University of Houston • University of Houston-Clear Lake • ISSO Annual Report Y2002—pp. 105-108
Magnetic Superconducting Coils for Space and Clinical Applications
Jarek Wosik (UH), Maged R. Kamel (UH), Lian Xue (UH), Vivek Chavla (UH), Sarah Hirsch (UH), and Nathan Withers (UH)
Abstract
The development of a small MRI system has been identified by the Science Working Group of
the Human Research Facility on ISS as one of the most desired new research tools to be
used during long duration flights to document the changes in muscle volume and to assess
the effectiveness of in-flight muscle atrophy countermeasures. One of the limitations when
using surface probes for human imaging is their relatively small field of view. There are
two approaches to increasing the field of view of a probe. One is to increase the area
being imaged by switching among multiple coils, which are specially arranged to minimize
their mutual inductances. The second method is to simultaneously acquire the signals from
mutually isolated receiver coils. The latter approach, phased array, provides the enhanced
signal-to-noise ratio of small surface coils over a field-of-view normally associated with
large volume coils. UH researchers proposed to enhance SNR even further by using
superconducting phased arrays.
THE LIMIT OF OPERATION MAY NOT BE DEFINED BY SIGNAL-TO-NOISE ratio (SNR) alone but also by the time available for image acquisition. Until recently, further increase of the MRI acquisition speed was limited by the speed at which the field gradients could switch. However, hardware speed has increased to the point where the main limitations are now physiological. Faster gradient switching used for imaging or/and applying more rf power per unit/time causes nerve stimulation and heating. Two new image processing techniques, SMASH and SENSE (SENSitive Encoding), have thus been developed to overcome these limitations. These and related methods employ arrays of rf receiver coils and can reduce image acquisition time, in theory, by the number of array elements. Unfortunately, such methods have their own limitation. Specifically, for a given field of view, as the number of elements increases, the resulting smaller coil sizes reduce the signal-to-noise ratio (SNR).
UH researchers proposed to overcome this limitation and dramatically improve SNR with the use of high temperature superconductors (HTS) in elements of a radio-frequency (rf) array. Here we take advantage of the fact that, as the rf probe sizes become smaller, their resistive losses largely determine the overall system SNR. HTS materials nearly eliminate such losses.
In order to estimate the SNR gain due to the use of an HTS coil or a cold normal metal coil, we can examine the fields produced by the coil and nature of the losses in two cases. The room temperature normal metal coil (at T = 300K) must have the same size and shape as the cooled coils in order to produce an identical distribution of the electromagnetic field. Assuming for the extreme case that the coil resistance is close to zero, we can estimate a gain of SNR, shown in the following equation:
We can see from the equation that as the conductor resistance of a coil is negligible, the gain is dependent only on the ratio of Rc (coil resistance) and Rb, (body resistance) at room temperature. If the resistances are equal, the gain will be 42 percent (3dB). We consider this as the lowest limit of the gain sufficiently worthwhile for using a superconductor or cooled copper coil.
Phased arrays can be used to allow small coils to cover a large region of interest while preserving the improved SNR.
To see the effect of substituting a metal coil for a superconducting coil, we have compared the performance of a single 8×2-in. coil with four 2×2-in. coils covering the same area as the single coil. For purposes of simulation, the coils were aligned against a cylindrical phantom (diameter = 127 mm, length = 254 mm, with body conductivity _ = 0.7 S/m.) 5 mm away from its surface. A phantom model of such size is sufficient to simulate an MRI of a human neck or leg. Fig. 1 shows such a configuration.
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Figure 1. Shown are (a) a single 2×8-in. coil facing a cylindrical phantom and (b) an array of four 2×2-in. coils facing the same phantom, but electromagnetically isolated one from the other
The SNR was calculated for both HTS and normal metal arrays of four 2×2-in. coils. The result is shown in Figs. 2-4. For comparison, results of SNR for an 8×2-in. single coil that replaced the 4-coil array are also shown in Fig. 4.
Across section of the SNR distributions, 5 mm from the surface of the phantom cylinder, is displayed in Fig. 4, and a cross section perpendicular to the coil at the center is shown in Fig. 5. Peaks seen in the SNR surface plots near the coils are attributed to the higher sensitivity caused by the close proximity of the coils.
From the simulations, we obtain about a 70% gain (~4.6dB) in SNR near the surface adjacent to the array by substituting superconducting phase array coils, but only a 16% gain (~1.3dB) from the single coil.
Discussion and Conclusions
In recent years, the design of phased arrays for parallel acquisition [IN MRI APPLICATION]
has become the subject of a great deal of research. The drive for faster and faster
acquisition rates calls for arrays with a larger number of receiving elements. As the
number of array elements increases and their size continues to decrease, conductive losses
become more dominant. These losses can overwhelm any SNR gains expected from the use of
smaller coils that express less body noise. At room temperature, it has been shown that
the noise attributed to these increasing conductor losses can actually result in a lower
SNR at a given depth, during phased array acquisition.1-7 The use of cryogenically cooled
copper/HTS coils can extend the depth at which SNR gains can be achieved through phased
array acquisition.
The potential SNR gain using large arrays increases with the number of elements: SNR gain increased significantly when the single coil (N = 1) was replaced with four coils (N = 4), and it would increase more with eight coils (N = 8) or even sixteen (N = 16). Thus, the potential advantage of cryogenically-cooled receiving arrays with a large number of elements becomes even greater. These SNR gains can be used in consort with parallel imaging to achieve higher accelerations.
Figure 2. Simulations of signal-to-noise ratio (SNR) in the cross section provide a sensitivity map of the cross sections marked within the cylinders of Fig. 1(a). (See the 2×8-in. coil (a) in Fig. 1. This represents calculations for both copper and superconducting coils. The (a) map represents a 2×8-in. coil, fabricated from copper and kept at room temperature; The (b) map represents a 2×8-in. coil fabricated from superconducting materials and kept at 77 K.
Figure 3. Simulations of signal-to-noise ratio (SNR) in the cross section provide a sensitivity map of the cross sections marked within the cylinders of Fig. 1(b). See the four coils in Fig. 1(b). This figure represents calculations for both copper and superconducting coils. The (a) map represents four 2×2-in. coils, fabricated from copper and kept at room temperature. The (b) map represents four 2×2-in. coils fabricated from superconducting materials and kept at 77 K.
Figure 4. A comparison of SNR for each of the four cases, shown in Figs. 2 and 3, demonstrates the significant gain one can achieve by replacing normal metal coils with superconducting coils and by utilizing an array of four coils instead of one.
Figure 5. Relative SNR versus depth of the field of view is shown at the 0 point of the width of Fig. 4. Calculations are for both the 2×8-in. coil and the array of four 2×2-in. coils.
References
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6J. Wosik, F. Wang, L.-M. Xie, M. Strikovski, K. Nesteruk, M. Bilgen, and P. A. Narayana.
"High-Tc Superconducting Surface Coil for 2 Tesla Magnetic Resonance Imaging
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Publications
Cherukuri, P., J. Wosik, M. Naghavi, and J. T. Willerson. "Intravascular MRI Coils
for Imaging Atherosclerosis," Proc., 11th Annual Meeting of the International
Society for Magnetic Resonance in Medicine, Toronto, Canada, July 10-16, 2003.
Wosik, J., K. Nesteruk, L.-M Xie, J. A. Bankson, M. Kamel, J. D. Hazle. "A Novel 200
MHz Phased Array," Proc., 11th Annual Meeting of the International Society for
Magnetic Resonance in Medicine, Toronto, Canada, July 10-16, 2003.
Wosik, J., K. Nesteruk, L.-M Xie, M. Kamel, J. A. Bankson, J. D. Hazle, and M. Naghavi.
"SNR Gain of Cooled Surface Coils," Proc., 11th Annual Meeting of the
International Society for Magnetic Resonance in Medicine, Toronto, Canada, July 10-16,
2003.
Wosik, J., L.-M. Xie, K. Nesteruk, L. Xue, K. Maged, J. A. Bankson, and J. D. Hazle.
"Superconducting Phased Arrays for Research and Clinical MRI Applications," IEEE
Trans. on Applied Superconductivity 13.1(2) (2003).
Wosik, J., K. Nesteruk, L.-M Xie, J. A. Bankson, M. Kamel, M. Naghavi, J. D. Hazle, and J.
T. Willerson. "A Novel Intravascular Quarter-Wave Length Resonator for Imaging
Atherosclerosis," Proc., Annual Meeting of the International Society for
Magnetic Resonance in Medicine, Honolulu, Hawaii, May 20-24, 2002. 678.
Presentations
Wosik, J. "Superconducting Phased Array for MRI Application," 7th International
Conference on Materials and Mechanisms of Superconductivity and High Temperature
Superconductors, Rio de Jainero, Brazil, May 25-30, 2003.
Wosik, J., K. Nesteruk, L.-M. Xie, L. Xue, J. A. Bankson, and M. Naghavi.
"Superconducting 200 MHz ‘Phased’ Array for Magnetic Resonance Imaging
Applications," American Physical Society March Meeting, Austin, TX, March 3-7, 2003.
Xie, L.-M., J. Wosik, and P. Przuslupski. "Abnormal Dynamic Permeability of Nd1
-xSrxMnO3 Thin Films at Microwave Frequencies,"
American Physical Society March Meeting, Austin, TX, March 3-7, 2003.
Xue, L., J. Wosik, L.-M. Xie, and D. Chan. "Ferromagnetic Resonance of
Superparamagnetic Iron Oxide Nanoparticles for Biomedical Applications," American
Physical Society March Meeting, Austin, TX, March 3-7, 2003.
Funding and proposals
Miller, Jr., J. H. and J. Wosik. "Dielectric Spectroscopy for the Detection of
Biological and Chemical Warfare Agents." $198,525 feasibility study (April 23,
2003-Oct. 23, 2003) and $783,000, two years.
Investigative Team UH PI: Jarek Wosik, Ph.D., Research Associate Professor Graduate Research Assistant: Maged R. Kamel Graduate Student: Lian Xue Graduate Student: Vivek Chawla Undergraduate Student: Sarah Hirsch Undergraduate Student: Nathan Withers |
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Table of Contents
Institute for Space Systems Operations - Y2002
Annual Report
Copyright © 2003
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