Apparatus for measuring blood flow

An apparatus for measuring blood flow in which an optical system guides a coherent light beam to impinge on a stationary measurement spot of a measurement plane of an in vivo tissue. A light-receiving optical system focuses a light image of the measurement plane in the vicinity of an image point conjugate with the stationary measurement spot. The light-receiving optical system shares at least part of the optical components of the guiding optical system. Photodetectors are disposed at prescribed positions perpendicular to the optical axis of the light-receiving optical system, and in the vicinity of the image point, for measuring light scattered from the in vivo tissue to provide blood flow information. The blood flow information is related to blood flowing at a depth within the in vivo tissue. A visible image of the measurement plane is converted into a video image. The blood flow information corresponding to the position of the stationary measurement spot is superimposed on the video image and displayed to obtain a composite visible image.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an apparatus for measuring blood flow, and more 
particularly to an apparatus for noninvasively obtaining blood flow 
information within in vivo tissue while providing the capability to 
superimpose the blood flow information on a video image of the tissue to 
obtain a composite visible image. 
2. Description of the Prior Art 
Laser Doppler apparatuses or laser speckle apparatuses have ben marketed as 
apparatuses for noninvasively obtaining information on blood flow. These 
apparatuses measure blood flow factors by directing a laser beam into the 
blood steam, picking up light scattered by erythrocytes moving in the 
blood, and analyzing the frequency spectrum of the received light for 
determining the frequency gradient. An apparatus based on this method is 
disclosed, for example, in Japanese Patent Public Disclosure Sho 
60(1985)-203235. 
Such conventional apparatuses use a laser beam projecting probe and a light 
receiving probe, both constituted of optical fibers, and the measurement 
depths within the in vivo tissue is regulated by adjusting the distance 
between the probes (See Fujii et al., Measurement of blood flow in skin 
using laser beam phenomenon (V) (Japan Laser Medical Magazine), Vol. 6, 
No. 3 (January 1986)). 
An example of the arrangement used is shown in FIG. 4. A laser beam is 
directed into in vivo tissue P from an optical fiber F1 and the scattered 
light is received by an optical fiber F2. The intensity of the light 
received by the light receiving fiber F2 is governed by its distance from 
the beam projecting fiber F1. 
Assuming the tissue to be a perfect light scattering body, Fujii et al. 
approximated the intensity of the light received by the light receiving 
fiber F2 as shown below (Fujii et al., Evaluation of skin blood flow using 
laser speckle phenomena (VII) (The Journal of Japan Society for Laser 
Medicine), Vol. 7, No. 3 (January 187)). 
EQU Im=I.sub.0 Exp {-.gamma.(R1+r2)} (1) 
where 
Im: intensity of received light 
I.sub.0 : intensity of irradiating beam 
.gamma.: coefficient of attenuation owing to absorption and scattering 
r1: distance between end surface P1 of beam projecting fiber and light 
scattering particles (erythrocytes) 
r2: distance between end surface P2 of light receiving fiber and light 
scattering particles (erythrocytes) 
This equation being that of an ellipse having its foci at the points P1 and 
P2 where the light enters and leaves the tissue, it can be seen that the 
length of the light path for which scattered light can be received 
increases with increasing distance between the points P1 and P2. In other 
words, scattered light from deeper parts of the tissue can be received by 
increasing the distance between the optical fibers F1 and F2. 
As this conventional arrangement requires the optical fiber probes to be 
brought in contact with the tissue with respect to which measurement is 
being conducted, it is apt to have undesirable effects on the patient, 
such as making him or her feel uneasy or uncomfortable. 
Scanning a large measurement region using the conventional arrangement 
involves the troublesome work of repeatedly repositioning the measurement 
probes and, moreover, requires the position information to be recorded 
after each repositioning. The measurement work is thus complicated and 
laborious. 
SUMMARY OF THE INVENTION 
An object of the present invention is therefore to provide an apparatus 
capable of obtaining blood flow information from a depth within in vivo 
tissue in a contactless manner without using optical fibers or the like, 
displaying while simultaneously a video image of the measurement region of 
the in vivo tissue, and capable of enabling positional information 
relating to a large measurement region of the in vivo tissue to be 
acquired simply and quickly during the measurement. 
In accordance with the present invention, an apparatus for measuring a 
blood flow comprises a scanning optical system for scanning a measurement 
plane of an in vivo tissue in a desired pattern with a scanning spot 
formed by a coherent light beam so that the coherent light beam impinges 
on a stationary measurement spot of the in vivo tissue a light receiving 
optical system for focusing an image of the measurement plane in the 
vicinity of an image point conjugate with a scanning spot formed by the 
scanning optical system, the light receiving system sharing at least a 
part of the optical components of the scanning optical system, at least 
one photodetecting means disposed at prescribed positions in the vicinity 
of the image point for measuring light scattered from the in vivo tissue 
to provide blood flow information related to blood flowing at a depth 
within the in vivo tissue, video imaging means for receiving a visible 
image of the measurement plane and converting it into a video image, and 
image superimposing means for superposing information relating to the 
position of the scanning spot on the video image and displaying the video 
image and the blood flow information as a composite visible image. 
Owing to these features, the apparatus for measuring blood flow according 
to the invention enables measurement of various data relating to blood 
flow at a depth within in vivo tissue to be carried out in a totally 
contactless manner, to be accomplished in respect of a large measurement 
region simply and at high speed while monitoring the measurement position 
on the visible image, and to be conducted without the need for mechanical 
positioning. In addition, it is structurally simple and inexpensive to 
manufacture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will now be described in detail on the basis of the preferred 
embodiment illustrated in the drawings. 
FIG. 1 shows the basic arrangement of the optical system of an apparatus 
for measuring blood flow embodying the present invention. A coherent light 
source such as a laser beam emitted by a laser beam source 3 passes 
through the center hole of a perforated mirror 4 and the center of a lens 
1, is reflected in turn by mirrors 5a, 5b of a galvanoscanner 5 disposed 
for conducting two-dimensional scanning, and thus scans a measurement 
region P of in vivo tissue. 
A laser beam source which emits a near infrared beam is used so as to 
minimize the amount of the laser beam light absorbed by the in vivo 
tissue. The measurement region P is further illuminated by white light 
from a source not shown in the figures. 
A dichromatic mirror 7 disposed between the galvanoscanner 5 and the 
measurement region P transmits near infrared light and reflects light of 
other wavelengths toward a TV camera 8 so as to enable the TV camera 8 to 
pick up a visible image of the measurement region P. 
The TV camera 8 transmits the picked-up measurement region P image (of 
non-near infrared light reflected by the dichromatic mirror 7) to an image 
memory 14 for synthesizing the visible image (picture) from the TV camera 
8 and a light image obtained from a light receiving section 6 to be 
described later. 
Since the in vivo tissue constituting the measurement region P is a light 
scattering body, it emits scattered light. The scattered light from blood 
corpuscles within the tissue blood vessels is partially scattered and 
partially absorbed by the tissue. 
Light scattered by the tissue passes through the dichromatic mirror 7 and 
the lens 1 and is reflected by the perforated mirror 4 through a lens 2 to 
form a light image at a light-receiving or image plane of the light 
receiving section 6. The perforated mirror 4 is disposed between the 
lenses 1 and so as to prevent the laser beam from the laser beam source 3 
from reaching the light receiving section 6. The image focusing system 
constituted by the lens 1, the lens 2 and the perforated mirror 4 is 
required to have a focal length that is large enough to avoid image 
blurring owing to oscillation of the mirrors 5a, 5b. For this purpose, 
therefore, the focal arrangement illustrated in FIG. 3 is established as 
regards the light receiving system at the focal plane and at the object 
surface. (The galvanoscanner 5 is represented schematically in FIG. 3.) 
An example of the arrangement of the light receiving section is shown in 
FIG. 2. 
As shown in this figure, photosensors a-d are disposed at positions offset 
from the scanning spot. Differently from conventional apparatuses for 
measuring blood flows based on the laser speckle or laser Doppler 
principle, the apparatus for measuring blood flow according to this 
invention does not use optical fiber probes and, therefore, the depth of 
the measurement region is governed by the distance at the light receiving 
plane (image plane) between the scanning spot and the photosensors a-d, 
the numerical aperture of the lens 1 and the area of the sensors. 
Placing sensors at four locations as shown in FIG. 2 makes it possible to 
receive information from a single scanning spot at four points and thus 
reduces the measurement time. The blood flow distribution can be measured 
at different depths by varying the distance between the scanning spot and 
the light receiving points and the so obtained information can be 
displayed on, for example, a display 15 in the manner described below. 
The video image (picture) output by the TV camera 8 and laser beam scanning 
position information obtained from the galvanoscanner 5 are synthesized in 
the image memory 14 in accordance with the procedures indicated in FIG. 5 
and the result is output to the display 15. 
The procedures of FIG. 5 are executed by a control unit 16 (constituted as 
a computer or the like) for overall control of the apparatus for measuring 
blood flow. 
The procedure begins in step S1 in which the picture output by the TV 
camera 8 is read into the image memory 14. In the following step S2 the 
current laser beam scanning position is determined from position control 
information obtained from the galvanoscanner 5, whereafter a calculation 
is conducted in step S3 to determine what position within the picture 
output by the TV camera 8 the scanning position corresponds to. 
The procedure then moves to step S4 in which, based on the result of the 
calculation in step S3, information indicating the current scanning 
position (in the form of a dot or the like) is superimposed on the TV 
camera picture in the image memory 14 and the result is displayed on the 
display 15. If a plurality of light receiving elements (photosensors) are 
disposed at the light receiving section 6 in the aforesaid manner, the 
corresponding positions are also displayed. 
In the following step S5, blood flow data is measured in a manner to be 
explained later and, if desired, also superimposed on the TV camera 
picture appearing on the display 15. 
The procedure then moves through steps S6 to S8 for controlling the 
scanning position, and if there is a change in the scanning position the 
procedures of steps S1 to S5 are repeated. 
In such an arrangement, the beam (spot) of light output by the laser beam 
source 3 is deflected by galvanoscanner 5 so as to scan the measurement 
region P and scattered light from the measurement region P passes back 
through the lenses 1 and 2 of the scanning optical system to form a light 
image of the area in the vicinity of the scanning spot on the light 
receiving section 6. The four photosensors a-d disposed on the light 
receiving section 6 at prescribed distances from the light image of the 
scanning spot receive only that part of the scattered light reaching their 
respective positions. Using the variation in this scattered light as the 
main information regarding blood flow at the prescribed depth, it is then 
possible by analysis to determine the blood flow within the measurement 
region. 
If a beam including light of more than one wavelength is used, the 
differences among the scattered amounts of the different wavelength lights 
can be analyzed as spectral information for determining the oxygen 
saturation at prescribed points within a given plane. 
On the other hand, where a beam (spot) from a laser beam source is scanned 
in a prescribed manner and the dynamic spectral signals from the 
individual scanned points are analyzed, it becomes possible to determine 
the blood flow velocity distribution at a given depth within the in vivo 
tissue. 
As methods for analyzing oxygen saturation and blood flow velocity are well 
known, only the basic principles will be discussed briefly here. 
The oxygen saturation is determined from the amount of light the blood 
absorbs. Lambert-Beer's law defines the absorbance A of a solution in 
terms of incident light I.sub.in and transmitted light I.sub.out as 
EQU A=log (I.sub.in /I.sub.out)=ECL 
(1) 
where E is the absorbance coefficient at the wavelength of the light used, 
C is the substance concentration, I.sub.in is the amount of incident 
light, L is the length of the optical path and I.sub.out is the amount of 
transmitted light. 
In the case of blood, since the absorbance is almost totally accounted for 
by hemoglobin, C in the aforesaid equation can be considered to be the 
hemoglobin concentration. 
The coefficient of absorbance of hemoglobin varies with the saturation S 
and the wavelength as follows: 
EQU E=Er-S (Er-Eo) 
where Er and Eo are the absorbance coefficients when S=0 and S=1, 
respectively. 
Where A.sub.1 and A.sub.2 are the absorbances at wavelengths .lambda.1 and 
.lambda.2 
EQU A.sub.1 /Cl.sub.1 =E.sub.1 =Er.sub.1 -S (Er.sub.1 -Eo.sub.1) 
EQU A.sub.2 /CL.sub.2 =E.sub.2 =Er.sub.2 -S (Er.sub.2 -Eo.sub.2) 
If the length of the optical path is the same for both wavelengths, 
EQU L=L.sub.1 =L.sub.2 
and, therefore, the absorbance ratio R for .lambda.1 and .lambda.2 can be 
expressed as 
##EQU1## 
The oxygen saturation S can therefore be expressed as 
EQU S={E.sub.r1 =RE.sub.r2 }/{E.sub.r1 -E.sub.o1)-R(E.sub.r2 -E.sub.o2)} 
whereby the oxygen saturation can be obtained from the ratio of the 
absorbances at two different wavelengths. 
The oxygen saturation can therefore be determined by irradiating the tissue 
with light of two different wavelengths in the aforesaid manner. 
Blood flow velocity is measured by utilizing the fact that, when 
erythrocytes are irradiated by a laser beam, the frequency at which the 
intensity of the light scattered thereby varies is a function of their 
velocity. The light scattered by erythrocytes moving within an in vivo 
blood vessel forms what is referred to as a "boiling speckle" pattern. It 
is known that when the intensity variation of such speckles is frequency 
analyzed with respect to a given light receiving region on the tissue 
surface, the velocity of the erythrocytes is found to be related to the 
distribution of the power spectrum as shown in FIGS. 6a and 6b. When the 
erythrocyte velocity is high, the spectral distribution extends to higher 
frequencies. Taking advantage of this fact, the blood flow velocity is 
determined from the gradient of the power spectrum. Nohira, Shintomi, 
Ohura, Fujii, Asakura et al. determined blood flow velocity by finding the 
ratio of the absolute values of 40 Hz and 640 Hz signals. (See, for 
example, Evaluation of skin blood flow using laser speckle phenomena (III) 
and (IV), The Journal of Japan Society for Laser Medicine, Vol. 5, No. 3.) 
In the present invention, since the galvanoscanner 5 is used both for 
scanning and as part of the light receiving system, the received light 
field is scanned simultaneously with the scanning spot. The embodiment of 
the invention described earlier, for example, is characterized in that the 
scanning spot and photosensors can, as a combined pair, be disposed for 
contactless scanning with the photosensors maintained in a fixed interval, 
whereby it becomes possible to position the measurement point while 
visually observing the measurement region on the display 15. With the beam 
projecting fiber and beam receiving fiber of the conventional apparatus 
for measuring blood flow referred to earlier, it is not possible to 
combine the measured results with measurement point position information, 
even by conducting measurement at a large number of points. In contrast, 
the apparatus for measuring blood flow according to the present invention 
is able to provide blood flow or oxygen saturation information including 
positional information. 
Differently from conventional apparatuses of this type, it thus enables 
measurement while carrying out completely contactless positional scanning. 
Moreover, once the object with respect to which measurement is to be 
conducted and the light receiving apparatus have been positioned, it 
becomes possible by laser beam scanning to carry out the measurement 
simply and quickly while visually observing a large measurement region. In 
the embodiment described in the foregoing the simultaneous measurement of 
scattered light data associated with a single scanning spot at four light 
receiving points (more if the number of sensors is increased) enables the 
measurement to be conducted at high speed. 
In addition, since the scanning optical system and the light receiving 
optical system use some components in common, the apparatus becomes 
simpler in structure and, accordingly, less expensive to fabricate. 
While in the embodiment set out in the foregoing the light receiving 
section was explained as having four photosensors disposed around the 
scanning spot at the center, the number of photosensors and their 
locations can be changed variously in line with the desired measurement 
depth and the like. It is also possible to constitute the photosensors of 
the light receiving section as a photomultiplier tube equipped with a mask 
for defining a plurality of light receiving positions. With this 
arrangement, the number of positions of the photosensors can be easily 
varied by using different masks. 
While the invention has been described with reference to a preferred 
embodiment, it will be understood by those skilled in the art that various 
changes may be made and equivalents ay be substituted for elements thereof 
without departing from the scope of the invention. In addition, many 
modifications may be made to adapt a particular situation or material to 
the teachings of the invention without departing from the essential scope 
thereof. Therefore, it is intended that the invention should not be 
limited to the particular embodiment disclosed as the best mode 
contemplated for carrying out the invention, but that the invention will 
include all embodiments falling within the scope of the appended claims.