Lance DelaBarre

7T vs. 4T: RF power, homogeneity, and signal-to-noise comparison in head images

Signal‐to‐noise ratio (SNR), RF field (B1), and RF power requirement for human head imaging were examined at 7T and 4T magnetic field strengths. The variation in B1 magnitude was nearly twofold higher at 7T than at 4T (∼42% compared to ∼23%). The power required for a 90° pulse in the center of the head at 7T was approximately twice that at 4T. The SNR averaged over the brain was at least 1.6 times higher at 7T compared to 4T. These experimental results were consistent with calculations performed using a human head model and Maxwells equations. Magn Reson Med 46:24–30, 2001.

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.

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.

Proton T2 relaxation study of water, N-acetylaspartate, and creatine in human brain using Hahn and Carr-Purcell spin echoes at 4T and 7T.

Carr‐Purcell and Hahn spin‐echo (SE) measurements were used to estimate the apparent transverse relaxation time constant (T  †2 ) of water and metabolites in human brain at 4T and 7T. A significant reduction in the T  †2 values of proton resonances (water, N‐acetylaspartate, and creatine/phosphocreatine) was observed with increasing magnetic field strength and was attributed mainly to increased dynamic dephasing due to increased local susceptibility gradients. At high field, signal loss resulting from T  †2 decay can be substantially reduced using a Carr‐Purcell‐type SE sequence. Magn Reson Med 47:629–633, 2002.

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.

Detunable transverse electromagnetic (TEM) volume coil for high-field NMR

Most high‐field MRI systems do not have the actively detuned body coils that are integral to clinical systems operating at 1.5T and lower field strengths. Therefore, many clinical applications requiring homogeneous volume excitation in combination with local surface coil reception are not easily implemented at high fields. To solve this problem for neuroimaging applications, actively detunable transverse electromagnetic (TEM) head coils were developed to be used with receive‐only surface coils for signal‐to‐noise ratio (SNR) gains and improved spatial coverage from homogeneously excited regions. These SNR and field of view (FOV) gains were achieved by application of a detunable TEM volume coil to human brain imaging at 4T. Magn Reson Med 47:990–1000, 2002.

Eliminating spurious lipid sidebands in 1H MRS of breast lesions.

Detecting metabolites in breast lesions by in vivo 1H MR spectroscopy can be difficult due to the abundance of mobile lipids in the breast which can produce spurious sidebands that interfere with the metabolite signals. Two‐dimensional J‐resolved spectroscopy has been demonstrated in the brain as a means to eliminate these artifacts from a large water signal; coherent sidebands are resolved at their natural frequencies, leaving the noncoupled metabolite resonances in the zero‐frequency trace of the 2D spectrum. This work demonstrates that using the zero‐frequency trace—or equivalently the average of spectra acquired with different echo times—can be used to separate noncoupled metabolite signals from the lipid‐induced sidebands. This technique is demonstrated with simulations, phantom studies, and in several breast lesions. Compared to the conventional approach using a single echo time, echo time averaging provides increased sensitivity for the study of small and irregularly shaped lesions. Magn Reson Med 48:215–222, 2002.

BISTRO: an outer-volume suppression method that tolerates RF field inhomogeneity.

A technique is described for performing frequency‐selective signal suppression with a high degree of tolerance to RF field inhomogeneity. The method is called B1‐insensitive train to obliterate signal (BISTRO). BISTRO consists of multiple amplitude‐ and frequency‐modulated (FM) pulses interleaved with spoiler gradients. BISTRO was developed for the purpose of accomplishing band‐selective signal removal, as in water suppression and outer‐volume suppression (OVS), in applications requiring the use of an inhomogeneous RF transmitter, such as a surface coil. In the present work, Bloch simulations were used to illustrate the principles and theoretical performance of BISTRO. Its performance for OVS was evaluated experimentally using MRI and spectroscopic imaging of phantoms and in vivo animal and human brain. By using FM pulses featuring offset‐independent adiabaticity, BISTRO permitted high‐quality, broadband suppression with one (or two) discrete borders demarcating the edge(s) of the suppression band. Simulations and experiments demonstrated the ability to operate BISTRO with reasonably attainable peak RF power levels and with average RF energy deposition similar to other multipulse OVS techniques. Magn Reson Med 45:1095–1102, 2001.