Device and Method for Examining Metabolic Autoregulation

Abstract

The invention relates to a method and to a device suitable therefor, for examining the metabolic autoregulation of the retinal vessels in a patient's eye. Over a baseline phase (BP), a stimulation phase (SP), in which the individual retinal perfusion pressure in the eye is decreased in a normalized manner by the targeted application of a stimulation pressure to the eye, and a posterior phase (NP), a video sequence of images of the retina is captured, from which images signals describing the local vascular perfusion can be derived, from which signals metabolic vascular reactions are derived and recorded in order to examine the vascular reactions of the large retinal vessels e.g. on the basis of vessel diameter signals (D(t,x,y) and in order to examine the capillary vessels e.g. on the basis of spectrally normalized quotient signals.

Claims

1. A device for examining metabolic autoregulation of retinal vessels in a patient's eye (A), said device comprising: a unit for generating and applying stimulation pressure acting on the eye (A); imaging unit being a modified retinal camera with a digital image sensor, the imaging unit serving to generate video sequence of images of retina, to each of the images being assigned two color channels; a unit for generating spectrally normalized quotient signals which derives quotient signals (Q(t,x,y)) from intensity signals of the color channels, the quotient signals (Q(t,x,y)) being utilized to infer a vascular reaction and the metabolic autoregulation of the capillaries of the retinal vessels; or imaging unit using laser scanning technology or optical coherence tomography for generating a video sequence of images of the retina derive images signals corresponding to local vessel diameters, local blood velocities, local blood flows or local capillary densities of the capillaries or/and the large vessels; a tonometer for measuring an intraocular pressure (IOP) in the eye (A), said intraocular pressure (IOP) changing as a function of a stimulation pressure (SD) applied by the unit for generating and applying the stimulation pressure; and a sensor for measuring the stimulation pressure (SD) comprised in the unit for generating and applying a stimulation pressure the sensor serving to assign a stimulation pressure value to a respective measured intraocular pressure value (IOP) and to each image of the video sequence.

2. The device according to claim 1, wherein the unit for generating and applying a stimulation pressure comprises a pressure applicator which can be attached to a patient's head, in a fixed manner with respect to the patient's eye (A) outside of a cornea and outside of a light path of the imaging unit in a pressure-free planar contact with the eye (A).

3. The device according to claim 1, wherein the imaging unit is the spectrally modified retinal camera having an illumination beam path, and wherein -a double band-pass filter with a spectral range in red light and a spectral range in green light is disposed in the illumination beam path.

4. A method for examining metabolic autoregulation of retinal vessels in a patient's eye (A), the method comprising: recording a video sequence of images of the retinal vessels during a baseline phase (BP) which does not affect the eye (A); recording the video sequence of images of the retinal vessels during a stimulation phase (SP), increasing an intraocular pressure (IOP) by a predetermined change intraocular pressure value (dIOP.sub.s) by applying and increasing a stimulation pressure (SD) acting on the eye (A), and maintaining the intraocular pressure (IO) at a stimulation intraocular pressure value (IOP.sub.s) for a stimulation period (T); recording the video sequence of images of the retinal vessels during a posterior phase (NP) which does not affect the eye (A), and deriving signals corresponding to local vascular perfusion from the images of the video sequence; carrying out the increase by the predetermined change intraocular pressure value (dIOP.sub.s) starting from a measured resting intraocular pressure value (IOP.sub.0) if, during the baseline phase (BP), measurement criteria for a spontaneous venous collapse at an optic nerve head are determined on the retina; carrying out the increase by the predetermined change intraocular pressure value (dIOP.sub.s) starting from an increased intraocular pressure value (IOP.sub.RVP), if, during an increase in intraocular pressure (IOP), measurement criteria for spontaneous venous collapse at the optic nerve head are determined on the retina when the increased intraocular pressure value (IOP.sub.RVP) is reached; and carrying out the increase by the predetermined change intraocular pressure value (dIOP.sub.s) on the basis of the measured resting intraocular pressure value (IOP.sub.0) if no spontaneous venous collapse is detected, not even during the increase in intraocular pressure (IOP) at the optic nerve head on the retina.

5. The method according to claim 4, wherein the signals corresponding to the vascular reaction of the retinal vessels for arterial and venous vessels are vessel diameter signals (D(t,x,y)), and wherein the signals describing capillary vessels are spectrally normalized quotient signals (Q(t,x,y)) which are acquired and recorded in parallel.

6. The method according to claim 4, further comprising measuring at least a second intraocular pressure value after measuring the resting intraocular pressure value (IOP.sub.0) the stimulation pressure (SD), and determining for the eye (A) an individual relationship between the intraocular pressure values (IOP) and the respectively associated stimulation pressure values for setting the stimulation intraocular pressure value (IOP.sub.s) via an individually associated stimulation pressure value (SD.sub.s).

7. The method according to claim 4, further comprising increasing the stimulation pressure (SD) by at least 1 mmHG per second with the start of the stimulation phase (SP) to rapidly increase the resting intraocular pressure value (IOP.sub.0) by the predetermined change intraocular pressure value (dIOP.sub.s) to the stimulation intraocular pressure value (IOP.sub.s).

8. The method according to claim 4, further comprising terminating the stimulation phase (SP) after the stimulation period (T) has elapsed, and the stimulation pressure (SD) is reduced to zero.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention will be explained in more detail below on the basis of drawings with reference to an exemplary embodiment of a device and of a method using a modified conventional retinal camera. In the drawings:

[0035] FIG. 1 shows a block diagram of a device according to the invention;

[0036] FIG. 2 shows an embodiment of a unit for generating and applying a stimulation pressure;

[0037] FIG. 3A shows a time sequence for the examination when the retinal perfusion pressure rPP results from the resting intraocular pressure value IOP.sub.0;

[0038] FIG. 3B shows a time sequence for the examination when the retinal perfusion pressure rPP results from an increased intraocular pressure value IOP.sub.RVP, which corresponds to an RVP greater than the resting intraocular pressure IOP.sub.0;

[0039] FIG. 4A shows a time sequence of a vessel diameter signal for an artery and a vein during the three examination phases, and

[0040] FIG. 4B shows a time sequence of a quotient signal during the three examination phases.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] As shown in a block diagram in FIG. 1, a device according to the invention comprises at least one unit for generating and applying a stimulation pressure 1, an imaging unit 2 with a digital image sensor, a tonometer 3, a computing and control unit 4, an input and output unit 5, a results image storage unit 7, a data and image evaluation unit 8, a signal analysis unit 9, a unit for generating spectrally normalized quotient signals 10 and a unit for generating vessel diameter signals 11.

[0042] The unit for generating and applying a stimulation pressure 1 comprises, as schematically shown in FIG. 2, a pressure generating unit 1.2, a holder 1.3 and a pressure applicator 1.1, which is fixed to the patient's right and left eyes A laterally (temporally), in each case, via the holder 1.3, which preferably resembles a pair of glasses. The pressure applicator 1.1 can be placed in pressure-free, planar contact with the patient's eye A to be examined.

[0043] The pressure applicator 1.1 serves to apply a stimulation pressure SD onto the patient's eye A to be examined and is preferably a small pneumatic balloon, but could also be, for example, a die, a suction cup or an hydraulic system.

[0044] In contrast to pressure applicators known from prior art devices or methods, the embodiment of the pressure applicator 1.1 as a small pneumatic balloon has a number of advantages. For instance, there is a much lower risk of injury from sharp rims, which are produced on the edges of pressure applicators 1.1 made of metals, plastic materials or ceramics or other solid materials. Moreover, the soft surface of the balloon is much more pleasant for the patient during the examination. In addition, the uniform expansion of the balloon in all directions avoids transverse forces which may result in falsified measurement results.

[0045] Connected to the pressure applicator 1.1 is the pressure generating unit 1.2, by which the stimulation pressure SD can be generated, increased, decreased or kept constant.

[0046] In order to generate the stimulation pressure SD in a controlled manner, the unit for generating and applying a stimulation pressure 1 is connected to the computing and control unit 4. Depending on the pressure applicator 1.1 selected, the unit for generating and applying a stimulation pressure 1 may comprise, as the pressure generating unit 1.2, for example, a pump, a system consisting of a pneumatic cylinder and a piston or/and control electronics for linear drives. Advantageously, the pressure generating unit 1.2 is a pneumatic system consisting of a pneumatic cylinder and a piston which can be moved in the pneumatic cylinder by a linear drive. By moving the piston, the air contained in the pneumatic system is compressed or dilated, causing the pressure in the pressure applicator 1.1 either to increase or to decrease. The pressure generating unit 1.2 may advantageously comprise components for defined adjustment of the increase or decrease in stimulation pressure SD. Possible designs for this include, for example, systems consisting of various throttle valves and magnetic valves, or suitable control electronics which allow different speeds of adjustment of a linear drive.

[0047] The pressure generating unit 1.2 comprises a sensor for measuring the stimulation pressure SD. Depending on the design of the unit, pressure sensors, force sensors or distance sensors may be used, for example.

[0048] The pressure generating unit 1.2 advantageously also comprises a component enabling a sudden drop in the stimulation pressure SD. For this purpose, one or more magnetic valves may be used, for example, by which the system is suddenly vented in an emergency.

[0049] The holder 1.3 serves to couple the pressure applicator 1.1 directly to the patient's head and may be, for example, a pair of glasses, a head band or a bracket placed on the patient's head. Preferably, the holder 1.3 is implemented in the form of a pair of glasses. In order to improve the coupling of the pressure applicator 1.1 to the patient's head, the holder 1.3 is advantageously provided with a further component, such as a glasses lace, a rubber band or a mechanically adjustable fixing means.

[0050] In order to achieve individually adaptable positioning of the pressure applicator 1.1, in particular adjustability of the direction from which the pressure applicator 1.1 is adducted to the patient's eye A, the pressure applicator 1.1 is attached to the holder 1.3 on the patient's eye A in an individually adjustable manner, preferably via height adjustment, distance adjustment and angle adjustment.

[0051] The optical access (light path) to the retina for the imaging unit 2 must not be impeded and/or blocked by any of the components contained in the unit for generating and applying a stimulation pressure 1.

[0052] In this case, the tonometer 3 is advantageously a modified rebound tonometer. It is connected to the computing and control unit 4 and to the data and image evaluation unit 8 via signaling pathways. It is integrated into the device and fully automatically controlled by it. Via the connection to the computing and control unit 4, automatically performed measurements of the intraocular pressure IOP are triggered upon reaching previously defined measurement criteria. The obtained intraocular pressure values IOP are transmitted to the computing and control unit 4 and are synchronized there to a time signal for further processing. The intraocular pressure values IOP synchronized to the time signal are transmitted to the data and image evaluation unit 8 for storage and further processing.

[0053] The computing and control unit 4 has a signal connection to the unit for generating and applying a stimulation pressure 1, the imaging unit 2, the tonometer 3 and the input and output unit 5, the data and image evaluation unit 8, the signal analysis unit 9, the unit for generating spectrally normalized quotient signals 10 and the unit for generating vessel diameter signals 11. The function of the data and image evaluation unit 8, which is connected via signal lines to the computing and control unit 4, the results image storage unit 7, the input and output unit 5, the imaging unit 2, the unit for generating and applying a stimulation pressure 1, the signal analysis unit 9, the unit for generating spectrally normalized quotient signals 10 and the unit for generating vessel diameter signals 11, will be explained by describing the method.

[0054] The results image storage unit 7, which is connected via a signal connection to the imaging unit 2, the input and output unit 5, and the data and image evaluation unit 8, is used to store or temporarily store measured values or signals reflecting the vascular reactions in result images such as the mapping image.

[0055] Signals that are transmitted to the signal analysis unit 9 for analysis are, for example, vessel diameter signals D(t,x,y). They are formed as a time- and location-dependent vessel diameter change of individual vascular segments or the average vessel diameter averaged for a vessel section formed from several vascular segments in the unit for generating vessel diameter signals 11. Other such signals over a defined area of a pixel or group of pixels (measurement location) can be e.g. an averaged unnormalized brightness signal or/and an averaged spectrally normalized quotient signal Q(t,x,y).

[0056] The input and output unit 5 is used by an examiner U to input data and control commands and to represent and output the respective examination results. During the examination, the stimulation pressure values SD and the video sequences can be represented online on the monitor. The examiner U can observe the video sequences on a monitor belonging to the input and output unit 5 and have the finished examination results presented. The video sequences are preferably presented to the examiner U together with measurement results for tracking and monitoring the adjustment of the imaging unit 2 during the examination procedure.

[0057] For example, the imaging unit 2 is a spectrally modified retinal camera that generates video sequences of images with two color channels of the retina.

[0058] Advantageously, the imaging unit 2 has a dual band-pass filter in its illumination beam path, e.g. with one spectral range in the red light and one spectral range in the green light, and the spectral components are directed to separate areas or different chips of a digital image sensor, resulting in a red and a green image, each of which can be understood as one of the two color channels of an image.

[0059] Alternatively, the imaging unit 2 can have a digital image sensor in which each pixel is formed by at least two subpixels of different spectral sensitivity, e.g. red and green. Each of the subpixels of a pixel of the digital image sensor generates one of two color channels of the images of the video sequence. The unit for generating spectrally normalized quotient signals 10 forms quotients, pixel by pixel, from the red and green color intensity signals of the color channels of the images or of the subpixels which together form a pixel. The pixels in successive images must correspond to the same measurement location. This results in a spectrally normalized quotient image, wherein illumination-side differences are eliminated by spectral normalization. The red backscattered light, which substantially penetrates blood, serves as the reference wavelength in this case, with light in the green light range being strongly absorbed by blood and reflecting the blood volume in a retinal region. Thus, the spectrally normalized quotient signal Q(t,x,y) describes, regardless of illumination, the blood volume in a capillary region. The resulting quotient image sequence of the retina is stored in the unit for generating spectrally normalized quotient signals 10 and then transmitted to the signal analysis unit 9 as spectrally normalized quotient signal Q(t,x,y) in accordance with the method steps. Any other imaging device that can produce images with multiple color channels can also be used as imaging unit 2.

[0060] For the purposes of the invention, it is irrelevant how and over which wavelengths the spectral normalization takes place, provided that illumination-independent signals are produced which describe the blood volume in a tissue volume.

[0061] The signal analysis unit 9 is connected to the input and output unit 5, the data and image evaluation unit 8, the computing and control unit 4, and the imaging unit 2. Its mode of operation will be explained later by describing an exemplary embodiment of the method according to the invention.

[0062] The unit for generating spectrally normalized quotient signals 10 is connected to the input and output unit 5, the data and image evaluation unit 8 and the imaging unit 2. As already explained, the unit for generating spectrally normalized quotient signals 10 serves to eliminate the influence of the illumination intensity on the signals generated for the examination.

[0063] The unit for generating vessel diameter signals 11 is connected to the input and output unit 5, the data and image evaluation unit 8 and the imaging unit 2. The unit for generating vessel diameter signals 11 determines vessel diameters in at least one selected vessel, segment by segment along the vessel sections, as well as image by image in the preferably green color channel of the video sequence or optionally in the quotient image. The time sequence of the vessel diameters of the individual vascular segments is then used to generate vessel diameter signals D(t,x,y) which are fed to the signal analysis unit 9.

[0064] The device need not necessarily include both a unit for generating spectrally normalized quotient signals 10 and a unit for generating vessel diameter signals 11 nor need it necessarily generate spectrally normalized quotient images and spectrally normalized quotient signals Q(t,x,y) derived from them. The proposals presented in this exemplary embodiment represent advantageous embodiments.

[0065] As to performing a method according to the invention, explanations were already given in the description of the essence of the invention and in the description of the functioning of the device, which are to be attributed to the following description of the process sequence, even if not explicitly referred to in each case.

[0066] In preparation for the examination, the examiner U attaches the pressure applicator 1.1 to the patient's head such that the pressure applicator 1.1 slightly touches the patient's eye A in the temporal canthus, without applying pressure.

[0067] Then the examiner U adjusts the tonometer 3 to the eye A such that automatic tonometer measurements are possible in parallel with imaging by the modified retinal camera and an evaluable image of the retina, including the optic nerve head (ocular fundus), can be provided on the monitor of the input and output unit 5.

[0068] The video sequences of images provided by the imaging unit 2 are automatically examined for sufficient image quality. If the image quality is not sufficient, the examiner U is prompted via the input and output unit 5 to correct the image quality by adjusting the modified retinal camera forming the imaging unit 2.

[0069] Then the examiner U starts the examination process with a first phase, the so-called baseline phase BP.

[0070] The intraocular pressure IOP is measured automatically and the measured intraocular pressure value IOP, equal to the resting intraocular pressure value IOP.sub.0, is stored as an initial value.

[0071] Subsequently or during this operation, a video sequence of images, to which two color channels are assigned, is generated using the modified retinal camera over the duration of three phases of the examination. In addition to vessel diameter signals D(t,x,y), spectrally normalized quotient signals Q(t,x,y) in particular are formed from these images to evaluate the metabolic autoregulation of capillaries.

[0072] In the baseline phase BP, all signals are formed without increasing the stimulation pressure SD. They serve as reference values for a later evaluation of the signals via the subsequent stimulation phase SP and the posterior phase NP.

[0073] Prior to this, the video sequences of retinal images generated by the imaging unit 2 are analyzed by the data and image evaluation unit 8 for image shifts or rotations between temporally successive images and corrected in such a way that a movement-corrected video sequence is generated in which identical retinal points overlap in the images. All signals are generated on the basis of this movement-corrected video sequence.

[0074] To examine arterial and venous vessels, vessel diameter signals D(t,x,y) are formed from the movement-corrected video sequence. For this purpose, the arterial and venous vessels in the area of the papilla (optic nerve head) are selected and their measurement location is stored. The unit for generating vessel diameter signals 11 then accesses the stored arterial and venous vessels and determines vessel diameters along the vessels, segment by segment and image by image, the values of which are assigned to the measurement location on the retina and the time or the respective image and stored, and forms a time- and location-dependent vessel diameter signal D(t,x,y). Individual pixels or pixel groups can be assigned to the measurement locations.

[0075] The data and image evaluation unit 8 forms green color intensity signals and red color intensity signals from the color channels of the images of the video sequence or intensity values of the assigned pixels or subpixels, respectively, and forms spectrally normalized quotient signals Q(t,x,y) whose time sequence is assigned to the pixels at the measurement location on the retina, to a time signal to which all measurement results are also synchronized in the form of measured values or signals, or to the images.

[0076] All signals formed are fed to the signal analysis unit 9 and recorded (stored) in parallel as time sequences.

[0077] The computing and control unit 4 assigns all current SD values to a time signal, which is set to ZERO with the first START signal for the examination and to which all original and derived video sequences, images, quotient images and signals are synchronized or temporally assigned.

[0078] During the baseline phase BP and the subsequent increase of the intraocular pressure IOP at the beginning of the stimulation phase SP, the signal analysis unit 9 monitors all signals with regard to the objective measurement criteria defined below:

[0079] The signals on the selected optic nerve head are monitored for the occurrence of a spontaneous venous collapse. The following objective measurement criteria are used:

[0080] The diameters of individual venous vascular segments on the optic nerve head begin to pulsate significantly more than in the past or more than most venous vascular segments on the optic nerve head. The threshold factor for the increase in pulsation amplitude, which determines when the measurement criterion has occurred, is set at 3, but can be adjusted differently based on experimental investigations.

[0081] The signals, i.e. the quotient signal Q(t,x,y) and/or red color intensity signal and/or green color intensity signal, from the area of the optic nerve head increase significantly in their pulse amplitude compared to the past or to the signals of neighboring pixels. The threshold factor for the increase in pulsation amplitude, which determines when the measuring criterion has occurred, is set at 3, but can be adjusted differently on the basis of experimental investigations or between the different signals.

[0082] If the signal analysis unit 9 already detects during the baseline phase BP that at least one of the measurement criteria for venous collapse is met, see FIG. 3A, the resting intraocular pressure value IOP.sub.0 is equated to the retinal venous blood pressure within the eyeball rPv. In this case, the resting intraocular pressure value IOP.sub.0 determines the retinal perfusion pressure rPP and the above formula 1 is used, as in the prior art. A desired stimulation intraocular pressure value IOP.sub.s results from the addition of the resting intraocular pressure value IOP.sub.0 and a predetermined change intraocular pressure value dIOP.sub.s for stimulation of the eye A.

IOP.sub.s=IOP.sub.0+dIOP,

[0083] Not only in this case, but also if no spontaneous venous collapse has been detected, the stimulation phase SP is started via the computing and control unit 4. A START signal is fed to the unit for generating and applying a stimulation pressure 1 and the increase in stimulation pressure SD in the pressure applicator 1.1 is triggered. The stimulation pressure SD in the pressure applicator 1.1 should increase by at least 1 mmHg per second to rapidly increase the resting intraocular pressure value IOP.sub.0 by the predetermined change intraocular pressure value dIOP, to the stimulation intraocular pressure value IOP.sub.s.

[0084] During the increase of the stimulation pressure values SD, at least one further intraocular pressure measurement is performed, indicated as IOP.sub.1 in FIG. 3A, in order to determine the individual correlation of the causative stimulation pressure SD for eye A as a function of the achieved intraocular pressure IOP or of its change, SD=f(IOP), and to calculate a stimulation pressure value SD.sub.s associated with the stimulation intraocular pressure value IOP.sub.s, which must ultimately be reached by the unit for generating and applying a stimulation pressure 1 and kept constant over a stimulation period T. For increased accuracy, further IOP values can be determined during the SD increase, which are included in the calculation of the individual correlation SD=f(IOP).

[0085] In the absence of spontaneous venous collapse during the baseline phase BP, the signal analysis unit 9 continues its monitoring for the onset of spontaneous venous collapse during the SD increase, see FIG. 3B.

[0086] As soon as the signal analysis unit 9 detects spontaneous venous collapse on the optic nerve head during the SD increase, the increased intraocular pressure value IOP.sub.RVP then associated with the retinal venous pressure outside the eyeball RVP is measured by the computing and control unit 4 and a specific change in intraocular pressure dIOP.sub.RVP is determined from the difference with the resting intraocular pressure IOP.sub.0, i.e. dIOP.sub.RVP=IOP.sub.RVP−IOP.sub.0 or IOP.sub.0+dIOP.sub.RVP=RVP. According to the described formula 2 and the individual correlation SD=f(IOP), a new stimulation pressure value SD.sub.Snew=f(dIOP.sub.RVP) to be set is then calculated in order to increase the intraocular pressure IOP, equal to the RVP, to a newly calculated stimulation intraocular pressure value IOP.sub.s by changing it by the given change intraocular pressure value dIOP.sub.s.

[0087] If the venous collapse did not occur until the predetermined stimulation intraocular pressure value IOP.sub.s was reached or until the associated stimulation pressure value SD.sub.s was reached and consequently no intraocular pressure value IOP.sub.RVP could be determined for the RVP, the stimulation intraocular pressure value IOP.sub.s is set for the case IOP>RVP (formula 1) and no new stimulation intraocular pressure value IOP.sub.s is calculated and generated by setting a new associated stimulation pressure value SD.sub.s.

[0088] After the stimulation period T has elapsed, the stimulation phase SP is terminated and the stimulation pressure SD is suddenly reduced to zero.

[0089] The recording of the signals is continued for a period of the posterior phase NP and then terminated.

[0090] The recorded signals are printed out as a description for metabolic autoregulation, as e.g. diameter signals D(t,x,y) shown in FIG. 4A or quotient signals Q(t,x,y) shown in FIG. 4B, or characteristic measurement values, such as minima and maxima of the signals in the stimulation phase SP and from the posterior phase NP, are assigned to the measurement locations in a false color-coded manner and output as a mapping image superimposed on one of the video images of the retina (ocular fundus).

[0091] The reactions of the large vessels (arteries, veins) do not necessarily have to be examined together with the reactions of the capillaries, as in the exemplary embodiment described above. For certain medical questions, the examination of only one of the vessel types, i.e. arteries, capillaries or veins, is sufficient.

[0092] Furthermore, the method can also be performed semi-quantitatively by the examiner U manually determining the IOP values, in which case the corresponding longer pauses in the SD increase must be realized.

[0093] In a further embodiment of the invention, the individual correlation SD=f(IOP) could be determined separately from the stimulation in order to avoid possible tonographic effects and influences on vascular reactions.

LIST OF REFERENCE NUMERALS

[0094] 1 unit for generating and applying a stimulation pressure [0095] 1.1 pressure applicator [0096] 1.2 pressure generating unit [0097] 1.3 holder [0098] 2 imaging unit [0099] 3 tonometer [0100] 4 computing and control unit [0101] 5 input and output unit [0102] 7 results image storage unit [0103] 8 data and image evaluation unit [0104] 9 signal analysis unit [0105] 10 unit for generating spectrally normalized quotient signals [0106] 11 unit for generating vessel diameter signals [0107] A eye [0108] U examiner [0109] rPP retinal perfusion pressure (value) [0110] rPP.sub.s stimulation perfusion pressure [0111] rPa retinal arterial blood pressure [0112] rPv retinal venous blood pressure (within the eyeball) [0113] RVP retinal venous blood pressure (value) outside the eyeball [0114] SD stimulation pressure (value) [0115] IOP intraocular pressure (value) [0116] IOP.sub.0 resting intraocular pressure (value) [0117] dIOP.sub.s predetermined change intraocular pressure (value) [0118] IOP.sub.s stimulation intraocular pressure value [0119] SD.sub.s stimulation pressure value associated with the stimulation intraocular pressure value IOP.sub.s [0120] BP baseline phase [0121] SP stimulation phase [0122] NP posterior phase [0123] T stimulation period [0124] Q(t,x,y) (spectrally normalized) quotient signal [0125] D(t,x,y) vessel diameter signal