Magnetic Resonance in Medicine最新文献

Purpose: To demonstrate improved image quality and lesion conspicuity in prostate diffusion-weighted imaging (DWI) using an inside-out nonlinear gradient coil that provides locally strong gradients (200-500 500 mT/m) at typical prostate positions.

Theory and methods: Before applying the nonlinear gradient coil to DWI with Echo Planar Imaging (EPI) readout, we investigated geometric distortion and eddy currents, and proposed necessary corrections. We then developed two DWI protocols (b_max = 1000 and 3000 s/mm²) with minimized echo time (TE) and tested them on volunteers and patients. We validated apparent diffusion coefficient (ADC) maps from the nonlinear gradient acquisition against the reference (linear gradients only). We quantified improvements in signal-to-noise ratio (SNR), lesion contrast-to-noise ratio (CNR), and lesion-to-normal-tissue contrast ratio in the compartmental map of restricted diffusion.

Results: Corrections effectively reduced nonlinear-gradient DWI artifacts. ADC maps from linear- and nonlinear-gradient-encoded studies agreed well, with a normalized root-mean-square-error of ∼10%, a common level of ADC variation. TE was significantly reduced from 57 to 42-47 ms for moderate b-values (≤ 1000 s/mm²) and from 72 to 42-54 ms for high b-values (≤ 3000 s/mm²). Consequently, SNR increased by 3%-38% (median 16%, p < 0.01) and 7%-38% (median 26%, p < 0.01), respectively. Lesion CNR improved by a median of 133% at b = 2000 s/mm² and 217% at b = 3000 s/mm². The restricted diffusion component in lesions was more conspicuous at short TE, with a median 23% increase in lesion-to-normal-tissue contrast ratio (p = 0.02).

Conclusion: The inside-out nonlinear gradient coil enhances prostate DWI.

Purpose: Dorsal rhizotomy, or spinal dorsal nerve root lesioning, is a surgical procedure used to treat intractable nerve pain by selectively severing sensory afferent nerve roots. This study aimed to evaluate whether multiparametric MRI, including diffusion tensor imaging (DTI), quantitative magnetization transfer (qMT), chemical exchange saturation transfer (CEST), and relayed nuclear Overhauser enhancement (rNOE), can sensitively detect structural and biochemical changes in the intact spinal cord following a focal dorsal nerve root section in a nonhuman primate model.

Methods: In four squirrel monkeys, unilateral dorsal nerve roots at cervical segments C4 and C5 were surgically transected. MRI data were collected using a 9.4 T scanner with a custom saddle-shaped transmit-receive quadrature coil before and 1 week after lesioning. DTI-derived fractional anisotropy (FA), axial diffusivity (AD), and radial diffusivity (RD); qMT-derived pool size ratio (PSR); and CEST and rNOE effects extracted from five-pool Lorentzian fitting of Z-spectrum, were quantified across seven regions of interest.

Results: At the lesioned dorsal nerve root bundles, FA, PSR, and rNOE (-1.6 ppm) values decreased, while RD and CEST (3.5 ppm) increased, consistent with fiber degeneration, demyelination, and inflammation. Similar, though less pronounced, changes were observed in the dorsal root entry zone, particularly within the first week post-lesion.

Conclusion: Multiparametric MRI enables region-specific detection of spinal cord pathology, including axonal degeneration, demyelination, and possible neuroinflammation, as early as 1 week after dorsal nerve root injury. These results demonstrate its promise for noninvasive monitoring of post-injury pathology and for evaluating therapeutic efficacy.

This article has been temporarily removed pending the resolution of a potential legal issue.

Purpose: To design 3D radial spiral phyllotaxis trajectories aimed at removing phase inconsistencies, improving image quality, and enhancing parametric mapping accuracy by acquiring nearly opposing spokes starting from both hemispheres in 3D radial k-space.

Methods: Two 3D radial trajectories, pole-to-pole and continuous spiral phyllotaxis, were developed and implemented on a 3T MRI scanner in a phase-cycled balanced steady-state free precession (bSSFP) and a spoiled gradient-echo (GRE) sequence. Image quality and k-space center phase variations were evaluated in a spherical phantom using the original and new radial phyllotaxis designs. T1/T2 was quantified and compared using phase-cycled bSSFP data acquired with the new radial trajectory designs, as well as the original phyllotaxis trajectory and a Cartesian trajectory as references, in both an MRI system phantom and the brains of three healthy volunteers. ECG-triggered whole-heart GRE data were acquired using the original and pole-to-pole phyllotaxis trajectories in three healthy volunteers and compared for image quality improvement.

Results: All 3D radial trajectory designs showed variations in the k-space center phase depending on the orientation of the readout spokes. Image quality improved when using the pole-to-pole and continuous phyllotaxis over the original trajectory. Scans using the original trajectory had higher T1/T2 estimation errors in comparison to the new trajectories and the Cartesian trajectory. The pole-to-pole and continuous trajectories improved T1/T2 maps of the brain and image quality for all cardiac images.

Conclusion: Acquiring nearly opposing spokes in 3D radial trajectory designs compensates phase inconsistencies without requiring additional corrections, which improves quantitative imaging and anatomical visualizations.

Purpose: To present a method for quantifying dissolved ¹²⁹Xe spectroscopy using singular value decomposition (SVD) and a dynamic red blood cell (RBC)/membrane ratio as a biomarker of disease.

Methods: A spectroscopic sequence was performed in 45 subjects (27 healthy, 12 dyspnea of unknown origin [DUO], and 6 pulmonary hypertension [PH]) consisting of 499 pulse/acquire experiments. SVD was used to construct a low-noise approximation of FID data, and time-domain curve-fitting was performed on all free induction decays allowing calculation of RBC (218 ppm) and membrane (197 ppm) signal amplitudes and the RBC/membrane ratio. RBC/membrane oscillation amplitudes were assessed using independent t-tests. An Analysis of Covariance (ANCOVA) test was performed to control for age and sex, followed by post hoc Tukey tests for pairwise comparisons.

Results: Independent t-tests demonstrated statistically significant differences in RBC oscillation amplitudes and RBC/membrane oscillation amplitudes among healthy subjects and DUO patients (p-value = 0.003 and p-value = 0.0008, respectively). An ANCOVA test was performed to control for age and sex and resulted in statistically significant differences among diseases (p = 3.12 × 10^-6). A post hoc pairwise Tukey test demonstrated statistical significance among healthy subjects and DUO patients (p = 3.12 × 10^-5) and among healthy and PH patients (p = 2.56 × 10^-5).

Conclusion: Dynamic RBC-to-membrane ratio measurements may yield useful physiological information related to overall lung health.

Purpose: To develop a double-tuned calf coil with excellent ¹H and ³¹P performance to study muscle physiology by interleaved ¹H/³¹P MRS and parallel imaging at 7 T.

Methods: The coil combines three ¹H transceive dipoles, six ¹H receive-only loops and three ³¹P transceive loops, arranged in a nested, half-cylindrical layout. Several different decoupling mechanisms were implemented to limit electromagnetic interactions within the ¹H and ³¹P arrays, between transmit and receive elements as well as ¹H-³¹P cross-coupling. A custom housing was designed for mechanical stability and ease of use. The ¹H and ³¹P performance were investigated on phantoms and demonstrated in vivo in an exemplary measurement with interleaved, localized ¹H/³¹P MRS in the gastrocnemius muscle of a healthy subject at rest, during exercise and recovery.

Results: On phantoms, compared to a double-tuned reference coil, the new coil showed 2.8-fold higher ¹H SNR, a stronger and more homogenous ¹H transmit field, and 27% lower ³¹P SNR. It enabled four-fold GRAPPA acceleration. In vivo the CH₂ resonance of creatine and PCr were fitted from the unaveraged ¹H and ³¹P spectra, which resulted in time courses with closely matching depletion and recovery constants.

Conclusion: These results highlight the coil's suitability for the target application of advanced metabolic calf muscle studies. Its high ¹H performance will enable ¹H MRS acquisitions with improved spatial and temporal resolution and enable the implementation of MRS sequences that had previously been limited by insufficient SNR, which will generate new insights into muscle physiology.

Purpose: Ultrashort echo time (UTE) MRI enables direct imaging of cortical bone and quantification of its water compartments via bicomponent T₂* modeling. However, conventional approaches require multiple separate dual-echo scans due to limitations in gradient power. This approach is prone to inter-scan inconsistencies such as motion and signal drift, which degrade fitting accuracy. This study proposes an interleaved dual-echo acquisition sequence that acquires multiple echo time (TE) images in a single scan to improve bicomponent T₂* quantification in cortical bone.

Methods: The proposed UTE sequence utilizes interleaved dual-echo acquisitions with flexible TE spacings. This sequence was tested on five healthy subjects' tibial midshafts and compared to conventional separate dual-echo scans with and without image registration. Bicomponent T₂* modeling was performed, and fitting accuracy was evaluated using normalized-root-mean-squared error (NRMSE). Three subjects were scanned three times to evaluate the scan repeatability.

Results: The interleaved method significantly reduced NRMSE (3.2% ± 2.3% vs. 6.2% ± 3.1%, p = 0.0231) and yielded lower and more stable T₂* (T_2s*; 0.50 ± 0.10 ms vs. 0.76 ± 0.13 ms, p < 0.0001) and fraction (F_s; 78.2 ± 5.1 vs. 84.2% ± 7.1%, p = 0.0006) of short T₂ components compared to separate scans without registration. Image registration had a minimal improvement on mapping results for separate scans. Parameter maps from the interleaved scans confirmed more homogeneous distributions of T_2s* and F_s with lower fitting errors. The much lower coefficients of variance of the interleaved scans demonstrated improved repeatability compared with separate scans.

Conclusion: The proposed interleaved UTE dual-echo sequence improves the robustness of bicomponent T₂* mapping of the cortical bone by reducing inter-scan inconsistencies.

Purpose: To develop a processing pipeline combining neuroimaging analysis tools with CEST postprocessing and utilize it in a pilot study probing differences between cognitively impaired (CI) Alzheimer's disease (AD) patients and cognitively normal (CN) individuals.

Methods: Eight subjects (4 biomarker confirmed CI with underlying AD and 4 CN) were scanned on a 3T MRI scanner. The processing pipeline included rigid motion correction and co-registration of the CEST images with the 3D T_1w images using Advanced Normalization Tools (ANTs). MTR_asym maps at 1 ppm, 2 ppm and 3.5 ppm were generated on a voxel-by-voxel basis. Each subject's 3D T1w images were processed with FreeSurfer to generate brain region specific ROIs, and the MTR_asym values were averaged over the generated ROIs. In addition, a 6-pool Lorentzian multi-peak model was fitted to averaged Z-spectrum data per ROI. Between-group (CI vs. CN) CEST effects were evaluated using Mann-Whitney U tests.

Results: Motion correction reduced artifacts and visually improved CEST results. Multiple cortical and white matter ROIs showed differences in MTR_asym values between CI and CN groups (p < 0.05). Overall, the mean MTR_asym and Lorentzian amplitudes were lower in the CI than in the CN group.

Conclusion: Our proposed pipeline enables robust data analysis using either MTR_asym or multi-pool Lorentzian quantification approaches. The results suggest feasibility for detecting CEST differences between CI and CN groups in brain regions previously implicated in AD, motivating future validation in larger cohorts.

Purpose: To develop and validate a rapid, fully automated method for shimming of the lung.

Methods: For field mapping of the lung, a custom 2D RF spoiled gradient echo sequence was used offering sub-millisecond echo times. At 3T, four echo-shifted coronal images were acquired with a minimum TE of 0.57 ms and an inter-echo spacing of 0.1 ms. Imaging was performed in free-breathing using three repetitions with a total scan time of 4.2 s. Subsequently, motion correction, field mapping, lung segmentation, and lung shim currents were calculated inline. Finally, the derived shim currents were saved locally, accessible by any custom sequence for application during scan preparation. The shimming method's performance was compared against the vendor's default settings in five healthy volunteers for functional lung imaging with matrix pencil decomposition using balanced steady-state free precession (bSSFP).

Results: Field mapping and shimming of the lung was successfully performed in all volunteers and derivation of shim currents took less than 10s. At 3T, subject-specific shimming reduced the mean frequency offset in the lung by up to 45 Hz and the frequency range (max-min) by up to 180 Hz. Overall, this improved bSSFP signal homogeneity, resulting in more uniform functional images.

Conclusions: This work demonstrates a robust, automated shimming solution for lung imaging at 3T, easily integrable into clinical workflows. The technique significantly enhances image quality and reliability for high-field, bSSFP-based pulmonary imaging. At lower field strengths, the method can possibly relax TR constraints, reducing SAR and peripheral nerve stimulation.

Purpose: Ferritin's iron core exhibits complex magnetic properties, as suggested by magnetometry and Mössbauer spectroscopy, which remain incompletely understood. In particular, the antiferromagnetic inner core could influence the accuracy of iron quantification using MRI and raise concerns about postmortem validation studies involving degraded ferritin cores.

Methods: Fresh postmortem brain samples from six deceased human subjects were analyzed using energy-filtered transmission electron microscopy (EFTEM) and electron energy loss spectrometry in scanning mode of the TEM (STEM-EELS) to visualize and quantify the iron cores of ferritin proteins and estimate their iron content. EFTEM findings were compared with results from mass spectrometry and R₂* mapping at 3T. Analyses focused on three gray matter regions including the frontal cortex, putamen, and globus pallidus.

Results: Autolysis led to a rapid degradation of ferritin molecules, with fewer than one-third remaining detectable via EFTEM 24 h postmortem. The degradation followed a single-exponential decay pattern, suggesting that almost the entire non-heme iron is stored in ferritin. However, R₂* relaxation rates did not follow this degradation pattern but instead correlated strongly with total iron content as measured by mass spectrometry.

Conclusion: R₂* mapping-derived magnetic susceptibility of ferritin appears to be independent of the structural and magnetic organization of its iron core and shows a linear relationship with total iron content. These findings support the interpretation of ferritin as a simple paramagnet at room temperature, without significant antiferromagnetic contributions. Consequently, susceptibility based postmortem studies focusing on iron accumulation are not affected by autolysis.