B interferometry for the calibration of RF transmitter arrays

Multiple‐channel RF transmission holds great promise for MRI, especially for human applications at high fields. For calibration it requires mapping the effective RF magnetic fields, B, of the transmitter array. This is challenging to do accurately and fast due to the large dynamic range of B and tight SAR constraints. In the present work, this problem is revisited and solved by a novel mapping approach relying on an interference principle. The B fields of individual transmitter elements are measured indirectly by observing their interference with a SAR‐efficient baseline RF field. In this fashion even small RF fields can be observed in the B ‐sensitive large‐flip‐angle regime. Based on a set of such experiments B maps of the individual transmitter channels are obtained by solving a linear inverse problem. Confounding relaxation and off‐resonance effects are addressed by an extended signal model and nonlinear fitting. Using the novel approach, 2D mapping of an 8‐channel transmitter array was accomplished in less than a minute. For validation it is demonstrated that mapping results do not vary with T₁ or parameters of the mapping sequence. In RF shimming experiments it is shown that the measured B maps accurately reflect the linearity of RF superposition. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

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.     

B1‐based specific energy absorption rate determination for nonquadrature radiofrequency excitation

The current gold standard to estimate local and global specific energy absorption rate for MRI involves numerically modeling the patient and the transmit radiofrequency coil. Recently, a patient‐individual method was presented, which estimated specific energy absorption rate from individually measured B₁ maps. This method, however, was restricted to quadrature volume coils due to difficulties distinguishing phase contributions from radiofrequency transmission and reception. In this study, a method separating these two phase contributions by comparing the electric conductivity reconstructed from different transmit channels of a parallel radiofrequency transmission system is presented. This enables specific energy absorption rate estimation not only for quadrature excitation but also for the nonquadrature excitation of the single elements of the transmit array. Though the contributions of the different phases are known, unknown magnetic field components and tissue boundary artifacts limit the technique. Nevertheless, the high agreement between simulated and experimental results found in this study is promising. B₁‐based specific energy absorption rate determination might become possible for arbitrary radiofrequency excitation on a patient‐individual basis. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Short echo spectroscopic imaging of the human brain at 7T using transceiver arrays

Recent advances in magnet technology have enabled the construction of ultrahigh‐field magnets (7T and higher) that can accommodate the human head and body. Despite the intrinsic advantages of performing spectroscopic imaging at 7T, increased signal‐to‐noise ratio (SNR), and spectral resolution, few studies have been reported to date. This limitation is largely due to increased power deposition and B₁ inhomogeneity. To overcome these limitations, we used an 8‐channel transceiver array with a short TE (15 ms) spectroscopic imaging sequence. Utilizing phase and amplitude mapping and optimization schemes, the 8‐element transceiver array provided both improved efficiency (17% less power for equivalent peak B₁) and homogeneity (SD(B₁) = ±10% versus ±22%) in comparison to a transverse electromagnetic (TEM) volume coil. To minimize the echo time to measure J‐modulating compounds such as glutamate, we developed a short TE sequence utilizing a single‐slice selective excitation pulse followed by a broadband semiselective refocusing pulse. Extracerebral lipid resonances were suppressed with an inversion recovery pulse and delay. The short TE sequence enabled visualization of a variety of resonances, including glutamate, in both a control subject and a patient with a Grade II oligodendroglioma. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Phase‐sensitive sodium B1 mapping

Quantitative sodium MRI requires accurate knowledge of factors affecting the sodium signal. One important determinant of sodium signal level is the transmit B₁ field strength. However, the low signal‐to‐noise ratio typical of sodium MRI makes accurate B₁ mapping in reasonable scan times challenging. A new phase‐sensitive B₁ mapping technique has recently been shown to work better than the widely used dual‐angle method in low‐signal‐to‐noise ratio situations and over a broader range of flip angles. In this work, the phase‐sensitive B₁ mapping technique is applied to sodium, and its performance compared to the dual‐angle method through both simulation and phantom studies. The phase‐sensitive method is shown to yield higher quality B₁ maps at low signal‐to‐noise ratio and greater consistency of measurement than the dual‐angle method. An in vivo sodium B₁ map of the human breast is also shown, demonstrating the phase‐sensitive method's feasibility for human studies. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Parallel traveling‐wave MRI: A feasibility study

Traveling‐wave magnetic resonance imaging utilizes far fields of a single‐piece patch antenna in the magnet bore to generate radio frequency fields for imaging large‐size samples, such as the human body. In this work, the feasibility of applying the “traveling‐wave” technique to parallel imaging is studied using microstrip patch antenna arrays with both the numerical analysis and experimental tests. A specific patch array model is built and each array element is a microstrip patch antenna. Bench tests show that decoupling between two adjacent elements is better than ‐26‐dB while matching of each element reaches ‐36‐dB, demonstrating excellent isolation performance and impedance match capability. The sensitivity patterns are simulated and g‐factors are calculated for both unloaded and loaded cases. The results on B sensitivity patterns and g‐factors demonstrate the feasibility of the traveling‐wave parallel imaging. Simulations also suggest that different array configuration such as patch shape, position and orientation leads to different sensitivity patterns and g‐factor maps, which provides a way to manipulate B₁ fields and improve the parallel imaging performance. The proposed method is also validated by using 7T MR imaging experiments. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Automatic high‐order shimming using parallel columns mapping (PACMAP)

This work presents a new automatic high‐order shimming method that maps the B₀ field using a group of parallel columns. We found that a pair of four columns in two separate slices could determine an optimal correction field comprising the spherical harmonic terms up to the third‐order. The technique of multiple stimulated echoes was incorporated into the method, allowing the use of at least eight shots to accomplish field mapping. The shim currents were first determined in the logic frame by assuming that the slices were in axial planes, and then uniquely converted into the physical frame where the slices could be at any oblique angle, by using a spherical harmonics rotation transformation. This method thus works regardless of slice orientation. It was demonstrated on a 3T scanner equipped with a complete set of second‐order harmonic shim coils. Both phantom and in vivo experiments showed that this newly introduced high‐order shimming method is an effective and efficient way to reduce field inhomogeneity for a region of imaging slices. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Rapid B1 mapping using orthogonal,equal‐amplitude radio‐frequency pulses

We present a new phase‐based method for mapping the amplitude of the radio‐frequency field (B₁) of a transmitter coil in three‐dimension. This method exploits the noncommutation relation between rotations about orthogonal axes. Our implementation of this principle in the current work results in a simple relation between the phase of the final magnetization and the flip angle (FA). In this study, we focus on FAs less than 90°. Our method is rapid and easy to implement compared with the existing B₁ mapping schemes. The mapping sequence can be simply obtained by adding to a regular three‐dimensional gradient‐echo sequence a magnetization preparation radio‐frequency pulse of the same FA but orthogonal in phase to the excitation radio‐frequency pulse. This method is demonstrated capable of generating reliable maps of the B₁ field within 1 min using FAs no larger than 60°. We show that it is robust against T₁, small chemical shift, and mild background inhomogeneity. This method may especially be suitable for B₁ mapping in situations (e.g., long‐T₁ and hyperpolarized‐gas imaging) where magnitude‐based methods are not readily applicable. A noise calculation of the FA map using this method is also presented. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

7T head volume coils: Improvements for rostral brain imaging

Purpose

To improve the performance of 7T head coils over the rostral head regions. Due to radiofrequency (RF) field/tissue interactions, the RF magnetic field profile produced by 7T volume head coils is very inhomogeneous, with enhanced sensitivity near the center of the human brain and substantially reduced in the periphery.

Materials and Methods

Two head‐sized quadrature volume coils of similar diameters but substantially different lengths (17 and 10 cm) were constructed and tested using a 7T Varian Inova system.

Results

Experimental data demonstrated that by using a shorter volume head‐sized coil or simply by partially moving a head out of the coil, coil efficiency near the top of a head can be improved by 20%. The homogeneity also improved, largely resulting from an increase in peripheral B₁ values. This resulted in 10%–20% variation in axial slices located near the top of a head.

Conclusion

We have demonstrated a less deeply positioned head or substantially shorter volume coil can significantly improve coil performance and homogeneity for the rostral head at ultrahigh magnetic fields (7T and above). For studies that target superior brain regions, this coil arrangement can be highly effective. J. Magn. Reson. Imaging 2009;29:461–465. © 2009 Wiley‐Liss, Inc.     

Four‐channel surface coil array for 300‐MHz pulsed EPR imaging: Proof‐of‐concept experiments

Time‐domain electron paramagnetic resonance imaging is currently a useful preclinical molecular imaging modality in experimental animals such as mice and is capable of quantitatively mapping hypoxia in tumor implants. The microseconds range relaxation times (T₁ and T₂) of paramagnetic tracers and the large bandwidths (tens of MHz) to be excited by electron paramagnetic resonance pulses for spatial encoding makes imaging of large objects a challenging task. The possibility of using multiple array coils to permit studies on large sized object is the purpose of the present work. Toward this end, the use of planar array coils in different configurations to image larger objects than cannot be fully covered by a single resonator element is explored. Multiple circular surface coils, which are arranged in a plane or at suitable angles mimicking a volume resonator, are used in imaging a phantom and a tumor‐bearing mouse leg. The image was formed by combining the images collected from the individual coils with suitable scaling. The results support such a possibility. By multiplexing or interleaving the measurements from each element of such array resonators, one can scale up the size of the subject and at the same time reduce the radiofrequency power requirements and increase the sensitivity. Magn Reson Med 71:853–858, 2014. © 2013 Wiley Periodicals, 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.     

Calculating T2 in images from a phased array receiver

Images reconstructed from multielement, phased array coils and presented as the square root of the sum of the squares of the signals received by the individual elements have a distribution of signal and noise that distorts the relationship between the image intensity and the underlying signal. The distortion is accentuated for long echo times for which the signal‐to‐noise ratio (SNR) may be low. When measuring T₂ or T this signal distortion leads to biased estimates of these parameters. We demonstrate this effect and its dependence on the image SNR and the number of elements in a phased array coil. We evaluated the effects of four techniques for calculating T₂ from data acquired in phased array coils (log transform, least squares, lookup table correction, and maximum likelihood [ML] estimation). The ML estimation gave the most accurate T₂ in the presence of this bias. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Heuristic linear mapping of physiological parameters in dynamic contrast-enhanced MRI without T₁ measurement and contrast agent concentration

Purpose:

To present a novel heuristic linear mapping method to individually estimate physiological parameters for Tofts model without T₁ measurement and contrast agent concentration.

Materials and Methods:

A linear relationship was used for k_ep mapping through a heuristic time intensity curve (TIC) shape factor (TSF). K^trans maps were subsequently estimated using k_ep maps and another approximate linear model derived from the Tofts model. Twenty‐seven patients with head‐and‐neck squamous cell carcinoma received dynamic contrast‐enhanced magnetic resonance imaging (DCE‐MRI). Physiological parameters maps were obtained using this heuristic linear mapping method and compared to the maps obtained by the normal nonlinear least‐square fitting with T₁ measurement.

Results:

High linearity (R² >0.95) between k_ep and TSF was found in all patients for k_ep <5/min. This linearity is robust for TSF timepoint selection. The k_ep maps generated by this linear fitting were highly consistent with those by the normal nonlinear approach (P > 0.05). The K^trans maps were consistent with the normally derived maps in pattern distribution but the absolute value might be scaled due to the assumption of the reference K^trans value.

Conclusion:

This novel method generates reliable and consistent physiological parameter maps with significantly lower computation complexity than the multiparameter nonlinear fitting. The DCE‐MRI scan time can be greatly shortened without T₁ mapping. J. Magn. Reson. Imaging 2012;35:916–925. © 2011 Wiley Periodicals, Inc.     

Simultaneous B0‐ and B1+‐Map acquisition for fast localized shim,frequency, and RF power determination in the heart at 3 T

High‐field (≥3 T) cardiac MRI is challenged by inhomogeneities of both the static magnetic field (B₀) and the transmit radiofrequency field (B₁+). The inhomogeneous B fields not only demand improved shimming methods but also impede the correct determination of the zero‐order terms, i.e., the local resonance frequency f₀ and the radiofrequency power to generate the intended local B₁+ field. In this work, dual echo time B₀‐map and dual flip angle B₁+‐map acquisition methods are combined to acquire multislice B₀‐ and B₁+‐maps simultaneously covering the entire heart in a single breath hold of 18 heartbeats. A previously proposed excitation pulse shape dependent slice profile correction is tested and applied to reduce systematic errors of the multislice B₁+‐map. Localized higher‐order shim correction values including the zero‐order terms for frequency f₀ and radiofrequency power can be determined based on the acquired B₀‐ and B₁+‐maps. This method has been tested in 7 healthy adult human subjects at 3 T and improved the B₀ field homogeneity (standard deviation) from 60 Hz to 35 Hz and the average B₁+ field from 77% to 100% of the desired B₁+ field when compared to more commonly used preparation methods. 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.     

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.     

Noise figure limits for circular loop MR coils

Circular loops are the most common MR detectors. Loop arrays offer improved signal‐to‐noise ratios (SNRs) and spatial resolution, and enable parallel imaging. As loop size decreases, loop noise increases relative to sample noise, ultimately dominating the SNR. Here, relative noise contributions from the sample and the coil are quantified by a coil noise figure (NF), NF_coil, which adds to the conventional system NF. NF_coil is determined from the ratio of unloaded‐to‐loaded coil quality factors Q. Losses from conductors, capacitors, solder joints, eddy currents in overlapped array coils, and the sample are measured and/or computed from 40 to 400 MHz using analytical and full‐wave numerical electromagnetic analysis. The Qs are measured for round wire and tape loops tuned from 50 to 400 MHz. NF_coil is determined as a function of the radius, frequency, and number of tuning capacitors. The computed and experimental Qs and NF_coils agree within ～10%. The NF_coil values for 3 cm‐diameter wire coils are 3 dB, 1.9 dB, 0.8 dB, 0.2 dB, and 0.1 dB, at 1T, 1.5T, 3T, 7T, and 9.4T, respectively. Wire and tape perform similarly, but tape coils in arrays have substantial eddy current losses. The ability to characterize and reliably predict component‐ and geometry‐associated coil losses is key to designing SNR‐optimized loop and phased‐array detectors. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

SPECIAL semi‐LASER with lipid artifact compensation for 1H MRS at 7 T

The measurement of full metabolic profiles at ultrahigh fields including low concentrated or fast‐relaxing metabolites is usually achieved by applying short echo time sequences. One sequence beside stimulated echo acquisition mode that was proposed in this regard is spin echo full intensity‐acquired localized spectroscopy. Typical problems that are still persistent for spin echo full intensity‐acquired localized spectroscopy are B₁ inhomogeneities especially for signal acquisition with surface coils and chemical shift displacement artifacts due to limited B₁ amplitudes when using volume coils. In addition, strong lipid contaminations in the final spectrum can occur when only a limited number of outer volume suppression pulses is used. Therefore, an adiabatic short echo time (= 19 ms) spin echo full intensity‐acquired localized spectroscopy semilocalization by adiabatic selective refocusing sequence is presented that is less sensitive to strong B₁ variations and that offers increased excitation and refocusing pulse bandwidths than regular spin echo full intensity acquired localized spectroscopy. Furthermore, the existence of the systematic lipid artifact is identified and linked to unfavorable effects due to the preinversion localization pulse. A method to control this artifact is presented and validated in both phantom and in vivo measurements. The viability of the proposed sequence was further assessed for in vivo measurements by scanning 17 volunteers using a surface coil and moreover acquiring additional volume coil measurements. The results show well‐suppressed lipid artifacts, good signal‐to‐noise ratio, and reproducible fitting results in accordance with other published studies. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.