Second-order, high-temperature superconducting gradiometer for magnetocardiography in unshielded environment

Abstract

By employing high-temperature superconducting quantum interference device ͑SQUID͒ magnetometers, we have assembled a second-order gradiometer for magnetocardiography ͑MCG͒ in unshielded environment. With this high-temperature superconductor ͑HTS͒ SQUID system, we demonstrated its diagnostic relevance for MCG in terms of signal-to-noise ratio, spatial resolution, frequency bandwidth, rejection of environmental disturbances, and long-term stability. The electronically balanced gradiometer consists of three HTS radio-frequency SQUIDs with superconducting coplanar resonators, mounted in axial gradiometric arrangement with a baseline of 7.5 cm. The system achieves a common mode rejection for axial homogeneous fields of about 10 4 without any mechanical balancing, and a white noise about 130 fT/ͱHz at 77 K, with an 8ϫ8 mm 2 flux pickup area. MCG maps above volunteers' chests have been recorded in unshielded environment in a bandwidth of about 130 Hz. We showed the influence of several notch filters ͑suppressing the power line frequency͒ on the quality of the MCG signals. © 2000 American Institute of Physics. ͓S0003-6951͑00͒03307-6͔Magnetocardiographic ͑MCG͒ measurements using superconducting quantum interference device ͑SQUID͒ sensors are usually performed in magnetically shielded rooms, to reduce the influence of electromagnetic disturbances from the environment. 1,2 However, the high cost of such a room represents a major economic obstacle for the widespread application of magnetocardiography. Therefore, MCG gradiometer systems operating in unshielded environment would be highly desirable.High-temperature superconductor ͑HTS͒ SQUID gradiometers of adequate sensitivity ͑magnetic field resolution͒ and good disturbance rejection could offer a lower system and operating cost than their low-temperature ͑LTS͒ equivalents. Until present, first-and second-order HTS SQUID gradiometers, measuring axial or tangential fields, were demonstrated and MCG data recorded without magnetic shielding. [3][4][5] In these measurements, low-pass filters with a cutoff frequency of below 30 Hz were used to reduce the power line interference at 50 Hz, or a 60 Hz notch filter was used. 5 However, the frequency components of the MCG higher than 50 and 60 Hz contain significant information. 2,6 For clinical diagnostics, a MCG system has to have a bandwidth wider than 100 Hz, so that further development is warranted. Electrocardiographic ͑ECG͒ instruments have bandwidths of 100 Hz or more.In addition, a diagnostically meaningful map of magnetic field above the patient's chest has to be constructed of many sensing points ͑e.g., arranged in a 6ϫ6 grid͒. Hence, a sufficient long-term stability of measurement is required if sequential mapping is necessary. This is especially critical when using a single measuring point system. In this case, mapping can take, typically, about 40 min.In this letter, we present an improved second-order HTS SQUID gradiometer addressing the requirements listed above. To demonstrate the diagnostic relevance of this system,...

A simplified HTc rf SQUID to analyze the human cardiac magnetic field

“…Many impressive studies, recording animals, or fetus, or children, or adults MCG using HT c SQUID, have their own criterions for extracting valid signals under different environmental condition. [9][10][11][12]24,27 It is suggested that each system has its own characteristics. Since then, most efforts for early diagnosis of IHD and myocardial viability are still underway.…”

“…1,[3][4][5][6][7][8][9][10][11][12] The first-order axial gradiometer recording results are in less distortion than the second-order axial gradiometer with the same baseline for HT c SQUID MMCG system in a MSR. 1,6,12 The drawback of such gradiometers is impossible to keep in parallel with each other strictly. Considering the flexibility and stability of gradiometer, software gradiometer is an available way to obtain the high quality MCG signals that has been demonstrated by some impressive researches using various digital signal processing methods.…”

“…5,7,8 Now high performance high temperature SQUIDs (HT c ) can achieve a E-mail: zhangchen2010@pku.edu.cn, tfk616@sina.com, maping@pku.edu.cn, zizhaogan@pku.edu. magnetic field sensitivities of about a few tens of fT/Hz 1/2 in the white noise region, [9][10][11][12] which is sufficient for MCG sensor devices. From a practical point of view, HT c SQUID MCG systems have great advantages in lower cost, easier to handle and more widely available in hospitals compared to LT c SQUIDs.…”

HTS SQUID gradiometer using substrate resonators operating in an unshielded environment - A portable MCG system

“…This disturbance cannot easily be filtered because it lies within the major MCG signal frequency range. The use of a notch filter would either cause signal distortions or result in signal loss [4]. This spurious signal may have a large spatial phase change.…”

“…Reported were first and second order axial electronic rf SQUID gradiometers with mechanical balance adjustment [2], [3], and without [4], as well as dc SQUID gradiometers with planar first order gradiometric antennas [5]. However, in contrast to the presently tested concept, these instruments operated with a fixed baseline length and, in most cases, with conventional rather than adaptive balancing [6].…”