《医学磁共振》杂志论文的写作及审稿

《医学磁共振》杂志(MRM或官方简称“Magn Reson Med”)是一本国际性期刊,迄今为止已收到来自38个不同国家(和地区)的稿件.近年来,越来越多的中国学者关注我们的杂志.本文是Magn Reson Med杂志主编应邀在澳大利亚墨尔本召开的2012年国际磁共振年会上“聚焦中国”的发言稿,将向大家介绍《医学磁共振》杂志的概况、审稿流程、对审稿人的要求以及杂志社给作者的一些写作建议和投稿的注意事项.     

Development and Characterization of An Unshielded PatLoc Gradient Coil for Human Head Imaging

A cylindrical head gradient insert for human imaging with non‐linear spatial encoding magnetic fields (SEMs) has been designed, optimized and successfully integrated with a modified 3T clinical MR system. This PatLoc (parallel acquisition technique using localized gradients) SEM coil uses SEMs that resemble second‐order magnetic shim fields, but with much higher amplitude as well as the possibility for rapid switching. This work describes the optimization of a coil design and measurement methods to characterize its SEMs, induced self‐eddy currents and concomitant fields. Magnetic field maps of the SEMs are measured and it is demonstrated that the induced self‐eddy current magnetic fields are small and can be compensated. A method to measure concomitant fields is presented and those fields are compared to simulated data. Finally, in vivo human images acquired using the PatLoc system are presented and discussed. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 43B: 111–125, 2013     

Design and implementation of an FPGA‐based timing pulse programmer for pulsed‐electron paramagnetic resonance applications

The design, construction, and implementation of a field‐programmable gate array (FPGA)‐based pulse programmer for pulsed‐electron paramagnetic resonance experiments is described. The FPGA pulse programmer offers advantages in design flexibility and cost over previous pulse programmers, which are based on commercial digital delay generators, logic pattern generators, and application‐specific integrated circuit designs. The FPGA pulse progammer features a novel transition‐based algorithm and command protocol, which is optimized for the timing structure required for most pulsed magnetic resonance experiments. The algorithm was implemented by using a Spartan‐6 FPGA (Xilinx), which provides an easily accessible and cost effective solution for FPGA interfacing. An auxiliary board was designed for the FPGA‐instrument interface, which buffers the FPGA outputs for increased power consumption and capacitive load requirements. Device specifications include: Nanosecond pulse formation (transition edge rise/fall times, ≤3 ns), low jitter (≤150 ps), large number of channels (16 implemented; 48 available), and long pulse duration (no limit). The hardware and software for the device were designed for facile reconfiguration to match user experimental requirements and constraints. Operation of the device is demonstrated and benchmarked by applications to one‐dimensional electron spin echo envelope modulation and two‐dimensional hyperfine sublevel correlation (HYSCORE) experiments. The FPGA approach is transferrable to applications in nuclear magnetic resonance (magnetic resonance imaging), and to pulse perturbation and detection bandwidths in spectroscopies up through the optical range. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 43B: 100‐109, 2013     

Cross-correlation: an fMRI signal-processing strategy

The discovery of functional MRI (fMRI), with the first papers appearing in 1992, gave rise to new categories of data that drove the development of new signal-processing strategies. Workers in the field were confronted with image time courses, which could be reshuffled to form pixel time courses. The waveform in an active pixel time-course was determined not only by the task sequence but also by the hemodynamic response function. Reference waveforms could be cross-correlated with pixel time courses to form an array of cross-correlation coefficients. From this array of numbers, colorized images could be created and overlaid on anatomical images. An early paper from the authors' laboratory is extensively reviewed here (Bandettini et al., 1993. Magn. Reson. Med. 30:161-173). That work was carried out using the vocabulary of vector algebra. Cross-correlation methodology was central to the discovery of functional connectivity MRI (fcMRI) by Biswal et al. (1995. Magn. Reson. Med. 34:537-541). In this method, a whole volume time course of images is collected while the brain is nominally at rest and connectivity is studied by cross-correlation of pixel time courses.     

A circuit for synchronizing external stimuli and events to the pulse sequence of a clinical magnetic resonance scanner

Interfacing experiments to a clinical magnetic resonance imaging scanner which require synchronization with an imaging pulse sequence can be challenging unless access to a trigger‐out is available. We present a simple, alternative approach to synchronize an experiment to a scanner using an external trigger to drive the scanner through the peripheral optical pulse rate monitor and cardiac trigger available on most clinical imagers. A trigger circuit and pulse rate monitor interface are described for applying a stimulus in the form of a pulsed voltage to a sample at a specific time in a spin‐echo, echo planar imaging sequence. The apparatus and approach could be used for many other types and numbers of experimental stimuli or events. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 50–52, 2014     

Retrospective respiratory motion correction for navigated cine velocity mapping.

In general, high spatial and temporal resolutions in cine cardiac imaging require long scan times, making breath-hold acquisition impossible in many cases. To enable free-breathing cardiac imaging, methods such as navigator gating were developed to reduce image artifacts due to respiratory motion. Nevertheless, residual image blurring is seen in images acquired late in the cardiac cycle. Image blurring itself hampers accurate blood flow quantification, especially in vessels exhibiting high flows during diastole. In the present work, the navigator gating and slice tracking method was extended by using navigator information to correct for in-slice motion components throughout the cardiac cycle. For this purpose, a standard two-dimensional (2D) cine phase contrast sequence with navigator gating and slice position correction was used, and navigator information was recorded along with the raw k-space data. In postprocessing, in-plane motion components arising from respiration during the actual data acquisition were estimated and corrected according to the Fourier shift theorem. In phantom experiments, the performance of the correction algorithm for different slice angulations with respect to the navigator orientation was validated. In vivo, coronary flow measurements were performed in 9 healthy volunteers. The correction algorithm led to considerably improved vessel sharpness throughout the cardiac cycle in all measured subjects [increase in vessel sharpness: 16+/-11% (mean+/-SD)]. Furthermore, these improvements resulted in increased volume flow rates [16+/-13% (mean+/-SD)] after retrospective correction indicating the impact of the method. It is concluded that retrospective respiratory motion corrections for navigated cine two-dimensional (2D) velocity mapping can correct for in-plane motion components, providing better image quality for phases acquired late in the cardiac cycle. Therefore, this method holds promise in particular for free-breathing coronary flow quantification.     

A complex way to compute fMRI activation

In functional magnetic resonance imaging, voxel time courses after Fourier or non-Fourier "image reconstruction" are complex valued as a result of phase imperfections due to magnetic field inhomogeneities. Nearly all fMRI studies derive functional "activation" based on magnitude voxel time courses [Bandettini, P., Jesmanowicz, A., Wong, E., Hyde, J.S., 1993. Processing strategies for time-course data sets in functional MRI of the human brain. Magn. Reson. Med. 30 (2): 161-173 and Cox, R.W., Jesmanowicz, A., Hyde, J.S., 1995. Real-time functional magnetic resonance imaging. Magn. Reson. Med. 33 (2): 230-236]. Here, we propose to directly model the entire complex or bivariate data rather than just the magnitude-only data. A nonlinear multiple regression model is used to model activation of the complex signal, and a likelihood ratio test is derived to determine activation in each voxel. We investigate the performance of the model on a real dataset, then compare the magnitude-only and complex models under varying signal-to-noise ratios in a simulation study with varying activation contrast effects.     

Modeling both the magnitude and phase of complex-valued fMRI data

In MRI and fMRI, images or voxel measurement are complex valued or bivariate at each time point. Recently, (Rowe, D.B., Logan, B.R., 2004. A complex way to compute fMRI activation. NeuroImage 23 (3), 1078-1092) introduced an fMRI magnitude activation model that utilized both the real and imaginary data in each voxel. This model, following traditional beliefs, specified that the phase time course were fixed unknown quantities which may be estimated voxel-by-voxel. Subsequently, (Rowe, D.B., Logan, B.R., 2005. Complex fMRI analysis with unrestricted phase is equivalent to a magnitude-only model. NeuroImage 24 (2), 603-606) generalized the model to have no restrictions on the phase time course. They showed that this unrestricted phase model was mathematically equivalent to the usual magnitude-only data model including regression coefficients and voxel activation statistic but philosophically different due to it derivation from complex data. Recent findings by (Hoogenrad, F.G., Reichenbach, J.R., Haacke, E.M., Lai, S., Kuppusamy, K., Sprenger, M., 1998. In vivo measurement of changes in venous blood-oxygenation with high resolution functional MRI at .95 Tesla by measuring changes in susceptibility and velocity. Magn. Reson. Med. 39 (1), 97-107) and (Menon, R.S., 2002. Postacquisition suppression of large-vessel BOLD signals in high-resolution fMRI. Magn. Reson. Med. 47 (1), 1-9) indicate that the voxel phase time course may exhibit task related changes. In this paper, a general complex fMRI activation model is introduced that describes both the magnitude and phase in complex data which can be used to specifically characterize task related change in both. Hypotheses regarding task related magnitude and/or phase changes are evaluated using derived activation statistics. It was found that the Rowe-Logan complex constant phase model strongly biases against voxels with task related phase changes and that the current very general complex linear phase model can be cast to address several different hypotheses sensitive to different magnitude/phase changes.     

A comparison of prospective and retrospective respiratory navigator gating in 3D MR coronary angiography

A comparison between the prospective and retrospective respiratory navigator gating in MR coronary angiography was performed with eight normal subjects. A three-dimensional (3D) ECG-gated fast gradient echo pulse sequence was used for image data acquisition. The results show that the MR coronary angiography obtained using retrospective gating retains a considerable amount of motion artifacts. In this study, the images acquired using prospective navigator gating demonstrated significantly reduced motion artifacts (p = 0.009), improved vessel visibility (p = 0.021) with reduced imaging time (p = 0.013) compared to the images obtained using retrospective navigator gating.     

Dependence of NMR noise line shapes on tuning,matching, and transmission line properties

The tuning and matching conditions of rf circuits, as well as the properties of the transmission lines connecting these to the preamplifier, have direct consequences for NMR probe sensitivity and as for the optimum delivery of rf power to the sample. In addition, tuning/matching conditions influence radiation damping effects, which manifest themselves as fast signal flip‐back and line broadening effects, and can lead to concentration‐dependent frequency shifts. Previous studies have also shown that the appearance of spin‐noise and absorbed circuit noise signals heavily depended on tuning settings. Consequently, all these phenomena are linked together. The mutual connections and interdependences of these effects are highlighted and reviewed here. © 2014 The Authors Concepts in Magnetic Resonance, Part B: Magnetic Resonance Engineering Published by Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 1–11, 2014     

Using nonlinear models in fMRI data analysis: model selection and activation detection

There is an increasing interest in using physiologically plausible models in fMRI analysis. These models do raise new mathematical problems in terms of parameter estimation and interpretation of the measured data. In this paper, we show how to use physiological models to map and analyze brain activity from fMRI data. We describe a maximum likelihood parameter estimation algorithm and a statistical test that allow the following two actions: selecting the most statistically significant hemodynamic model for the measured data and deriving activation maps based on such model. Furthermore, as parameter estimation may leave much incertitude on the exact values of parameters, model identifiability characterization is a particular focus of our work. We applied these methods to different variations of the Balloon Model (Buxton, R.B., Wang, E.C., and Frank, L.R. 1998. Dynamics of blood flow and oxygenation changes during brain activation: the balloon model. Magn. Reson. Med. 39: 855-864; Buxton, R.B., Uluda?, K., Dubowitz, D.J., and Liu, T.T. 2004. Modelling the hemodynamic response to brain activation. NeuroImage 23: 220-233; Friston, K. J., Mechelli, A., Turner, R., and Price, C. J. 2000. Nonlinear responses in fMRI: the balloon model, volterra kernels, and other hemodynamics. NeuroImage 12: 466-477) in a visual perception checkerboard experiment. Our model selection proved that hemodynamic models better explain the BOLD response than linear convolution, in particular because they are able to capture some features like poststimulus undershoot or nonlinear effects. On the other hand, nonlinear and linear models are comparable when signals get noisier, which explains that activation maps obtained in both frameworks are comparable. The tools we have developed prove that statistical inference methods used in the framework of the General Linear Model might be generalized to nonlinear models.     

Characterization of a dedicated double loop,endoluminal coil for anal sphincter MR imaging at 1.5 T and 3 T

MRI has proven its usefulness in the prediction of surgical anterior anal repair that cannot be done with the reference endosonographic exam. Conventional endorectal coils are often based on a single loop coil design and do not possess satisfactory radial uniformity which could impede the correct assessment of the anal sphincter. In this study, several double loop endorectal coils were designed, built, and assessed in simulations, on phantoms and in vivo. The optimum was found for a 50°–70° double loop endorectal coil which presents a better radial uniformity especially at close distance from the coil where the SNR is the highest. First in vivo experiments proved enhanced readability of the MR exam for the radiologist. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 39–49, 2014     

An advanced,integrated large‐volume high‐pressure autoclave and 1h/13c double‐tuned resonator for chemistry and materials nuclear magnetic resonance spectroscopy and microscopy investigations

Nuclear magnetic resonance spectroscopy and imaging are well‐established tools in chemistry, physics, and life sciences. Nevertheless, most applications are performed at room temperature and atmospheric pressure. To study the processes in supercritical fluids, sample containers and coils have to be redesigned to especially allow for higher pressures up to several hundred times the atmospheric pressure. In this study, we present a setup for performing spectroscopic and imaging experiments on wood immersed in supercritical CO₂ at up to 20 MPa for drying. A magnetic resonance‐compatible autoclave as well as a double‐tuned ¹H/¹³C‐birdcage coil was designed and a setup for regulating pressure and storing gases was assembled. We were able to successfully perform measurements on the wood and water during the drying process and gaininsights into the displacement of water and its chemical reactions with the highly pressurized CO₂. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 43B: 49–58, 2013     

Multislice localized parallel excitation for EPI applications in humans

In this work, the opportunities and challenges for the use of parallel transmission in combination with 2D RF pulses designed on EPI‐based excitation trajectories for diffusion‐weighted imaging (DWI) with reduced FOV are presented and analyzed in detail. The use of localized excitation allows for shortening of the EPI read‐out, which is especially important for EPI applications outside of the brain. DWI is chosen as a practically important and relevant example demonstrating the key aspects of 2D spatial selection. The properties of accelerated pulses are explored experimentally in phantoms for two different schemes, in which the thickness of the excited limited slices is encoded either along the frequency or phase encoding directions of the excitation trajectory. The feasibility of application of parallel transmission for MR imaging in humans is analyzed based on several pilot experiments. Although the parallel transmission acceleration is demonstrated to work in some examples in the spinal cord and abdomen, the results also uncover a number of challenges. Nonetheless, the reduction of FOV in the phase encoding direction of the read‐out train along with the associated substantial shortening of the minimum echo train length and reduction of geometric distortions motivates further search for an advantageous use of the parallel transmit technology in EPI applications. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 153–173, 2015     

2D B0 mapping of micro solenoids with and without FC‐84 and SU‐8 at 9.4 T

This work addresses the effect of susceptibility matching improvement of micro‐solenoid coil materials on decreasing the B0 deviation in MR imaging of mass‐limited samples at high Tesla animal scanners. For this purpose, I investigated the effect of improving the solenoids of 1 and 0.5 mm diameters “susceptibility matching” by surrounding them in FC‐84 and SU‐8. Comparing 2D B0 maps of solenoids of 1 mm show that the mean value of B0 deviation has decreased by factors of 15.6 and 4.72 for the coils embedded with FC‐84 and SU‐8 respectively. Likewise, the mean of B0 deviation has decreased by factors of 13.15 and 5.27 for the solenoids of 0.5 mm diameter embedded in FC‐84 and SU‐8, respectively. MR images acquired by the solenoids 0.5 and 1 mm are clearly verifying the role of using susceptible materials in the coil structure in reducing the geometrical artifacts due to B0 deviation. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 69–77, 2015     

Analytical expressions for magnetic fields and field gradients of current‐carrying triangles

We present an analytical method for calculating magnetic field gradients generated by arbitrary triangulated surfaces. Our work builds upon the results published by Pissanetzky and Xiang, who presented formulas for calculating the magnetic field of current‐carrying faceted surfaces. We show that the analytical gradient expressions can be computed considerably faster than finite field value differences. We also find that the aforementioned published expressions for the magnetic field can be simplified and optimized substantially. Closer inspection of the algorithms, for both field and gradient, reveals a number pathological parameter constellations, which require special treatment. We present a detailed discussion on this. Our results can be directly applied in the optimization of complex magnetic field coils, such as magnetic resonance gradient coils. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 18–25, 2014     

Magnetic resonance coronary angiography using navigator echo gated real-time slice following

Navigator echo gating allows for the elimination of breath-holding in MR imaging by providing a real-time monitor of respiratory position to gate image acquisition. In this study we examined the advantages and utility of real-time, navigator echo gated slice following technique in 2D magnetic resonance coronary angiography of patients with coronary artery disease. Thirteen patients with coronary artery disease were examined. MR images of the right coronary artery (RCA) were obtained parallel to the atrioventricular groove to image long sections of the RCA in a small number of slices. In-plane resolution was 0.7 × 0.9 mm and 2–6 signals were averaged to support this high spatial resolution. Targeted maximum intensity projection (MIP) images were generated from the slices to present the RCA in a single image. All patients had x-ray angiograms available for comparison with the MR images. Using the navigator echo gated real-time slice following technique, MRI successfully obtained images in 11 of 13 cases. The technique failed in two patients with irregular breathing patterns. The average length of the RCA seen in the 11 successful MR exams was 61 mm and the average length seen in the x-ray angiograms was 80 mm. Eight patients were determined to be without disease in the RCA by x-ray angiography, and all eight were correctly identified as normal on the MRI exam. In the three patients who had a sucessful MRI exam and were determined to have disease in the RCA by x-ray angiography, MRI identified the lesion in two cases. In the third case MRI indicated a discrete lesion and x-ray angiography indicated diffuse disease without a focal lesion. Navigator echo gating improves patient tolerance, provides aligned sections of coronaries over multiple slices, and allows for improved resolution through signal averaging. This preliminary patient study suggests that navigator echo gated magnetic resonance coronary angiography may play a role in evaluating coronary artery disease.     

MR coronary artery imaging with 3D motion adapted gating (MAG) in comparison to a standard prospective navigator technique.

Magnetic resonance coronary angiography (MRCA) has been proven to be feasible for imaging of the proximal and medial portions of the three main coronary arteries. Free breathing techniques allow for high resolution imaging but prolong scan time. This could potentially be shortened by improving the efficiency, robustness and accuracy of the navigator gating algorithm. Aim of this study was to determine the feasibility, efficiency, and image quality of a new motion compensation algorithm (3D-MAG) for coronary artery imaging with navigator techniques. In 21 patients the coronaries were imaged in plane with a 3D k-space segmented gradient echo sequence. A T2 preparation prepulse was used for suppression of myocardial signal, during free breathing and a navigator technique with using real time slice following and a gating window of 5 mm was applied to suppress breathing motion artefacts. Imaging was performed with standard gating and compared to 3D-MAG. Image quality was visually compared, contrast-to-noise and signal-to-noise ratio were calculated, the length of visualized coronary arteries was measured and scan duration and scan efficiency were calculated. Standard navigator imaging was feasible in 19 of 21 (90.5%) patients 3D-MAG in 21/21 (100%). Scan efficiency and duration was significantly improved with 3D-MAG (p < .05) without change in image quality. 3D-MAG is superior to conventional navigator correction algorithms. It improves feasibility and scan efficiency without reduction of image quality. This approach should be routinely used for MR coronary artery imaging with navigator techniques.     

Numerical methods and software tools for simulation,design, and resonant mode analysis of radio frequency birdcage coils used in MRI

Design of magnetic resonance imaging (MRI) radiofrequency (RF) coils using lumped circuit modeling based techniques begins to fail at high frequencies, and therefore more accurate models based on the electromagnetic field calculations must be used. Field calculations are also necessary to understand the interactions between the RF field and the subject inside the coil. Furthermore, observing the resonance behavior of the coil and the fields at the resonance frequencies have importance for design and analysis. In this study, finite element method (FEM) based methods have been proposed for accurate time‐harmonic electromagnetic simulations, estimation of the tuning capacitors on the rungs or end rings, and the resonant mode analysis of the birdcage coils. Capacitance estimation was achieved by maximizing the magnitude of the port impedance at the desired frequency while simultaneously minimizing the variance of RF magnetic field in the region of interest. In order for the proposed methods to be conveniently applicable, two software tools, resonant mode and frequency domain analyzer (RM‐FDA) and Optimum Capacitance Finder (OptiCF), were developed. Simulation results for the validation and verification of the software tools are provided for different cases including human head simulations. Additionally, two handmade birdcage coils (low‐pass and high‐pass) were built and resonance mode measurements were made. Results of the software tools are compared with the measurement results as well as with the results of the lumped circuit modeling based method. It has been shown that the proposed software tools can be used for accurate simulation and design of birdcage coils. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 13–32, 2015