Clinical usability of 3D gradient-echo-based ultrashort echo time imaging: Is it enough to facilitate diagnostic decision in real-world practice?

BACKGROUND: With recent advances in magnetic resonance imaging (MRI) technology, the practical role of lung MRI is expanding despite the inherent challenges of the thorax. The purpose of our study was to evaluate the current status of the concurrent dephasing and excitation (CODE) ultrashort echo-time sequence and the T1-weighted volumetric interpolated breath-hold examination (VIBE) sequence in the evaluation of thoracic disease by comparing it with the gold standard computed tomography (CT). METHODS: Twenty-four patients with lung cancer and mediastinal masses underwent both CT and MRI including T1-weighted VIBE and CODE. For CODE images, data were acquired in free breathing and end-expiratory images were reconstructed using retrospective respiratory gating. All images were evaluated through qualitative and quantitative approaches regarding various anatomical structures and lesions (nodule, mediastinal mass, emphysema, reticulation, honeycombing, bronchiectasis, pleural plaque and lymphadenopathy) inside the thorax in terms of diagnostic performance in making specific decisions. RESULTS: Depiction of the lung parenchyma, mediastinal and pleural lesion was not significant different among the three modalities (p > 0.05). Intra-tumoral and peritumoral features of lung nodules were not significant different in the CT, VIBE or CODE images (p > 0.05). However, VIBE and CODE had significantly lower image quality and poorer depiction of airway, great vessels, and emphysema compared to CT (p < 0.05). Image quality of central airways and depiction of bronchi were significantly better in CODE than in VIBE (p < 0.001 and p = 0.005). In contrast, the depiction of the vasculature was better for VIBE than CODE images (p = 0.003). The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were significant greater in VIBE than CODE except for SNRlung and SNRnodule (p < 0.05). CONCLUSIONS: Our study showed the potential of CODE and VIBE sequences in the evaluation of localized thoracic abnormalities including solid pulmonary nodules.

Assessment of Pulmonary Ventilation Using 3D Ventilation Flow-Weighted and Ventilation-Weighted Maps From 3D Ultrashort Echo Time (UTE) MRI.

BACKGROUND: Three-dimensional (3D) ventilation flow-weighted (VFW) maps together with 3D ventilation-weighted (VW) maps may help to better assess pulmonary function. PURPOSE: To investigate the use of 3D VFW and VW maps for evaluating pulmonary ventilation function. STUDY TYPE: Prospective. POPULATION: Two patients (one male, 85 years old; one female, 64 years old) with chronic obstructive pulmonary disease (COPD) and nine healthy subjects (all male; 23-27 years). FIELD STRENGTH/SEQUENCE: 3-T, 3D radial UTE imaging. ASSESSMENT: 3D VFW and VW maps were calculated from 3D UTE MRI by voxel-wise subtraction of respiratory phase images. Their validation was tested in nine healthy volunteers using slow/deep and fast/shallow breathing conditions. Additional validation was performed by comparison with single photon emission computed tomography (SPECT) ventilation maps of one healthy participant. For comparison, gravity dependence of anterior-posterior regional ventilation was assessed by one-dimensional plot of the mean signal intensity for each coronal slice. Structural similarity index measure was also calculated. Finally, VW maps and VFW maps of two COPD patients were evaluated for emphysema lesions with reference to CT images. STATISTICAL TESTS: Wilcoxon sign-rank tests for regional Ventilation and Ventilation flow, analysis of variance, post-hoc t-tests and Bonferroni correction, coefficient of variation, Kullback-Liebler divergence. A P-value <0.05 was considered statistically significant. RESULTS: The validation of 3D VFW and VW maps was shown by statistically significant differences in ventilation flow and ventilation between the breathing conditions. Additionally, UTE-MRI and SPECT-based ventilation maps showed gravitational dependence in the anteroposterior direction. When applied to patients with COPD, the use of 3D VFW and VW maps was able to differentiate between two patients with different phenotypes. DATA CONCLUSION: The use of 3D VFW and VW maps can provide regional information on ventilation function and potentially contribute to assessment of COPD subtypes and disease progression. EVIDENCE LEVEL: 2 TECHNICAL EFFICACY: Stage 1.

In vivo direct imaging of neuronal activity at high temporospatial resolution.

There has been a long-standing demand for noninvasive neuroimaging methods that can detect neuronal activity at both high temporal and high spatial resolution. We present a two-dimensional fast line-scan approach that enables direct imaging of neuronal activity with millisecond precision while retaining the high spatial resolution of magnetic resonance imaging (MRI). This approach was demonstrated through in vivo mouse brain imaging at 9.4 tesla during electrical whisker-pad stimulation. In vivo spike recording and optogenetics confirmed the high correlation of the observed MRI signal with neural activity. It also captured the sequential and laminar-specific propagation of neuronal activity along the thalamocortical pathway. This high-resolution, direct imaging of neuronal activity will open up new avenues in brain science by providing a deeper understanding of the brain's functional organization, including the temporospatial dynamics of neural networks.

Neural correlates of tactile hardness intensity perception during active grasping.

While tactile sensation plays an essential role in interactions with the surroundings, relatively little is known about the neural processes involved in the perception of tactile information. In particular, it remains unclear how different intensities of tactile hardness are represented in the human brain during object manipulation. This study aims to investigate neural responses to various levels of tactile hardness using functional magnetic resonance imaging while people grasp objects to perceive hardness intensity. We used four items with different hardness levels but otherwise identical in shape and texture. A total of Twenty-five healthy volunteers participated in this study. Before scanning, participants performed a behavioral task in which they received a pair of stimuli and they were to report the perceived difference of hardness between them. During scanning, without any visual information, they were randomly given one of the four objects and asked to grasp it. We found significant blood oxygen-level-dependent (BOLD) responses in the posterior insula in the right hemisphere (rpIns) and the right posterior lobe of the cerebellum (rpCerebellum), which parametrically tracked hardness intensity. These responses were supported by BOLD signal changes in the rpCerebellum and rpIns correlating with tactile hardness intensity. Multidimensional scaling analysis showed similar representations of hardness intensity among physical, perceptual, and neural information. Our findings demonstrate the engagement of the rpCerebellum and rpIns in perceiving tactile hardness intensity during active object manipulation.

Line scan-based rapid magnetic resonance imaging of repetitive motion.

Two-dimensional (2D) line scan-based dynamic magnetic resonance imaging (MRI) is examined as a means to capture the interior of objects under repetitive motion with high spatiotemporal resolutions. The method was demonstrated in a 9.4-T animal MRI scanner where line-by-line segmented k-space acquisition enabled recording movements of an agarose phantom and quail eggs in different conditions-raw and cooked. A custom MR-compatible actuator which utilized the Lorentz force on its wire loops in the scanner's main magnetic field effectively induced the required periodic movements of the objects inside the magnet. The line-by-line k-space segmentation was achieved by acquiring a single k-space line for every frame in a motion period before acquisition of another line with a different phase-encode gradient in the succeeding motion period. The reconstructed time-course images accurately represented the objects' displacements with temporal resolutions up to 5.5 ms. The proposed method can drastically increase the temporal resolution of MRI for imaging rapid periodic motion of objects while preserving adequate spatial resolution for internal details when their movements are driven by a reliable motion-inducing mechanism.

An equal-TE ultrafast 3D gradient-echo imaging method with high tolerance to magnetic susceptibility artifacts: Application to BOLD functional MRI.

PURPOSE: To develop an ultrafast 3D gradient echo-based MRI method with constant TE and high tolerance to B0 inhomogeneity, dubbed ERASE (equal-TE rapid acquisition with sequential excitation), and to introduce its use in BOLD functional MRI (fMRI). THEORY AND METHODS: Essential features of ERASE, including spin behavior, were characterized, and a comparison study was conducted with conventional EPI. To demonstrate high tolerance to B0 inhomogeneity, in vivo imaging of the mouse brain with a fiber-optic implant was performed at 9.4 T, and human brain imaging (including the orbitofrontal cortex) was performed at 3 T and 7 T. To evaluate the performance of ERASE in BOLD-fMRI, the characteristics of SNR and temporal SNR were analyzed for in vivo rat brains at 9.4 T in comparison with multislice gradient-echo EPI. Percent signal changes and t-scores are also presented. RESULTS: For both mouse brain and human brain imaging, ERASE exhibited a high tolerance to magnetic susceptibility artifacts, showing much lower distortion and signal dropout, especially in the regions involving large magnetic susceptibility effects. For BOLD-fMRI, ERASE provided higher temporal SNR and t-scores than EPI, but exhibited similar percent signal changes in in vivo rat brains at 9.4 T. CONCLUSION: When compared with conventional EPI, ERASE is much less sensitive, not only to EPI-related artifacts such as Nyquist ghosting, but also to B0 inhomogeneity including magnetic susceptibility effects. It is promising for use in BOLD-fMRI, providing higher temporal SNR and t-scores with constant TE when compared with EPI, although further optimization is needed for human fMRI.

Strategies for rapid reconstruction in 3D MRI with radial data acquisition: 3D fast Fourier transform vs two-step 2D filtered back-projection.

For 3D radial data reconstruction in magnetic resonance imaging (MRI), fast Fourier transform via gridding (gFFT) is widely used for its fast processing and flexibility. In comparison, conventional 3D filtered back projection (cFBP), while more robust against common radial k-space centering errors, suffers from long computation times and is less frequently used. In this study, we revisit another back-projection reconstruction strategy, namely two-step 2D filtered back-projection (tsFBP), as an alternative 3D radial MRI reconstruction method that combines computational efficiency and certain error tolerance. In order to compare the three methods (gFFT, cFBP, and tsFBP), theoretical analysis was performed to evaluate the number of computational steps involved in each method. Actual reconstruction times were also measured and compared using 3D radial-MRI data of a phantom and a human brain. Additionally, the sensitivity of tsFBP to artifacts caused by radial k-space centering errors was compared with the other methods. Compared to cFBP, tsFBP dramatically improved the reconstruction speed while retaining the benefit of tolerance to the radial k-space errors. Our study therefore suggests that tsFBP can be a promising alternative to the conventional back projection method for 3D radial MRI reconstruction.

k-Space deep learning for reference-free EPI ghost correction.

PURPOSE: Nyquist ghost artifacts in echo planar imaging (EPI) are originated from phase mismatch between the even and odd echoes. However, conventional correction methods using reference scans often produce erroneous results especially in high-field MRI due to the nonlinear and time-varying local magnetic field changes. Recently, it was shown that the problem of ghost correction can be reformulated as k-space interpolation problem that can be solved using structured low-rank Hankel matrix approaches. Another recent work showed that data driven Hankel matrix decomposition can be reformulated to exhibit similar structures as deep convolutional neural network. By synergistically combining these findings, we propose a k-space deep learning approach that immediately corrects the phase mismatch without a reference scan in both accelerated and non-accelerated EPI acquisitions. THEORY AND METHODS: To take advantage of the even and odd-phase directional redundancy, the k-space data are divided into 2 channels configured with even and odd phase encodings. The redundancies between coils are also exploited by stacking the multi-coil k-space data into additional input channels. Then, our k-space ghost correction network is trained to learn the interpolation kernel to estimate the missing virtual k-space data. For the accelerated EPI data, the same neural network is trained to directly estimate the interpolation kernels for missing k-space data from both ghost and subsampling. RESULTS: Reconstruction results using 3T and 7T in vivo data showed that the proposed method outperformed the image quality compared to the existing methods, and the computing time is much faster. CONCLUSIONS: The proposed k-space deep learning for EPI ghost correction is highly robust and fast, and can be combined with acceleration, so that it can be used as a promising correction tool for high-field MRI without changing the current acquisition protocol.

A new ultrafast 3D gradient echo-based imaging method using quadratic-phase encoding.

PURPOSE: To propose a novel 3D ultrafast gradient echo-based MRI method, dubbed RASE, using quadratic-phase encoding. THEORY AND METHODS: Several characteristics of RASE, including spin behaviors, spatial resolution, SNR, and reduction of susceptibility-induced signal loss, were analytically described. A way of compensating for TE variation was suggested in the quadratic phase-encoding direction. Lemon, in vivo rat and mouse images were demonstrated at 9.4T, including a feasibility study for DCE-MRI as one of promising applications. RESULTS: RASE was successfully demonstrated by lemon and in vivo rat brain imaging, showing a good robustness to field inhomogeneity. Contribution of the quadratic phase to signal enhancement in a range of magnetic susceptibilities was also evaluated by simulation. Taking a geometric mean of 2 phantom data acquired with opposite gradient polarities effectively compensated for the effect of TE variation. Preliminary DCE-MRI results were also presented, showing that RASE could more accurately estimate Gd concentration than FLASH. CONCLUSION: RASE offers a shorter effective TE, having less sensitivity to field inhomogeneity and T2* effects, much less Nyquist ghosting or chemical-shift artifacts than gradient echo EPI (GE-EPI). We highly anticipate that RASE can be an alternative to GE-EPI in many applications, particularly those requiring high spatial and temporal resolutions in a broad volume coverage.

Gradient-echo and spin-echo blood oxygenation level-dependent functional MRI at ultrahigh fields of 9.4 and 15.2 Tesla.

PURPOSE: Sensitivity and specificity of blood oxygenation level-dependent (BOLD) functional MRI (fMRI) is sensitive to magnetic field strength and acquisition methods. We have investigated gradient-echo (GE)- and spin-echo (SE)-BOLD fMRI at ultrahigh fields of 9.4 and 15.2  Tesla. METHODS: BOLD fMRI experiments responding to forepaw stimulation were performed with 3 echo times (TE) at each echo type and B0 in α-chloralose-anesthetized rats. The contralateral forelimb somatosensory region was selected for quantitative analyses. RESULTS: At 9.4 T and 15.2 T, average baseline T2* (n = 9) was 26.6 and 17.1 msec, whereas baseline T2 value (n = 9) was 35.7 and 24.5 msec, respectively. Averaged stimulation-induced ΔR2* was -1.72 s-1 at 9.4 T and -3.09 s-1 at 15.2 T, whereas ΔR2 was -1.19 s-1 at 9.4 T and -1.97 s-1 at 15.2 T. At the optimal TE of tissue T2* or T2 , BOLD percent changes were slightly higher at 15.2 T than at 9.4 T (GE: 7.4% versus 6.4% and SE: 5.7% versus 5.4%). The ΔR2* and ΔR2 ratio of 15.2 T to 9.4 T was 1.8 and 1.66, respectively. The ratio of the macrovessel-containing superficial to microvessel-dominant parenchymal BOLD signal was 1.73 to 1.76 for GE-BOLD versus 1.13 to 1.19 for SE-BOLD, indicating that the SE-BOLD contrast is less sensitive to macrovessels than GE-BOLD. CONCLUSION: SE-BOLD fMRI improves spatial specificity to microvessels compared to GE-BOLD at both fields. BOLD sensitivity is similar at the both fields and can be improved at ultrahigh fields only for thermal-noise-dominant ultrahigh-resolution fMRI.

Gradient-echo-based 3D submillisecond echo time pulmonary MR imaging: a preliminary usability study on clinical and preclinical MR scanners.

OBJECTIVE: To preliminarily investigate a technical feasibility of a submillisecond echo time concurrent-dephasing-and-excitation (CODE) sequence for pulmonary MRI on clinical and preclinical MR scanners Methods: CODE imaging (echo time, 0.14 ~ 0.18 ms) was performed with American College of Radiology phantom at 3 T, 7 healthy volunteers at 1.5 and 3 T, 10 rabbits at 3 T, and 2 rodents at 9.4 T. Signal-to-noise ratio was compared in phantom. Image quality of human MRI was visually assessed on a 5-point scale for comparison between CODE and conventional lung MRI sequences. Visibility of bronchi, subcentimeter nodules, and MR air-bronchogram were assessed in animal studies. RESULTS: In phantom study, signal-to-noise ratio was higher with CODE than with original three-dimensional ultrashort-echo time sequence (106.71 ± 4.32 vs 91.66 ± 3.54; p < 0.001). Image quality of human MRI was better with CODE than with conventional MRI sequences (p ≤ 0.002). Bronchi remained traceable up to the fifth bronchial generation in CODE images in rabbits and rodents. 95.2% of metastatic nodules (diameter, 1.5 ± 0.4 mm) and 93.8% of MR air-bronchogram (diameter, 0.9 ± 0.2 mm) in rabbits. CONCLUSION: Submillisecond echo time pulmonary MRI was technically feasible by using CODE on various MR scanners. Advances in knowledge: CODE can be a practical alternative for lung MRI on both clinical and pre-clinical scanners, without challenges of free-induction-decay-based ultrashort-echo time sequences.

In-vivo Visualization of Iron Oxide Enhancement in Focal Pulmonary Inflammatory Lesions Using a Three-Dimensional Radial Gradient-Echo-Based Ultrashort Echo Time Sequence: A Preliminary Study.

Objective: To preliminarily evaluate technical feasibility of a dual-echo ultrashort echo time (UTE) subtraction MR imaging by using concurrent dephasing and excitation (CODE) sequence for visualization of iron-oxide enhancement in focal inflammatory pulmonary lesions. Materials and Methods: A UTE pulmonary MR imaging before and after the injection of clinically usable superparamagnetic iron-oxide nanoparticles, ferumoxytol, was conducted using CODE sequence with dual echo times of 0.14 ms for the first echo and 4.15 ms for the second echo on 3T scanner in two rabbits concurrently having granulomatous lung disease and lung cancer in separate lobes. A mean ratio of standardized signal intensity (SI) was calculated for comparison of granulomatous lesion and cancer at first echo, second echo, and subtracted images. Lesions were pathologically evaluated with Prussian blue and immunohistochemistry staining. Results: Post-contrast subtracted CODE images visualized exclusive enhancement of iron oxide in granulomatous disease, but not in the cancer (mean ratio of SI, 2.15 ± 0.68 for granulomatous lesion versus 1.00 ± 0.07 for cancer; p value = 0.002). Prussian blue and corresponding anti-rabbit macrophage IgG-staining suggested an intracellular uptake of iron-oxide nanoparticles in macrophages of granulomatous lesions. Conclusion: Dual-echo UTE subtraction MR imaging using CODE sequence depicts an exclusive positive enhancement of iron-oxide nanoparticle in rabbits in focal granulomatous inflammatory lesions.

Neural Activity Patterns in the Human Brain Reflect Tactile Stickiness Perception.

Our previous human fMRI study found brain activations correlated with tactile stickiness perception using the uni-variate general linear model (GLM) (Yeon et al., 2017). Here, we conducted an in-depth investigation on neural correlates of sticky sensations by employing a multivoxel pattern analysis (MVPA) on the same dataset. In particular, we statistically compared multi-variate neural activities in response to the three groups of sticky stimuli: A supra-threshold group including a set of sticky stimuli that evoked vivid sticky perception; an infra-threshold group including another set of sticky stimuli that barely evoked sticky perception; and a sham group including acrylic stimuli with no physically sticky property. Searchlight MVPAs were performed to search for local activity patterns carrying neural information of stickiness perception. Similar to the uni-variate GLM results, significant multi-variate neural activity patterns were identified in postcentral gyrus, subcortical (basal ganglia and thalamus), and insula areas (insula and adjacent areas). Moreover, MVPAs revealed that activity patterns in posterior parietal cortex discriminated the perceptual intensities of stickiness, which was not present in the uni-variate analysis. Next, we applied a principal component analysis (PCA) to the voxel response patterns within identified clusters so as to find low-dimensional neural representations of stickiness intensities. Follow-up clustering analyses clearly showed separate neural grouping configurations between the Supra- and Infra-threshold groups. Interestingly, this neural categorization was in line with the perceptual grouping pattern obtained from the psychophysical data. Our findings thus suggest that different stickiness intensities would elicit distinct neural activity patterns in the human brain and may provide a neural basis for the perception and categorization of tactile stickiness.

Inter-vender and test-retest reliabilities of resting-state functional magnetic resonance imaging: Implications for multi-center imaging studies.

This prospective multi-center study aimed to evaluate the inter-vendor and test-retest reliabilities of resting-state functional magnetic resonance imaging (RS-fMRI) by assessing the temporal signal-to-noise ratio (tSNR) and functional connectivity. Study included 10 healthy subjects and each subject was scanned using three 3T MR scanners (GE Signa HDxt, Siemens Skyra, and Philips Achieva) in two sessions. The tSNR was calculated from the time course data. Inter-vendor and test-retest reliabilities were assessed with intra-class correlation coefficients (ICCs) derived from variant component analysis. Independent component analysis was performed to identify the connectivity of the default-mode network (DMN). In result, the tSNR for the DMN was not significantly different among the GE, Philips, and Siemens scanners (P=0.638). In terms of vendor differences, the inter-vendor reliability was good (ICC=0.774). Regarding the test-retest reliability, the GE scanner showed excellent correlation (ICC=0.961), while the Philips (ICC=0.671) and Siemens (ICC=0.726) scanners showed relatively good correlation. The DMN pattern of the subjects between the two sessions for each scanner and between three scanners showed the identical patterns of functional connectivity. The inter-vendor and test-retest reliabilities of RS-fMRI using different 3T MR scanners are good. Thus, we suggest that RS-fMRI could be used in multicenter imaging studies as a reliable imaging marker.

Human Brain Activity Related to the Tactile Perception of Stickiness.

While the perception of stickiness serves as one of the fundamental dimensions for tactile sensation, little has been elucidated about the stickiness sensation and its neural correlates. The present study investigated how the human brain responds to perceived tactile sticky stimuli using functional magnetic resonance imaging (fMRI). To evoke tactile perception of stickiness with multiple intensities, we generated silicone stimuli with varying catalyst ratios. Also, an acrylic sham stimulus was prepared to present a condition with no sticky sensation. From the two psychophysics experiments-the methods of constant stimuli and the magnitude estimation-we could classify the silicone stimuli into two groups according to whether a sticky perception was evoked: the Supra-threshold group that evoked sticky perception and the Infra-threshold group that did not. In the Supra-threshold vs. Sham contrast analysis of the fMRI data using the general linear model (GLM), the contralateral primary somatosensory area (S1) and ipsilateral dorsolateral prefrontal cortex (DLPFC) showed significant activations in subjects, whereas no significant result was found in the Infra-threshold vs. Sham contrast. This result indicates that the perception of stickiness not only activates the somatosensory cortex, but also possibly induces higher cognitive processes. Also, the Supra- vs. Infra-threshold contrast analysis revealed significant activations in several subcortical regions, including the pallidum, putamen, caudate and thalamus, as well as in another region spanning the insula and temporal cortices. These brain regions, previously known to be related to tactile discrimination, may subserve the discrimination of different intensities of tactile stickiness. The present study unveils the human neural correlates of the tactile perception of stickiness and may contribute to broadening the understanding of neural mechanisms associated with tactile perception.

Full analytical solution of the bloch equation when using a hyperbolic-secant driving function.

PURPOSE: The frequency-swept pulse known as the hyperbolic-secant (HS) pulse is popular in NMR for achieving adiabatic spin inversion. The HS pulse has also shown utility for achieving excitation and refocusing in gradient-echo and spin-echo sequences, including new ultrashort echo-time imaging (e.g., Sweep Imaging with Fourier Transform, SWIFT) and B1 mapping techniques. To facilitate the analysis of these techniques, the complete theoretical solution of the Bloch equation, as driven by the HS pulse, was derived for an arbitrary state of initial magnetization. METHODS: The solution of the Bloch-Riccati equation for transverse and longitudinal magnetization for an arbitrary initial state was derived analytically in terms of HS pulse parameters. The analytical solution was compared with the solutions using both the Runge-Kutta method and the small-tip approximation. RESULTS: The analytical solution was demonstrated on different initial states at different frequency offsets with/without a combination of HS pulses. Evolution of the transverse magnetization was influenced significantly by the choice of HS pulse parameters. The deviation of the magnitude of the transverse magnetization, as obtained by comparing the small-tip approximation to the analytical solution, was < 5% for flip angles < 30 °, but > 10% for the flip angles > 40 °. CONCLUSION: The derived analytical solution provides insights into the influence of HS pulse parameters on the magnetization evolution. Magn Reson Med 77:1630-1638, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Nuclear paramagnetism-induced MR frequency shift and its implications for MR-based magnetic susceptibility measurement.

PURPOSE: To investigate the 1 H spin contribution (0.004 parts per million (ppm)) to the water magnetic susceptibility and discuss its implications for high-precision phase mapping and tissue susceptibility measurement. METHODS: Free induction decay (FID) signals were acquired at 3 Tesla (T) and 9.4T from thin square phantoms at a range of tip angles. The FID frequency shift was examined at a high resolution ( < 0.01 Hz) for different phantom orientations relative to the main magnetic field (B0 ). B0 maps on an axial and a coronal slice of a spherical phantom were obtained at 3T to examine the tip angle and orientation dependence at the 0.001 ppm level. RESULTS: A frequency shift of about 0.3 Hz was observed between tip angles of 10 ° and 90 ° when the thin phantom was normal to B0 at 3T, whereas the shift changed sign and was halved in magnitude when the phantom's face was parallel to B0 . At 9.4T, the effect size increased proportionately. The orientation-dependent frequency shift was also observed in the B0 map experiment. These observations agree with theoretical frequency shift due to longitudinal 1 H spin polarization. CONCLUSION: Magnetic susceptibility contribution from the nuclear paramagnetism should be taken into account in the interpretation of high-precision phase and susceptibility mapping in MRI. Magn Reson Med 77:848-854, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

A radial sampling strategy for uniform k-space coverage with retrospective respiratory gating in 3D ultrashort-echo-time lung imaging.

The purpose of this work was to develop a 3D radial-sampling strategy which maintains uniform k-space sample density after retrospective respiratory gating, and demonstrate its feasibility in free-breathing ultrashort-echo-time lung MRI. A multi-shot, interleaved 3D radial sampling function was designed by segmenting a single-shot trajectory of projection views such that each interleaf samples k-space in an incoherent fashion. An optimal segmentation factor for the interleaved acquisition was derived based on an approximate model of respiratory patterns such that radial interleaves are evenly accepted during the retrospective gating. The optimality of the proposed sampling scheme was tested by numerical simulations and phantom experiments using human respiratory waveforms. Retrospectively, respiratory-gated, free-breathing lung MRI with the proposed sampling strategy was performed in healthy subjects. The simulation yielded the most uniform k-space sample density with the optimal segmentation factor, as evidenced by the smallest standard deviation of the number of neighboring samples as well as minimal side-lobe energy in the point spread function. The optimality of the proposed scheme was also confirmed by minimal image artifacts in phantom images. Human lung images showed that the proposed sampling scheme significantly reduced streak and ring artifacts compared with the conventional retrospective respiratory gating while suppressing motion-related blurring compared with full sampling without respiratory gating. In conclusion, the proposed 3D radial-sampling scheme can effectively suppress the image artifacts due to non-uniform k-space sample density in retrospectively respiratory-gated lung MRI by uniformly distributing gated radial views across the k-space.

Identification and reduction of image artifacts in non-contrast-enhanced velocity-selective peripheral angiography at 3T.

PURPOSE: To identify and reduce image artifacts in non-contrast-enhanced velocity-selective (VS) magnetization-prepared peripheral MR angiography (MRA) at 3T. METHODS: To avoid signal loss in the arteries, double and quadruple refocused VS excitation pulse sequences were designed that were robust to a wide range of B0 and B1 offset. To suppress stripe artifact and background signal variation, we successively applied two VS preparations with excitation profiles shifted by half the period of the stripes. VS-MRA using single, double, and quadruple refocused VS preparations was tested in healthy subjects and a patient. RESULTS: In the regions of large B0 and B1 offsets, arterial signal loss was yielded by single refocused VS preparation, but was avoided with double or quadruple refocused preparations. Compared with single VS preparation, the two consecutive preparations with shifted excitation profiles substantially reduced the stripe artifact and background signal variation, as demonstrated by increased mean and decreased standard deviation of relative contrast-to-noise ratio. The proposed VS-MRA identified multilevel disease in the femoral arteries of the patient, as validated by digital subtraction angiography. CONCLUSION: Two multiple refocused VS magnetization preparations with shifted excitation profiles yield artifact-free peripheral angiograms at 3T. Magn Reson Med 76:466-477, 2016. © 2015 Wiley Periodicals, Inc.