WO2006056739A2

WO2006056739A2 - Surgical tag, magnetometer, and associated system

Info

Publication number: WO2006056739A2
Application number: PCT/GB2005/004338
Authority: WO
Inventors: Anthony James Maxwell; Stavros Michael Stivaros
Original assignee: Quantum Medical Technology Limited
Priority date: 2004-11-23
Filing date: 2005-11-10
Publication date: 2006-06-01
Also published as: GB2435591A; GB0711582D0; WO2006056739A3

Abstract

Disclosed is a marker, dimensioned for insertion via an introduction device into tissue of the human or animal body, for marking a site for possible further examination and/or treatment, comprising: a magnetic material, for enabling the marker to be located from outside the body, using a suitable detector. Also disclosed is a magnetometer for detecting the presence of a magnetic field, wherein the magnetometer is arranged to perform an operation whereby the effects of any ambient magnetic field may be disregarded in any subsequent detecting operations.

Description

Surgical Tag, Magnetometer, and associated system

The present invention relates to a surgical marker or tag for marking a location in tissue for possible further procedures or investigation. The marker finds particular, but not exclusive, use in soft contiguous or non-hollow tissue, such as breast tissue or liver tissue. It also relates to a magnetometer (also known as a gaussmeter) . In particular, the present invention relates to a three- axis magnetometer. Embodiments of the present invention find particular, but not exclusive, use in the field of scanning for surgical marker devices. Other uses outside the field of surgery will be apparent to the skilled man.

During medical examinations, it is often desirable to tag or mark the position of a site of interest such that it may subsequently be re-located for further investigation or treatment, possibly by using some therapeutic or diagnostic technique. Often the site of interest to be marked is the site of some abnormality, such as a suspected tumour.

Using breast tissue as an example, the sequence of events culminating in final treatment are summarised below. The patient may present via a number of routes e.g. an abnormality having been found on a screening examination; by virtue of symptoms; due to an abnormality found on routine or targeted imaging e.g. X-ray (mammography), ultrasound, magnetic resonance imaging or nuclear medicine studies; or due to a biochemical abnormality. Further assessment may then include additional imaging using the aforementioned modalities with biopsy of the tissue of interest.

During percutaneous biopsy, a device is sometimes used to mark the area biopsied such that it can be relocated at a later date. Such a device is shown in Figure 1. This device 20, is a clip which comprises a single short

(approx 10mm) piece of wire, which is bent around a central point to resemble the Greek letter alpha (α) . Other shapes, such as s-shaped or Greek letter omega (Ω) devices, are also used. These devices may or may not be enclosed in a plastics material. Such devices are sometimes known by one of the following names: Tissue marker clip/coil/device; marker clip/coil/device; breast biopsy clip; biopsy site marker/clip/coil/device; or by reference to specific device type as used in the biopsy process itself, for example the Mammotome Micromark device.

This device 20 is used to mark the site of an abnormality during image guided biopsy, particularly in cases where there is a risk of difficulty in locating the site subsequently, as can happen when the abnormality is small and could be removed substantially or entirely by biopsy sampling.

To position the clip 20, it is passed through a rigid or flexible introducer needle, cannula or tube which may be inserted into the tissue using image guidance following removal of the biopsy device. Some marker devices are designed to be passed through the bore of the biopsy device itself prior to its removal from the tissue, e.g. the Micromark clip for the Mammotome biopsy system. Being a metal clip, it is visible using X-ray imaging. Variants are available which are visible on ultrasound. Suitable materials for the device include stainless steel or titanium. To increase the visibility of the clip under ultrasound, it may have bioabsorbable gel or fibre pellets introduced with it, or a bioabsorbable gel or fibre sponge pad attached to, or integrated with, it. Suitable materials include resorbable polylactic acid, polyglycolic acid, gelatine and collagen.

Continuing treatment of a suspected breast abnormality will then depend on the results of the pathological analysis of the biopsy specimen.

If, following the percutaneous biopsy, a patient requires surgical removal of tissue from the region of the biopsy site, then a wire localisation device 10 may need to be inserted to aid in the actual surgical excision process. A suitable prior art device is known as a Reidy breast localisation wire, and is shown in Figure 2. This Reidy wire, or similar wire-localisation device, can be inserted under X-ray or ultrasound guidance, using the clip 20 as a target.

The Reidy device 10 comprises a crosswire member 12 and an elongate wire 14 which extends from the crosswire member. The Reidy device 10 is introduced into the breast tissue, through a needle and is positioned adjacent to the abnormality. The wire 14 is left with its end protruding through the skin so that it provides a locator for the position of the crosswire member 12. The Reidy device 10 is usually inserted under X-Ray (mammography) , ultrasound guidance or magnetic resonance imaging guidance.

The crosswire member 12 forms a stable anchor in the soft breast tissue, comprising, as it does, four distinct end points. The localisation wire usually comprises braided stainless steel wire, although variants exist which use a solid wire.

Various alternative devices, similar to the Reidy device are also used. In particular, a device known as a Kopans wire is commonly used. This device comprises a wire which features a simple hook or barb at its distal end, for securing the wire in the breast. The device resembles a tick. Still further devices are used which have differently shaped tips, including curved tips, and wires which are re-positionable. Wire localisation devices are often referred to as hook wires .

After the wire localisation device is inserted, prior to surgery, the patient is left with the fine wire protruding from the breast, and surgery is normally performed a short time after the insertion of the wire localisation device. These devices, for example the Reidy device, should not be left in-situ for long before needing removal as they protrude through the skin, posing an infection risk and causing discomfort.

During the surgery, which may be for excision of tissue for either diagnostic or therapeutic purposes, the surgeon makes an incision in the skin over the estimated position of the crosswire member 12. This estimated position, which may be obtained by location with ultrasound followed by skin marking or by reference to mammograms taken after insertion of the wire, is liable to inaccuracy. The surgeon dissects the breast tissue in the direction of the tip of the crosswire wire member 12. It is often difficult for the surgeon to feel the crosswire member 12 within the tissue, and consequently it may be difficult for the surgeon to ascertain precisely where to resect tissue. It is usually desirable for the surgeon to remove the tissue around the crosswire member 12 in a single piece, with a predetermined minimum distance from the resection plane to the crosswire member 12, and this minimum distance may be different in different axes within the breast. Failure of the surgeon to accurately ascertain the location of the crosswire member 12 within the tissue may result in the removal of inappropriate tissue, or inadequate or excessive volumes of tissue being excised.

In some circumstances the surgeon may make the skin incision at the point where the wire 14 projects from the skin and then trace the path of the wire down towards the crosswire member 12. This often presents similar problems for the surgeon in correctly ascertaining the appropriate resection planes.

A prime reason for the use of wire localisation devices in breast surgery is that it is difficult, if not impossible, to use mammographic or ultrasound equipment during surgery. Hence, it is more convenient to mark the site of the abnormality in a separate procedure, prior to surgery.

Magnetometers are known in the prior art and are used to measure the strength of a magnetic field in a particular region. Often, such devices include a processor/readout unit to which is coupled a probe, which includes the actual sensor. Such devices may be used to measure the earth' s magnetic field, and are often used to provide compass functionality. As such, the devices are intended to measure relatively dispersed rather than localised fields .

A typical prior art magnetometer is provided Metrolab Instruments SA, and is known as a DC 3-axis Hall Magnetometer (THM 7025) . However, the use of such a device in locating implanted magnetic markers is not practicable for several reasons which will become clear in the following description.

A problem with such prior art magnetometers is that they are incapable of accurately detecting magnetic field strengths which may be associated with magnetic surgical implants or markers .

It is an aim of embodiments of the present invention to address this and other shortcomings with prior art surgical tags and/or magnetometers, whether described herein or not.

According to an aspect of the present invention, there is provided a marker, dimensioned for insertion via an introduction device into tissue of the human or animal body, for marking a site for possible further examination and/or treatment, comprising: a magnetic material, for enabling the marker to be located from outside the body, using a suitable detector. Preferably, the marker is substantially encapsulated in a substantially biologically inert material.

Preferably, the substantially biologically inert material comprises a plastics material.

Preferably, the introduction device is one of a rigid or flexible introducer needle, a cannula or a tube.

Preferably, the marker has a maximum diameter of 2mm and a maximum length of 15mm.

More preferably, the marker has a maximum diameter of 1.5mm and a maximum length of 6mm.

Preferably, the magnetic material comprises a NdFeB (Neodymium Iron Boron) -type magnet.

Preferably, the marker additionally includes an anchor for co-operation or engagement with the tissue.

Preferably, the anchor comprises a length of wire which extends beyond the substantially biologically inert material.

Preferably, the wire comprises one of stainless steel, titanium or nitinol.

Preferably, the wire is configured to provide a plurality of barbs for engaging with the tissue.

Preferably, the anchor is integrally formed from the substantially biologically inert material . Preferably, the wire further comprises a localisation wire.

Preferably, the marker further comprises a material visible under ultrasound.

According to another aspect of the present invention there is provided a magnetometer for detecting the presence of a magnetic field, wherein the magnetometer is arranged to perform an operation whereby the effects of any ambient magnetic field may be disregarded in any subsequent detecting operations .

Preferably, the magnetometer is arranged to perform a calibration operation to measure the ambient magnetic field such that said measured ambient magnetic field can be subtracted from any subsequent measurements.

Preferably, the calibration operation includes the operation of moving a probe of the magnetometer through a range of movements in three mutually perpendicular axes so that the probe substantially fully experiences the ambient magnetic field.

Preferably, the magnetometer comprises a processor unit coupled to a probe unit.

Preferably, the probe comprises a three-axis magneto- resistive sensor. Preferably, the processor unit is arranged to receive signals from the probe unit indicative of the strength of a measured magnetic field.

Preferably, the magnetometer is provided with sensory feedback means for indicating the strength of a measured magnetic field.

Preferably, the sensory feedback means comprise one or more of: a numerical readout, a bar-graph display, and an audio transducer.

Preferably, the audio transducer comprises a speaker for emitting a tone, the pitch of which is indicative of the measured magnetic field strength.

Preferably, the magnetometer is operable is a calibration mode and also in a measurement mode.

According to a further aspect of the present invention, there is provided a calibration method whereby a magnetometer is rendered insensitive to an ambient magnetic field.

According to a further aspect of the present invention, there is provided a method of locating a previously implanted tag, according to an aspect of the preset invention, using a magnetometer, according to another aspect of the present invention.

According to a still further aspect of the present invention, there is provided a system comprising a magnetometer according to a previous aspect and at least one tag according to a previous aspect.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which:

Figure 1 shows a first prior art marker device;

Figure 2 shows a second prior art marker device;

Figure 3 shows a marker device according to an embodiment of the invention;

Figure 4 shows a marker device according to an embodiment of the invention;

Figure 5 shows a marker device according to an embodiment of the invention;

Figure 6 shows a marker device according to an embodiment of the invention;

Figure 7 shows a marker device according to an embodiment of the invention;

Figure 8 shows a marker device according to an embodiment of the invention;

Figure 9 shows a marker device according to an embodiment of the invention; Figure 10 shows a marker device according to an embodiment of the invention;

Figures lla-f show various marker device according to embodiments of the invention;

Figure 12 shows a typical scan of a breast in an attempt to locate an implanted marker;

Figure 13 shows a second iteration of a scan;

Figure 14 shows a third iteration of a scan;

Figure 15 shows a scan after an incision has been made in the breast;

Figures lβa and 16b show different measured responses depending on magnet orientation;

Figure 17 shows a schematic block diagram of a magnetometer according to an embodiment of the present invention;

Figure 18 shows a flowchart illustrating a magnetometer calibration process according to an embodiment of the invention; and

Figure 19 shows a flowchart illustrating a magnetometer seek process according to an embodiment of the present invention.

Figure 3 shows the most basic embodiment of the present invention 50, which comprises a magnetic material 52 encapsulated in a substantially biologically inert material 54. The marker 50 is dimensioned such that it is suitable for insertion into suspect tissue via a suitable introduction device, which may comprise one of a rigid or flexible introducer needle, a cannula or a tube. In order to be introduced in such a manner, the device should be no larger than approximately 2mm in diameter and 15mm in length. In practice, a preferred size of marker has a diameter of approximately 1.5mm and a length of approximately 6mm.

Physically larger, and hence stronger, magnets may be used with patients having larger breasts, since the marker is likely to be positioned at a greater distance from the surface of the skin. The surgeon in each case can make a decision on the most appropriate size of device to use depending on the morphology of each patient .

A preferred form of magnetic material is Neodymium Iron Boron (NdFeB) , which can be compression moulded for example, as manufactured by Kane Magnetics

(www. kanemagnetics . com) , amongst other manufacturers, under the product name ^ΛNeomag C . Alternative forms of rare earth magnet which are injection moulded or sintered may also be suitable. Another preferred magnet composition is Samarium Cobalt (SaCo) or Aluminium Nickel Cobalt Iron

(Alnico) . In addition to rare earth magnets, ceramic magnets or composite ferrite magnets may also be considered.

Magnets composed of the materials referred to above are particularly advantageous as they offer very high magnetic field strengths, allowing smaller magnets to be used. Magnets of the type NdFeB have a typical remanence (Br) figure of 1080-1430 mT, and SaCo have typical values in the range 900-108OmT, rendering them particularly suitable for use in embodiments of the present invention.

Certain types of magnet, particularly those comprising rare earth metals may be toxic, and in order to implant them into the human or animal body, they need to be encapsulated in some form of substantially biologically inert material, such as a plastics material and/or plated with an inert metal, such as gold. Other magnets, e.g. ceramic magnets may be implanted directly as they have no toxicity.

Figure 4 shows a device 100 according to another embodiment of the present invention. The device includes a first anchor 110 for securing the marker 100 in position in the relevant soft tissue of the patient. The anchor 110 comprises a plurality of (in this case, four) barbs 120 which secure the marker firmly in position by embedding the marker in the tissue adjacent to the site of interest.

The anchor 110 of Figure 4 is formed from two short pieces of wire. The two wires are interwound to form a substantially X-shaped member. Typically, the unconstrained device 100, i.e. outside the body, is of the order of 3-5mm long, and of the order of 5-lOmm wide i.e. in the unconstrained device, a typical distance between barb-ends positioned at a given end of the device will be in the range 5 - 10mm.

Positioned at the intersection of the two wires of the anchor 110 is a magnetic element 130. The magnetic element may comprise a rare earth magnet, as described previously. Rare earth magnets have a relatively high magnetic field strength, when compared to more common ferrous magnets, which means that a relatively small magnet can be used to achieve a given magnetic field strength.

The magnet 130 is in the form of a hollow cylinder, and is held in position by the resilient force provided by the outward splaying of the barbs of the anchor. Alternatively, or additionally, the magnet 130 can be secured to the anchor 110 by using a suitable adhesive, a soldering process, crimped retention beads or ferrules, or the magnet may be integrally formed with the anchor. For the latter option, the wire anchor members may be embedded in the magnet during the compression forming of the magnet itself, resulting in a strong, unitary construction. The magnet may be embedded in a suitable plastics material, with the anchoring barbs extending beyond the plastics material to embed in the tissue.

Other configurations of the marker are envisaged, and other possible forms are illustrated in Figures 5 - 10. These alternative embodiments use a non-bored magnet, rather than the hollow form used previously. It should be noted that there are no restrictions on the topology of magnet used, and the embodiments described are merely exemplary, as will be readily understood by the skilled reader.

Figure 5 shows a magnetic marker 200 comprising a magnet 210, composed as has been described previously, encapsulated in a biologically inert material 220 preferably a plastics material. Suitable plastics materials for coating the magnet include nylon, PTFE, polyurethane and prylene (polypropylene) . Embedded in the plastics material at the manufacturing stage are a pair of wire barbs 230, each formed from a simple twisted piece of wire, and the twisted portion being embedded in the plastics material 220. Once implanted, the wire barbs will secure the marker in the soft tissue.

The magnet 210 itself is a cylinder having a diameter and length of the order of a few millimetres.

Figure 6 shows a further embodiment of a marker 250 where the twisted wire barbs of the marker in Figure 4 are replaced with a pair of hooks 260, one at each end of the marker 250. In this embodiment, the hooks 260 will spring out when the marker is released from the end of the needle through which it is implanted. This ensures a good anchoring of the marker.

Figure 7 shows a still further embodiment of a marker 300 where the anchors comprise a pair of coiled wire members 310, each protruding from an end of the marker 300. The coils 310 act to embed the marker in the surrounding tissue.

Other forms of anchor are possible, in addition to those described herein, and each type may be beneficial in different circumstances or in different tissue types.

In particular, these include hybrid forms of device which additionally include a localisation wire, as used in the prior art localisation wires as previously described. An embodiment of this is shown in Figure 8, where both ends of the marker 350 include an anchor 360, similar to that shown in Figure 4. One end of the marker further includes an embedded anchor 370 for a localisation wire 380 which extends from the marker. The benefit of this form of marker is that it combines features of embodiments of the present invention with features of prior art localisation wires, and so enables a surgeon to have the additional option of using the localisation wire 380 as a confirmatory location identifier, which is of particular use while learning how to interpret the positional data supplied by the magnetometer used to locate the marker 350.

Figure 9 shows a further embodiment of a marker 400 which also includes features of the prior art, namely the localisation wire. In this case, the anchor is a barb 410, similar to that shown in Figure 5. The other end is provided with a localisation wire 430 which is firmly embedded in the plastics material 220 with an anchor 420, which comprises a short integral coil of wire.

Figure 10 shows a further embodiment of the marker 450, comprising an additional part 460 coupled to the magnetic portion of the marker by a short length of wire 470. The additional part 460 may further be integrated with the marker to form a unitary marker. The additional part 460 comprises an amount of bioabsorbable gel or fibre as described previously, which is visible on ultrasound and so provides a further means of locating the marker when implanted. In addition, these ultrasound visible markers may have incorporated iodine or barium to further improve their radio-opacity for x-ray localisation or gadolinium amongst other compounds for enhanced visualisation with magnetic resonance imaging.

Further embodiments of the invention are presented in Figures lla-f. These embodiments of the invention differ from the previously described embodiments in that the anchors are formed from the substantially biologically inert material 52 which encapsulates the magnet 50. These particular embodiments clearly require fewer steps in their manufacture and may therefore be cheaper to produce.

Other forms of anchor are possible, in addition to those described above, and different types may be beneficial in different circumstances or in different tissue types.

In the following description, reference will be made to the marker 100 of Figure 4, but note that any embodiment of the present invention, as shown in Figures 3 - 11, or otherwise, may be substituted therefor as desired, and the description should be construed accordingly.

The marker 100 may be introduced into the tissue under investigation in the same way as the marker 20 of Figure 1 i.e. it is inserted, under X-Ray or ultrasound guidance, down a suitable needle, the end of which is positioned near the site of interest. The marker is propelled down the bore of the needle via a plunger, obturator or pusher rod, so that the marker is ejected from the tip of the needle into position. As soon as the marker 100 leaves the needle, or as the needle is withdrawn leaving the now exposed marker in situ, the resilient barbs or other anchors 110 spring from the constrained configuration encountered in the needle, and embed themselves securely in the surrounding tissue. The example of a needle is given as the deployment device, but this may be any mechanism by which the marker is introduced into the region of interest for example a cannula or a flexible tube which could be used in conjunction with a vacuum assisted biopsy device.

Once the marker 100 is implanted, the device which was used to insert it can be removed, leaving no external sign of the marker. Of course, there may be a small scar on the surface of the skin, but this, in itself, will give no real indication of the position of the marker.

In order to locate the marker again, a device which is sensitive to magnetic fields - a magnetometer - may be used to provide information identifying the location of the marker 100.

A suitable magnetometer will have sufficient sensitivity to detect an implanted marker, and in a preferred embodiment, will be insensitive to any ambient magnetic fields. Such a magnetometer will be described later.

Embodiments of the invention are particularly useful for marking sites of interest in non-hollow soft tissue or organs, but find particular, but not exclusive, use in the breast or liver.

Once the marker is in position, it can be located in a subsequent procedure by scanning the approximate area of implantation with a magnetometer, and iteratively homing in on its exact location. A typical scanning procedure is shown in Figure 12. In this figure, there is shown a breast 500 into which a marker 100 according to an embodiment of the invention has previously been implanted, as described. When, in a later procedure, it is desired to locate the position of the marker, a suitable detector 600 is passed over the breast 500 in an ordered back and forth, or zig-zag, pattern 510 as shown. The detector, which is preferably a suitable magnetometer, is equipped with, or attached to, suitable indicating equipment which provides information to the operator about the proximity of the embedded marker. The information may be a numerical readout giving an indication of the distance from the marker. Instead, or additionally, some other form of feedback to the operator may be provided via an audible tone which alters in pitch or amplitude to indicate proximity to the marker, or a series of visual indicators, such as a bar-graph display.

The primary back and forth search pattern 510 will provide a good first approximation as to the location of the marker, and hence the site of interest.

By repeating the back and forth scan pattern 520 in a direction perpendicular to the first scan, a better approximation of the position may be made. Such a scan 520 is shown in Figure 13. The scan area 520 is more localised as the approximate location of the marker will be apparent from the earlier scan, shown in Figure 10. Any number of repeated back and forth scan patterns may be made at increasing degrees of obliquity from the first to increase the accuracy of localisation. To further improve the accuracy of the location, a scan pattern as shown in Figure 14 may be used. In this way, the operator begins a few centimetres away from the likely location identified in the previous scan, and moves across the likely location to a point opposite. This scan is then repeated a number of times from different points around the circumference of a circle centred on the likely location. If necessary the estimated marker position can be revised and a repeat scan performed over the new estimated location. In this way, a more accurate assessment of the position may be made.

Once the likely location of the marker has been determined using the detector 600, a suitable mark may be placed on the surface of the skin.

Once the operator is satisfied that the best approximation of the location has been obtained, an incision can be made above the location of the marker. This is shown in Figure 15 where a suitable incision 530 has been made and retractors 540 have been used to keep the incision site open.

In order to ensure that the operator or surgeon is aware of the current distance from the marker, and hence the site of interest, it may be necessary to iteratively scan 550 the incision site 530 as dissection proceeds, so that a continually updated marker location can be obtained. By using a combination of the intensity of the measured magnetic field and the response of the magnetometer 600 to small changes in the position of the probe, the operator can obtain distance information. In this way, the surgeon will be able to more accurately plan the resection planes. Once the incision has been extended to within the desired distance from the marker 100, the surgeon may excise tissue as appropriate, usually including the marker. The excised tissue may be scanned with the magnetometer to confirm that the marker has been successfully removed, and to obtain an estimate of the distance of the marker from the resection margins. Alternatively, in some instances, it is desirable to retain the marker in position in the breast, for example the marker may also find use in patients where the site of previous biopsy needs to be localised for microwave ablation therapy, interstitial laser therapy, or ultrasound ablation.

Depending on the actual orientation of the marker 100 in the breast tissue, the magnetometer 600 may indicate one or more peaks in the response as the detector is passed over the site of the marker. This occurs depending on whether the detector encounters a pole end of the magnet 210, where there is a greater magnetic flux density, or whether it passes over the magnet such that it detects both poles in a single pass. Figures lβa and b provide an illustration of this phenomenon. In each case, the x-axis 650 indicates position in a given linear direction and the y-axis 640 illustrates relative magnetic field strength.

Figure 16a shows a detector 600 passing over a pole of the magnet 210 in the direction of the arrow. The curve 610 beneath the magnet represents the intensity of the magnetic field as detected by the detector and indicated by the indicating equipment. The magnetic field is strongest immediately above the pole and so a peak is measured at that point. If the marker is lying in such a way that the detector passes over both poles of the magnet 210, fully or partially, then a different response will be identified. Figure 16b shows the case where the detector passes over both poles in the direction of the arrow. The curve 620 beneath the magnet 210 represents the measured magnetic flux density as indicated by the detector equipment. In this case, two clearly defined peaks are displayed, with a relative null between the two.

Of course, in practice, the magnetic marker may lie in any orientation when in-situ and a typical response may well lie between the two extremes illustrated in Figures 16a and 16b. With a little practice, the user will recognise the different types of response which are possible, and this will assist in identifying the true location of the marker.

Clearly, embodiments of the present invention provide an improved means by which a suspect area in the soft contiguous tissue of the human or animal body may be marked for further examination. Particular embodiments of the invention have been described, but it will be apparent to the skilled man that different marker configurations are possible which conform to the overall inventive concept disclosed herein.

In order to locate a previously implanted tag or marker, a magnetometer having the requisite sensitivity to the tag and insensitivity to the ambient magnetic field is desirable. Figure 17 shows a schematic of a suitable magnetometer 800, according to an embodiment of the invention, comprising a processing and display unit 900, to which is attached a probe 1000. The probe comprises a generally elongate member, which houses a three-axis magneto-resistive sensor 1010. A suitable sensor Is provided by Honeywell under the part number HMC1023. The three outputs of the sensor 1010 are each connected to an input of an operational amplifier 1020, also located within the probe 1000.

The sensor 1010 advantageously provides a three-axis sensor where all three sensors are co-located. In certain prior art magnetometers, one of the disadvantages is that three separate sensor devices are provided with the drawback that the physical separation between the sensors has a tangible effect on the measured magnetic field.

The probe 1000 is connected to the processing and display unit 900 via a suitable length of electrical cable.

The processing and display unit 900 comprises a microcontroller 910 which receives the amplified outputs from the sensor 1010, processes them, and acts to display the results in a suitable form to a user of the apparatus.

The outputs from operational amplifier 1020 pass along the electrical cable and, upon termination in the processing and display unit 900, are further amplified in operational amplifier 920, although further amplification is not required in all instances. The outputs of said operational amplifier 920 are each fed into one of a plurality of analogue inputs of the micro controller 910. The analogue inputs are adapted to receive one or more analogue signal levels, which are converted to digital values using one or more on-chip analogue to digital converters (ADCs) .

A suitable micro controller is the PIC16F877, available from Micro Chip Inc. The micro controller 910 acts upon the digital values which are output from the ADCs to calculate the strength of a magnetic field sensed by sensor 1010.

A difference between a magnetometer according to an embodiment of the present invention and prior art magnetometers is the ability of embodiments of the invention to be calibrated to negate the effect of the local ambient magnetic field, which is, in general, largely due to the earth's magnetic field. Advantageously, the calibration process need not be performed before every occasion of using the magnetometer but, if the magnetometer is moved from a particular location, or it is noticed that the results show any sign of drift, then it may well be necessary to re-calibrate the device as and when needed.

The mode of operation of the magnetometer 800 is selected via switch 930, which is used to select either a calibration mode or an operational or seek mode of the device .

In calibration mode, it is necessary for a user of the device to rotate the probe 1000 through a full range of movements in all three axes so that the probe 1000 is able to fully experience the ambient field in all 3 axes . The microprocessor 910 is then able to perform the calibration algorithm. Figure 18 shows a flow chart which details the steps performed in the calibration process, by the microprocessor 910.

The flow chart in Figure 18 commences with the step 1100 of the user selecting calibration mode by operation of switch 930. This instructs the microprocessor 910 to execute the code corresponding to the calibration process.

The next step 1110 requires the user to rotate the probe 300 through a full range of movement in all three axes (x, y, z) so that the probe is exposed in all possible orientations to the ambient magnetic field.

At the next step 1120, the microprocessor 910 is operable to record the maximum and minimum values measured for the magnetic field for each of the three channels. The values recorded are stored in temporary memory.

At the next step 1130, the microprocessor is operable to subtract the minimum value for each channel from the maximum value for each channel to calculate the range of values measured for each of the channels.

At the next step 1140, the microprocessor is operable to calculate the mean of the maximum and minimum values for each channel .

At the next step 1150, the microprocessor is operable to display the mean and range values for each channel on display device 960, which displays numerical values, so that the operator is able to easily see when the displayed values cease to change with repeated rotation of the probe. This indicates that the probe has been rotated sufficiently to fully experience the ambient field in all three mutually perpendicular axes.

Once the displayed values cease to change, the microprocessor 910 stores the mean and range values for each channel into non-volatile memory 940. The non¬ volatile memory is preferably an EEPROM.

As can be seen from the procedure illustrated in Figure 18, the calibration method is very simple and requires nothing more from the user than merely rotating the probe through a full range of movements in all three mutually perpendicular axes.

The user will know when the procedure is complete as further movements of the probe will not result in any change in the reading displayed by the processing unit 900. Once this has process been completed, the microprocessor has made a measurement of the ambient magnetic field associated with the current location of the magnetometer. As such, the measured ambient magnetic field, the details of which are stored in the non-volatile memory 940, can be effectively subtracted from any subsequent measurements made using the magnetometer, such that the effects of the ambient magnetic field can be disregarded.

The calibration procedure therefore effectively allows for the presence of an ambient magnetic field, which is usually mainly due to the earth' s magnetic field, although nearby electrical equipment can have an affect. It also compensates for any variability in the sensitivity of the three separate channels in the sensor. A particular use for a magnetometer according to an embodiment of the invention is for locating previously implanted surgical markers, as described earlier.

It has been found that prior art techniques for marking the location of suspected abnormality in breast tissue suffer from a number of drawbacks . One of the main drawbacks encountered with such prior art markers, is the fact that a localisation wire is often left protruding through the surface of the skin after the marker has been implanted. As such, follow-up surgery is required soon after implantation of the marker or else the protruding wire can pose an infection risk. The surgical markers according to embodiments of the present invention comprise a strong magnetic element coupled to an anchor device which is implanted into the breast via a needle, commonly a biopsy needle.

Advantageously, such a magnetic marker does not require any protruding wires through the skin and so can be implanted and left in situ for some time before any follow-up operation is required.

Upon developing the magnetic marker according to embodiments of the present invention, the inventors were unable to locate a magnetometer having the required characteristics and functionality to locate such an implanted magnetic marker. As a result, embodiments of the present invention were created in order to specifically address this requirement. In particular, the functionality offered by embodiments of the present invention in cancelling and negating the effects of the ambient magnetic field allow a suitable magnetometer to be constructed.

In use, the probe 1000 of the magnetometer 800 is passed iteratively over the breast in order to locate the site of the magnetic marker. The iterative nature of the scanning process allows a closer approximation to the true location of the marker to be obtained.

The use of the magnetometer to locate a previously implanted magnetic surgical tag or marker is only one possible use of the magnetometer and further uses will be apparent to the skilled man. For instance, the magnetometer may find application in manufacturing processes where embedded magnetic tags may be used for product tracking.

Figure 19 shows a flow chart illustrating the operation of the magnetometer in seek or scan mode i.e. the mode where the magnetometer is used to detect a local magnetic device or element.

The first step 1200 in the flow chart shows the calibrate/seek switch 930 being set into seek mode.

In the next step 1210, the microcontroller 910 records measured values from each of the three channels of the sensor 1010. These values are recorded in temporary memory.

At the next step 1220, the microprocessor 910 acts to subtract the reading in the temporary memory for each channel from the mean value stored during a previous calibration process. The microprocessor then takes the modulus of the calculation, thereby ensuring that the result is always positive in value.

The next step 1230 involves the microprocessor 910 correcting the measured values for differences in the sensitivity of the three sensors. This is done by multiplying the measured value for each channel by a predetermined fixed value and dividing by the range value for that channel, previously determined during the calibration process.

The next step 1240 involves the microprocessor calculating the Root Mean Square (RMS) value of the adjusted measurements from each channel of the probe 1000. This is done in the normal way by calculating the square of the adjusted value for each channel, summing the three squared values and then calculating the square root of the resulting sum.

At the next step 1250, the microprocessor 910 is operable to subtract the resultant RMS value from the fixed value already used in step 1230 above, and then takes the modulus of the result, ensuring that it is therefore always positive in value.

The final step 1260 involves displaying the value calculated in the previous step 1250 on a suitable display device. The processor unit 900 may be equipped with one or a plurality of display devices. The embodiment shown in Figure 17 includes an LED bar-graph display 950, comprising a series of individual LED devices which are progressively illuminated to indicate the strength of the detected field. The embodiment of Figure 17 also comprises an alphanumeric display 960, which is arranged to show a numerical value related to the strength of the measured magnetic field.

Additionally, a different form of sensory feedback may be presented to the user. The embodiment of Figure 17 includes a speaker 990 which emits an audible tone which varies in frequency depending upon the strength of the measured magnetic field. The value calculated in step 1260 is applied to a Digital to Analogue Converter (DAC) 970, which in turn applies its analogue output signal to a Voltage Controlled Oscillator (VCO) 980. The output of the VCO is then used to drive, directly or via an intermediate amplifier, the speaker 990. By responding to the tone of the sound emitted by the speaker, a user of the apparatus 800 may be able to move the probe 1100 around the area under investigation without having to look away at the display device (s) 950, 960.

In a preferred embodiment, additional range information may be provided to the user which indicates an approximate absolute distance to the magnet from the probe. It is not generally possible to give a precise figure as there are several variables which have an appreciable effect on the measured magnetic field strength. For instance, the amount and type of tissue between the probe and the magnet can influence the measured value. Also, depending on the orientation of the magnet with respect to the probe, a different field strength may be measured.

In order to calibrate the probe, a calibration device comprising a magnet of known strength encapsulated in a sphere of known radius is provided. In a calibration mode, the probe can be placed adjacent the sphere and the sphere rotated throughout its full range of positions so that the magnetometer can take a series of readings and can then associate a given field strength reading with a particular distance (in this case, the distance of the magnet from the probe i.e. a distance equivalent to the radius of the sphere) .

In an enhancement of a magnetometer according to the present invention, the device 800 can be adapted to differentiate between North and South poles of magnets. In effect, it is immaterial whether a particular detected pole in North or South, but the ability to distinguish between two detected poles, which may be quite close together, can be useful.

This can be achieved by adapting step 1250 of the method, shown in Figure 19. The result of the RMS calculation calculated there is compared with the fixed value found in step 1230. Instead of taking the modulus of the result, as described previously, the microcontroller 910 is arranged to indicate a first pole in a first manner and a second pole in a different manner. For instance, a red indicator LED can be used to indicate cases where the result of the RMS calculation is above the fixed value, and a green indicator LED can be used to indicate cases where the result is below the fixed value. In this way, a user of the apparatus is provided with feedback allowing them to distinguish between the two poles of the magnet. This can assist in locating an implanted surgical tag. The null point, which generally coincides with the centre of the tag, is generally situated between the two detected poles. By providing different feedback to the user depending on which pole is detected, it is less likely that the poles will be confused, resulting in more accurate detection of the true position of the tag.

Since the magnet of the tag is unlikely to be positioned in an orientation where each pole produces an equal amplitude reading on the magnetometer, a further enhancement of the magnetometer allows an estimate to be made of the expected reading when the probe lies over the centre of the tag.

This is achieved by the microcontroller recording the maximum value associated with each pole of the magnet as the user scans the probe over the likely area of implantation. At the same time, the user marks on the skin of the patient the positions associated with each measure peak value. The microcontroller then acts to compare the amplitude of the two measured peaks and calculates the reading that will correspond to the probe being closest to the centre of the magnet. This point will be between the null point and the strongest peak.

The user then scans along a line joining the two peaks, from weak to strong, until the measure reading corresponds to the calculated target value. Once the reading is equal to the target value, audible feedback can be provided to the user. Alternatively, or additionally, a graphic indication of the presently measure signal can be displayed, e.g. in bar-graph form, so that it can be seen at a glance when the measured value equals the target value, which can be marked on the bar-graph.

If the magnet is located end-on to the probe, then only a single peak may be recorded, but the microcontroller can be arranged to account for this by calculating the target reading to close to the measured peak reading.

It has been found empirically that the relationship between the true centre of the magnet and the measured peak amplitudes is not a simple linear one, and can be affected by a number of factors, such as distance of the magnet from the probe, the ambient field strength and the design of the probe itself.

In practice, an approximate formulaic relationship can be derived by calibration of the system with a particular combination of probe and tag. This approximate relationship can be represented in the form of a look up table, or a totally empirical approach, using only a look up table, may be used.

The above development may be further enhanced by providing approximate distance information to the user. In practice, it is difficult to provide accurate information on range, since there are so many factors which can have an effect. However, an estimate can be provided by taking the highest reading measured during a scan of the area of interest. This is corrected by a correction factor which is related to the angle of inclination of the magnet relative to the plane of the scan. The correction factor is determined on the basis of the ratio of the measured peaks of the two poles of the magnet, and is derived empirically. The resulting value can be referenced in a look up table to provide approximate range information, displayed e.g. as a range of values, such as 10-15mm, 15-2Omm etc.

To make sure that the range value displayed is meaningful, it is necessary to calibrate the system prior to its first use in a new location, since the ambient field strength can have an effect on the calculation.

It is possible to provide a user with a ^λmap' of the scanned area in the form of a 2-d display. The probe may be supplemented with an optical or mechanical tracking system, such as a rolling ball as used in a computer mouse. The area of interest e.g. a breast, can then be scanned, starting from a reference point, e.g. the nipple, by moving the probe across the surface. The resulting measured field strength can be displayed on a screen, with the measure peaks shown as particular icons and variations in the field strength depicted as shades of colour.

Any one or more of the above described enhancements can be included with a magnetometer according to an embodiment of the invention.

A magnetometer according to an embodiment of the invention finds particular use, as mentioned, in the field of surgical tag location. As such, a magnetometer may be supplied for use with one or more tags, suitable for implantation in the human or animal body.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series ^' of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment (s) . The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings) , or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A marker, dimensioned for insertion via an introduction device into tissue of the human or animal body, for marking a site for possible further examination and/or treatment, comprising:

a magnetic material, for enabling the marker to be located from outside the body, using a suitable detector.

2. A marker as claimed in claim 1 wherein the marker is substantially encapsulated in a substantially biologically inert material .

3. A marker as claimed in claim 2 wherein the substantially biologically inert material comprises a plastics material.

4. A marker as claimed in any preceding claim wherein the introduction device is one of a rigid or flexible introducer needle, a cannula or a tube.

5. A marker as claimed in any preceding claim wherein the marker has a maximum diameter of 2mm and a maximum length of 15mm.

6. A marker as claimed in claim 5 wherein the marker has a maximum diameter of 1.5mm and a maximum length of βmm.

7. A marker as claimed in any preceding claim wherein the magnetic material comprises a NdFeB (Neodymium Iron Boron) -type magnet.

8. A marker as claimed in any preceding claim wherein the marker additionally includes an anchor for co¬ operation with the tissue.

9. A marker as claimed in claim 8 wherein the anchor comprises a length of wire which extends beyond the substantially biologically inert material .

10. A marker as claimed in claim 9 wherein the wire comprises one of stainless steel, titanium or nitinol.

11. A marker as claimed in claim 8 or 9 wherein the wire is configured to provide a plurality of barbs for engaging with the tissue.

12. A marker as claimed in claim 8 wherein the anchor is integrally formed from the substantially biologically inert material .

13. A marker as claimed in any preceding claim, further comprising a localisation wire.

14. A marker as claimed in any preceding claim further comprising material visible under ultrasound.

15. A magnetometer for detecting the presence of a magnetic field, wherein the magnetometer is arranged to perform an operation whereby the effects of any ambient magnetic field may be disregarded in any subsequent detecting operations .

16. A magnetometer according to claim 15 wherein the magnetometer is arranged to perform a calibration operation to measure the ambient magnetic field such that said measured ambient magnetic field can be subtracted from any subsequent measurements.

17. A magnetometer according to claim 16 wherein the calibration operation includes the operation of moving a probe of the magnetometer through a range of movements in three mutually perpendicular axes so that the probe substantially fully experiences the ambient magnetic field.

18. A magnetometer as claimed in any of claims 15 - 17 comprising a processor unit coupled to a probe unit.

19. A magnetometer as claimed in any f claims 15 - 18 wherein the probe comprises a three-axis magneto-resistive sensor.

20. A magnetometer as claimed in claim 18 or 19 wherein the processor unit is arranged to receive signals from the probe unit indicative of the strength of a measured magnetic field.

21. A magnetometer as claimed in any of claims 15 - 20 wherein the magnetometer is provided with sensory feedback means for indicating the strength of a measured magnetic field.

22. A magnetometer as claimed in claim 21 wherein the sensory feedback means comprise one or more of: a numerical readout, a bar-graph display, and an audio transducer.

23. A magnetometer as claimed in claim 22 wherein the audio transducer comprises a speaker for emitting a tone, the pitch of which is indicative of the measured magnetic field strength.

24. A magnetometer as claimed in any of claims 15 - 23 wherein the magnetometer is operable is a calibration mode and also in a measurement mode.