Practical design of multi‐channel MOSFET RF transmission system for 7 T animal MR imaging

In this work, we developed and tested a multi‐channel radio frequency (RF) transmission system with compact metal‐oxide semiconductor field effect transistor (MOSFET) amplifiers for parallel excitation in 7 T animal MRI scanner. The system is composed of a multi‐channel RF controller and four independent RF power amplifiers. Each power amplifier contains two amplification stages. The design was validated by simulation and bench test. The power gain for the amplifier is 18.7 dB at 300 MHz, demonstrating the sufficient amplification capability of the transmission system for small animal parallel excitation applications at 7 T. This compact RF power amplifier can be potentially used for on‐coil amplification in multichannel RF array system. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 191–200, 2015     

Image‐based estimation method for field inhomogeneity in brain echo‐planar images with geometric distortion using k‐space textures

Echo‐planar imaging (EPI) can suffer from geometrical distortion due to magnetic field inhomogeneity. To correct the geometric distortions in EPI, a magnetic field map is used. Our purpose was to develop a novel image‐based method for estimating the field inhomogeneity map from the distorted EPI image and T1‐weighted image of the brain using k‐space textures. Based on magnetic resonance imaging physics, our method synthesizes the distorted image to match the measured EPI image through the generating process of EPI image by updating the estimated field inhomogeneity map. The estimation process was performed to minimize the cost function, which was defined by the synthesized EPI image and the measured EPI image with geometric distortion, using an iterative conjugate gradient algorithm. The proposed method was applied to simulation and human data. To evaluate the performance of the proposed method quantitatively, we used the normalized root mean square error (NRMSE) between the ground truth and the results estimated by our proposed method. In simulation data, the values of the NRMSE between the ground truth and the estimated field inhomogeneity map were <0.08. In both simulation and human data, the estimated EPI images were very similar to input EPI images, and the NRMSE values between them were <0.09. The results of the simulated and human data demonstrated that our method produced a reasonable estimation of the field inhomogeneity map. The estimated map could be used for distortion correction in EPI images. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 142–152, 2015     

Universal radio‐frequency receive‐coil connector: Concept and validation at 3 T

In most clinical magnetic resonance imaging systems, only commercial receive coils with the appropriate connector and encoding can be plugged in. When willing to use a dedicated receive coil for a specific study which cannot be achieved with commercial coils, the researcher faces the connecting issue related to the specificity of the proprietary connector. In this work, a universal device is proposed which allows for the connection of any single channel dedicated coil on any magnetic resonance (MR) system, as long as it is provided with at least one commercial receive coil. Technical feasibility of the universal connecting device was demonstrated on a 3 T MR clinical imager. The device included an independent active decoupling circuit while signal transmission to the data cabinet was achieved by electromagnetic coupling with a commercial receive coil plugged to the MR device. The universal connecting device was notably characterized in terms of signal‐to‐noise ratio (SNR) and compared to the standard connection. Image SNR was comparable using both means of connection. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 125–133, 2015     

Maximally spaced projection sequencing in electron paramagnetic resonance imaging

Electron paramagnetic resonance imaging (EPRI) provides 3D images of absolute oxygen concentration (pO₂) in vivo with excellent spatial and pO₂ resolution. When investigating such physiologic parameters in living animals, the situation is inherently dynamic. Improvements in temporal resolution and experimental versatility are necessary to properly study such a system. Uniformly distributed projections result in efficient use of data for image reconstruction. This has dictated current methods such as equal‐solid‐angle (ESA) spacing of projections. However, acquisition sequencing must still be optimized to achieve uniformity throughout imaging. An object‐independent method for uniform acquisition of projections, using the ESA uniform distribution for the final set of projections, is presented. Each successive projection maximizes the distance in the gradient space between itself and prior projections. This maximally spaced projection sequencing (MSPS) method improves image quality for intermediate images reconstructed from incomplete projection sets, enabling useful real‐time reconstruction. This method also provides improved experimental versatility, reduced artifacts, and the ability to adjust temporal resolution post factum to best fit the data and its application. The MSPS method in EPRI provides the improvements necessary to more appropriately study a dynamic system. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 33–45, 2015     

Real‐time image reconstruction for pulse EPR oxygen imaging using a GPU and lookup table parameter fitting

The importance of tissue oxygenation has led to a great interest in methods for imaging pO₂ in vivo. Electron paramagnetic resonance imaging (EPRI) provides noninvasive, near absolute 1 mm‐resolved 3D images of pO₂ in the tissues and tumors of living animals. Current EPRI image reconstruction methods tend to be time consuming and preclude real‐time visualization of information. Methods are presented to significantly accelerate the reconstruction process in order to enable real‐time reconstruction of EPRI pO₂ images. These methods are image reconstruction using graphics processing unit (GPU)‐based 3D filtered back‐projection and lookup table parameter fitting. The combination of these methods leads to acceleration factors of over 650 compared to current methods and allows for real‐time reconstruction of EPRI images of pO₂ in vivo. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 46–57, 2015     

A fast and simple method for calibrating the flip angle in hyperpolarized 13C MRS experiments

Hyperpolarized ¹³C Magnetic resonance represents a promising modality for in vivo studies of intermediary metabolism of bio‐molecules and new biomarkers. Although it represents a powerful tool for metabolites spatial localization and for the assessment of their kinetics in vivo, a number of technological problems still limits this technology and needs innovative solutions. In particular, the optimization of the signal‐to‐noise ratio during the acquisitions requires the use of pulse sequences with accurate flip angle calibration, which is performed by adjusting the transmit power in the prescan step. This is even more critical in the case of hyperpolarized studies, because the fast decay of the hyperpolarized signal requires precise determination of the flip angle for the acquisition. This work describes a fast and efficient procedure for transmit power calibration of magnetic resonance acquisitions employing selective pulses, starting from the calibration of acquisitions performed with non‐selective (hard) pulses. The proposed procedure employs a simple theoretical analysis of radiofrequency pulses by assuming a linear response and can be performed directly during in vivo studies. Experimental MR data validate the theoretical calculation by providing good agreement. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 78–84, 2015     

A more accurate tuning‐matching technique for NMR probes using wobulation and variable phase shifter

For optimizing sensitivity and RF excitation, high resolution NMR probes have to be tuned at the Larmor frequency and matched to 50 Ω‐transmission lines to which they are connected. For achieving this setting, one usually resorts to the wobulation function which consists in sweeping the frequency of the power source while monitoring and minimizing the reflected power taken from a directional coupler through adjusting the tuning and matching capacitors. However, the presence of a transmission‐reception switch between the directional coupler and the probe leads to the appearance of extra loss and reactance, and affects the determined values of the previous mentioned capacitors. Even if this situation is usually satisfactory, the true tuning‐matching condition is required for optimal NMR sensitivity and for reliable spectra analyses when nonlinear effects are present (radiation damping, spin‐noise experiments, etc.). To circumvent the need of network analyzer directly attached to the probe for finding this optimal tuning‐matching condition, in the current work, we report a procedure which combines wobulation and choice of a transmission‐line phase shift obtained by a variable phase shifter inserted between the probe and the switch. The technique was successfully validated with help of a network analyzer. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 59–68, 2015     

Circuit level simulation of MRI receive chain using excitation derived from images

We introduce a novel component‐level simulation method with which to characterize the performance of the electronic interfaces and circuitry of a magnetic resonance imaging (MRI) receiver. The Matlab‐based toolbox first reconstructs idealized coil free induction decay signals from real MR images, then uses these signals to drive a transistor‐level and extracted‐layout simulation of the desired receiver architecture. The simulated receiver's outputs are read back into the toolbox, which then reconstructs the MR images. By comparing the reconstructed images with the original ones, important design characteristics, such as the circuit's noise figure, linearity, phase noise, and inter‐channel coupling, can be investigated in terms of image quality. To validate the method, a new MRI receiver architecture is designed and simulated. The architecture is a 16‐channel multiplexer, designed for a commercially available 0.35 μm CMOS technology, and employs both frequency‐division and time‐division multiplexing to form a combined output signal. The SNR degradation and the circuit's linearity determined from the MR images compare well with the SNR and linearity point predictions from conventional circuit simulations. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 102–113, 2015     

A novel acoustically quiet coil for neonatal MRI system

Magnetic resonance imaging (MRI) acoustic exposure has the potential to elicit physiological distress and impact development in preterm and term infants. To mitigate this risk, a novel acoustically quiet coil was developed to reduce the sound pressure level experienced by neonates during MR procedures. The new coil has a conventional high‐pass birdcage radio frequency design, but is built on a framework of sound abating material. We evaluated the acoustic and MR imaging performance of the quiet coil and a conventional body coil on two small footprint neonatal intensive care unit MRI systems. Sound pressure level and frequency response measurements were made for six standard clinical MR imaging protocols. The average sound pressure level, reported for all six imaging pulse sequences, was 82.2 dBA for the acoustically quiet coil, and 91.1 dBA for the conventional body coil. The sound pressure level values measured for the acoustically quiet coil were consistently lower, 9 dBA (range 6–10 dBA) quieter on average. The acoustic frequency response of the two coils showed a similar harmonic profile for all imaging sequences. However, the amplitude was lower for the quiet coil, by as much as 20 dBA. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 107–114, 2015     

Theoretical analysis of magnetic wall decoupling method for radiative antenna arrays in ultrahigh magnetic field MRI

Radiative antenna techniques, e.g., dipole and monopole, have been proposed for radiofrequency (RF) coil array designs in ultrahigh field MRI to obtain stronger B₁ field and higher signal‐to‐noise ratio (SNR) gain in the areas deep inside human head or body. It is known that element decoupling performance is crucial to SNR and parallel imaging ability of array coil and has been a challenging issue in radiative antenna array designs for MR imaging. Magnetic wall or induced current elimination (ICE) technique has proven to be a simple and effective way of achieving sufficient decoupling for radiative array coils experimentally. In this study, this decoupling technique for radiative coil array was analyzed theoretically and verified by a simulation study. The decoupling conditions were derived and obtained from the theory. By applying the predicated decoupling conditions, the isolation of two radiative elements could be improved from about ? 8 dB to better than ? 35 dB. The decoupling performance has also been validated by current distribution along the radiative elements and magnetic field profiles in a water phantom. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 183–190, 2015     

Approaching ultimate intrinsic SNR in a uniform spherical sample with finite arrays of loop coils

We investigated to what degree and at what rate the ultimate intrinsic (UI) signal‐to‐noise ratio (SNR) may be approached using finite radiofrequency detector arrays. We used full‐wave electromagnetic field simulations based on dyadic Green's functions to compare the SNR of arrays of loops surrounding a uniform sphere with the ultimate intrinsic SNR (UISNR), for increasing numbers of elements over a range of magnetic field strengths, voxel positions, sphere sizes, and acceleration factors. We evaluated the effect of coil conductor losses and the performance of a variety of distinct geometrical arrangements such as “helmet” and “open‐pole” configurations in multiple imaging planes. Our results indicate that UISNR at the center is rapidly approached with encircling arrays and performance is substantially lower near the surface, where a quadrature detection configuration tailored to voxel position is optimal. Coil noise is negligible at high field, where sample noise dominates. Central SNR for practical array configurations such as the helmet is similar to that of close‐packed arrangements. The observed trends can provide physical insights to improve coil design. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 53–65, 2015     

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     

Transmit‐only/receive‐only radiofrequency coil configuration for hyperpolarized 129Xe MRI of rat lungs

We report a novel radiofrequency (RF) transmit‐only/receive‐only (TO/RO) coil configuration providing excellent transmit B₁₊ field uniformity as well as high sensitivity for hyperpolarized ¹²⁹Xe MR lung imaging of rats at 3T (35.34 MHz). The TO/RO coil configuration consisted of two separate components: (i) a high‐pass birdcage transmit coil which produces a homogeneous B₁₊ magnetic field, (ii) a saddle‐shaped single‐turn receive‐only surface coil that couples closely to the rat lung. On transmit, the receive‐only coil is decoupled from the transmit coil using a detuning circuit. On receive, the bird‐cage coil is deactivated through the use of PIN diodes. The sensitivity and uniformity of the saddle‐shaped receive coil were optimized solving the Biot‐Savart equation using 3D finite element modeling. The electrical performance of the new TO/RO configuration in transmit/receive (T/R) mode was compared with a commercial T/R birdcage coil of similar diameter, which was considering to be the gold standard for conventional T/R mode imaging. Experimental results in phantoms confirm that our novel TO/RO coil configuration provides a factor of three increase in SNR without compromising B₁ transmit uniformity compared with the commercial T/R birdcage coil configuration. The novel TO/RO coil was successfully tested for in vivo rat lung imaging. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 115–124, 2015     

在宏观静止中感受微观运动:磁共振扩散加权成像临床应用的若干认识

DWI利用磁共振特殊序列突出了扩散引起的散相位作用,在宏观成像中反映活体组织中水分子的微观扩散运动,有必要将病变的扩散特征作为重要的影像学特征来加以认识.采用ASSET-EPI技术使DWI的空间分辨率和图像质量得到提高;通过增加信号平均次数,可基本消除EPI伪影及ASSET相关伪影;应用分次屏气采集的方法可获得较为满意的腹部DWI图像.高b值(800～1000 s/mm2)的DWI已经在乳腺、胃、肝脏、直肠、子宫、前列腺等领域的临床应用获得成功,显示出良好的应用前景.临床研究结果表明,DWI对可在一定程度上显示未出现明显形态学改变的病变区域;表观弥散系数(ADC)值作为量化指标可为肉眼不易区分的病变提准确判断;以病变区的相对值作为定量指标,在反映病变组织特征方面具有一定的临床应用价值;DWI联合其他MRI技术,有助于客观评价DWI的临床价值,并可在更深层次上拓展对疾病的认识.     

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     

A novel switched frequency 3He/1H high‐pass birdcage coil for imaging at 1.5 tesla

The ability to produce hyperpolarized noble gases ³He and ¹²⁹Xe has opened up exciting possibilities for pulmonary magnetic resonance imaging (MRI). We have recently built a hyperpolarizer with the goal of using hyperpolarized ³He gas for MRI in neonatal lungs in a dedicated small foot‐print 1.5 T MR scanner developed at our institution and sited in our Neonatal Intensive Care Unit. Although hyperpolarized gas imaging can provide unique insights into lung ventilation, acinar microstructure, and gas‐exchange dynamics, there is an undiminished need for ¹H MRI of the lung to provide anatomic references, B₁ and B₀ maps, and ¹H images of lung parenchyma. To address this need, we designed, built and tested a novel radiofrequency body coil that provides a high‐pass birdcage coil that can be used for both ³He and ¹H frequencies (48.65 and 63.86 MHz, respectively, at 1.5 T). To switch between frequencies, the birdcage coil has a large mechanical actuator that simultaneously changes the capacitance between every rung of the birdcage. Advantages of this coil design include: 1) quadrature excitation and reception at the ³He and ¹H frequencies, 2) identical B₁ field maps for ³He and ¹H imaging, 3) excellent signal‐to‐noise ratio and B₁ homogeneity at both frequencies, and 4) rapid (10–20 s) switching times between ³He and ¹H operation. This report provides details of the coil's design and fabrication. Images of hyperpolarized ³He and ¹H in phantoms and ex vivo rabbit lungs demonstrate the image quality obtained with the coil. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 174–182, 2015     

An RF breast coil for 0.2T MRI

Breast cancer is the most frequently diagnosed cancer in women. High field studies have shown the diagnostic value of breast MRI, but the examination costs greatly exceed those of competing conventional mammography. Low field MRI offers typical MRI contrast at substantially lower cost, but has suffered from lower spatial resolution. Specificity of breast MRI can potentially be increased by acquiring MR imaging with higher spatial or temporal resolution, but the signal‐to‐noise ratio (SNR) achievable in a given imaging time becomes limiting. SNR for the particular pulse sequence and magnet field strength is strongly influenced by the characteristics of the radio‐frequency coil. An optimal breast coil should yield excellent SNR but also generate a homogeneous B₁ field, while allowing imaging of the both breasts simultaneously and maintaining patient comfort. RF receiver coil design is a key determinant of image quality, thus to address this we have designed and constructed a low field breast imaging coil. The coil was tested with a 4‐post 0.2T MRI providing high quality breast images. Designed and constructed saddle rf coil allows to obtain good quality image of the breast using low 0.2 T MRI system within 2 minutes. The coil provides patient comfort as breast compression is not required and minimizes artefacts caused by respiration or motion. A high contrast, low‐cost and pain‐free breast examination using optimized low field MRI system has the potential to serve a large patient population for whom current technologies have deficiencies. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 3–7, 2016     

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     

MR弥散加权成像对颅脑非缺血性病变的诊断价值

目的探讨磁共振弥散加权成像(DWI)在颅脑非缺血性病变中的诊断价值。方法收集手术病例/临床证实的颅脑非缺血性病变42例,包括颅脑肿瘤25例、感染性病变8例、血肿5例、多发性硬化4例,均作了常规MRIT2WI、T1WI、增强扫描及DWI,分析其DWI信号表现。结果25例颅脑肿瘤、8例感染性病变、5例血肿、4例多发性硬化病例,病变在DWI上呈高或稍高信号。结论结合常规MRIT2WI、T1WI及增强扫描图像,DWI在颅脑非缺性病变的定性诊断中具有重要价值。