A simple method for mapping the B1 field distribution of linear RF coils

Inhomogeneity of the radio frequency (RF) field B₁ leads to intensity variations in MR images and to spatial dependence of spectral line amplitudes. In this paper, a simple method of measuring the B₁ field components of an unsegmented linear coil is described. The method is designed for the coils operating up to 20 MHz. The B₁ field distribution is replaced by the static magnetic field caused by DC current flowing through the coil. The technique involves rotating the coil 90° so that measured B₁ component is aligned with B₀ and measuring the shift of resonance frequency using a spectroscopic imaging sequence. Experimental results were in good agreement with the theoretical computations.     

A correction method for streak artifacts in gradient-echo EPI using spin-echo EPI reference data

Objective

To analyze the streak artifacts in a gradient-echo echo planar imaging (GE-EPI) sequence and to propose a correction method for the Nyquist ghost artifacts that does not cause streak artifacts in the GE-EPI imaging.

Materials and methods

Several GE-EPI imaging experiments with various reference scans, using both GE-EPI and SE-EPI scan data, were performed to analyze the streak artifacts and to investigate the spin dephasing phenomena of the GE-EPI reference scan. In addition, the analysis based on the spin dephasing was undertaken in order to demonstrate that the SE-EPI reference data can be used for the correction of the GE-EPI main scan data.

Results

The experimental results confirmed that the improvement of the reference data using either signal averaging or a large flip angle cannot guarantee perfect correction of the streak artifact if the noise is not completely removed. Due to the main field inhomogeneity, the spins of the GE-EPI reference data were dephased in multiple echo signals. The proposed correction method, which uses a SE-EPI reference scan for the GE-EPI images, eliminates the N/2 ghost artifacts without producing streak artifacts.

Conclusion

It is believed that the proposed phase error correction scheme can improve the EPI performance in high field MRIs with higher magnetic field inhomogeneities.     

Artifacts in CSI-measurements caused by the drift of the static magnetic field

In chemical shift resolved spectroscopic imaging (CSI) temporal changes in the static magnetic field (drift) can lead to distortions of the phase encoding process. This can result in localization artifacts. The extent of the artifact depends on the size of the drift, the number of acquisitions, as well as on the combination of the size of the field of view and the number of phase encoding gradient steps. Furthermore, it is affected by the succession of the phase encoding gradients. Precautions are described which allow substantial minimization of the artifact.     

Regional proton nuclear magnetic resonance spectroscopy differentiates cortex and medulla in the isolated perfused rat kidney

Volume-localized proton nuclear magnetic resonance spectroscopy was used as an assay of regional biochemistry in the isolated perfused rat kidney. This model eliminated artifacts caused by respiratory and cardiac motion experiencedin vivo. Immersion of the kidney under its venous effluent reduced the susceptibility artifacts evoked by tissue-air interfaces. The rapid acquisition with relaxation enhancement imaging sequence was used for scout imaging. This gave excellent spatial resolution of the cortex, outer medulla, and inner medulla. Spectra were then acquired in 10 minutes using the volume-selective multipulse spectroscopy sequence from voxels with a volume of approximately 24 μL located within the cortical or medullary regions. Spectral peaks were assigned by the addition of known compounds to the perfusion medium and by comparison with spectra of protein-free extracts of cortex and medulla. The medullary region spectra were characterized by signals from the osmolytes betaine, glycerophosphorylcholine, and inositol. The spectra from the cortex were more complex and contained lesser contributions from osmolytes.     

Monitoring and correcting spatio-temporal variations of the MR scanner’s static magnetic field

The homogeneity and stability of the static magnetic field are of paramount importance to the accuracy of MR procedures that are sensitive to phase errors and magnetic field inhomogeneity. It is shown that intense gradient utilization in clinical horizontal-bore superconducting MR scanners of three different vendors results in main magnetic fields that vary on a long time scale both spatially and temporally by amounts of order 0.8–2.5 ppm. The observed spatial changes have linear and quadratic variations that are strongest along the z direction. It is shown that the effect of such variations is of sufficient magnitude to completely obfuscate thermal phase shifts measured by proton-resonance frequency-shift MR thermometry and certainly affect accuracy. In addition, field variations cause signal loss and line-broadening in MR spectroscopy, as exemplified by a fourfold line-broadening of metabolites over the course of a 45 min human brain study. The field variations are consistent with resistive heating of the magnet structures. It is concluded that correction strategies are required to compensate for these spatial and temporal field drifts for phase-sensitive MR protocols. It is demonstrated that serial field mapping and phased difference imaging correction protocols can substantially compensate for the drift effects observed in the MR thermometry and spectroscopy experiments.     

Segmentation and quantification of adipose tissue by magnetic resonance imaging

In this brief review, introductory concepts in animal and human adipose tissue segmentation using proton magnetic resonance imaging (MRI) and computed tomography are summarized in the context of obesity research. Adipose tissue segmentation and quantification using spin relaxation-based (e.g., T1-weighted, T2-weighted), relaxometry-based (e.g., T1-, T2-, T2*-mapping), chemical-shift selective, and chemical-shift encoded water–fat MRI pulse sequences are briefly discussed. The continuing interest to classify subcutaneous and visceral adipose tissue depots into smaller sub-depot compartments is mentioned. The use of a single slice, a stack of slices across a limited anatomical region, or a whole body protocol is considered. Common image post-processing steps and emerging atlas-based automated segmentation techniques are noted. Finally, the article identifies some directions of future research, including a discussion on the growing topic of brown adipose tissue and related segmentation considerations.     

Alleviating artifacts in 1H MRI thermometry by single scan spatiotemporal encoding

Objective

Recent years have seen an increased interest in combining MRI thermometry with devices capable of destroying malignancies by heat ablation. Expected from the MR protocols are accurate and fast thermal characterizations, providing real time feedback on restricted tissue volumes and/or rapidly moving organs like liver. This article explores the potential advantages of relying on spatiotemporally encoded (SPEN) sequences for retrieving real-time thermometric images based on the water’s proton resonance frequency (PRF) shifts.

Materials and methods

Hybrid spatiotemporal/k-space encoding single-scan MRI experiments were implemented on animal and human scanners, and their abilities to deliver single- and multi-slice real-time thermometric measurements based on PRF-derived phase maps in phantoms and in vivo, were compared against echo planar imaging (EPI) and gradient-echo counterparts.

Results

Under comparable acquisition conditions, SPEN exhibited advantages vis-à-vis EPI in terms of dealing with inhomogeneous magnetic field distortions, with shifts arising due to changes in the central frequency offsets, with PRF distributions, and for zooming into restricted fields-of-view without special pulse sequence provisions.

Conclusion

This work confirms the ability of SPEN sequences, particularly when implemented under fully-refocused conditions, to exploit their built-in robustness to shift- and field-derived inhomogeneities for monitoring thermal changes in real-time under in vitro and in vivo conditions.     

Optimized EPI for fMRI studies of the orbitofrontal cortex: compensation of susceptibility-induced gradients in the readout direction

Object Most functional magnetic resonance imaging (fMRI) studies record the blood oxygen leveldependent (BOLD) signal using gradient-echo echo-planar imaging (GE EPI). EPI can suffer from substantial BOLD sensitivity loss caused by magnetic field inhomogeneities. Here, BOLD sensitivity losses due to susceptibility- induced gradients in the readout (RO) direction are characterized and a compensation approach is developed. Materials and Methods Based on a theory describing the dropout mechanism, an EPI sequence was optimized for maximal BOLD sensitivity in the orbitofrontal cortex (OFC) using a specific combination of an increased spatial resolution in the RO direction and a reduced echo time. Using measured BOLD sensitivity maps and a breath hold experiment, the model and compensation approach were tested. Results Using typical fMRI EPI parameters, susceptibility-induced gradients in the RO direction caused dropouts in the OFC and the inferior temporal lobe. Optimizing the echo time and spatial resolution effectively reduced the dropout as predicted by the theory. Conclusion The model-based compensation approach effectively reduces BOLD sensitivity losses due to susceptibility-induced gradients in the RO direction. It retains the high temporal resolution of single-shot EPI and can be readily combined with methods for the compensation of susceptibility-induced field gradients in the phase-encoding and through-plane direction.     

Interaction between grounding pads used for RF ablation therapy and magnetic resonance imaging

Objectives: To characterize artifacts and imaging problems in the presence of conductive grounding pads for RF ablation therapy as well as potential heating problems due to induction of eddy currents in the pads. Strategies for avoidance of those problems are developed. Materials and methods: Underlying principles of interactions between grounding pads and MR imaging are reported. Influential parameters, e.g., orientation in relation to the magnetic field, shape of the grounding pad, sequence type (spin-echo versus gradient echo) and magnetic field strength (0.2 T, 1.5 T, 3 T) were varied in systematic phantom studies. Heating effects due to induced eddy currents were estimated theoretically and measured by infrared imaging in an adapted set-up. Results: MR imaging artifacts are markedly dependent on the orientation and geometrical shape of the grounding pads. Visible signal extinction artifacts were more pronounced using spin-echo techniques than in gradient echo images and increased for higher field strengths. Suitable incisions in the grounding pad reduced eddy currents markedly and minimized image artifacts. Heating problems due to induced eddy currents by the RF transmitted for MR imaging were excluded by phantom measurements. Conclusions: Suitable positioning of the grounding pads and adaptation of their geometry provide clearly reduced artifacts in MR imaging.     

阳极加载稳恒磁场对PEMFC性能的影响

在工作面积为26.5 mm×26.5 mm的质子交换膜燃料电池(PEMFC)的阳极侧加载不同方向(与阳极侧平行、垂直)、不同强度(0、210、310、390 m T)的稳恒磁场,测出稳恒磁场源的磁场空间分布,得到在PEMFC阳极侧加载磁场对电池性能的影响。实验结果发现:一定强度范围内的磁场能够提高PEMFC的工作性能,但在不同磁场方向与不同磁场强度下,得到的PEMFC工作性能提高幅度有差异。当在PEMFC阳极侧加载垂直磁场时,其最大输出功率提高更大,平行磁场其次。当在阳极侧加载390 mT的磁场时,其PEMFC最大输出功率密度可以达到73.38 mW/cm~2。实验同时发现一定强度的加载磁场还可以提升PEMFC输出功率的稳定性。     

1H spectroscopic imaging of acute head injury—evidence of diffuse axonal injury

Using single slice two-dimensional spectroscopic imaging (SI), nine acute head injury patients and six controls have been successfully scanned. The problems presented by the need for ITU monitoring of these patients during MR scanning was overcome using MR compatible monitoring equipment. In previous studies of head injury which used proton spectroscopy, single voxel localisation procedures have meant that the spatial extent of the spectral data has been limited. With spectral data from a whole axial slice, we have been able to identify NAA abnormalities in regions remote to any T2 visible lesions This suggests that SI (of NAA in particular) will be useful for the diagnosis of diffuse axonal injury.     

稳恒磁场处理对聚乙烯/炭系填料复合材料电导特性的影响

在聚乙烯(PE)、低密度聚乙烯(LDPE)/炭黑、LDPE/石墨、LDPE/碳纳米管的热压成型过程中施加稳恒磁场,研究了磁场处理对聚乙烯/炭系填料复合材料电导特性的影响。研究结果表明:磁场处理能导致聚乙烯的结晶度提高,聚乙烯和LDPE/炭黑复合材料的体积电阻率增加;磁场在石墨片层内和碳纳米管中"诱导"形成感应磁矩,使得石墨片层和碳纳米管分别沿垂直于磁场方向和平行于磁场方向在LDPE中取向,从而导致LDPE/石墨复合材料沿平行于磁场方向的电导率减小,而LDPE/碳纳米管复合材料沿平行于磁场方向的电导率增加。     

Chemical-shift micro-imaging of subcutaneous lesions

A chemical-shift imaging technique was used for the study of small subcutaneous lesions. This study concerns micro-imaging of two females suffering from a tenosynovial giant cell tumor and an epidermal cyst. High-resolution water, fat and chemical-shift artifact-free images were obtained on a whole-body MR unit (1.5 T) equipped with a 23-mm microscopy surface coil and standard gradients (23 mT/m). A significant improvement in signal-to-noise ratio was achieved by reducing the receiver bandwidth to values below ±10 kHz. The image data sets were acquired with resolution 0.1×0.13 mm in the plane, slice thickness 0.5 mm and with acquisition time less than 3 min. Spatial resolution, fat suppression, image texture and edge delineation were improved on spectroscopic images compared with those on conventional MR images.     

Combined acquisition technique (CAT) for high-field neuroimaging with reduced RF power

Object

Clinical 3 T MRI systems are rapidly increasing and MRI systems with a static field of 7 T or even more have been installed. The RF power deposition is proportional to the square of the static magnetic field strength and is characterized by the specific absorption rate (SAR). Therefore, there exist defined safety limits to avoid heating of the patient. Here, we describe a hybrid method to significantly reduce the SAR compared to a turbo-spin-echo (TSE) sequence.

Materials and methods

We investigate the potential benefits of a combined acquisition technique (CAT) for high-field neuroimaging at 3 and 7 T. The TSE/EPI CAT experiments were performed on volunteers and patients and compared with standard TSE and GRASE protocols. Problems and solutions regarding T2 weighted CAT imaging are discussed.

Results

We present in vivo images with T2 and proton density contrast obtained on 3 and 7 T with significant SAR reduction (up to 60 %) compared with standard TSE. Image quality is comparable to TSE but CAT shows fewer artifacts than a GRASE sequence.

Conclusion

CAT is a promising candidate for neuroimaging at high fields up to 7 T. The SAR reduction allows one to shorten the waiting time between two excitations or to image more slices thereby reducing the overall measurement time.     

Diffusion-weighted echo planar MR imaging of the neck at 3 T using integrated shimming: comparison of MR sequence techniques for reducing artifacts caused by magnetic-field inhomogeneities

Objective

Our objective was to compare available techniques reducing artifacts in echo planar imaging (EPI)-based diffusion-weighed magnetic resonance imaging MRI (DWI) of the neck at 3 Tesla caused by B0-field inhomogeneities.

Materials and methods

A cylindrical fat–water phantom was equipped with a Maxwell coil allowing for additional linear B0-field variations in z-direction. The effect of increasing strength of this superimposed gradient on image quality was observed using a standard single-shot EPI-based DWI sequence (sEPI), a zoomed single-shot EPI sequence (zEPI), a readout-segmented EPI sequence (rsEPI), and an sEPI sequence with integrated dynamic shimming (intEPI) on a 3-Tesla system. Additionally, ten volunteers were examined over the neck region using these techniques. Image quality was assessed by two radiologists. Scan durations were recorded.

Results

With increasing strength of the external gradient, marked distortions, signal loss, and failure of fat suppression were observed using sEPI, zEPI, and rsEPI. These artifacts were markedly reduced using intEPI. Significantly better in vivo image quality was also observed using intEPI compared with the other techniques. Scan time of intEPI was similar to sEPI and zEPI and shorter than rsEPI.

Conclusion

The use of integrated 2D shim and frequency adjustment for EPI-based DWI results in a significant improvement in image quality of the head/neck region at 3 Tesla. Combining integrated shimming with rsEPI or zEPI can be expected to provide additional improvements.

     

In vivo 2D magnetic resonance spectroscopy of small animals

Localized in vivo NMR spectroscopy, chemical shift imaging or multi-voxel spectroscopy are potentially useful tools in small animals that are complementary to MRI, adding biochemical information to the mainly anatomical data provided by imaging of water protons. However the contribution of such methods remains hampered by the low spectral resolution of the in vivo 1D spectra. Two-dimensional methods widely developed for in vitro studies have been proposed as suitable approaches to overcome these limitations in resolution. The different homonuclear and heteronuclear sequences adapted to in vivo studies are reviewed. Their specific contributions to the spectral resolution of spectroscopic data and their limitations for in vivo investigations are discussed. The applications to experimental models of pathological processes or pharmacological treatment in mainly brain and muscle are presented. According to their combined sensitivity, acquisition duration and spatial resolution, the heteronuclear 2D experiments, which are mainly used for ¹H detected-¹³C spectroscopy after administration of ¹³C-labeled compounds, appear to be less efficient than ¹H detected-¹³C 1D methods at high field. However, the applications of 2D proton homonuclear methods show that they remain the best tools for in vivo studies when an improved resolution is required.     

Subchondral bone and cartilage thickness from MRI: effects of chemical-shift artifact

Magnetic resonance imaging (MRI) is the modality of choice for visualizing and quantifying articular cartilage thickness. However, difficulties persist in MRI of subchondral bone using spoiled gradient-echo (SPGR) and other gradient-echo sequences, primarily due to the effects of chemical-shift artifact. Fat-suppression techniques are often used to reduce these artifacts, but they prevent measurement of bone thickness. In this report, we assess the magnitude of chemical-shift effects (phase-cancellation and misregistration artifacts) on subchondral bone and cartilage thickness measurements in human femoral heads using a variety of pulse sequence parameters. Phase-cancellation effects were quantified by comparing measurements from in-phase images (TE=13.5 ms) to out-of-phase images (TE=15.8 ms). We also tested the assumption of the optimal in-phase TE by comparing thickness measures at small variations on TE (13.0, 13.5 and 14.0 ms). Misregistration effects were quantified by comparing measurements from water+fat images (water-only+fat-only images) to the measurements from in-phase (TE=13.5) images. A correction algorithm was developed and applied to the in-phase measurements and then compared to measurements from water+fat images. We also compared thickness measurements at different image resolutions. Results showed that both phase-cancellation artifact and misregistration artifact were significant for bone thickness measurement, but not for cartilage thickness measurement. Using an in-phase TE and correction algorithm for misregistration artifact, the errors in bone thickness relative to water+fat images were non-significant. This information may be useful for developing pulse sequences for optimal imaging of both cartilage and subchondral bone. Electronic Publication     

Lateralization of amygdala activation in fMRI may depend on phase-encoding polarity

Object

Susceptibility artifacts along the phase-encoding (PE) direction impact the activation pattern in the amygdala and may lead to systematic asymmetries. We implemented a triple-echo echo-planar imaging (EPI) sequence, acquiring opposite PE polarities along left?Cright PE direction in a single shot, to investigate its effects on amygdala lateralization.

Materials and Methods

Twelve subjects viewed emotional faces to evoke amygdala activation.

Results and Conclusion

A region of interest analysis revealed that the lateralization of amygdala responses depended on the PE polarity thus representing a pure method artifact. Alternating PE with multi-echo EPI reduced the artifact. Lateralized fMRI activation in areas with magnetic field inhomogeneities need to be interpreted with caution.     

实心圆柱式永磁同步电机转子偏心气隙磁场的空间和频率特性

针对两极平行充磁实心圆柱式永磁同步电机(SCPMSM),分析转子偏心对气隙磁场空间和频率特性的影响。通过引入转子静态偏心和动态偏心磁导修正系数,建立了转子偏心气隙磁场的数学模型,分析了空载和负载情形下转子静态偏心以及动态偏心气隙磁场的空间和频率特性。以1台2极12槽SCPMSM为例,对转子偏心气隙磁场特性进行了有限元分析,验证了理论分析的正确性。     

Online motion correction for diffusion-weighted segmented-EPI and FLASH imaging

This paper explores the application of online motion correction using navigator echoes to the segmented-EPI and FLASH techniques. In segmented EPI this has the advantage over post-acquisition correction that the position in k-space of each segment is no longer subject to arbitrary shifts caused by rotation. In diffusion-weighted FLASH it has the advantage that the full magnetisation can be utilised in comparison to other methods of eliminating the sensitivity to bulk motion, in which the sensitivity is halved. Healthy subjects were investigated on a 3 T whole-body system in which the hardware has been modified so that navigator echoes can be recorded on a personal computer which generates the necessary magnetic field gradient correction pulses and shifts in the Larmor frequency within 800 s. ECG triggering was used to avoid the period of non-rigid-body brain motion. Two orthogonal navigator echoes were employed. For segmented EPI it was found essential to minimise the T2* weighting of the navigator echoes to about 10 ms to obtain reliable results. High quality images were obtained for both methods examined. Online motion correction brings direct benefits to both the diffusion-weighted segmented-EPI and FLASH techniques.