An optimized geometry has been proposed for superconducting sensing coils that are used in conjunction with superconducting quantum interference devices (SQUIDs) in magnetic resonance imaging (MRI), magneto- encephalography (MEG), and related applications in which magnetic fields of small dipoles are detected. In designing a coil of this type, as in designing other sensing coils, one seeks to maximize the sensitivity of the detector of which the coil is a part, subject to geometric constraints arising from the proximity of other required equipment. In MRI or MEG, the main benefit of maximizing the sensitivity would be to enable minimization of measurement time.
In general, to maximize the sensitivity of a detector based on a sensing coil coupled with a SQUID sensor, it is necessary to maximize the magnetic flux enclosed by the sensing coil while minimizing the self-inductance of this coil. Simply making the coil larger may increase its self-inductance and does not necessarily increase sensitivity because it also effectively increases the distance from the sample that contains the source of the signal that one seeks to detect. Additional constraints on the size and shape of the coil and on the distance from the sample arise from the fact that the sample is at room temperature but the coil and the SQUID sensor must be enclosed within a cryogenic shield to maintain superconductivity.
One element of an approach to increasing sensitivity without appreciably increasing size is to increase the number of turns, subject to the requirement to maintain an impedance match with the SQUID sensor. On the other hand, simply increasing the number of turns also increases the self-inductance. One way to effect a substantial reduction in inductance without reducing the number of turns is to make the coil of a thicker wire and/or to shape the wire to other than a commonly available circular or square cross section, as described in "Improved Sensing Coils for SQUIDs" (NPO-44397), NASA Tech Briefs, Vol. 31, No. 10 (October, 2007), page 26. On the other hand, the allowable increase in size and change in shape of the wire is limited by the above-mentioned geometric constraints pertaining to enclosure in the cryogenic shield and distance from the sample.
Taking all of the aforementioned considerations into account, it was found that for both SQUID MRI and SQUID MEG, the optimum or nearly optimum coil geometry would be realized by constructing each turn of the coil in the form of a thin washer. Moreover, in the case of a four-turn coil in a second-order gradiometer arrangement (see figure), it would be beneficial to axially separate the two middle turns in order to further reduce the self-inductance. It has been estimated that for an MRI coil designed according to the present optimized geometry, the increase in sensitivity over that of the corresponding conventional wire-wound coil would make it possible to reduce measurement time by a half.
This work was done by Byeong Ho Eom, Konstantin Penanen, and Inseob Hahn of Caltech for NASA's Jet Propulsion Laboratory.
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Refer to NPO-44629, volume and number of this NASA Tech Briefs issue, and the page number.