Device and method for determining a force applied to a probe tip

Abstract

A device capable of determining a force applied to a tip of an electrical impedance spectroscopy probe, comprising: an elongated probe comprising a probe tip attached to a handle, the probe tip having a substantially flat distal end for contacting human or animal tissue; a load cell located in the handle and capable of measuring a force Floadcell applied axially along a longitudinal axis when the probe tip is in contact with human or animal tissue; an accelerometer located in the handle for measuring a gravity vector Aaxial; processing means for compensating the mass of the probe tip using the measured force and the gravity vector to produce a calibrated measurement of the force F applied to the probe tip.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of apparatuses and methods for determining a force applied to a tip of a probe, such as an electrical impedance spectroscopy probe. The claimed apparatuses and methods can improve the measurement of the electrical conductivity of human or animal tissue, particularly but not exclusively cervical tissue, for determining the likelihood of pre-term birth. BACKGROUND

[0002] Pre-term birth is the cause of two-thirds of perinatal deaths in infants without structural malformations. It places a huge economic burden on scarce health resources, as each very pre-term infant born costs tens of thousands of pounds in neonatal care. When born before 28 weeks of gestation, 1 in 4 infants has a disability. These disabilities can cost hundreds of thousands of pounds per year to treat. Families also suffer a huge psychological burden, with one parent often having to give up work to care for the disabled child. Despite improvements in survival rates for pre-term babies, the rate of pre-term birth is increasing, currently accounting for 7-12% of all newborns. There is no reliable means of identifying women who will pre-term birth. Current methods for identifying women at high risk of pre-term birth, such as cervical ultrasound and fetal fibronectin assays, have limited accuracy in women without a history of pre-term birth. A technique that reliably predicts pre-term birth through universal screening is therefore highly desirable.

[0003] Electrical impedance spectroscopy (EIS) is a known technique that can be used to assess cervical precancerous lesions, as set out, for example, in WO2006 / 12910 (Brown and Tidy) and WO2006 / 129116 (Brown and Tidy). Other publications relating to the study of the cervix include:

[0004] Avis (1996). In vitro multifrequency electrical impedance measurements and modelling of the cervix in late pregnancy. Physiol Meas 17 Suppl 4A: A97

[0005] Brown (2000). Relation between tissue structure and imposed electrical current flow in cervical neoplasia. Lancet 355 (9207): 892

[0006] Gandhi (2006). Comparison of human uterine cervical electrical impedance measurements derived using two tetrapolar probes. Biomed Eng Online 5:62

[0007] Gandhi (2006). Electrical impedance spectroscopy of the cervix in non-pregnant and pregnant women. Eur J Obstet Gynecol Reprod Biol 129: 145

[0008] Hoe et al. (2004) Measuring Bioimpedance in the Human Uterine Cervix: Towards Early Detection of Preterm Labor. Preceedings of the 26 th Annual Conference of the IEEE EMBS San Francisco, CA, USA September 1-5 2004.

[0009] Jokhi (2009). Reproducibility and repeatability of measuring the electrical impedance of the pregnant human cervix. Biomed Eng Online 8: 10; and

[0010] Jokhi (2009). The role of cervical Electrical Impedance Spectroscopy in the prediction of the course and outcome of induced labour. BMC Pregnancy Childbirth 9: 40

[0011] The present applicant has investigated the value of using EIS to measure the "resistance" of the cervix to "very small currents" (in other words, the electrical conductivity or bioimpedance of the cervical tissue) to detect changes that can precede pre-term birth. A series of preliminary studies on women at high risk of pre-term birth showed predictive accuracy for pre-term birth before 37 weeks and 34 weeks. However, significant measurement errors were observed using EIS technology and it is desirable to improve the accuracy and repeatability of the measurements. One possible cause of the measurement errors in EIS technology is the difficulty in ensuring a consistent pressure on the cervical tissue by the EIS probe. The above-mentioned paper by Hoe et al. addresses this problem by using a constant force spring to enable more consistent measurements by a range of applied contact forces. The mucus layer on the cervix affects the tissue conductivity, adding further error.

[0012] It is an object of the present invention to address the shortcomings associated with the known prior art. SUMMARY

[0013] Aspects and embodiments of the present invention provide apparatus and methods as claimed in the appended claims.

[0014] According to an aspect of the present invention, there is provided apparatus capable of determining a force applied to a tip of a probe (e.g. an electrical impedance spectroscopy probe), comprising:

[0015] an elongate probe comprising a probe tip attached to a handle, the probe tip having a substantially flat distal end for contacting human or animal tissue;

[0016] a load cell located in the handle and capable of measuring a force F axially along a longitudinal axis when the probe tip is in contact with the human or animal tissue loadcell ;

[0017] an accelerometer located in the handle for measuring a gravity vector A axial ;

[0018] processing means for using the measured force and gravity vector to compensate for the mass of the probe tip to produce a calibrated measurement of the force F applied to the probe tip;

[0019] display means for indicating the calibrated measurement of the force to a user.

[0020] In an embodiment, the processing means is further capable of determining the electrical conductivity of the human or animal tissue to which the distal end of the probe tip is applied. Preferably, the human or animal tissue is cervical tissue.

[0021] In an embodiment, the load cell comprises four strain gauges in a bridge configuration.

[0022] In one embodiment, the accelerometer is an analogue three-axis MEMS accelerometer.

[0023] The processing means can comprise an analogue to digital converter to digitise the outputs of the force element and the accelerometer.

[0024] In embodiments, the calibrated measure of the force F = F loadcell - A axial * (M tip + M load ), where A axial is the output of the accelerometer aligned in the axial direction of the probe tip, M tip is the mass of the probe tip and M load is the free mass of the force element and other parts connected to the force element such as a connector for the probe tip.

[0025] Preferably, the display means is capable of indicating a real-time calibrated measure of the force applied to the probe tip.

[0026] In embodiments, the display means comprises a threshold indication indicating whether a force that is too large or too small is being applied to the probe tip.

[0027] The apparatus can further comprise recording means for recording the measure to facilitate repeatable application of the force to the probe tip.

[0028] According to another aspect of the application, there is provided a method of determining a force applied to a tip of a probe, such as an electrical impedance spectroscopy probe, using an apparatus as claimed in any preceding claim, the method comprising the steps of:

[0029] obtaining a raw force element output;

[0030] obtaining a raw accelerometer output;

[0031] obtaining a mass of the probe tip;

[0032] obtaining an area of a distal end of the probe tip;

[0033] applying the distal end of the probe tip to human or animal tissue and measuring a force F loadcell axially along the longitudinal axis when the probe tip is in contact with the human or animal tissue;

[0034] measuring a gravitational vector A axial using the accelerometer;

[0035] using the display device to indicate to a user the calibrated measurement of force.

[0036] using the display device to indicate to a user the calibrated measurement of force.

[0037] Further features are defined in the appended claims.

[0038] It is expressly intended that various aspects, embodiments, examples and alternatives set forth in the foregoing paragraphs, in the claims, and / or in the following description and drawings, and that various features thereof, can be employed independently or in various combination with one another. That is, the various embodiments and / or features can be interchanged and still be in keeping with one or more of the aspects of the application. The Applicant reserves the right to change any originally filed claim, or a claim accordingly submitted, including the right to be specified as amended by the Examining Officer in any allowance or re-examination certificate or any amendment issued after initiation of the examination or re-examination process, including the right to amend depending on any feature of any other claim to which it is then dependent, although originally claimed differently.

[0039] Throughout the description and claims of this specification, the words "comprise", "contain", "include" and "comprising", "containing", "including" and "comprises" and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense, unless the context requires otherwise. Specifically, the use of "comprising" or "including" in the description is not to be interpreted in an excluding or exhaustive sense.

[0040] Throughout the description and claims of this specification, the singular encompasses the plural unless the context requires otherwise. Specifically, where the indefinite article is used, such as "a" or "an", such terms should be construed to be considering the plural as well as the singular, unless the context requires otherwise. BRIEF DESCRIPTION OF DRAWINGS

[0041] One or more embodiments of the application will now be described, by way of example only, with reference to the accompanying drawings in which:

[0042] Figure 1 is a schematic representation of an elongate probe for use in embodiments of the application. DETAILED DESCRIPTION

[0043] Figure 1An elongate probe 1 is shown with a handle 3 attached to a probe tip 2. The probe tip can be attached to the handle using a standard commercial connector arrangement 4. The probe tip 2 can be removably attached to the handle 3 so that the clinician can select and change different probe tips for different patients. The probe tip 2 has a substantially flat distal end 5 which includes an arrangement of electrodes, for example a known type of tetrapole arrangement for use in electrical impedance spectroscopy.

[0044] A force cell 6 is located in the handle 3. A three-axis MEMS accelerometer 7 is also provided in the handle 3 of the probe 1 and is therefore in fixed relationship to both the probe tip 2 and the force cell 6.

[0045] The EIS technique involves placing the distal end 5 of the probe tip 2 against the tissue whose conductivity is desired to be measured. As the difference in applied pressure significantly affects the results, the pressure applied by the clinician when the probe tip is placed against the tissue is important. Changes in cervical tissue measured as an indicator of pre-term birth are more subtle and therefore more susceptible to the applied pressure than pre-cancerous changes which are more commonly measured by EIS techniques. It is therefore important not only to be able to repeat the same applied pressure when taking successive measurements, but also to apply the pressure within a predetermined threshold.

[0046] The apparatus described herein facilitates this by providing a display to the clinician or other user which indicates the applied pressure and indicates whether the applied pressure is within the desired range. This can be done by a bar graph or traffic light indicator, for example showing a green light when the applied pressure is within the desired range. Alternatively or in addition, an audible alarm or other signal can be provided.

[0047] The apparatus described herein is able to compensate for the mass of the probe tip 2 in order to measure (and display) a calibrated measure of the applied force which is more accurate than a direct measure of the force applied at the probe tip. The probe tip 2 has a known mass which due to the action of gravity can exert a force which significantly affects the accuracy of a direct measurement (depending on the orientation of the probe).

[0048] In order to determine the applied pressure, it is only necessary to measure the applied force at the probe tip 2, as the probe tip has a known area at its distal end 5 (and P = F / A).

[0049] The force cell 6 measures the force applied axially along the longitudinal axis of the probe tip. This force is equal to the force applied to the probe tip plus the mass of the tip multiplied by the gravitational force and resolved axially. The mass of the probe tip is known, and the local gravitational vector relative to the probe A axial is measured using the accelerometer. Therefore, a calibrated measure of the force applied to the probe tip can be obtained in this way, which compensates for the mass of the tip.

[0050] In the illustrated embodiment, the force cell 6 comprises four foil strain gauges arranged in a bridge configuration. This bridge can be excited by a square wave pulse burst at 1 kHz frequency, which allows detection of small resistance changes in the force cell bridge with both low power requirements and low sensitivity to DC drift. The output of the bridge can be amplified and filtered by a Sallen and Key circuit, the output of which can be sampled multiple times per cycle by the microcontroller's analog to digital converter circuit. Other methods of measuring force electrically will be suitable for this application and are understood by those skilled in the art.

[0051] In the illustrated embodiment, the accelerometer 7 is an "analog device" three axis MEMS device whose output is suitable for direct connection to the microcontroller's analog to digital converter circuit. The only signal processing required is a simple linear calibration for zero and range. The calibrated three axis output of the accelerometer 7 is operated through a rotation matrix so that it can be accurately aligned with the longitudinal axis of the probe 1. Other methods of measuring the gravity vector resolved to the longitudinal axis of the probe will be suitable for this application and are understood by those skilled in the art.

[0052] The mass of the probe tip can be measured on a balance and stored in the EEPROM within the probe tip 2. The probe tip 2 can be easily replaced because the probe 1 can read the mass for each specific probe tip 2 from its EEPROM. The free mass of the force cell 6 can be found through a calibration procedure in which the probe 1 is held in two orientations. By requiring the user to hold the probe 1 in different positions, the known mass of the probe tip 2 and the output of the accelerometer 7 can be used to make a fully automatic force cell calibration.

[0053] A display device (not shown) gives feedback to the clinician as to whether the force applied to the probe tip 2 is within acceptable limits. The display device can be a five LED bar graph in which the central LED is highlighted to indicate the desired pressure and as the pressure increases, progressively more LEDs are lit. The range of pressure thresholds required to light each of the LEDs is programmable.

[0054] When the pressure is within acceptable limits, the probe 1 can be set to take EIS measurements only. The LED bar graph is simple to use and allows the hand held probe 1 to maintain the EIS measurements within the + - 6% limits of the desired force when being measured.

[0055] A processing device is provided, for example in the form of a PC that can record patient information, guide the clinician through the measurement process, control the probe, analyze and save the results in a database.

[0056] Although the above description pertains to EIS probes, the device and force measurement techniques described herein can be used in other applications. For example, the device can be used to determine the force applied during joint surgery.

[0057] The force applied to the probe tip is used to assess the fit quality of the new joint. Other applications can be envisioned.

Claims

1. An apparatus capable of determining the force applied to the tip of a probe, such as an electrical impedance spectroscopy probe, the apparatus comprising: An elongated probe, including a probe tip attached to a handle, the probe tip having a substantially flat distal end for contacting human or animal tissue; A force-measuring element, located in the handle, is capable of measuring the force F applied axially along the longitudinal axis when the probe tip contacts the human or animal tissue. loadcell ; An accelerometer, located in the handle, is used to measure the gravity vector A. axial ; A processing device for using the measured force and gravity vector to compensate for the mass of the probe tip, so as to generate a calibrated measurement of the force F applied to the probe tip; A display device is used to indicate the calibrated measurement of the force to the user.

2. The apparatus according to claim 1, wherein, The processing device can also determine the conductivity of human or animal tissue to which the distal end of the probe tip is applied.

3. The apparatus according to claim 2, wherein, The human or animal tissue in question is cervical tissue.

4. The apparatus according to any one of the preceding claims, wherein, The force measuring element comprises four strain gauges arranged in a bridging configuration.

5. The apparatus according to any one of the preceding claims, wherein, The accelerometer is an analog triaxial MEMS accelerometer.

6. The apparatus according to any one of the preceding claims, wherein, The processing device includes an analog-to-digital converter to digitize the outputs of the force measuring element and the accelerometer.

7. The apparatus according to any one of the preceding claims, wherein, The calibrated measurement of the force F = F loadcell –A axial * (M tip + M load ), where A axial The output of the accelerometer, M, is aligned in the axial direction of the probe tip. tip It is the mass of the probe tip and M load It is the free mass of the force measuring element and other parts connected to the force measuring element, such as the connector for the probe tip.

8. The apparatus according to any one of the preceding claims, wherein, The display device is capable of indicating a real-time calibrated measurement of the force applied to the probe tip.

9. The apparatus according to any one of the preceding claims, wherein, The display device includes a threshold indicator that indicates whether too much or too little force is being applied to the probe tip.

10. The apparatus according to any one of the preceding claims, further comprising a recording means for recording measurements to facilitate the repeatable application of force to the probe tip.

Images