We present methods to compute the imbalance in a gradiometer of arbitrary shape due to imperfections in its geometry, eddy currents induced in the radio-frequency interference shield, and screening currents induced in the modules of the superconducting quantum interference devices (SQUIDs). As an example, the methods are applied to evaluate the maximum expected initial imbalance of second-and third-order axial gradiometers in a measuring setup designed for fetal magnetocardiography. Mechanical imperfections in this specific setup appear to have the largest effect: the field imbalance is 2 10 2 ; the first-order gradient imbalance is 10 3 m; the second-order gradient imbalance is 10 4 m 2 . In the example, the imbalance caused by the other effects is one order smaller.
Absrruc+Two different concepts for gradiometer formation were tested applying high-temperature rf SQUIDS operated at 77 K in liquid nitrogen. All gradiometer systems are fully based on magnetometers. The first concept applies a compensating magnetometer at different positions to actively cancel the magnetic field at the location of other magnetometers. These magnetometers were arranged in an axial direction. I n parts, a system of superconducting plates was used to align the relative magnetic orientation of the magnetometers. The outputs of these sensors were used to form a highly balanced electronic gradiometer. The second concept is based on electronic noise cancellation. A set of three magnetometers arranged in an axial direction was used to form an electronic second-order gradiometer. Different types of reference systems based on HTS-SQUID magnetometers and fluxgate sensors were applied to the gradiometer signal for achieving a high common mode rejection of the environmental disturbances. The performance of the different systems is demonstrated in a magnetically unshielded environment as well as in a shielded environment and the common mode rejection of homogeneous magnetic fields is measured. To demonstrate the performance of the systems, biomagnetic measurements have been performed in shielded and unshielded environments.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.