Carl J. Snyder

Transmit and receive transmission line arrays for 7 Tesla parallel imaging.

Transceive array coils, capable of RF transmission and independent signal reception, were developed for parallel, 1H imaging applications in the human head at 7 T (300 MHz). The coils combine the advantages of high‐frequency properties of transmission lines with classic MR coil design. Because of the short wavelength at the 1H frequency at 300 MHz, these coils were straightforward to build and decouple. The sensitivity profiles of individual coils were highly asymmetric, as expected at this high frequency; however, the summed images from all coils were relatively uniform over the whole brain. Data were obtained with four‐ and eight‐channel transceive arrays built using a loop configuration and compared to arrays built from straight stripline transmission lines. With both the four‐ and the eight‐channel arrays, parallel imaging with sensitivity encoding with high reduction numbers was feasible at 7 T in the human head. A one‐dimensional reduction factor of 4 was robustly achieved with an average g value of 1.25 with the eight‐channel transmit/receive coils. Magn Reson Med 53:434–445, 2005.

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.

Whole-body imaging at 7T: Preliminary results

The objective of this study was to investigate the feasibility of whole‐body imaging at 7T. To achieve this objective, new technology and methods were developed. Radio frequency (RF) field distribution and specific absorption rate (SAR) were first explored through numerical modeling. A body coil was then designed and built. Multichannel transmit and receive coils were also developed and implemented. With this new technology in hand, an imaging survey of the “landscape” of the human body at 7T was conducted. Cardiac imaging at 7T appeared to be possible. The potential for breast imaging and spectroscopy was demonstrated. Preliminary results of the first human body imaging at 7T suggest both promise and directions for further development. Magn Reson Med 61:244–248, 2009.

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.

Efficient high-frequency body coil for high-field MRI.

The use of body coils is favored for homogeneous excitation, and such coils are often paired with surface coils or arrays for sensitive reception in many MRI applications. While the body coils physical size and resultant electrical length make this circuit difficult to design for any field strength, recent efforts to build efficient body coils for applications at 3T and above have been especially challenging. To meet this challenge, we developed an efficient new transverse electromagnetic (TEM) body coil and demonstrated its use in human studies at field strengths up to 4T. Head, body, and breast images were acquired within peak power constraints of <8 kW. Bench studies indicate that these body coils are feasible to 8T. RF shimming was used to remove a high‐field‐related cardiac imaging artifact in these preliminary studies. Magn Reson Med 52:851–859, 2004.

Whole-body imaging at 7T

The objective of this study was to investigate the feasibility of whole‐body imaging at 7T. To achieve this objective, new technology and methods were developed. Radio frequency (RF) field distribution and specific absorption rate (SAR) were first explored through numerical modeling. A body coil was then designed and built. Multichannel transmit and receive coils were also developed and implemented. With this new technology in hand, an imaging survey of the “landscape” of the human body at 7T was conducted. Cardiac imaging at 7T appeared to be possible. The potential for breast imaging and spectroscopy was demonstrated. Preliminary results of the first human body imaging at 7T suggest both promise and directions for further development. Magn Reson Med 61:244–248, 2009.

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.

A 32-Channel Lattice Transmission Line Array for Parallel Transmit and Receive MRI at 7 Tesla

Transmit and receive RF coil arrays have proven to be particularly beneficial for ultra‐high‐field MR. Transmit coil arrays enable such techniques as B1+ shimming to substantially improve transmit B1 homogeneity compared to conventional volume coil designs, and receive coil arrays offer enhanced parallel imaging performance and SNR. Concentric coil arrangements hold promise for developing transceiver arrays incorporating large numbers of coil elements. At magnetic field strengths of 7 tesla and higher where the Larmor frequencies of interest can exceed 300 MHz, the coil array design must also overcome the problem of the coil conductor length approaching the RF wavelength. In this study, a novel concentric arrangement of resonance elements built from capacitively‐shortened half‐wavelength transmission lines is presented. This approach was utilized to construct an array with whole‐brain coverage using 16 transceiver elements and 16 receive‐only elements, resulting in a coil with a total of 16 transmit and 32 receive channels. Magn Reson Med 63:1478–1485, 2010.

7 Tesla (T) human cardiovascular magnetic resonance imaging using FLASH and SSFP to assess cardiac function: validation against 1.5 T and 3 T

We report the first comparison of cardiovascular magnetic resonance imaging (CMR) at 1.5 T, 3 T and 7 T field strengths using steady state free precession (SSFP) and fast low angle shot (FLASH) cine sequences. Cardiac volumes and mass measurements were assessed for feasibility, reproducibility and validity at each given field strength using FLASH and SSFP sequences. Ten healthy volunteers underwent retrospectively electrocardiogram (ECG) gated CMR at 1.5 T, 3 T and 7 T using FLASH and SSFP sequences. B1 and B0 shimming and frequency scouts were used to optimise image quality. Cardiac volume and mass measurements were not significantly affected by field strength when using the same imaging sequence (P > 0.05 for all parameters at 1.5 T, 3 T and 7 T). SSFP imaging returned larger end diastolic and end systolic volumes and smaller left ventricular masses than FLASH imaging at 7 T, and at the lower field strengths (P < 0.05 for each parameter). However, univariate general linear model analysis with fixed effects for sequence and field strengths found an interaction between imaging sequence and field strength (P = 0.03), with a smaller difference in volumes and mass measurements between SSFP and FLASH imaging at 7 T than 1.5 T and 3 T. SSFP and FLASH cine imaging at 7 T is technically feasible and provides valid assessment of cardiac volumes and mass compared with CMR imaging at 1.5 T and 3 T field strengths. Copyright