Magnetic Resonance in Medicine最新文献

Purpose: 4D flow MRI provides comprehensive evaluation of cardiovascular flow. One major limitation of intracardiac 4D flow is poor blood-myocardial contrast. 2D phase contrast using balanced steady state free precession (PC-SSFP) methods have been demonstrated to provide accurate velocity, with enhanced contrast and SNR. In this work, we extended our 2D PC-SSFP to 4D flow at clinical field strengths, and tested it for diastolic evaluation.

Methods: The 4D flow sequence with four-point encoding was modified to have 0th and 1st moment gradient nulling over each TR to achieve bSSFP contrast. Pixel-wise velocities were validated in a flow phantom at 3 T. Mitral inflow peak velocity (E, A, e') and stroke volume (SV) were compared in 14 scan (13 healthy subjects at 3 T, with one subject scanned again at 1.5 T) with standard 2D and 4D flow GRE methods.

Results: In phantom study, 4D flow bSSFP strongly agreed with GRE, with r > 0.9 in all three directions. Significantly improved SNR (42.4 ± 24.7 vs. 16.9 ± 8.5) and blood-tissue CNR (9.7 ± 3.3 vs. 2.3 ± 1.5) were found in vivo. 4D flow bSSFP measured comparable E (limits of agreement 1.3 ± 14.6 cm/s, r = 0.88), A (0.3 ± 11.3 cm/s, r = 0.95), e' (1.3 ± 3.4 cm/s, r = 0.71), and SV (-2.6 ± 9.7 mL, r = 0.91) vs. GRE approach, and showed similar agreement with 2D methods (r = 0.64-0.91). A study at 1.5 T suggested its potential applicability at lower field strength, with reduced susceptibility to off-resonance artifacts.

Conclusion: Our 4D flow bSSFP method is feasible, achieving improved SNR, CNR, and accurately measuring mitral velocity and volume at clinical field strengths.

Purpose: The intravoxel incoherent motion (IVIM) model is commonly used to separate the effects of motion related to diffusion and blood microcirculation (perfusion) on the MR signal. Depending on the encoding time (T), it is possible to probe the different temporal regimes of blood motion, which resemble a ballistic flow at short T and a pseudo-diffusion at long T. The purpose of this work was to derive an encoding-time-dependent analytical model for flow-compensated IVIM and to estimate the corresponding microvascular IVIM parameters in healthy brain.

Theory and methods: An encoding-time-dependent analytical IVIM model was derived for flow-compensated/non-flow-compensated (FC/NC) double diffusion encoding (DDE) from the Langevin equation and validated using simulations. Eleven healthy participants were scanned to estimate microvascular IVIM parameters (blood velocity ν and blood correlation time τ) in healthy brain using the proposed model, with T = 50-100 ms.

Results: The IVIM parameters were estimated to be τ = 123.1 ± 50 ms, ν = 1.51 ± 0.76 mm/s, perfusion fraction f = 4.75 ± 1.94%, and tissue diffusion coefficient D = 0.91 ± 0.32 μm²/ms in the healthy human brain, although simulations indicate a positive bias for τ. For very short/long T, the proposed model approaches established models for the ballistic/diffusive regimes. Pseudocode for the derivation of the analytical model is presented to facilitate a transfer to other gradient waveforms or pulse sequences.

Conclusion: An encoding-time-dependent analytical IVIM model is presented for FC/NC DDE. In vivo results and simulations indicate that IVIM experiments with encoding times typical for clinical MRI scanners probe an intermediate to ballistic blood flow regime in the brain.

Purpose: Extend the universal pulse GRAPE formalism to pulses with a defined spectral response, and apply the concept to spatial selection.

Methods: We added Bloch simulations at several frequencies for each voxel to the pulse calculation to create universal spectrally-selective GRAPE pulses. With a superimposed constant gradient field spatial selection was achieved. The method was tested in slice- and slab-selective imaging experiments.

Results: Universal spatially-selective GRAPE pulses increased FA homogeneity and SNR. In 2D gradient echoes, the SNR could be increased by approximately 6% compared to CP pulses, and in a slab-selective TSE sequence, the SNR increased by 29% against k ${}_T$ -spokes pulses. Additionally, the slab-selective GRAPE pulse proved to be more robust against $B_0$ deviations and is significantly shorter in comparison to k ${}_T$ -spokes pulses while maintaining a similar FA homogeneity.

Conclusion: Spatially-selective universal GRAPE pulses exhibit superior performance compared to k ${}_T$ -spokes pulses. These short and robust pTx pulses hold potential for enhancing a wide range of imaging applications, thereby advancing 7T MRI technology closer to clinical use.

Purpose: A modified interleaved flyback (miFB) approach is introduced, designed to mitigate flow artifacts caused by alternating readout polarities in Echo Planar Imaging (EPI), while preserving acquisition efficiency.

Methods: We propose reconstructing odd and even echoes of 3D-EPI separately. To this end, the respective missing lines are acquired in interleaved shots with inverted polarity and an additional gradient pre-lobe. Thereby, high scan efficiency is maintained compared to unsampled flyback gradients. Our miFB approach is additionally combined with gradient moment smoothing and compared to the interleaved dual-echo with acceleration (IDEA) method in phantom and in vivo scans at 7 Tesla.

Results: Our results demonstrate a significant reduction in ghosting and signal dropout using the miFB approach, yielding comparable image quality to non-EPI acquisitions while reducing acquisition time by approximately half.

Conclusion: The miFB approach offers a substantial reduction in flow artifacts, allowing for decreased acquisition times in TOF-MRA.

Purpose: To investigate how the relaxation rates (R₁, R₂) and asymmetry indices (AI), derived from phase-cycled balanced steady-state free precession (pc-bSSFP) data, depend on the orientation of white matter (WM) fiber tracts at different field strengths.

Methods: Phase-cycled bSSFP data acquired at 3 and 9.4T in the healthy human brain were processed using motion-insensitive rapid configuration relaxometry (MIRACLE) and a frequency response analysis to derive R₁, R₂, and AI values, respectively. Fractional anisotropy (FA) and fiber-to-field angle (θ) were estimated based on 3T diffusion tensor imaging. The orientation dependence of R₁, R₂, and AI in WM was characterized using literature model fits as well as Monte Carlo random walk simulations to explore the influence of field strength and susceptibility effects.

Results: R₂ and AI exhibited a pronounced orientation dependence while the influence of anisotropy on R₁ was weaker, but noticeable. The observed anisotropy increased systematically from 3 to 9.4T. Literature models assuming either a susceptibility or a generalized magic angle effect described the R₂ and AI anisotropy to a high degree (R² ≥ 0.99). The calculated partial contributions of susceptibility to R₂ anisotropy increased from 24.0%-39.0% at 3T to 77.0%-87.1% at 9.4T. The Monte Carlo simulations were able to reproduce the characteristics of R₂ anisotropy, but not its strength.

Conclusion: Microstructure-driven relaxation anisotropy considerably affects pc-bSSFP relaxometry, in particular R₂. The findings indicate that R₂ anisotropy is driven by susceptibility at ultra-high fields whereas additional mechanisms likely contribute at lower field strengths.

Purpose: In the course of diffusion, water molecules encounter varying values for the relaxation-time properties of the underlying tissue. This factor, which has rarely been accounted for in diffusion MRI (dMRI), is modeled in this work, allowing for the estimation of the gradient of relaxation-time properties from the dMRI signal.

Methods: With the aim of mining the dMRI data for information about spatial variations in tissue relaxation-time properties, a new mathematical relationship between the diffusion signal and the spatial gradient of the image is derived, enabling the estimation of the latter from the former. The hypothesis was validated on human brain dMRI images from three datasets: the public Human Connectome Project Young Adults database, 10 healthy volunteers and 1 ex vivo sample scanned in-house with stimulated-echo diffusion encoding and a long diffusion time of 1 s (which we have made publicly available), and three subjects from the public Multi-TE database. The effects of the confounding factor of "fiber continuity" were furthermore measured.

Results: The spatial image gradient estimated from the diffusion signal was compared to the gold-standard spatial gradient approximated using the finite difference method. The former gradient was significantly related to the latter in all datasets (i.e., with a difference significantly smaller than chance), with an effect distinct from fiber continuity.

Conclusion: The results support the hypothesized relationship between within-voxel dMRI signal and image gradient, with an effect that was not explainable by the confounding factor of fiber continuity.

Purpose: To develop and evaluate an ²H/¹H coil configuration that enables deuterium metabolic imaging at 7 T while preserving high-quality ¹H anatomical imaging.

Methods: An 16-channel ²H high-impedance receive array was combined with a ²H transmit birdcage and a 16-channel dual-row ¹H transceiver array. Electromagnetic simulations are used to assessed B₁ ⁺ efficiency and SAR. Phantom and in vivo experiments were performed on a Philips 7T system to evaluate transmit efficiency, signal-to-noise ratio, and signal coverage.

Results: Simulations results match with experimental ones, and they demonstrated good transmit efficiency, receive sensivity and image quality. In vivo 2H 3DMRSI map demonstrated whole brain coverage.

Conclusion: The proposed coil configuration enables robust 2H metabolic imaging at 7T while preserving good 1H performance.

Purpose: This study aims to assess the repeatability and reproducibility of qBOLD+QSM (QQ) oxygen extraction fraction (OEF) measurements across 3 and 1.5 T.

Methods: The effects of field strength on signal to noise ratio (SNR) and OEF sampling time were experimentally assessed in 14 healthy subjects using repeated scans performed at 3 and 1.5 T. Whole-brain and regional OEF values were analyzed using Bland-Altman and correlation between repeated scans and across field strengths.

Results: Whole-brain and regional OEF values showed strong agreement between repeated scans at the same field strength, with minimal differences (≤ 0.74%) and high correlation (r > 0.92). Across field strengths, comparisons similarly showed small mean differences (≤ 1.51%) and strong correlations (r > 0.95).

Conclusion: QQ-OEF has good repeatability and reproducibility at both 3 and 1.5 T. Good performance at 1.5 T may arise from accurate noise modeling and longer sampling times at lower field strengths.

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.