Adam Maunder

Experimental verification of SNR and parallel imaging improvements using composite arrays.

Composite MRI arrays consist of triplets where two orthogonal upright loops are placed over the same imaging area as a standard surface coil. The optimal height of the upright coils is approximately half the width for the 7 cm coils used in this work. Resistive and magnetic coupling is shown to be negligible within each coil triplet. Experimental evaluation of imaging performance was carried out on a Philips 3 T Achieva scanner using an eight‐coil composite array consisting of three surface coils and five upright loops, as well as an array of eight surface coils for comparison. The composite array offers lower overall coupling than the traditional array. The sensitivities of upright coils are complementary to those of the surface coils and therefore provide SNR gains in regions where surface coil sensitivity is low, and additional spatial information for improved parallel imaging performance. Near the surface of the phantom the eight‐channel surface coil array provides higher overall SNR than the composite array, but this advantage disappears beyond a depth of approximately one coil diameter, where it is typically more challenging to improve SNR. Furthermore, parallel imaging performance is better with the composite array compared with the surface coil array, especially at high accelerations and in locations deep in the phantom. Composite arrays offer an attractive means of improving imaging performance and channel density without reducing the size, and therefore the loading regime, of surface coil elements. Additional advantages of composite arrays include minimal SNR loss using root‐sum‐of‐squares combination compared with optimal, and the ability to switch from high to low channel density by merely selecting only the surface elements, unlike surface coil arrays, which require additional hardware. Copyright

Calibrated Layer-Stripping Technique for Level and Permittivity Measurement With UWB Radar in Metallic Tanks

A method to measure liquid level and electrical properties based on ultra-wideband pulsed radar is developed in this paper. Current methods of material property measurement using free-space radar typically use computationally intensive frequency-domain analysis or finite time-domain methods. The method presented is modified from layer-stripping algorithms and includes several improvements over previous techniques, such as an antenna array that allows measurement in a metallic tank environment and a method of calibration that characterizes path-loss and near-field effects for accurate amplitude-distance prediction. The method presented here also estimates the material loss properties and uses accumulated power with noise compensation to predict reflected pulse power. The method extends the use of pulsed radar for liquid-level measurement in tanks to the evaluation of liquid permittivity and the estimation of liquid height in liquids consisting of multiple layers. The accuracy of the presented method is evaluated using a transmitting single antenna element, a four-element antenna array, and an eight-element antenna array for measurement in a metallic tank environment. Accuracy is improved with larger antenna arrays, but the calibration becomes more critical. The accuracy for varying layer heights and materials is investigated to demonstrate the method reliability.

Stray Capacitance Between Magnetic Resonance Imaging Coil Elements: Models and Application to Array Decoupling

The role of capacitive coupling between RF coil elements of MRI arrays is studied extensively in this paper and comprehensive models are obtained. At higher MRI frequencies, capacitive coupling greatly impacts coil coupling and therefore cannot be neglected. We compare lumped and distributed capacitance models and show that they correctly predict impedance variations due to stray and intentional mutual capacitance. The distributed model takes into account the dispersed nature of the mutual impedance, and it is more accurate than the lumped model. The value of stray capacitance obtained by fitting simulated results is shown to depend strongly on substrate and phantom permittivity, while being mostly invariant to changes in frequency. A novel result of this investigation is a method of completely eliminating mutual impedance between adjacent coils, including the mutual resistance. Until recently it was believed that only mutual reactance could be eliminated with lossless passive circuits. The SNR performance of two and four coil arrays with varying degrees of coupling is compared, showing that there is no SNR degradation when the mutual resistance is eliminated in addition to mutual reactance. This result has applications in arrays used in transmission, where mutual resistance leads to losses that degrade transmit efficiency.

The noise factor of receiver coil matching networks in MRI

In typical MRI applications the dominant noise sources in the received signal are the sample, the coil loop and the preamplifier. We hypothesize that in some cases (e.g. for very small receiver coils) the matching network noise has to be considered explicitly. Considering the difficulties of direct experimental determinations of the noise factor of matching networks with sufficient accuracy, it is helpful to estimate the noise factor by calculation. A useful formula of the coil matching network is obtained by separating commonly used coil matching network into different stages and calculating their noise factor analytically by a combination of the noise from these stages. A useful formula of the coil matching network is obtained. ADS simulations are performed to verify the theoretical predictions. Thereafter carefully-designed proof-of-concept phantom experiments are carried out to qualitatively confirm the predicted SNR behavior. The matching network noise behavior is further theoretically investigated for a variety of scenarios. It is found that in practice the coil matching network noise can be improved by adjusting the coil open port resonant frequency.

Method and system for wireless and single-conductor power and data transmission

This paper proposes and outlines a novel method for wirelessly transmitting power using a combination of inductive coupling and single conductor power transfer. The power is wirelessly transmitted from a source to a single conductor, using a double ended coil and a single ended coil respectively. From the conductor, the power is similarly transmitted to multiple receivers. The Qi standard is implemented, in order to guaranty the portability, reliability, and interoperability of the system.

Application of UWB Arrays for Material Identification of Multilayer Media in Metallic Tanks

In this paper, a compact eight-element antenna array is designed for ultra-wide band (UWB) pulsed radar in highly reflective metallic environments, such as storage and transport tankers. The impact of the sidewalls in metallic tanks is to increase noise and create spurious signals due to sidewall reflection that limits accuracy and causes liquid level and material property measurement errors. The designed Vivaldi antenna and array are characterized in terms of the transient energy patterns and the signal fidelity given in terms of the off-angle signal correlation. Comparison with horn antennas is made. Antenna transient signals are calculated from the frequency-domain data. The 10-dB transient energy pattern beamwidth in the E-plane is improved from 130° to 50° using the eight-element array versus single element. Also, the signal correlation and fidelity for off-broadside angles is greatly reduced. The reflection data from a multilayer of canola oil on metal-backed marble is processed with a calibrated layer-stripping technique for material properties. The transient radiation patterns of the array are compared to that of two horn antennas with similar dimensions. The noise/interference and measurement error is shown to be greatly reduced using an array compared to horn antennas in a realistic tank environment.

Parasitic capacitance in MRI coil arrays: Models and application to array decoupling

The role of capacitive coupling in MRI RF coils has not been studied extensively largely because of the lack of comprehensive models in the literature. At higher MRI frequencies capacitive coupling greatly impacts coil coupling and therefore cannot be neglected. We provide a model that takes into account the distributed nature of the mutual and series capacitance on coils. It correctly predicts impedance variation due to parasitic and intentional mutual capacitance. In addition, the fitted capacitance is shown to depend strongly on substrate and phantom permittivity, while being mostly invariant to changes in frequency. A method of completely eliminating mutual impedance between coils results from this investigation.

Tunable open ended planar spiral coil for wireless power transmission

A novel wireless power transmission (WPT) system with adaptive frequency tuning and a novel primary coil is presented. The primary transmitting coil is an open-ended single conductor spiral coil, where the traditional inductive resonator would include a secondary ground conductor. In addition, an Open-ended Spiral Coil (OSC) tuning technique has been applied to adjust the resonance frequency of the transmitting resonator to enhance the efficiency of the WPT system. The tuning method is found to improve the output DC voltage 60 percent after tuning the resonance frequency in nominal distance from 2cm up to 20cm with an excitation voltage of 20V at 13.54 MHz.

Correlation-Based UWB Radar for Thin Layer Resolution

Liquid level measurement and material identification with ultrawideband (UWB) free-space radar is considered in this letter. Traditional level measurement methods cannot distinguish between overlapping pulses, and their accuracy in identifying thin layers of material is limited by the width of the pulse used. A thin layer resolution algorithm is introduced to estimate the amplitudes of overlapping pulses, which in turn increases the accuracy in thin layer identification. The proposed thin layer resolution algorithm is shown to outperform spectral estimation methods such as multiple signal classification (MUSIC). A complete test setup was developed in laboratory, and the proposed signal processing method was tested on measurement data. The thin layer resolution method can recognize a 1-cm top layer of oil that corresponds to an overlapping of 66% between the first two reflected pulses.

Comparison of high-density composite and surface coil arrays for MRI of spherical imaging volumes

The array configuration that maximizes the SNR in magnetic resonance imaging depends on the number and arrangement of RF coil elements. A comparison of composite coils (a surface coil in combination with two upright coils) and surface coils in arrays with 18, 36, and 54 elements is presented at 128 MHz. Realistic multi-port noise modeling shows that the combined SNR can depend strongly on matching network and preamplifier noise. Arrays of composite coils are shown to provide higher SNR than surface coil arrays at depth in the phantom as the number of elements increases.