A new gradiometer scheme uses middle loops as sensing elements in low-field superconducting quantum interference device (SQUID) magnetic resonance imaging (MRI). This design of a second order gradiometer increases its sensitivity and makes it more uniform, compared to the conventional side loop sensing scheme with a comparable matching SQUID. The space between the two middle loops becomes the imaging volume with the enclosing cryostat built accordingly.

The Middle-Loop Sensing Gradiometer concept shows a schematic illustration of cryostat wall geometry (cut-off view). The imaging volume is in between two middle loops, outside the cryostat at room temperature.
For optimal coupling to SQUID, the inductance of the gradiometer must be matched to that of the SQUID input coil. Previously, a second-order gradiometer was designed with optimized wire shape and split middle loops to increase the turn number for increased sensitivity, and/or the size for a larger field of view while keeping the inductance matched. This design was described in “Optimized Geometry for Superconducting Sensing Coils” (NPO-44629), NASA Tech Briefs, Vol. 32, No. 1 (January 2008), p. 26.

In a typical configuration of a SQUID MRI, the sensitivity of a gradiometer is a rapidly decreasing function of the distance from the sensing loops. This results in severe non-uniformity of sensitivity and signal-to-noise ratio (SNR) in the image. This problem can be solved by using two second-order gradiometers positioned at the opposite sides of the imaging volume, with two SQUIDs, one per gradiometer. This is not cost-effective since SNR improves only by a square root of two at the center of the imaging volume.

The new design, depicted in the figure, uses a single second-order gradiometer where the middle loops are used for sensing. Both the SNR and the uniformity of the gradiometer are greatly improved. In this scheme, the space between the middle loops becomes the imaging volume with the enclosing cryostat built accordingly.

Because of the sensing middle loops at both ends of the imaging volume, the sensitivity at the center of the imaging volume is twice that of conventional geometry with the same SQUID noise. Only about half of the induced energy is lost in the non-sensing loops in the new scheme. The symmetric placement of the sensing loops gives more uniform sensitivity. There is no inductance matching penalty associated with the new configuration, because the geometry and the inductance remain to be that of a single second-order gradiometer.

This work was done by Konstantin Penanen, Inseob Hahn, and Byeong Ho Eom of Caltech for NASA’s Jet Propulsion Laboratory.

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Refer to NPO-45720, volume and number of this NASA Tech Briefs issue, and the page number.