=6$ , reducing scan time to 2.5 minutes.

Conclusion: We demonstrate a robust $T_2$ -weighted technique that provides reliable water-fat-silicone imaging in 2.5 minutes, enabling uniform breast protocols for patients with and without silicone implants.

Purpose: Brain MR imaging at 10.5 T ultra-high field offers significant improvements in signal-to-noise ratio (SNR), but faces challenges with B₁ ⁺ inhomogeneity. Parallel-transmission (pTx) can be used to achieve a more uniform RF field distribution, but necessitates the use of B₁ ⁺ calibration in the region of interest. This study explores a universal B₁ ⁺ shim solution on 10.5 T that could eliminate the need for time-consuming subject-specific B₁ ⁺ calibration.

Methods: B₁ ⁺ data from 7 participants (19 sessions) were used to develop the universal B₁ ⁺ shim, which was then validated against traditional subject-specific approaches using T₁-weighted MP2RAGE structural images in 5 participants (6 sessions). Statistical comparisons of tissue and subcortical segmentations were conducted using popular neuroimaging tools SPM and FreeSurfer, respectively.

Results: The universal shim rapidly converged with a small training dataset, likely due to consistent positioning and the simplicity of B₁ ⁺ shimming used for head imaging. Whole-brain tissue segmentation showed no statistically significant differences between universal and subject-specific solutions, with only minor variations near the ventricles and inferior brain regions in the detailed subcortical segmentation. The proposed universal B₁ ⁺ shim reduces examination time by removing the need for separate data acquisition and optimization.

Conclusion: These findings suggest that the universal B₁ ⁺ shim is a viable substitute for subject-specific approaches, offering a more efficient solution for neuroimaging applications. Additionally, it confirms that 10.5 T MRI can produce reliable structural brain imaging data, paving the way for broader adoption of ultra-high field MRI in neuroimaging research.

Purpose: Metabolite concentrations can be determined from proton magnetic resonance spectroscopy (¹H-MRS) data using water as an internal reference. This calculation requires tissue-specific water T₁ and T₂ relaxation constants and proton density (PD). Although literature values are commonly used, these vary with age and within clinical conditions, potentially introducing variability or masking metabolite effects. The introduction of rapid multi-dynamic multi-echo (MDME) imaging to generate multiparametric maps allows fast measurement of these parameters for each individual within a single acquisition.

Methods: ¹H-MRS and MDME data were collected from 26 healthy volunteers (aged 18-40 years). The agreement between metabolite concentrations derived using individually measured T₁, T₂ and PD values and literature-based values was assessed. A sensitivity analysis was also used to determine the impact of extended value ranges on metabolite concentrations.

Results: Using a MDME sequence to determine individually measured T₁, T₂, and PD values for tissue correction was successful. Strong agreement between metabolite concentrations calculated using literature and measured values was seen, although concentrations calculated using literature values tended to be slightly higher than when using measured values. The sensitivity analysis showed T₁ relaxation contributed most strongly to the calculated concentration variability.

Conclusion: This study demonstrates the feasibility of using a MDME acquisition to acquire individual- specific parameter values for tissue correction. This provides a fast, effective method to acquire individual relaxation parameters, which will be highly relevant for populations where these parameters will vary (such as the elderly, pediatrics or with clinical diagnoses).

Purpose: To create a frequency-modulated pulse for slice selection of arbitrary, uniform flip angle even when B₀ and B₁ are inhomogeneous, without utilizing B₀ gradients THEORY AND METHODS: An amplitude-modulated hyperbolic secant pulse (AM_HS1) was derived from Hoult's B₁-selective imaging method which utilizes one component, B_1y, as a gradient and another, B_1x, for selective excitation. The frequency sweep in AM_HS1 was produced by a time-dependent amplitude-modulation of B_1x, defining the band of nutation frequencies to select along the B_1y gradient. Resilience of slice selection to B₀ and B₁ inhomogeneities was investigated by simulations. Slice- and slab-selective imaging were demonstrated experimentally in phantoms and rat brain in vivo using surface coils.

Results: Simulations of AM_HS1 demonstrated slice inversion despite B₀ and B₁ inhomogeneities. When operating sub-adiabatically to produce excitation flip angles < 180° with a single coil, the flip angle across the slice varied because both B_1x and B_1y gradients were present. This problem was corrected by scaling B_1x(t) by the normalized frequency-sweep and phase-modulation functions. Slice selection using only a B₁ gradient was demonstrated on phantoms using a pair of AM_HS1 pulses transmitted with a surface coil. By using a low-flip angle AM_HS1 with B_1y refocusing lobes, slice- and slab-selective excitation was realized in 3D gradient-echo imaging of rat brain in vivo at 9.4 T.

Conclusion: By implementing frequency modulation in a second rotating frame, B₁-selective excitation and inversion are feasible, even when B₀ and B₁ are nonuniform.

Purpose: Emissions generated during magnetic resonance imaging (MRI)-including gradient coil induced electric fields and radiofrequency coil induced heating near nerve fiber-may alter neural activation inside patients. This study investigates the combined effects of these emissions on vagus nerve activation in the presence of cuff electrodes.

Methods: Electromagnetic, thermal, and neurophysiological simulations were performed to quantify activation thresholds under MRI-induced fields. The study examined the impact of gradient field exposure and RF-induced heating, particularly for the trapezoidal waveform of the gradient coil with short pulse duration.

Results: The results indicate that the presence of the cuff electrode significantly reduces the activation threshold under gradient field exposure, while RF-induced heating further decreases the threshold for stimulations with short pulse durations. In some scenarios, the reduced neuron activation threshold can be lower than peripheral nerve stimulation limits defined in IEC 60601-2-33.

Conclusion: These findings indicate the potential risk of unintended vagus nerve stimulation in MRI environments, emphasizing the need for safety considerations in patients with implantable vagus nerve stimulators.

Purpose: Our goal is to develop and validate a practical protocol that guides users in identifying and suppressing electromagnetic noise in low-field MRI systems, enabling operation near the thermal noise limit.

Methods: We present a systematic, stepwise methodology that includes diagnostic measurements, hardware isolation strategies, and good practices for cabling and shielding. Each step is validated with corresponding noise measurements under increasingly complex system configurations, both unloaded and with a human subject present.

Results: Noise levels were monitored through the incremental assembly of a low-field MRI system, revealing key sources of EMI and quantifying their impact. Final configurations achieved noise within 1.5 $\times$ the theoretical thermal bound with a subject in the scanner. Image reconstructions illustrate the direct relationship between system noise and image quality.

Conclusion: The proposed protocol enables low-field MRI systems to operate close to fundamental noise limits in realistic conditions. The framework also provides actionable guidance for the integration of additional system components, such as gradient drivers and automatic tuning networks, without compromising signal-to-noise ratio (SNR).

Purpose: To mitigate artifacts related to motion and field changes in high-resolution $T_2^{*}$ -weighted human brain imaging using servo navigation at ultra-high fields up to 11.7 T.

Methods: MR-based servo navigators were integrated into a segmented 3D-EPI sequence to allow for prospective correction of involuntary head motion and first-order shim changes. Seven subjects were scanned with whole-brain protocols at 0.3 mm isotropic resolution with and without correction at 7 and 11.7 T. Validation was performed on detailed brain vasculature in scans with involuntary motion.

Results: Blurring of small veins was reduced by servo navigation for all subjects and across field strengths. In case of involuntary large motion, the method preserved image quality, while uncorrected motion led to severe artifacts. In case of microscopic motion, reduced blurring and shading in the frontal lobe demonstrate the additional benefit of prospective field drift correction.

Conclusion: Servo-navigated segmented 3D-EPI improves 0.3 mm isotropic whole-brain $T_2^{*}$ -weighted imaging under realistic motion and field changes within 5.5 to 11 min scan time at 11.7 and 7 T.

Purpose: Functional MRI (fMRI) in awake rodents presents countless valuable opportunities for researchers to probe questions that may not be accessible through anesthetized models, such as voluntary locomotion. The commonly used echo planar imaging (EPI) sequence is highly sensitive to motion that occurs even outside of the imaging plane. Recently, zero echo time sequences have been adopted for fMRI to address this challenge.

Methods: This study proposes a robust and reproducible protocol for longitudinal imaging of awake mice during spontaneous locomotion, using an implanted headpiece, incremental training, zero TE fMRI, reinforcement learning, and a custom treadmill module. Locomotion is known to have wide-ranging effects on brain activity and can alter neurovascular coupling, making it critical to understand this aspect of natural behavior.

Results: We present results from 10 trained mice across three different fMRI scanning sessions, finding minimal head motion across scans (average framewise displacement matched anesthetized EPI (p > 0.05)), consistent resting-state functional connectivity across subjects and scans, and evidence of a minimal stress response at the group and individual level. We also demonstrate little difference on signal quality during locomotion and altered functional connectivity and spatiotemporal dynamics during locomotion compared to rest.

Conclusions: This work establishes a new benchmark for awake rodent fMRI, enabling the direct investigation of naturalistic behaviors like locomotion and their whole-brain correlates without the confounding effects of anesthesia or excessive restraint.