Current-density imaging using ultra-low-field MRI with zero-field encoding

Electric current density can be measured noninvasively with magnetic resonance imaging (MRI). Determining all three components of the current density, however, requires physical rotation of the sample or current injection from several directions when done with conventional methods. However, the emerging technology of ultra-low-field (ULF) MRI, in which the signal encoding and acquisition is conducted at a microtesla-range magnetic field, offers new possibilities. The low applied magnetic fields can even be switched off completely within the pulse sequence, increasing the flexibility of the available sequences. In this article, we present a ULF-MRI sequence designed for obtaining all three components of a current-density pattern without the need of sample rotations. The sequence consists of three steps: prepolarization of the sample, signal encoding in the current-density-associated magnetic field without applying any MRI fields, and spatial encoding in a microtesla-range field using any standard ULF-MRI sequence. The performance of the method is evaluated by numerical simulations. The method may find applications, e.g., in noninvasive conductivity imaging of tissue.     

Current-density imaging using ultra-low-field MRI with adiabatic pulses

Magnetic resonance imaging (MRI) allows measurement of electric current density in an object. The measurement is based on observing how the magnetic field of the current density affects the associated spins. However, as high-field MRI is sensitive to static magnetic field variations of only the field component along the main field direction, object rotations are typically needed to image three-dimensional current densities. Ultra-low-field (ULF) MRI, on the other hand, with B₀ on the order of 10–100 μT, allows novel MRI sequences. We present a rotation-free method for imaging static magnetic fields and current densities using ULF MRI. The method utilizes prepolarization pulses with adiabatic switch-off ramps. The technique is designed to reveal complete field and current-density information without the need to rotate the object. The method may find applications, e.g., in conductivity imaging. We present simulation results showing the feasibility of the sequence.     

Optimization of spoiler gradients in flash MRI

The FLASH technique for fast magnetic resonance (MR) imaging often employs strong magnetic field gradients, called spoiler gradients, to dephase the transverse magnetization after it has been measured. Otherwise, image artifacts can develop. The effectiveness of spoiler gradients at suppressing these artifacts was evaluated experimentally on two-dimensional MR images of a uniform phantom and patients. It was informative to compare the magnetization immediately before the RF excitation in each phase encoding step. Only spoiler gradients in the slice selection direction were effective. Spoiler gradients that decreased steadily from a large amplitude in the first phase encoding step to zero in the last minimized the transverse magnetization and suppressed the image artifact, without changing the image contrast.     

Distortion-free magnetic resonance imaging in the zero-field limit

MRI is a powerful technique for clinical diagnosis and materials characterization. Images are acquired in a homogeneous static magnetic field much higher than the fields generated across the field of view by the spatially encoding field gradients. Without such a high field, the concomitant components of the field gradient dictated by Maxwell’s equations lead to severe distortions that make imaging impossible with conventional MRI encoding. In this paper, we present a distortion-free image of a phantom acquired with a fundamentally different methodology in which the applied static field approaches zero. Our technique involves encoding with pulses of uniform and gradient field, and acquiring the magnetic field signals with a SQUID. The method can be extended to weak ambient fields, potentially enabling imaging in the Earth’s field without cancellation coils or shielding. Other potential applications include quantum information processing and fundamental studies of long-range ferromagnetic interactions.     

On concomitant gradients in low-field MRI

Growing interest in magnetic resonance imaging (MRI) at ultra-low magnetic fields (ULF, approximately muT fields) has been motivated by several advantages over its counterparts at higher magnetic fields. These include narrow line widths, the possibility of novel imaging schemes, reduced imaging artifacts from susceptibility variations within a sample, and reduced system cost and complexity. In addition, ULF NMR/MRI with superconducting quantum interference devices is compatible with simultaneous measurements of biomagnetic signals, a capability conventional systems cannot offer. Acquisition of MRI at ULF must, however, account for concomitant gradients that would otherwise result in severe image distortions. In this paper, we introduce the general theoretical framework that describes concomitant gradients, explain why such gradients are more problematic at low field, and present possible approaches to correct for these unavoidable gradients in the context of a non-slice-selective MRI protocol.     

一种通用的MRI计算机模拟软件

报道一套基于解释脉冲程序的MRI的计算机模拟软件,可适用于任意脉冲序列成象方法的计算机模拟,该软件包括成象物理模型的建立,成象物理过程的模拟,基于K空间的时域FID信号的重组,多维傅立叶交换重建图象,多维滤波反投影重建图象.图象显示等功能模块;分析了成象的物理过程的基本原理和软件的设计思想.并以GE成象和SEPI快速成象方法为例进行了模拟,实现了图象重建.     

Correction of concomitant gradient artifacts in experimental microtesla MRI

Magnetic resonance imaging (MRI) suffers from artifacts caused by concomitant gradients when the product of the magnetic field gradient and the dimension of the sample becomes comparable to the static magnetic field. To investigate and correct for these artifacts at very low magnetic fields, we have acquired MR images of a 165-mm phantom in a 66-microT field using gradients up to 350 microT/m. We prepolarize the protons in a field of about 100 mT, apply a spin-echo pulse sequence, and detect the precessing spins using a superconducting gradiometer coupled to a superconducting quantum interference device (SQUID). Distortion and blurring are readily apparent at the edges of the images; by comparing the experimental images to computer simulations, we show that concomitant gradients cause these artifacts. We develop a non-perturbative, post-acquisition phase correction algorithm that eliminates the effects of concomitant gradients in both the simulated and the experimental images. This algorithm assumes that the switching time of the phase-encoding gradient is long compared to the spin precession period. In a second technique, we demonstrate that raising the precession field during phase encoding can also eliminate blurring caused by concomitant phase-encoding gradients; this technique enables one to correct concomitant gradient artifacts even when the detector has a restricted bandwidth that sets an upper limit on the precession frequency. In particular, the combination of phase correction and precession field cycling should allow one to add MRI capabilities to existing 300-channel SQUID systems used to detect neuronal currents in the brain because frequency encoding could be performed within the 1-2 kHz bandwidth of the readout system.     

核磁共振螺旋快速成像的新进展

核磁共振快速成像能在几十毫秒内获取数据，对运动器官作适时显示，并在功能成像的研究等方面具有常规成像不能替代的优点，是核磁共振成像的发展方向．螺旋快速成像对硬件的要求较低，近年来方法上的改善，已使其趋于实用．文章简要介绍了螺旋快速成像原理及网格重建算法．     

磁共振成象数据在486微机上的处理

把BRUKER磁共振成象(MRI)数据传输到486高档微机上后,经过一系列适当的编程处理,可在微计算机上进行数据压缩和文件格式转换,借助当前的通用图象处理软件系统,可进行图象的校正、增强、编辑、合成等处理,非常有效地扩展一般NMR成象谱仪图象处理软硬件的系统功能.另外,本方法也可用于其它类似谱仪的处理,使普通NMR谱仪用户的添加成象设备后所面临的计算机软硬件系统图象处理功能不足的问题得到解决.     

Objective quantification of intervertebral disc volume properties using MRI in idiopathic scoliosis surgery

The aim of this study was to quantify from magnetic resonance imaging (MRI) the volume and hydration variation of the intervertebral disc in the lumbar spine before and after surgery in severe idiopathic scoliosis cases. MRI data were posttreated using a custom-made image processing software to semiautomatically derive volume properties of disc, annulus fibrosus and nucleus pulposus. The nucleus-disc volume ratio was also an indicator of the hydration level. The MRI that was performed in the clinical protocol concerned 28 patients with severe idiopathic scoliosis; four types of surgical instrumentations were used. The reliability of the three-dimensional reconstruction process was initially verified using an intraoperator reproducibility test. Original preoperative data on disc volume properties were then derived. Postoperative volume variations were quantified in discs below spine fusion, taking into account the level of arthrodesis and the disc location. It showed that the postoperative volume criteria increased significantly for nucleus, disc and nucleus-disc volume ratio and that some magnitude modulation could be conditioned by the location of surgical instrumentation. It tended to prove that the recovery of balanced physiological positioning and inherent biomechanical loads could induce a restored hydration of disc, which should favor the remodeling of free segments. This work was the first report to deal with the consequences of scoliosis surgery on subjacent disc in terms of volume and hydration properties. The clinical outcome will follow based on the patient cohort follow-up at 1 year after surgery.     

并行MRI图像重建算法比较及软件实现

首先介绍了不加速的并行MRI图像重建方法,然后对加速的并行MRI的4种图像重建算法进行了比较,得出结论：加速因子相同时,重建质量上,GRAPPA和SENSE的重建质量最好,SMASH的重建质量次之, PILS算法对线圈位置要求极高,重建质量最差;重建速度上,SMASH的重建速度最快,其次是SENSE和PILS,GRAPPA的重建速度最慢. 当加速因子变大时,所有算法重建质量都变差. 最后介绍了算法实现软件,该软件可以读入原始数据,显示数据采集轨迹,计算线圈灵敏度,选择图像重建方法,分析和比较重建图像质量. 该软件为我国在MRI成像领域提供了一个学习和进一步研究图像重建算法的有力工具.     

A novel acquisition-reconstruction algorithm for surface magnetic resonance imaging

In U-shaped, hand-size magnetic resonance surface scanners, imaging is performed along only one spatial direction, with the application of just one gradient (one-dimensional imaging). Lateral spatial resolution can be obtained by magnet displacement, but, in this case, resolution is very poor (on the order of some millimeters) and cannot be useful for high-resolution imaging applications. In this article, an innovative technique for acquisition and reconstruction of images produced by U-shaped, hand-size MRI surface scanners is presented. The proposed method is based on the acquisition of overlapping strips and an analytical reconstruction technique; it is capable of arbitrarily improving spatial lateral resolution without either using a second magnetic field gradient or making any assumptions about the imaged sample extension. Numerical simulations on synthetic images are reported demonstrating the method functionalities. The presented method also makes it possible to use U-shaped, hand-size MRI surface scanners for high-resolution biomedical applications, such as the imaging of skin lesions.     

An evaluation of reconstructed ACL impingement on PCL using MRI

Using magnetic resonance imaging (MRI), we evaluated 42 reconstructed anterior cruciate ligaments (ACLs) for impingement on corresponding posterior cruciate ligaments (PCLs) in the knee-extended position. Thirty-one single-bundle ACL reconstructions and 11 double-bundle ACL reconstructions were performed. MR examinations were performed at 3 and 12 months after arthroscopic ACL reconstructions. Using oblique axial MRIs of reconstructed ACLs, we evaluated the shape of the PCL and divided them into two groups: an impingement-positive group and an impingement-negative group. Using sagittal images, we measured the PCL index (Liu's method) and examined the correlation between the degree of impingement and the index. At 3 months after surgery, 14 of 31 single-bundle ACL reconstructions and 5 of 11 double-bundle ACL reconstructions were regarded as positive impingement on PCLs. At 12 months after surgery, 17 of 31 single-bundle ACL reconstructions and 5 of 11 double-bundle ACL reconstructions were regarded as positive impingement on PCLs. At 3 months in single-bundle reconstructions and at 3 and 12 months in all reconstructions, the PCL index of the impingement-positive group was significantly lower than that of the negative group. This study indicated that reconstructed ACLs in the impingement-positive group pressed the PCLs more posteriorly than did the impingement-negative group.     

Stationary wavelet transform for under-sampled MRI reconstruction

In addition to coil sensitivity data (parallel imaging), sparsity constraints are often used as an additional l_p-penalty for under-sampled MRI reconstruction (compressed sensing). Penalizing the traditional decimated wavelet transform (DWT) coefficients, however, results in visual pseudo-Gibbs artifacts, some of which are attributed to the lack of translation invariance of the wavelet basis. We show that these artifacts can be greatly reduced by penalizing the translation-invariant stationary wavelet transform (SWT) coefficients. This holds with various additional reconstruction constraints, including coil sensitivity profiles and total variation. Additionally, SWT reconstructions result in lower error values and faster convergence compared to DWT. These concepts are illustrated with extensive experiments on in vivo MRI data with particular emphasis on multiple-channel acquisitions.     

In vitro assessment of MRI issues at 3-Tesla for a breast tissue expander with a remote port

Objectives

A patient with a breast tissue expander may require a diagnostic assessment using magnetic resonance imaging (MRI). To ensure patient safety, this type of implant must undergo in vitro MRI testing using proper techniques. Therefore, this investigation evaluated MRI issues (i.e., magnetic field interactions, heating, and artifacts) at 3-Tesla for a breast tissue expander with a remote port.

Methods

A breast tissue expander with a remote port (Integra Breast Tissue Expander, Model 3612-06 with Standard Remote Port, PMT Corporation, Chanhassen, MN) underwent evaluation for magnetic field interactions (translational attraction and torque), MRI-related heating, and artifacts using standardized techniques. Heating was evaluated by placing the implant in a gelled-saline-filled phantom and MRI was performed using a transmit/receive RF body coil at an MR system reported, whole body averaged specific absorption rate of 2.9-W/kg. Artifacts were characterized using T1-weighted and GRE pulse sequences.

Results

Magnetic field interactions were not substantial and, thus, will not pose a hazard to a patient in a 3-Tesla or less MRI environment. The highest temperature rise was 1.7 °C, which is physiologically inconsequential. Artifacts were large in relation to the remote port and metal connector of the implant but will only present problems if the MR imaging area of interest is where these components are located.

Conclusions

A patient with this breast tissue expander with a remote port may safely undergo MRI at 3-Tesla or less under the conditions used for this investigation. These findings are the first reported at 3-Tesla for a tissue expander.     

基于联合正则化及压缩传感的MRI图像重构

基于压缩传感的MRI图像重构利用图像稀疏的先验知识能从很少的投影值重构原图像。目前MRI重构算法只利用MRI图像稀疏性表示或只利用基于其局部光滑性的先验知识,重构效果不理想。针对此问题,结合两种先验知识,提出一种基于联合正则化及压缩传感的MRI图像重构方法。利用块坐标下降法将求解联合正则化问题转化为交替求解二次凸优化、稀疏正则化和全变差正则化三个简单的优化问题。并提出分别采用共轭梯度法、二元自适应收缩法以及梯度下降法对以上优化问题求解。实验结果表明,该算法重构效果比现有算法有明显地提高。     

Electric field gradients in niobium pentachloride dimer

Electric field gradient tensors at the bridging chlorine sites in niobium pentachloride dimer have been evaluated using point-charge model with de Wette’s planewise summation method for rapid convergence of the lattice sums. The results are discussed based on the experimental results and the existing theory on dimeric molecules.     

Definition of displacement probability and diffusion time in q-space magnetic resonance measurements that use finite-duration diffusion-encoding gradients

In q-space diffusion NMR, the probability P(r,td) of a molecule having a displacement r in a diffusion time td is obtained under the assumption that the diffusion-encoding gradient g has an infinitesimal duration. However, this assumption may not always hold, particularly in human MRI where the diffusion-encoding gradient duration delta is typically of the same order of magnitude as the time offset Delta between encoding gradients. In this case, finite-delta effects complicate the interpretation of displacement probabilities measured in q-space MRI, and the form by which the signal intensity relates to them. By considering the displacement-specific dephasing, , of a set of spins accumulating a constant displacement vector r in the total time Delta+delta during which diffusion is encoded, the probability recovered by a finite-delta q-space experiment can be interpreted. It is shown theoretically that a data analysis using a modified q-space index q=gammadeltaetag, with gamma the gyromagnetic ratio and eta=square root (Delta-delta/3)/(Delta+delta), recovers the correct displacement probability distribution if diffusion is multi-Gaussian free diffusion. With this analysis, we show that the displacement distribution P(r,texp) is measured at the experimental diffusion-encoding time texp=Delta+delta, and not at the reduced diffusion time tr=Delta-delta/3 as is generally assumed in the NMR and MRI literature. It is also shown that, by defining a probability P(y,Delta) that a time tdeltac then eta is not equal to square root (Delta-delta/3)/(Delta+delta) which implies that we can no longer obtain the correct displacement probability from the displacement distribution. In the case that /g/=18 mT/m and Delta-delta=5 ms, the parameter deltac in ms is given by "deltac=0.49a2+0.24" where a is the sphere's radius expressed in microm. Simulation of q-space restricted diffusion MRI experiments indicate that if eta=square root (Delta-delta/3)/(Delta+delta), the recovered displacement probability is always better than the Gaussian approximation, and the measured diffusion coefficient matches the diffusion coefficient at time texp=Delta+delta better than it matches the diffusion coefficient at time tr=Delta-delta/3. These results indicate that q-space MRI measurements of displacement probability distributions are theoretically possible in biological tissues using finite-duration diffusion-encoding gradients provided certain compartment size and diffusion encoding gradient duration constraints are met.     

Ultrasound-assisted modulation of concomitant polymorphism of curcumin during liquid antisolvent precipitation

Curcumin polymorphs were found to precipitate concomitantly during liquid antisolvent precipitation. While, commercially available curcumin exists in a monoclinic form, the curcumin particles when precipitated in presence of additives and ultrasound were either found to be the mixtures of orthorhombic (Form 3) and monoclinic form (Form 1) or were found to be in orthorhombic form (Form 3) or monoclinic form (Form 1). The experimentally observed particle morphologies did not match clearly with the predicted BFDH morphologies of curcumin and the experimentally observed morphologies were more elongated as compared to the predicted BFDH morphologies. At lower ultrasonic irradiation times, the monoclinic form (Form 1) was found to dominate the mixture of particles. However, an increase in ultrasonic irradiation time was found to increase the percentage of orthorhombic form (Form 3) in the particles indicating that the increase in ultrasonic energy facilitates formation of orthorhombic form over the monoclinic form, irrespective of the additive used. These results therefore suggest that the ultrasonic energy can be effectively used to manipulate the polymorphic outcome of the precipitation.