JP2001061798A - Fundus blood flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable measurement with high accuracy by calculating a blood flow velocity by fitting the result of a frequency analysis of a photodetecting output signal when the blood flow velocity is calculated by receiving the scattered light generated from the particles in a blood vessel by measuring light with a photodetecting means and analyzing the photodetecting output signal by a data processing means. SOLUTION: In the case of the measurement of the blood flow velocity of the fundus oculi Ea, a focusing knob of an operation part 33 is first regulated to cooperatively move a transmission type liquid crystal plate 6, focusing lenses 10 and 16 and a focusing unit 18 by a drive means, by which the focusing of the fundus oculi image is executed. The measurement is thereafter started and during the measurement, a measuring beam is held on the blood vessel by the action of a tracking control section 31 and the scattered and reflected light thereof is reflected by a dichroic mirror 26 and is received through a diaphragm 29 by a photomultiplier 30. The photodetection output signal of this photomultiplier 30 is subjected to a frequency analysis and the resulted FFT wavelength is analyzed in the data processing section 34. The blood flow velocity of the fundus oculi Ea is then calculated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a fundus blood flow meter for measuring a blood flow velocity in a blood vessel on the fundus utilizing the Doppler phenomenon.

[0002]

2. Description of the Related Art Conventionally, in a Doppler type fundus blood flow meter, as shown in, for example, U.S. Pat. No. 5,106,184, a blood vessel to be measured is irradiated with measurement light, and particles such as red blood cells flowing in the blood vessel. An optical beat signal obtained by mixing the Doppler-shifted scattered signal light and the non-Doppler-shifted scattered reference light from the blood vessel wall and surrounding tissue is received by two light receivers from two directions, and FFT is performed.
(Fast Fourier Transform) Analyzes waveforms. At this time, assuming that the flow of the blood flow in the blood vessel is a Poiseuille flow as shown in FIG. 8, the cutoff which is the maximum Doppler shift amount corresponding to the maximum flow velocity at the center of the blood vessel as shown in FIGS. The off frequencies Δfmax1 and Δfmax2 are obtained, and the maximum blood flow velocity is obtained from these values.

[0003]

However, in the above-mentioned conventional example, since two light receivers are used,
The area of the light receiving pupil for one light receiver becomes smaller, the amount of light beat signal entering the light receiver becomes smaller, the S / N ratio of the output signal of the light receiver deteriorates, and the measurement accuracy decreases. Further, since two expensive photomultipliers are used as the light receiver, there is a problem that the price rises.

An object of the present invention is to solve the above-mentioned problems,
An object of the present invention is to provide an inexpensive fundus blood flow meter by using one light receiver and improving the SN ratio of an output signal thereof to obtain high measurement accuracy.

[0005]

According to the present invention, there is provided a fundus blood flow meter according to the present invention for irradiating a blood vessel on a fundus of a subject's eye with a measuring light, At least one light receiving means for receiving scattered light generated from the internal particles, a light receiving pupil forming member for forming a light receiving pupil of the light receiving means, and a light receiving output signal from the light receiving means is analyzed to calculate a blood flow velocity. A fundus blood flow meter having data processing means and information storage means for storing information on the shape of the light receiving pupil or information on a theoretical power spectrum shape of a light receiving output signal calculated based on the shape of the light receiving pupil; The data processing means analyzes the information stored in the information storage means and fits the frequency analysis result of the received light output signal to the angle and frequency formed by the reference axis of the device and the blood vessel to obtain a blood sample. And calculating the velocity.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the embodiments shown in FIGS. 1 to 7. FIG. 1 shows a configuration diagram of a fundus blood flow meter according to the embodiment, such as a tungsten lamp which emits white light. On the illumination optical path from the observation light source 1 consisting of
Condenser lens 3, for example, a band-pass filter 4 that transmits only light in the yellow range, a ring slit 5 provided at a position substantially conjugate with the pupil of the eye E, and a fixation target display element that is movable along the optical path , A transmission type liquid crystal plate 6, a relay lens 6, a perforated mirror 8, and a band pass mirror 9 that transmits light in the yellow range and almost reflects other light beams are sequentially arranged to form a fundus illumination optical system. ing. In addition,
The ring slit 5 is for separating the fundus illumination light and the fundus observation light in the anterior segment of the eye E to be examined, and its shape and number do not matter as long as it forms a necessary light shielding area.

A fundus observation optical system is provided behind the perforated mirror 8, and a focus lens 10, a relay lens 11, a scale plate 12, and an eyepiece 13 which are movable along an optical path are sequentially arranged. It has reached eye e.

On the optical path in the reflection direction of the band-pass mirror 9, an image rotator 14, a double-side polished galvanometric mirror 15 having a rotation axis perpendicular to the paper surface, and a lower reflecting surface 1 of the galvanometric mirror 15 are arranged.
A second focus lens 16 that is movable along the optical path is disposed in the reflection direction of 5a, a lens 17 is disposed in the reflection direction of the upper reflection surface 15b, and a focus unit 18 that is movable along the optical path is disposed. I have.

Note that the front focal plane of the lens 17 is
A galvanometric mirror 15 having an asymmetric shape on the pupil is arranged on this focal plane. In addition, the galvanometric mirror 15
A concave mirror 19 is arranged concentrically on the optical axis behind the upper surface, and the upper reflecting surface 15b of the galvanometric mirror 15 is provided.
The relay optics forms an image of the upper reflecting surface 15b and the lower reflecting surface 15a of the galvanometric mirror 15 by -1 times so that the laser beam reflected by the mirror passes through the notch of the galvanometric mirror 15. The system is configured.

In the focus unit 18, a dichroic mirror 20, a condensing lens 21, and a measuring light source 22 such as a laser diode are sequentially arranged on the same optical path as the lens 17, and on the optical path in the reflection direction of the dichroic mirror 20. A mask 23 and a mirror 24 are arranged, and the focus unit 18 surrounded by the dotted line can be integrally moved in the arrow direction. Further, the mirror 24
A tracking light source 25 that emits, for example, green light having a different brightness from other light sources is arranged on the optical path in the incident direction of.

On the optical path in the direction of reflection of the lower reflecting surface 15a of the galvanometric mirror 15, a dichroic mirror 26, a magnifying lens 27, and a two-dimensional image pickup device 28 with an image intensifier are provided behind the second focus lens 16. Are sequentially arranged to form a blood vessel detection system. A stop 29 and a photomultiplier 30 that form a light receiving pupil with a circular hole are arranged on the optical path in the reflection direction of the dichroic mirror 26, and a light receiving optical system for measurement is configured. Although all the optical paths are shown on the same plane for the sake of illustration, the aperture 29 and the photomultiplier 30 are arranged in a direction perpendicular to the plane of the drawing.

The output of the two-dimensional image pickup device 28 is connected to a tracking control unit 31.
Is connected to the galvanometric mirror 15 and further to a system control unit 32 for controlling the entire apparatus. The outputs of the photomultiplier 30 and the operation unit 33 are connected to the system control unit 32, and the output of the system control unit 32 is connected to the data processing unit 34.

The white light emitted from the observation light source 1 passes through the condenser lens 3, only the yellow wavelength light is transmitted by the band pass filter 4, and the light flux passing through the ring slit 5 illuminates the transmission type liquid crystal 6 from behind. , Are reflected by a perforated mirror 8 through a relay lens 7. After that, only the light in the yellow region passes through the band-pass mirror 9 and the objective lens 2
, And once formed as a ring slit image on the pupil of the eye E to be examined, the fundus oculi Ea is almost uniformly illuminated. At this time, a fixation target is displayed on the transmissive liquid crystal plate 6, projected onto the fundus oculi Ea of the eye E by illumination light, and presented to the eye E as a target image.

The reflected light from the fundus Ea returns along the same optical path,
The light is taken out from the pupil as a fundus observation light beam, passes through the center opening of the perforated mirror 8, the focus lens 10 and the relay lens 11, forms an image as a fundus image Ea 'on the scale plate 12, and is then brought into eye contact with the examiner's eye e. Since the observation is made through the lens 13, the alignment of the apparatus can be performed while observing the fundus image Ea '.

The measuring light emitted from the measuring light source 22 passes eccentrically above the condenser lens 21 and passes through the dichroic mirror 20. On the other hand, the tracking light emitted from the tracking light source 25 is reflected by the mirror 24, is shaped into a desired shape by the mask 23, is further reflected by the dichroic mirror 20, and is condensed by the condensing lens 21 to the center of the opening of the mask 23. Is superimposed on the measurement light which is formed in a spot shape at a position conjugate with the measurement light.

The measurement light and the tracking light are transmitted through a lens 17.
Through the upper reflecting surface 1 of the galvanometric mirror 15
5b, reflected once by the concave mirror 19,
It is returned to the galvanometric mirror 15 again. Here, the two light beams reflected by the upper reflecting surface 15b of the galvanometric mirror 15 by the function of the relay optical system are returned to the positions of the cutout portions of the galvanometric mirror 15, and are reflected by the galvanometric mirror 15. Heads for the image rotator 14 without any problem.

The two light beams deflected by the band-pass mirror 9 in the direction of the objective lens 2 via the image rotator 14 are irradiated on the fundus Ea of the eye E through the objective lens 2. At this time, the tracking light is
In order to illuminate a rectangular area covering the blood vessel including the measurement point, its size is 300 to 5 in the blood vessel running direction.
The measurement light is shaped into a circular spot of about 50 to 120 μm about the thickness of the blood vessel to be measured, or an elliptical shape having a longitudinal direction in the blood vessel running direction. I have.

The scattered and reflected light from the fundus oculi Ea is collected again by the objective lens 2, reflected by the band-pass mirror 9, passes through the image rotator 14, and passes through the galvanometric mirror 1
5 is reflected by the lower reflecting surface 15a of the focusing lens 1
6, the measurement light and the tracking light are separated by the dichroic mirror 26.

The tracking light is transmitted through the dichroic mirror 26 and is formed on the two-dimensional image pickup device 28 by the magnifying lens 27 as a blood vessel image which is larger than the fundus image Ea 'by the fundus observation optical system. The imaging range at this time is almost the same size as the irradiation range of the tracking light. This blood vessel image signal is input to the tracking control unit 31 and is converted into a blood vessel position signal. The tracking control section 31 uses this signal to control the rotation angle of the galvanometric mirror 15 and perform tracking of the blood vessel.

A part of the scattered reflected light on the fundus oculi Ea due to the measurement light and the tracking light passes through the band-pass mirror 9 and is guided to the fundus observation optical system behind the perforated mirror 8.
The tracking light forms an image on the scale plate 12 as a bar-like indicator, and the measurement light forms an image as a spot image at the center of the indicator. These images are the eyepiece 1
3 and is observed together with the fundus image Ea ′ and the optotype image.
At this time, the spot image of the measurement beam is observed superimposed on the center of the indicator. The indicator is the operation unit 33
By rotating the galvanometric mirror 15, the one-dimensional movement on the fundus oculi Ea can be achieved.

At the time of measurement, the examiner first focuses the fundus image. When the focus knob of the operation unit 33 is adjusted, the transmission type liquid crystal plate 6, the focus lenses 10, 16 and the focus unit 18 are moved along the optical path in conjunction with the drive unit (not shown). When the fundus image is in focus,
Transmissive liquid crystal plate 6, scale plate 12, two-dimensional image sensor 28
Is simultaneously conjugated with the fundus oculi Ea.

After adjusting the focus of the fundus image, the examiner guides the line of sight of the eye E to change the observation area, and operates the operation unit 33 to move the blood vessel to be measured to an appropriate position. . The system control unit 32 controls the transmission type liquid crystal plate 6 to move the optotype image, and rotates the image rotator 14 to move the optical axis of the fundus blood flow meter and the photomultiplier in the running direction of the blood vessel to be measured. The operation is performed so that the lines connecting the centers of the pliers 30 are parallel. At this time, the two-dimensional image sensor 2 is rotated by rotating the galvanometric mirror 15.
The vertical direction of the eight pixel array and the moving direction of the measurement beam are simultaneously adjusted in a direction perpendicular to the blood vessel at right angles to the vertical direction.

The examiner starts tracking and checks the quality of the tracking, and then presses a measurement switch of the operation unit 33 to start measurement. During this measurement, the measurement beam is held on the blood vessel by the action of the tracking control unit 31, but the scattered reflected light is reflected by the dichroic mirror 26,
The light is received by the photomultiplier 30 through the aperture 29. The output of the photomultiplier 30 is output to the system control unit 32, where frequency analysis such as FFT processing is performed. The FFT waveform thus obtained is output to the data processing unit 3
4, the blood flow velocity of the fundus oculi Ea is determined.

FIG. 2 is a flowchart of the operation of the data processing section 34. First, in step S1, a process such as smoothing is performed on an FFT waveform obtained by subjecting a signal from the photomultiplier 30 to FFT processing.

FIG. 3 shows the relationship between the measurement site V of the blood vessel Ev to be measured, the measurement light source 22, the photomultiplier 30, and the light receiving pupil A formed by the stop 29. The light receiving pupil A is not limited to the aperture 29 but may be formed by a mirror or a sensor unit of the photomultiplier 30.
The angle of the blood vessel formed by the optical axis O of the fundus blood flow meter as a reference of the angle and the traveling direction of the blood vessel Ev to be measured is defined by θ, and the optical axis O and the incident direction of the measurement light source 22 to the measurement site V are formed. The incident angle is γ ₀ , and the light receiving angle between the optical axis O and the light receiving direction from the measurement site V to the light receiving pupil A is β.

FIG. 4 shows a positional relationship and a plan view of the light receiving pupil A. The light receiving pupil A has a circular shape with a radius R, and β ₀ is a light receiving angle at the center C of the light receiving pupil A. Here, the blood vessel angle θ in FIG. 3 is a variable that varies depending on the traveling direction of the blood vessel Ev, and the light receiving angle β in FIG. 3 is a variable that varies depending on the light receiving direction among the angles β1 to β2 in FIG. Note that these angles θ, γ ₀ , and β are those converted to angles in the human eye.

Next, in FIG. 2, of the possible angle ranges θs to θe such as 75 to 105 degrees, the initial angle θs is set to a temporarily determined blood vessel angle θ, and a predetermined frequency N
In the range Fs to Fe, the first frequency Fs is set as the provisionally determined cutoff frequency, and the FF at this time is set in step S2.
A fitting curve which is a model shape of the T waveform is obtained.

If the light receiving pupil A is a point, the ideal model shape of the FFT waveform is such that the power spectrum drops discontinuously at the frequency Fs as shown by the broken line FL in FIG. Is not discontinuous at the frequency Fs because the light receiving pupil A has an area.
It has a shape that sharply falls along a certain curve like a T waveform.

Among the optical beat signals received by the photomultiplier 30, the cutoff frequency Δf of the FFT by the optical beat signal in the light receiving direction at the light receiving angle β is the light receiving angle β _0.
Cut-off frequency Δf ₀ and the angle θ in, γ _0, β, β
_{By 0} , it is represented by the following equation. Δf = Δf ₀ · │ {cos (θ + β) + cos (θ-γ ₀ )} / {cos (θ + β ₀ ) + cos (θ-γ ₀ )} │… (1)

The power spectrum P of the FFT of the optical beat signal formed from the light receiving angle β in the light receiving pupil A
Is proportional to the area of that portion, and the following relational expression holds, where L is the distance between the plane formed by the light receiving pupil A and the measurement site V. P∝ [R ^2- {L (tanβ-tanβ ₀ )} ² ] ^1/2 … (2)

The power spectrum P of the FFT of the optical beat signal formed on the entire light receiving pupil A is represented by the light receiving angles β1 to β1 shown in FIG.
The power spectrum P up to β2 is superimposed, and F
The shape of a curve in which the power spectrum P of the FT waveform continuously drops at the cutoff frequency, that is, the theoretical cutoff shape can be obtained. In this embodiment, the theoretical cutoff shape is obtained by calculation, but a method of storing the numerical value of the theoretical cutoff shape for each blood vessel angle may be used.

A broken line FL1 shown in FIG. 6 is obtained by calculating a fitting curve when a temporarily determined blood vessel angle θs and a temporarily determined cutoff frequency Fs are used for fitting the frequency analysis result of the received light output signal. It is. The curve portion of the theoretical cutoff shape was determined by the above-described superposition. Assuming that the frequency at which the theoretical cutoff shape starts to fall is Fa, the power spectrum PS1 lower than this frequency Fa is the actual FF shown by the solid line SL.
The average value of the power spectrum P lower than the frequency Fa of the T waveform is used. Further, assuming that the frequency at the end of the theoretical cutoff shape is Fb, the power spectrum PS2 higher than this frequency Fb is the actual FF
The average value of a portion of the T waveform due to sufficiently high frequency noise is used. Since the method of obtaining these average values is simple in calculation, it can be calculated in a short time.

As described above, PS2 is added as a noise component to the theoretical cutoff shape so that the end of the theoretical cutoff shape is PS2, and further enlarged such that the height of the theoretical cutoff shape is PS1-PS2. Alternatively, a fitting curve is created by performing reduction. The cut-off frequency portion of the fitting curve has a shape close to the actual FFT waveform, so that the cut-off frequency can be determined more accurately, and a highly accurate blood flow velocity can be obtained.

FIG. 7 shows another method for obtaining a fitting curve. The power spectrum PS2 higher than the curve portion of the theoretical cutoff shape and the frequency Fb
Is similar to that of FIG. 6, but the method of obtaining the power spectrum PS1 lower than the frequency Fa is different. Here, a power spectrum PS2 is calculated from a value obtained by integrating a part obtained by subtracting the power spectrum PS2 considered as a noise component from the power spectrum P of the actual FFT waveform shown by the solid line SL over all frequencies and the power spectrum P of the fitting curve. The power spectrum PS1 is determined so that the value obtained by integrating the subtracted portion at a frequency lower than the frequency Fb is equal, that is, the sum of the power spectra P is equal.

In this case, the provisional cutoff frequency Fs
Is far from the cutoff frequency of the actual FFT waveform, the shape of the fitting curve is significantly different from the actual FFT waveform, but as the cutoff frequency approaches, the shape of the fitting curve also approaches the actual FFT waveform. That is, according to this method, the convergence is improved when the frequency at which the numerical value is minimized in step S4 in FIG. 2, and the cutoff frequency can be more easily obtained.

In step S3 of FIG. 2, the sum of squares T ₀ of the difference between the value of the power spectrum P between the fitting curve obtained as described above and the actual FFT waveform is obtained. Steps S2 and S3 are also performed at a predetermined interval for the frequency next to the frequency Fs, and the fitting to the frequency is repeatedly performed until the final frequency Fe. Next, steps S2 and S3 are similarly performed for each frequency at a predetermined interval between Fs and Fe for a blood vessel angle next to the blood vessel angle θs at a predetermined interval, and repeated until the final blood vessel angle θe. And fitting to the blood vessel angle θ is performed. Finally, the value of the sum of squares T ₀ using the blood vessel angle θ and the frequency N as variables is obtained.

Thereafter, in step S4, the minimum value of the sum of squares T ₀ is obtained, and the blood vessel angle θ at that time is set as the blood vessel angle θv of the measurement site V, and the frequency N is set as the cutoff frequency Δfmax at the light receiving angle β ₀ . At this time, the fitting curve has a shape closer to the FFT waveform as shown by the solid line FL0 in FIG. 6, and it can be seen that the cutoff frequency is accurately obtained.

Next, in step S5, the maximum blood flow velocity Vmax is obtained by the following equation. Here, λ is the wavelength of the measurement light beam, and n is the refractive index of the measurement site.

Vmax = λ · Δfmax / {n│cos (θv + β ₀ ) + cos (θv-γ ₀ ) │} (3) In this manner, the cutoff frequency can be obtained with high accuracy, so that The maximum blood flow velocity Vmax can be obtained. If the S / N ratio of the received light output signal is sufficiently good even when two light receivers are used, the two received light output signals are subjected to the above-mentioned fitting with respect to the blood vessel angle and the frequency so that the two equal light receiving signals are obtained. Since the maximum blood flow velocity is obtained, the maximum blood flow velocity can be obtained with higher accuracy by averaging them.

[0040]

As described above, the fundus blood flow meter according to the present invention provides information on the shape of the light receiving pupil for limiting the light receiving area of the light receiving means, or a light receiving output signal calculated based on the shape of the light receiving pupil. The information of the theoretical power spectrum shape is stored, and the information is used to fit the frequency analysis result of the received light output signal to the angle and frequency formed by the reference axis of the device and the blood vessel, thereby setting the cutoff frequency. By calculating, the blood flow velocity can be calculated by one light receiver. As a result, since the area of the light receiving pupil for one light receiving device can be increased, the SN ratio of the light receiving output signal is improved, and a more accurate blood flow velocity can be easily calculated. Also, even if an expensive photomultiplier is used as this light receiver, the number of used photomultipliers can be reduced to one.
It is possible to achieve miniaturization at low cost.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a fundus blood flow meter of the present embodiment.

FIG. 2 is a flowchart of a data processing operation.

FIG. 3 is an explanatory diagram of a relationship between a measurement site, a measurement light source, a photomultiplier, and a light receiving pupil.

FIG. 4 is an explanatory diagram of a positional relationship of a light receiving pupil.

FIG. 5 is a graph of a fitting curve when a light receiving pupil is a point.

FIG. 6 is a graph of a fitting curve in which the effect of the area of the light receiving pupil is considered.

FIG. 7 is a graph of a fitting curve according to another method.

FIG. 8 is an explanatory diagram of a flow velocity distribution in a blood vessel.

FIG. 9 is a graph showing an FFT waveform of an optical beat signal.

FIG. 10 is a graph showing an FFT waveform of an optical beat signal.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Observation light source 4 Bandpass filter 5 Ring slit 6 Transmission liquid crystal plate 8 Perforated mirror 9 Bandpass mirror 14 Image rotator 15 Galvanometric mirror 18 Focus unit 19 Concave mirror 20, 26 Dichroic mirror 22 Measurement light source 25 Tracking light source 28 two-dimensional image sensor 29 aperture 30 photomultiplier 31 tracking control unit 32 system control unit 33 operation unit 34 data processing unit

Claims (2)

[Claims] 1. A measuring light irradiating means for irradiating a blood vessel on a fundus of a subject's eye with measuring light, at least one light receiving means for receiving scattered light generated from intravascular particles by the measuring light,
A light receiving pupil forming member for forming a light receiving pupil of the light receiving means, a data processing means for analyzing a light receiving output signal from the light receiving means to calculate a blood flow velocity, and information relating to the shape of the light receiving pupil or the light receiving pupil. In a fundus blood flow meter having information storage means for storing information of the theoretical power spectrum shape of the received light output signal calculated based on the shape, the data processing means analyzes the information stored in the information storage means, A fundus blood flow meter, wherein a blood flow velocity is calculated by fitting a frequency analysis result of the received light output signal to an angle and a frequency formed by a reference axis of a device and the blood vessel. 2. The data processing means calculates a specific shape of a theoretical power spectrum of the received light output signal using information stored in the information storage means.
2. A fundus blood flow meter according to item 1.