Methods for Calibrating and Identifying Tonometer Probes

Abstract

A method for calibrating and identifying a tonometer probe includes the steps of: a) generating an initial magnetic impulse on a probe of ferromagnetic material via a first electromagnetic coil using known impulse parameters; b) generating a magnetic impulse via the electromagnetic coil to reverse probe speed; c) repeating steps a)-b) for N cycles; d) measuring a resultant induced voltage in the measuring electromagnetic coil from the probe motion; e) comparing the resultant induced voltage in the measuring electromagnetic coil to a reference value; and f) rejecting the probe if the induced voltage measured by the measuring electromagnetic coil differs from the reference value after N cycles.

Description

[Technical field]

[0001] The present invention relates to a method for calibrating and identifying a tonometer probe for intraocular pressure measurements, and in particular to the magnetic saturation and induced voltage of the probe compared to known reference values. [Background technology]

[0002] Measuring intraocular pressure (IOP) is important for the assessment of ocular health. Glaucoma is a chronic progressive blinding disease that is monitored by measuring intraocular pressure and treated by lowering intraocular pressure.

[0003] Intraocular pressure is measured using a device called a tonometer. Goldman Applanation Tonometry (GAT) remains the reference standard tonometry and involves measuring the force required to flatten the cornea using a prism of known surface area. However, GAT is limited in its use, requiring the injection of anesthetic drops, bulky and expensive equipment, and difficult patient positioning. Additional methods of tonometry have been described, such as US Pat. No. 5,399,433, which uses a non-contact method, where compressed air is sprayed onto the cornea and tissue deformation is then measured.

[0004] Another method, known as rebound tonometry, is described in US Pat. No. 5,399,633, in which a metal probe is set up to be operated by a solenoid and impacts the eye, the subsequent change in its movement being recorded and used to derive the intraocular pressure.

[0005] Rebound tonometry utilizes a probe made of ferromagnetic material that acts as a plunger in a solenoid housed within the tonometer. As a ferromagnetic material, the probe becomes magnetized when exposed to a magnetic field and behaves like a magnet.

[0006] The degree of magnetization is determined by the magnetic parameters of the probe material, including magnetic saturation, magnetic remanence, magnetic induction, permeability, and magnetic field strength, and can be called its "magnetization value." The magnetization value of a ferromagnetic material can range anywhere between 0 and a maximum or saturation value. In the presence of an external magnetic field, the magnetization value tends to approach the saturation value.

[0007] A short DC induction pulse by an induction coil creates a magnetic field and sets the probe into motion, after which the same induction coil becomes the measurement coil. The acceleration and velocity, collectively called the "drive mechanism" of the probe, are proportional to the magnetization value of the probe. This is true if the mass of the probe and the pulse duration remain constant. The magnetic probe moving within the solenoid induces a potential difference between the solenoid coil terminals, known as the induced voltage. The induced voltage produced is proportional to the magnetization value of the probe and its velocity.

[0008] When the probe strikes the ocular surface and bounces off, the induced voltage generated is proportional to the IOP. This also means that the derived IOP depends on the magnetization value of the probe material. Therefore, the measurement of the induced voltage is critical for the success of rebound tonometry, and this parameter cannot be compromised. To properly convert the induced voltage to IOP in a consistent and reliable manner, the ferromagnetic probe material must be nearly or completely saturated, i.e., approach the saturation value.

[0009] If the probe material is not fully saturated, the drive mechanism of the probe will change from measurement to measurement, leading to the algorithm used to derive an erroneous IOP.

[0010] Since ferromagnetic materials lose magnetization value over time, a method is needed to bring the probe material to near or full saturation. Furthermore, different ferromagnetic materials have different magnetization saturation value curves, as shown in Figure 1. Therefore, probes made of different ferromagnetic materials may have different magnetization saturation value curves and subsequently result in erroneous IOP results as determined by the algorithm. Furthermore, there is a possibility of damage to the eye if the magnetization value of the probe results in an increase in the probe speed and impact force on the eye.

[0011] While rebound tonometry is a clinically useful technique, the correct probe must be used for the measurement. Specifically, the use of an incorrect measurement probe from an alternative rebound tonometer device or the selection of other materials for use in the tonometer can have devastating consequences in the form of inaccurate IOP measurements or damage to the eye resulting in loss of vision and / or loss of the eye. Further increasing this concern is the growing popularity of home rebound tonometers where patients load their own probes and check their own intraocular pressure without the assistance of a licensed provider. Human factors engineering is a field that considers human capabilities and limitations for the design of safe systems and devices. It assumes that all humans, even licensed health care providers, are fallible. Thus, a safety method is needed for both calibration and subsequent identification of the correct probe before the device allows the user, either a health care provider or a patient, to make a measurement. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] U.S. Patent No. 5,299,573 [Patent Document 2] U.S. Patent No. 6,093,147 Summary of the Invention

[0013] The present invention provides a method for calibrating and identifying a tonometer probe made wholly or partially of a ferromagnetic material. The method uses a tonometer device to calibrate the probe and determine whether the probe is suitable for use in a rebound tonometer based on its magnetization saturation value and the induced voltage specific to the input parameters of the device. More specifically, the probe must be brought to its near or full magnetization saturation value during a calibration cycle, and then after setting the probe for operation, it must be identified as having the correct magnetization saturation value based on the induced voltage specific to the input parameters of the device. This verification must be done before starting measurements on a patient.

[0014] The method includes the steps of: a) generating an initial magnetic impulse on the probe via a first electromagnetic coil using known impulse parameters; b) generating a magnetic impulse via the electromagnetic coil to reverse the probe velocity; c) repeating steps a)-b) for N cycles based on a known reference number of cycles; d) measuring a resultant induced voltage on the measurement electromagnetic coil from the Nth cycle of probe motion; e) comparing the resultant induced voltage to a reference value; and f) rejecting the probe if the induced voltage measured by the measurement electromagnetic coil differs from the reference value after N cycles, e.g., deviates from the reference value by more than at least 0.1 millivolts after N cycles.

[0015] The comparison of the resultant induced voltage to the reference value may occur either before or after generating the reverse impulse. For example, instead of the above order of steps, the method may include the following order: a) generating an initial magnetic impulse on the probe via a first electromagnetic coil using known impulse parameters, b) measuring the resultant induced voltage on the measurement electromagnetic coil from the motion of the probe, c) comparing the resultant induced voltage to a reference value, d) generating a magnetic impulse via the electromagnetic coil to reverse the probe speed, e) repeating steps a)-d) for N cycles until the induced voltage approaches, equals, or exceeds the reference value, and f) rejecting the probe if the induced voltage measured by the measurement electromagnetic coil differs from the reference value after N cycles, e.g., if the induced voltage after N cycles is at least more than 0.1 millivolts away from the reference value.

[0016] The coil providing the initial magnetic impulse may be the same coil used to measure the resultant induced voltage. Additionally, a single coil system may be used to provide an impulse to reverse the probe velocity using the same electromagnetic coil as the measurement electromagnetic coil and the initial magnetic impulse coil. Alternatively, a separate coaxial electromagnetic coil may be used to measure the resultant induced voltage. A separate coaxial coil may also provide an impulse to reverse the probe velocity, or yet another coaxial electromagnetic coil may be used to provide an impulse to reverse the probe velocity. According to certain embodiments, both the coil providing the initial magnetic impulse and a separate coil may measure the resultant induced voltage. Additionally, the resultant induced voltage may be measured in multiple coaxial electromagnetic coils, such as three or more coils. Additionally, multiple electromagnetic coils may be used to provide an impulse to reverse the probe velocity.

[0017] The device can be set to use the minimum number of cycles necessary to achieve probe saturation, and therefore terminate the cycles when probe saturation is achieved. Alternatively, the device may be set to run a set number of cycles, which may be greater than the minimum number of cycles necessary to reach saturation. The number of cycles may also be varied with each calibration based on probe magnetization, so that the number of cycles may be varied to meet criteria specified by the tonometer design inputs.

[0018] If comparing the measured resultant induced voltage to a reference value reveals that the induced voltage is more than a certain number of volts away from the known reference value after N cycles, the probe is rejected. To notify the user that the probe has been rejected, the device may prevent subsequent measurements. Additionally or alternatively, the device may present a notification to the user to notify the user when the induced voltage is not close to or equal to the known reference value after N cycles. The notification may be, for example, an audible notification or a visual notification. [Brief description of the drawings]

[0019] [Figure 1] 1 is a graphical representation of magnetization saturation curves of various materials. [Diagram 2] FIG. 1 is a perspective view of a coaxial two-coil system with a probe inserted. [Diagram 3] 1 is a graphical representation of the voltage applied to two separate coils. [Figure 4] 1 is a graphical representation of induced voltage over time for four different probe scenarios. [Diagram 5] 1 is a graphical representation of induced voltage over time for four different probe scenarios. [Figure 6] 1 is a graphical representation of induced voltage over time for four different probe scenarios. [Figure 7] 1 is a graphical representation of induced voltage over time for four different probe scenarios. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The method described herein solves the problem of probe calibration and identification for use in a rebound tonometer for measuring intraocular pressure. More specifically, by examining the probe's magnetization saturation value and an induced voltage specific to the device's input parameters, a device used with the rebound tonometer can confirm the presence of a suitable probe or reject an inappropriate probe.

[0021] An example of a calibration and identification device 20 incorporated into a rebound tonometer that can be used to carry out the present methods is shown in Figure 2. The device 20 has two coaxial solenoid coils 22 and a probe 24 made partially or completely of a ferromagnetic material. The methods described herein may also be carried out using a similar device having a different number of solenoid coils 22, such as only one solenoid coil 22. In either case, the device 20 determines whether the probe 24 is suitable for use in a rebound tonometer.

[0022] The probe 24 is inserted into the shaft 26 in one or more of the solenoid coils 22, as shown in FIG. 2. The device 20 then generates an initial magnetic impulse on the probe 24 using known impulse parameters. For example, the device 20 can provide a controlled, predetermined voltage and pulse duration to the outermost electromagnetic coil 22a near the insertion point. According to other embodiments, the inner electromagnetic coil 22b further away from the insertion point can provide the impulse. The impulse sets the probe 24 into motion, driving it out of the solenoid. However, since it is not desirable to completely expel the probe 24 from the device 20, after a very short interval, a voltage is provided to a second coil 22 spaced apart from the first coil 22, thus reversing the velocity vector of the probe and returning the probe 24 to or near its initial starting point in the shaft 26 of the device 20. According to certain embodiments, both the initial impulse and the reverse impulse can be generated by the same coil 22. According to other embodiments, both the initial impulse and the reverse impulse can be generated by multiple coils 22.

[0023] FIG. 3 is a graph showing voltage and pulse duration in a two-coil system during one cycle. During the short interval where no voltage is applied, the probe has been given an initial impulse and has not yet been given a reverse impulse, so that the probe is moving at a certain speed. As a ferromagnetic material, the movement of the probe in the solenoid results in an induced voltage between the terminals of the measurement coil. In this particular embodiment, the front coil or the outermost coil is used as the measurement coil. In another embodiment, the rear coil or the inner coil can be used as the measurement coil. In yet another embodiment, both the front coil and the rear coil can be used as the measurement coil. In yet another embodiment, a system of multiple measurement coils, such as three or more coils, can be used.

[0024] After the initial impulse and before the reverse impulse, the device "reads" the induced voltage through one or more measurement coils. When the probe is near or at full saturation, the induced voltage must be equal to or within an acceptable range of values ​​entered into the device by the manufacturer if the probe is a suitable probe for use with the device. The induced voltage reference value is specific to that device and is determined by the manufacturer from various design inputs including voltage, pulse duration, probe material, probe manufacturing process, probe magnetization value, probe weight, and required speed and force, to name a few. Comparing the induced voltage to the reference value that the device "expects" as the manufacturer designs the device to be safe and effective ensures that the correct probe, with the correct material and magnetization value, is loaded into the device and calibrated. The comparison of the resulting induced voltage to the reference value may be done either before or after generating the reverse impulse. Once the probe is fully saturated, as shown in Figure 4, one cycle may be sufficient to ensure the calibration and identification of the probe.

[0025] However, as mentioned above, ferromagnetic materials can lose magnetization value over time. If the probe material is not yet fully saturated, the initial impulse may result in an induced voltage that is lower than the reference value. If the calibration is completed after this single cycle, and if the resulting induced voltage on the measurement electromagnetic coil does not approach or equal its reference value, the induced voltage and subsequent IOP determinations will be erroneous during the measurement cycle performed on the patient's eye. An example of this is shown in Figure 5.

[0026] Therefore, there must be a requirement for the device to check the induced voltage on the measurement electromagnetic coil to ensure accurate probe material and to ensure the probe is fully saturated. Providing an additional calibration cycle in which the probe is exposed to a short magnetic field when set to operate by the solenoid and then reversed back to or near the starting position will successively raise the magnetization value of the probe until it is nearly or fully saturated. This will take "N" cycles, which, as previously mentioned, will depend on many parameters. During the development of the probe, it can be determined how many cycles it will take to bring the probe to a saturated value if it is not fully magnetized. For example, in certain embodiments, N can be between 1 and 20, or between 4 and 10, or between 5 and 8. More than N cycles can be performed since the probe cannot be oversaturated. Once the probe reaches a point of near or full saturation, if the device upon comparison reads the induced voltage on the measurement electromagnetic coil near or at the expected reference value, then the calibration and identification is complete and it is safe to proceed with measuring patients using this probe. This scenario is illustrated in Figure 6 for a theoretical material and device. In the illustrated example, the device utilizes more cycles than the minimum required to achieve probe saturation. In other embodiments, the device may use the minimum number of cycles required to achieve probe saturation, and therefore terminate the cycles when probe saturation is achieved. The number of cycles may also vary with each calibration, as may be set according to one or more of the parameters or design inputs.

[0027] If comparing the measured resultant induced voltage to a reference value reveals that the induced voltage is more than a certain number of volts away from the known reference value after N cycles, the probe is rejected. Just as the number of cycles (N) depends on many parameters, the acceptable difference between the induced voltage and the known reference value also depends on many parameters. For example, the acceptable difference between the induced voltage and the known reference value may be up to 0.1 millivolts, or up to 1 millivolt, or up to 0.5 volts. Thus, the device may reject the probe if the induced voltage measured by the measuring electromagnetic coil is at least 0.1 millivolts away from the reference value after N cycles, or at least more than 1 millivolt, or at least more than 0.5 volts. To inform the user that the probe has been rejected, the device may prevent subsequent measurements. Additionally or alternatively, the device may present a notification to the user to inform the user when the induced voltage is not close to or equal to the known reference value after N cycles. The notification may be an audible notification, such as a beep, or the notification may be visual, such as providing a sentence or symbol on an electronic display.

[0028] In another example, consider a device that does not have a probe identification process. If a user inserts a probe, wire, or material that has a larger magnetization value when fully saturated, and the device does not have a safety check compared to the reference value, and the device impulse parameters remain the same, the probe speed and induced voltage may be higher (Figure 7). Not only will the IOP be inaccurate, but the impact force on the eye may be greater than the safety threshold and cause eye damage. For example, the calibration cycle to bring the probe into saturation may be variable, since there is no way to predict the current magnetization value of the probe after it has been left in the case for an extended period of time, and thus, without comparison to the reference value, the device may overshoot the reference safety value upon saturation of alternative probe materials without proper device feedback.

[0029] The description and figures contained herein show specific embodiments to teach those skilled in the art how to make and use the best options. For the purpose of teaching the principles of the invention, some conventional aspects have been simplified or omitted. Those skilled in the art will appreciate variations from these embodiments that fall within the scope of the invention. Those skilled in the art will also appreciate that the features described above can be combined in various ways to form multiple embodiments. As a result, the present invention is not limited to the specific embodiments described above, but only by the claims and their equivalents.

Claims

1. 1. A method for calibrating and identifying a tonometer probe, comprising: a) generating an initial magnetic impulse on a probe of ferromagnetic material via a first electromagnetic coil using known impulse parameters; b) generating a magnetic impulse via an electromagnetic coil to reverse the probe velocity; c) repeating steps a) to b) for N cycles based on a known reference number of cycles; d) measuring the resultant induced voltage on the measurement electromagnetic coil from the Nth cycle of probe motion; e) comparing the resulting induced voltage on the measuring electromagnetic coil with a reference value; f) rejecting the probe if the induced voltage measured by the measurement electromagnetic coil differs from the reference value after N cycles.

2. The method of claim 1 , wherein the resultant induced voltage is measured in the same coil that provided the initial magnetic impulse.

3. The method of claim 1 , wherein the resultant induced voltage is measured in a separate coaxial electromagnetic coil.

4. The method of claim 1 , wherein the resultant induced voltage is measured both in the same coil that provided the initial magnetic impulse and in a separate coaxial electromagnetic coil.

5. The method of claim 1 , wherein the resultant induced voltages are measured in a plurality of coaxial electromagnetic coils.

6. 2. The method of claim 1, including using a single coil system and providing the impulse to reverse probe velocity using the same electromagnetic coil as the measurement electromagnetic coil and the initial magnetic impulse coil.

7. The method of claim 1 , wherein the impulse to reverse the probe velocity is provided by a second electromagnetic coil.

8. The method of claim 1 , wherein the impulses to reverse probe velocity are provided by a plurality of electromagnetic coils.

9. 2. The method of claim 1, wherein the number of cycles used to saturate the probe is equal to the minimum number of cycles required to reach saturation.

10. The method of claim 1 , wherein the number of cycles used to saturate the probe is greater than the minimum number of cycles required to reach saturation.

11. The method of claim 1 , wherein the number of cycles used to saturate the probe varies with each calibration.

12. 2. The method of claim 1, comprising rejecting the probe if the induced voltage measured by the measurement electromagnetic coil is at least more than 0.1 millivolts away from the reference value after N cycles.

13. 2. The method of claim 1, wherein if the induced voltage measured by the measurement electromagnetic coil does not approach or equal the reference value after N cycles, further measurements are not accepted.

14. The method of claim 1 , further comprising: presenting a notification to a user if the induced voltage measured by the measurement electromagnetic coil is not close to or equal to the reference value after N cycles.

15. The method of claim 14 , wherein the notification is an audible notification.

16. The method of claim 14 , wherein the notification is a visual notification.

17. 1. A method for calibrating and identifying a tonometer probe, comprising: a) generating an initial magnetic impulse on a probe of ferromagnetic material via a first electromagnetic coil using known impulse parameters; b) measuring the resulting induced voltage on the measurement electromagnetic coil from the probe movement; and c) comparing the resulting induced voltage on the measuring electromagnetic coil with a reference value; and d) generating a magnetic impulse via an electromagnetic coil to reverse the probe velocity; e) repeating steps a) to d) for N cycles until the induced voltage measured by the measuring electromagnetic coil approaches, equals, or exceeds the reference value; f) rejecting the probe if the induced voltage measured by the measurement electromagnetic coil differs from the reference value after N cycles.

18. 20. The method of claim 17, wherein the resultant induced voltage is measured in the same coil that provided the initial magnetic impulse.

19. The method of claim 17 , wherein the resultant induced voltage is measured in a separate coaxial electromagnetic coil.

20. 20. The method of claim 17, wherein the resultant induced voltage is measured both in the same coil that provided the initial magnetic impulse and in a separate coaxial electromagnetic coil.

21. The method of claim 17 , wherein the resultant induced voltages are measured in a plurality of coaxial electromagnetic coils.

22. 20. The method of claim 17, including using a single coil system and providing the impulse to reverse probe velocity using the same electromagnetic coil as the measurement electromagnetic coil and the initial magnetic impulse coil.

23. The method of claim 17 , wherein the impulse to reverse the probe velocity is provided by a second electromagnetic coil.

24. The method of claim 17 , wherein the impulses to reverse probe velocity are provided by a plurality of electromagnetic coils.

25. 18. The method of claim 17, wherein the number of cycles used to saturate the probe is equal to the minimum number of cycles required to reach saturation.

26. 20. The method of claim 17, wherein the number of cycles used to saturate the probe is greater than the minimum number of cycles required to reach saturation.

27. 20. The method of claim 17, wherein the number of cycles used to saturate the probe varies with each calibration.

28. 20. The method of claim 17, comprising rejecting the probe if the induced voltage measured by the measurement electromagnetic coil is at least more than 0.1 millivolts away from the reference value after N cycles.

29. 18. The method of claim 17, wherein if the induced voltage measured by the measurement electromagnetic coil does not approach or equal the reference value after N cycles, further measurements are not accepted.

30. 20. The method of claim 17, further comprising: presenting a notification to a user if the induced voltage measured by the measurement electromagnetic coil is not close to or equal to the reference value after N cycles.

31. The method of claim 30 , wherein the notification is an audible notification.

32. The method of claim 30 , wherein the notification is a visual notification.

33. 18. The method of claim 17, wherein generating the magnetic impulse through the electromagnetic coil to reverse a probe velocity occurs before comparing the resulting induced voltage on the measurement electromagnetic coil with the reference value.

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