Can Akgun

B1 destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil

RF behavior in the human head becomes complex at ultrahigh magnetic fields. A bright center and a weak periphery are observed in images obtained with volume coils, while surface coils provide strong signal in the periphery. Intensity patterns reported with volume coils are often loosely referred to as “dielectric resonances,” while modeling studies ascribe them to superposition of traveling waves greatly dampened in lossy brain tissues, raising questions regarding the usage of this term. Here we address this question experimentally, taking full advantage of a transceiver coil array that was used in volume transmit mode, multiple receiver mode, or single transmit surface coil mode. We demonstrate with an appropriately conductive sphere phantom that destructive interferences are responsible for a weak B1 in the periphery, without a significant standing wave pattern. The relative spatial phase of receive and transmit B1 proved remarkably similar for the different coil elements, although with opposite rotational direction. Additional simulation data closely matched our phantom results. In the human brain the phase patterns were more complex but still exhibited similarities between coil elements. Our results suggest that measuring spatial B1 phase could help, within an MR session, to perform RF shimming in order to obtain more homogeneous B1 in user‐defined areas of the brain. Magn Reson Med, 2005.

9.4T human MRI: preliminary results.

This work reports the preliminary results of the first human images at the new high‐field benchmark of 9.4T. A 65‐cm‐diameter bore magnet was used together with an asymmetric 40‐cm‐diameter head gradient and shim set. A multichannel transmission line (transverse electromagnetic (TEM)) head coil was driven by a programmable parallel transceiver to control the relative phase and magnitude of each channel independently. These new RF field control methods facilitated compensation for RF artifacts attributed to destructive interference patterns, in order to achieve homogeneous 9.4T head images or localize anatomic targets. Prior to FDA investigational device exemptions (IDEs) and internal review board (IRB)‐approved human studies, preliminary RF safety studies were performed on porcine models. These data are reported together with exit interview results from the first 44 human volunteers. Although several points for improvement are discussed, the preliminary results demonstrate the feasibility of safe and successful human imaging at 9.4T. Magn Reson Med, 2006.

Local B1+ shimming for prostate imaging with transceiver arrays at 7T based on subject-dependent transmit phase measurements

High‐quality prostate images were obtained with transceiver arrays at 7T after performing subject‐dependent local transmit B1 (B1+) shimming to minimize B1+ losses resulting from destructive interferences. B1+ shimming was performed by altering the input phase of individual RF channels based on relative B1+ phase maps rapidly obtained in vivo for each channel of an eight‐element stripline coil. The relative transmit phases needed to maximize B1+ coherence within a limited region around the prostate greatly differed from those dictated by coil geometry and were highly subject‐dependent. A set of transmit phases determined by B1+ shimming provided a gain in transmit efficiency of 4.2 ± 2.7 in the prostate when compared to the standard transmit phases determined by coil geometry. This increased efficiency resulted in large reductions in required RF power for a given flip angle in the prostate which, when accounted for in modeling studies, resulted in significant reductions of local specific absorption rates. Additionally, B1+ shimming decreased B1+ nonuniformity within the prostate from (24 ± 9%) to (5 ± 4%). This study demonstrates the tremendous impact of fast local B1+ phase shimming on ultrahigh magnetic field body imaging. Magn Reson Med 59:396–409, 2008.

High field magnetic resonance

This chapter deals with data and concepts relevant to high magnetic fields with the primary focus on efforts related to probing brain function and neurochemistry utilizing imaging and spectroscopy capabilities. One of the most important accomplishments in magnetic resonance imaging (MRI) research over the past years is the introduction of methods that can map the areas of altered neuronal activity in the brain, that is, functional MRI or fMRI. The most commonly used method of fMRI is based on blood oxygen level dependent (BOLD) contrast which is sensitive to the presence of deoxyhemoglobin. In an fMRI experiment, images are collected subsequent to signal excitation and echo formation, either by a gradient reversal or application of a refocusing radio frequency (RF) pulse. During the delay after excitation and before echo formation, it is possible to apply a pair of gradient pulses with opposing or same polarity depending on whether the experiment is a gradient recalled echo or a spinecho experiment, respectively.

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. Magn Reson Med, 2008.

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  1+ ) homogeneity was far superior for the trESA, the maximum achievable B  1+ 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  1+ 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.

Dynamically applied B1+ shimming solutions for non-contrast enhanced renal angiography at 7.0 Tesla.

The purpose of this study was to detail a strategy for performing non‐contrast enhanced renal magnetic resonance angiography studies at 7.0 T. It is demonstrated that with proper B  1+ management, these studies can be successfully performed at ultrahigh field within local specific absorption rate constraints. An inversion prepared gradient echo acquisition, standard for non‐contrast renal magnetic resonance angiography studies, required radiofrequency pulse specific B  1+ shimming solutions to be dynamically applied to address the field dependent increases in both B0 and B  1+ inhomogeneity as well as to accommodate limitation in available power. By using more efficient B  1+ shimming solutions for the inversion preparation and more homogeneous solutions for the excitation, high quality images of the renal arteries were obtained without venous and background signal artifacts while working within hardware and safety constraints. Finite difference time domain simulations confirmed in vivo measurements with respect to B  1+ distributions and homogeneity for the range of shimming strategies used and allowed the calculation of peak local specific absorption rate values normalized by input power and B  1+ . Increasing B  1+ homogeneity was accompanied by decreasing local specific absorption rate per Watt and increasing maximum local specific absorption rate per [B  1+ ]2, which must be considered, along with body size and respiratory rate, when finalizing acquisition parameters for a given individual. Magn Reson Med, 2013.

Comparison between eight- and sixteen-channel TEM transceive arrays for body imaging at 7 T.

Eight‐ and sixteen‐channel transceive stripline/TEM body arrays were compared at 7 T (297 MHz) both in simulation and experiment. Despite previous demonstrations of similar arrays for use in body applications, a quantitative comparison of the two configurations has not been undertaken to date. Results were obtained on a male pelvis for assessing transmit, signal to noise ratio, and parallel imaging performance and to evaluate local power deposition versus transmit B1 (B1+). All measurements and simulations were conducted after performing local B1+ phase shimming in the region of the prostate. Despite the additional challenges of decoupling immediately adjacent coils, the sixteen‐channel array demonstrated improved or nearly equivalent performance to the eight‐channel array based on the evaluation criteria. Experimentally, transmit performance and signal to noise ratio were 22% higher for the sixteen‐channel array while significantly increased reduction factors were achievable in the left–right direction for parallel imaging. Finite difference time domain simulations demonstrated similar results with respect to transmit and parallel imaging performance, however, a higher transmit efficiency advantage of 33% was predicted. Simulations at both 3 and 7 T verified the expected parallel imaging improvements with increasing field strength and showed that, for a specific B1+ shimming strategy used, the sixteen‐channel array exhibited lower local and global specific absorption rate for a given B1+. Magn Reson Med, 2011.

Parallel transmit pulse design for patients with deep brain stimulation implants

Specific absorption rate (SAR) amplification around active implantable medical devices during diagnostic MRI procedures poses a potential risk for patient safety. In this study, we present a parallel transmit (pTx) strategy that can be used to safely scan patients with deep brain stimulation (DBS) implants.

Novel multi-channel transmission line coil for high field magnetic resonance imaging

Radiofrequency (RF) coils are the antenna-like devices used in magnetic resonance imaging (MRI) to inductively excite and receive the nuclear magnetic resonance (NMR) signal in anatomy. This nuclear magnetic induction is most efficient at the field strength dependent Larmor frequency for a nuclear species. Coils must resonate at Larmor frequencies of 300 MHz or more to take advantage of the signal-to-noise benefits of 7T+ MRI. In high water content tissue dielectrics however, the wavelengths at these frequencies are 12cm and less, significantly shorter than human anatomic dimensions. One consequence of these short wavelengths is a highly non-uniform RF excite field. In this investigation, we aim to mitigate this problem through a novel coil element design. The traditional microstrip line element is modified into a multi-section alternating impedance configuration to homogenize the magnetic field over the coil length. Feasibility of this approach is numerically simulated, and then empirically validated by phantom and human imaging.