Probe

Abstract

A probe 2 is disclosed that comprises a magnetic field sensor 3, an elastic element 10 and a permanent magnet 11 for detecting a magnetic object on or inside of a body. The magnet 11 is connected the

Description

Field The present invention relates to a probe for particular, but not exclusive use, in 5 surgery or therapy. Background Sentimag (RTM) is a platform which can be used to locate magnetic material which may be introduced into a body, for example, for locating a tumour. The magnetic 10 material may take the form of a pellet or a tracer solution. Aspects of the Sentimag (RTM) are described in US 11 592 501 Bl. Summary According to a first aspect of the present invention there is provided a probe comprising a magnetic field sensor, a resilient member, and a permanent magnet. The permanent magnet is operably coupled to the magnetic field sensor via the resilient member so as to allow the permanent magnet to move relative to the magnetic field sensor. This can provide a sensitive magnetic field sensor. The magnetic field sensor may comprise a diamond having defect centres. The resilient member may comprise a piece of foam material. The probe may further comprise a rigid member, wherein the magnetic field sensor is attached to the rigid member, the resilient member is attached to the rigid member and the permanent magnet is attached to the resilient member. The probe may be configured to be handheld. The probe may further comprise an optical fibre having an end, wherein the magnetic field sensor is attached to the end of the optical fibre. The probe may further comprise at least one reflector attached to an opposite side of the magnetic field sensor from the optical fibre so as to reflect light towards the optical fibre. The probe may further comprise thermally conductive paste covering the magnetic field sensor. The probe may further comprise reflective foil enveloping the end of the optical fibre and the magnetic field sensor. According to a second aspect of the present invention there is provided a probe, comprising a magnetic field sensor comprising a diamond having defect centres, and a coil wound around the magnetic field sensor, responsive to an alternating signal, to generate an alternating magnetic field. According to a third aspect of the present invention there is provided a probe, comprising a permanent magnet arranged to apply an Inhomogeneous field to a given volume and a diamond having defect centres for sensing magnetic field in a volume which includes the given volume. According to a fourth aspect of the present invention there Is provided a system comprising the probe of first, second or third aspect, and a measurement system for performing a measurement of the magnetic field sensor. The measurement system may be configured to perform an optically detected magnetic resonance measurement of defect centres. According to a fifth aspect of the present invention there Is provided apparatus comprising a detector having first and second photodiodes, a beam splitter for splitting a first signal into a first path towards the detector, a dichroic beam splitter for directing a second signal into a second path towards the second photodiode of the detector and a movable neutral density filter disposed In the first path between the beam splitter and the first photodiode. The movable neutral density filter may be a servo-driven neutral density filter. According to a sixth aspect of the present invention there is provided use of a diamond having defect centres as a magnetic field sensor in surgery or therapy. Brief Description of the Drawings Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic block diagram of a sensing system which includes a probe for use in surgery; Figure 2 is a schematic side view of a probe which includes a first sensing arrangement which includes a magnetic field sensor, a resilient member, and a magnet; Figure 3 schematically illustrates change in separation between a magnetic field sensor and a magnet; Figure 4 is a schematic side view of a probe which includes a magnetic field sensor and a coil for generating an alternating magnetic field; Figure 5 is a schematic side view of a probe which includes a magnetic field sensor and a fixed nanomagnet for generating local magnet field; Figure 6 is schematic block diagram of a balancing system; Figure 7 is a schematic side view of a diamond and the end of an optical fibre; and Figure 8 is a schematic side view of the diamond and optical fibre shown in Figure 7 with an arrangement to improve light collection and to increase thermal conduction. Detailed Description Referring to Figure 1, a sensing system 1 is shown for use in surgery, diagnosis or other activity requiring or benefiting from precise positioning or location of an object. The sensing system 1 comprises a probe 2 (or "sensor head") which includes at least one magnetic field sensor 3 (or "magnetometer") for directly or indirectly sensing an object 4 in the form of a magnetic seed (or simply "seed") lying on, at or beneath a surface 5 of a body 6, such as human patient, subject or specimen (which may or may not be biological). The seed 4 can take the form of bead or particle of ferromagnetic material, ferrimagnetic material or paramagnetic material. The seed 4 may be solid or instead may be a tracer solution that could be injected. The tracer solution may be iron oxide nanoparticles in water. The probe 2 comprises a probe body B that can take the form of an instrument which can be handheld, that is, capable of being wielded (in other words, held and used) by hand (not shown). The probe 2 can take the form of an endoscope. For example, the probe 2 can be inserted into an orifice or passage, such as, the mouth or throat. The i ..ned during an Invasive procedure, such as laparoscopic surgery. In this case it would be inserted into a patient though a surgical cut. This cut could be small, as In keyhole surgery, or larger in non-keyhole surgery. The magnetic field sensor 3 takes the form of a diamond 7 containing an ensemble of defect centres 8, in particular, nitrogen vacancy centres (or "NV centres"). As will be explained in more detail later, the probe 2 can Include a resilient member 10, such as a spring or a metal cantilever or a block of resilient material (for example, foam consisting of an elastomeric material, rubber or soft plastic), coupled to a permanent magnet 11. The probe 2 can include a coil 12 (Figure 4) for generating an alternating magnetic field or a magnetic field pulse 13 in a given volume 14 which can be used to excite a seed 4 whose response can be detected by the magnetic field sensor 3. The probe 2 can include a permanent magnet 16 (Figure 5) for applying a bias field 17 in a given volume 18 and the magnetic field sensor 3 can be used to sensing the magnetic field in a volume 19 which includes the given volume 18. The system 1 includes a measurement system 21 for measuring optically detected magnetic resonance (ODMR.) of a defect centre 8 in the diamond 7. The measurement system 21 is coupled to the probe 2 via an optical fibre 22 (or other light transmission arrangement) and a microwave transmission arrangement 23, for example, in the form of a co-axial cable. At the end of the probe 2, near the diamond 7, a resonant microwave structure or impedance-transforming structure (not shown) can be used to increase the microwave power at the diamond without increasing the heating of the probe head. This can take the form of a planar resonant structure, an energy recycling / phase correcting Inline capacitor or a transformer to drive a current loop more efficiently from 50 ohm near the probe head. To join the optical fibre 22 to the diamond 7 with efficient optical excitation and collection, one or more small lenses (not shown) can be used, such as ball lenses which may have a diameter of, for example, between 0.5 mm and 3 mm. The measurement system 21 includes an optical system 31 which includes a laser (or other light source) 32 (Figure 6) for generating an excitation signal 33 (Figure 6), a ^>4 (Figure 6) comprising two photodiodes 35i, 352 (Figure 6) for comparing a sample of the excitation signal 33 and a response signal 37 from the diamond 7, and other optic components 36 (Figure 6), 38 (Figure 6), 40 (Figure 6). The measurement system 31 also includes a microwave system 41 which includes a microwave source (not shown) which generates a microwave signal 42. The optical and microwave systems 21, 31 are controlled by a control system 50, for example, in the form of a computer system. As explained earlier, the magnetic field sensor 3 is based on NV centres 8 in diamond 7. The diamond 7 has a diameter (or maximum dimension) of between 0.1 mm and 2 mm, for example, between 0.3 mm and 0.8 mm or less. The diamond 7 can be smaller than 0.1 mm. Using a smaller diamond 7 can improve spatial resolution. A smaller sensor head 2 can be brought closer to the subject 6 and allows sensing of smaller objects 4 thereby improving spatial resolution. The diamond 7 is located on the end of an optical fibre 22. A green excitation signal 33 can be delivered to the diamond 7, and a red fluorescence 37 can be collected from the diamond 7 via the optical fibre 22. Probe Referring to Figure 2, the magnetic field sensor 7, in this case a diamond having a rectangular cuboid shape, is attached (for instance using an adhesive or a clamp) to a rigid support 9. A rigid support 9 may take the form of a plate, a block, a 'U'-shaped bracket or a circumferential frame of non-magnetic material, such as plastic or nonferrous metal. The rigid support 9 runs down at least one side of the magnetic field sensor 7. In some cases, such as a pair of plates or blocks, the 'U'-shaped bracket or a circumferential frame, the rigid support 9 can run down opposite sides of the magnetic field sensor 7. The rigid support 9 is attached, at a distal end via the resilient member 10, to the permanent magnet 11 so that the permanent magnet 11 is separated by a gap 12 having a gap size, s. The permanent magnet 11 can move along an axis, in this case the z axis, relative to the magnetic sensor 7 and so the gap size, s, can vary. Movement of the permanent magnet 11 in other axes, namely x and y axes, may be constrained. The permanent magnet 11 may take the form of a cantilever beam, that is, attached to the resilient member 10 at one end or edge. The permanent magnet 11 may, however, take the form of a supported beam, attached to the resilient member 10 (or resilient members 10) at two ends or edges. The permanent magnet 11 takes the form of a magnet, such as a rare-earth magnet such as a neodymium (NdFeB) magnet or a samarium-cobalt (SmCo) magnet, or other strong permanent magnet, for example, capable of producing a magnetic field greater than or equal to 0.5 T. The stronger the magnet 11, the stronger the force the magnet 11 experiences from the object 4. Also, a stronger magnet 11 produces a larger effect for the magnetometer 7 to detect. The magnetic susceptibility of the object 4 changes how much the permanent magnet 11 is attracted to the object 4. The magnetic field sensor 7 detects that the permanent magnet 11 has moved. Thus, as the probe 2 is moved relative to the object 4, for example, as it brought closer to the object 4, moved over the object 4 and / or tilted with respect to the object 4, the magnetic field sensor 7 generates a signal corresponding to the size of the gap 12. Referring also to Figure 3, as the probe 2 is moved over the surface 5 of the body 6 and passes over the object 4, the permanent magnet 11 is attracted to the object 4 and moves, for example, closer to the object 4. The magnetic field sensor 7 detects that the permanent magnet 11 has moved. If the sensor 7 is calibrated, then this can be used to determine a distance, d, between the probe 2 (specifically, the magnet 11) and the object 4. Using three orthogonally arranged magnetic field sensor arrangements (for example, each with respective permanent magnet), a probe 2 can be used to determine a set of three orthogonal distances. Referring again to Figure 1, the sensor head 2 can take the form of a cylinder having a diameter, D, of between 1 and 8 mm, for example, 3 mm. A larger sensor head 2 can be used to accommodate more than one diamond 7. For example, an array of diamonds 7 can be used to be provide an image (not shown), with each diamond 7 providing a pixel. Using a plurality of diamonds 7 enables magnetic gradiometry and magnetic tensor gradiometry. The sensor head 2 can have a length, L, of, for example, between 5 to 20 cm thereby making it convenient to hold. A microwave cable 23 can run next to the optical fibre 22. At the end of the microwave cable 23 can be provided with a microwave delivery system such as an antenna (not shown) arranged to excite the diamond 7. The microwave delivery system (not shown) can take the form of a loop or coil of wire wrapped around the diamond 7. The system can be used for vector magnetometry. For example, tensor gradiometry can be performed using multiple diamonds or with single diamond, and reference is made to Alex Newman et. al.: "Tensor gradiometry with a diamond magnetometer" arXiv:2307.05438vl (11 July 2023) which is incorporated herein by reference. As explained earlier, using a stronger permanent magnet helps to increase the force that the magnet 11 experiences from the object 4 and a greater movement for the magnetometer 7 to detect. This also allows a seed having a weaker magnetic seed, that is a seed having a lower value of remanence, to be used. Using a smaller, more sensitive magnetometer 7 it is possible to achieve sensitivity of 30 pT / VHz. Better sensitivity allows magnetic objects 4 to be detected further away. If the seed 4 has known magnetic properties (for example, a known remanence), then it is possible to calibrate the probe 2 with the known seed 4 and, thus, determine a distance, d, between the probe 2 and seed 3 and the orientation of the seed 4. During assembly of the sensor head 2, in particular, while the diamond 7 is being glued to the optical fibre 22 or the glue is drying, photoluminescence can be measured which allows the diamond 7 to re-positioned so as to maximize the photoluminescence signal. Procedures and uses As explained earlier, the sensing system 1 can be used in surgery and / or diagnosis. For example, a unmagnetized seed 4 can be located in a patient or subject (for example, by injection or ingestion) and can be magnetised prior to measurement by applying a strong (for example, >0.5T) magnetic field, for example, using a eo^^oooot- o- electromagnet. The procedure may be performed using the magnetised seed 4 and the sensing system 1. After the procedure, the seed can be demagnetised, for example, by applying an alternating field which decreases in strength. The sensing system 1 can be used during magnetic resonance imaging (MRI) and in magnetic induction tomography (MIT). Diamond-based magnetometry enjoys a high dynamic range which makes it of particular use in high magnetic field environments. Referring to Figure 4, in magnetic induction tomography, the probe 2 can be provided with an electromagnet 13 at the end of the sensor head 2 which is used to generate an ac magnetic field or magnetic field pulses 14 which Is applied to a metal seed 4. Using a sensitive diamond-based magnetometer in the probe 2 allows detection of a weakly magnetic seed 4. As explained earlier, an arrangement employing a movable permanent magnet and a sensitive diamond-based magnetometer can be used to detect a weakly magnetic seed indirectly via the movement of the permanent magnet. A sensitive diamond-based magnetometer can be used in a probe 2 to allow direct detection of a weakly magnetic seed 4. Referring to Figure 5, a seed 4 may be weakly ferromagnetic (for example, it may be made from stainless steel). A small permanent bias magnet 16 can be used to Inject magnetic flux 17 into the seed 4. The seed 4 distorts the flux 17 which is then detected by the magnetic field sensor 7. Further details can be found in WO 2022 096891 Al which is incorporated herein by reference. In addition to or instead of the permanent bias magnet 16, an electromagnet (not shown) can be used to inject time-varying magnetic flux into the seed or the tracer solution. This can be used to vary the induced magnetisation in the material being detected, particularly when detecting paramagnetic materials. Materials can be detected by their phase shifts in the signal or data. Balancing system As explained earlier, the system 1 (Figure 1) uses ODMR-based measurement of a diamond 7 employing a signal detector in the form of a balanced photodetector —Photodiodes which are used to compare a sample of the excitation signal and a response signal. This approach, however, can suffer from drift in the intensities of each signal which can lead to imbalance. Referring to Figure 6, a system 51 for balancing excitation and photoluminescence signals 33, 37 for a signal detector 34 is shown. The signal detector 34 includes first and second photodiodes 35i, 35?. The first photodiode 35i is used for detecting the green excitation signal 33 and the second photodiode 35z is used for detecting the red photoluminescence signal 37. The system 51 includes a beam splitter 36 for splitting the green excitation signal 33 into a first path 37 towards the detector 34, and a dichroic beam splitter 38 for directing the photoluminescence signal 37 into a second path 39 to the second photodiode 35? of the detector 34. The system 51 includes a servo-controlled neutral-density filter 40 in the first path 37 between the beam splitter 36 and the first photodiode 35i. The servo-controlled neutral-density filter 40 is configured to adjust the amount of green light 33 reaching the balanced detector 34. The neutral-density filter could be darker on one end than the other, with the servo moving this filter so that more or less light gets through depending on the position of the filter with respect to the fixed light path. In particular, the servo-controlled neutral-density filter 40 Is arranged to keep the amount of green light 33 reaching the detector 34 to be about the same (e.g., to within 5%) as the amount of red light 37 reaching the detector 34 over a timescale, T, of about 10 seconds. This can help to avoid the problem of the detector 38 suffering from drift in the intensity of the two input beams 33, 37. The detector 34 subtracts one beam intensity from the other to remove noise (such as, laser intensity noise) that changes on a timescale faster than one Hertz. Other modifications Referring to Figure 7, the diamond 7 and the optical fibre 22 are shown. The diamond 7 is a cube having a side length, a, of, for example, 0.5 mm. The diamond 7 is placed on the end 61 of a bare optical fibre 22 which has a diameter, 0, of, for example, 0.9 mm. The diamond 7 is secured to the end of the optical fibre 22 using optically transmissive glue (not shown). The diamond 7 has a back face (or the optical fibre 22. Referring to Figure 8, a reflective foil 63 (such as aluminium foil) is attached to the back 62 of the diamond 7 to reflect red photoluminescence back into the fibre 22. The diamond 7 can be surrounded in silicone thermal grease 64 (herein also referred 5 to as "paste" or "compound") having high thermal conductivity. Reflective foil 65 is tightly wrapped round the entire end portion of the fibre 22 and the diamond 7 for light tightness. The reflective foil 65 is glued and taped in place thereby creating a good thermal connection between the diamond 7 and the foil 65. 10 This approach helps to keep the probe 2 cool. For example, using 250 mW of 532 nm green laser power from a 532 GEM laser (from Novanta Photonics), 33 dBm of microwave power coupled to the diamond via a 0.2 mm copper wire connected directly to the diamond, a probe tip operating temperature is measured to be about 40 °C. Also, using this set up sensing system 1 exhibited a sensitivity of 3 nT / VHz. 15 It will be appreciated that many modifications may be made to the embodiments hereinbefore described.

Claims

1. A probe (2), comprising:• a magnetic field sensor (3);• a resilient member (10); and■ a permanent magnet (11);wherein the permanent magnet is operably coupled to the magnetic field sensor via the resilient member so as to allow the permanent magnet to move relative to the magnetic field sensor.

2. The probe of claim 1, wherein the magnetic field sensor (3) comprises a diamond (7) having defect centres (8).

3. The probe of claim 1 or 2, wherein the resilient member (10) comprises a piece of foam material.

4. The probe of claim 1, 2 or 3, further comprising: ■ a rigid member (9),wherein the magnetic field sensor (3) is attached to the rigid member (7), the resilient member (10) is attached to the rigid member and the permanent magnet is attached to the resilient member.

5. The probe of claim 1 or any one of claims 2 to 4, wherein the probe (2) is configured to be handheld.

6. The probe of claim 1 or any one of claims 2 to 5, further comprising:• an optical fibre (22) having an end (61),wherein the magnetic field sensor (3) is attached to the end of the optical fibre.

7. The probe of claim 6, further comprising:• at least one reflector (63) attached to an opposite side of the magnetic field sensor (3) from the optical fibre so as to reflect light towards the optical fibre.

8. The probe of claim 6 or 7, further comprising:• thermally-conductive paste (64) covering the magnetic field sensor (3).

9. The probe of claim 8, further comprising:• reflective foil (65) enveloping the end of the optical fibre and the magnetic field sensor (3).

10. A probe, comprising:■ a magnetic field sensor (3) comprising a diamond (7) having defect centres (8); and• a coil (13) wound around the magnetic field sensor, responsive to an alternating signal, to generate an alternating magnetic field.

11. A probe, comprising:• a permanent magnet arranged to apply an inhomogeneous field to a given volume; and• a diamond having defect centres for sensing magnetic field in a volume which includes the given volume.

12. A system comprising:■ the probe of any one of claims 1 to 11; and■ a measurement system (21) for performing a measurement of the magnetic field sensor.

13. The system of claim 12, wherein the magnetic field sensor (3) comprises a diamond (7) having defect centres (8) and the measurement system (21) is configured to perform an optically detected magnetic resonance measurement of defect centres.

14. A apparatus comprising:■ a detector (34) having first and second photodiodes (35i, 35?);• a beam splitter (36) for splitting a first signal into a first path (37) towards the detector;• a dichroic beam splitter (36) for directing a second signal into a second path (39) towards the second photodiode (35z) of the detector; and■ a movable neutral density filter (40) disposed in the first path (37) between the beam splitter and the first photodiode (35i).

15. Use of a diamond (7) having defect centres (8) as a magnetic field sensor in surgery or therapy.

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