GB2558210A

GB2558210A - System and apparatus for ferromagnetically screening prone patients

Info

Publication number: GB2558210A
Application number: GB1621893.5A
Authority: GB
Inventors: Wray Goodyear Simon; Nicholas Keene Mark
Original assignee: Metrasens Ltd
Current assignee: Metrasens Ltd
Priority date: 2016-12-21
Filing date: 2016-12-21
Publication date: 2018-07-11
Also published as: GB201621893D0

Abstract

A ferromagnetic detection apparatus (FMDS) 100 for screening a prone patient (possibly prior to an MRI exam) comprises: a first magnetic sensor 104 in a housing 102, the sensor measuring an ambient magnetic field or gradient within a detection volume and producing a corresponding measurement signal; signal processing means for electronically filtering the sensor signal to produce a detection signal indicative of a magnetic object moving through the detection zone; a detector 106 for examining the detection signal to determine the presence or absence of a ferromagnetic object in the detection zone; and a warning device 108 for emitting an alert when a ferromagnetic object is determined to be present. The housing is connected to a support mechanism 114, 116 enabling it to be movably positionable in at least a direction with a substantially vertical component, in use. A method comprises locating a patient on a non-ferromagnetic gurney 112, configuring a ferromagnetic screening apparatus horizontally and adjusting its height so the patient may pass closely beneath, moving the gurney beneath the screening apparatus, and observing the response of the apparatus.

Description

(54) Title of the Invention: System and apparatus for ferromagnetically screening prone patients Abstract Title: Ferromagnetically screening prone patients prior to an MRI examination (57) A ferromagnetic detection apparatus (FMDS) 100 for screening a prone patient (possibly prior to an MRI exam) comprises: a first magnetic sensor 104 in a housing 102, the sensor measuring an ambient magnetic field or gradient within a detection volume and producing a corresponding measurement signal; signal processing means for electronically filtering the sensor signal to produce a detection signal indicative of a magnetic object moving through the detection zone; a detector 106 for examining the detection signal to determine the presence or absence of a ferromagnetic object in the detection zone; and a warning device 108 for emitting an alert when a ferromagnetic object is determined to be present. The housing is connected to a support mechanism 114, 116 enabling it to be movably positionable in at least a direction with a substantially vertical component, in use. A method comprises locating a patient on a non-ferromagnetic gurney 112, configuring a ferromagnetic screening apparatus horizontally and adjusting its height so the patient may pass closely beneath, moving the gurney beneath the screening apparatus, and observing the response of the apparatus.

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Figure 10 (PRIOR ART)

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SYSTEM AND APPARATUS FOR FERROMAGNETICALLY SCREENING PRONE PATIENTS

The present invention relates to a ferromagnetic detection apparatus for screening prone patients. It also relates to a ferromagnetic screening system using such a ferromagnetic detection apparatus and a method of screening.

Prior to an MRI examination it is important that all ferromagnetic objects are removed from the patient. Ferromagnetic objects may be attracted to the MRI scanner’s magnet causing a projectile accident. Small ferromagnetic objects such as hair pins and bra under-wires, for example, may cause serious artefacts in the MRI image that can prevent diagnosis. In this case the object needs to be removed and the patient rescanned, which uses additional time. In addition, it is important that any implants that may be ferromagnetic are known about. Although these cannot be removed, knowledge of them is essential because they may be unsafe in an MRI, or require precautions in order to carry out a scan in a safe manner.

Ferromagnetic detection systems (FMDS) have recently been developed to detect ferromagnetic materials prior to MRI. By way of background, FMDS comprise a plurality of magnetic sensors arranged to detect fluctuations in the ambient magnetic field caused by ferromagnetic objects moving within their locality. The fluctuations are detected using a thresholding detector the output of which is coupled to visual and/or audible alerting devices.

FMDS have two roles. Entry control FMDS are normally mounted at an MRI doorway and warn of large ferromagnetic objects entering that could be a major hazard. They are not sensitive to the very small objects patients may carry in. Patient screening FMDS are used to check patients. They are more sensitively set and the patient is required to rotate once close to the unit in order that all parts of the body are brought close to the sensors in the FMDS, in order to maximise detection performance. An example FMDS is disclosed in the applicant’s own prior application, published as WO 2013/001292.

To be screened using a patient screening FMDS, the patient needs to rotate in front of the unit. Whilst this is no problem for able-bodied patients, those with limited or no ambulatory ability cannot perform this manoeuvre. The current solution is to use a non-magnetic wheelchair or gurney and wheel the patient past in two opposite directions. This provides some screening capability but the proximity of the patient to the FMDS is poorer than for ambulatory patients so the effectiveness is lower. The alternative is to not screen the patient at all, although this is clearly not desirable for the aforementioned reasons.

For stroke patients, an early diagnosis is essential to the success of the treatment. Time is a critical factor in minimising brain damage. Diffusion-weighted MRI is the method of choice to assess a stroke, and it is imperative to get a patient as quickly as possible into the scanner. The patients are generally unable to undergo a normal MRI preparation routine, not least because of the time it takes, but in particular they may lack the ability to complete the questionnaire and interviews aimed at determining whether they have MRI-unsafe implants or loose ferromagnetic objects about their person. There is a specific need for the rapid ferromagnetic assessment of stroke patients prior to MRI, but also a general need to improve the ferromagnetic screening of non-ambulatory or otherwise prone patients.

Patients may be non-ambulatory due to a physical infirmity or due to them being under sedation or general anaesthetic. An MRI examination requires the patient to be still for long periods of time. Children in particular find this difficult and many will be sedated. In these cases, the transfer of the patient from a ward to an MRI scanner will be made using a gurney with the patient in a prone position.

The present invention addresses the need for ferromagnetic screening MRI patients who may be prone through either being non-ambulatory, sedated, or for any other reason.

According to a first aspect of the invention, there is provided a ferromagnetic detection apparatus for screening prone patients, the ferromagnetic detection apparatus comprising:

a first magnetic sensor arranged in a sensor housing, the or each magnetic sensor in use measuring an ambient magnetic field or gradient within a first volume of space forming a zone of detection and produces a corresponding measurement signal;

signal processing means for electronically filtering the measurement signal of the first magnetic sensor to produce a detection signal indicative of the presence of a magnetic object moving through the zone of detection;

a detector for examining the detection signal to determine the presence or absence of a ferromagnetic object in the zone of detection; and a warning device for emitting an alert signal when it is determined that a ferromagnetic object is present;

wherein the sensor housing is connected to a support mechanism enabling the sensor housing to be movably positionable at least in a direction with a substantially vertical component, in use.

The ferromagnetic detection apparatus is therefore suited for use in screening patients who are not capable of ambulatory movement or are otherwise incapacitated. In the event of a detection, the apparatus will alert a user to the presence of a ferromagnetic object.

The sensor housing may be movably positionable in at least two and preferably three dimensions. More preferably, the sensor housing may have four, five, or six degrees of freedom.

The sensor housing may enable the sensor housing to rotate between a generally vertical orientation and a generally horizontal orientation.

The support mechanism may comprise an articulating arm. Furthermore, the support mechanism may connect the sensor housing to a mount.

The mount may be fixable to a wall, floor, or ceiling or may otherwise be freestanding. The mount may preferably be provided fixed to a wall. This allows the mount to be positioned in any necessary configuration whilst being easily storable.

In an advantageous arrangement, the sensor housing may be positionable in an upright configuration suitable for screening an upright or ambulatory patient.

Preferably, the apparatus may include a second magnetic sensor which in use measures an ambient magnetic field or gradient within a second volume of space that at least partially overlaps the first volume of space, the first and second volumes of space forming the zone of detection, the second magnetic sensor producing a corresponding measurement signal, the signal processing means electronically filtering and combining the measurement signals from the first and second volumes of space to provide a detection signal indicative of the presence of a magnetic object moving through the zone of detection. Such an arrangement can be readily extended to three or four or more magnetic sensors.

By an overlapping region of two localised volumes of space we mean a space in a known region which is close to the apparatus, typically within 1 metre of the apparatus within which a person to be screened can be located. Each of the sensors may respond to ferromagnetic objects that are moving outside of the that space, but generally will be less sensitive for objects that are further away.

Preferably the first and second sensors may be spaced apart such that, with the sensor housing in a first configuration the first and second sensors are positioned such that they can screen a width of a patient in a prone position and with the sensor housing in a second configuration the first and second sensors are positioned such that they can screen a height or the majority of a height of a patient in a standing position.

In the second configuration the two sensors may be aligned one above the other along a substantially vertical axis and in the first configuration may be aligned along a substantially horizontal axis. They may be spaced apart by at least 30cm or more along the axis. The sensor housing may be elongate with the long axis of the housing parallel to the axis along which the sensors are spaced. The sensors may be located towards or at respective opposite ends of the sensor housing.

By substantially vertical axis we mean that the vertical component of the spacing between the two sensors is considerably greater than the horizontal component of the spacing. By substantially horizontal axis we mean that the horizontal component of the spacing between the two sensors is considerably greater than the vertical component of the spacing.

The sensor housing may be a unitary housing that contains both sensors, or may comprise two discrete sub housings that are connected together directly or indirectly, each containing one of the sensors.

The signal processing means may include one or more filters and the output signals from the two sensors may be passed through the one or more filters. The filters may include a low-pass filter, and may include a high-pass filter. These may be combined in a band-pass filter. The high pass filter may be configured to remove frequencies above, say, 5 Hz which typically correspond to changes in magnetic field caused by nearby electrical appliances. The low pass filter may be configured to remove frequencies below, say, 1 Hz, which mainly correspond to the background magnetic field produced by the earth which changes very slowly over time.

The apparatus may include a user-operable input means which enables the warning device to be disabled by a user without powering down the magnetic sensors.

Where the output of the sensors is passed through a filter the user operable input means may enable the warning device to be disabled without powering down the filter. Such filters can take a long time to stabilise, so ensuring they are not powered down can reduce the time taken for the apparatus to be enabled when the user operable input means is operated. Disabling the warning device removes the unwanted feeling experienced by some MRI qualified personnel that they are constantly being monitored, and prevents false alarms being issued when they pass close to the apparatus with ferromagnetic objects during their day to day work when they are not in fact using the apparatus to screen a patient.

Operating the user operable input means may alter the value of a relatively low current electrical signal, the apparatus disabling or enabling the warning device according to the value of that signal. By relatively low current electrical signal we mean a signal of less than 1 mA such as a digital logic signal whereby the input means changes the logic level of the signal according to whether the user has enabled of disabled the warning device. By relatively low current we mean a current that is considerably lower than the current drawn by the signal processing means or warning device.

The signal may be carried from the user operable input device along twisted pair cables.

The applicant has appreciated that it is desirable to minimise the current flowing through cables to and from the input device because the changing signal may generate a magnetic flux which might be picked up by the sensors leading to false warning signals being generated. The lower the current the smaller the field and so the less likely this is to happen.

The apparatus may include a primary power supply which provides power to the magnetic sensors.

The apparatus may include a secondary power supply which provides at an output power for the warning device, the output of the power supply being switchable between a level at which the power to the warning device is enabled and a level at which power to the warning device is disabled, the condition of the output being dependent upon the value of the signal from the user-operable input means. For instance, the power supply may include a digital control circuit which may receive the signal from the user operable input means at an input terminal which causes the output of the power supply to switch on or off.

The second power supply may be switchable independently of the first power supply, ensuring that the power to the magnetic sensors and the filters (where present) is not interrupted.

The secondary power supply may be fed from a power output of the first power supply. Both the first power supply and the second power supply may comprise DCDC converters and each may provide a dual polarity DC output signal, having a positive and negative output voltage,

A third power supply may be provided. This may provide power to the first power supply and the second power supply where present. It may comprise a battery, or may comprise an AC-DC converter which may take an input from a mains supply and provide as an output a DC signal suitable for input to the other power supplies.

The output of the power supply or supplies may be connected to the signal processing means and warning device through a twisted pair cable. This may comprise two conductive cables which are surrounded by respectively insulating sleeves and which are then twisted together along their length. Each conductive cable may in turn comprise two individual strands of conductive wire which are twisted together along their length within the insulating sleeves. The twisting helps to minimise the magnetic flux that is radiated from the cable towards the sensors, reducing or eliminating false warnings.

In an alternative to the user operable input means producing a relatively low level signal that is used to turn the output of a secondary power supply on or off, the user operable input means could comprise a switch that is arranged in series between at least one output of the first power supply, or second power supply where present, and the warning device. Therefore, rather than a low level signal the user device will interrupt the relatively higher current supply fed to the warning device. This, however, for the reasons already stated may not be preferred depending on the relative positions of the power supply and the user operable input means as it is desirable to minimise the amount of cable within the apparatus along which high currents travel, and clearly the power would need to run from the location of the power supply to the switch and then on to the warning device. In any event, even if the user operable input means and power supply are conveniently close together it is generally not desirable to tap into the output cables from the power supply as this can create unwanted localised magnetic fields that may cause false warnings.

The user operable input means may be a physically activated switch or button or a sensing switch that requires no physical contact.

The sensor housing may be elongate and pole-shaped. The sensor housing may define a single void within which the signal processing means, sensors, and power supply or supplies are provided but could define two or more voids which may be interconnected. The power supply or supplies, sensors, signal processing means and optionally the warning means may be located within the sensor housing.

When in use, the user operable input means may be located on the housing at a height where it can be pressed by a user of average height (between 5 and 6 foot tall) without having to stoop down, and the warning device may be located at the top of the housing.

When in a the configuration suitable for screening a standing patient, the magnetic sensors may have an overlapping detection zone which lies to the front of the housing and encompasses a volume which extends from floor level up to at least 2.1 metres and outward from the housing by at least 1.2 metres. This defines a zone which will encompass the whole of a 99th percentile person standing in front of the housing, so that any ferromagnetic objects they are carrying, wearing or have inside their body will be detected by the signal processing means,

This arrangement of magnetic sensors ensures that the apparatus can be used in a configuration suitable for an ambulatory or standing patient as well as a nonambulatory or prone patient, when the sensor housing is placed in the required configuration.

Where the housing comprises a pole, one of the magnetic sensors may be located at, or close to the top of the pole and the other one at or close to the bottom of the pole, and the user operable input may be located conveniently at or close to the midpoint of the pole. They may be generally aligned with a common axis that passes through both sensors.

The first and second sensors and the signal processing means may be configured as a gradiometer.

In one arrangement, the apparatus may include a third magnetic sensor and the first, second and third magnetic sensors may be configured as a second order gradiometer.

The third sensor may be located towards the middle of the pole, midway between the first and second magnetic sensors. All three sensors may be aligned with a common axis. The use of three sensors may provide an apparatus which is relatively more sensitive to nearby ferromagnetic objects and less sensitive to distant objects and which gives a larger change in output for a given movement of a nearby object. This is especially advantageous in a busy or compact environment where the potential for interfering magnetic fields from people and equipment moving nearby is high. In addition, it is also advantageous for an apparatus that may be temporarily located within a room, as less time has to be spent ensuring that it is positioned far enough away from potential interfering magnetic fields that could produce false warnings. With a less sensitive two sensor apparatus more care is needed during set up.

According to a second aspect of the invention there is provided a ferromagnetic screening system for screening prone patients, the ferromagnetic screening system comprising:

a ferromagnetic detection apparatus according to the first aspect of the invention; and a non-ferromagnetic gurney.

The provision of a non-ferromagnetic gurney allows the ferromagnetic detection apparatus to be used with a non-ambulatory or prone patient without risking a false alarm caused by a ferromagnetic component of the gurney.

The non-ferromagnetic gurney may comprise aluminium, brass, bronze, and/or nonmetals such as nylon or composite materials.

The non-ferromagnetic gurney may have a frame formed of aluminium or composite materials. It may have fittings formed of brass, bronze, aluminium, and/or non-metals such as nylon or composite materials.

To avoid conductive loops in the non-ferromagnetic gurney, insulation sections may be included. Such a composition may be particularly useful where the gurney comprises aluminium.

According to a third aspect of the invention, there is provided a method of ferromagnetically screening prone patients, comprising the steps of:

locating a patient on a non-ferromagnetic gurney;

configuring a ferromagnetic screening apparatus substantially horizontally and adjusting its height such that the patient on the gurney may pass closely beneath;

moving the non-ferromagnetic gurney beneath the ferromagnetic screening apparatus; and observing the response of the ferromagnetic screening apparatus.

The patient may preferably pass within 3 cm of the ferromagnetic screening apparatus.

The invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a ferromagnetic screening system in accordance with the second aspect of the invention, as viewed from a side-direction of a patient;

Figure 2 is the ferromagnetic screening system of Figure 1 as viewed from a head-direction of the patient;

Figure 3 is the ferromagnetic screening system of Figure 1 as viewed from above the patient;

Figure 4 is a ferromagnetic detection apparatus as used in the system of Figure 1, the system being used for screening an ambulatory patient;

Figure 5 is the ferromagnetic detection apparatus of Figure 4 shown firstly in a configuration suitable for screening an ambulatory patient and secondly in a configuration suitable for screening a non-ambulatory or prone patient;

Figure 6 is a schematic showing the electric circuit of the ferromagnetic detection apparatus of Figure 4;

Figure 7 is a schematic showing an alternative electric circuit of the ferromagnetic detection apparatus of Figure 4;

Figure 8 is an illustration of the arrangement of three sensors in the apparatus of Figure 1 connected as a second order gradiometer;

Figure 9 is an illustration of the arrangement of two sensors in the apparatus of Figure 1 connected as a first order gradiometer;

Figure 10 is a prior art ferromagnetic detection apparatus, shown from above an ambulatory patient; and

Figure 11 is a view of an alternative configuration of the ferromagnetic detection apparatus of Figure 4 shown firstly in a configuration suitable for screening an ambulatory patient and secondly in a configuration suitable for screening a non-ambulatory or prone patient

Referring firstly to Figure 10, there is shown a prior art ferromagnetic detection apparatus 2. The apparatus includes an elongate pole-shaped sensor housing 4 including a plurality of magnetic sensors which are capable of detecting ferromagnetic objects which are passed close by the sensor housing. The sensors are part of a detector circuit which processes signals in order to detect the objects. As shown, a patient 6 is required to rotate in front of the sensor housing 4 in order that the sensor housing 4 may be brought close enough to each part of the patient 6 in order to properly detect any ferromagnetic objects on their person.

Whilst such an apparatus 2 is suitable for use when considering the needs of ambulatory patients 6 who are capable of both supporting themselves and rotating in front of the sensor housing 4, it is totally unsuitable for detecting ferromagnetic objects on patients who are incapable of such things. This is because the sensors are not able to be brought into close enough proximity to the patient 6. Therefore, an apparatus according to the present invention is required.

Figures 1 to 5 show a ferromagnetic detection apparatus 100 in accordance with the present invention. The ferromagnetic detection apparatus 100 comprises a sensor housing 102 including at least one and preferably a plurality of magnetic sensors 104. The magnetic sensors 104 are configured to detect small variations in the ambient magnetic field, these variations being associated with ferromagnetic objects passing through a zone of detection surrounding the sensor housing 102. The magnetic sensors 104 may be of any type but those with a high sensitivity are clearly preferable.

Signal processing means in the form of a detector circuit 106 are also included within the sensor housing 102, although they may instead be remote from the sensor housing 102, for example being mounted on a wall adjacent to the sensor housing. The signal processing means may combine outputs from the magnetic sensors 104 such that the magnetic sensors 104 form a gradiometer which is capable of rejecting the influence of interference or general magnetic noise. This is well known in the art.

The outputs of the magnetic sensors 104 may be filtered and amplified by corresponding parts of the signal processing means in order to reject powerline frequencies and static magnetic fields. These signals may be further processed by a detector within the detector circuit 106 which determines whether the signals are above or below a predetermined threshold magnitude.

A warning device 108 is positioned towards one end of the sensor housing 102, in communication with the detector circuit 106. When the detector indicates that the output of the signals is above the predetermined threshold magnitude, the detector triggers the warning device 108 which emits a warning, which may be audible or visual. The warning may also be a warning signal which is transmitted to a remote location such as a receiver.

The sensor housing 102 is preferably elongate with either a straight or curving structure. In order to scan patients 110 who are prone on a gurney 112 or other surface, the sensor housing 102 must be horizontally arranged such that it may screen the full width of a patient 110 passing beneath it. The length of the sensor housing 102 is preferably between 80 and 140 cm. Furthermore, the sensor housing 102 needs to be positioned horizontally at a height whereby the patient 110 on the gurney 112 may pass underneath with a small gap.

In order to allow the sensor housing 102 to be positioned horizontally whilst still being capable of being stowed away, the sensor housing 102 is attached to a mount 114 by an articulating arm 116. The mount 114 depicted in the present embodiment is a wall mount 114, although it could instead be a floor or ceiling mount or a freestanding mount, if desired. The mount 114 may house the detector circuit or other signal processing means and may include a battery if not connected to mains electricity or as a backup.

The articulating arm 116 allows movement of the sensor housing 102 in threedimensions due to the three rotational joints between the mount 114, two arm portions

118a, 118b, and sensor housing 102. Other types of joint or configurations of arm 116 are also usable, and will be apparent to the skilled person. This allows the sensor housing 102 to be positioned adjacent to wall, if wall-mounted, when not in use, which is highly-advantageous where space is at a premium.

Furthermore, the articulating arm can allow the apparatus 100 to function in a second configuration 100b, as shown in Figures 4 and 5. The second configuration allows the apparatus 100 to be used as a normal upright ferromagnetic screener for patients 110 who are able to stand whilst being able to be moved into the first configuration 100a when a prone patient 110 must be screened. Of course, it is also possible to mount the apparatus 100 to the ceiling or floor. Figure 11 shows the apparatus mounted to the ceiling in a first configuration 200a and in a second configuration 200b.

Instead of the articulating arm 116 of the present embodiment, it may also be possible to provide other means of movement of the sensor housing 102. Such means may include sliding mounts on one, two, or three axes, or other forms support mechanism. At a minimum, the movement means are required to position the sensor housing 102 at a variable height relative to a patient 110, in order to allow for different sized patients 110 and differently configured gurneys 112.

In order to provide a complete system, it is also necessary to consider the gurney 112 used to transport the patient. So that the system works effectively, the gurney 112 must not cause ferromagnetic detections as this would interfere with the detection relating to the patient 110, nullifying the effects of the use of the ferromagnetic detection apparatus 100. Gurneys which contain any mild steel will be magnetic and not suitable. All general purpose gurneys are in this category and are therefore unsuitable. However, whilst so-called “MRI conditional” gurneys are low enough in their magnetic content for safe use near the MRI magnet, they still contain steel components which are sufficient to trigger an FMDS. Thus, a specialised gurney 112 is required which includes no ferromagnetic materials and no substantial electrically connected loops that could cause magnetically-induced fields.

“MR Conditional” gurneys that can be used in MRI are somewhat magnetic due to the use of steel ball bearings in the castors and some steel fixings such as nuts, bolts and small parts of mechanisms. Furthermore, they commonly use austenitic stainless steels as their main structural material. This is nominally non-magnetic, but may become magnetic where the component has been cold worked such as on bends. These features are enough to render “MR Conditional” gurneys unfit for the purpose of this invention. A gurney that will not trigger a ferromagnetic detection needs to have no steel components at all. The frame may be aluminium or composite materials. Fixings can be brass, bronze, aluminium or non-metals such as nylon or composites. Care should be taken with brass fixings due to the iron contamination they may contain. Marine bronzes are strong and non-magnetic (few iron impurities) but expensive.

Castors pose the biggest challenge. Ceramic bearings or bronze spindles can be entirely non-magnetic. Where the structure of the gurney is aluminium it is preferable there are no large conducting loops. If a conductive loop passes through a magnetic gradient, then eddy currents will flow according to Faraday’s law of induction. These currents will have an associated magnetic field that may be detectable by an FMDS. This may be the case in the vicinity of an MRI magnet. To avoid conductive loops in the design of a gurney constructed from aluminium, insulation sections are required.

It will be appreciated that the person pushing the gurney should have no ferromagnetic material about their person as this may cause a false alert.

The gurney should be pushed or pulled its whole length underneath the FMDS so the entire length of the patient is examined. The FMDS should be set to a vertical height such that the patient passes as close as possible beneath the FMDS without touching it. A minimum clearance of between 1 cm and 3 cm is ideal. Clearance of greater than 3 cm is non-ideal resulting in a loss of sensitivity. The portion of the patient beneath the FMDS where a detected signal is strongest is the likely location of the object detected.

Now referring to the generalised circuit diagram 120 illustrated in Figure 6, the electronic circuit of the ferromagnetic detection apparatus 100 comprises a main power supply unit PSU 1 which receives incoming power from a remote power supply (not shown but denoted by the phrase “power in”. This may comprise an AC-DC power pack that is located outside of the housing 102 and connects through a power lead and plug to a suitable electrical outlet. The PSU 1 converts the incoming power to positive and negative DC voltages of +12V/-12V which are output to positive and negative supply lines in the housing 102.

Three sensors 104 are provided in the housing, although in some arrangements there may be only two sensors 104. The sensors 104 each comprise a sensitive flux gate magnetometer which is sensitive to changes in magnetic flux in a localised region of space in the vicinity of the sensor. The sensors 104 are all sensitive over a common overlapping region of space. They are more sensitive to objects very close to the sensor within that space than they are to objects further away in that space. Other types of sensor could be provided, such as magneto-resistive sensors, magnetoimpedance sensors, Hall Effect sensors, SQUIDS, gas-cell total field sensors, or a galvanic coil sensor.

Each sensor 104 outputs a respective measurement signal that is a measurement of the magnetic field incident upon the sensor. The measurement signal from each sensor 104 is passed to a signal processing device forming readout electronics 122. In an alternative arrangement, two sensors 104 could be provided but three are preferred as they can be configured to operate as a second order gradiometer. The configuration required for both the preferred embodiment with three sensors 104 and the alternative with two sensors 104 in illustrated in Figures 8 and 9, respectively, where ή is a unit vector defining the sensing direction of a sensor and B represents the magnetic field present at the sensor.

With two sensors 104, the sensors 104 are arranged to sense in opposite directions and their outputs are summed as shown in Figure 9. An alternative would be for the sensors 104 to sense in the same direction and their outputs differenced (not shown) as this is equivalent. In either of these ways, they are connected so as to define a first order gradiometer whose output is of the form:

Giitpisi oc (¾ — S₂),n

If ή defines the x direction (extending radially away from the pole) and y defines a baseline extending vertically down through the sensors 104, the output is therefore the first order derivative of the field component of B in the x direction with respect to the y direction, often expressed as 6B_x/6y.

With the preferred three sensors 104, the sensors 104 are arranged with Βχ and B₃sensing in the same direction and B₂ sensing in the opposite direction with a relative gain of two with respect to Βχ and B₃. The outputs are summed as shown in Figure 8. Alternatively, the sensors 104 could all sense in the same direction with B₂ given a relative gain of minus-two. With either way the output is of the form:

cc (¾ + S₃. - 2¾)...^

If ή defines the x direction (extending radially away from the pole) and y defines a baseline extending vertically down through the sensors, the output is therefore the second order derivative of the field component of B in the x direction with respect to the y direction, often expressed as 5²B_x/5y², giving a much higher uniformity in the y direction and a much lower sensitivity to distant objects relative to the same object position at a distance compared with the two sensor arrangement.

Since the apparatus 100 will typically be fixed in position when in use for most of the time the sensors 104 will register a largely unchanging magnetic field due to the earth, and unchanging first and second order gradients. These constitute a large offset on the output of the sensor 104. This constant offset can be removed using a high pass filter 124 in the readout electronics 122. The sensors 104 will also likely measure regular changes in the magnetic field associated with the power supply for electrical equipment located near the sensors 104 which will cause the output to vary at the supply frequency and its harmonics. This can also be filtered out using a low pass filter 124 in the signal processing device. The filters 124 collectively constitute a band-pass filter 124 to perform these functions. The filtered output of the filters 124 will therefore take a low value, ideally zero, in its steady state.

The sensors 104 and readout electronics 122 receive power from PSU 1. Depending on the complexity of the filters 124 that are used there may be a considerable delay between the sensors 104 and readout electronics 122 being switched on and the filtered output signal settling to a steady state. It is therefore important that the power to the sensors 104 and the readout electronics 122, at least the filtering stage, is kept on at all times when the apparatus 100 might be imminently be needed for prescreening.

If a ferromagnetic object is carried, or pulled or pushed, by a person close to the sensors 104 the ambient magnetic field will be altered causing a change in the output of the sensor 104. That change will pass through the filter 124 and be amplified by an amplifier within the readout electronics 122. In order to trigger an alarm the signal size is compared with a predetermined threshold. Because the signal may be positive or negative, the threshold (set by passing the signal through a threshold detector within the readout electronics 122) consists of a rectification stage followed by a comparator that has a circuit to provide a threshold voltage. Alternatively, separate comparators are used for positive and negative signals with the outputs combined to give a single alarm signal instead of a rectifier and a single comparator. An optional latch (not shown) may be provided which holds the value of the signal output from the comparator for a predetermined period - perhaps up to 1 second.

The output of the comparator is arranged to have logic level ‘zero’ for the state where the signal does not exceed the threshold, and level ‘one’ for the state when the signal has exceeded the threshold. Once an object has passed out of range of the sensors 104, the logic level returns to zero, once the signal level drops below the threshold. In practice, it may be preferable that the alarm continues for an elapsed time defined by a reset delay and a latch such as a flip-flop that maintains the output at logic one until the switch/button is pressed.

The output of the latch is passed from the readout electronics 122 to an input of the warning device, which comprises a display electronics circuit 126 that drives the display 128. It has been found to be beneficial, although not essential, that both a visual and audible alarm are provided.

In addition, the rectified magnetic signal that feeds into the comparator may pass to the warning device to control a bar graph indicator.

If the sensors 104 have a digital output, or if a digitizer is placed immediately after the sensors 104, then the electronic functions described, including the filters 124, rectifier, comparator, latches and delays, may be done with a digital processor rather than in analogue electronics.

The warning device is powered from a second power supply PSU 2. The power supply PSU 2 has an input port which receives a signal from the user operable device and depending on the state of the signal will power up or power down the display electronics 126. When powered down, the display 128 will not issue a warning regardless of the value of the latched signal output from the readout electronics 122. Notably, PSU 2 can be powered down without powering down PSU 1, so that the warning device can be disabled without cutting power to the sensors 104 and filters

124.

A modified electronic circuit which can be used is shown in Figure 7 of the drawings. Where parts are the same as those used in the circuit of Figure 6 the same reference numbers have been used for clarity and the associated description above applies. The key difference is the omission of the second power supply unit PSU 2, and the location of the switch in the power supply line from PSU 1 to the display electronics 126. Thus, rather than switching a relatively low current that in turn disables a power supply to the display electronics 126, the switch breaks the current flowing to the display electronics 126 directly. This has the disadvantage that a break in the power supply line could cause unwanted magnetic fields to be generated unless care is taken during assembly but does have the benefit of reducing the number and complexity of the components that are needed.

Claims

1. A ferromagnetic detection apparatus for screening prone patients, the ferromagnetic detection apparatus comprising:

wherein the sensor housing is connected to a support mechanism enabling the sensor housing to be movably positionable in at least a direction with a substantially vertical component, in use.

2. A ferromagnetic detection apparatus according to claim 1, wherein the sensor housing is movably positionable in at least two and preferably three dimensions.

3. A ferromagnetic detection apparatus according to claim 1 or claim 2, wherein the sensor housing has four, five, or six degrees of freedom.

4. A ferromagnetic detection apparatus according to any one of the preceding claims, wherein the support mechanism comprises an articulating arm.

5. A ferromagnetic detection apparatus according to any one of the preceding claims, wherein the support mechanism connects the sensor housing to a mount.

6. A ferromagnetic detection apparatus according to claim 5, wherein the mount is fixable to at least one of: a wall, a floor and a ceiling.

7. A ferromagnetic detection apparatus according to claim 5, wherein the mount is fixed to a wall.

8. A ferromagnetic detection apparatus according to claim 5, wherein the mount is free-standing.

9. A ferromagnetic detection apparatus according to any one of the preceding claims, wherein the sensor housing is positionable in an upright configuration suitable for screening an upright or ambulatory patient.

10. A ferromagnetic detection apparatus according to any one of the preceding claims, further comprising a second magnetic sensor which in use measures an ambient magnetic field or gradient within a second volume of space that at least partially overlaps the first volume of space, the first and second volumes of space forming the zone of detection, the second magnetic sensor producing a corresponding measurement signal, the signal processing means electronically filtering and combining the measurement signals from the first and second volumes of space to provide a detection signal indicative of the presence of a magnetic object moving through the zone of detection.

11. A ferromagnetic detection apparatus according to claim 10, wherein the first and second sensors are spaced apart such that, with the sensor housing in a first configuration the first and second sensors are positioned spaced apart along a substantially horizontal axis such that they can screen a width of a patient in a prone position and with the sensor housing in a second configuration the first and second sensors are positioned spaced apart along a substantially vertical axis such that they can screen a height or the majority of a height of a patient in a standing position.

12. A ferromagnetic detection apparatus according to claim 10 or claim 11, wherein the first and second sensors and the signal processing means are configured as a gradiometer.

13. A ferromagnetic detection apparatus according to any one of claims 10 to 12, wherein the signal processing means includes one or more filters and the output signals from the two sensors are passed through the said one or more filters.

14. A ferromagnetic detection apparatus according to claim 13, wherein the filters include a low-pass filter, a high-pass filter, and/or a band-pass filter.

15. A ferromagnetic detection apparatus according to any one of the preceding claims, further comprising a user-operable input means which enables the warning device to be disabled by a user without powering down the magnetic sensors.

16. A ferromagnetic detection apparatus according to any one of the preceding claims, further comprising a primary power supply which provides power to the magnetic sensors.

17. A ferromagnetic detection apparatus according to claim 16, further comprising a secondary power supply which provides at an output power for the warning device, the output of the power supply being switchable between a level at which the power to the warning device is enabled and a level at which power to the warning device is disabled, the condition of the output being dependent upon the value of the signal from the user-operable input means.

18. A ferromagnetic detection apparatus according to any one of the preceding claims, wherein the sensor housing is elongate and pole-shaped.

19. A ferromagnetic detection apparatus according to claim 18, wherein one of the magnetic sensors is located at or close to the top of the pole and the other magnetic sensor is located at or close to the bottom of the pole.

20. A ferromagnetic detection apparatus according to claim 18 or claim 19 when dependent upon claim 13, wherein the user-operable input is located at or close to the midpoint of the pole.

21. A ferromagnetic detection apparatus according to any one of the preceding claims, wherein the sensor housing defines a single void.

22. A ferromagnetic detection apparatus according to any one of the preceding claims, wherein the sensor housing defines two or more voids which are interconnected.

23. A ferromagnetic detection apparatus according to any one of the preceding claims, wherein the signal processing means, sensors, and power supply or supplies are located within the sensor housing.

24. A ferromagnetic screening system for screening prone patients, the ferromagnetic screening system comprising:

a ferromagnetic detection apparatus according to any preceding claim; and a non-ferromagnetic gurney.

25. A ferromagnetic screening system according to claim 24, wherein the nonferromagnetic gurney comprises aluminium, brass, bronze, and/or non-metals such as nylon or composite materials.

26. A ferromagnetic screening system according to claim 24 or claim 25, wherein the non-ferromagnetic gurney includes a frame formed of aluminium or composite materials.

27. A ferromagnetic screening system according to any one of claims 24 to 26, wherein the non-ferromagnetic gurney includes fittings formed of brass, bronze, aluminium, and/or non-metals such as nylon or composite materials.

28. A ferromagnetic screening system according to any one of claims 24 to 27, wherein the non-ferromagnetic gurney includes insulation sections for the prevention or limitation of conductive loops.

29. A method of ferromagnetically screening prone patients, comprising the steps of:

locating a patient on a non-ferromagnetic gurney;

30. A method according to claim 29, wherein the patient passes within 3 cm of the ferromagnetic screening apparatus.

Intellectual

Property

Office

Application No: GB1621893.5