Self-contained hand-held ultrasonic instrument for ophthalmic use

Abstract: Disclosed is a completely portable, hand-held digital ul­ trasonic instrument about the size and shape of an ordinary pen. The housing is contoured such that it can be easily grasped in the manner of a writing pen. An activation button is located on the interior dorsal surface in close approximation to the index fingertip of the user. A liquid crystal display prov­ ides a digital readout of the corneal thickness on the barrel of the instrument housing. The instrument incorporates a 10 MHz or 20 MHz solid phase piezoelectric transducer, a micro­ processor, a gate array, a hybrid analog circuit, a liquid crystal display, batteries and a removable battery cover. The 20 MHz transducer is attached to a swan neck connector which is attached to the instrument housing. When a 10 MHz transdu­ cer is used, it is attached to a truncated conical connector which is also attached to the instrument housing. In addition, a pinjack connector is available to link the unit to other elec­ tronic media such as a microcomputer, personal computer, or printer.

Description: Myopia (near-sightedness) is the most common optical refractive error. Over 133 million individuals in the United States bear some form of refractive correction, either in the form of spectacles or contact lenses. Recently, surgical correction of refractive errors, or refractive surgery, has gained increasing popularity. The most popular form of refractive surgery is called radial keratotomy (RK). In this surgery, a series of radial incisions of precise depth are placed in the cornea, reflecting a change in its curvature and hence its refractive power. RK surgery was initially regarded with great skepticism by the ophthalmic community, however its effectiveness and relative safety have been demonstrated in the last few years. Accordingly, an increasing number of ophthamologists have been performing this operation. The length and number of incisions required in an RK operation varies according to the degree of optical correction desired, and the depth of the incision is critical to the satisfactory outcome of the surgery. Inadequate depth will result in undercorrection, whereas excessive depth will result in overcorrection or perforation of the cornea and potentially serious harm to the eye. Accurate measurement of the corneal thickness is therefore essential to safe and successful RK surgery. Instruments that measure corneal thickness are referred to as pachymeters. Early pachymeters were purely optical devices and had been shown to be imprecise. Ultrasonic technology is now routinely used to measure corneal thickness. All current instruments employ an ultrasonic probe which contacts the eye and which is connected via cable to a desk unit which is about the size of an oscilloscope. Some units have a smaller oscilloscope screen that displays the transducer tracings while other more recent units simply display a digital readout of the corneal thickness. The most commonly performed operation in ophthalmology is cataract surgery. A cataract is an opacification of the biological lens inside the eye. In cataract extraction, one of the several techniques is used to remove the opacified lens material. Once removed, it is possible to implant an artificial lens in order to restore optical integrity to the eye, eliminating the need for thick cataract glasses or contact lenses. When cataract surgery is performed, several measurements are necessary in order to calculate the precise power of the intraocular lens (10L) to be implanted. The important variables are corneal curvature (keratometry), size of the eye (axial length), knowledge of where inside the eye the inner ocular lens is to be implanted (interior or posterior chamber). Measurement of axial length is performed ultrasonically by a device referred to as a biometric ruler. Through such an instrument, the major internal structures of the eye can be imaged and their dimensions measured. Of importance in the measurement of true axial length is the distance from the cornea to the retina along the visual axis of the eye. The early biometric rulers employed a piezoelectric transducer, in a hand-held probe attached via a cable to an oscilloscope. It was necessary for the individual performing the examination to visually assess the oscilloscope signals and identify an axial scan. Second generation instruments still require the examiner to detect the pattern consistent with an axial scan. Then, electronics are used to calculate and display the desired measurement in millimeters. It would be desirable to have a self-contained, hand-held, digital, ultrasonic biometric ruler instrument for displaying the axial length of the eye using microprocessor technology to assess the echo wave-forms and displaying digitally a readout representing as accurately as possible the true axial length of the eye. US-4,413,629 discloses a large hand held ultrasonic probe intended for detecting foetal heart beats in pregnant women. DE-A-3,338,745 discloses a large ultrasonic device for measuring dimensions of the eye which has a separate probe attached to the base unit by a cable. The present invention as defined in claim 1 relates to a completely portable, hand held digital ultrasonic instrument which can be about the size and shape of an ordinary pen. The housing is contoured such that it can be easily grasped in the manner of a writing pen. In the later described preferred embodiments an activation button is located on the interior dorsal surface in close approximation to the index fingertip of the user. A liquid crystal display provides a digital readout of the corneal thickness on the barrel of the instrument housing. The instrument incorporates a 10 MHz or 20 MHz solid phase piezoelectric transducer, a microprocessor, a gate array, a hybrid transceiver, a liquid crystal display, batteries and a removable battery cover. The pachymeter uses a 20 MHz transducer that is attached to a swan neck connector which is attached to the insrument housing. The biometric ruler uses a 10 MHz transducer that is attached to a truncated conical connector which is also attached to the instrument housing. In addition, a pinjack connector is available to link the unit to other electronic media such as a microcomputer, personal computer, or printer. When the 10 MHz transducer is used, the program in the microprocessor is changed to accommodate the different echo patterns that are measured when the axial length of the eye is measured as opposed to the corneal thickness. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A is a perspective view of the instrument as a pachymeter with a 20 MHz transducer and Figure 1B shows the biometric ruler with a 10 MHz transducer. Figure 2A shows a digital display for use with a 20 MHz transducer and Figure 2B shows a digital display for use with a 10 MHz transducer. Figure 3 is a top view of the pachymeter with the top cover removed. Figure 4 is a side view of the pachymeter showing the various location of the component parts. Figure 5 is a perspective view of the pachymeter as it would be held by a user when the thickness of the cornea is measured. Figure 6 is a perspective view of the biometric ruler as it would be held by a user when the axial length of the eye is measured. Figure 7 shows a pachymeter with a 20 MHz transducer in contact with the eye as held open by a lid retractor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figure 1A, the instrument comprises a housing 20 which is contoured such that it can be easily grasped, in the manner of a writing pen. The tip of the instrument is the piezoelectric ultrasonic transducer 22 mounted on a plastic standoff 34. The other functioning components of the instrument include an activation button 26, located on the anterior dorsal surface in close approximation to the index fingertip of the user, a liquid crystal display 28 (LCD), a reset button (not shown), a removable battery cover 30, and a pinjack connector 32 (Figure 4). The 20 MHz transducer 22 comprises a plastic, ultrasonically transparent, contact head 34, and a recessed piezoelectric ultrasonic transducer 22. This cone shaped contact head 34 is attached to the transducer 22 which is attached to the swan neck connector 24 which connects to the main housing 20 of the pachymeter 36. The backing of the transducer is a 0.127mm (.005 inch) piece of tungsten-loaded epoxy which is an acoustically inert material. The piezoelectric transducer element has a frequency of 20 MHz. It is activated by a hybrid transceiver which connects to a microprocessor via a gate array. One to three pulses of current are delivered to the transducer from the hybrid transceiver. The first echo corresponds to the front corneal surface. The second echo corresponds to the back corneal surface. The time between the two corneal echoes is proportional to the corneal thickness. Because sound travels at a rate of approximately 1640 meters/sec through corneal tissue, the time between the emitted signal and the reflected echoes can be converted to millimeters, using standard formulae. The active transducer element is opaque and is centered at the posterior edge of the cone shaped contact head 34. The contact head is made of a clear acrylic material that allows the instrument tip to be better visualized under the operating microscope 60 (Figure 5). The small size of the transducer 22 and the contact head 34 minimizes the area of the corneal surface which is obscured by the transducer 22. In addition, the swan neck connector 24 is designed such that it is rather easy for the user to place the contact head 34 into contact with the corneal surface perpendicularly to the visual axis shown by dotted line 62 in Figure 5. In essence, the combined clear acrylic contact head 34 and the swan neck connector 24 both facilitate perpendicular alignment of the transducer 22 to the corneal surface. The transducer must be aligned perpendicular to the corneal surface in order to obtain detectable echoes from the inner corneal surface adjacent to the aqueous humor. In addition, the shortest echographic distance corresponds to the actual corneal thickness. If the transducer 22 is slightly off perpendicular to the visual axis, then spurious readings may result. To calibrate a pachymeter with a 20 MHz transducer, an acrylic multistepped calibration block, which simulates the corneal interfaces, is used which is placed on a flat surface. The pachymeter 36 is then held perpendicularly to the surface of the block and the activation switch 26 is pressed once and then released. A beeper and miniature speaker provide a series of clicks followed by a beep. The pachymeter reading from the surface will be displayed on the display 28. This output should be within .01 mm of the calibration block measurement printed on its surface. When the instrument is used as a biometric ruler, the same housing 20 is used. The tip of the instrument is a piezoelectric ultrasonic transducer element (not shown) mounted within a standoff 38 as shown in Figure 1B. A special acoustically mismatched damping material is utilized to prevent "ringing" of the ultrasonic transducer which might preclude interpretation or detection of early echoes. Other functional components of the instrument include an activation button 26 located on the anterior dorsal surface in the close approximation to the index fingertip of the user, liquid crystal display 28, a reset button (not shown), and a removable battery cover 30. The transducer head 42 is a focusing element with a focal length of 24 millimeters, corresponding to an area near the retina of the eye. The diameter of the focusing element is 7.62mm (0.3 inch). In the center of the focusing element is a light-emitting diode 40. The patient is asked to look at the light-emitting diode 40, which has been activated, as the probe is placed in contact with the corneal surface. This allows the patient to center the eye on the transducer, facilitating axial measurements. The average speed of sound through the eye is approximately 1560 meters/second. The time between the emitted signal, corresponding to the corneal surface, and reflected echoes are then converted into millimeters using standard formulae. One to three pulses of current from the hybrid transceiver are delivered to activate the transducer. After sending out the ultrasonic signal, the transducer is used in a detection mode. The first echo corresponds to the transducer-corneal interface. All subsequent echoes are measured relative to this. The second echo, found within a "window" of 1.5 to 5 millimeters from the corneal surface, corresponds to the anterior lenticular surface. The third echo, falling into a window of 1.5 to 6.5 millimeters behind the anterior lenticular echo, corresponds to the posterior lenticular surface. The fourth echo, located 18.5 to 29.0 millimeters behind the corneal surface echo, corresponds to the retinal surface. The fifth echo, 0.29 to 2.5 millimeters behind the retina, corresponds to the scleral surface. Echoes which occur outside the given windows will be ignored. When the second echo through the fifth echo are detected simultaneously there is optimal alignment. The four annunciator bars 56, Figure 2B, separately indicate the presence of these four echos. The 10 MHz Transducer is calibrated by use of an acrylic block with multiple interfaces which simulates a standardized eye. This is placed on a flat surface and the transducer head 42 is then held perpendicular to the surface of the block and the activation switch 26 is pressed once and then released. A series of clicks will sound, following which there will be a beep. The biometric ruler reading from this source will be demonstrated on the display 28. This output should be within 0.1 millimeters of the calibration block measurement printed on its surface. Figure 2A shows typical information that is displayed on the digital readout 28 when the 20 MHz transducer is used and Figure 2B shows typical information displayed when the 10 MHz transducer is substituted for the 20 MHz transducer of Figure 1A. Figure 3 shows a top view of the instrument with the top cover 42 (Figure 1A) removed. The batteries 40 are located to the rear of the instrument. The liquid crystal display 28 is connected to a circuit board 44 (Figure 4) and is located adjacent to the microprocessor 46. A gate array 48 is located on the same circuit board 44 between the microprocessor 46 and the hybrid transceiver 54. The activation switch 26 is located between the hybrid receiver 54 and the connector 24. The total length of the unit is approximately 184mm (7.25 inches) and it weighs approximately 57 grams (2 ounces). Figure 4 shows a sectional view of the housing 20 of the hand-held pachymeter 36 with the various components installed. The same housing 20 and component parts shown in Figure 4 are used when the instrument is used as a biometric ruler except that the connector 38 and its associated parts are substituted for the connector 24 and its associated parts. At the rear end of the casing is located a reset switch 50 and a Murata (Registered Trade Mark) beeper 52 is located directly beneath the microprocessor 46 on the opposite side of circuit board 44. There is a pinjack 32 located directly below the printed circuit board 44. This pinjack 32 is used to send data such as corneal thickness or axial length from the instrument, whether used as a pachymeter or biometric ruler, to outside instruments such as a microcomputer, a personal computer or printer. All elements of the instrument are connected to a multilayered circuit board 44. Mounted off the circuit board are four silver oxide batteries 41. Mounted on the circuit board is the discrete circuitry related to transducer signal processing. Also on the circuit board are connectors to the display 28, the reset button 50, and the RS232 pinjack 32. Figure 5 shows the pachymeter 36 with a 20 MHz transducer 22 being used for measuring the thickness of the cornea. The user is able to look through a microscope 60 and at the same time grasp the pachymeter 36 in the manner shown in Figure 5. The shape of the swan neck connector 24 allows the user 58 to position the transducer element 22 on the central axis of the cornea, shown by dotted line 62, and still be able to view the patient's eye through the microscope 60. Dotted line 82 shows the perpendicular tangent of the cornea to the visual axis shown by line 62. The central axis of the pachymeter 36, shown by dotted line 80, is approximately 27° from the tangent of the cornea shown by line 82. The swan neck connecter allows the pachymeter 36 to be comfortably held in a position in which the transducer 22 is aligned with the visual axis of the eye and the pachymeter clears the patient's eyebrow 84 and allows the user to comfortably rest his hand on the patient's forehead. A clearer view of what the user sees when viewing the transducer 22 in contact with the eye 65 is shown in Figure 7. The lid retractor 64 holds the patient's eyelids in an open position to facilitate the thickness measurement. The size of the transducer 22 along with the swan neck connector 24 allows the user to partially view the optical zone as shown by dotted lines 66 in Figure 7. The optical zone marks the central limit of the radial corneal cuts performed during radial keratotomy surgery. This zone, usually 3 to 4.5 millimeters in diameter, is demarcated by the surgeon by the superficial application of a calibrated trephine. Figure 6 shows a user holding the biometric ruler as it would be held when making an axial length measurement. The biometric ruler unit 68 is held perpendicular to the patient's eye 70 along the visual axis of the patient's eye shown by dotted line 72. The activation button 26, Figure 1B, is depressed and the measurement sequence is initiated. The results of the successful measurement are displayed on the LCD 28 as shown in Figure 2B. In the case of aphakia, the anterior and posterior lenticular spikes are not available. Axial length calculation is required for the placement of secondary lens implants with appropriate power. Because it is impossible to align multiple interfaces to ensure axial scan, the reliability of such measurements is less than could be expected in aphakic patients. Therefore, in order to initiate an aphakia mode measurement, the activation button 26 is pressed twice in rapid succession. The patient is then asked to look at LED emitter 40 (Figure 1B) in the middle of the focusing element 42 (Figure 1B). Once the retinal and scleral spikes are identified using thresholding identification algorithms, the maximum length is sought in a manner analogous to the evaluation performed in the phakic examination. The results are displayed on the display 28, sent to the pinjack 32, and the annunciators 56, corresponding to the retinal and scleral spikes, are illuminated on the display 28. The biometric ruler 68 as shown in Figure 6 weighs approximately 57 grams (2 ounces) and is approximately 184mm (7.25 inches) in length. When the activation switch has not been depressed for two minutes, the microprocessor 46 and transducer elements are automatically turned off in order to reduce power consumption and preserve the battery life. A small discrete circuit performs this function and also responds to depression of the activation button by activating the electronic elements and the transducer.

Claims: A self-contained hand-held ultrasonic instrument for measuring ophthalmic thicknesses comprising a housing (20) with an elongated body adapted to be grasped in the manner of a writing pen, said body having a top surface and having depressions toward the distal end to facilitate grasping said instrument, a swan neck connector (24) having a conically tapering neck that is angled away from said top surface to a footplate shaped area and containing a transducer (22) connected to said footplate area, said connector being connected to the distal end of said housing and said transducer (22) being disposed so that its surface is at an angle between 10° and 45° off the central axis (80) of said elongated body, a digital display (28) for displaying ophthalmic thicknesses located on the top surface of said body and said body housing an interconnected circuit comprising power supply, gate array, microprocessor, transmitter/receiver and activation button (26). A self-contained hand-held ultrasonic instrument for measuring the axial length of an eye comprising a housing (20) with an elongated body adapted to be grasped in the manner of a writing pen said body having a top surface and having depressions toward the distal end to facilitate grasping said instrument, a truncated conical connector which narrows distally along the axis of the elongated body and containing a transducer, said connector being connected to the distal end of said housing and said connector being fabricated from an acoustically mismatched damping material to allow better visualization of cornea applanation by the transducer, a digital display (28) for displaying the axial length of an eye located on the top surface of said body and said body housing an interconnected power supply, gate array, microprocessor, transmitter/receiver and activation button (26).

Inventors: FELDON STEVEN, WALLACE DAVID, FELDON, STEVEN, WALLACE, DAVID
Applicants: MENTOR O & O INC, MENTOR O & O, INC
Application year: 1986
Main classification: A61B8
IPC codes: A61F9, A61B8, A61B3