Optimizing surface coils and the self-shielded gradiometer

Abstract

A computational method for optimizing both single- and two-layer surface coils is presented that is relatively easy to implement in practice using either a variational approach or through standard numerical matrix diagonalization techniques. The technique is applied, in particular, to develop a self-shielded gradiometer that is relatively immune to radiofrequency (rf) interference from distant sources with a minimal compromise to its ability to sense rf from nearby sources, properties necessary for nuclear quadrupole resonance detection of buried land mines. Results of the optimization procedure are evaluated using finite-element numerical techniques and measurements on prototype coils for a number of configurations. These results show that practical coils can be constructed that approximate the theoretically optimized coils quite well. In addition, the trade off between the surface coil sensitivity and noise immunity is presented for the self-shielded gradiometer configuration.

Mobile single-sided NMR

“…Figure-eight coils are gradiometer coils by their very nature. They are insensitive to far field noise, while simple current loops are not, but two and more can be arranged coaxially to form gradiometer coils with B 1 perpendicular to the face of the coil [231][232][233][234].…”

The noise immunity of gradiometer coils for14N NQR land mine detection: Practical limitations

“…This must be done, of course, without a significant decrease of the coil Q. Initial work along these lines is discussed elsewhere [15]. In that work, consideration is also given to the coil's immunity to the presence of near-field (and possibly even near-coil) objects.…”

TNT detection with 14N NQR: Multipulse sequences and matched filter

“…The noise from external sources is dominant in remote detection, but can be successfully removed with gradiometric coils or additional antennas [14][15][16]. The internal noise however, mainly the coil's thermal noise, will remain, and its size is usually much bigger than the detected signal [2].…”