Practical design of a 4 Tesla double-tuned RF surface coil for interleaved 1H and 23Na MRI of rat brain

MRI is proving to be a very useful tool for sodium quantification in animal models of stroke, ischemia, and cancer. In this work, we present the practical design of a dual-frequency RF surface coil that provides (1)H and (23)Na images of the rat head at 4 T. The dual-frequency RF surface coil comprised of a large loop tuned to the (1)H frequency and a smaller co-planar loop tuned to the (23)Na frequency. The mutual coupling between the two loops was eliminated by the use of a trap circuit inserted in the smaller coil. This independent-loop design was versatile since it enabled a separate optimisation of the sensitivity and RF field distributions of the two coils. To allow for an easy extension of this simple double-tuned coil design to other frequencies (nuclei) and dimensions, we describe in detail the practical aspects of the workbench design and MRI testing using a phantom that mimics in vivo conditions. A comparison between our independent-loop, double-tuned coil and a single-tuned (23)Na coil of equal size obtained with a phantom matching in vivo conditions, showed a reduction of the (23)Na sensitivity (about 28 %) because of signal losses in the trap inductance. Typical congruent (1)H and (23)Na rat brain images showing good SNR ((23)Na: brain 7, ventricular cerebrospinal fluid 11) and spatial resolution ((23)Na: 1.25 x 1.25 x 5mm(3)) are also reported. The in vivo SNR values obtained with this coil were comparable to, if not better than, other contemporary designs in the literature.     

Noninvasive (1)H and (23)Na nuclear magnetic resonance imaging of ancient Egyptian human mummified tissue

Historic mummies are a unique example of the human desire for immortality. Therefore, it is not surprising that modern diagnostic imaging has been widely applied to study them. Yet, magnetic resonance imaging (MRI) of such old remains has never been successfully achieved in a noninvasive way without rehydration. Furthermore, the impact of artificial mummification as done in ancient Egypt by natron (a blend of NaCl, Na(2)CO(3), NaHCO(3) and NaP(2)SO(4)) on human tissue with a particular focus on the sodium spatial distribution has never been addressed. Here, we show for the very first time completely noninvasive (1)H and (23)Na imaging of an ancient Egyptian mummified finger by nuclear magnetic resonance (NMR). Protons could be visualized by NMR only in the tissue close to surface and sodium primarily in the bone, while computer tomography images both, soft tissue and bone but does not distinguish between different chemical elements. The selective enrichment of sodium in the bone may by due to postmortem incorporation of (23)Na into the tissue by natron-based mummification because our reference measurement of a historical finger not subjected to artificial mummification showed no sodium signal at all. Our results demonstrate not only the general feasibility of nonclinical MRI to visualize historic dry human tissues but also shows the specific (1)H and (23)Na spatial distributions in such mummy tissue, which is particularly interesting for archeology and may open up a new application for MRI.     

Use of 1H/23Na and 1H/31P double frequency tuned birdcage coils to study in vivo carbon tetrachloride-induced hepatotoxicity in rats.

In vivo 1H and 23Na magnetic resonance imaging (MRI) and 31P magnetic resonance spectroscopy (MRS) techniques were used to study CCl4-induced acute hepatotoxicity in rats in situ. One or two hours following exposure to CCl4, a localized edematous region was detected in the liver by 1H MRI. The CCl4-induced edema was localized in a region surrounding the hepatic portal vein. With the use of a 23Na/1H double frequency tuned bird-cage imaging coil an increase in Na+ ion flux was also observed in the same region as the edematous region detected by 1H-MRI. Pretreatment with alpha-phenyl-tert-butyl nitrone (PBN), a free radical spin trap, 30 min prior to CCl4 exposure, was found to reduce the CCl4-induced edematous response in the liver observed in either 1H or 23Na-NMR images. Inhibition of the CCl4-induced edematous response in rat liver by PBN demonstrates that free radical intermediates, arising from the metabolism of CCl4, are possibly the key causal agents in the initiation of the edematous response. In addition, with the use of a 31P/1H double frequency tuned bird-cage imaging/spectroscopy coil, localized 31P spectra (ISIS) were obtained from the regions of CCl4-induced "tissue damage" observed in the 1H-MRI images. The most notable changes observed from the 31P spectra were an increase in inorganic phosphate (Pi) and a decrease in hepatocytosolic pH in the CCl4-treated rat livers in comparison to saline-treated control livers.     

结合基于梯度的振铃评价算法的总变分最小化图像分块复原法

为了消除退化函数随空间变化发生变化模糊图像分块复原法子块之间的不平滑拼接缝,提出了一种结合了基于梯度的振铃评价算法梯度振铃评价(GRM)的总变分(TV)最小化分块复原法.根据图像分布及退化类型将模糊图像划分为矩形、环形或其他形状的子块,图像子块之间要留有一定的重叠区;然后对每一个图像子块进行复原,GRM方法是基于图像梯度结构相似度的图像质量评价算法,以GRM作为TV复原算法迭代过程中的收敛条件,可以更好地控制复原图像的振铃;最后去除复原图像子块含振铃波纹的重叠区,拼接得到完整图像.并以矩形分块及环形分块为例,证明该方法可以很好地抑制图像边界振铃效应,克服分块复原法本身的缺陷,得到拼接平滑的完整图像.     

Feasibility of proton chemical shift imaging with a stereotactic headframe

To prove feasibility of proton chemical shift imaging ((1)H CSI) during stereotactic procedure, authors performed (1)H CSI in combination with a stereotactic headframe and selected targets according to local metabolic information, evaluated the pathologic results. The (1)H CSI directed stereotactic biopsy was performed in four patients. (1)H CSI and conventional Gd-enhancement stereotactic MRI were performed simultaneously after the fitting of a stereotactic headframe. Focal areas of increased phosphocholine(Cho)/phosphocreatine(Cr) and Lactate/Cr ratios were selected as target sites in the stereotactic MR images. (1)H CSI is possible with the stereotactic headframe in place. Pathologic samples taken from areas of increased Cho/Cr ratios and decreased NAA/Cr ratios provided information upon increased cellularity, mitoses and cellular atypism, and facilitated diagnosis. Pathologic samples taken from areas of increased Lac/Cr ratio showed predominant feature of necrosis. (1)H CSI was feasible with the stereotactic headframe in place. The final pathologic results obtained were concordant with the local metabolic information from (1)H CSI. We believe that (1)H CSI-directed stereotactic biopsy has the potential to significantly improve the accuracy of stereotactic biopsy targeting.     

Iterative reconstruction of radially-sampled 31P bSSFP data using prior information from 1H MRI

The purpose of this study is to improve direct phosphorus (³¹P) MR imaging. Therefore, 3D density-adapted radially-sampled balanced steady-state free precession (bSSFP) sequences were developed and an iterative approach exploiting additional anatomical information from hydrogen (¹H) data was evaluated. Three healthy volunteers were examined at B₀ = 7 T in order to obtain the spatial distribution of the phosphocreatine (PCr) intensities in the human calf muscle with a nominal isotropic resolution of 10 mm in an acquisition time of 10 min. Three different bSSFP gradient schemes were investigated. The highest signal-to-noise ratio (SNR) was obtained for a scheme with two point-reflected density-adapted gradients. Furthermore, the conventional reconstruction based on a gridding algorithm was compared to an iterative method using an ¹H MRI constraint in terms of a segmented binary mask, which comprises prior knowledge. The parameters of the iterative approach were optimized and evaluated by simulations featuring ³¹P MRI parameters. Thereby, partial volume effects as well as Gibbs ringing artifacts could be reduced. In conclusion, the iterative reconstruction of ³¹P bSSFP data using an ¹H MRI constraint is appropriate for investigating regions where sharp tissue boundaries occur and leads to images that represent the real PCr distributions better than conventionally reconstructed images.     

Monitoring chemotherapeutic response in RIF-1 tumors by single-quantum and triple-quantum-filtered (23)Na MRI, (1)H diffusion-weighted MRI and PET imaging

The effects of 5-fluorouracil (5FU, 150 mg/kg, ip) on subcutaneously implanted radiation-induced fibrosarcoma (RIF-1) tumors were monitored by in vivo (1)H MRI to evaluate the water apparent diffusion coefficient (ADC), by single-quantum (SQ) and triple-quantum-filtered (TQF) (23)Na MRI to evaluate compartmental Na(+) content and by positron emission tomography (PET) to evaluate 2-[(18)F]fluoro-2-deoxy-d-glucose (FDG) uptake in the tumor. The MRI experiments were performed on untreated control and treated mice once before and then daily for 3 days after treatment. The PET experiments were performed on separate groups of age- and tumor-volume-matched animals once before and then 3 days after treatment. Tumor volumes significantly decreased in treated animals 2 and 3 days posttreatment. At the same time points, in vivo MRI measurements showed an increase in both total tissue SQ (23)Na signal intensity (SI) and water ADC in treated tumors while control tumors showed no change in these parameters. TQF (23)Na SI and FDG uptake were significantly lower in treated tumors compared with control tumors 3 days after 5FU treatment. The correlated increases in total tissue (23)Na SI and water ADC following chemotherapy reflect an increase in extracellular space, while the lower TQF (23)Na SI and FDG uptake in treated tumors compared with control tumors suggest a shift in tumor metabolism from glycolysis to oxidation and/or a decrease in cell density.     

In vivo K, Na and H MR imaging using a triple resonant RF coil setup

The maintenance of a gradient of potassium and sodium ions across the cell membranes is essential for the physiological function of the mammal organism. The measurement of the spatial distribution of pathologically changing ion concentrations of ²³Na and ³⁹K with magnetic resonance imaging offers a promising approach in clinical diagnostics to measure tissue viability. Existing studies were focused mainly on ²³Na imaging as well as spectroscopy with only one post-mortem study for ³⁹K imaging. In this paper a triple resonant RF coil setup for the rat head at 9.4 T is presented for imaging of both nuclei (²³Na and ³⁹K) and the acquisition of anatomical proton images in the same experiment without moving the subject or the RF coil. In vivo MR images of ³⁹K and ²³Na in the rat brain were acquired as well as anatomical proton images in the same scanning session.     

23Na and (1)H NMR microimaging of intact plants

(23)Na NMR microimaging is described to map, for the first time, the sodium distribution in living plants. As an example, the response of 6-day-old seedlings of Ricinus communis to exposure to sodium chloride concentrations from 5 to 300 mM was observed in vivo using (23)Na as well as (1)H NMR microimaging. Experiments were performed at 11.75 T with a double resonant (23)Na-(1)H probehead. The probehead was homebuilt and equipped with a climate chamber. T(1) and T(2) of (23)Na were measured in the cross section of the hypocotyl. Within 85 min (23)Na images with an in-plane resolution of 156 x 156 micrometer were acquired. With this spatial information, the different types of tissue in the hypocotyl can be discerned. The measurement time appears to be short compared to the time scale of sodium uptake and accumulation in the plant so that the kinetics of salt stress can be followed. In conclusion, (23)Na NMR microimaging promises great potential for physiological studies of the consequences of salt stress on the macroscopic level and thus may become a unique tool for characterizing plants with respect to salt tolerance and salt sensitivity.     

A novel algorithm for the reduction of undersampling artefacts in magnetic resonance images

An innovative algorithm is presented which is effective in reducing the truncation artefacts occurring in magnetic resonance images due to missing k-space samples. The algorithm works first by filling the incomplete matrix of coefficients with zeroes and then adjusting, by an iterative process, the missing coefficients by performing a reduction of the undersampling artefacts. Then, this set of coefficients is used as a basis for a superresolution algorithm that estimates the missing coefficients by modeling the data as a linear combination of increasing and decreasing exponential functions using Prony's method. In fact, the Prony's method consists of the interpolation of a given data set with a sum of exponential functions: the MRI signals can be well represented as a sum of exponential functions and the missing data can be extrapolated by this representation. The algorithm has been proven to perform better than either a simple algorithm, which detects and then reduces the undersampling artefacts, or an algorithm that models the measured data with approximation functions. The presented algorithm is quite simple and is applicable both to missing rows (phase-frequency acquisitions) and to radial-missing angle (acquisition from projections) undersampling. Experimental results are reported; comparisons, made between the results obtained using the presented algorithm and the alternative methods described above, clearly demonstrate the superiority of the algorithm.     

Sodium T2*-weighted MR imaging of acute focal cerebral ischemia in rabbits

Changes in T2*-weighted tissue sodium (23Na) signal following acute ischemia may help to identify necrotic tissue and estimate the duration of ischemia. Sodium signal was monitored in a rabbit model of acute (0-4 h) focal cerebral ischemia, using gradient echo 23Na MR images (echo time = 3.2 ms) acquired continuously in 20-min intervals on a 4-Tesla MRI. 2,3,5-Triphenyl-tetrazolium chloride staining was used to identify regions of necrosis. In necrotic tissue, average 23Na image signal intensity decreased by 11% +/- 8% during the first 40 min of ischemia followed by a linear increase (0.19%/min) to 25% +/- 14% greater than baseline after 4 h of ischemia. The time course of 23Na signal change observed in necrotic tissue following focal ischemia in this rabbit model is consistent with an initial decrease in 23Na T2* relaxation time followed by an increase in tissue sodium concentration and provides further evidence that tissue 23Na signal may offer unique information regarding tissue viability that is complementary to other MR imaging techniques.     

23 Na磁共振成像的初步实验研究

介绍了用于²³Na磁共振成像实验的收发一体表面线圈的设计、制作原理与实际制作. 用制作的²³Na表面线圈在Bruker Biospec 47/30磁共振成像仪上采集了不同浓度NaCl溶液,盐腌制鹌鹑蛋和大鼠头部的²³Na密度像,为深入开展23Na磁共振成像的生物医学应用研究奠定基础.     

An improved Richardson-Lucy algorithm based on local prior

Ringing is one of the most common disturbing artifacts in image deconvolution. With a totally known kernel, the standard Richardson-Lucy (RL) algorithm succeeds in many motion deblurring processes, but the resulting images still contain visible ringing. When the estimated kernel is different from the real one, the result of the standard RL iterative algorithm will be worse. To suppress the ringing artifacts caused by failures in the blur kernel estimation, this paper improves the RL algorithm based on the local prior. Firstly, the standard deviation of pixels in the local window is computed to find the smooth region and the image gradient in the region is constrained to make its distribution consistent with the deblurring image gradient. Secondly, in order to suppress the ringing near the edge of a rigid body in the image, a new mask was obtained by computing the sharp edge of the image produced using the first step. If the kernel is large-scale, where the foreground is rigid and the background is smoothing, this step could produce a significant inhibitory effect on ringing artifacts. Thirdly, the boundary constraint is strengthened if the boundary is relatively smooth. As a result of the steps above, high-quality deblurred images can be obtained even when the estimated kernels are not perfectly accurate. On the basis of blurred images and the related kernel information taken by the additional hardware, our approach proved to be effective.     

An adaptive super-resolution method based on regional pixel information and ringing artifacts suppression

Multi-frame image super-resolution (SR) aims to utilize information from a set of low-resolution (LR) images to compose a high-resolution (HR) one. In this paper, a novel multi-frame image super-resolution algorithm is proposed based on regional pixel information and ringing artifacts suppression. Firstly, a new regularization term which adopts Regional Adaptive Weight Coefficients (RAWC) is produced to keep edges and flat regions. After detailed analysis, an iterative process is given for image reconstruction. Then an adaptive term according to the local variance of iterative correction image is designed to evaluate the ringing artifacts. Finally, the original iteration is updated by adding the restraint term for better visual effects and lower noise of reconstructive HR image. Thorough experimental results show the proposed algorithm is effective for SR reconstruction and ringing artifacts suppression.     

High-resolution ultrahigh-field MRI of stroke

BACKGROUND: Ultrahigh-field MRI at 8 T offers unprecedented resolution for imaging brain structures and microvasculature. OBJECTIVE: The aim of this study is to apply high-resolution MRI for stroke imaging and to characterize findings at 1.5 and 8 T. METHODS: Seventeen subjects with minor ischemic infarcts were studied using T2-weighted gradient echo (GE) and rapid acquisition with relaxation enhancement (RARE) images at 8 T with resolution up to 200 microm. In 10 subjects, T1- and T2-weighted fast spin echo (FSE) and fluid-attenuated inversion recovery (FLAIR) images were also acquired at 1.5-T MRI. RESULTS: The 8-T images showed infarcts as sharply demarcated areas of high-signal intensity (n=21) and revealed more infarctions than 1.5-T images (n=14) (P<.003). The low-signal intensity areas that surrounded infarctions were suggestive of hemosiderin deposits. The 8-T characteristics of microvessels terminating within the infractions were distinct from normal vasculature. The 8-T images revealed an angioma at the site of a second stroke, not apparent on 1.5-T images. CONCLUSIONS: Ultrahigh-field MRI at 8 T is feasible for stroke imaging. The 8-T MRI visualized infarcts and microvasculature with high resolution, revealing infarcts and vascular pathologies that were not apparent at 1.5 T.     

GPU based parallel framework for receiver coil sensitivity estimation in SENSE reconstruction

Magnetic Resonance Imaging (MRI) uses non-ionizing radiations and is safer as compared to CT and X-ray imaging. MRI is broadly used around the globe for medical diagnostics. One main limitation of MRI is its long data acquisition time. Parallel MRI (pMRI) was introduced in late 1990's to reduce the MRI data acquisition time. In pMRI, data is acquired by under-sampling the Phase Encoding (PE) steps which introduces aliasing artefacts in the MR images. SENSitivity Encoding (SENSE) is a pMRI based method that reconstructs fully sampled MR image from the acquired under-sampled data using the sensitivity information of receiver coils. In SENSE, precise estimation of the receiver coil sensitivity maps is vital to obtain good quality images. Eigen-value method (a recently proposed method in literature for the estimation of receiver coil sensitivity information) does not require a pre-scan image unlike other conventional methods of sensitivity estimation. However, Eigen-value method is computationally intensive and takes a significant amount of time to estimate the receiver coil sensitivity maps. This work proposes a parallel framework for Eigen-value method of receiver coil sensitivity estimation that exploits its inherent parallelism using Graphics Processing Units (GPUs). We evaluated the performance of the proposed algorithm on in-vivo and simulated MRI datasets (i.e. human head and simulated phantom datasets) with Peak Signal-to-Noise Ratio (PSNR) and Artefact Power (AP) as evaluation metrics. The results show that the proposed GPU implementation reduces the execution time of Eigen-value method of receiver coil sensitivity estimation (providing up to 30 times speed up in our experiments) without degrading the quality of the reconstructed image.     

Temperature-dependent chemical shift and relaxation times of (23)Na in Na(4)HTm[DOTP

We describe the characterization of a (23)Na temperature-dependent chemical shift and relaxation rates in the complex, Na(4)HTm[DOTP]. This is the first characterization of a (23)Na temperature-dependent chemical shift in a nonmetallic sample. The (23)Na temperature-dependent chemical shift coefficient is approximately -0. 5 PPM/ degrees C for both an aqueous solution and a 6% agarose gel of this compound. This is 50 times the magnitude of the temperature-dependent chemical shift coefficient of water protons. The relaxation times, T(1), T(2f), and T(2s) increased by 0.1, 0.01, and 0.05 ms/ degrees C, respectively. Applications of these unique properties for designing an MRI technique for monitoring heat deposition in tissue and tissue phantoms are discussed.     

Dynamics of NaHSeO3 and NaHSeO4 single crystals by observation of 1H and 23Na spin-lattice relaxation

The 1H and 23Na spin-lattice relaxation times of NaHSeO3 and NaHSeO4 single crystals grown by the slow evaporation method were measured. The phase transitions of the two crystals were studied using differential scanning calorimetry (DSC), and the environments of the 1H and 23Na nuclei in these crystals were investigated by means of 1H and 23Na NMR spectroscopy. The DSC results show that NaHSeO3 crystals do not undergo phase transitions, whereas for NaHSeO4 crystals there is a small endothermic peak at 419 K. The dimers in NaHSeO3 stabilize its structure to the extent that there are no phase transitions even in the presence of disordered hydrogen bonds. In addition, the NMR results indicate that the temperature dependencies of T1 for the 1H and 23Na nuclei in the two hydrogen-bonded crystals-the NaHSeO3 crystal with 3-coordinated SeO3 and the NaHSeO4 crystal with 4-coordinated SeO4--are significantly different. The different trends in T1 for Na in the two crystals are accompanied by different shifts of the oxygen atoms from the 3-coordinated SeO3 and the 4-coordinated SeO4 species around the Na+ ions. Thus, although the two crystals have hydrogen-bonded structures, the differences between the hydrogen bonding of the 3-coordinated SeO3 in NaHSeO3 and the hydrogen bonding of the 4-coordinated SeO4 in NaHSeO4 result in different T1 temperature dependences and different phase transitions.     

基于指数律分布的图像快速盲复原算法

针对现有图像盲复原迭代算法多存在耗时较长和难以保证收敛性等问题,提出一种改进的快速算法.首先根据指数律重建原始图像的频谱,然后利用原始图像和降质图像的频谱关系,采用多方向综合估计方法得到点扩散函数.多方向综合估计方法可降低估计误差,增加算法的稳定性.最后利用得到的点扩散函数和维纳滤波法进行图像重建.与现有算法的对比实验结果表明,针对适合大量成像系统的G类点扩散函数,本算法可以得到更准确的点扩散函数估计,且降低了振铃效应的影响,取得更好的图像复原效果.     

EASY: A simple tool for simultaneously removing background,deadtime and acoustic ringing in quantitative NMR spectroscopy—Part I: Basic principle and applications

Elimination of Artifacts in NMR SpectroscopY (EASY) is a simple but very effective tool to remove simultaneously any real NMR probe background signal, any spectral distortions due to deadtime ringdown effects and -specifically- severe acoustic ringing artifacts in NMR spectra of low-gamma nuclei. EASY enables and maintains quantitative NMR (qNMR) as only a single pulse (preferably 90°) is used for data acquisition. After the acquisition of the first scan (it contains the wanted NMR signal and the background/deadtime/ringing artifacts) the same experiment is repeated immediately afterwards before the T1 waiting delay. This second scan contains only the background/deadtime/ringing parts. Hence, the simple difference of both yields clean NMR line shapes free of artefacts.In this Part I various examples for complete ¹H, ¹¹B, ¹³C, ¹⁹F probe background removal due to construction parts of the NMR probes are presented. Furthermore, ²⁵Mg EASY of Mg(OH)₂ is presented and this example shows how extremely strong acoustic ringing can be suppressed (more than a factor of 200) such that phase and baseline correction for spectra acquired with a single pulse is no longer a problem. EASY is also a step towards deadtime-free data acquisition as these effects are also canceled completely. EASY can be combined with any other NMR experiment, including 2D NMR, if baseline distortions are a big problem.