We describe the design and testing of a quadrature transmit, eight-channel receive array RF coil configuration for the acquisition of images of the entire human spinal column at 7T. Imaging... Show moreWe describe the design and testing of a quadrature transmit, eight-channel receive array RF coil configuration for the acquisition of images of the entire human spinal column at 7T. Imaging parameters were selected to enable data acquisition in a clinically relevant scan time. Large field-of-view (FOV) scanning enabled sagittal imaging of the spine in two or three-stations, depending upon the height of the volunteer, with a total scan time of between 10 and 15min. A total of 10 volunteers have been scanned, with results presented for the three subjects spanning the range of heights and weights, namely one female (1.6m, 50kg), one average male (1.8m, 70kg), and one large male (1.9m, 100kg). Show less
Vossen, M.; Teeuwisse, W.; Reijnierse, M.; Collins, C.M.; Smith, N.B.; Webb, A.G. 2011
We describe the design and testing of a quadrature transmit, eight-channel receive array RF coil configuration for the acquisition of images of the entire human spinal column at 7 T. Imaging... Show moreWe describe the design and testing of a quadrature transmit, eight-channel receive array RF coil configuration for the acquisition of images of the entire human spinal column at 7 T. Imaging parameters were selected to enable data acquisition in a clinically relevant scan time. Large field-of-view (FOV) scanning enabled sagittal imaging of the spine in two or three-stations, depending upon the height of the volunteer, with a total scan time of between 10 and 15 min. A total of 10 volunteers have been scanned, with results presented for the three subjects spanning the range of heights and weights, namely one female (1.6 m, 50 kg), one average male (1.8 m, 70 kg), and one large male (1.9 m, 100 kg). (C) 2010 Elsevier Inc. All rights reserved. Show less
The major radiofrequency engineering challenges of high-field MR neuroimaging are as follows: (1) to produce a strong, homogeneous transmit B, field, while remaining within regulatory guidelines... Show moreThe major radiofrequency engineering challenges of high-field MR neuroimaging are as follows: (1) to produce a strong, homogeneous transmit B, field, while remaining within regulatory guidelines for tissue power deposition and (2) to receive the signal with the maximum signal-to-noise and the greatest flexibility in terms of utilizing the benefits of parallel imaging. Borrowing from developments in electromagnetic hyperthermia, the first challenge has been met by the use of transmit arrays, in which the input power to each element of the array can be varied in terms of magnitude and phase. Optimization of these parameters, as well as the form of the applied RF pulse, leads to very homogeneous B, fields throughout the brain. The design of large receive arrays, using impedance-mismatched preamplifiers and geometrical overlap for interelement isolation, has resulted in significant sensitivity improvements as well as large acceleration factors in parallel imaging. 0 2010 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 20, 2-13, 2010; Published online in Wiley InterScience (wwwintersciencemiley.com). DOI 10.1002/ima.20219 Show less
Using a small resonant loop to produce a longitudinal traveling wave on a human 7-T system allows MR to be performed over the entire volume of the human leg. We have used this capability to perform... Show moreUsing a small resonant loop to produce a longitudinal traveling wave on a human 7-T system allows MR to be performed over the entire volume of the human leg. We have used this capability to perform localized proton MR spectroscopy of the lipid composition of muscle in volunteers with a coil placed similar to 30 cm away from the region of interest. Spectra with a reasonable signal-to-noise ratio can be acquired in a clinically relevant data acquisition time of less than 5 min using the loop in transmit/receive mode, maintaining the full flexibility to acquire spectra from any part of the calf and/or thigh. If a local receive coil is used in combination with the remote transmit coil, then the signal-to-noise improves significantly, as expected. Magn Reson Med 63:297-302, 2010. (C) 2010 Wiley-Liss, Inc. Show less
A comparison of methods to decrease RF power dissipation and related heating in conductive samples using passive conductors surrounding a sample in a solenoid coil is presented. Full-Maxwell finite... Show moreA comparison of methods to decrease RF power dissipation and related heating in conductive samples using passive conductors surrounding a sample in a solenoid coil is presented. Full-Maxwell finite difference time domain numerical calculations were performed to evaluate the effect of the passive conductors by calculating conservative and rnagnetically-induced electric field and magnetic field distributions. To validate the simulation method, experimental measurements of temperature increase were conducted using a solenoidal coil (diameter 3 mm), a saline sample (10 mM NaCl) and passive copper shielding wires (50 mu m diameter). The temperature increase was 58% lower with the copper wires present for several different input powers to the coil. This was in good agreement with simulation for the same geometry, which indicated 57% lower power dissipated in the sample with conductors present. Simulations indicate that some designs should be capable of reducing temperature increase by more than 85%. (C) 2009 Elsevier Inc. All rights reserved. Show less
It is important to accurately characterize the heating of tissues due to the radiofrequency energy applied during MRI. This has led to an increase in the use of numerical methods to predict... Show moreIt is important to accurately characterize the heating of tissues due to the radiofrequency energy applied during MRI. This has led to an increase in the use of numerical methods to predict specific energy absorption rate distributions for safety assurance in MRI. To ensure these methods are accurate for actual MRI coils, however, it is necessary to compare to experimental results. Here, we report results of some recent efforts to experimentally map temperature change and specific energy absorption rate in a phantom and in vivo where the only source of heat is the radiofrequency fields produced by the imaging coil. Results in a phantom match numerical simulation well, and preliminary results in vivo show measurable temperature increase. With further development, similar methods may be useful for verifying numerical methods for predicting specific energy absorption rate distributions and in some cases for directly measuring temperature changes and specific energy absorption rate induced by the radiofrequency fields in MRI experiments. Magn Reson Med 63:218-223, 2010. (C) 2009 Wiley-Liss, Inc. Show less
