CA1117643A - Apparatus for measuring the axial length of an eye - Google Patents
Apparatus for measuring the axial length of an eyeInfo
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- CA1117643A CA1117643A CA000338432A CA338432A CA1117643A CA 1117643 A CA1117643 A CA 1117643A CA 000338432 A CA000338432 A CA 000338432A CA 338432 A CA338432 A CA 338432A CA 1117643 A CA1117643 A CA 1117643A
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- echo
- gate
- eye
- triggered
- retinal
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Abstract
ABSTRACT
APPARATUS FOR MEASURING THE
AXIAL LENGTH OF AN EYE
An apparatus for measuring the axial length of the eye is disclosed. The apparatus comprises a transducer adapted to transmit repetitive ultrasonic pulses along the ocular axis of the eye of a patient and to receive echo pulses reflected from the retina of the eye. A fixed gain amplifier is connected to a transducer for amplifying the reflected echo pulses. An automatic gain controlled ampli-fier is also connected to the transducer for amplifying the reflected echo pulses. Control means are coupled to the automatic gain controlled amplifier for gradually increasing the gain of the amplifier to a predetermined maximum gain.
First and second gate circuits are coupled to the output of the fixed and automatic gain controlled amplifier, respecti-vely, and adapted to pass logic signals triggered by retinal echo pulses exceeding first and second predetermined thre-sholds. A digital counter is connected to the second gate circuit and adapted to display the axial length of the eye as a function of the distance travelled by the retinal echo pulses. A gate width generator is connected to the second gate circuit for generating a time slot during which echo pulses originating from the posterior wall of the eye can be received. A retinal echo triggered gate width generator is connected between the first and second gate circuits and is responsive to the first gate circuit for enabling the se-cond gate circuit to pass logic signals triggered by retinal echo pulses exceeding the second threshold in the time slot generated by the gate width generator. The output of the retinal echo triggered gate width generator is also connec-ted to a latching circuit which is connected to the first gate circuit for blocking the first gate circuit immediately after receipt of the first logic signal triggered by a reti-nal echo pulse, thereby preventing mistriggering of the re-tinal echo triggered gate width generator circuit by echoes originating from structures behind the retina.
APPARATUS FOR MEASURING THE
AXIAL LENGTH OF AN EYE
An apparatus for measuring the axial length of the eye is disclosed. The apparatus comprises a transducer adapted to transmit repetitive ultrasonic pulses along the ocular axis of the eye of a patient and to receive echo pulses reflected from the retina of the eye. A fixed gain amplifier is connected to a transducer for amplifying the reflected echo pulses. An automatic gain controlled ampli-fier is also connected to the transducer for amplifying the reflected echo pulses. Control means are coupled to the automatic gain controlled amplifier for gradually increasing the gain of the amplifier to a predetermined maximum gain.
First and second gate circuits are coupled to the output of the fixed and automatic gain controlled amplifier, respecti-vely, and adapted to pass logic signals triggered by retinal echo pulses exceeding first and second predetermined thre-sholds. A digital counter is connected to the second gate circuit and adapted to display the axial length of the eye as a function of the distance travelled by the retinal echo pulses. A gate width generator is connected to the second gate circuit for generating a time slot during which echo pulses originating from the posterior wall of the eye can be received. A retinal echo triggered gate width generator is connected between the first and second gate circuits and is responsive to the first gate circuit for enabling the se-cond gate circuit to pass logic signals triggered by retinal echo pulses exceeding the second threshold in the time slot generated by the gate width generator. The output of the retinal echo triggered gate width generator is also connec-ted to a latching circuit which is connected to the first gate circuit for blocking the first gate circuit immediately after receipt of the first logic signal triggered by a reti-nal echo pulse, thereby preventing mistriggering of the re-tinal echo triggered gate width generator circuit by echoes originating from structures behind the retina.
Description
~117643 APPAR~TUS FO~ MEASURING THE
. _ AXI~L LENGTH OF AN ~YE
_ This invention relates to an apparatus for effec-ting rapid and accurate ultrasonic measurements of the axial length of the eye.
A compact and simple ultrasonic instrument (echo-oculometer) for measuring the axial length and anteriorchamber depth of the eye has been described in the litera-ture by Mortimer et al. in the Proceedings of the 11th Intl.
Conf. on Medical and Biol. Engng. 1976 pp. 508-509 and by Mortimer et al. in the Canadian J. Ophthal. Volume 12, 1977 pp. 318-320.
Advantages of this instrument over conventional A-Scan devices employing cathode ray tubes for display are that a display of the A-Scan is not essential, the results are immediately available on a counter and are expressed in convenient numerical units.
The above echo-oculometer utilizes techniques si-milar to those used in the echo-encep~alo~raph invented by Hudson et al. and described in U.S. Patent No. 3,872,858 issued ~arch 25, 1~75 and its corresponding Canadian Patent No. 973,632 issued August 26, 1975. The echo-oculometer employs a transducer which emits a short pulse of ultrasound ~117643 aimed along the ocular axis~ The echoes returning from the various surfaces within the eye are received by ~he same transducer and the time required for the sound pulse to re-turn is converted to a length measurement. Two range gates consisting of electronic logic circuits allow echoes from particular interfaces to be sel~cted and the corresponding time interval to be measured~ The retinal echo is selected for the axial length and the anterior lens echo is selected for the anterior chamber depth measurement. The statisti-cal accuracy of the determinations may be increased by averaging several readings.
Three important features of the above echo-oculo-meter device are that it employs a slow gain sweep, a crys-tal controlled time base of a particular frequency, and a special delay circuit which determines the time at which the counter starts to count.
In contrast to the echo-encephalograph which em-ploys a second fast gain sweep (functioning analogously to the time varied gain or TGC of the conventional A and B
scan equipment), the oculometer has only a slow gain sweep since compensation for tissue attenuation is not needed.
For the slow gain sweep, the gain does not vary significan-tly during the time a given pulse is transmitted and its echoes are recei~ed. Rather the receiver gain increases from a transmitted pulse to the next until the range-gated signal exceeds a predetermined threshold, stopping the counter (displaying the accumulated count), or until the maximum recei~er gain level is reached whereupon the mea-~li7643 surement cycle ls aut~matically repeated.
The gain sweep has two functions. It i5 supposedto compensate for differences in the amplitudes of the re-ceived echoes (in different eyes) and, more important, it is supposed to insure that the strongest echo detected in the range gate inter~al selected will be the first to ex-ceed the detection threshold thereby stopping the counter.
Key assumptions made in the operation of the abo-Ye echo-oculometer device are that (1I when the beam is di-rected along the axis of the eye, the first echo in theselected range gate will be the largest echo, and (2) if the beam is directed off axis, the echoes received will be too weak (due to the inclination of the reflecting interface and the transducer directivity) to exceed the range gate de-tection threshold.
While these assumptions are generally true for theanterior lens echoes (,anterior chamber depth), research and clinical experience conclusively show that this is not al-ways true for the much more important retinal echoes (axial length). In a significant number of cases, other interfaces and structures behind the retina give rise to the largest echoes. This can result in errors in the axial length de-terminations of as much as 3mm which corresponds to an error in the lens power determinations of about 8 or 9 diopters.
An error of this magnitude is altogether unacceptable.
Furthermore, lt must be emphasized that although this pro-blem is much more common for the case of off-axis beam in-cidence, it will still sometimes occur when the beam is pro-1~7~3 perly aligned.
Finally, if readings are taken with the beam im-properly aligned, the accuracy of the axial length determi-nations will be decreased either due to the problem just described or in the case where the retinal echo does stop the counter, the fact~ that a chord shorter than the axial diameter is being measured.
One method of counteracting these problems would bé to increase the beam directivity but this is subject to both theoretical and practical limitations. Another ap-proach might be to lower the upper limit of the swept gain.
However, there are ob~ious constraints since the instrument must accommodate a considerable range of ultrasonic and geometrical characteristic~ for different eyes~
Another method would be to require the presence of the anterior and posterior lens echoes (either one but preferably both) of a magnitude equal or greater than some specified fraction of the retinal echo threshold (typically ~ or more) as a necessary condition for a valid reading.
In fact, the standard A-scan technique (used in determining the axial length) consists of insuring that both the ante-rior and posterior lens echoes are simultaneously present together with the retinal echo, and then maximizing the two lens echoes while maintaining a good clean and large retinal echo. The technique is illustrated in detail by Leary in Ultrasonics April 1967, pp. 84-87, Under normal conditions, the symmetry of the eye is such that following the above procedure will insure good axial alignment.
~lq643 Implementing the abo~e lens echo conditions ln the form of electronic circuits to assure that readings are obtained only under conditions of good alignment is straight forward and is obvious to those skilled in the art. Howe-ver~ while this would greatly reduce the likelihood of in-correct ~riggerlng by structures behind the retina, it does not entirely eliminate the problem. Also the difficulty of achieving exact alignment without reference to an A-Scan can make the actual obtaining of readings very problematic.
lQ Certainly adding the lens echo condition reduces the speed with which valid readings can be obtained. This i5 an im-portant consideration when dealing with older or uncoopera-tive patients.
It is therefore the object of the present inven-tion to provide an apparatus for effecting rapid and accu-rate ultrasonic measurements of the axial length of the eye.
The apparatus, in accordance with the invention, comprises a transducer adapted to transmit repetitive ultra-sonic pulses along the ocular axis of the eye of a patient and receive echo pulses reflected from the retina of the eye, a fixed gain amplifier connected to the transducer for amplifying such reflected echo pulses, an automatic gain controlled amplifier also connected to the transducer for amplifying the reflected echo pulses, control means coupled to the automatic gain controlled amplifier for gradually increasing the gain of the amplifier during a measurement cycle, first and second gate circuits controlled by the output of the fixed and automatic gain controlled amplifier, 11176~3 respectively, and adapted to pass logic signals triggered by retinal echo pulses exceeding first and second predeter-mined thresholds, a digital counter connected to the second gate circuit and adapted to display the axial length of the eye as a function of the distance travelled by the retinal echo pulses, a gate dela~ initiated by a slow clock, a gate width generator connected to the gate delay for generating a time slot during which echo pulses originating from the posterior wall of the eye can be received, a latching cir-cuit responsive to the gate delay for enabling the firstgate circuit to pass logic signals triggered by echo pulses exceeding the first threshold, a retinal echo triggered gate width generator interconnecting the first and second gate circuits and responsive to the first gate circuit for ena-bling the second gate circuit to pass logic signals trigge-red by retinal echo pulses exceeding the second threshold in the time slot generated by the gate width generator, the output of the retinal echo triggered gate width generator being also connected to the latching circuit for blocking the first gate circuit immediately after receipt of the first logic signal triggered by a retinal echo pulse, there-b~ preventing mistriggering of the retinal echo triggered gate width generator by echoes originating from structures behind the retina~
The above control means includes a slow ramp gene-rator adapted to generate a ramp voltage which is applied to the automatic gain controlled ampllfier in such a manner that the gain of the amplifier varies from a minimum at the 76~3 start of the measurement cycle to a preset maximum after a number of cycles~
A pulser is connected to the transducer for ap-plying sharp hlgh voltage splkes to such transducer to shock excite lt so as to direct an ultrasonic pulse into ~ne eye being examined. The pulser is triggered by the slow clock~
A fast clock, operating at a frequency in MHz which is l/2 times the average velocity of ultxasound in the eye expres-sed in units of 0.1 mm per microsecond, provides the coun-ter time base and is used to synchronize the slow clock.
A pulser delay is located between the slow clockand the pulser to insure that the counter starts counting at the correct time~
The invention will now be disclosed, by way of example, with reference to the accompanying drawings in which:
Figure l is a block diagram of an exemplary embo-diment of the invention;
Figure 2 is a series of waveforms produced at va-rious points in the block diagram of Figure l; and Figure 3 is a circuit diagram of the echo-oculome-ter constructed according to the present invention.
Referring to Figures l and 2, there is shown a fast clock 10 which generates a continuous signal at a fre-quency in MHz which is 1/2 times the average velocity of ul-trasound in the eye ~1553 m/sec~ in units of 0~1 mm per mi-crosecond, that is 7.765 MHzt,and has a sinusoidal waveform as illustrated at A tn Figure 2. The clock lO feeds a si-1~76~3 gnal to a counter-display 12 and to a slow clock 14. The slow clock 14 generates a square ~ave at a frequency of about 60 Hz as shown at B in Figure 2 of the drawings. The operation of the slow clock 14 is synchronized to the fast clock 10. The output slgnal of the slow clock is applied to a pulser delay 1~ which generates a repetitive pulse signal such as shown at C in Figure 2~ The pulser delay 16 trig-gers a pulser 18 on the positive going edge of the waveform C and at the same time resets the counter 12. The counter begins counting on the negative going edge of the waveform C to compensate for the propagation time of the ultrasonic pulse from the transducer to the eye and from the eye to the transducer. Pulser 18 generates a sharp high voltage spike, such as shown at D in Figure 2, which is used to shock exci-te a transducer 2Q to direct an ultrasonic pulse through theeye being examined. This pulse travels through the eye and is reflected by the various surfaces and returned to the transducer. These echo pulses are picked up by the transdu-cer and converted back to electrical signals which are di-rected to a preamplifier 22 providing an output such asshown at E in Figure 2. The output of the preamplifier 22 is applied to a fixed gain amplifier 24 providing an ampli-fied output such as shown at F in Figure 2 and to an automa-tic gain control amplifier 26 providing an amplified output such as shown at G, H or I in Figure 2 depending on the gain of the amplifier as controlled by the voltage V applied to its automatic gain control terminal, The output of the fi-xed galn amplifier 24 is fed to a first gate circuit 28 1:~L1764~3 through a comparator 30 which sets a signal threshold level for the output of amplifier 24 as illustrated by a dashed line through waveform F in Figure 2. Similarly, the output of the automatic gain control amplifier 26 is fed to a se-cond gate clrcuit 32 t~rough a comparator 34 which sets asignal threshold level for the output of amplifier 26 as illustrated by a dashed line through waveform I in Figure 2.
The pr~sent apparatus is capable of measuring not only the axial length (AL) of an eye by detecting the reti-nal echo pulses but also the anterior chamber (AC) depth bydetectIng the anterior lens echo pulses. As mentioned pre-viously, the measurement of the anterior lens echoes is not the object of the present invention, therefore the portion of the circuitry which is concerned with the measurement of the retinal lens echo will be primarily disclosed. In or-der to permit gate circuits 28 and 32 to pass logic signals triggered by the echo pulses which are reflected from the desired surfaces of the eye, there is provided a gate delay 36 which is triggered by the positive leading edge of the signal B appearing at the output of the slow clock 14. Gate delay 36 provides an output waveform such as illustrated at L in Figure 2, and triggers the gate width generator 38 on the positive edge of the waveform L. If the anterior cham-ber depth was to be measured, the delay would of course be much shorter so as to allow gating of the echo pulses origi-nating from the anterior lens surface of the eye. A func-tion switch 37 is- provided for selecting which one of the measurements is to be performed by the apparatus. The gate ~'7ti~3 width generator 38 generates a signal M as shown in Figure
. _ AXI~L LENGTH OF AN ~YE
_ This invention relates to an apparatus for effec-ting rapid and accurate ultrasonic measurements of the axial length of the eye.
A compact and simple ultrasonic instrument (echo-oculometer) for measuring the axial length and anteriorchamber depth of the eye has been described in the litera-ture by Mortimer et al. in the Proceedings of the 11th Intl.
Conf. on Medical and Biol. Engng. 1976 pp. 508-509 and by Mortimer et al. in the Canadian J. Ophthal. Volume 12, 1977 pp. 318-320.
Advantages of this instrument over conventional A-Scan devices employing cathode ray tubes for display are that a display of the A-Scan is not essential, the results are immediately available on a counter and are expressed in convenient numerical units.
The above echo-oculometer utilizes techniques si-milar to those used in the echo-encep~alo~raph invented by Hudson et al. and described in U.S. Patent No. 3,872,858 issued ~arch 25, 1~75 and its corresponding Canadian Patent No. 973,632 issued August 26, 1975. The echo-oculometer employs a transducer which emits a short pulse of ultrasound ~117643 aimed along the ocular axis~ The echoes returning from the various surfaces within the eye are received by ~he same transducer and the time required for the sound pulse to re-turn is converted to a length measurement. Two range gates consisting of electronic logic circuits allow echoes from particular interfaces to be sel~cted and the corresponding time interval to be measured~ The retinal echo is selected for the axial length and the anterior lens echo is selected for the anterior chamber depth measurement. The statisti-cal accuracy of the determinations may be increased by averaging several readings.
Three important features of the above echo-oculo-meter device are that it employs a slow gain sweep, a crys-tal controlled time base of a particular frequency, and a special delay circuit which determines the time at which the counter starts to count.
In contrast to the echo-encephalograph which em-ploys a second fast gain sweep (functioning analogously to the time varied gain or TGC of the conventional A and B
scan equipment), the oculometer has only a slow gain sweep since compensation for tissue attenuation is not needed.
For the slow gain sweep, the gain does not vary significan-tly during the time a given pulse is transmitted and its echoes are recei~ed. Rather the receiver gain increases from a transmitted pulse to the next until the range-gated signal exceeds a predetermined threshold, stopping the counter (displaying the accumulated count), or until the maximum recei~er gain level is reached whereupon the mea-~li7643 surement cycle ls aut~matically repeated.
The gain sweep has two functions. It i5 supposedto compensate for differences in the amplitudes of the re-ceived echoes (in different eyes) and, more important, it is supposed to insure that the strongest echo detected in the range gate inter~al selected will be the first to ex-ceed the detection threshold thereby stopping the counter.
Key assumptions made in the operation of the abo-Ye echo-oculometer device are that (1I when the beam is di-rected along the axis of the eye, the first echo in theselected range gate will be the largest echo, and (2) if the beam is directed off axis, the echoes received will be too weak (due to the inclination of the reflecting interface and the transducer directivity) to exceed the range gate de-tection threshold.
While these assumptions are generally true for theanterior lens echoes (,anterior chamber depth), research and clinical experience conclusively show that this is not al-ways true for the much more important retinal echoes (axial length). In a significant number of cases, other interfaces and structures behind the retina give rise to the largest echoes. This can result in errors in the axial length de-terminations of as much as 3mm which corresponds to an error in the lens power determinations of about 8 or 9 diopters.
An error of this magnitude is altogether unacceptable.
Furthermore, lt must be emphasized that although this pro-blem is much more common for the case of off-axis beam in-cidence, it will still sometimes occur when the beam is pro-1~7~3 perly aligned.
Finally, if readings are taken with the beam im-properly aligned, the accuracy of the axial length determi-nations will be decreased either due to the problem just described or in the case where the retinal echo does stop the counter, the fact~ that a chord shorter than the axial diameter is being measured.
One method of counteracting these problems would bé to increase the beam directivity but this is subject to both theoretical and practical limitations. Another ap-proach might be to lower the upper limit of the swept gain.
However, there are ob~ious constraints since the instrument must accommodate a considerable range of ultrasonic and geometrical characteristic~ for different eyes~
Another method would be to require the presence of the anterior and posterior lens echoes (either one but preferably both) of a magnitude equal or greater than some specified fraction of the retinal echo threshold (typically ~ or more) as a necessary condition for a valid reading.
In fact, the standard A-scan technique (used in determining the axial length) consists of insuring that both the ante-rior and posterior lens echoes are simultaneously present together with the retinal echo, and then maximizing the two lens echoes while maintaining a good clean and large retinal echo. The technique is illustrated in detail by Leary in Ultrasonics April 1967, pp. 84-87, Under normal conditions, the symmetry of the eye is such that following the above procedure will insure good axial alignment.
~lq643 Implementing the abo~e lens echo conditions ln the form of electronic circuits to assure that readings are obtained only under conditions of good alignment is straight forward and is obvious to those skilled in the art. Howe-ver~ while this would greatly reduce the likelihood of in-correct ~riggerlng by structures behind the retina, it does not entirely eliminate the problem. Also the difficulty of achieving exact alignment without reference to an A-Scan can make the actual obtaining of readings very problematic.
lQ Certainly adding the lens echo condition reduces the speed with which valid readings can be obtained. This i5 an im-portant consideration when dealing with older or uncoopera-tive patients.
It is therefore the object of the present inven-tion to provide an apparatus for effecting rapid and accu-rate ultrasonic measurements of the axial length of the eye.
The apparatus, in accordance with the invention, comprises a transducer adapted to transmit repetitive ultra-sonic pulses along the ocular axis of the eye of a patient and receive echo pulses reflected from the retina of the eye, a fixed gain amplifier connected to the transducer for amplifying such reflected echo pulses, an automatic gain controlled amplifier also connected to the transducer for amplifying the reflected echo pulses, control means coupled to the automatic gain controlled amplifier for gradually increasing the gain of the amplifier during a measurement cycle, first and second gate circuits controlled by the output of the fixed and automatic gain controlled amplifier, 11176~3 respectively, and adapted to pass logic signals triggered by retinal echo pulses exceeding first and second predeter-mined thresholds, a digital counter connected to the second gate circuit and adapted to display the axial length of the eye as a function of the distance travelled by the retinal echo pulses, a gate dela~ initiated by a slow clock, a gate width generator connected to the gate delay for generating a time slot during which echo pulses originating from the posterior wall of the eye can be received, a latching cir-cuit responsive to the gate delay for enabling the firstgate circuit to pass logic signals triggered by echo pulses exceeding the first threshold, a retinal echo triggered gate width generator interconnecting the first and second gate circuits and responsive to the first gate circuit for ena-bling the second gate circuit to pass logic signals trigge-red by retinal echo pulses exceeding the second threshold in the time slot generated by the gate width generator, the output of the retinal echo triggered gate width generator being also connected to the latching circuit for blocking the first gate circuit immediately after receipt of the first logic signal triggered by a retinal echo pulse, there-b~ preventing mistriggering of the retinal echo triggered gate width generator by echoes originating from structures behind the retina~
The above control means includes a slow ramp gene-rator adapted to generate a ramp voltage which is applied to the automatic gain controlled ampllfier in such a manner that the gain of the amplifier varies from a minimum at the 76~3 start of the measurement cycle to a preset maximum after a number of cycles~
A pulser is connected to the transducer for ap-plying sharp hlgh voltage splkes to such transducer to shock excite lt so as to direct an ultrasonic pulse into ~ne eye being examined. The pulser is triggered by the slow clock~
A fast clock, operating at a frequency in MHz which is l/2 times the average velocity of ultxasound in the eye expres-sed in units of 0.1 mm per microsecond, provides the coun-ter time base and is used to synchronize the slow clock.
A pulser delay is located between the slow clockand the pulser to insure that the counter starts counting at the correct time~
The invention will now be disclosed, by way of example, with reference to the accompanying drawings in which:
Figure l is a block diagram of an exemplary embo-diment of the invention;
Figure 2 is a series of waveforms produced at va-rious points in the block diagram of Figure l; and Figure 3 is a circuit diagram of the echo-oculome-ter constructed according to the present invention.
Referring to Figures l and 2, there is shown a fast clock 10 which generates a continuous signal at a fre-quency in MHz which is 1/2 times the average velocity of ul-trasound in the eye ~1553 m/sec~ in units of 0~1 mm per mi-crosecond, that is 7.765 MHzt,and has a sinusoidal waveform as illustrated at A tn Figure 2. The clock lO feeds a si-1~76~3 gnal to a counter-display 12 and to a slow clock 14. The slow clock 14 generates a square ~ave at a frequency of about 60 Hz as shown at B in Figure 2 of the drawings. The operation of the slow clock 14 is synchronized to the fast clock 10. The output slgnal of the slow clock is applied to a pulser delay 1~ which generates a repetitive pulse signal such as shown at C in Figure 2~ The pulser delay 16 trig-gers a pulser 18 on the positive going edge of the waveform C and at the same time resets the counter 12. The counter begins counting on the negative going edge of the waveform C to compensate for the propagation time of the ultrasonic pulse from the transducer to the eye and from the eye to the transducer. Pulser 18 generates a sharp high voltage spike, such as shown at D in Figure 2, which is used to shock exci-te a transducer 2Q to direct an ultrasonic pulse through theeye being examined. This pulse travels through the eye and is reflected by the various surfaces and returned to the transducer. These echo pulses are picked up by the transdu-cer and converted back to electrical signals which are di-rected to a preamplifier 22 providing an output such asshown at E in Figure 2. The output of the preamplifier 22 is applied to a fixed gain amplifier 24 providing an ampli-fied output such as shown at F in Figure 2 and to an automa-tic gain control amplifier 26 providing an amplified output such as shown at G, H or I in Figure 2 depending on the gain of the amplifier as controlled by the voltage V applied to its automatic gain control terminal, The output of the fi-xed galn amplifier 24 is fed to a first gate circuit 28 1:~L1764~3 through a comparator 30 which sets a signal threshold level for the output of amplifier 24 as illustrated by a dashed line through waveform F in Figure 2. Similarly, the output of the automatic gain control amplifier 26 is fed to a se-cond gate clrcuit 32 t~rough a comparator 34 which sets asignal threshold level for the output of amplifier 26 as illustrated by a dashed line through waveform I in Figure 2.
The pr~sent apparatus is capable of measuring not only the axial length (AL) of an eye by detecting the reti-nal echo pulses but also the anterior chamber (AC) depth bydetectIng the anterior lens echo pulses. As mentioned pre-viously, the measurement of the anterior lens echoes is not the object of the present invention, therefore the portion of the circuitry which is concerned with the measurement of the retinal lens echo will be primarily disclosed. In or-der to permit gate circuits 28 and 32 to pass logic signals triggered by the echo pulses which are reflected from the desired surfaces of the eye, there is provided a gate delay 36 which is triggered by the positive leading edge of the signal B appearing at the output of the slow clock 14. Gate delay 36 provides an output waveform such as illustrated at L in Figure 2, and triggers the gate width generator 38 on the positive edge of the waveform L. If the anterior cham-ber depth was to be measured, the delay would of course be much shorter so as to allow gating of the echo pulses origi-nating from the anterior lens surface of the eye. A func-tion switch 37 is- provided for selecting which one of the measurements is to be performed by the apparatus. The gate ~'7ti~3 width generator 38 generates a signal M as shown in Figure
2. Signal M is fed to gate 32 and controls the time slo.
during which gate 32 is opened. Gate width generator 38 is also responsive to function switch 37 for selection of the desired measurement to be performed. The output L of gate delay 36 is also applied to a spike generator 40 which gene-rates a signal shown at N in Figure 2. Signal N is fed to a latching circuit 42 which generates a signal P, as shown in Figure 2, for controlling the opening of gate circuit 28.
Latching circuit 42 is reset by the output C of the pulser delay at the beginning of each cycle.
A retinal echo triggered gate width generator 44 is connected between gates 28 and 32 and is triggered by output signal Q of gate circuit 28 when a retinal echo si-gnal exceeding the threshold of comparator 30 is present.The retinal echo triggered gate width generator 44 provides an output R, as shown in ~igure 2, which is applied to the gate circuit 32 to permit the gate to pass logic signals triggered by echo signals originating from automatic gain control amplifier 26 exceeding the threshold of comparator 34. The output R is also fed to the latching circuit 42 to cause the latching circuit to immediately disable gate circuit 28 after receipt of the first logic signal triggered by an echo signal originating from the fixed gain amplifier 24. Thus, gate circuit 28 is latched out immediately after the retinal echo pulses are detected to prevent retriggering of 44 thereby pre~enting gate 32 from passing logic signals triggered by echo pulses originating from structures behind 1~76~3 the retina~ The retinal echo triggered gate width genera-tor 44 is dlsabled by the function switch 37 during anterior chamber measurement because it is not required.
The output T of gate 32 is fed to a display dura-S tion circuit 46 whic~ provides an output U to stop the coun-ter and display,for a few seconds, the distance travelled by the retinal echo pulse as an indication of the axial length of the eye.
The gain of the fixed gain amplifier 24 is set by an amount approximately 10 to 14 dB greater (3 to 5 times greater) than the maximum gain of the automatic gain control amplifier 26, This insures that the retinal echo signal will be of su~ficient amplitude to exceed the threshold of the comparator 30 and that the retinal echo triggered gate width generator 44 will not be mistriggered by an echo pulse of greater amplitude originating from structures behind the retina.
The gain of the automatic gain control amplifier 26 is varied by a slow ramp generator 48 which generates a voltage of increasing negative amplitude V starting from a minimum value at the beginning of the measurement up to a maxim~m value set by a comparator 50. The output U of the display duration circuit 46 is applied to the slow clock to enable the same, and to the slow ramp generator 48 to reset the slow ramp generator voltage V to its mlnimum value, when the automatic gain control amplifier has sufficient gain to pass the signals exceeding the threshold set by comparator 34~
ill76~3 The invention will now be disclosed with reference to the more detailed circuit diagram of Figure 3 which is intended to give a better understanding of the invention but not to limit t~e scope thereof. The non-detailed blocks as well as the circuit diagrams outlined in Figure 3 by broken lines carry the same references as the corresponding blocks of Figure l.
The fast clock 10 is a conventional crystal con-trolled oscillator operating at a frequency of 7.765 MHz as mentioned previously. The output A of the fast clock is fed to the counter-display 12 which is a conventional digital counter capable of displaying a count when energized to do so. A suitable example of such a counter is RCA No.
ICAN-6733. The counter-display is therefore operated by the fast clock to indicate directly the axial length of the eye.
For synchronizing of the clocks, the output A of the fast clock is also applied to the clock input C of a conventional type D flip~flop 60 which acts as a slow clock. The timing period of the slow clock is about 60 Hz as mentioned pre-viously and is determined by a resistor Rl connected between terminals D and Q of the flip-flop and capacitor Cl connec-ted between terminal D and ground. Flip-flop 60 may be ena-bled by clamping terminal D through diode Dl as it will be disclosed later.
The output of the slow clock is applied to pulser delay 16 through coupling capacitor C2 and resistor R2. The pulser delay is comprised of a CMOS NOR gate 62 and an in-~rerter 64. Gate 62 has a first input connected to the slow 1~76~3 clock and a second input connected to the output of inver-ter 64~ The output of gate 62 is connected to the input of the inverter 64 through capacitor C3. A positive potential V+ is also applied to t~e input of the inverter through re-sistors R3 and R4~ Gate 62 and inverter 64 form a wellknown monostable circuit. The output of the pulser delay i5 as shown at C in Figure 2 of the drawings.
The pulser 18, which is energized from a conven-tional high voltage source 66, is triggered on the rising edge of output C of the pulser delay and produces a sharp high voltage spike D which is used to shock excite the transducer 20 to direct an ul~rasonic pulse into the eye of the person being examined. This pulse travels through the eye and is reflected by various surfaces of the eye, as men-tioned previously, and returns to the transducer. The echopulses are detected by the transducer and converted back to electric signals which are fed to preamplifier 22. The out-put of preamplifier 22 is applied to a fixed hi-gain ampli-fier 24 and to an automatic gain control amplifier 26. The above mentioned circuit elements 18, 22, 24 and 26 are con-ventional and need not be disclosed in detail.
The output F of fixed hi~gain amplifier 24 is full-wave rectified by diodes D2 and D3 and clamped to the voltage level determined by the voltage d vider resistors R5, R6 connected across a source V ; The clamped signal ~Gate trig2 is fed to one of the inputs of a two input NOR
gate 68. T~e echo signal detection threshold voltage is equal to the difference between the NOR gate logic threshold 1~7643 and the above clamping voltage. In this particular embodi-ment, therefore, the resistor network R5 and R6 and the lo-gic threshold (approximately ~V+ for CMOS logic) constitute essentially the equivalent of the comparator 30 while the S NOR gate 68 constitutes the gatlng circuit 28.
Resistors R7 and R8 and capacitor C4 provide lo-pass flltering for the echo signals which improves the de-tection performance. While not essential, full-wave recti-fication of the signal simplifies lo pass flltering of the signal (in order to improve the signal to noise).
In a manner similar to the preceeding, the output (G, H, I) of the automatic gain control amplifier is clam-ped to the voltage level determined by the voltage divider, resistors R9 and R10, connected across the V+ source. ~he clamped signal (stop) is fed to one lnput of the NOR gate circuit 70. R9 and Rl0 and the NOR gate logic threshold thus form the equivalent of the comparator 34, and the three input NOR gate 70 corresponds to the gating circui~ 32.
The gain of the amplifier 26 is Yariable and de-pends on the "sweep" voltage applied to its AGC terminal asit will be disclosed later.
A diode D4 is connected across the resistor R9 to protect the CMOS against overvoltage.
The output B of the slow clock 14 is also applied to the gate delay 36. Gate delay 36 comprises a C~OS ~OR
gate 72 and an inverter 74 which are interconnected in th~
same manner as in pulser delay 16 to form a monostable.
Gate 72 has a first input connected to the slow clock 14 and a second input connected to the output of invertex 74.
The output of gate 72 is connected to the input of the in-verter 74 through a capacitor C5. A posltive potential V+
is also applied to the input of the inverter through resis-tors Rll-R14. The time constant of the monostable is con-trolled by resistors Rll-R14 and capacitor C5. The output L of the monostable is as shown in Figure 2 of the draw-ings. Gate delay 36 may also be used for anterior chamber -- measurement and, in such a case, the time constant of the lQ R-C circuit may be changed by clamping the connecting point of resistors R12 and R13 to the voltage source V'+ through a diode D5. The clamping action is performed by operating a switch in function switch 37 (Figure 1). As mentioned pre-viously, the purpose of the gate delay 36 is to delay the operation of the gate width generator 38 which sets the time slot during which the desired echo is to be detected.
The output L of the gate delay 36 is applied, through a capacitor C6, to the gate width generator 38 which comprises a CMOS NOR gate 76 and an inverter 78. The gate width generator is a monostable of the same type as the one of the pulser delay 16 and the gate delay 36. The first in-put of the gate 76 is connected to output L of the gate de-lay and its second input is connected to the output of in-verter 78. The output of gate 76 is connected to the input of inverter 78 through a capacitor C7. A positive potential source V~ is also connected to the inpu~ of inverter 78 through resistors R16-Rl9. The time constant of the mono-stable is set by resistors R16-Rl9 and capacitor C7. The output M of the monostable is as shown in Figure 2 of the drawings~
The output M of the gate width monostable 38 de-termines the length of time during which the echo returned from the posterior part of the eye will be detected. Such output M is applied to the gate circuit 32 as it will be disclosed later. The gate width generator 38 may also be used for anterior chamber measurement and, in such a case, the time constant of the R-C circuit will be chanyed by clamping the connecting point of resistors R17 and R18 to V'~ through a diode D6. This is performed by function switch 37.
The output L of the gate delay 36 is also applied to a latching circuit 42 through a spike generator 40 formed by resistor R15 and capacitor C6. In the embodiment disclo-sed, latching circuit 42 is a conventional type D flip-flop 80~ The output N of the spike generator is applied to the reset terminal of flip-flop 80 to reset the output of the flip-flop to low at the beginning of the time slot during which theechoes returned from theposterior part of the eye are to be detected r as indicated by waveform P in Figure 2 of the drawings.
The output P of the latching circuit is applied to the CMOS NOR gate 68 to enable the gate. When no echo signal ~amplified by fixed gain amplifier 24) exceeds the threshold set by comparator 30r the output Q of gate 68 remains low.
CMOS NOR gates 68 and 70 are interconnected by a retinal echo triggered (R.E.T.l gate width generator 44 com-~1176~;~
prising a CMOS NOR gate 82 and an inver~er 84 which operateas a monostable~ Gate 82 has a first input connected to ground through resistor R20, a second input connected to the output Q of gate 68 and a third input connected to the out-put of in~erter 84~ The output of gate 82 is connected tothe input of inverter 88 through capacitor C8. The lnput of the inverter 84 is also connected to a positive potential source Vl through resistor R21. The inverter is protected against overvoltage by diode D7. The input of the inverter is normally high as it is connected to V~ and its output low, so that when the output Q of gate 68 is low (no echo signal exceeding the threshold of comparator 30), the output R of gate 82 is high. However, when an echo signal excee-ding the threshold level is received, the output Q of gate 68 turns high and the outpu~ R of gate 82 turns low. The output R o~ gate 82 is applied to terminal C (clear) of flip-flop 80 to turn output Q of the flip-flop high to imme-diately block gate 68 and so prevent the R.E.T. gate width generator from being retriggered after it has been triggered 2Q by a retinal echo signal. Thus, output Q is only a narrow spike such as shown in Figure 2 of the drawings. After ap-proximately one microsecond as set by resistor R21 and capa-citor C8, the output of inverter 84 returns to low and the output of gate 82 to high thereby blocking gate 70. As a result, any subsequent echos which exceed the threshold of comparator 34 are prevented from trig~ering gate 3~ If no retinal echo s~gnal exceeding the threshold level of the comparator 30 is detected, flip-flop 80 is set ~Q = 1) by the output C of pulser delay 16 at the beginning of the next cycle to block gate 68 and so prevent the latching circuit from being activated by echo signals detected in the gate delay interval which would disable the R.E~T. gate width generator prematurely. The R ~ E ~ T ~ gate width generator may also be disabled by applying a positive voltage ~'+ to the first input of gate 82. This may be done by a switch of function switch 37 when making an anterior chamber measure-ment as the R.E.T. gate width generator is not needed for such an operation.
When a retinal echo pulse is detected, the output of the R.E.~. gate width generator is applied to the first input of the three input NOR gate 70. As long as the reti-nal echo signal amplified by the automatic gain control (AGC) amplifier does not exceed the threshold set by compa-rator 34, the clamped output signal stop applied to the se-cond input of gate 70 is logical~ ~ig~ and the output T o~
ga~a 7Q is low~
As mentioned previously, the gain of the AGC am-2Q plifier is controlled by a slow ramp generator 48 and a com-parator 50. The slow ramp generator i5 a conventional Mil-ler integrator comprising a resistor-capacitor charging net-work consisting of resistor R23 connected to the inverting terminal of an operational amplifier 86 and a capacitor C10 connected between the inverting terminal and the output ter-minal of the operational amplifier. The output "Sweep" of the operational amplifier provides a linear time~base volta-ge varying from a minimum voltage of say 5V to a maximum vol-1~76~3 tage of say 12V under the control of comparator sa~ Thecomparator 50 comprises an operational amplifier 88 having its inverting terminal connected to source V+ through resis-tors R24 and R25 and its inverting terminal connected to the junction of resistors R26 and R27 which are connected in se-ries with a diode D10 between the "sweep" output of the ope-rational amplifier 86 and ground. A resistor R28 is connec-ted between the non-inverting terminal and the output termi-nal of the operational amplifier 88 for controlling the gain lQ thereof in known manner. The output of operational ampli-fier 88 is connected to the non-inverting terminal of the operational amplifier 86 through a coupling resistor R29.
The "sweep" output of the ramp generator is shown at V in Figure 2 of the drawings but not on the same time scale as the other waveforms. The time lapse between tl and t2 is about 0.7 sec and between tS and t6 about 2 sec. The ampli-tudes of the echo signals at times t3, t4 and t5 are shown at G, H and I in Figure 2 of the drawings. At the beginning of the measurement cycle, the gain of the AGC amplifier 26 is low but it gradually increases until at time t5 the reti-nal echo exceeds the threshold set by comparator 34. At such time, the "Stop" input of gate 70 turns low and, if the other two inputs of gate 70 are also low (in the time slots generated by the gate width generator 38 and the R~E~T. gate width generator 44~, the output T of gate 70 turns high.
The output T of gate 70 is applied to a display duration monostable circuit comprising a CMOS NOR gate 90 and inverter 92. ~he first input of gate 90 i5 connected 11~76~3 to the output of gate 70 and its second input is connected to the output of the in~erter 92. The output of the gate 90 is connected to the input of inverter 92 through a capa-citor Cll. A positi~e potential source v~ is also connected to the input of inverter 92 through resistor R30. A protec-tive diode Dll is connected across resistor R30 to protect the inverter agai~st overvoltage. The output of the mono-stable is shown at U in Figure 2 of the drawings. The out-put signal U is applied to the "Display enable" terminal of the coun~er and display 12 to stop the counter and show the axial length of the eye for a period of time determined by the time constant of resistor R30 and capacitor Cll~ The output U of the inverter 92 is applied to an inverter 94 which produces the output "Display enable" which is applied to the slow clock 14 to enable the slow clock. The output "Display enable" is also applied to the inverting terminal of operational amplifier 86 through diode D12 to reset the gain sweep ramp voltage generator 48.
It will be seen from the above description that the counter 12 will be stopped and the display turned on on-ly if (1) the output of the automatic gain amplifier 26 ex-ceeds the threshold set by the comparator 34 and if (2) this occurs during the short time interval that the gate circuit 32 ~or CMOS NOR gate 70) is enabled by the R.E.~. gate width generator 44 as indicated by waveform R in Figure 2 of the drawings. If the beam is off-axis, the retinal echo will not have enough amplitude to stop the counter (which is then automatically reset on the next pulse transmitted). As a 11~76~3 result, the echo-oculometer in accordance with the present invention behaves as if the beam directivity was much grea-ter than it actually is. More importantly, mistriggering by echoes from structures behind the retina is eliminated by the latching circuit which prevents the R~E.T. gate width generator from ~eing retriggered by echoes from structures behind the retina after it has been triggered by the retinal echo pulses~ It is also important to note that the signal E
appearing at the output of fixed gain amplifier 24 is ampli-lQ fied by an amount approximately 10 to 14 dB greater than themaximum gain of the AGC amplifier 26 to make sure that the first echo signal exceeding the threshold of the comparator 30 is positively a retinal echo signal and not an echo si-gnal originating from a structure behind the retina as would happen if the retina echo was of lower amplitude than the threshold set by the comparator 30.
Although the invention has been disclosed with re-ference to a workable embodiment shown in Figure 3 of the drawings, it is to be understood that other detailed circuit diagrams could be used for the blocks of Figure 1 and that the invention is not limited to such detailed circuit dia-grams.
during which gate 32 is opened. Gate width generator 38 is also responsive to function switch 37 for selection of the desired measurement to be performed. The output L of gate delay 36 is also applied to a spike generator 40 which gene-rates a signal shown at N in Figure 2. Signal N is fed to a latching circuit 42 which generates a signal P, as shown in Figure 2, for controlling the opening of gate circuit 28.
Latching circuit 42 is reset by the output C of the pulser delay at the beginning of each cycle.
A retinal echo triggered gate width generator 44 is connected between gates 28 and 32 and is triggered by output signal Q of gate circuit 28 when a retinal echo si-gnal exceeding the threshold of comparator 30 is present.The retinal echo triggered gate width generator 44 provides an output R, as shown in ~igure 2, which is applied to the gate circuit 32 to permit the gate to pass logic signals triggered by echo signals originating from automatic gain control amplifier 26 exceeding the threshold of comparator 34. The output R is also fed to the latching circuit 42 to cause the latching circuit to immediately disable gate circuit 28 after receipt of the first logic signal triggered by an echo signal originating from the fixed gain amplifier 24. Thus, gate circuit 28 is latched out immediately after the retinal echo pulses are detected to prevent retriggering of 44 thereby pre~enting gate 32 from passing logic signals triggered by echo pulses originating from structures behind 1~76~3 the retina~ The retinal echo triggered gate width genera-tor 44 is dlsabled by the function switch 37 during anterior chamber measurement because it is not required.
The output T of gate 32 is fed to a display dura-S tion circuit 46 whic~ provides an output U to stop the coun-ter and display,for a few seconds, the distance travelled by the retinal echo pulse as an indication of the axial length of the eye.
The gain of the fixed gain amplifier 24 is set by an amount approximately 10 to 14 dB greater (3 to 5 times greater) than the maximum gain of the automatic gain control amplifier 26, This insures that the retinal echo signal will be of su~ficient amplitude to exceed the threshold of the comparator 30 and that the retinal echo triggered gate width generator 44 will not be mistriggered by an echo pulse of greater amplitude originating from structures behind the retina.
The gain of the automatic gain control amplifier 26 is varied by a slow ramp generator 48 which generates a voltage of increasing negative amplitude V starting from a minimum value at the beginning of the measurement up to a maxim~m value set by a comparator 50. The output U of the display duration circuit 46 is applied to the slow clock to enable the same, and to the slow ramp generator 48 to reset the slow ramp generator voltage V to its mlnimum value, when the automatic gain control amplifier has sufficient gain to pass the signals exceeding the threshold set by comparator 34~
ill76~3 The invention will now be disclosed with reference to the more detailed circuit diagram of Figure 3 which is intended to give a better understanding of the invention but not to limit t~e scope thereof. The non-detailed blocks as well as the circuit diagrams outlined in Figure 3 by broken lines carry the same references as the corresponding blocks of Figure l.
The fast clock 10 is a conventional crystal con-trolled oscillator operating at a frequency of 7.765 MHz as mentioned previously. The output A of the fast clock is fed to the counter-display 12 which is a conventional digital counter capable of displaying a count when energized to do so. A suitable example of such a counter is RCA No.
ICAN-6733. The counter-display is therefore operated by the fast clock to indicate directly the axial length of the eye.
For synchronizing of the clocks, the output A of the fast clock is also applied to the clock input C of a conventional type D flip~flop 60 which acts as a slow clock. The timing period of the slow clock is about 60 Hz as mentioned pre-viously and is determined by a resistor Rl connected between terminals D and Q of the flip-flop and capacitor Cl connec-ted between terminal D and ground. Flip-flop 60 may be ena-bled by clamping terminal D through diode Dl as it will be disclosed later.
The output of the slow clock is applied to pulser delay 16 through coupling capacitor C2 and resistor R2. The pulser delay is comprised of a CMOS NOR gate 62 and an in-~rerter 64. Gate 62 has a first input connected to the slow 1~76~3 clock and a second input connected to the output of inver-ter 64~ The output of gate 62 is connected to the input of the inverter 64 through capacitor C3. A positive potential V+ is also applied to t~e input of the inverter through re-sistors R3 and R4~ Gate 62 and inverter 64 form a wellknown monostable circuit. The output of the pulser delay i5 as shown at C in Figure 2 of the drawings.
The pulser 18, which is energized from a conven-tional high voltage source 66, is triggered on the rising edge of output C of the pulser delay and produces a sharp high voltage spike D which is used to shock excite the transducer 20 to direct an ul~rasonic pulse into the eye of the person being examined. This pulse travels through the eye and is reflected by various surfaces of the eye, as men-tioned previously, and returns to the transducer. The echopulses are detected by the transducer and converted back to electric signals which are fed to preamplifier 22. The out-put of preamplifier 22 is applied to a fixed hi-gain ampli-fier 24 and to an automatic gain control amplifier 26. The above mentioned circuit elements 18, 22, 24 and 26 are con-ventional and need not be disclosed in detail.
The output F of fixed hi~gain amplifier 24 is full-wave rectified by diodes D2 and D3 and clamped to the voltage level determined by the voltage d vider resistors R5, R6 connected across a source V ; The clamped signal ~Gate trig2 is fed to one of the inputs of a two input NOR
gate 68. T~e echo signal detection threshold voltage is equal to the difference between the NOR gate logic threshold 1~7643 and the above clamping voltage. In this particular embodi-ment, therefore, the resistor network R5 and R6 and the lo-gic threshold (approximately ~V+ for CMOS logic) constitute essentially the equivalent of the comparator 30 while the S NOR gate 68 constitutes the gatlng circuit 28.
Resistors R7 and R8 and capacitor C4 provide lo-pass flltering for the echo signals which improves the de-tection performance. While not essential, full-wave recti-fication of the signal simplifies lo pass flltering of the signal (in order to improve the signal to noise).
In a manner similar to the preceeding, the output (G, H, I) of the automatic gain control amplifier is clam-ped to the voltage level determined by the voltage divider, resistors R9 and R10, connected across the V+ source. ~he clamped signal (stop) is fed to one lnput of the NOR gate circuit 70. R9 and Rl0 and the NOR gate logic threshold thus form the equivalent of the comparator 34, and the three input NOR gate 70 corresponds to the gating circui~ 32.
The gain of the amplifier 26 is Yariable and de-pends on the "sweep" voltage applied to its AGC terminal asit will be disclosed later.
A diode D4 is connected across the resistor R9 to protect the CMOS against overvoltage.
The output B of the slow clock 14 is also applied to the gate delay 36. Gate delay 36 comprises a C~OS ~OR
gate 72 and an inverter 74 which are interconnected in th~
same manner as in pulser delay 16 to form a monostable.
Gate 72 has a first input connected to the slow clock 14 and a second input connected to the output of invertex 74.
The output of gate 72 is connected to the input of the in-verter 74 through a capacitor C5. A posltive potential V+
is also applied to the input of the inverter through resis-tors Rll-R14. The time constant of the monostable is con-trolled by resistors Rll-R14 and capacitor C5. The output L of the monostable is as shown in Figure 2 of the draw-ings. Gate delay 36 may also be used for anterior chamber -- measurement and, in such a case, the time constant of the lQ R-C circuit may be changed by clamping the connecting point of resistors R12 and R13 to the voltage source V'+ through a diode D5. The clamping action is performed by operating a switch in function switch 37 (Figure 1). As mentioned pre-viously, the purpose of the gate delay 36 is to delay the operation of the gate width generator 38 which sets the time slot during which the desired echo is to be detected.
The output L of the gate delay 36 is applied, through a capacitor C6, to the gate width generator 38 which comprises a CMOS NOR gate 76 and an inverter 78. The gate width generator is a monostable of the same type as the one of the pulser delay 16 and the gate delay 36. The first in-put of the gate 76 is connected to output L of the gate de-lay and its second input is connected to the output of in-verter 78. The output of gate 76 is connected to the input of inverter 78 through a capacitor C7. A positive potential source V~ is also connected to the inpu~ of inverter 78 through resistors R16-Rl9. The time constant of the mono-stable is set by resistors R16-Rl9 and capacitor C7. The output M of the monostable is as shown in Figure 2 of the drawings~
The output M of the gate width monostable 38 de-termines the length of time during which the echo returned from the posterior part of the eye will be detected. Such output M is applied to the gate circuit 32 as it will be disclosed later. The gate width generator 38 may also be used for anterior chamber measurement and, in such a case, the time constant of the R-C circuit will be chanyed by clamping the connecting point of resistors R17 and R18 to V'~ through a diode D6. This is performed by function switch 37.
The output L of the gate delay 36 is also applied to a latching circuit 42 through a spike generator 40 formed by resistor R15 and capacitor C6. In the embodiment disclo-sed, latching circuit 42 is a conventional type D flip-flop 80~ The output N of the spike generator is applied to the reset terminal of flip-flop 80 to reset the output of the flip-flop to low at the beginning of the time slot during which theechoes returned from theposterior part of the eye are to be detected r as indicated by waveform P in Figure 2 of the drawings.
The output P of the latching circuit is applied to the CMOS NOR gate 68 to enable the gate. When no echo signal ~amplified by fixed gain amplifier 24) exceeds the threshold set by comparator 30r the output Q of gate 68 remains low.
CMOS NOR gates 68 and 70 are interconnected by a retinal echo triggered (R.E.T.l gate width generator 44 com-~1176~;~
prising a CMOS NOR gate 82 and an inver~er 84 which operateas a monostable~ Gate 82 has a first input connected to ground through resistor R20, a second input connected to the output Q of gate 68 and a third input connected to the out-put of in~erter 84~ The output of gate 82 is connected tothe input of inverter 88 through capacitor C8. The lnput of the inverter 84 is also connected to a positive potential source Vl through resistor R21. The inverter is protected against overvoltage by diode D7. The input of the inverter is normally high as it is connected to V~ and its output low, so that when the output Q of gate 68 is low (no echo signal exceeding the threshold of comparator 30), the output R of gate 82 is high. However, when an echo signal excee-ding the threshold level is received, the output Q of gate 68 turns high and the outpu~ R of gate 82 turns low. The output R o~ gate 82 is applied to terminal C (clear) of flip-flop 80 to turn output Q of the flip-flop high to imme-diately block gate 68 and so prevent the R.E.T. gate width generator from being retriggered after it has been triggered 2Q by a retinal echo signal. Thus, output Q is only a narrow spike such as shown in Figure 2 of the drawings. After ap-proximately one microsecond as set by resistor R21 and capa-citor C8, the output of inverter 84 returns to low and the output of gate 82 to high thereby blocking gate 70. As a result, any subsequent echos which exceed the threshold of comparator 34 are prevented from trig~ering gate 3~ If no retinal echo s~gnal exceeding the threshold level of the comparator 30 is detected, flip-flop 80 is set ~Q = 1) by the output C of pulser delay 16 at the beginning of the next cycle to block gate 68 and so prevent the latching circuit from being activated by echo signals detected in the gate delay interval which would disable the R.E~T. gate width generator prematurely. The R ~ E ~ T ~ gate width generator may also be disabled by applying a positive voltage ~'+ to the first input of gate 82. This may be done by a switch of function switch 37 when making an anterior chamber measure-ment as the R.E.T. gate width generator is not needed for such an operation.
When a retinal echo pulse is detected, the output of the R.E.~. gate width generator is applied to the first input of the three input NOR gate 70. As long as the reti-nal echo signal amplified by the automatic gain control (AGC) amplifier does not exceed the threshold set by compa-rator 34, the clamped output signal stop applied to the se-cond input of gate 70 is logical~ ~ig~ and the output T o~
ga~a 7Q is low~
As mentioned previously, the gain of the AGC am-2Q plifier is controlled by a slow ramp generator 48 and a com-parator 50. The slow ramp generator i5 a conventional Mil-ler integrator comprising a resistor-capacitor charging net-work consisting of resistor R23 connected to the inverting terminal of an operational amplifier 86 and a capacitor C10 connected between the inverting terminal and the output ter-minal of the operational amplifier. The output "Sweep" of the operational amplifier provides a linear time~base volta-ge varying from a minimum voltage of say 5V to a maximum vol-1~76~3 tage of say 12V under the control of comparator sa~ Thecomparator 50 comprises an operational amplifier 88 having its inverting terminal connected to source V+ through resis-tors R24 and R25 and its inverting terminal connected to the junction of resistors R26 and R27 which are connected in se-ries with a diode D10 between the "sweep" output of the ope-rational amplifier 86 and ground. A resistor R28 is connec-ted between the non-inverting terminal and the output termi-nal of the operational amplifier 88 for controlling the gain lQ thereof in known manner. The output of operational ampli-fier 88 is connected to the non-inverting terminal of the operational amplifier 86 through a coupling resistor R29.
The "sweep" output of the ramp generator is shown at V in Figure 2 of the drawings but not on the same time scale as the other waveforms. The time lapse between tl and t2 is about 0.7 sec and between tS and t6 about 2 sec. The ampli-tudes of the echo signals at times t3, t4 and t5 are shown at G, H and I in Figure 2 of the drawings. At the beginning of the measurement cycle, the gain of the AGC amplifier 26 is low but it gradually increases until at time t5 the reti-nal echo exceeds the threshold set by comparator 34. At such time, the "Stop" input of gate 70 turns low and, if the other two inputs of gate 70 are also low (in the time slots generated by the gate width generator 38 and the R~E~T. gate width generator 44~, the output T of gate 70 turns high.
The output T of gate 70 is applied to a display duration monostable circuit comprising a CMOS NOR gate 90 and inverter 92. ~he first input of gate 90 i5 connected 11~76~3 to the output of gate 70 and its second input is connected to the output of the in~erter 92. The output of the gate 90 is connected to the input of inverter 92 through a capa-citor Cll. A positi~e potential source v~ is also connected to the input of inverter 92 through resistor R30. A protec-tive diode Dll is connected across resistor R30 to protect the inverter agai~st overvoltage. The output of the mono-stable is shown at U in Figure 2 of the drawings. The out-put signal U is applied to the "Display enable" terminal of the coun~er and display 12 to stop the counter and show the axial length of the eye for a period of time determined by the time constant of resistor R30 and capacitor Cll~ The output U of the inverter 92 is applied to an inverter 94 which produces the output "Display enable" which is applied to the slow clock 14 to enable the slow clock. The output "Display enable" is also applied to the inverting terminal of operational amplifier 86 through diode D12 to reset the gain sweep ramp voltage generator 48.
It will be seen from the above description that the counter 12 will be stopped and the display turned on on-ly if (1) the output of the automatic gain amplifier 26 ex-ceeds the threshold set by the comparator 34 and if (2) this occurs during the short time interval that the gate circuit 32 ~or CMOS NOR gate 70) is enabled by the R.E.~. gate width generator 44 as indicated by waveform R in Figure 2 of the drawings. If the beam is off-axis, the retinal echo will not have enough amplitude to stop the counter (which is then automatically reset on the next pulse transmitted). As a 11~76~3 result, the echo-oculometer in accordance with the present invention behaves as if the beam directivity was much grea-ter than it actually is. More importantly, mistriggering by echoes from structures behind the retina is eliminated by the latching circuit which prevents the R~E.T. gate width generator from ~eing retriggered by echoes from structures behind the retina after it has been triggered by the retinal echo pulses~ It is also important to note that the signal E
appearing at the output of fixed gain amplifier 24 is ampli-lQ fied by an amount approximately 10 to 14 dB greater than themaximum gain of the AGC amplifier 26 to make sure that the first echo signal exceeding the threshold of the comparator 30 is positively a retinal echo signal and not an echo si-gnal originating from a structure behind the retina as would happen if the retina echo was of lower amplitude than the threshold set by the comparator 30.
Although the invention has been disclosed with re-ference to a workable embodiment shown in Figure 3 of the drawings, it is to be understood that other detailed circuit diagrams could be used for the blocks of Figure 1 and that the invention is not limited to such detailed circuit dia-grams.
Claims (8)
1. An apparatus for measuring the axial length of an eye comprising:
a) a transducer adapted to transmit repetitive ultrasonic pulses along the ocular axis of the eye of a pa-tient and to receive echo pulses reflected from the retina of the eye;
b) a fixed gain amplifier connected to said transducer for amplifying said reflected echo pulses;
c) an automatic gain controlled amplifier also connected to said transducer for amplifying said reflected echo pulses;
d) control means coupled to said automatic gain controlled amplifier for gradually increasing the gain of the amplifier during a measurement cycle;
e) first and second gate circuits coupled to the output of the fixed and automatic gain controlled amplifier, respectively, and adapted to pass logic signals triggered by retinal echo pulses exceeding first and second predeter-mined thresholds;
f) a digital counter connected to the second gate circuit and adapted to display the axial length of the eye as a function of the distance travelled by the retinal echo pulses;
g) a slow clock;
h) a gate delay initiated by said slow clock;
i) a gate width generator connected to said gate delay for generating a time slot during which echo pulses originating from the posterior wall of the eye can be recei-ved;
j) a latching circuit responsive to said gate de-lay for enabling said first gate circuit to pass logic si-gnals triggered by echo pulses from the posterior wall of the eye exceeding said first threshold;
k) a retinal echo triggered gate width generator interconnecting said first and second gate circuits and res-ponsive to said first gate circuit for enabling said second gate circuit to pass logic signals triggered by retinal echo pulses exceeding said second threshold in the time slot ge-nerated by said gate width generator, the output of said re-tinal echo triggered gate width generator being also connec-ted to the latching circuit for blocking the first gate cir-cuit immediatly after receipt of the first logic signal triggered by a retinal echo pulse, thereby presenting mis-triggering of the retinal echo triggered gate width genera-tor by echoes originating from structures behind the retina.
a) a transducer adapted to transmit repetitive ultrasonic pulses along the ocular axis of the eye of a pa-tient and to receive echo pulses reflected from the retina of the eye;
b) a fixed gain amplifier connected to said transducer for amplifying said reflected echo pulses;
c) an automatic gain controlled amplifier also connected to said transducer for amplifying said reflected echo pulses;
d) control means coupled to said automatic gain controlled amplifier for gradually increasing the gain of the amplifier during a measurement cycle;
e) first and second gate circuits coupled to the output of the fixed and automatic gain controlled amplifier, respectively, and adapted to pass logic signals triggered by retinal echo pulses exceeding first and second predeter-mined thresholds;
f) a digital counter connected to the second gate circuit and adapted to display the axial length of the eye as a function of the distance travelled by the retinal echo pulses;
g) a slow clock;
h) a gate delay initiated by said slow clock;
i) a gate width generator connected to said gate delay for generating a time slot during which echo pulses originating from the posterior wall of the eye can be recei-ved;
j) a latching circuit responsive to said gate de-lay for enabling said first gate circuit to pass logic si-gnals triggered by echo pulses from the posterior wall of the eye exceeding said first threshold;
k) a retinal echo triggered gate width generator interconnecting said first and second gate circuits and res-ponsive to said first gate circuit for enabling said second gate circuit to pass logic signals triggered by retinal echo pulses exceeding said second threshold in the time slot ge-nerated by said gate width generator, the output of said re-tinal echo triggered gate width generator being also connec-ted to the latching circuit for blocking the first gate cir-cuit immediatly after receipt of the first logic signal triggered by a retinal echo pulse, thereby presenting mis-triggering of the retinal echo triggered gate width genera-tor by echoes originating from structures behind the retina.
2. An echo-oculometer as defined in claim 1, wherein said control means includes a slow ramp generator adapted to generate a ramp voltage which is applied to the variable gain amplifier in such a manner that the gain of the ampli-fier varies from a minimum at the start of the measurement cycle to a preset maximum gain at the end of the measurement cycle.
3. An echo-oculometer as defined in claim 1, further comprising a fast clock adapted to generate a signal of a frequency which is proportional to the average velocity of ultrasound in the eye and provides a time base for the coun-ter.
4. An echo-oculometer as defined in claim 3, further comprising a pulser responsive to said slow clock and con-nected to said transducer for applying sharp high voltage spikes to said transducer to shock excite the transducer to produce said ultrasonic pulses, and wherein the slow clock is responsive to said fast clock for controlling the rate of said repetitive ultrasonic pulses.
5. An echo-oculometer as defined in claim 4, further comprising a pulser delay interconnecting said slow clock to said pulser for controlling the time interval between the time the ultrasonic pulses are generated and the time the counter starts counting.
6. An echo-oculometer as defined in claim 2, wherein the gain of said fixed gain amplifier is approximately 10 to 14 dB greater than the maximum gain of the automatic gain controlled amplifier to make sure that any retinal echo signal will exceed the first threshold thereby preven-ting mistriggering of the retinal echo triggered gate width generator circuit by echoes originating from structures be-hind the retina.
7. An echo-oculometer as defined in claim 1, further comprising a display duration circuit interconnecting said second gate circuit and said digital counter and display for allowing display of the axial length of the eye for a predetermined interval.
8. An echo-oculometer as defined in claim 1, further comprising comparators interconnecting said fixed gain and automatic gain controlled amplifiers to said first and se-cond gate circuits, respectively, for setting up said first and second thresholds.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000338432A CA1117643A (en) | 1979-10-25 | 1979-10-25 | Apparatus for measuring the axial length of an eye |
| JP3255980A JPS5663332A (en) | 1979-10-25 | 1980-03-14 | Measuring device for axial length of eye |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000338432A CA1117643A (en) | 1979-10-25 | 1979-10-25 | Apparatus for measuring the axial length of an eye |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1117643A true CA1117643A (en) | 1982-02-02 |
Family
ID=4115432
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000338432A Expired CA1117643A (en) | 1979-10-25 | 1979-10-25 | Apparatus for measuring the axial length of an eye |
Country Status (2)
| Country | Link |
|---|---|
| JP (1) | JPS5663332A (en) |
| CA (1) | CA1117643A (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4564018A (en) * | 1982-10-28 | 1986-01-14 | Storz Instrument Company | Ultrasonic system for obtaining ocular measurements |
| CN101799538A (en) * | 2010-03-01 | 2010-08-11 | 李文强 | Digitalized ultrasonic receiving conditioning circuit |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5349889A (en) * | 1976-10-16 | 1978-05-06 | Gen Corp | Supersonic eye axis length measuring apparatus |
-
1979
- 1979-10-25 CA CA000338432A patent/CA1117643A/en not_active Expired
-
1980
- 1980-03-14 JP JP3255980A patent/JPS5663332A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| JPS5663332A (en) | 1981-05-29 |
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