Proposal for a permanent magnet system with a constant gradient mechanically adjustable in direction and strength

A design for a permanent magnet system is proposed that generates spatially homogeneous, constant magnetic field gradients, thus creating conditions suitable for MRI without gradient coils and amplifiers. This is achieved by superimposing a weak Halbach quadrupole on a strong Halbach dipole. Rotation of either the quadrupole or the entire magnet assembly can be used to generate two‐dimensional images via filtered backprojection. Additionally, the mutual rotation of two quadrupoles can be used to scale the resulting gradient. If both gradients have identical strength the gradient can even be made to vanish. The concept is demonstrated by analytical considerations and FEM simulations. However, a demonstration on a working prototype is still pending. © 2016 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 41–48, 2016     

Estimation of occupational exposure to static magnetic fields due to usual movements in magnetic resonance units

The purpose of this work is to estimate the induced electric field E due to movements of magnetic resonance (MR) workers in static magnetic field. Different operator walking speeds and paths are taken into account, based on real‐world scenarios for a 3T MR scanner. The spectral components of magnetic field gradient are estimated to verify the compliance with the specific International Commission on Non‐Ionizing Radiation Protection safety guidelines for MR workers. The induced electric fields on the chosen paths are estimated using a previously described software tool based on an analytical model. Finally, a real situation is considered in which an operator performs the patient preparation for a brain MR examination. From the spectral components of the motion‐induced magnetic field gradient, it is possible to see that the highest values are concentrated on frequencies below 1 Hz. For the basic paths considered, the worst‐case scenario is a walking perpendicular to the magnet cylinder along the x‐axis. For the example of a real path, the critical zone where the worker exposure is highest is highlighted. The results of this study could be used for training MR workers to follow correct behavior in the scanner room to avoid high‐exposure scenarios. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 75–81, 2015     

Performance of a novel NMR apparatus with a solenoidal tape‐shaped antenna and a split‐type superconducting magnet

Since 2003, the authors have been developing a new configuration NMR that consists of a solenoid‐type antenna and a split‐type superconducting magnet to improve the signal‐to‐noise ratio (SNR). The SNR (standard 0.1% ethylbenzene) of the system reached 9,850 in 2009. Refinement of the radiofrequency components, which include an antenna coil, a low‐temperature preamplifier, and a signal switch, led to a reduction in the system noise. In this study, the line shape of the spectrum was improved by reducing the residual magnetization of the antenna coil using a low‐magnetic sheet laminated with a tungsten sheet and a copper sheet. The measured SNR showed a good agreement with the predicted value, and the result shows the validity of this approach to improve the SNR based on the theoretical prediction. In this article, the outline and the performance of the NMR system are reported. © 2013 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 43B: 79‐89, 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     

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     

Acoustic analysis for a split MRI system using FE method

In some MRI‐LINAC (Magnetic Resonance Imaging and Linear Accelerator) hybrid systems, the MRI scanner is split into two parts to form a central gap for the accommodation of the patient or a LINAC. Little is known about the acoustic characteristics of the split gradient coil structure needed for this system; however, it is believed to be very different from its typical configurations. It is important to develop dedicated numerical methods for the characterization of the unique acoustic properties, to provide engineering solutions for the noise attenuation for such a new system. In this article, we modeled the acoustic fields of a split MRI system and traditional gradient structures using the finite element method. The models were validated against acoustic experimental results obtained from a conventional MRI scanner. The acoustic field distribution analysis showed that the average sound pressure levels in the central gap were lower than those in the cylindrical tunnels of the split MRI system at most frequencies. This was also true when both the x coils or z coils were energized independently. Thus, if the patient bed is placed perpendicular to the axis of the main magnet of the split MRI system, the patient will be subjected to relatively lower acoustic intensities compared with that if the patient bed is placed parallel to the axis of the main magnet. Further work is planned to reduce the sound level in the central gap where the patient bed may be placed for this split system. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 85–96, 2015     

Subthreshold CMOS transistors are largely immune to magnetic field effects when operated above 11 T

We report on measurements of n‐channel field‐effect transistor structures with multiple drain/source contacts, fabricated in a commercial complementary metal‐oxide‐semiconductor (CMOS) technology, at high magnetic field strengths. The focus lies on field dependent effects such as the Hall effect to better understand how these key electronic devices behave when their embedding circuits are immersed in strong magnetic fields. In addition, measurement results of relevant electrical parameters for circuit design, i.e., the saturation current of I_DS as well as the slope of I_DS and threshold voltage V_t will be shown. The results are in agreement with theory and it is possible to predict the Hall voltage as well as the channel resistance. This allows to simulate noise behavior of amplifiers using silicon MOSFETs in high magnetic fields. The measurements show that, electrically, CMOS transistors operated in subthreshold mode are largely immune to magnetic field effects when operated above 11 T. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 97–105, 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     

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     

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     

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     

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     

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     

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     

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     

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     

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     

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     

Quadrature birdcage coil with distributed capacitors for 7.0 T magnetic resonance data acquisition of small animals

High static magnetic field magnetic resonance imaging (MRI) is commonly used for preclinical studies in rodents. In this context, minimization of coil losses is mandatory to scan samples that are small compared to the radiofrequency wavelength in the medium. In this study we construct a radiofrequency (RF) birdcage probe with distributed capacitors, operating in quadrature, tailored for 7.0T ¹H MRI of small animals. The design eliminates the need for extra electrical components on the probe structure and affords a high SNR, a uniform field (homogeneity of 93% in the axial plain of the phantom) and a coil sensitivity of 9.8 . Feasibility experiments of mouse imaging are conducted and the competitive capability of a 7.0 T human system equipped with the proposed coil is demonstrated in both body and brain preclinical imaging. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 83–88, 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