Improved homogeneity of the transmit field by simultaneous transmission with phased array and volume coil

Purpose:

To improve the homogeneity of transmit volume coils at high magnetic fields (≥4 T). Due to radiofrequency (RF) field/tissue interactions at high fields, 4 T to 8 T, the transmit profile from head‐sized volume coils shows a distinctive pattern with relatively strong RF magnetic field B₁ in the center of the brain.

Materials and Methods:

In contrast to conventional volume coils at high field strengths, surface coil phased arrays can provide increased RF field strength peripherally. In theory, simultaneous transmission from these two devices could produce a more homogeneous transmission field. To minimize interactions between the phased array and the volume coil, counter rotating current (CRC) surface coils consisting of two parallel rings carrying opposite currents were used for the phased array.

Results:

Numerical simulations and experimental data demonstrate that substantial improvements in transmit field homogeneity can be obtained.

Conclusion:

We have demonstrated the feasibility of using simultaneous transmission with human head‐sized volume coils and CRC phased arrays to improve homogeneity of the transmit RF B₁ field for high‐field MRI systems. J. Magn. Reson. Imaging 2010;32:476–481. © 2010 Wiley‐Liss, Inc.     

Eigenmode analysis of transmit coil array for tailored B1 mapping

In vivo radiofrequency (RF) field B₁ mapping represents an essential prerequisite for parallel transmit applications. However, the large dynamic range of the transmit fields of the individual coil elements challenges the accuracy of MR‐based B₁ mapping techniques. In the present work, the B₁ mapping error and its impact on the RF performance are studied based on a coil eigenmode analysis. Furthermore, the linear properties of the transmit chain are exploited to virtually adjust the weighting of the different coil eigenmodes in the B₁ mapping procedure, resulting in considerably reduced mapping errors. In addition, the weighting of the eigenmodes is tailored to potential target applications, e.g., specific absorption rate (SAR) reduced RF shimming or multidimensional RF pulses, resulting in improved RF performance. The basic theoretic principles of the concept are elaborated and validated by corresponding simulations. Furthermore, results on B₁ mapping and RF shimming experiments, performed on phantoms and in vivo using a 3‐T scanner equipped with an eight‐channel transmit/receive body coil, are presented to prove the feasibility of the approach. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

MRI using radiofrequency magnetic field phase gradients

Conventionally, MR images are formed by applying gradients to the main static magnetic field (B₀). However, the B₀ gradient equipment is expensive, power‐hungry, complex, and noisy and can induce eddy currents in nearby conducting structures, including the patient. Here, we describe a new silent, B₀ gradient‐free MRI principle, Transmit Array Spatial Encoding (TRASE), based on phase gradients of the radio‐frequency (RF) field. RF phase gradients offer a new method of k‐space traversal. Echo trains using at least two different RF phase gradients allow spin phase to accumulate, causing k‐space traversal. Two such RF fields provide one‐dimensional imaging and three are sufficient for two‐dimensional imaging. Since TRASE is a k‐space method, analogues of many conventional pulse sequences are possible. Experimental results demonstrate one‐dimensional and two‐dimensional RF MRI and slice selection using a single‐channel, transmit/receive, 0.2 T, permanent magnet, human MR system. The experimentally demonstrated spatial resolution is much higher than that provided by RF receive coil array sensitivity encoding alone but lower than generally achievable with B₀ gradients. Potential applications are those in which one or more of the features of simplified equipment, lower costs, silent MRI, or the different physics of the image formation process are particularly advantageous. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Transmit B1‐field correction at 7T using actively tuned coupled inner elements

When volume coils are used for ¹H imaging of the human head at 7T, wavelength effects in tissue cause a variation in intensity, that is typically brighter at the center of the head and darker in the periphery. Much of this image nonuniformity can be attributed to variation in the effective transmit B₁ field, which falls by ～ 50% to the left and right of center at mid‐elevation in the brain. Because most of this B₁ loss occurs in the periphery of the brain, we have explored use of actively controlled, off‐resonant loop elements to locally enhance the transmit B₁ field in these regions. When tuned to frequencies above the NMR frequency, these elements provide strong local enhancement of the B₁ field of the transmit coil. Because they are tuned off‐resonance, some volume coil detuning results, but resistive loading of the coil mode remains dominated by the sample. By digitally controlling their frequency offsets, the field enhancement of each inner element can be placed under active control. Using an array of eight digitally controlled elements placed around a custom‐built head phantom, we demonstrate the feasibility of improving the B₁ homogeneity of a transmit/receive volume coil without the need for multiple radio frequency transmit channels. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

A flexible 32‐channel receive array combined with a homogeneous transmit coil for human lung imaging with hyperpolarized 3He at 1.5 T

Parallel imaging presents a promising approach for MRI of hyperpolarized nuclei, as the penalty in signal‐to‐noise ratio typically encountered with ¹H MRI due to a reduction in acquisition time can be offset by an increase in flip angle. The signal‐to‐noise ratio of hyperpolarized MRI generally exhibits a strong dependence on flip angle, which makes a homogeneous B₁⁺ transmit field desirable. This paper presents a flexible 32‐channel receive array for ³He human lung imaging at 1.5T designed for insertion into an asymmetric birdcage transmit coil. While the 32‐channel array allows parallel imaging at high acceleration factors, the birdcage transmit coil provides a homogeneous B₁⁺ field. Decoupling between array elements is achieved by using a concentric shielding approach together with preamplifier decoupling. Coupling between transmit coil and array elements is low by virtue of a low geometric coupling coefficient, which is reduced further by the concentric shields in the array. The combination of the 32‐channel array and birdcage transmit coil provides ³He ventilation images of excellent quality with similar signal‐to‐noise ratio at acceleration factors R = 2 and R = 4, while maintaining a homogeneous B₁⁺. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

High‐resolution MRI of the carotid arteries using a leaky waveguide transmitter and a high‐density receive array at 7 T

A setup for 7T MRI of the carotid arteries in the neck was designed and constructed. Separate dedicated arrays were used for transmit and receive. For the transmit array, single‐side adapted dipole antennas were mounted on a dielectric pillow, which was shown to serve as a leaky waveguide, efficiently distributing B₁ into the neck. Risk assessment was performed by simulations. Phantom measurements were performed to establish optimal positions of the antennas on the pillow. Using two antennas, a dual transmit setup was created. In vivo B₁⁺ maps with different shim configurations were acquired to assess transmit performance. This effective transmit array was used in combination with a dedicated 30 channel small element receive coil. High‐resolution in vivo turbo spin echo images were acquired to demonstrate the excellent performance of the setup. Magn Reson Med 69:1186–1193, 2013. © 2012 Wiley Periodicals, Inc.     

Complex B1 mapping and electrical properties imaging of the human brain using a 16‐channel transceiver coil at 7T

The electric properties of biological tissue provide important diagnostic information within radio and microwave frequencies, and also play an important role in specific absorption rate calculation which is a major safety concern at ultrahigh field. The recently proposed electrical properties tomography (EPT) technique aims to reconstruct electric properties in biological tissues based on B₁ measurement. However, for individual coil element in multichannel transceiver coil which is increasingly utilized at ultrahigh field, current B₁‐mapping techniques could not provide adequate information (magnitude and absolute phase) of complex transmit and receive B₁ which are essential for electrical properties tomography, electric field, and quantitative specific absorption rate assessment. In this study, using a 16‐channel transceiver coil at 7T, based on hybrid B₁‐mapping techniques within the human brain, a complex B₁‐mapping method has been developed, and in vivo electric properties imaging of the human brain has been demonstrated by applying a logarithm‐based inverse algorithm. Computer simulation studies as well as phantom and human experiments have been conducted at 7T. The average bias and standard deviation for reconstructed conductivity in vivo were 28% and 67%, and 10% and 43% for relative permittivity, respectively. The present results suggest the feasibility and reliability of proposed complex B₁‐mapping technique and electric properties reconstruction method. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

RF Surface Receive Array Coils: The Art of an LC Circuit

The radiofrequency (RF) receive array coil is a complicated device with many inductors and capacitors and serves as one of the most critical magnetic resonance imaging (MRI) electronic devices. It directly determines the achievable level of signal‐to‐noise ratio (SNR). Simply put, however, the RF coil is nothing but an LC circuit. The receive array coil was first proposed more than 20 years ago, evolving from a simple arrangement with a few electronic channels to a complicated system of 128 channels, enabling highly sophisticated parallel imaging, at different field strengths. This article summarizes the basic concepts pertaining to RF receive coil arrays and their associated SNR and reviews the theories behind the major components of such arrays. This includes discussions of the intrinsic SNR of a receive coil, the matching circuits, low‐noise preamplifiers, coupling/decoupling amongst coils, the coupling between receive and transmit coils, decoupling via preamplifiers, and baluns. An 8‐channel receive array coil on a cylindrical former serves as a useful example for demonstrating various points in the review. J. Magn. Reson. Imaging 2013;38:12–25. © 2013 Wiley Periodicals, Inc.     

Design and evaluation of an RF front‐end for 9.4 T human MRI

At the field strength of 9.4 T, the highest field currently available for human MRI, the wavelength of the MR signals is significantly shorter than the size of the examined structures. Even more than at 7 T, constructive and destructive interferences cause strong inhomogeneities of the B₁ field produced by a volume coil, causing shading over large parts of the image. Specialized radio frequency hardware and B₁ management methods are required to obtain high‐quality images that take full advantage of the high field strength. Here, the design and characteristics of a radio frequency front‐end especially developed for proton imaging at 9.4 T are presented. In addition to a 16‐channel transceiver array coil, capable of volume transmit mode and independent signal reception, it consists of custom built low noise preamplifiers and TR switches. Destructive interference patterns were eliminated, in virtually the entire brain, using a simple in situ radio frequency phase shimming technique. After mapping the B profile of each transmit channel, a numerical algorithm was used to calculate the appropriate transmit phase offsets needed to obtain a homogeneous excitation field over a user defined region. Between two and three phase settings are necessary to obtain homogeneous images over the entire brain. Magn Reson Med, 2011. &© 2011 Wiley‐Liss, Inc.     

RF excitation using time interleaved acquisition of modes (TIAMO) to address B1 inhomogeneity in high‐field MRI

As the field strength and, therefore, the operational frequency in MRI is increased, the wavelength approaches the size of the human head/body, resulting in wave effects, which cause signal decreases and dropouts. Several multichannel approaches have been proposed to try to tackle these problems, including RF shimming, where each element in an array is driven by its own amplifier and modulated with a certain (constant) amplitude and phase relative to the other elements, and Transmit SENSE, where spatially tailored RF pulses are used. In this article, a relatively inexpensive and easy to use imaging scheme for 7 Tesla imaging is proposed to mitigate signal voids due to B ${}_{\text{1}}^{\text{+}}$ field inhomogeneity. Two time‐interleaved images are acquired using a different excitation mode for each. By forming virtual receive elements, both images are reconstructed together using GRAPPA to achieve a more homogeneous image, with only small SNR and SAR penalty in head and body imaging at 7 Tesla. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Performance of external and internal coil configurations for prostate investigations at 7 T

Three different coil configurations were evaluated through simulation and experimentally to determine safe operating limits and evaluate subject size‐dependent performance for prostate imaging at 7 T. The coils included a transceiver endorectal coil (trERC), a 16‐channel transceiver external surface array (trESA) and a trESA combined with a receive‐only ERC (trESA+roERC). Although the transmit B₁ (B) homogeneity was far superior for the trESA, the maximum achievable B is subject size dependent and limited by transmit chain losses and amplifier performance. For the trERC, limitations in transmit homogeneity greatly compromised image quality and limited coverage of the prostate. Despite these challenges, the high peak B close to the trERC and subject size‐independent performance provides potential advantages especially for spectroscopic localization where high‐bandwidth radiofrequency pulses are required. On the receive side, the combined trESA+roERC provided the highest signal‐to‐noise ratio and improved homogeneity over the trERC resulting in better visualization of the prostate and surrounding anatomy. In addition, the parallel imaging performance of the trESA+roERC holds strong promise for diffusion‐weighted imaging and dynamic contrast‐enhanced MRI. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Computational and experimental optimization of a double‐tuned 1H/31P four‐ring birdcage head coil for MRS at 3T

Purpose

To optimize the homogeneity and efficiency of the B₁ magnetic field of a four‐ring birdcage head coil that is double‐tuned at the Larmor frequencies of both ³¹P and ¹H and optimized to acquire magnetic resonance spectroscopy (MRS) data at 3T for the study of infants.

Materials and Methods

We developed a finite difference time domain (FDTD) tool in‐house to iteratively compute and seek the range of geometric and electromagnetic parameters of a dual‐tuned, four‐ring birdcage coil that would produce the desired resonance patterns, optimize homogeneity of the B₁‐field, and maximize efficiency of the coil. To demonstrate the validity of our computational results, we constructed three RF coils: one dual‐tuned coil that was based on the calculated optimized parameters and two single‐tuned coils that had dimensions similar to those of the dual‐tuned coil, but tuned at the Larmor frequencies of both ³¹P and ¹H, respectively. We then tested and compared the performances of the dual‐tuned coil and single‐tuned coils at both of these frequencies.

Results

We found that a dual‐tuned, four‐ring birdcage coil with a diameter of 180 mm, an inner birdcage length of 100–300 mm, and an outer birdcage length of 25–100 mm produces the desired resonance patterns. For the use of this coil with human infants, optimization of the homogeneity of the B₁ field, combined with improved coil efficiency, yielded a dual‐tuned birdcage coil with diameter of 180mm, an inner birdcage length of 150 mm, an outer birdcage length of 25 mm, and corresponding inner and outer capacitances of 17.2 pF and 7.6 pF, respectively. The experimental results from a constructed coil having the sameparameters with the modeled coil agreed well with the computational results from the modeled coil. This optimized design overcame the deficiencies of existing dual‐tuned, four‐ring birdcage coils.

Conclusion

The homogeneity and efficiency of the B₁ field for ³¹P/¹H dual‐tuned, four‐ring birdcage coils can be optimized well using our FDTD tool, especially at high static magnetic fields (B₀). J. Magn. Reson. Imaging 2009;29:13–22. © 2008 Wiley‐Liss, Inc.     

Human brain imaging at 9.4 T using a tunable patch antenna for transmission

For human brain imaging at ultrahigh fields, the traveling wave concept can provide a more uniform B₁⁺ field over a larger field of view with improved patient comfort compared to conventional volume coils. It suffers, however, from limited transmit efficiency and receive sensitivity and is not readily applicable in systems where the radiofrequency shield is too narrow to allow for unattenuated wave propagation. Here, the near field of a capacitively adjustable patch antenna for excitation is combined with a receive‐only array at 9.4 T. The antenna is designed in compact size and placed in close proximity to the subject to improve the transmit efficiency in narrow bores. Experimental and numerical comparisons to conventional microstrip arrays reveal improved B₁⁺ homogeneity and longitudinal coverage, but at the cost of elevated local specific absorption rate. High‐resolution functional and anatomical images demonstrate the use of this setup for in vivo human brain imaging at 9.4 T. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

A geometrically adjustable 16-channel transmit/receive transmission line array for improved RF efficiency and parallel imaging performance at 7 Tesla.

A novel geometrically adjustable transceiver array system is presented. A key feature of the geometrically adjustable array was the introduction of decoupling capacitors that allow for automatic change in capacitance dependent on neighboring resonant element distance. The 16-element head array version of such an adjustable coil based on transmission line technology was compared to fixed geometry transmission line arrays (TLAs) of various sizes at 7T. The focus of this comparison was on parallel imaging performance, RF transmit efficiency, and signal-to-noise ratio (SNR). Significant gains in parallel imaging performance and SNR were observed for the new coil and attributed to its adjustability and to the design of the individual elements with a three-sided ground plane.     

A high‐throughput eight‐channel probe head for murine MRI at 9.4 T

Murine MRI studies are conducted on dedicated MR systems, typically equipped with ultra‐high‐field magnets (≥4.7 T; bore size: ～12–25 cm), using a single transmit‐receive coil (volume or surface coil in linear or quadrature mode) or a transmit‐receive coil combination. Here, we report on the design and characterization of an eight‐channel volume receive‐coil array for murine MRI at 400 MHz. The array was combined with a volume‐transmit coil and integrated into one probe head. Therefore, the animal handling is fully decoupled from the radiofrequency setup. Furthermore, fixed tune and match of the coils and a reduced number of connectors minimized the setup time. Optimized preamplifier design was essential for minimizing the noise coupling between the elements. A comprehensive characterization of transmit volume resonator and receive coil array is provided. The performance of the coil array is compared to a quadrature‐driven birdcage coil with identical sensitive volume. It is shown that the miniature size of the elements resulted in coil noise domination and therefore reduced signal‐to‐noise‐ratio performance in the center compared to the quadrature birdcage. However, it allowed for 3‐fold accelerated imaging of mice in vivo, reducing scan time requirements and thus increasing the number of mice that can be scanned per unit of time. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Design of a nested eight‐channel sodium and four‐channel proton coil for 7T knee imaging

The critical design aim for a sodium/proton coil is to maximize sodium sensitivity and transmit field homogeneity while simultaneously providing adequate proton sensitivity and homogeneity. While most dual‐frequency coils use lossy high‐impedance trap circuits or PIN diodes to allow dual‐resonance, we explored a nested‐coil design for sodium/proton knee imaging at 7 T. A stand‐alone eight‐channel sodium receive array was implemented without standard dual‐resonance circuitry to provide improved sodium signal‐to‐noise ratio. A detunable sodium birdcage was added for homogeneous sodium excitation and a four‐channel proton transmit‐receive array was added to provide anatomical reference imaging and B₀ shimming capabilities. Both additional modules were implemented with minimal disturbance to the eight‐channel sodium array by managing their respective resonances and geometrical arrangement. In vivo sodium signal‐to‐noise ratio was 1.2–1.7 times greater in the developed eight‐channel array than in a mononuclear sodium birdcage coil, whereas the developed four‐channel proton array provided signal‐to‐noise ratio similar to that of a commercial mononuclear proton birdcage coil. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Reduction of implant RF heating through modification of transmit coil electric field

In this work, we demonstrate the possibility to modify the electric‐field distribution of a radio frequency (RF) coil to generate electric field‐free zones in the body without significantly altering the transmit sensitivity. Because implant heating is directly related to the electric‐field distribution, implant‐friendly RF transmit coils can be obtained by this approach. We propose a linear birdcage transmit coil with a zero electric‐field plane as an example of such implant‐friendly coils. When the zero electric‐field plane coincides with the implant position, implant heating is reduced, as we demonstrated by the phantom experiments. By feeding RF pulses with identical phases and shapes but different amplitudes to the two orthogonal ports of the coil, the position of the zero electric‐field plane can also be adjusted. Although implant heating is reduced with this method, a linear birdcage coil results in a whole‐volume average specific absorption rate that is twice that of a quadrature birdcage coil. To solve this issue, we propose alternative methods to design implant‐friendly RF coils with optimized electromagnetic fields and reduced whole‐volume average specific absorption rate. With these methods, the transmit field was modified to reduce RF heating of implants and obtain uniform transmit sensitivity . Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Transmit/receive radiofrequency coil with individually shielded elements

A novel method for decoupling coil elements of transmit/receive (transceive) arrays is reported. Each element of a coil array is shielded both concentrically and radially to reduce the magnetic flux linkage between neighboring coils; this substantially reduces the mutual inductance between coil elements and allows them to behave independently. A six‐channel transceive coil was developed using this decoupling scheme and compared with two conventional decoupling schemes: the partial overlapping of adjacent elements and capacitive decoupling. The radiofrequency coils were designed to image the human head and were tested on a 7‐T Varian scanner. The decoupling, transmit uniformity, transmit efficiency, signal‐to‐noise ratio, and geometry factors were compared between coils. The individually shielded coil achieved higher minimum isolation between elements (2.7–4.0 dB) and lower geometry factors (2–14%) than the overlapped and capacitively decoupled coils, while showing a reduction in transmit efficiency (2.8–5.9 dB) and signal‐to‐noise ratio (up to 34%). No difference was found in the power absorbed by the sample during a 90° radiofrequency pulse. The inset distance of coil elements within their shields was then reduced, resulting in significant improvement of the transmit efficiency (1.3 dB) and signal‐to‐noise ratio (28%). The greatest asset of this decoupling method lies in its versatility: transceive coils can be created with elements of arbitrary shape, size, location, and resonant frequency to produce three‐dimensional conformal arrays. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Quantitative assessment of the effects of high‐permittivity pads in 7 Tesla MRI of the brain

The use of high‐permittivity materials has been shown to be an effective method for increasing transmit and receive sensitivity in areas of low‐signal intensity in the brain at high field. Results in this article show that the use of these materials does not increase the intercoil coupling for a phased array receive coil, does not have any detrimental effects on the B₀ homogeneity within the brain, and does not affect the specific absorption rate distribution within the head. Areas of the brain close to the pads exhibit significant increases (>100%) in transmit field efficiency, but areas further away show a less pronounced (～10%) decrease due to the homogenization of the transmit field and the loss introduced by the dielectric pads. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Flexible transceiver array for ultrahigh field human MR imaging

A flexible transceiver array, capable of multiple‐purpose imaging applications in vivo at ultrahigh magnetic fields was designed, implemented and tested on a 7 T MR scanner. By alternately placing coil elements with primary and secondary harmonics, improved decoupling among coil elements was accomplished without requiring decoupling circuitry between resonant elements, which is commonly required in high‐frequency transceiver arrays to achieve sufficient element‐isolation during radiofrequency excitation. This flexible array design is capable of maintaining the required decoupling among resonant elements in different array size and geometry and is scalable in coil size and number of resonant elements (i.e., number of channels), yielding improved filling factors for various body parts with different geometry and size. To investigate design feasibility, flexibility, and array performance, a multichannel, 16‐element transceiver array was designed and constructed, and in vivo images of the human head, knee, and hand were acquired using a whole‐body 7 T MR system. Seven Tesla parallel imaging with generalized autocalibrating partially parallel acquisitions (GRAPPA) performed using this flexible transceiver array was also presented. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.