A new type of gradiometer for the receiving circuit of magnetic induction tomography (MIT)

Abstract

Magnetic induction tomography (MIT) is a low-resolution imaging modality which aims at the three-dimensional (3D) reconstruction of the electrical conductivity in objects from alternating magnetic fields. In MIT systems the magnetic field perturbations to be detected are very small when compared to the excitation field (ppm range). The voltage which is induced by the excitation field in the receiver coils must be suppressed for providing sufficient dynamic range. In the past, two very efficient strategies were proposed: adjusted planar gradiometers (PGRAD) and the orientation of a receiver coil with respect to the excitation coil such that the net magnetic flow is zero (zero flow coil, ZFC). In contrast to the PGRAD no voltage is induced in the ZFC by the main field. This is advantageous because two comparatively high voltages in the two gradiometer coils can never be subtracted perfectly, thus leaving a residual voltage which is prone to drift. However, a disadvantage of the ZFC is the higher susceptibility to interferences from far RF sources. In contrast, in the gradiometer such interferences are cancelled to a high degree. We developed a new type of gradiometer (zero flow gradiometer, ZFGRAD) which combines the advantages of ZFC and PGRAD. All three systems were compared with respect to sensitivity and perturbation to signal ratio (PSR) defined as the ratio of the signal change due to a magnetic perturbation field at the carrier frequency and the signal change due to shifting a metallic sphere between two test points. The spatial sensitivity of the three systems was found to be very similar. The PSR of the ZFGRAD was more than 12 times lower than that of the ZFC. Finally, the feasibility of image reconstruction with two arrays of eight excitation coils and eight ZFGRAD, respectively, was shown with a single-step Gauss-Newton reconstructor and simulated measurement data generated for a cylindrical tank with a spherical perturbation. The resulting images show a clear, bright feature at the correct position of the sphere and are comparable to those with PGRAD arrays.

A Large and Quick Induction Field Scanner for Examining the Interior of Extended Objects or Humans

“…The flux density at the safety border toward the excitation side was set to the permissible SAR level of 10 µT at 1.5 MHz. Although a generally lower conductivity than 1% saline is known for human tissue [25], an upright standing human provides much more conducting volume to the measurement zone due to a more widely-extending y-direction. The obtained signal level from a person therefore was relatively strong and exploited the full linear scale of the receivers (Fig.…”

“…5(f) shows the calculated scanner signals from a single, small, and conductive cube without a conducting background For this scanner with moving test objects, the sensitivity for a local deviation inside an extended 3D object apparently does not depend strongly on the deviation's position. This finding is helpful, as static sensitivity fields for opposing coils tend to show heterogeneous sensitivity and obtain relatively weaker signals from a deviation in the middle of a conducting body (e.g., as shown in [3,25], and [26] and as seen for the sheet Fig. 5(e)).…”

The MAIN Shirt: A Textile-Integrated Magnetic Induction Sensor Array

“…To eliminate disturbance by the primary magnetic field, the measurement coil has to be precisely calibrated by a geometrical alignment (orthogonal or gradiometric arrangements) [7,8,16]. To overcome the necessity of this geometrical alignment and its problems regarding flexibility, the sensors integrated in the MAIN Shirt follow a slightly different approach using a single coil for field excitation and for field measurement.…”

Magnetic induction measurements with a six channel coil array for vital parameter monitoring

“…With a combination of two measurement coils as a (Planar/Axial)-Gradiometer (e.g. [5] and [6]), the measurement coils could be arranged in the same orientation as the excitation could but without measuring the excitation field. However, this requires an additional measurement coil and thus more space and volume of the array.…”