RU2720055C1 - Multichannel diagnostic system - Google Patents

Abstract

FIELD: registration of electromagnetic fields.

SUBSTANCE: multichannel diagnostic system includes a laser source of linearly polarized radiation of optical pumping with an optical multichannel splitter of pumping radiation; radiofrequency field generator; radio-frequency field coil; at least two atomic magnetometric sensors, each of which includes a polarization-supporting optical fibre for transmitting pumping radiation; a sensitive element comprising a circular polarization of pumping radiation; a working cell, a non-magnetic heater, a photodetector, a photocurrent amplifier; synchronous detector; and an integrator controlling the low-frequency pumping radiation modulator, and a multichannel signal processing and recording circuit. Atomic magnetometric sensors are installed so that angle between any two said sensors does not exceed 20°, and their orientation relative to direction of magnetic field vector provides maximum sensitivity for given type of sensors, wherein all sensors are in common radio-frequency field. Optical pumping circuit of each of said sensors includes pumping power control and light shift of magnetic resonance frequency low-frequency modulator, installed either directly after optical multichannel splitter before transmitting radiation optical fibre, or at the input of sensitive element after radiation transmitting optical fibre.

EFFECT: technical result consists in improvement of efficiency of diagnostic system due to increased accuracy of detection.

4 cl, 3 dwg

Description

The invention relates to medicine and neuroscience, and is intended for non-invasive registration of electromagnetic fields generated by activity of the human brain, with subsequent localization of active areas on the cerebral cortex for the diagnosis of a wide range of neurological disorders and the study of the principles of functioning of the human brain.

Currently, the urgent task is to study, interpret and model the principles of the brain with subsequent use for the diagnosis and development of treatment methods and preventive therapy of neurodegenerative diseases. The solution of these problems is impossible without functional neuroimaging techniques, the unique representative of which is magnetoencephalography (MEG). MEG allows non-invasively, i.e. without violating the integrity of the tissues, register the electrical activity of the brain with a temporal resolution of the order of units of milliseconds and a sub-centimeter spatial resolution. Currently, the MEG as the only technique that allows non-invasively and with reasonable accuracy to localize non-stationary neuronal sources is the de facto gold standard for non-invasive diagnostics of epilepsy in clinics with access to this equipment. In addition, MEG is actively used in the pharmacological industry to assess the effect of drugs on the functional state of the central nervous system, and, in contrast to functional MRI (fMRI), allows to study the fine spatio-temporal structure of changes in the electrical activity of the brain caused by drugs, both desired and secondary

Currently, in general, there are two trends in the development of non-invasive neuro-mapping techniques based on biomagnetometric information. The first is based on the construction of multichannel systems based on atomic magnetometers operating in a mode free from spin exchange broadening (SERF (Spin Exchange Relaxation Free)). The second and more traditional direction is the use of superconducting quantum interferometers (SQUID (Superconducting Quantum Interference Device). Also, the nearest technologies have been identified, but not related to SERF or SQUID sensors.

However, a significant drawback of SQUIDs used in modern multichannel magnetoencephalographs (from the Superconducting Quantum Interference Sensor) operating at ultra-low (helium) temperatures is the inability to bring them closer to the object of study at a distance shorter than the wall thickness of the Dewar vessel (~ 3 cm). The inability to adapt the geometry of the Dewar vessel to the shape and size of the subject’s head leads to even greater growth and uneven distribution of the sensor-brain distance and significant losses in the signal-to-noise ratio when registering the activity of neuronal sources. In addition, SQUID systems can only be used in expensive magnetically shielded rooms, the cost of which starts at 400 thousand US dollars, and require weekly refueling with expensive liquid helium, which makes the operation of such systems unacceptably expensive for the vast majority of clinical institutions.

The application of quantum optics technologies to magnetometry has led to the creation of new promising schemes that are comparable in sensitivity to SQUID systems and even surpass the latter. High sensitivity (~ 3-10 fTl / √Hz), small size, the ability to work in a wide range of external magnetic fields, up to the earth, the lack of need for deep cooling (for their operation it is enough to provide relatively little heat) open up the possibility of using these devices for non-invasive (without violating the integrity of tissues) registration of the electrical activity of biological objects, including the brain.

The construction of integrated multichannel magnetoencephalographic systems with adaptive geometry of an array of atomic magnetometric sensors (AMD) will allow us to achieve a new qualitative level of non-invasive functional neuroimaging. The resulting technology will increase the accuracy of preoperative mapping and will allow to reach a qualitatively new level in solving problems of identifying foci of epileptic activity, providing the possibility of more detailed differential diagnosis and planning of surgical intervention in the treatment of pharmacologically resistant epilepsy.

Atomic magnetometers with maximum sensitivity operate in the so-called SERF (Spin Exchange Relaxation Free) mode and, therefore, require ultra-weak external magnetic fields with the highest degree of spatial uniformity, which is currently achieved through the use of magnetically insulating cameras in combination with additional compensating coils located in close proximity to the subject. In order to overcome this obstacle, we conducted a study, as a result of which a laboratory prototype AM was developed, capable of working in nonzero fields (10-100 μT) and providing ultimate sensitivity comparable to SQUID.

Closest to the claimed invention is the well-known German patented solution of an optically pumped magnetometer, which allows its use in arrays of sensors in which there is no mutual influence of a separate magnetometer, and the method of operation of the device, including an array of magnetically pumped magnetometers. (German application No. 102013004385, IPC G01R 33/032; G01R 33/26, published on 09/19/2013).

The disadvantages of the known solution are its complexity and high cost, since its implementation requires the use in each channel of expensive and requiring fine adjustment of the acousto-optical (AOM) or electro-optical (EOM) modulator (indicated in the IM patent), providing 100% modulation at a Larmor frequency of tens or hundreds of kilohertz.

The objective of the claimed invention is to eliminate the disadvantages of known devices and systems and create a multi-channel diagnostic system for a new generation magnetoencephalograph (MEG), which allows non-invasively recording the electrical activity of the human brain with previously unavailable spatial and temporal accuracy. A new technology for measuring ultra-weak magnetic fields, which does not require the use of cryogenic equipment, will significantly reduce the cost of the device, make it accessible to a wide range of clinical and research institutions and provide superiority in the tasks of diagnosing neurological disorders and in cognitive research.

Thus, the technical result is to increase the efficiency of the diagnostic system by increasing the accuracy of detection and providing the possibility of operation of an atomic magnetometric sensor as part of a multichannel diagnostic system without interfering with the operation of nearby similar sensors.

The technical result is achieved due to the fact that in a multi-channel diagnostic system, including a laser source of linearly polarized optical pump radiation with an optical multi-channel splitter of pump radiation; RF field generator; RF coil; at least two atomic magnetometric sensors, each of which includes polarization-supporting optical fiber for transmitting pump radiation; a sensitive element comprising a circular polarizer for pump radiation; working cell, non-magnetic heater, photodetector, photocurrent amplifier; synchronous detectors; and integrators controlling low-frequency modulators of pump radiation (according to the number of sensors), and a multi-channel signal processing and recording circuit, atomic magnetometric sensors are installed so that the angle between any two of these sensors does not exceed 20 °, and their orientation with respect to the direction of the magnetic vector field provides the maximum sensitivity for this type of sensor, and all sensors are in a common radio frequency field, and the optical pump circuit of each of these sensors includes directs pump light power and the frequency shift of the magnetic resonance low frequency modulator mounted either directly after the multi-channel optical splitter optical fiber before the transmission light, or a sensor input radiation after transmitting fiber.

In addition, the low-frequency modulator may be, in particular, liquid crystal.

The system also includes a linearly polarized detection radiation laser source with an optical multi-channel detection radiation splitter, an atomic magnetometric sensor includes polarization-supporting optical fiber for transmitting detection radiation, and a sensitive element includes a balanced detector for changing the polarization angle of detection radiation;

In addition, the sensing element includes a coil of the radio frequency field.

The technical result, the achievement of which is ensured by the implementation of the entire set of essential features of the claimed invention, consists in eliminating interference in the operation of similar sensors located nearby, which is achieved by the following solutions:

1) the use of a resonant radio-frequency (RF) field common to all sensors, having a constant frequency, or tied in frequency to a magnetic field, measured by one or more sensors of a multichannel diagnostic system;

2) dynamic compensation of the shift of the center of the magnetic resonance line (occurring in each sensor due to variations in the local field, and proportional to these variations), carried out by closing feedback to control the intensity of the pump light, and as a result, the magnitude of the light shift of the magnetic resonance line.

3) the measurement of the magnitude of the magnetic field variations is made by measuring the variations in the pump power; in the case of using a low-frequency (0-100 Hz) intensity modulator (LCD liquid crystal modulator), the magnitude of the magnetic field variations is measured by measuring the variations of the LCD control signal.

The essence of the invention is illustrated by drawings, where

in FIG. 1 is a simplified block diagram of a classical atomic magnetometer (AM);

in FIG. 2 is a diagram of a known atomic magnetometric sensor (AMD) operating in the MEG;

in FIG. 3 shows an example of an atomic magnetometric sensor (AMD) circuit operating as part of an MEG for the claimed diagnostic system.

The figures in FIG. 1-3 include the following items:

1 - pump source (laser),

2 - circular polarizer,

3 - working cell,

4 - RF field coil,

5 - photodetector with amplifier,

6 - synchronous detector,

7 - integrator

8 - voltage controlled oscillator,

9 - liquid crystal (LCD) modulator.

In the description, the authors use the following concepts.

A magnetometric sensor (MD) is a device designed to measure magnetic field parameters or variations of these parameters.

A magnetometric sensor designed to operate as part of a magnetoencephalographic system (MEG) must be capable of measuring variations in the module or component of the magnetic field in the band 2 ÷ 100 Hz and with a variational sensitivity of 10 ÷ 20 fTl / √Hz; sensitivity requirements for magnetocardiographic systems (MCH) are an order of magnitude lower.

A magnetometer is a device that includes one or more MDs, control circuits, and recording readings.

A multichannel diagnostic system (MDS) is a collection of several MDs whose sensors are located as close to the surface (in the general case - curvilinear) of the studied object. The purpose of the MDS is to measure the local temporal variations of the magnetic field generated by the studied object in the locations of the sensors in a certain frequency band with a spatial resolution of <1 cm.

When using atomic magnetometric sensors (AMD) in multichannel diagnostic systems, namely, magnetoencephalographs (MEG) and magnetocardiographs (MAG), an important condition is their ability to work in an array of several (in the case of MEG - up to two hundred) nearby sensors, without interfering adjacent sensors. It is this arrangement of sensors that allows you to register spatially high-frequency harmonics of the magnetic field generated by neuronal sources, and thus provides a high spatial resolution for mapping brain functions.

The classical scheme of an atomic magnetometer (AM) is unsuitable for use in multichannel systems. Due to the heterogeneity of the magnetic field in the studied frequency region, the RF fields of different atomic magnetometric sensors (AMD) will differ. Since the radio-frequency (RF) field created around the AMD cell cannot be localized in the space of this cell, it will affect neighboring sensors, which will inevitably lead to their mutual influence.

The principle of operation of an atomic (or quantum) magnetometric sensor (AMD) with optical pumping is based on the ability of an atomic magnetic moment to precess in a magnetic field, like a top in the Earth's gravitational field.

The frequency ω _{0 of the} precession of the magnetic moment (the so-called Larmor frequency, or Larmor frequency) in the first approximation is proportional to the magnetic field induction B:

where γ is the gyromagnetic ratio, for cesium atoms (Cs) it is 2π⋅ (3.5 Hz / nT).

This value is a constant, which determines the absoluteness of AMD readings. However, the resonance frequency can be additionally shifted by external factors, such as pump radiation interacting with atoms.

In FIG. 3 shows an example of an atomic magnetometric sensor (AMD) circuit operating as part of an MEG for which the claimed invention is implemented. An atomic magnetometric sensor includes a pump source (laser) 1, a circular polarizer 2, a working cell 3, a coil of radio-frequency (RF) field 4, a photodetector 5 with an amplifier, a synchronous detector 6, an integrator 7, a voltage-controlled oscillator 8, a liquid crystal (LCD) modulator 9.

The sensitive element of AMD is a transparent (glass) cell 3 filled with paramagnetic (usually alkaline) atoms in the gas phase. To prevent the collision of atoms with the cell walls, an inert gas is added to it, or the cell walls are coated with a special coating from the inside.

AMD work includes the following steps:

1. Optical pumping by a circularly polarized resonant pumping light longitudinal with respect to the measured magnetic field is a process that builds the precession axis of all atomic magnetic moments along the magnetic field. As a result of pumping, all atomic moments precess around one axis, but with random phases.

2. Phasing of the precession of atomic magnetic moments, that is, matching the phases of their precession with the help of a resonant (that is, oscillating with a frequency ω≈ω ₀ ) radio-frequency (RF) field. As a result of phasing, all atomic moments precess around one axis with a frequency ω≈ω ₀ and the same phase determined by the phase of the radio field; a nonzero total rotating transverse magnetization of the medium appears.

3. Optical detection of rotating transverse magnetization of the medium by transverse light detection. Depending on the polarization of the detecting beam and the degree of its detuning from the center of the optical absorption line, interaction with rotating moments will either modulate the intensity of the detecting beam at a frequency ω≈ω ₀ , or will cause its polarization plane to swing at the same frequency. There are simplified (single-beam) schemes in which the pump and detection beams are combined into a single beam directed at 45 ° to the magnetic field. The algorithm of their work does not differ from that described above. There are also variations of the above-described circuit, in particular, SERF zero-field magnetometers, which also use a single-beam circuit, but which record the signal not at the Larmor frequency, but at the frequency of the external modulating field.

To measure the magnetic field, it is necessary to precisely bind the frequency of the radio frequency (RF) field ω to the Larmor frequency ω ₀ . After that, knowing ω ₀ , the induction of the magnetic field is calculated by the formula (1).

The binding is as follows: the modulation signal of the detecting beam (also called the magnetic resonance signal) at a frequency ω is detected by a synchronous detector; an RF field is used as a reference signal during detection.

With the right choice of the detection phase, the error signal that occurs when the frequency of the RF field is tuned from ω to ω ₀ has the form of a dispersion curve:

Where

Δω = ω-ω ₀

Ω is the Rabi frequency of the resonant RF field, proportional to its amplitude,

T ₁ , T ₂ - the longitudinal and transverse relaxation times of atomic moments;

k is a coefficient depending on the parameters of the system - the intensity of the pump radiation and detection, temperature, etc.

Formula (2) describes one projection of the magnetic resonance line. In the classical AM circuit, feedback (OS) is closed by the error signal S (Δω). The error signal is used to control the frequency of the RF field (Fig. 1); The OS constantly maintains the equality S (Δω) = 0, thereby ensuring that the condition Δω = 0 is satisfied. Thus, in this scheme, the frequency of the RF field ω is always equal to the Larmor frequency ω ₀ , and is proportional to the modulus of the magnetic field.

To solve this problem, the authors propose to use the property of pump radiation to shift the frequency of magnetic resonance ω ₀ (the so-called light shift of magnetic resonance). The magnitude of this shift is proportional to the intensity of the pump radiation. The magnitude of the light shift is especially strong in compact circuits using the effects of suppression of spin-exchange broadening in zero fields (SERF), and in nonzero fields in the “extended” states (stretched state). In particular, in the most efficient optical pumping schemes from the ground state (F-1/2) level and when interrogating the ground state (F + 1/2) levels, the typical values of the light shift by pump radiation (reduced to the magnetic field) are tens and hundreds NT This leads to the need to stabilize the pump intensity at the level of 10 ^-6 ÷ 10 ^-7 , which in itself is a non-trivial technical task. By including the light shift in (2), redefine Δω:

Where

Δω - frequency offset, defined above, under the formula (2)

I _p is the intensity of the pump radiation,

k _sh is the light shift coefficient, the value of which is almost constant at a given resonance line width and pump radiation frequency.

From (2) and (3) it follows that until the change in the pump intensity begins to destructively affect the AMD sensitivity, one can use the control of the light shift I _p ⋅k _sh to zero the error signal S (Δω). In this case, the feedback instead of the frequency of the RF field ω will control the intensity of the pump light I _p (Fig. 3). Since the dependence of the sensitivity of AMD on I _p has an optimum in which the derivative of sensitivity with respect to I _p is zero, small (within ± 10%) variations in intensity relative to the optimal value will not lead to any noticeable decrease in the sensitivity of AMD.

Since the spatial and temporal variations of the magnetic field in the region of a multichannel system in a magnetically shielded room do not exceed ± 10 nT, and typical light shifts of the magnetic resonance line are ~ 10 nT / mW, this method is particularly well applicable to highly sensitive AMD with compact (less than 1 cm ³ ) cells. Such AMDs use optical pump intensities of the order of 10 mW per sensor, and a 10% change in the pump light intensity (i.e., 1 mW) will allow us to reliably compensate for the temporal and spatial variations of the magnetic field in the region of the multichannel system, and in so doing will lead to deterioration in sensitivity not exceeding 2%.

Thus, during the operation of the MEG system, all AMD work in a common RF field, the frequency of which is the same for all sensors, and is tied to the average field value in the volume under study (or, on the contrary, the average field value is tied to the frequency of the RF field).

In contrast to the known solutions, zeroing the error signal S (Δω) in each sensor and keeping it at the center of the resonance line is carried out by controlling the pump light intensity I _p . The magnitude of the magnetic field variations is determined by the magnitude of the variations in the pump power.

The claimed system operates as follows.

In a multi-channel diagnostic system, atomic magnetometric sensors are set arbitrarily close to each other so that the angle between any two sensors does not exceed 20 °, and their orientation with respect to the direction of the magnetic field vector provides the maximum sensitivity for this type of sensors. In particular, for atomic magnetometers operating in a two-beam circuit (double-beam sensors) this means that the direction of the pump beam is oriented parallel to the magnetic field, and for atomic magnetometers working in a single-beam circuit (single-beam sensors), the direction of the pump beam is oriented at 45 ° to the magnetic field . The choice of the angle between the optical axes of adjacent sensors is less than 20 ° due to the geometry of the array of sensors, the bases of which should uniformly cover the scalp of the subject and be located as close as possible. Given the natural curvature of the scalp surface and the distance between the sensors not exceeding 2 cm, the angle between the axes of the sensors will not exceed 20 °.

Resonant laser radiation is generated and, passing it sequentially through a liquid crystal modulator 9, a circular polarizer 2, and parallel to the direction of the magnetic field through the working cell 3, optical atomic moments are pumped with light with an intensity that ensures maximum sensor sensitivity. Resonant radiation and tuning using the intensity of optical radiation provide maximum sensitivity in accordance with formulas (2) and (3) (E.B. Aleksandrov, A.K. Vershovskiy. Modern radio-optical methods of quantum magnetometry. - UFN, 2009, Volume 179, vol. 6, pp. 605-637).

In all sensors, the phases of the precession of magnetic moments are coordinated using a resonant radio frequency field directed perpendicularly to the magnetic field, which is common for all sensors, so that all atomic moments precess around the direction of the local field with a frequency ω close to the Larmor frequency of the precession in a given local magnetic field ω ₀ and the same phase determined by the phase of the radio frequency field. A second laser radiation is generated and optical detection of the magnetic resonance signal is carried out by passing this radiation through the cell in a direction perpendicular to the direction of the magnetic field.

At the photodetector, the intensity of the detection radiation transmitted through the cell of each sensor is detected and a component oscillating at the frequency of the Larmor precession is extracted from the resulting signal. The selected signal is detected on a synchronous detector, using the RF field signal as a reference, and selecting the phase of synchronous detection so that the resulting dependence of the voltage U on the frequency detuning ω-ω ₀ has an antisymmetric shape such that in the center of the magnetic resonance line U ~ (ω-ω ₀ ); the frequency band of synchronous detection in this case should be no less than the frequency band of the multichannel diagnostic system.

The extracted signal U ~ (ω-ω ₀ ) is used to control the liquid crystal modulator so that the full range of variation in the pump intensity is ± 10%. The voltage at the input of the liquid crystal modulator U _{M is} measured. Calibrated local magnetic fields B are supplied to the sensors, the voltage dependence U _M (B) is measured, and the linear dependence coefficient is calculated by fitting the linear regression model using the least squares method. The measured value U _M is recalculated into the magnitude of the local field induction in accordance with the linear regression dependence obtained in the previous step.

Implementation of the proposed method makes it possible to use a resonant radio frequency (RF) field common to all sensors, having a constant frequency, or tied in frequency to a magnetic field measured by one or more sensors of a multichannel diagnostic system, dynamic compensation of the shift of the center of the magnetic resonance line (occurring in each sensor due to variations of the local field, and proportional to these variations) carried out by closing feedback for control the intensity of the pump light, and as a result, the magnitude of the light shift of the magnetic resonance line. The measurement of the magnitude of the magnetic field variations is made by measuring the variations in the pump power; in the case of using a low-frequency (0-100 Hz) intensity modulator (LCM - liquid crystal modulator), the magnitude of the magnetic field variations is measured by measuring the variations of the LCD control signal. In addition, in this way, the previously unattainable possibility of the atomic magnetometric sensor (ADM) as part of a multichannel diagnostic system without interfering with the operation of arbitrarily close similar sensors is naturally ensured. An additional advantage is the possibility of using in each AMD not an expensive high-frequency acousto-optical modulator (AOM), but a relatively cheap liquid crystal (LCD) modulator operating in the frequency band 0 ÷ 100 Hz.

Claims (12)

1. Multichannel diagnostic system, including a laser source of linearly polarized optical pump radiation with an optical multi-channel splitter of pump radiation; RF field generator; RF coil; at least two atomic magnetometric sensors, each of which includes polarization-supporting optical fiber for transmitting pump radiation; a multi-channel signal processing and recording circuit, and a sensitive element comprising a circular polarizer for pump radiation; work cell; non-magnetic heater; photodetector; photocurrent amplifier; synchronous detector and integrator controlling the low-frequency pump radiation modulator, characterized in that atomic magnetometric sensors are installed in such a way that the angle between any two of these sensors does not exceed 20 °, and their orientation with respect to the direction of the magnetic field vector ensures the maximum sensitivity for this type of sensors, all sensors being in a common radio frequency field, and the optical pump circuit each of these sensors includes a low-frequency modulator that controls either the pump power and the light shift of the magnetic resonance frequency, either installed immediately after an optical multichannel splitter in front of the radiation transmitting optical fiber, or at the input of the sensing element after the radiation transmitting optical fiber. 2. The multi-channel diagnostic system according to claim 1, characterized in that the low-frequency modulator is liquid crystal. 3. The multi-channel diagnostic system according to claim 1, characterized in that it includes a laser source of linearly polarized detection radiation with an optical multi-channel splitter of detection radiation, an atomic magnetometric sensor includes a polarization-supporting fiber for transmitting detection radiation, and the sensitive element includes a balanced angle change detector polarization detection radiation. 4. The multichannel diagnostic system according to claim 1, characterized in that the sensitive element includes a coil of the radio frequency field.

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