WO2020120565A1

WO2020120565A1 - Magnetic field sensing device for high-strength magnetic fields

Info

Publication number: WO2020120565A1
Application number: PCT/EP2019/084625
Authority: WO
Inventors: Kasper JENSEN; Esben Thade PETERSEN; Vincent BOER; Eugene S. POLZIK; Hans Christian STÆRKIND
Original assignee: Københavns Universitet; Hvidovre Hospital
Priority date: 2018-12-14
Filing date: 2019-12-11
Publication date: 2020-06-18

Abstract

A magnetic field sensing device and a method for determining a magnetic field during medical imaging is disclosed, the method comprising: arranging a first sensor comprising a closed container containing alkali gas in the magnetic field; providing a laser beam (6) based on a reference control signal (68); modulating a first part (18) of the laser beam with a first control signal for provision of a first modulated laser beam; transmitting the first modulated laser beam through the alkali gas of the first sensor for provision of a first output signal; detecting, at a position away from the part of the magnetic field to be measured, the first output signal for provision of a first control input signal (46); providing the first control signal (44) based on the first control input signal (46); obtaining a first parameter indicative of the first control signal and optionally a first secondary parameter indicative of the first control input signal; and determining a magnetic field strength at the position of the first sensor based on the first parameter.

Description

MAGNETIC FIELD SENSING DEVICE FOR HIGH-STRENGTH MAGNETIC

FIELDS

The present disclosure relates to apparatus and method for sensing and determining magnetic fields and/or changes in high-strength magnetic field, in particular for medical imaging.

BACKGROUND

Instabilities in strong or high-strength magnetic fields can distort sensitive scanning procedures, such as performed with medical scanners employing a large magnetic field, such as Magnetic Resonance Imaging (MRI) scanners. Therefore, it is valuable to monitor changes in the magnetic field over time during such scanning procedures. This is not generally performed, due to the lack of sufficiently accurate and easy-to-use magnetic field sensors and apparatus.

SUMMARY

Accordingly, there is a need for apparatus and methods with improved

measurement/determination of magnetic field changes in high-strength magnetic fields.

A method for determining a magnetic field, e.g. during medical imaging, is disclosed, the method comprising arranging a first sensor, e.g. a non-metallic first sensor, comprising a closed container containing alkali gas in the magnetic field; providing a laser beam, e.g. based on a reference control signal; modulating a first part of the laser beam with a first control signal for provision of a first modulated laser beam; transmitting the first modulated laser beam through the alkali gas of the first sensor for provision of a first output signal; detecting, at a position away from the part of the magnetic field to be measured, the first output signal for provision of a first control input signal; providing the first control signal, e.g. based on the first control input signal; obtaining a first parameter indicative of the first control signal and/or a first secondary parameter indicative of the first control input signal; and determining a magnetic field strength at the position of the first sensor based on the first parameter and/or the first secondary parameter.

Further, a magnetic field sensing device for determining a magnetic field, e.g. during medical imaging is disclosed, the magnetic field sensing device comprising a laser device for emitting a laser beam, e.g. based on a reference control signal; a first light modulator for provision of a first modulated laser beam based on a first part of the laser beam; a first sensor, e.g. a non-metallic first sensor, comprising a closed container containing an alkali gas; a first controller for controlling the first light modulator with a first control signal, e.g. based on a first output signal from the first sensor; and a processing device configured to determine a magnetic field strength at the position of the first sensor based on a first parameter indicative of the first control signal and/or a first secondary parameter indicative of the first control input signal.

The present disclosure provides a high-precision measurement/determination of magnetic field changes in a high-strength magnetic field. In particular, the present disclosure allows for measurement/determination of magnetic field changes with sub-100 microT resolution. Further, the present disclosure allows for monitoring magnetic field changes (e.g. due to gradient switching imperfections and/or physiology such as breathing and/or motion) during medical imaging, such as scanning procedures, substantially without affecting the medical imaging procedure.

It is an important advantage of the present disclosure that large magnetic fields in the Tesla range (and/or variations therein) can be detected, optionally without distorting the magnetic field inside a medical scanner, e.g. an MRI scanner.

Further, the magnetic field sensing device advantageously does not emit radio frequency (RF) waves which may also disturb the medical imaging.

Further, the present disclosure advantageously allows for optical detection of the magnetic field, which is in particular useful when having multiple sensors, as the optical signals do not interfere with each other (for instance when propagating in separate fibers).

Further, the present disclosure provides an effective and fast detection of changes in magnetic field strengths. BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present invention will become readily apparent to those skilled in the art by the following detailed description of exemplary embodiments thereof with reference to the attached drawings, in which:

Fig. 1 schematically illustrates an exemplary magnetic field sensing device,

Fig. 2 is a flow diagram of an exemplary method according to the disclosure,

Fig. 3 is a graph showing measurement of magnetic field strength with an exemplary field sensing device and method,

Fig. 4 schematically illustrates an exemplary first controller and second controller,

Fig. 5 schematically illustrates an exemplary reference controller,

Fig. 6 schematically illustrates an exemplary magnetic field sensing device,

Fig. 7 schematically illustrates an exemplary sensor device, and

Fig. 8 illustrates light absorption/transmission at different temperatures.

DETAILED DESCRIPTION

Various exemplary embodiments and details are described hereinafter, with reference to the figures when relevant. It should be noted that the figures may or may not be drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the invention or as a limitation on the scope of the invention. In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated, or if not so explicitly described.

The present disclosure exploits the fact that the energy levels of atoms change when atoms are exposed to a magnetic field. This means that the resonance frequencies of the atoms also change. The changes depend on the magnetic field strength. By detecting and keeping track of the changes of a resonance frequency, it can be derived how the magnetic field is changing in time at the position of the atoms. A transition in Cs-133-atoms has been identified, where the change in resonance frequency is simply Dn = y B, where y = 13.992(6) GHz/T. The two states involved in the transition can be named differently depending on the strength of the magnetic field. In the Zeeman limit (weak magnetic field) the lower state is called 6²S_I/2, F = 4, rri_F = 4, and the upper state is called 6²P_3/2, F = 5, rri_F = 5. In the Paschen-Back limit (strong magnetic field) the lower state is called 6²S_I/2, m = 1/2, mi = 7/2, and the upper state is called

6²P_3/2, m = 3/2, mi = 7/2. It is to be noted that other transitions may be employed, such as a similar transition in Rb-85. In the Zeeman limit (weak magnetic field) the lower state is called 5²S_I/2, F = 3, rri_F = 3, and the upper state is called 5²P_3/2, F = 4, rri_F = 4. In the Paschen-Back limit (strong magnetic field) the lower state is called 5²S_I/2, m = 1/2, mi = 5/2, and the upper state is called 5²P_3/2, m = 3/2, mi = 5/2. This transition has y = 14.04(3) GHz/T. Similar transitions can be found and used in all the isotopes of all the alkali metals. They all have a y which is between 13.9 and 14.1 GHz/T.

A method for determining a magnetic field during medical imaging is disclosed. The method comprises arranging a first sensor, e.g. a non-metallic first sensor, comprising a closed container containing alkali gas in the magnetic field. The closed container of the sensors may be made of glass and/or other suitable materials, such as a semi-conductor material. In one or more exemplary methods/devices, the magnetic field strength is in the range from 6T to 12 T, such as 7T, 8T, 9T, 10T, or 1 1T, or any ranges therebetween. In one or more exemplary methods/devices, the magnetic field strength is less than 6T, such as in the range from 1T to 6T, e.g. about 1 5T or about 3T. In one or more exemplary methods/devices, the magnetic field strength is larger than 12T.

In the present context, a non-metallic sensor, e.g. non-metallic first sensor and/or non- metallic second sensor, is to be understood as a sensor where the sensor except the alkali element (in solid, liquid, and/or gas form) contained in the closed container is made of non-metallic materials.

The method comprises providing a laser beam with a laser device. The laser beam may be based on a reference control signal, e.g. from a reference controller optionally as disclosed herein.

The method comprises modulating a first part of the laser beam with a first control signal for provision of a first modulated laser beam. Accordingly, the method may comprise splitting or dividing the laser beam into a plurality of parts including a first part and optionally a reference part.

The method comprises transmitting the first modulated laser beam through the alkali gas of the first sensor for provision of a first output signal. T ransmitting the first modulated laser beam through the alkali gas of the first sensor for provision of a first output signal may comprise transmitting the first modulated laser beam through the alkali gas for a first time; redirecting the first modulated laser beam; and transmitting the first modulated laser beam through the alkali gas for a second time. In other words, the present method performs saturated absorption spectroscopy in the first sensor.

The method comprises detecting, at a position away from the part of the magnetic field to be measured, the first output signal for provision of a first control input signal. Thereby, the medical imaging and/or the magnetic field strength is not influenced by detector circuitry.

The method comprises providing the first control signal, optionally based on the first control input signal.

The method comprises obtaining a first parameter, also denoted P_1 , indicative of the first control signal and/or a first secondary parameter, also denoted P_1_2.

The first parameter may be a first frequency of the first control signal or a voltage/current value representative of the first frequency of the first control signal, such as a voltage value representative of the control voltage of a voltage controlled oscillator (VCO) of the first controller. The method comprises determining a magnetic field strength at the position of the first sensor based on the first parameter.

The first secondary parameter may be indicative of the first control input signal. In one or more exemplary methods, the first secondary parameter may be an output signal from a lock-in amplifier (LIA) module of the first controller, optionally wherein the first control input signal forms an input signal to the LIA module. The LIA module may comprise a mixer and a lowpass filter. In one or more exemplary methods, determining a magnetic field strength at the position of the first sensor is based on the first secondary parameter.

In one or more exemplary methods, the method comprises arranging a reference sensor comprising a closed container containing alkali gas outside the magnetic field; modulating a reference part of the laser beam with a reference modulation signal for provision of a modulated reference laser beam; transmitting the modulated reference laser beam through the alkali gas of the reference sensor for provision of a reference output signal; and providing the reference control signal based on the reference output signal. The purpose of this setup is to lock the laser frequency at a known frequency shift compared to an atomic resonance of the alkali gas of the reference sensor. The frequency shift is an integer multiple, n, of the reference modulation signal frequency, also denoted reference frequency V_Ref. The reference sensor may be arranged in an environment with zero or low magnetic field, e.g. less than 1 mT, less than 100 microT, or even less than 1 microT. The reference sensor may be arranged inside a magnetic shield.

Determining a magnetic field strength may be based on a reference frequency of the reference modulation signal.

Transmitting the modulated reference laser beam through the alkali gas of the reference sensor for provision of a reference output signal may comprise transmitting the modulated reference laser beam through the alkali gas for a first time; redirecting the modulated reference laser beam; and transmitting the modulated reference laser beam through the alkali gas for a second time. In other words, the present method performs saturated absorption spectroscopy in the reference sensor.

In one or more exemplary methods, the magnetic field strength B_1 at the first position is given by

B_1 = Dn / Y = (n v_Ref + m_1 Vi) / y,

wherein n is indicative of the sideband used for the reference part of the laser beam, V_Ref is the reference frequency of the reference modulation signal controlling the reference light modulator, m_1 is indicative of the sideband used for the first part of the laser beam, Vi is the first frequency of the first control signal of which P_1 is indicative, and y is 13.992(6) GHz/T and represents how the atomic resonance depends on the magnetic field strength. The sideband n (e.g. n=1 , 2, 3, 4, or 5) and V_Ref may be selected to adjust the magnetic field strength range that is to be measured. The sideband m_1 (e.g. m_1 =1 , 2, 3, 4, or 5) and the range of Vi may be selected to adjust the magnetic field strength range that is to be measured, e.g. in the first position.

B_1 = Dn / Y = (5 v_Ref + 1 Vi) / Y,

wherein V_Ref is the reference frequency of the reference modulation signal controlling the reference light modulator, Vi is the first frequency of the first control signal of which P_1 is indicative, and y is 13.992(6) GHz/T and represents how the atomic resonance depends on the magnetic field strength.

In one or more exemplary methods, providing the laser beam based on a reference control signal comprises locking the laser frequency with a known shift relative to the atomic reference resonance frequency.

In one or more exemplary methods, modulating the first part of the laser beam comprises modulating the first part of the laser beam to have a first frequency spectrum capable of driving a predetermined atomic transition in an alkali atom of the first sensor.

In one or more exemplary methods, the method comprises arranging a second sensor, e.g. a non-metallic second sensor, comprising a closed container containing alkali gas in the magnetic field; modulating a second part of the laser beam with a second control signal for provision of a second modulated laser beam; transmitting the second modulated laser beam through the alkali gas of the second sensor for provision of a second output signal; detecting, at a position away from the part of the magnetic field to be measured, the second output signal for provision of a second control input signal; providing the second control signal based on the second control input signal; obtaining a second parameter, also denoted P_2, indicative of the second control signal and optionally a second secondary parameter, also denoted P_2_2, indicative of the second control input signal; and determining a magnetic field strength at the position of the second sensor based on the second parameter. Determining a magnetic field strength at the position of the second sensor may also be based on the second secondary parameter. In one or more exemplary methods, the magnetic field strength B_2 at the second position is given by

B_2 = Dn / Y = (n V_Ref + m_2 V2) / y,

wherein n is indicative of the sideband used for the reference part of the laser beam, V_Ref is the reference frequency of the reference modulation signal controlling the reference light modulator, m_2 is indicative of the sideband used for the second part of the laser beam, V2 is the second frequency of the second control signal of which P_2 is indicative, and y is 13.992(6) GHz/T and represents how the atomic resonance depends on the magnetic field strength. The sideband m_2 (e.g. m_2=1 , 2, 3, 4, or 5) and the range of V2 may be selected to adjust the magnetic field strength range that is to be measured, e.g. in the second position.

Transmitting the second modulated laser beam through the alkali gas of the second sensor for provision of a second output signal may comprise transmitting the second modulated laser beam through the alkali gas for a first time; redirecting the second modulated laser beam; and transmitting the second modulated laser beam through the alkali gas for a second time. In other words, the present method performs saturated absorption spectroscopy in the second sensor.

In one or more exemplary methods, providing the first control signal based on the first control input signal comprises performing lock-in detection of an absorption frequency of the first output signal. This is also sometimes called FM (frequency modulated) spectroscopy. Accordingly, providing the first control signal based on the first control input signal may comprise performing frequency modulated spectroscopy.

In one or more exemplary methods, providing the reference control signal based on the reference output signal comprises performing lock-in detection of an absorption frequency of the reference output signal. This is also sometimes called FM (frequency modulated) spectroscopy. Accordingly, providing the reference control signal based on the reference control input signal (output of detector detecting the reference output signal) may comprise performing frequency modulated spectroscopy.

In one or more exemplary methods, providing the second control signal based on the second control input signal comprises performing lock-in detection of an absorption frequency of the second output signal. This is also sometimes called FM (frequency modulated) spectroscopy. Accordingly, providing the second control signal based on the second control input signal may comprise performing frequency modulated spectroscopy.

In one or more exemplary methods, the method comprises heating the first sensor to a first temperature, e.g. to a first temperature larger than 20°C, or larger than 35°C, e.g. in the range from 55°C to 75°C. In one or more exemplary methods, the first temperature is larger than 75°C. Thereby, the concentration of alkali gas is increased leading to increased absorption of the light around resonance, and thereby stronger signal.

In one or more exemplary methods, heating the first sensor to a first temperature comprises heating the first sensor with a first heating laser beam, e.g. from a first heating laser device. The first heating laser beam may be fed through a first heating fiber.

In one or more exemplary methods, the method comprises heating the second sensor to a second temperature, e.g. to a second temperature larger than 20°C, or larger than 35°C, e.g. in the range from 55°C to 75°C. In one or more exemplary methods, the second temperature is larger than 75°C. The first temperature and the second temperature may be the same or within a 15°C difference. In other words, the difference between the first temperature and the second temperature may be less than 15°C degrees, such as less than 10°C or even less than 5°C.

In one or more exemplary methods, heating the second sensor to a second temperature comprises heating the second sensor with a second heating laser beam, e.g. from a second heating laser device and/or the first heating laser device. The second heating laser beam may be fed through a second heating fiber. In one or more exemplary methods, heating the second sensor to a second temperature comprises splitting a heating laser beam into a plurality of heating laser beams including the first heating laser beam and the second heating laser beam.

A first power of the first heating laser beam/first heating laser device may be in the range from 0.2 W to 5 W, such as in the range from 0.4 W to 2 W e.g. 0.5 W or 1 W. A too high power may damage components and/or lead to a too high absorption in the sensor.

A second power of the second heating laser beam/second heating laser device may be in the range from 0.2 W to 5 W, such as in the range from 0.4 W to 2 W e.g. 0.5 W or 1 W. A too high power may damage components and/or lead to a too high absorption in the sensor.

In one or more exemplary methods, heating the first sensor to a first temperature comprises absorbing the first heating laser beam, or at least a part of such as more than 90% of the first heating laser beam, in a first absorption element arranged in thermal contact with the closed container of the first sensor. Thus, heat is transferred from the first absorption element to the closed container, e.g. via a thermal paste contacting the closed container and the first absorption element. The first absorption element may be an optical filter.

In one or more exemplary methods, heating the second sensor to a second temperature comprises absorbing the second heating laser beam, or at least a part of such as more than 90% of the second heating laser beam, in a second absorption element arranged in thermal contact with the second closed container of the second sensor. Thus, heat is transferred from the second absorption element to the second closed container, e.g. via a thermal paste contacting the second closed container and the second absorption element. The second absorption element may be an optical filter.

The alkali gas of sensors, such as the first sensor, second sensor, reference sensor, etc., may comprise one of cesium gas and rubidium gas. In one or more exemplary

methods/magnetic field sensing devices, the alkali gas of sensors, such as the first sensor, second sensor, reference sensor, etc., may comprise one of lithium (Li), sodium (Na), potassium (K).

Further, a magnetic field sensing device for determining a magnetic field, e.g. during medical imaging, is disclosed. The magnetic field sensing device comprises a laser device for emitting a laser beam, e.g. based on a reference control signal. The present disclosure exploits a stabilization of the laser frequency with a desired frequency shift compared to an atomic resonance of the alkali gas of the reference sensor. The frequency shift is an integer multiple, n, of the reference modulation signal frequency, also denoted reference frequency V_Ref. Further, a magnetic field sensing device for determining a magnetic field is disclosed, wherein the magnetic field sensing device is configured to perform the method as disclosed herein.

The magnetic field sensing device optionally comprises one or more beam splitters for splitting the laser beam into different parts, such as a reference part and a first part.

The magnetic field sensing device comprises a first light modulator for provision of a first modulated laser beam based on a first part of the laser beam. The first light modulator may be an electro-optic modulator, e.g. a phase modulator or an amplitude modulator.

The first light modulator modulates e.g. the phase of the first part of the laser beam. This introduces new frequency components above and below the carrier frequency of the first modulated laser beam. These new frequency components are called sidebands. The sidebands appear symmetrically on each side of the carrier with a frequency spacing equal to the modulation frequency (first frequency of the first control signal). The strength of the modulation determines the power in the different sidebands. With the right modulation strength, up to 34 % of the light power is transferred to each of the two first sidebands (34 % of the power in the first sideband above the carrier frequency, and 34 % of the power in the first sideband below the carrier frequency.

The magnetic field sensing device comprises a first sensor, e.g. a non-metallic first sensor comprising a closed container containing an alkali gas. The closed container may be made of glass. The alkali gas of the first sensor may comprise one of cesium gas, rubidium gas, or some other alkali gas.

The magnetic field sensing device optionally comprises one or more photo detectors including a first photo detector and/or a reference photo detector.

The magnetic field sensing device comprises a first controller for controlling the first light modulator with a first control signal based on a first output signal from the first sensor. In other words, an electrical first control input signal (from first photo detector) based on (optical) first output signal may be fed to the first controller, wherein the first controller is configured to control the first light modulator (with first control signal) based on the first control input signal (and thus based on the first output signal). The first controller may comprise a lock-in amplifier, an integrator and/or an oscillator, such as a voltage- controlled oscillator (VCO). An output of the VCO may be used as the first control signal. The controller electronics may be either digital or analog. The controller electronics may include direct measurements of the frequency that is sent to the respective light modulators, such as the first light modulator and the second light modulator, or an indirect measurement, e.g. by measurement of a VCO tuning voltage.

The magnetic field sensing device comprises a processing device configured to determine a magnetic field strength at the position of the first sensor based on a first parameter indicative of the first control signal. The processing device comprises a processor, a memory, and an interface.

In one or more exemplary magnetic field sensing devices, the magnetic field sensing device comprises a reference light modulator for provision of a modulated reference laser beam based on a reference part of the laser beam. The reference light modulator may be an electro-optic modulator. The magnetic field sensing device may comprise a reference sensor comprising a closed container containing alkali gas; and a reference controller for controlling the laser device based on a reference output signal from the reference sensor.

In other words, an electrical reference control input signal (from reference photo detector) based on (optical) reference output signal may be fed to the reference controller, wherein the reference controller is configured to control the laser device (with laser control signal) based on the reference control input signal (and thus based on the reference output signal).

In one or more exemplary magnetic field sensing devices, the magnetic field sensing device comprises a second light modulator for provision of a second modulated laser beam based on a second part of the laser beam. The second light modulator may be an electro-optic modulator, e.g. a phase modulator or an amplitude modulator. The magnetic field sensing device may comprise a second sensor, e.g. a non-metallic second sensor comprising a closed container containing alkali gas; and optionally a second controller for controlling the second light modulator with a second control signal based on a second output signal from the second sensor. In other words, an electrical second control input signal (from second photo detector) based on (optical) second output signal may be fed to the second controller, wherein the second controller is configured to control the second light modulator (with second control signal) based on the second control input signal (and thus based on the second output signal). The second controller may comprise a lock-in amplifier, an integrator and/or an oscillator, such as a voltage-controlled oscillator (VCO). An output of the VCO may be used as the second control signal.

In one or more exemplary magnetic field sensing devices, the processing device may be configured to determine a magnetic field strength at a position of the second sensor based on a second parameter indicative of the second control signal.

The magnetic field sensing device optionally comprises one or more optical fibers, e.g. including a (polarization maintaining) first input fiber for feeding the first modulated laser beam to the first sensor and/or a (polarization maintaining) reference input fiber for feeding the modulated reference laser beam to the reference sensor.

The one or more optical fibers of the magnetic field sensing device may comprise a first heating fiber. The first heating fiber may be configured to feed a first heating laser beam to the first sensor (sensor device), e.g. for heating the first sensor or at least the closed container to a first temperature.

The magnetic field sensing device may comprise one or more heating laser devices, such as a first heating laser device and optionally a second heating laser device. The first heating laser device may be configured to heat the first sensor (sensor device) by providing a first heating laser beam to the first sensor, e.g. via first heating fiber.

The one or more optical fibers of the magnetic field sensing device may comprise a second heating fiber. The second heating fiber may be configured to feed a second heating laser beam to the second sensor (sensor device), e.g. for heating the second sensor or at least the closed container to a second temperature. The second heating laser device may be configured to heat the second sensor (sensor device) by providing a second heating laser beam to the second sensor, e.g. via second heating fiber.

A sensor, such as one or more of the first sensor, the second sensor, and the reference sensor, may comprise a beam splitter, such as a polarizing beam splitter. A sensor, such as one or more of the first sensor, the second sensor, and the reference sensor, may comprise one or more (optical) filters and/or lenses. A sensor, such as one or more of the first sensor, the second sensor, and the reference sensor, may comprise one or more waveplates, such as a quarter-wave plate. A sensor, such as one or more of the first sensor, the second sensor, and the reference sensor, may comprise one or more reflectors. A reflector may be a mirror. The purpose of the optical components in the (reference, first, second, and so on) sensors is to perform saturated absorption

spectroscopy. The polarizing beam splitter (PBS) transmits only one component of linearly polarized light. When passing the quarter waveplate the first time, this linear polarization is transformed into circularly polarization. This drives and saturates the desired transition. Upon reflection by the mirror and attenuation by the optical filter, the laser beam again drives the transition but this time weaker. When passing the quarter waveplate for the second time, circular polarization is transformed into linear polarization, perpendicular to the original linear polarization. Because of the new polarization this is now reflected by the PBS and directed into an output fiber, such as a multimode output fiber. The purpose of the first (strong) laser beam (first time passing the alkali gas) is to saturate the Doppler broadened transition. The purpose of the second (weak) laser beam (second time passing the alkali gas) is to probe the Doppler broadened transition. When the two laser beams are addressing the same atoms with no velocity-components along the beam direction, the second laser beam is absorbed less because the transition is already saturated by the first laser beam. This effect is also seen in the so-called cross-over resonances, when the two laser beams address different transitions in atoms of the alkali gas with a non-zero velocity component along the beam direction. This gives a spectral feature with the natural linewidth, which is about a hundred times smaller than the Doppler broadened linewidth (at room temperature). A sensor, such as the first sensor and/or the second sensor may be embodied as a sensor device described herein. It is to be noted that a description of a sensor, such as the first sensor and/or the second sensor may apply to the sensor device and vice versa.

Further, a medical scanner is disclosed, wherein the medical scanner comprises a magnetic field sensing device as disclosed herein.

Also disclosed is a sensor device comprising a housing; a closed container containing an alkali gas; and a plurality of ports including an input port and an output port. The sensor device may comprise a heating port. The input port is configured for receiving an input fiber. The input port may be formed as an input connector for coupling with a fiber connector of the input fiber. The output port is configured for receiving an output fiber. The output port may be formed as an output connector for coupling with a fiber connector of the input fiber. The heating port is configured for receiving a heating fiber. The heating port may be formed as a heating connector for coupling with a fiber connector of the heating fiber.

The input port and the output port may be configured to receive the input fiber and the output fiber, respectively, such that the input fiber and the output fiber are parallel at the input port and the output port. The input port and the heating port may be configured to receive the input fiber and the heating fiber, respectively, such that the input fiber and the heating fiber are parallel at the input port and the output port. In other words, the housing may comprise a first side or surface comprising the input port, the output port, and the heating port if present. Thereby, arranging the sensor device in a limited space as in a medical scanner is facilitated.

The housing of the sensor device may comprise a base part and optionally a lid part. The housing may be made of an opaque material, such as a black material. The housing may be made of a thermoplastic polymer.

The sensor device may comprise an absorption element for absorbing at least a part of a heating laser beam from the heating fiber. The absorption element may be in thermal contact with the closed container optionally via a thermal paste. In other words, the sensor device may comprise a thermal paste arranged in thermal contact with the absorption element and the closed container. Thus, heat can in an effective way be transferred from the absorption element to the closed container allowing heating of the closed container with a heating laser beam fed through the heating fiber.

The sensor device may comprise or define a heating path for feeding a heating laser beam from the heating fiber towards the closed container, the heating path comprising one or more of a mirror, and the absorption element.

The sensor device may comprise or define an input path for feeding an input laser beam from the input fiber to the closed container. The input path may comprise one or more of a beamsplitter, a waveplate, a mirror, a lens, and an input coupling element. The input coupling element may comprise a plate or window optionally coated with an anti-reflection coating on the side or surface pointing away from the input fiber. Thereby is achieved that the input fiber does not disturb the sensitivity by acting as a cavity that has frequency- dependent transmission. In one or more exemplary magnetic field sensing device, the input fiber is coated with an anti-reflection coating at the tip connected to the sensor device.

The input path may be optically separated from the heating path. In other words, the heating laser beam is optionally prevented from entering the input path, thus preventing distortion of the input laser beam. Distortion of the input laser beam may lead to erroneous determination of magnetic field strength and is not desirable.

The sensor device may comprise or define an output path for feeding an output signal, e.g. output laser beam, from the closed container to the output fiber. The output path may comprise one or more of a beamsplitter, a waveplate, a mirror, a lens, and an output coupling element. The output coupling element may comprise a plate or window optionally coated with an anti-reflection coating on the side or surface pointing away from the output fiber. Thereby is achieved that the output fiber does not disturb the sensitivity by acting as a cavity that has frequency-dependent transmission. In one or more exemplary magnetic field sensing device, the output fiber is coated with an anti-reflection coating at the tip connected to the sensor device. The input path and the output path may share a beamsplitter and/or a waveplate.

The input coupling element may be coupled to the input fiber with an index-matching gel. The output coupling element may be coupled to the output fiber with an index-matching gel.

The output path may be optically separated from the heating path. In other words, the heating laser beam is optionally prevented from entering the output path, thus preventing distortion of the output laser beam. Distortion of the output laser beam may lead to erroneous determination of magnetic field strength and is not desirable.

A sensor device as disclosed may be used as or form the first sensor. A sensor device as disclosed may be used as or form the second sensor.

Fig. 1 shows an exemplary magnetic field sensing device for determining a magnetic field, e.g. during medical imaging. The magnetic field sensing device 2 comprises a laser device 4 for emitting a laser beam 6. The laser device 4 is configured for emitting the laser beam 6 based on a reference control signal 68. The magnetic field sensing device 2 comprises one or more beam splitters including reference beam splitter 10, optional first beam splitter 12, and optionally second beam splitter 14 for splitting the laser beam 6 into different parts, such as a reference part 16 of laser beam, first part 18 of laser beam, and second part 20 of laser beam.

The magnetic field sensing device 2 comprises a first light modulator 22 for provision of a first modulated laser beam 24 based on the first part 18 of the laser beam. A first input fiber 26 feeds the first modulated laser beam 24 to an input port of non-metallic first sensor 30. The non-metallic first sensor 30 comprises a closed container 32 made of glass and containing cesium gas. A first photo detector 34 receives first output signal 36 from the first sensor 30 via first output fiber 38. The first photo detector 34 is arranged outside medical scanner 40. The first sensor 30 optionally is a sensor device 600, see Fig. 7.

The magnetic field sensing device 2 comprises a first controller 42 for controlling the first light modulator 22 with a first control signal 44. The first control signal 44 is based on the first output signal 36 from the first sensor 30 by the first photo detector 34 detecting the first output signal 36 and outputting an electrical first control input signal 46 based on the first output signal 36. The first controller 42 is configured to control the first light modulator 22 with first control signal 44, wherein the first control signal 44 is based on the first control input signal 46 (and thus based on the first output signal 36). The first controller 42 is connected to frequency source 90D for provision of a local oscillator signal to the first controller 42.

The magnetic field sensing device comprises a processing device 48 configured to determine a magnetic field strength B_1 at the position of the first sensor based on a first parameter P_1 indicative of the first frequency of the first control signal and/or a first secondary parameter P_1_2. The magnetic field strength B_1 may be stored, e.g. in memory of the processing device 48, and/or transmitted via an interface of the processing device 48, e.g. in substantially real-time, to the medical scanner for correcting/adjusting operation of the medical scanner (such as magnetic field correction), the scanning procedure and/or medical imaging based on the detected magnetic field strength B_1. The magnetic field sensing device 2 comprises a reference light modulator 50 for provision of a modulated reference laser beam 52 based on the reference part 16 of the laser beam. A reference input fiber 54 feeds the modulated reference laser beam 52 to an input port of reference sensor 58, the reference sensor 58 comprising a closed container 60 made of glass and containing cesium gas. The reference sensor 58 and in particular the closed container 60 is arranged in an environment 61 with zero or low magnetic field, e.g. inside a magnetic shield. A reference photo detector 62 of the magnetic field sensing device receives reference output signal 64 from the reference sensor 58. The magnetic field sensing device 2 comprises a reference controller 66 for controlling the laser device with reference control signal 68 based on the reference output signal 64 from the reference sensor 58. The reference control signal 68 is based on the reference output signal 64 from the reference sensor 58 by the reference photo detector 62 detecting the reference output signal 64 and outputting an electrical reference control input signal 70 based on the reference output signal 64. The reference controller 66 is configured to control the laser device 4 with reference control signal 68, wherein the reference control signal 68 is based on the reference control input signal 70 (and thus based on the reference output signal 64). In other words, electrical reference control input signal 70 from reference photo detector 62 based on optical reference output signal 64 is fed to the reference controller 66, wherein the reference controller 66 is configured to control the laser device 4 (with reference control signal 68) based on the reference control input signal 70 (and thus based on the reference output signal 64). Thereby, the laser device 4 is tuned to a desired frequency. The reference controller 66 is connected to frequency source 90C for provision of a local oscillator signal to the reference controller 66.

The magnetic field sensing device 2 comprises a second light modulator 72 for provision of a second modulated laser beam 74 based on the second part 20 of the laser beam. A second input fiber 76 feeds the second modulated laser beam 74 to an input port of non- metallic second sensor 80. The non-metallic second sensor 80 comprises a closed container 82 made of glass and containing cesium gas. A second photo detector 84 receives second output signal 86 from the second sensor 80 via second output fiber 88. The second photo detector 84 is arranged outside medical scanner 40. The second sensor 80 optionally is a sensor device 600, see Fig. 7.

The magnetic field sensing device 2 comprises a second controller 92 for controlling the second light modulator 72 with a second control signal 94. The second control signal 94 is based on the second output signal 86 from the second sensor 80 by the second photo detector 84 detecting the second output signal 86 and outputting an electrical second control input signal 96 based on the second output signal 86. The second controller 92 is configured to control the second light modulator 72 with second control signal 94, wherein the second control signal 94 is based on the second control input signal 96 (and thus based on the second output signal 86). The second controller 92 is connected to frequency source 90E for provision of a local oscillator signal to the second controller 92.

The processing device 48 is optionally configured to determine a magnetic field strength B_2 at the position of the second sensor based on a second parameter P_2 indicative of the second frequency of the second control signal and/or a second secondary parameter P_2_2. The magnetic field strength B_2 may be stored, e.g. in memory of the processing device 48, and/or transmitted via an interface of the processing device, e.g. in

substantially real-time, to the medical scanner for correcting/adjusting operation of the medical scanner (such as magnetic field correction), the scanning procedure and/or medical imaging based on the detected magnetic field strength B_2.

It is to be noted that the field sensing device 2 may comprise three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen, fifteen, sixteen or more sensor assemblies (sensor, light modulator, photo detector, and controller). Further, a field sensing device with only a single first sensor assembly (refs 22, 26, 30, 34, 38, 42) in addition to reference sensor assembly (refs 50, 54, 58, 66) is also contemplated.

The optical setups after each light modulator 22, 50 perform saturated absorption spectroscopy with lock-in detection (this is also sometimes called FM (frequency modulated) spectroscopy) on the atoms in the closed container 32, 60. This is a way to measure if the laser light is resonant with the atomic resonance, or if it is a little lower or a little higher. The field sensing device 2 uses feedback that ensures that the laser beam parts are always resonant with the respective atomic resonances, also denoted locking. For this to work the laser device 4 is modulated at a frequency of e.g. 5 MHz, by frequency source 90A.

The cesium atoms of reference sensor 58 are placed in an environment with zero or low magnetic field 61 , e.g. less than 1 mT, less than 100 microT, or even less than 1 microT. The unchanged resonance frequency is denoted Vo. In the illustrated example, the fifth lower sideband of the reference light modulator 50 is then locked to the atomic resonance, i.e. V_L - 5 V_Ref = vo, or equivalently that vi_ = Vo + 5 V_Ref, where V_Ref is the frequency of control signal from frequency source 90B (e.g. about 20 GHz). The laser frequency is now significantly higher than the unchanged resonance by a known amount (on the order of 5 20 GHz = 100 GHz). The first sensor 30 is placed in an environment with a strong magnetic field B_1. The resonance frequency is denoted VBI and is a little higher than vi_. The first upper sideband of the first light modulator 22 is locked to this resonance with the first controller 42, and therefore:

VBI ⁼ VL + 1 Vi.

Inserting the expression for VL_:

VBI = Vo + 5 V_Ref + 1 Vi .

Written in terms of the frequency change this reads:

Dn = VBI - vo = 5 V_Ref + 1 Vi.

Relating the change in frequency to the magnetic field:

B_1 = Dn / Y = (5 v_Ref + 1 Vi) / y.

Now, when the magnetic field changes a little, and V_BI therefore changes, then the locking feedback to the first light modulator 22 ensures that Vi will change correspondingly. Notice that V_Ref is always the same and selected dependent on the application (and provided by frequency source 90B as control input to the reference light modulator 50). This frequency determines the lowest measurable magnetic field for a given configuration. The frequency Vi of the first control signal 44 is monitored (first parameter P_1 ) and the magnetic field is determined from the above expression.

The same applies for the second sensor which advantageously uses the same initial feedback/locking by the reference sensor assembly.

In one or more exemplary field sensing devices, the locking is performed in the first controller 42 by continuously providing a signal (such as an output from a LIA module) proportional to an error signal ei given by:

ei = (Vo + 5 VRef + 1 Vi ) - VB1 ,

and trying to make the error signal ei zero. The error signal ei is the difference between the light frequency, (vo + 5 VRef + 1 Vi ), and the atomic resonance in the first position of the magnetic field, V_BI . During rapid changes in the magnetic field, the error signal will not be zero, due to the finite speed of the feedback. Accordingly, the magnetic field for the first position may be based on a first secondary parameter, e.g. being an output signal of a LIA module, indicative of the error signal.

The magnetic field may be given as:

B_1 = Dn / Y = (5 v_Ref + 1 Vi - ei) / y.

The signal that is obtained from the saturated absorption spectroscopy with lock-in detection (this is also sometimes called FM (frequency modulated) spectroscopy), is merely indicative of whether the resonance frequency is a little lower or a little higher. If the light frequency of the first modulated laser beam and the atomic resonance of the atoms in the first sensor differ too much, no information about their relation is obtained. That is, if the error gets too big, the error signal disappears. In other words, the equation for the error ei is only true when ei is close to zero.

Notice that the magnetic field measured is an average of the volume given by the length of the alkali gas container, and the area of the laser beam passing through it. So by changing the length of the container and the area of the laser beam, the spatial resolution of the sensor can be changed to a desired resolution.

Fig. 2 is a flow diagram of an exemplary method for determining a magnetic field during medical imaging, the method 100 comprising arranging S102A a non-metallic first sensor comprising a closed container containing alkali gas in the magnetic field; providing S104 a laser beam based on a reference control signal; modulating S106A a first part of the laser beam with a first control signal for provision of a first modulated laser beam; transmitting S108A the first modulated laser beam through the alkali gas of the first sensor for provision of a first output signal; detecting S110A, at a position away from the part of the magnetic field to be measured, the first output signal for provision of a first control input signal, by means of lock-in detection; providing S112A the first control signal based on the first control input signal; obtaining S114A a first parameter P_1 indicative of the first control signal and optionally a first secondary parameter indicative of the first control input signal; and determining S1 16A a magnetic field strength B_1 at the position of the first sensor based on the first parameter P_1 and/or the first secondary parameter P_1_2. The method 100 comprises arranging S102B a reference sensor comprising a closed container containing alkali gas outside the magnetic field; modulating S106B a reference part of the laser beam with a reference modulation signal for provision of a modulated reference laser beam; transmitting S108B the modulated reference laser beam through alkali gas of the reference sensor for provision of a reference output signal; and determining S116B the reference control signal based on the reference output signal.

Providing S104 the laser beam based on a reference control signal optionally comprises locking S104A the laser frequency of the laser beam at a laser frequency, wherein a difference between the laser frequency and a reference resonance frequency of the reference sensor corresponds to a desired frequency shift, e.g. in the range from 90GHz to 1 10 GHz, e.g. in a 7T configuration.

Optionally, modulating S106A the first part of the laser beam comprises modulating the first part of the laser beam to have a first frequency spectrum capable of driving a predetermined atomic transition in an alkali atom of the first sensor.

The method 100 optionally comprises arranging S102C a non-metallic second sensor comprising a closed container containing alkali gas in the magnetic field; modulating S106C a second part of the laser beam with a second control signal for provision of a second modulated laser beam; transmitting S108C the second modulated laser beam through the alkali gas of the second sensor for provision of a second output signal;

detecting S1 10C, at a position away from the part of the magnetic field to be measured, the second output signal for provision of a second control input signal, e.g. by lock-in detection; providing S112C the second control signal based on the second control input signal; obtaining S114C a second parameter P_2 indicative of the second control signal and optionally a second secondary parameter P_2_2 indicative of the second control input signal; and determining S1 16C a magnetic field strength B_2 at the position of the second sensor based on the second parameter, and optionally also the second secondary parameter.

Optionally, providing S112A the first control signal based on the first control input signal comprises performing lock-in detection of an absorption frequency of the first output signal. Fig. 3 shows a result of a magnetometer test during an MRI scanning session using a field sensing device as disclosed herein. The graph shows the measured field. Small rapid changes on top of a large magnetic field are seen. An important part of an MRI sequence is a series of shifts in the magnetic field. Ideally, these shifts should be trapezoid variations. The spatial resolution of this magnetometer is set by an alkali gas container length of 5 mm and a laser beam diameter of 2 mm, i.e. a laser beam area of about 3.14 mm^A2.

Fig. 4 shows exemplary first controller 42 and second controller 92. The controller 42, 92 comprises a lock-in amplifier (LIA) module 400 configured to receive first control input signal 46 and second control input signal 96, respectively. The LIA module 400 receives a local oscillator signal 402 from a respective frequency source 90D, 90E. The controller 42, 92 comprises an integrator module 404 having an input connected to an output of the LIA module 400. The controller 42, 92 comprises a voltage controlled oscillator (VCO) 406 having an input connected to an output of the integrator module. The VCO 406 provides respective first control signal 44 and second control signal 96 to respective light modulators. The first parameter P_1 being the output of integrator module 404 of first controller 42 and second parameter P_2 being the output of integrator module 404 of second controller 92 is fed to processing device for determination of respective magnetic field strengths B_1 and B_2. Optionally, the first secondary parameter P_1_2 being the output of LIA module 400 of first controller 42 and second secondary parameter P_2_2 being the output of LIA module 404 of second controller 92 is fed to processing device for determination of respective magnetic field strengths B_1 and B_2 based on P_1_2 and P_2_2, respectively.

Fig. 5 shows exemplary reference controller 66. The reference controller 66 comprises a lock-in amplifier (LIA) module 500 configured to receive reference control input signal 70. The LIA module 500 receives a local oscillator signal 502 from a frequency source 90C. The reference controller 66 comprises an integrator module 504 having an input connected to an output of the LIA module 500. The integrator module 504 provides reference control signal 68 to the laser device for stabilizing the laser frequency of the laser beam 6.

Fig. 6 shows an exemplary magnetic field sensing device for determining a magnetic field, e.g. during medical imaging. The magnetic field sensing device 2A is similar to magnetic field sensing device 2 and additionally comprises a first heating laser device 510 for provision of a first heating laser beam and configured to heat the first closed container 32 of the first sensor 30 via a first heating fiber 512. The first sensor 30 optionally is a sensor device 600, see Fig. 7. The first heating fiber 512 of the magnetic field sensing device 2A is connected to the first sensor at the heating port of the first sensor 30, 600. The magnetic field sensing device 2A optionally comprises a second heating laser device 514 for provision of a second heating laser beam and configured to heat the second closed container 82 of the second sensor 80 via a second heating fiber 516. The second sensor 80 optionally is a sensor device 600, see Fig. 7. The second heating fiber 516 of the magnetic field sensing device 2A is connected to the second sensor at the heating port of the second sensor 80, 600.

The magnetic field sensing device 2A optionally comprises a reference heating laser device 518 for provision of a reference heating laser beam and configured to heat the closed container 60 of the reference sensor 58 via a reference heating fiber 520. The reference sensor 58 optionally is a sensor device 600, see Fig. 7. The reference heating fiber 520 of the magnetic field sensing device 2A is connected to the reference sensor at the heating port of the reference sensor 58, 600. It is to noted that heating of the reference sensor may be omitted in magnetic field sensing device 2A.

Fig. 7 shows an exemplary sensor device. The sensor device 600 may be used as first sensor 30 and/or second sensor 80 of magnetic field sensing device. The sensor device 600 comprises a housing 602; a closed container 604 containing an alkali gas, such as Cesium; and a plurality of ports including an input port 606, an output port 608, and optionally a heating port 610. The input port 606 is configured for receiving input fiber 612; the output port 608 is configured for receiving an output fiber 614; and the heating port 610 is, if present, configured for receiving a heating fiber 616. The housing 602 is made from an opaque, such as black, material, so that no light exits and/or travels between paths in the sensor device. This is important since high power laser light easily cause eye injuries and/or in order to prevent distortion of the laser beams used in the measurement.

The sensor device 600 comprises an absorption element 618 for absorbing at least a part of a heating laser beam from the heating fiber 616. The absorption element 618 is in thermal contact with the closed container 604 via a thermal paste arranged in thermal contact with the closed container 604 and the absorption element 618. A heating laser beam having a high power, such as larger than 0.2 W, enters the sensor device 600 through heating fiber 616 connected to the heating port 610. The heating laser beam is reflected in mirror 620 and is at least 90%, such as at least 95%, absorbed in the absorption element 618 to heat the closed container 604 of the sensor device. The heating laser beam is optically separated from the closed container 604 by the absorption element 618, the thermal paste, and the housing 602. Thus, light from the heating path which could disturb the measurement is prevented from entering the closed container 604. Further, the thermal paste is optionally arranged in the vicinity of optical windows of the closed container configured for allowing input laser beam and/or output laser beam to pass. Thereby, the alkali gas in the closed container is prevented from or has a reduced risk for condensing on the optical windows which would reduce laser beam transmission, in turn reducing laser beam transmission through the closed container or cell. Further, the overall temperature of the sensor device/closed container is increased, leading to an increased atomic density, and thereby stronger absorption of the input laser beam. In other words, the sensor device defines a heating path for feeding a heating laser beam from the heating fiber 616 towards the closed container 604, the heating path comprising the mirror 620 and the absorption element 618.

The sensor device 600 defines an input path for feeding an input laser beam from the input fiber 612 to the closed container 604. The input path 612 comprises a beamsplitter 622, a waveplate 624, a lens in the form of input lens 626, and an input coupling element 628, the input coupling element 628 comprising a window coated with an anti-reflection coating on the side or surface pointing away from the input fiber 612, i.e. on the side or surface pointing towards the input lens 626. The input path is optically separated from the heating path and thus, the heating laser beam or light therefrom is prevented from entering the input path, thus preventing distortion of the input laser beam, which may lead to erroneous determination of magnetic field strength.

The sensor device 600 defines an output path for feeding an output signal, e.g. output laser beam, from the closed container 604 to the output fiber. The output path comprises beamsplitter 622, waveplate 624, a mirror 630, a lens in the form of output lens 632, and an output coupling element 634, the output coupling element 634 comprising a window coated with an anti-reflection coating on the side or surface pointing away from the output fiber 614, i.e. on the side or surface pointing towards the output lens 632. The output path is optically separated from the heating path and thus, the heating laser beam or light therefrom is prevented from entering the output path, thus preventing distortion of the output laser beam, which may lead to erroneous determination of magnetic field strength.

Fig. 8 illustrates the effect of light absorption in a closed container/sensor with increased temperature. The graph shows that the transmission is heavily reduced (and therefore the absorption heavily increased) when the temperature in the closed container is increased from room temperature by laser heating. It is an important advantage of the present disclosure that laser heating allows for a local and directed heating of the closed container/sensor while the temperature of the scanning volume is substantially maintained (thus allowing sensing while a patient is being scanned).

With an array of magnetic field sensors or sensor devices according to the present disclosure, it should be possible to do corrections of MRI images, such that higher resolution and/or shorter scan time can be obtained.

EXAMPLE 1

Suppose that V_Ref = 19.3 GHz, e.g. as set by frequency source 90B, and that Vi can range from 0.9 to 2 GHz. And we lock the fifth lower sideband, to the atomic reference resonance frequency. In the case, where Vi = 0.9 GHz, the field sensing device is at the lowest magnetic field that the field sensing device is able to measure:

B_1 = (5 19.3 GHz + 0.9 GHz) / 13.99 GHz/T = 6.96 T.

In the case, where Vi = 2 GHz, the field sensing device is at the highest magnetic field that the field sensing device is able to measure:

B = (5 19.3 GHz + 2 GHz) / 13.99 GHz/T = 7.04 T.

Accordingly, the field sensing device has a measurement range of 7.00 ± 0.04 T.

EXAMPLE 2

Suppose that V_Ref = 24.12 GHz, e.g. as set by frequency source 90B, and that Vi can range from 0.9 to 2 GHz. And we now lock the fourth lower sideband, to the atomic reference resonance frequency. In the case, where Vi = 0.9 GHz, the field sensing device is at the lowest magnetic field that the field sensing device is able to measure:

B_1 = (4 24.12 GHz + 0.9 GHz) / 13.99 GHz/T = 6.96 T.

B = (4 24.12 GHz + 2 GHz) / 13.99 GHz/T = 7.04 T.

EXAMPLE 3

Suppose that V_Ref = 30.2 GHz, e.g. as set by frequency source 90B, and that Vi can range from 1.8 to 4 GHz. And we lock the fifth lower sideband, to the atomic reference resonance frequency. In the case, where Vi = 1.8 GHz, the field sensing device is at the lowest magnetic field that the field sensing device is able to measure:

B_1 = (5 30.2 GHz + 1.8 GHz) / 13.99 GHz/T = 10.92 T.

In the case, where Vi = 4 GHz, the field sensing device is at the highest magnetic field that the field sensing device is able to measure:

B = (5 30.2 GHz + 4 GHz) / 13.99 GHz/T = 1 1 .08 T.

Accordingly, the field sensing device has a measurement range of 1 1.00 ± 0.08 T

EXAMPLE 4

Suppose that V_Ref = 17.51 GHz, e.g. as set by frequency source 90B, and that Vi can range from 0.9 to 2 GHz. The fifth lower sideband of reference light modulator is now locked to the transition from 6²SI_/2, F = 3, ITIF = 3 to 6²P_3/2, F = 4, ITIF = 4. This transition is 8.94 GHz higher than the previous transition. In the case, where Vi = 0.9 GHz, the field sensing device is at the lowest magnetic field that the field sensing device is able to measure:

B_1 = (5 17.51 GHz + 8.94 GHz + 0.9 GHz) / 13.99 GHz/T = 6.96 T.

B = (5 17.51 GHz + 8.94 GHz + 2 GHz) / 13.99 GHz/T = 7.04 T. Accordingly, the field sensing device has a measurement range of 7.00 ± 0.04 T.

Also disclosed are methods and devices according to any one of the following items.

Item 1. A method for determining a magnetic field during medical imaging, the method comprising:

arranging a first sensor comprising a closed container containing alkali gas in the magnetic field;

providing a laser beam based on a reference control signal;

modulating a first part of the laser beam with a first control signal for provision of a first modulated laser beam;

transmitting the first modulated laser beam through the alkali gas of the first sensor for provision of a first output signal;

detecting, at a position away from the part of the magnetic field to be measured, the first output signal for provision of a first control input signal;

providing the first control signal based on the first control input signal;

obtaining a first parameter indicative of the first control signal and optionally a first secondary parameter indicative of the first control input signal; and

determining a magnetic field strength at the position of the first sensor based on the first parameter.

Item 2. Method according to item 1 , wherein determining a magnetic field strength at the position of the first sensor is also based on the first secondary parameter.

Item 3. Method according to any of items 1-2, wherein the method comprises:

arranging a reference sensor comprising a closed container containing alkali gas outside the magnetic field;

modulating a reference part of the laser beam with a reference modulation signal for provision of a modulated reference laser beam;

transmitting the modulated reference laser beam through the alkali gas of the reference sensor for provision of a reference output signal; and

providing the reference control signal based on the reference output signal. Item 4. Method according to item 3, wherein providing the laser beam based on a reference control signal comprises locking the laser frequency with a known shift relative to the atomic reference resonance frequency.

Item 5. Method according to any of items 1-4, wherein modulating the first part of the laser beam comprises modulating the first part of the laser beam to have a first frequency spectrum capable of driving a predetermined atomic transition in an alkali atom of the first sensor.

Item 6. Method according to any of items 1-5, wherein the method comprises:

arranging a second sensor comprising a closed container containing alkali gas in the magnetic field;

modulating a second part of the laser beam with a second control signal for provision of a second modulated laser beam;

transmitting the second modulated laser beam through the alkali gas of the second sensor for provision of a second output signal;

detecting, at a position away from the part of the magnetic field to be measured, the second output signal for provision of a second control input signal;

providing the second control signal based on the second control input signal; obtaining a second parameter indicative of the second control signal and optionally a second secondary parameter indicative of the second control input signal; and

determining a magnetic field strength at the position of the second sensor based on the second parameter.

Item 7. Method according to item 6, wherein determining a magnetic field strength at the position of the second sensor is based also on the second secondary parameter.

Item 8. Method according to any of items 1-7, wherein providing the first control signal based on the first control input signal comprises performing lock-in detection of an absorption frequency of the first output signal.

Item 9. Method according to any of items 1-8, the method comprising heating the first sensor to a first temperature larger than 35°C, e.g. in the range from 55°C to 75°C. Item 10. Method according to item 9, wherein heating the first sensor comprises heating the first sensor with a first heating laser beam from a first heating laser device through a first heating fiber.

Item 11. Method according to item 10, wherein a first power of the first heating laser beam is in the range from 0.2 W to 5 W, such as in the range from 0.4 W to 2 W e.g. 0.5 W or 1 W.

Item 12. Method according to any of items 1-1 1 as dependent on item 6, the method comprising heating the second sensor to a second temperature larger than 35°C, e.g. in the range from 55°C to 75°C.

Item 13. Method according to item 12, wherein heating the second sensor comprises heating the second sensor with a second heating laser beam from a second heating laser device or the first heating laser beam through a second heating fiber.

Item 14. Method according to item 13, wherein a second power of the second heating laser beam is in the range from 0.2 W to 5 W, such as in the range from 0.4 W to 2 W e.g. 0.5 W or 1 W.

Item 15. Method according to any of items 1 -14, wherein the alkali gas of the first sensor comprises cesium gas.

Item 16. Method according to any of items 1-15 as dependent on item 6, wherein the alkali gas of the second sensor comprises cesium gas.

Item 17. Sensor device comprising:

a sensor housing;

a closed container containing an alkali gas;

an input port for receiving an input fiber; and

an ouput port for receiving an output fiber.

Item 18. Sensor device according to item 17, the sensor device comprising a heating port for receiving a heating fiber. Item 19. Sensor device according to item 18, the sensor device comprising an absorption element for absorbing at least a part of a heating laser beam from the heating fiber, wherein the absorption element is in thermal contact with the closed container optionally via a thermal paste.

Item 20. Sensor device according to item 19, the sensor device comprising a heating path for feeding the heating laser beam from the heating fiber towards the closed container, the heating path comprising one or more of a mirror, and the absorption element.

Item 21. Sensor device according to any of items 17-20, the sensor device comprising an input path for feeding an input laser beam from the input fiber to the closed container, the input path comprising one or more of a beamsplitter, a waveplate, and an input coupling element.

Item 22. Sensor device according to item 21 , wherein the heating path is optically separated from the input path.

Item 23. Sensor device according to any of items 17-22, the sensor device comprising an output path for feeding an output signal, e.g. output laser beam, from the closed container to the output fiber, the output path comprising one or more of a beamsplitter, a waveplate, and an output coupling element.

Item 24. Sensor device according to item 23, wherein the heating path is optically separated from the output path.

Item 25. A magnetic field sensing device for determining a magnetic field during medical imaging, the magnetic field sensing device comprising:

a laser device for emitting a laser beam based on a reference control signal;

a first light modulator for provision of a first modulated laser beam based on a first part of the laser beam;

a first sensor comprising a closed container containing an alkali gas;

a first controller for controlling the first light modulator with a first control signal based on a first output signal from the first sensor; and

a processing device configured to determine a magnetic field strength at the position of the first sensor based on a first parameter indicative of the first control signal. Item 26. Magnetic field sensing device according to item 25, the magnetic field sensing device comprising:

a reference light modulator for provision of a modulated reference laser beam based on a reference part of the laser beam;

a reference sensor comprising a closed container containing alkali gas; and a reference controller for controlling the laser device based on a reference output signal from the reference sensor.

Item 27. Magnetic field sensing device according to any of items 25-26, the magnetic field sensing device comprising:

a second light modulator for provision of a second modulated laser beam based on a second part of the laser beam;

a second sensor comprising a closed container containing alkali gas; and a second controller for controlling the second light modulator with a second control signal based on a second output signal from the second sensor; and

wherein the processing device is configured to determine a magnetic field strength at a position of the second sensor based on a second parameter indicative of the second control signal.

Item 28. Magnetic field sensing device according to any of items 25-27, wherein the first sensor is a sensor device according to any of items 17-24.

Item 29. Magnetic field sensing device according to any of items 25-28 as dependent on item 27, wherein the second sensor is a sensor device according to any of items 17-24.

Item 30. Magnetic field sensing device according to any of items 25-29, the magnetic field sensing device comprising a first heating laser device for provision of a first heating laser beam and optionally configured to heat the closed container of the first sensor, such as via a first heating fiber.

Item 31. Magnetic field sensing device according to any of items 25-30, the magnetic field sensing device comprising a second heating laser device for provision of a second heating laser beam and optionally configured to heat the closed container of the second sensor, such as via a second heating fiber. Item 32. Medical scanner comprising a magnetic field sensing device according to any of items 25-31.

The use of the terms“first”,“second”,“third” and“fourth”,“primary”,“secondary”,“tertiary” etc. does not imply any particular order, but are included to identify individual elements. Moreover, the use of the terms“first”,“second”,“third” and“fourth”,“primary”,“secondary”, “tertiary” etc. does not denote any order or importance, but rather the terms“first”, “second”,“third” and“fourth”,“primary”,“secondary”,“tertiary” etc. are used to distinguish one element from another. Note that the words“first”,“second”,“third” and“fourth”, “primary”,“secondary”,“tertiary” etc. are used here and elsewhere for labelling purposes only and are not intended to denote any specific spatial or temporal ordering.

Furthermore, the labelling of a first element does not imply the presence of a second element and vice versa.

It may be appreciated that Figs. 1-8 comprise some modules or operations which are illustrated with a solid line and some modules or operations which are illustrated with a dashed line. The modules or operations which are comprised in a solid line are modules or operations which are comprised in the broadest example embodiment. The modules or operations which are comprised in a dashed line are example embodiments which may be comprised in, or a part of, or are further modules or operations which may be taken in addition to the modules or operations of the solid line example embodiments. It should be appreciated that these operations need not be performed in order presented.

Furthermore, it should be appreciated that not all of the operations need to be performed. The exemplary operations may be performed in any order and in any combination.

It is to be noted that the word "comprising" does not necessarily exclude the presence of other elements or steps than those listed.

It is to be noted that the words "a" or "an" preceding an element do not exclude the presence of a plurality of such elements.

It should further be noted that any reference signs do not limit the scope of the claims, that the exemplary embodiments may be implemented at least in part by means of both hardware and software, and that several "means", "units" or "devices" may be

represented by the same item of hardware.

The various exemplary methods, devices, and systems described herein are described in the general context of method steps processes, which may be implemented in one aspect by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers in networked environments. A computer-readable medium may include removable and non removable storage devices including, but not limited to, Read Only Memory (ROM), Random Access Memory (RAM), compact discs (CDs), digital versatile discs (DVD), etc. Generally, program modules may include routines, programs, objects, components, data structures, etc. that perform specified tasks or implement specific abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of program code for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps or processes.

Although features have been shown and described, it will be understood that they are not intended to limit the claimed invention, and it will be made obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the claimed invention. The specification and drawings are, accordingly to be regarded in an illustrative rather than restrictive sense. The claimed invention is intended to cover all alternatives, modifications, and equivalents.

LIST OF REFERENCES

2, 2A magnetic field sensing device

4 laser device

6 laser beam

10 reference beam splitter

12 first beam splitter

14 second beam splitter

16 reference part of laser beam

18 first part of laser beam

20 second part of laser beam

22 first light modulator

24 first modulated laser beam

26 first input fiber

30 first sensor

32 first closed container

34 first photo detector

36 first output signal

38 first output fiber

40 medical scanner

42 first controller

44 first control signal

46 first control input signal

48 processing device

50 reference light modulator

52 modulated reference laser beam

54 reference input fiber

58 reference sensor

60 closed container of reference sensor

61 environment with zero or low magnetic field

62 reference photo detector

64 reference output signal

66 reference controller

68 reference control signal

70 reference control input signal

72 second light modulator 74 second modulated laser beam

76 second input fiber

80 second sensor

82 second closed container

84 second photo detector

86 second output signal

88 second output fiber

90A frequency source

90B frequency source

90C frequency source

90 D frequency source

90 E frequency source

92 second controller

94 second control signal

96 second control input signal

97 beam splitter

98 waveplate

99 filter

99A reflector/mirror

100 method for determining a magnetic field during medical imaging

S102 arrange

S102A arranging a non-metallic first sensor comprising a closed container containing alkali gas in the magnetic field

S102B arranging a reference sensor comprising a closed container containing alkali gas outside the magnetic field

S102C arranging a non-metallic second sensor comprising a closed container containing alkali gas in the magnetic field

S104 providing a laser beam based on a reference control signal

S104A locking the laser frequency of the laser beam at a laser frequency

S106 modulate

S106A modulating a first part of the laser beam with a first control signal for provision of a first modulated laser beam

S106B modulating a reference part of the laser beam with a reference modulation signal for provision of a modulated reference laser beam S106C modulating a second part of the laser beam with a second control signal for provision of a second modulated laser beam

S108 transmit

S108A transmitting the first modulated laser beam through the alkali gas of the first sensor for provision of a first output signal

S108B transmitting the modulated reference laser beam through alkali gas of the reference sensor for provision of a reference output signal

S108C transmitting the second modulated laser beam through the alkali gas of the second sensor for provision of a second output signal

S110 detect

S110A detecting, at a position away from the part of the magnetic field to be measured, the first output signal for provision of a first control input signal, by means of lock-in detection

S110C detecting, at a position away from the part of the magnetic field to be measured, the second output signal for provision of a second control input signal, by means of lock-in detection

S112 provide

S112A providing the first control signal based on the first control input signal

S112C providing the second control signal based on the second control input signal S114 obtain

S114A obtaining a first parameter indicative of the first control signal and optionally a first secondary parameter indicative of the first control input signal

S114C obtaining a second parameter indicative of the second control signal and optionally a second secondary parameter indicative of the second control input signal

S116 determine

S116A determining a magnetic field strength at the position of the first sensor based on the first parameter and optionally the first secondary parameter

S116B determining/providing the reference control signal based on the reference output signal

S116C determining a magnetic field strength at the position of the second sensor based on the second parameter and optionally the second secondary parameter

400 lock-in amplifier module

402 local oscillator signal

404 integrator module

406 voltage-controlled oscillator 500 lock-in amplifier module

502 local oscillator signal

504 integrator module

510 first heating laser device

512 first heating fiber

514 second heating laser device

516 second heating fiber

518 reference heating laser device

520 reference heating fiber

600 sensor device

602 housing

604 closed container

606 input port

608 ouput port

610 heating port

612 input fiber

614 output fiber

616 heating fiber

618 absorption element

620 mirror of heating path

622 beamsplitter

624 waveplate

626 input lens

628 input coupling element

630 mirror

632 output lens

634 output coupling element

Claims

1. A method for determining a magnetic field during medical imaging, the method comprising:

providing a laser beam based on a reference control signal;

providing the first control signal based on the first control input signal;

2. Method according to claim 1 , wherein determining a magnetic field strength at the position of the first sensor is also based on the first secondary parameter.

3. Method according to any of claims 1-2, wherein the method comprises:

providing the reference control signal based on the reference output signal.

4. Method according to claim 3, wherein providing the laser beam based on a reference control signal comprises locking the laser frequency with a known shift relative to the atomic reference resonance frequency.

5. Method according to any of claims 1-4, wherein modulating the first part of the laser beam comprises modulating the first part of the laser beam to have a first frequency spectrum capable of driving a predetermined atomic transition in an alkali atom of the first sensor.

6. Method according to any of claims 1-5, wherein the method comprises:

7. Method according to claim 6, wherein determining a magnetic field strength at the position of the second sensor is based also on the second secondary parameter.

8. Method according to any of claims 1-7, wherein providing the first control signal based on the first control input signal comprises performing lock-in detection of an absorption frequency of the first output signal.

9. Method according to any of claims 1-8, the method comprising heating the first sensor to a first temperature larger than 35°C, e.g. in the range from 55°C to 75°C.

10. Method according to claim 9, wherein heating the first sensor comprises heating the first sensor with a first heating laser beam from a first heating laser device through a first heating fiber.

1 1. Method according to any of claims 1-10, wherein the alkali gas of the first sensor comprises cesium gas.

12. A magnetic field sensing device for determining a magnetic field during medical imaging, the magnetic field sensing device comprising:

a laser device for emitting a laser beam based on a reference control signal;

a first sensor comprising a closed container containing an alkali gas;

a processing device configured to determine a magnetic field strength at the position of the first sensor based on a first parameter indicative of the first control signal.

13. Magnetic field sensing device according to claim 12, the magnetic field sensing device comprising:

14. Magnetic field sensing device according to any of claims 12-13, the magnetic field sensing device comprising:

15. Medical scanner comprising a magnetic field sensing device according to any of claims 12-14.