Nikolai I. Avdievich

RF shimming for spectroscopic localization in the human brain at 7 T.

Spectroscopic imaging of the human head at short echo times (≤15 ms) typically requires suppression of signals from extracerebral tissues. However, at 7 T, decreasing efficiency in B  1+ generation (hertz/watt) and increasing spectral bandwidth result in dramatic increases in power deposition and increased chemical shift registration artifacts for conventional gradient‐based in‐plane localization. In this work, we describe a novel method using radiofrequency shimming and an eight‐element transceiver array to generate a B  1+ field distribution that excites a ring about the periphery of the head and leaves central brain regions largely unaffected. We have used this novel B  1+ distribution to provide in‐plane outer volume suppression (>98% suppression of extracerebral lipids) without the use of gradients. This novel B  1+ distribution is used in conjunction with a double inversion recovery method to provide suppression of extracerebral resonances with T1s greater than 400 ms, while having negligible effect on metabolite ratios of cerebral resonances with T1s > 1000 ms. Despite the use of two adiabatic pulses, the high efficiency of the ring distribution allows radiofrequency power deposition to be limited to 3‐4 W for a pulse repetition time of 1.5 sec. The short echo time enabled the acquisition of images of the human brain, displaying glutamate, glutamine, macromolecules, and other major cerebral metabolites. Magn Reson Med, 2010.

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.

Resonant inductive decoupling (RID) for transceiver arrays to compensate for both reactive and resistive components of the mutual impedance.

Transceiver surface coil arrays improve transmit performance (B1/√kW) and B1 homogeneity for head imaging up to 9.4 T. To further improve reception performance and parallel imaging, the number of array elements must be increased with a corresponding decrease in their size. With a large number of small interacting antennas, decoupling is one of the most challenging aspects in the design and construction of transceiver arrays. Previously described decoupling techniques using geometric overlap, inductive or capacitive decoupling have focused on the elimination of the reactance of the mutual impedance only, which can limit the obtainable decoupling to –10 dB as a result of residual mutual resistance. A novel resonant inductive decoupling (RID) method, which allows compensation for both reactive and resistive components of the mutual impedance between the adjacent surface coils, has been developed and verified experimentally. This method provides an easy way to adjust the decoupling remotely by changing the resonance frequency of the RID circuit through the adjustment of a variable capacitor. As an example, a single‐row (1 × 16) 7‐T transceiver head array of n = 16 small overlapped surface coils using RID decoupling between adjacent coils was built. In combination with overlapped coils, the RID technique achieved better than –24 dB of decoupling for all adjacent coils. Copyright

Safety testing and operational procedures for self-developed radiofrequency coils

The development of novel radiofrequency (RF) coils for human ultrahigh‐field (≥7 T), non‐proton and body applications is an active field of research in many MR groups. Any RF coil must meet the strict requirements for safe application on humans with respect to mechanical and electrical safety, as well as the specific absorption rate (SAR) limits. For this purpose, regulations such as the International Electrotechnical Commission (IEC) standard for medical electrical equipment, vendor‐suggested test specifications for third party coils and custom‐developed test procedures exist. However, for higher frequencies and shorter wavelengths in ultrahigh‐field MR, the RF fields may become extremely inhomogeneous in biological tissue and the risk of localized areas with elevated power deposition increases, which is usually not considered by existing safety testing and operational procedures. In addition, important aspects, such as risk analysis and comprehensive electrical performance and safety tests, are often neglected. In this article, we describe the guidelines used in our institution for electrical and mechanical safety tests, SAR simulation and verification, risk analysis and operational procedures, including coil documentation, user training and regular quality assurance testing, which help to recognize and eliminate safety issues during coil design and operation. Although the procedure is generally applicable to all field strengths, specific requirements with regard to SAR‐related safety and electrical performance at ultrahigh‐field are considered. The protocol describes an internal procedure and does not reflect consensus among a large number of research groups, but rather aims to stimulate further discussion related to minimum coil safety standards. Furthermore, it may help other research groups to establish their own procedures. Copyright

J-refocused coherence transfer spectroscopic imaging at 7 T in human brain.

Short echo spectroscopy is commonly used to minimize signal modulation due to J‐evolution of the cerebral amino acids. However, short echo acquisitions suffer from high sensitivity to macromolecules which make accurate baseline determination difficult. In this report, we describe implementation at 7 T of a double echo J‐refocused coherence transfer sequence at echo time (TE) of 34 msec to minimize J‐modulation of amino acids while also decreasing interfering macromolecule signals. Simulation of the pulse sequence at 7 T shows excellent resolution of glutamate, glutamine, and N‐acetyl aspartate. B1 sufficiency at 7 T for the double echo acquisition is achieved using a transceiver array with radiofrequency (RF) shimming. Using an alternate RF distribution to minimize receiver phase cancellation in the transceiver, accurate phase determination for the coherence transfer is achieved with rapid single scan calibration. This method is demonstrated in spectroscopic imaging mode with n = 5 healthy volunteers resulting in metabolite values consistent with literature and in a patient with epilepsy. Magn Reson Med, 2010.

Occipital levels of GABA are related to severe headaches in migraine.

Although GABA has figured prominently in theories of migraine pathogenesis,1 brain levels of this transmitter have not been directly measured in migraineurs. This is of importance since, in migraine, neurophysiologic events account for brain hyperexcitability and subcortical disinhibition.2,3 Magnetic resonance spectroscopy (MRS) measures levels of metabolites in the human brain and may map regional changes in their levels.4 Accordingly, herein we used MRS to measure the levels of GABA in individuals with migraine with aura (MA), migraine without aura (MO), and in controls. ### Methods. Individuals with MA (n = 9), MO (n = 10), and controls without headache (n = 9) were enrolled. Participants could not use migraine preventive drugs or any other on a daily basis. Subjects prospectively collected their headache information over 1 month, using a daily headache calendar. Disability was assessed with the Migraine Disability Assessment Questionnaire (MIDAS). Migraineurs were imaged no less than 72 hours after their last headache attack. Data were acquired at 4T using a MRS INOVA system. Details of the methods are described elsewhere.5 Briefly, a volume of 3 × 3 × 1.5 cm3 was used positioned within the midline of the occipital lobe using inversion recovery gradient echo images. Two 8.5-minutes acquisition were averaged to …

4 T actively detunable transmit/receive transverse electromagnetic coil and 4-channel receive-only phased array for 1H human brain studies

The design and construction of a 4 T transverse electromagnetic (TEM) transmit/receive head coil and a four‐channel phased array receive‐only RF system are described. To enable both high‐resolution imaging of the entire brain and high‐resolution spectroscopic imaging, active PIN diode decoupling was used in both the TEM resonator and each surface coil in the array. This configuration allows for both transmission and reception from the volume coil as well as reception from the phased array. The surface coils were decoupled by overlapping the coils and using preamplifier decoupling. Since at high frequencies construction of a lumped element matching quarter wavelength transformer, an important component of the preamplifier decoupling, becomes difficult, a transmission line approach was used. The system was tested and compared to a TEM volume transmit/receive head coil. A four‐ to sixfold improvement in signal‐to‐noise ratio from the sensitive volume of the array was achieved. Magn Reson Med 52:1459–1464, 2004.

Evaluation of transmit efficiency and SAR for a tight fit transceiver human head phased array at 9.4 T

Ultra‐high field (UHF, ≥7 T) tight fit transceiver phased arrays improve transmit (Tx) efficiency (B1+/√P) in comparison with Tx‐only arrays, which are usually larger to fit receive (Rx)‐only arrays inside. One of the major problems limiting applications of tight fit arrays at UHFs is the anticipated increase of local tissue heating, which is commonly evaluated by the local specific absorption rate (SAR). To investigate the tradeoff between Tx efficiency and SAR when a tight fit UHF human head transceiver phased array is used instead of a Tx‐only/Rx‐only RF system, a single‐row eight‐element prototype of a 400 MHz transceiver head phased array was constructed. The Tx efficiency and SAR of the array were evaluated and compared with that of a larger Tx‐only array, which could also be used in combination with an 18‐channel Rx‐only array. Data were acquired on the Siemens Magnetom whole body 9.4 T human MRI system.

7T head volume coils: Improvements for rostral brain imaging

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.

OPEN HALF VOLUME QUADRATURE TRANSVERSE ELECTROMAGNETIC COIL FOR HIGH FIELD MAGNETIC RESONANCE IMAGING

A half‐volume quadrature head transverse electromagnetic (TEM) coil has been constructed for 4 T imaging applications. This coil produces a sufficiently large homogeneous B1 field region for the use as a volume coil. It provides superior transmission efficiency, resulting in significantly lower power deposition, as well as greater sensitivity and improved patient comfort and accessibility compared with conventional full‐volume coils. Additionally, this coil suppresses the RF penetration artifact that distorts the RF magnetic field profile and alters the intensity in high‐field images recorded with linear surface and volume coils. These advantages make it possible to apply this device as an efficient transmit/receive coil for high‐field imaging with a restricted field of view. Magn Reson Med 53:937–943, 2005.