First-order SQUID gradiometer with electronic ...

First-order SQUID gradiometer with electronic subtraction for magnetocardiography

Baltag Octavian

Rau Miuta Carmina

Faculty of Medical Bioengineering UMF ”Grigore. T. Popa” Iasi, Romania, [email protected]

Faculty of Electrical Engineering ”Gh. Asachi” Technical University of Iași Iasi, Romania [email protected]

Abstract—The paper presents theoretical and experimental results of the research concerning the design and properties of a 1st-order SQUID gradiometer with electronic subtraction and the recording of the first magnetocardiogram (MCG) performed in Romania, at the Bioelectromagnetism Laboratory of the Faculty of Biomedical Engineering. The MCG recording was possible by using a complex installation composed from a non ferromagnetic shielded room placed in the centre of the large (4 x 4 x 4) m, triaxial Helmholtz coils system. The Helmholtz system ensures the static compensation of the geomagnetic field and the active shielding too, by using the magnetic field negative feedback. . Magnetocardiography (MCG) is a noninvasive and risk-free technique for contactless surface mapping of the magnetic fields generated by the electrical activity of the heart. The difficulty of recording of a magnetocardiogram is determined by the very low value of biomagnetic field (10-15- 10-12)T. Even the strongest of such biomagnetic fields, the one produced by the human heart, barely reaches 100 pT outside the thorax, less than a millionth of the geomagnetic field value and orders of magnitude weaker than environmental magnetic noise fields. In this context, the detection and recording of these fields require an extremely sensitive device such as SQUID biomagnetometer and special conditions. Performances of the installation are presented. Keywords -magnetocardiography, magnetically shielded room, SQUID gradiometer, biomagnetic fields.

I.

INTRODUCTION

The magnetocardiography (MCG) is a non-invasive technique used to detect and to measure at the surface of human body the magnetic field generated by the heart electric activity. The graphical recording of the heart magnetic field variations is named magnetocardiogram. The maximum value of the heart magnetic field is about 10-10T for an adult and about 0.5•10-12T for a foetus, i.e. values much smaller than the geomagnetic field value (5•10-5T) [1], [2]. Whereas the magnetic fields produced by the electric activity of the living organisms are extremely weak, their detection is carried out under special conditions. These imply the diminution, within the working environment, of all the fields that might interfere with the measured field, as well as the utilization of high resolution investigation equipment, as SQUID device, the standard detector of the biomagnetic fields. The creation of an

environment with almost zero magnetic activity can be accomplished by various means: shielding (passive shielding) [3], [4], compensation (active shielding, active magnetic screen, active compensation) [5] – [13] or mixed systems consisting of shielded rooms with walls made of permalloy, mumetal or aluminium layers, together with coil systems that compensate and control the external magnetic field [14] – [18]. The first recording of the heart magnetic field using a SQUID magnetometer and a magnetically shielded room were carried out in 1969 by Cohen, Edelsack and Zimmerman. The recorded magnetocardiographic signal went beyond the expectations, being some order better than that recorded with coils [19]. The first SQUID biomagnetometers had 1 – 7 channels and a spatial resolution within the range 3 mm – 5 cm [20], needing a longer time to obtain the magnetic field distribution map. The utilization of multi-channels SQUID systems (the number of channels exceeds nowadays some hundreds) resulted in the improvement of investigation conditions and more credible data [21] – [23]. There have been also created systems which make use of superconducting gradiometers of the first and the second order, which permitted to carry out biomagnetic measurements in magnetically unshielded rooms. Zimmerman made the first SQUID gradiometer for MCG measurements in unshielded rooms [24], these being later experimented by other research groups from Germany, Japan, China [25] – [28]. There are also systems that work in unshielded environment [29]. II.

METHOD AND MATERIALS

The ferromagnetic shielding and the active compensation are standard methods used at low frequency. At high frequency, the shielding method (eddy current absorption) makes use of high electric conductivity materials [30]. For shielded rooms made of ferromagnetic material, the shielding is accomplished due to the fact that the magnetic flux prefers the path with the highest value of the magnetic permeability. In the case of shielded rooms made of non-ferromagnetic materials (Cu, Al), the shielding is based on the Lenz law. The time variable electromagnetic field induces in the conducting material eddy currents which, in turn, generate a field opposed to the external field. Simultaneously, energy absorption through eddy currents takes place. In the frequently used mixed

systems, consisting of the shielded room and the active compensation systems with orthogonal coils, the field control is carried out with vectorial magnetometers. The assembly consisting of coil system, magnetometer and a power electronic circuit, works in a negative feedback loop, performing the external field compensation down to values which permit the accurate operation of a SQUID biomagnetometer In order to detect the heart magnetic field and to record a magnetocardiogram, the chosen solution was to elaborate a mixed system, consisting of a shielded room made of nonferromagnetic materials, and a triaxial Helmholtz coils system for external magnetic field compensation and dynamic control. The system is located in the sole Laboratory of Bioelectromagnetism existing in the country, within the Faculty of Medical Bioengineering, Iasi. The installation for biomagnetometry measurements consists of a shielded room (3x2x2) m made of non-ferromagnetic material (12 mm thick aluminum plates), located in the centre of a triaxial Helmholtz coil system, Fig. 1.

The magnetometer sensors are connected such that to obtain three channels for the 1st order gradient: G1 =

dB( z ) dz

(1)

and one channel for the 2nd order gradient: G2 =

d 2 B( z ) dz 2

(2)

with electronic subtraction. The SQUID gradientmeter accomplished like this is installed in the center of the shielded room and of the Helmholtz coil system, above a bed that can be moved in a horizontal plane. Fig. 2 presents the three SQUID sensors vertically oriented on z direction for detecting the B (z) component of the magnetic field. By electronic subtraction of the output of the three sensors on realize a complex axial gradiometer composed from: •

two 1st order gradiometers with short baseline, ∆z = 4 cm

•

one 1st order gradiometer with long baseline, ∆z = 8 cm

•

one 2nd order gradiometer with short baseline, ∆z = 4 cm.

The first order gradients are expressed by the relations:

G11 = Figure 1. Outside and inside view of the biomagnetometry installation

The triaxial Helmholtz coil system permits to compensate for the low frequency magnetic fields along the three directions. Each coil has two windings. The first winding is used for the manual compensation of the continuous component of the vector magnetic field along the three directions (X, Y, Z) within the range ±40 A/m. The second winding serves to compensate the alternating and slowly variable components of the magnetic field within the range ±40 A/m, making use of a system controlled by a triaxial magnetometer which operates in the negative feedback loop [31], [32]. The utilized biomagnetometer is of HTS SQUID type. This was converted to a SQUID gradientmeter for measuring the vertical component of the magnetic field gradient, Fig. 2. In order to reduce the interference of the environmental noise, gradiometric sensor arrangements are commonly used.

G2 =

B z1

SQUID Electronic unit

Electronic subtraction

G31



G11

G2

(4)

•

geomagnetic field with levels about 3.7x10-5 T (vertical component),

•

slowly variable magnetic fields produced by moving vehicles in the area surrounding the laboratory or daily variations of the geomagnetic fields, during intense magnetic storms these fields can reach intensities of about 0,5µT.

•

alternating magnetic fields with the frequency of 50 Hz with variable intensity within the range (10 ÷ 200) nT;

•

even and odd harmonics of the mains frequency having a lower intensity than the fundamental frequency

Low pass filter 38 Hz MCG

Figure 2. SQUID gradiometer

Bz1 − 2 Bz 2 + Bz 3 ∆z

After electronic subtraction, the gradient signals are filtered through of low-pass filter with the cut-off frequencies of 38 Hz, 60 Hz and 120 Hz. The gradientmeter pass-band is DC ÷ 5 kHz, with the adjusting possibility within the ranges: 0.3 Hz, 500 Hz, 1000 Hz and 5000 Hz. The noise of the SQUID sensor is 50 fT / Hz1/2 at the frequency of 1000 Hz. The global sensitivity of the gradientmeter is 14.4⋅10-9 T/V. The gradientmeter operates within an electromagnetic environment characterized by continuous and alternating magnetic fields:

G21 Electronic subtraction

4 cm

Bz 2


4 cm


(3)

and the second order gradients are given by:

SQUID sensors

Bz 3

Bz1 − Bz 2 , 1 Bz 2 − Bz 3 , 1 Bz1 − Bz 3 G3 = G2 = ∆z 2∆z ∆z

In order to improve the signal to noise ratio of a magnetocardiogram, it is recommended to carry out

measurements during the time intervals when the daily geomagnetic activity is minimum, and the geomagnetic disturbances due to the human activity are also minimum (during night time). III.

electronic data processing is recommended, using standard methods for signal filtering and averaging.

EXPERIMENTAL RESULTS

In order to carry out measurements meant to record a magnetocardiogram, the following operations were executed: •

diminish the geomagnetic field down to the level of some nanotesla, by injecting currents in the triaxial Helmholtz system;

•

activate the system for automated compensation of the variable magnetic field through negative feedback;

•

calibrate the SQUID gradientmeter using a standard Helmholtz coil as a field and gradient generator.

The subject is lying in supine position on the mobile bed, with the SQUID gradientmeter fixed over the subject’s thorax; the gradientmeter’s sensor, which is situated in its lowed part, is located at a distance of 2 cm from the thorax. Simultaneously with magnetocardiogram recording, an electrocardiogram was executed, with standard derivations. The utilized electrodes are non-magnetic. The magnetograms were measured over regions which correspond to the anatomical landmarks of the chest: the 5th left intercostal space, on the mid-clavicular line and in the xiphoid process zone. Figure 3 presents a magnetocardiogram and an electrocardiogram, simultaneously recorded, from an adult subject of 66, without pathological cardiac antecedents For this recording, the sensor was positioned over the lower region of the thorax, in front of the xiphoid process, with the specification that the subject was in apnea after forced inspiration. This region was chosen because the component of the magnetic field produced by the cardiac dipole, perpendicular to the thorax, has the largest value. The thorax motions during breathing can determine the shift of the position of the heart magnetic moment through the spatial displacement of the heart inside the thorax; this shift of the heart magnetic moment determines the modulation of the MCG signal. For this reason, the patient presented an episode of controlled apnea during recording. The recording was carried out opposite to the right xiphoid process in apnea condition. The gradient of the measured field has the value of 21.6 pTpp. The noise level is of 10 pTpp. The noise spectrum contains both even and odd harmonics of the mains frequency: 100 Hz and 150 Hz. The gradientmeter frequency band is DC ÷ 5000 Hz. The signal level at the gradientmeter output was of 21, 6 pTpp. In order to record the magnetocardiogram, an electronic filter was used with the bandwidth of (0.05 – 38) Hz. On the filtered magnetocardiogram, one can notice the existence of the QRS complex, as well as of the P and T waves corresponding to a standard electrocardiogram and it can noticed the temporal coincidence of these two signals. The filtered magnetocardiogram is presented in Figure 3. In order to improve the magnetocardiogram recording and interpretation,

Figure 3. Unfiltered and filtered MCG signals and standard ECG signal

IV.

CONCLUSIONS

The magnetogram recording was possible by the realization of a complex biomagnetometric installation which provides environmental conditions necessary to the operation of the SQUID biomagnetometer. The magnetoeneter was converted in a first order gradiometer with electronic subtraction, to improve the signal to noise ratio. The global sensitivity of the gradientmeter is 14.4⋅10-9 T/V. The gradient of the measured field has the value of 21.6 pTpp. The noise level is of 10 pTpp. The signal level at the gradientmeter output was of 21, 6 pTpp. The recorded magnetocardiographic signal corresponds to the ECG signal from both morphological and temporal points of view. The recording was carried out on the anterior thorax, in apnea condition, both the resistivity, position of the lungs also the orientation and location of the heart change during the respiratory cycle and these changes of the heart magnetic moment determines the modulation of the MCG signal. On the filtered magnetocardiogram, one can notice the existence of the QRS complex, as well as of the P and T waves, and the temporal and morphological coincidence between MCG signal and ECG signal.

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