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Volume Coil (volume + coil)

Distribution by Scientific Domains

Medical Sciences	100%

Selected Abstracts

Improved homogeneity of the transmit field by simultaneous transmission with phased array and volume coil

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2010
Nikolai I. Avdievich PhD
Abstract Purpose: To improve the homogeneity of transmit volume coils at high magnetic fields (,4 T). Due to radiofrequency (RF) field/tissue interactions at high fields, 4 T to 8 T, the transmit profile from head-sized volume coils shows a distinctive pattern with relatively strong RF magnetic field B1 in the center of the brain. Materials and Methods: In contrast to conventional volume coils at high field strengths, surface coil phased arrays can provide increased RF field strength peripherally. In theory, simultaneous transmission from these two devices could produce a more homogeneous transmission field. To minimize interactions between the phased array and the volume coil, counter rotating current (CRC) surface coils consisting of two parallel rings carrying opposite currents were used for the phased array. Results: Numerical simulations and experimental data demonstrate that substantial improvements in transmit field homogeneity can be obtained. Conclusion: We have demonstrated the feasibility of using simultaneous transmission with human head-sized volume coils and CRC phased arrays to improve homogeneity of the transmit RF B1 field for high-field MRI systems. J. Magn. Reson. Imaging 2010;32:476,481. © 2010 Wiley-Liss, Inc. [source]

High-field actively detuneable transverse electromagnetic (TEM) coil with low-bias voltage for high-power RF transmission

MAGNETIC RESONANCE IN MEDICINE, Issue 6 2007
Nikolai I. Avdievich
Abstract The design and construction of a 4T (170 MHz) transverse electromagnetic (TEM) actively detuneable quadrature head coil is described. Conventional schemes for active detuning require high negative bias voltages (>300 V) to prevent leakage of RF pulses with amplitudes of 1,2 kW. To extend the power handling capacity and avoid the use of high DC bias voltages, we developed an alternate method of detuning the volume coil. In this method the PIN diodes in the detuning circuits are shorted when the RF volume coil is tuned, and negatively biased with ,12 V when the coil is detuned. To preserve the high QU/QL ratio of the TEM coil, we modified the method of Nabetani and Watkins (Proceedings of the 13th Annual Meeting of ISMRM, Kyoto, Japan, 2004, abstract 1574) by utilizing a high-impedance (,200 ,), lumped-element, quarter-wavelength transformer. A QU of 500 was achieved for the detuneable TEM, such that incorporation of the detuning network had minimal effect (<1 dB) on the performance of the coil in vivo. Magn Reson Med 57:1190,1195, 2007. © 2007 Wiley-Liss, Inc. [source]

A method for interleaved acquisition of a vascular input function for dynamic contrast-enhanced MRI in experimental rat tumours

NMR IN BIOMEDICINE, Issue 3 2004
Dominick J. O. McIntyre
Abstract Dynamic contrast-enhanced MRI is widely used for the evaluation of the response of experimental rodent tumours to antitumour therapy, particularly for the newly developing antiangiogenic and antivascular agents. However, standard models require a time-course for the plasma concentration of contrast agent (usually referred to as the arterial input function) to calculate the transfer constant Ktrans from the dynamic time-course data. Ideally, the plasma concentration time-course should be measured during each experiment to obtain the most accurate measure of Ktrans. This is technically difficult in rodents, so assumed values are generally used. A method is presented here using interleaved acquisitions from a tail coil to obtain the plasma concentration simultaneously with DCE-MRI data obtained from a solenoid coil around the tumour. The SNR of the resulting vascular input function data is high compared with methods using a volume coil to acquire plasma concentrations from the aorta and vena cava. Copyright © 2004 John Wiley & Sons, Ltd. [source]