U.S. patent application number 14/274898 was filed with the patent office on 2014-09-04 for imaging apparatus and imaging method using optical coherence tomography.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. The applicant listed for this patent is CANON KABUSHIKI KAISHA. Invention is credited to Nobuhito Suehira.
Application Number | 20140247426 14/274898 |
Document ID | / |
Family ID | 41119939 |
Filed Date | 2014-09-04 |
United States Patent
Application |
20140247426 |
Kind Code |
A1 |
Suehira; Nobuhito |
September 4, 2014 |
IMAGING APPARATUS AND IMAGING METHOD USING OPTICAL COHERENCE
TOMOGRAPHY
Abstract
Provided is an imaging apparatus using Fourier-domain optical
coherence tomography, the imaging apparatus removing noises caused
by the autocorrelation component of returning light to obtain a
high-resolution tomographic image. A first switching unit 17
switches a first state in which returning light 12 is combined with
reference light (a state in which the returning light 12 is
conducted to a combining unit 22) and a second state different from
the first state (a state in which the light path of the returning
light 12 is blocked or changed). A controlling unit 18 controls the
switching unit 17 to change the first and the second state. A
interferometric information acquiring unit 19 acquires
interferometric information on the returning light 12 and the
reference light 14 using the reference light 14 or the returning
light 12 detected by the detecting unit 16 in the second state and
the combined light 15.
Inventors: |
Suehira; Nobuhito;
(Kawasaki-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CANON KABUSHIKI KAISHA |
Tokyo |
|
JP |
|
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
41119939 |
Appl. No.: |
14/274898 |
Filed: |
May 12, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12991734 |
Nov 9, 2010 |
8760664 |
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PCT/JP2009/062642 |
Jul 6, 2009 |
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14274898 |
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Current U.S.
Class: |
351/206 ;
351/246 |
Current CPC
Class: |
A61B 5/7257 20130101;
A61B 3/1025 20130101; A61B 5/0066 20130101; G01N 21/4795 20130101;
A61B 5/0073 20130101; A61B 3/102 20130101 |
Class at
Publication: |
351/206 ;
351/246 |
International
Class: |
A61B 3/10 20060101
A61B003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 7, 2008 |
JP |
2008-177158 |
Jun 25, 2009 |
JP |
2009-151483 |
Claims
1-16. (canceled)
17. An imaging apparatus for imaging an object using Fourier domain
optical coherence tomography, the imaging apparatus comprising: a
detecting unit for detecting combined light comprising a returning
light from the object and a reference light; a switching unit for
switching between a first state in which the detecting unit can
detect the combined light and a second state in which the detecting
unit can detect the reference light; an acquiring unit for
acquiring interferometric information on the returning light and
the reference light by using the combined light detected by the
detecting unit in the first state and the reference light detected
by the detecting unit in the second state; and a controlling unit
for controlling the switching unit such that the reference light is
detected in the second state after processing for acquiring the
interferometric information is started and the second state is
switched to the first state after the reference light is detected
in the second state.
18. The imaging apparatus according to claim 17, wherein the object
is an eye, and wherein the imaging apparatus further comprises a
scanning unit for scanning a measuring light in the retina of the
eye with a vicinity of the cornea of the eye as a fulcrum.
19. The imaging apparatus according to claim 17, wherein the
switching unit is configured such that one of a light path of a
measuring light to be irradiated on the object and a light path of
the returning light can be blocked, and wherein the second state is
in the blocked state.
20. The imaging apparatus according to claim 17, wherein the
switching unit is configured such that the transmittance of one of
a measuring light to be irradiated on the object and the returning
light can be controlled.
21. The imaging apparatus according to claim 17, further
comprising: a light quantity detecting unit for detecting the light
quantity of a measuring light to be irradiated on the object; and a
comparing unit for comparing the detected light quantity with a
predetermined value, wherein when the detected light quantity is
different from the predetermined value, the switching unit switches
to the second state by blocking the measuring light.
22. The imaging apparatus according to claim 17, wherein the
switching unit is configured such that one of the light path of a
measuring light to be irradiated on the object and the light path
of the returning light can be changed, and wherein the second state
is the changed state.
23. The imaging apparatus according to claim 17, wherein the
controlling unit controls the switching unit based on a
predetermined timing, and the combined light is detected by the
detecting unit at the timing of the first state.
24. The imaging apparatus according to claim 17, further comprising
a light quantity detecting unit for detecting the quantity of a
measuring light to be irradiated on the object, wherein the
controlling unit is configured such that the quantity of the
measuring light is detected after processing for acquiring the
interferometric information is started, the reference light is
detected in the second state after the quantity of the measuring
light is detected, and the second state is switched to the first
state after the reference light is detected in the second
state.
25. The imaging apparatus according to claim 17, further
comprising: a light source for emitting light; a splitting unit for
splitting light from the light source into the reference light and
a measuring light to be irradiated on the object; a subtracting
unit for subtracting the autocorrelation component of the reference
light and the autocorrelation component of the returning light from
the combined light detected by the detecting unit; a standardizing
unit for standardizing the subtraction result by the
autocorrelation component of the reference light; a transforming
unit for Fourier transforming the standardization result; and an
image acquiring unit for obtaining a tomographic image of the
object to be detected.
26. An imaging method for imaging an object using a Fourier domain
optical coherence tomography, the imaging method comprising:
detecting combined light comprising a returning light from the
object and a reference light; switching between a first state in
which the combined light can be detected and a second state in
which the reference light can be detected; and acquiring
interferometric information on the returning light and the
reference light by using the combined light detected in the first
state and the reference light detected in the second state, wherein
the reference light is detected in the second state after
processing for acquiring the interferometric information is started
and the second state is switched to the first state after the
reference light is detected in the second state.
27. The imaging method according to claim 26, wherein the object is
an eye, and wherein the method further comprises scanning a
measuring light in the retina of the eye with a vicinity of the
cornea of the eye as a fulcrum.
28. The imaging method according to claim 26, further comprising:
emitting light from a light source; splitting the light from the
light source into the reference light and a measuring light to be
irradiated on the object; subtracting the autocorrelation component
of the reference light and the autocorrelation component of the
returning light from the combined light; standardizing the
subtraction result by the autocorrelation component of the
reference light; Fourier-transforming the standardization result;
and obtaining a tomographic image of the object to be detected.
29. The imaging method according to claim 26, further comprising:
detecting the light quantity of a measuring light to be irradiated
on the object; comparing the detected light quantity with a
predetermined value; and switching to the second state by blocking
the measuring light when the detected light quantity is different
from the predetermined value.
30. A non-transitory computer-readable storage medium storing a
program for causing a computer to execute the imaging method
according to claim 26.
31. The imaging method according to claim 26, further comprising
detecting the light quantity of a measuring light to be irradiated
on the object, wherein the light quantity of the measuring light is
detected after processing for acquiring the interferometric
information is started, the reference light is detected in the
second state after the light quantity of the measuring light is
detected, and the second state is switched to the first state after
the reference light is detected in the second state.
32. The imaging apparatus according to claim 17, further
comprising: an image acquiring unit which acquires, from
information of a tomography image of the object based on the
intensity of the combined light, a tomography image in which
information based on the intensity of the reference light is
reduced.
33. The imaging apparatus according to claim 32, further
comprising: a subtracting unit which subtracts the information
which is based on the intensity of the autocorrelation component of
the reference light, from the information of the tomography image
of the object which is based on the intensity of the combined
light, wherein the image acquiring unit acquires the reduced
tomography image based on the subtraction result of the subtracting
unit.
34. An imaging apparatus for imaging an object a retina of an eye
using Fourier-domain optical coherence tomography, the imaging
apparatus comprising: a scanning unit for scanning a measuring
light on the retina, with a cornea of the eye as a fulcrum; a
detecting unit for detecting combined light comprising a returning
light from the retina and a reference light; a first switching unit
for switching between a first state in which the detecting unit can
detect the combined light and a second state in which the detecting
unit can detect the reference light; an interferometric information
acquiring unit for acquiring interferometric information on the
returning light and the reference light by using the combined light
detected by the detecting unit in the first state and the reference
light detected in the second state; a light quantity detecting unit
for detecting the quantity of a measuring light to be irradiated on
the retina; and a comparing unit for comparing the detected light
quantity with a predetermined value, wherein when a measuring light
quantity is different from the predetermined value, the first
switching unit switches to the second state by blocking the
measuring light.
35. An imaging method for imaging a retina of an eye using a
Fourier domain optical coherence tomography, the imaging method
comprising: scanning a measuring light on the retina, with a cornea
of the eye as a fulcrum; switching between (a) a first state in
which combined light comprising a returning light from the retina
and a reference light can be detected and (b) a second state in
which the reference light can be detected; obtaining
interferometric information on the returning light and the
reference light by using the combined light detected in the first
state and the reference light detected in the second state;
detecting the light quantity of the measuring light to be
irradiated on the retina; and changing to the second state by
blocking the measuring light when the light quantity is different
from a predetermined light quantity.
36. An imaging apparatus for imaging a retina of an eye using
Fourier-domain optical coherence tomography, comprising: a scanning
unit for scanning a measuring light on the retina, with a cornea of
the eye as a fulcrum; a detecting unit for detecting one of (a)
reference light and (b) combined light into which returning light
obtained by radiating measuring light to the retina and reference
light corresponding to a measuring light are combined; a first
switching unit for switching between a first state in which the
detecting unit can detect the combined light and a second state in
which the detecting unit can detect the reference light; a
determining unit for determining whether or not a measuring light
quantity is no more than a predetermined value; a controlling unit
for controlling the first switching unit such that in a case where
the determining unit determines that the measuring light quantity
is no more than the predetermined value, the first switching unit
switches from the second state to the first state; and a
tomographic image acquiring unit for acquiring, after the detecting
unit detects the combined light in the first state, a tomographic
image in which information based on an intensity of the reference
light is cut down from information of a tomographic image obtained
based on an intensity of the combined light.
37. The imaging apparatus according to claim 36, further
comprising: a subtraction unit for subtracting information based on
an intensity of autocorrelation component of the reference light
from the information of the tomographic image obtained based on the
intensity of the combined light, wherein the tomographic image
acquiring unit acquires the cut down tomographic image based on a
subtraction result of the subtraction unit.
38. The imaging apparatus according to claim 36, further
comprising: a variable transmittance material which is provided on
a light path of the reference light and which is capable of
changing a transmittance of the reference light, wherein the
tomographic image acquiring unit acquires the tomographic image in
which information based on an intensity of the reference light
after passing through the variable transmittance material is cut
down from information of the tomographic image obtained based on
the intensity of the combined light.
39. The imaging apparatus according to claim 38, wherein the
variable transmittance material is an ND filter.
40. The imaging apparatus according to claim 36, wherein the
controlling unit controls the first switching unit such that the
first switching unit changes a light path of the measuring light to
switch from the first state to the second state.
41. The imaging apparatus according to claim 40, wherein the
controlling unit controls the scanning unit such that the scanning
unit can deviate a measuring light from the light path of the
measuring light to switch from the first state to the second
state.
42. The imaging apparatus according to claim 36, wherein the first
switching unit comprises a shutter which is provided on a light
path of a measuring light and which is capable of shutting off the
measuring light, and wherein the controlling unit controls the
shutter such that the shutter shuts off the measuring light to
switch from the first state to the second state.
43. The imaging apparatus according to claim 36, further
comprising: a second switching unit for switching, when the first
switching unit switches to the state in which the detecting unit
can detect the combined light, between the first state and a state
in which the detecting unit can detect the returning light, wherein
the tomographic image acquiring unit acquires the tomographic image
in which information based on an intensity of autocorrelation
component of the reference light and information based on an
intensity of autocorrelation component of the returning light are
subtracted from information of the tomographic image obtained based
on the intensity of the combined light.
44. The imaging apparatus according to claim 43, wherein the second
switching unit comprises a variable transmittance unit for changing
a transmittance of the reference light, and wherein the controlling
unit controls the variable transmittance unit such that the
variable transmittance unit changes a transmittance of the
reference light to switch from the first state i to the state in
which the detecting unit can detect the returning light.
45. The imaging apparatus according to claim 44, wherein the
variable transmittance unit is an ND filter.
46. The imaging apparatus according to claim 37, further
comprising: a standardizing unit for standardizing the subtraction
result by the information based on the intensity of autocorrelation
component of the reference light; and a transforming unit for
Fourier-transforming a standardization result by the standardizing
unit, wherein the tomographic image acquiring unit acquires the cut
down tomographic image based on a transforming result of the
transforming unit.
47. The imaging apparatus according to claim 36, further
comprising: a light quantity detecting unit for detecting a
quantity of a measuring light, wherein the controlling unit
controls the first switching unit such that the first switching
unit switches, when the quantity of light detected by the light
quantity detecting unit in the state in which the detecting unit
can detect the combined light exceeds the predetermined value, from
the first state to the second state.
48. An imaging method for imaging a retina of an eye using a
Fourier-domain optical coherence tomography, the imaging method
comprising: scanning a measuring light on the retina, with a cornea
of the eye as a fulcrum; detecting, by using a detecting unit, one
of (a) reference light and (b) combined light into which returning
light obtained by radiating measuring light to the retina and
reference light corresponding to a measuring light are combined;
determining whether or not a measuring light quantity is no more
than a predetermined value; switching, in a case where in the
determining step it is determined that the measuring light quantity
is no more than the predetermined value, to a first state in which
the combined light can be detected from a second state in which the
reference light can be detected; and acquiring, after the detecting
unit detects the combined light in the first state, a tomographic
image in which information based on an intensity of the reference
light is cut down from information of a tomographic image obtained
based on an intensity of the combined light.
49. The imaging method according to claim 48, further comprising:
subtracting information based on an intensity of autocorrelation
component of the reference light from the information of the
tomographic image obtained based on the intensity of the combined
light, wherein the tomographic image acquiring acquires the cut
down tomographic image based on a result of the subtraction.
50. A non-transitory computer-readable storage medium storing a
program for causing a computer to execute the imaging method
according to claim 48.
51. The imaging apparatus according to claim 36, wherein when the
measuring light quantity exceeds the predetermined value, the
controlling unit processes error handling.
52. The imaging apparatus according to claim 51, wherein when the
measuring light quantity exceeds the predetermined value, the
controlling unit stops detection of light by the detecting unit as
the processing of error handling.
53. The imaging apparatus according to claim 51, wherein when the
measuring light quantity exceeds the predetermined value, the
controlling unit returns the scanning unit to an initial position
as the processing of error handling.
54. The imaging apparatus according to claim 51, wherein the first
switching unit comprises a reference mirror located on a light path
for the reference light, and wherein when the measuring light
quantity exceeds the predetermined value, the controlling unit
returns the reference mirror to an initial position as the
processing of error handling.
55. The imaging apparatus according to claim 36, wherein when the
measuring light quantity exceeds the predetermined value, the
controlling unit outputs an error message on a display unit.
56. The imaging apparatus according to claim 36, wherein the
controlling unit controls the first switching unit such that: when
the measuring light quantity is within a predetermined range, the
first switching unit switches from the second state to the first
state, and when, in the state which the detecting unit can detect
the combined light, the measuring light quantity is not within a
predetermined range, the first switching unit switches from the
first state to the second state.
57. The imaging apparatus according to claim 56, wherein the
predetermined range is 680 .mu.W to 700 .mu.W.
58. The imaging apparatus according to claim 36, wherein the
predetermined value is 700 .mu.W.
59. (canceled)
60. The imaging apparatus according to claim 47, further comprising
a splitting unit for splitting the measuring light into two lights,
wherein the light quantity detecting unit detects a quantity of one
of the lights as the quantity of the measuring light.
61. The imaging method according to claim 48, further comprising
switching, when the measuring light quantity exceeds the
predetermined value, from the first state to the second state.
62. The imaging method according to claim 48, further comprising
processing and error handling when the measuring light quantity
exceeds the predetermined value.
63. The imaging method according to claim 48, further comprising
outputting an error message on a display unit when the measuring
light quantity exceeds the predetermined value.
64. The imaging method according to claim 48, wherein the
predetermined value is 700 .mu.W.
65. (canceled)
66. A non-transitory computer-readable storage medium storing a
program for causing a computer to execute the imaging method
according to claim 35.
67. A non-transitory program causing a computer to execute the
imaging method according to claim 35.
68. The imaging apparatus according to claim 17, wherein the first
state, the second state, and a third state are switched such that
the first state is followed by the second state and the third
state.
69. The imaging apparatus according to claim 17, wherein the first
state, the second state, and a third state are switched such that
the first state, the second state, and the third state are
continuously and repeatedly changed.
70. The imaging method according to claim 26, wherein the
controlling unit is configured to control the first switching unit
and the second switching unit such that the first state is followed
by the second state and a third state.
71. The imaging method according to claim 26, wherein the
controlling unit is configured to control the first switching unit
and the second switching unit so as to continuously and repeatedly
change the first state, the second state, and a third state.
Description
TECHNICAL FIELD
[0001] The present invention relates to an imaging apparatus and an
imaging method using optical coherence tomography, specifically for
an imaging apparatus and an imaging method used for observing the
eye and the skin using the optical coherence tomography.
BACKGROUND ART
[0002] There has been put into practical use an imaging apparatus
using optical coherence tomography (OCT) taking advantage of
coherence of low coherence light (hereinafter referred to as OCT
apparatus). The imaging apparatus can obtain a tomographic image
with resolution on the order of a wavelength of light incident on
an object, which enables providing a high-resolution tomographic
image of an object to be detected.
[0003] Light from a light source is split into measuring light and
reference light by a splitting light path such as a beam splitter.
An object to be detected such as the eyes is first irradiated with
the measuring light through a measuring light path. The light
returning from the object to be detected is then conducted to a
detecting position through a detecting light path. The term
"returning light" refers to reflected light and scattered light
including information on an interface in the direction in which the
object to be detected is irradiated with light. The reference light
is reflected by a reference mirror through a reference light path
to conduct the reflected light to the detecting position. The
detection and analysis of the coherent light of the returning light
and the reference light with a detector provides a tomographic
image of the object to be detected.
[0004] Japanese Patent Application Laid-Open No. H11-325849
discloses an OCT configuration in which the position of a reference
mirror is discontinuously changed three times in measuring one
point of an object to be detected to obtain wavelength spectra and
then the calculation of the spectra provides a tomographic
image.
[0005] Furthermore, there is disclosed a Fourier-Domain OCT
apparatus (hereinafter referred to as FD-OCT apparatus) in which
wavelength spectra are obtained with a reference mirror of the OCT
apparatus fixed and a tomogram is measured by Fourier
transformation, in a paper; A. F. Fercher, C. K. Hitzenberger, G.
Kamp, S. Y. El-Zaiat, Opt. Commun. 117, 43-48, (1995). The FD-OCT
apparatus includes a system using a spectroscope (SD-OCT:Spectral
Domain OCT) and a system sweeping the wavelength of a light source
(SS-OCT:Source Swept-OCT).
DISCLOSURE OF THE INVENTION
[0006] It requires more time for the OCT apparatus disclosed in
Japanese Patent Application Laid-Open No. H11-325849 to perform
measurement than for the FD-OCT apparatus capable of obtaining
tomographic images collectively in the depth direction, because the
OCT apparatus moves the reference mirror.
[0007] On the other hand, the FD-OCT apparatus can use the fixed
reference mirror in collectively obtaining tomographic images in
the depth direction. However, the autocorrelation components of the
reference light and the returning light are included in the
combined light of the reference light and the returning light as
noise. For avoiding this, the reference mirror located in an
optically equivalent position may be away from the object to be
detected to be isolated from these components. Also for avoiding
this, coherence gate may be away from the object to be detected in
order to be isolated from those components.
[0008] However, the reference mirror located in an optically
equivalent position being away from the object sometimes decreases
measuring sensitivity (sensitivity of the sensor), and the
autocorrelation components of the reference light and the returning
light need to be removed from the combined light to perform high
accuracy measurement. In particular, the autocorrelation component
of the returning light is varied with locations to be measured, so
that the autocorrelation component of the returning light needs to
be sequentially obtained to be removed from the combined light.
[0009] The present invention has been made in view of the above
problems and it is an object of the present invention to provide an
optical tomographic imaging apparatus configured in the following
manner and a method of imaging an optical tomographic image.
[0010] According to the present invention, an optical tomographic
imaging apparatus is provided which splits light from a light
source into measuring light and reference light, conducts the
measuring light to an object to be detected through a measuring
light path and conducts the reference light to a reference mirror
through a reference light path and images a tomographic image of
the object to be detected using returning light based on the
measuring light reflected or scattered by the object to be
detected, the reference light reflected by the reference mirror and
combined light based on the returning light and the reference light
including: units for controlling the transmittance of light
arranged in each of the measuring light path and the reference
light path; a controlling unit for controlling time interval of
change in the transmittance of light in the units for controlling
the transmittance of light based on a set profile; a unit for
obtaining wavelength spectra data of each of the returning light,
reference light and combined light which is based on light from the
light source and obtained by the control of the time interval based
on the profile; and a calculating unit in which wavelength spectra
data of the obtained returning light, reference light and combined
light is used to calculate at least any of the optical
components.
[0011] A method of imaging an optical tomographic image according
to the present invention includes the steps of: splitting light
from a light source into measuring light and reference light,
conducting the measuring light to an object to be detected and
conducting the reference light to a reference mirror; and imaging a
tomographic image of the object to be detected using returning
light based on the measuring light reflected or scattered by the
object to be detected, the reference light reflected by the
reference mirror and combined light based on the returning light
and the reference light; and in that units for controlling the
transmittance of light arranged in each of a measuring light path
for conducing the measuring light and a reference light path for
conducting the reference light are controlled based on a profile in
which a time interval is set to obtain the returning light, the
reference light and the combined light based on the light from the
light source, and wavelength spectra data acquired from each of the
obtained returning light, reference light and combined light is
used to calculate at least any of the optical components.
[0012] Another imaging apparatus using Fourier-domain optical
coherence tomography includes:
[0013] a light source for emitting light;
[0014] a splitting unit for splitting light from the light source
into reference light and measuring light;
[0015] a detecting unit for detecting combined light into which the
returning light obtained by radiating the measuring light to an
object to be detected and the reference light are combined;
[0016] one of a first switching unit for switching between a first
state in which the detecting unit can detect the combined light and
a second state in which the detecting unit can detect the reference
light, and a second switching unit for switching between the first
state and a third state in which the detecting unit can detect the
measuring light;
[0017] a controlling unit for controlling one of the first
switching unit to switch between the first state and the second
state, and the second switching unit to switch between the first
state and the third state; and
[0018] a interferometric information acquiring unit for acquiring
interferometric information on the returning light and the
reference light by using the combined light detected by the
detecting unit in the first state and one of the reference light
detected in the second state and the measuring light detected in
the third state.
[0019] Another imaging method using a Fourier domain optical
coherence tomography includes:
[0020] generating a light;
[0021] splitting the light into a reference light and a measuring
light;
[0022] irradiating the measuring light to an object to be
detected;
[0023] detecting the combined light obtained by combining the
reference light and the returning light obtained by the
irradiation;
[0024] switching between a first state in which the combined light
can be detected and a second state in which the reference light can
be detected, or between the first state and a third state in which
the measuring light can be detected; and
[0025] obtaining interferometric information on the returning light
and the reference light by using the combined light detected in the
first state and one of the reference light detected in the second
state and the measuring light detected in the third state.
[0026] According to the present invention, the removal of the
autocorrelation component of the reference light allows providing a
high accuracy tomographic image.
[0027] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIGS. 1A and 1B are schematic diagrams for describing
imaging apparatus using optical coherence tomography according to
the present embodiment.
[0029] FIGS. 2A and 2B are schematic diagrams for describing
optical systems of the imaging apparatus using the optical
coherence tomography according to examples 1 and 2.
[0030] FIGS. 3A, 3B, 3C, 3D and 3E are timing charts for describing
control profiles for reference and returning lights in examples 1
to 3.
[0031] FIG. 4 is a flow chart for measurement in the example 1.
[0032] FIG. 5 is a schematic diagram for describing the structure
of tomogram and a relationship of reflectivity and transmittance in
the example 1.
[0033] FIG. 6 is a flow chart of measurement in the example 2.
[0034] FIG. 7 is a schematic diagram for describing an optical
system for an image apparatus using optical coherence tomography in
the example 3.
[0035] FIG. 8 is a flow chart of measurement in the example 3.
BEST MODES FOR CARRYING OUT THE INVENTION
First Embodiment
[0036] An imaging apparatus using a Fourier domain optical
coherence tomography according to the present embodiment is
described below with reference to FIGS. 1A and 1B. FIG. 1A
illustrates a Michelson type interferometer and FIG. 1B illustrates
a Mach-Zehnder type interferometer. In FIG. 1A, a splitting unit 21
and a combining unit 22 is configured of common members. In FIG.
1B, on the other hand, the splitting unit 21 and the combining unit
22 are configured of different members.
[0037] The present invention is not limited to the apparatus
presented by either of FIGS. 1A and 1B in which switching unit 17
(also referred as the first switching unit) such as a shutter is
located on the measuring light pass. The switching unit (also
referred as the second switching) such as shutter may also be
located on the reference light pass. The apparatus may include only
one of the first switching unit and the second switching unit, and
the apparatus may include both of them.
[0038] The FD-OCT (Fourier Domain OCT) includes two types that is
SD-OCT (Spectral Domain OCT) and SS-OCT (Source Swept-OCT).
Although in this embodiment, the following explanation is
accompanied with SD-OCT type, the invention may also include the
apparatus using SS-OCT.
[0039] (Imaging Apparatus)
[0040] A light source 20 generates light (low coherence light). A
super luminescent diode (SLD) can be applied to the light source
20. An amplified spontaneous emission (ASE) can also be applied to
the light source 20. In addition, ultrashort pulse laser such as
titanium sapphire laser can also be applied to the light source 20.
Any others that can generate low coherence light may be applied to
the light source 20. The wavelength of light generated from the
light source 20 is 400 nm to 2 .mu.m, which is not limited in
particular. The resolution to the depth direction increases as the
bandwidth of the wavelength becomes wide. Generally, when the
central wavelength is 850 nm, the resolution is 6 .mu.m within the
wavelength of 50 nm and the resolution is 3 .mu.m within the
wavelength of 100 nm.
[0041] A splitting unit 21 splits light from the light source 20
into reference light 14 and measuring light 23. A beam splitter or
a fiber coupler can be applied to the splitting unit 21. Any others
that can split light may be applied to the splitting unit 21. A
ratio of the splitting may also be selected as proper to the object
to be detected.
[0042] A detecting unit (spectroscope) 16 detects combined light 15
of returning light 12 obtained by radiating the measuring light 23
to the object to be detected 11 (including living body parts such
as eye of fundus) and the reference light 14 (reflected by a
reference unit 13 such as mirrors to be set on the reference light
path. The measuring light 23 may be radiated to the object to be
detected 11 by an optical unit such as rens located on the
measuring light path. The detecting unit 16 has a spectroscopic
device for dispersing the combined light 15 (for example, a prism
109 in FIG. 2A). The spectroscopic device is a diffraction grating
or a prism and any others that can disperse light may be used. The
detecting unit 16 has a sensor (for example, an imaging device 110
in FIG. 2A) for detecting the light dispersed by the spectroscopic
device. The sensor is a line sensor or a two dimensional sensor and
any others that can detect light may be used.
[0043] A switching unit 17 switches between a first state in which
the detecting unit 16 can detect the combined light 15 (a state in
which the returning light 12 is guided to the combining unit 22)
and a second state in which the detecting unit can detect the
reference light 14. The switching unit 17 is also referred as a
first switching unit in which the first switching unit is located
on the measuring light path. A switching unit of this embodiment
may be located on the reference light path as described above,
which is referred as a second switching unit. The first switching
unit and the second switching unit may be located on the same
apparatus. The second switching unit can switch between the first
state and a third state in which the detecting unit 16 can detect
the measuring light.
[0044] The switching unit 17 is desirably configured such that the
light path of the measuring light 23 or the returning light 12 can
be blocked. At this point, the second state is in a blocked state.
The switching unit 17 is desirably configured such that the
transmittance of the measuring light 23 or the returning light 12
can be controlled. In those cases, a shutter (described later) can
be applied to the switching unit 17, but any others that can block
the light path may be applied.
[0045] The switching unit 17 may be configured such that the light
path of the measuring light 23 or the returning light 12 can be
changed. At this point, the second state is in the changed state.
In this case, an optical scanning unit (for example, an XY scanner
104 in FIG. 2A) for scanning the measuring light 23 on the object
to be detected 11, for example, can be applied to the switching
unit 17, but any others that can change the light path may be
applied.
[0046] A controlling unit 18 (the first switching unit) controls
the switching unit 17 to change the first state and the second
state. The controlling unit 18 can control the switching unit (the
second switching unit) to switch between the first state and the
third state. The controlling unit 18 may control both of the first
switching unit and the second switching unit to switch the first
state, the second state and the third state. The controlling unit
18 may include two controlling device to control both of the first
switching unit and the second switching unit, and may be configured
on a single device to control the first switching unit and the
second switching unit. Desirably, the controlling unit 18 controls
the switching unit 17 based on a predetermined timing (for example,
FIGS. 3A to 3E). At this point, the combined light 15 is detected
by the detecting unit 16 at the timing of the first state.
[0047] An interferometric information acquiring unit 19 acquires
interferometric information (coherence components, an equation 8
described later) on the returning light 12 and the reference light
14 using the reference light 14 detected by the detecting unit 16
in the second state and the combined light 15. It is desirable that
the autocorrelation component (an equation 1 described later) of
the reference light 14 and the autocorrelation component (an
equation 2 described later) of the returning light 12 are
subtracted from the combined light 15 (an equation 7 described
later) detected by the detecting unit 16. Desirably, the
subtraction result (an equation 8 described later) is standardized
by the autocorrelation component of the reference light 14. The
Fourier transformation of the standardization result (an equation 9
described later) provides the tomographic image of the object to be
detected 11.
[0048] This allows sequentially providing the returning light 12
which may vary with locations to be measured. Thereby, the
autocorrelation component of the returning light 12 can be removed
from the combined light 15.
[0049] The autocorrelation component is a component by the
reference light itself or by the returning light itself, which is
included in addition to the coherence component of reference light
and the returning light. The combined light of the reference light
and the returning light include the autocorrelation component in
addition to their coherence component. In order to detect the
coherent component, the autocorrelation component of the reference
light, which is a component having relatively large amount of light
quantity compared to the coherence component, is desirably
subtracted from the combined light.
[0050] The OCT apparatus desirably includes a light quantity
detecting unit (for example, a detector 803 in FIG. 7) for
detecting the quantity of the measuring light and a comparing unit
(not shown) for comparing the detected light quantity with a
predetermined value. When the detected light quantity is different
from the predetermined value (predetermined light quantity) or the
detected light quantity is out of the range of the predetermined
light quantity, the switching unit 17 desirably switches the first
state to the second state. As a result, when the light quantity of
the measuring light is different from the predetermined light
quantity (or is out of the range of the predetermined light
quantity), the measuring light is not radiated out of the imaging
apparatus (described later in an embodiment 3).
[0051] (Imaging Method)
[0052] An imaging method using a Fourier domain optical coherence
tomography according to the present embodiment at least includes
the steps described below.
[0053] a) a step of generating a light.
[0054] b) a step of splitting the light into a reference light and
a measuring light.
[0055] c) a step of irradiating the measuring light to an object to
be detected.
[0056] d) a step of detecting the combined light obtained by
combining the reference light and the returning light obtained by
the irradiation.
[0057] e) a step of switching between a first state in which the
detecting unit 16 can detect the combined light and a second state
in which the detecting unit can detect the reference light, or
between the first state and a third state in which the detecting
unit 16 can detect the measuring light.
[0058] f) a step of obtaining interferometric information on the
returning light and the reference light by using the combined light
detected in the first state and one of the reference light detected
in the second state and the measuring light detected in the third
state.
[0059] By using this method, the autocorrelation component of the
reference light or the returning light is removed from the combined
light, thereby enabling an obtaining of high accuracy
interferometric information.
[0060] In the step e), the changing may be among the first state,
the second state and the third state.
[0061] In order to obtain still accurate tomographic information,
the method desirably includes the following steps:
[0062] g) a step of subtracting the autocorrelation component of
the reference light and the autocorrelation component of the
returning light from the combined light.
[0063] h) a step of standardizing the subtracted value by the
autocorrelation component of the reference light.
[0064] i) a step of Fourier transforming the standardized
result.
[0065] j) a step of obtaining the tomographic image of the object
to be detected.
[0066] Above steps g) to j) are desirably executed by a subtracting
unit, a standardizing unit, a transforming unit, and a tomographic
image acquiring unit, respectively. Each of the above unit is not
necessarily divided into different processor such as CPU, but a
single processor may include each of them.
[0067] When the method includes further steps below, the measuring
light may not be radiated out of the imaging apparatus when the
light quantity of the measuring light is different from the
predetermined light quantity (or out of the range of the
predetermined light quantity):
[0068] k) a step of detecting the light quantity of the measuring
light.
[0069] l) a step to change to the second state when the light
quantity is different from the predetermined light quantity.
Second Embodiment
[0070] An optical tomographic imaging apparatus according to
another embodiment is described below with reference to FIG. 2A.
Light from a light source 101 through a splitting light path is
split into measuring light 112 and reference light 114. The
measuring light 112 is conducted to the object 106 to be detected
through a measuring light path and a returning light 113 of the
measuring light reflected or scattered by the object to be detected
106 is conducted to a detection position through a detecting light
path. On the other hand, the reference light 114 is conducted to a
reference mirror 115 through a reference light path and the
reference light reflected by the reference mirror 115 is conducted
to the detection position. The use of combined light of the
returning light 113 and the reference light 114 which are conducted
to the detection position images the tomographic image of the
object to be detected. At this point, units 117-1 and 117-2 for
controlling the transmittance of light are arranged on the
reference light path and the measuring light path respectively. The
unit for controlling the transmittance of light is configured to
control time interval of change in the transmittance of light based
on the profile set by a control unit 111. A unit for acquiring
wavelength spectrum data 108 is configured to acquire the
wavelength spectrum data of the returning light 113, the reference
light 114 and the combined light based on the light from the light
source that are acquired by controlling time interval based on the
profile. A calculating unit 111 is configured to use the wavelength
spectrum data of the acquired returning light, the reference light
and the combined light to calculate at least any of those optical
components.
[0071] The units 117-1 and 117-2 for controlling the transmittance
of light may be configured with an optical switching device for
switching the transmittance and cutoff of light. In addition, the
optical switching device may be configured with either of a
mechanical or an electrical shutter. The mechanical or the
electrical shutter may be configured to enable the transmittance of
light to be controlled. The control unit 111 may be configured to
enable control based on the profile in which the acquiring time for
the returning light is set longer than the acquiring time for the
reference light. Furthermore, the control unit 111 may be adapted
to include an optical amplifier (for example, an optical amplifier
517 in FIG. 2B) for amplifying the returning light.
[0072] A method of imaging an optical tomographic image according
to the present embodiment can be configured as follows. The
foregoing units 117-1 and 117-2 for controlling the transmittance
of light are controlled based on the profile with the set time
interval to obtain the returning light 113, the reference light 114
and the combined light which are based on the light from the light
source. The wavelength spectrum data obtained from the acquired
returning light, the reference light and the combined light is used
to calculate at least any of those optical components.
[0073] In calculating the optical component, the wavelength
spectrum data obtained from the acquired returning light, the
reference light and the combined light is used to enable the
subtraction of the autocorrelation components of the reference
light and the returning light from the component of the combined
light. Furthermore, in calculating a component of the light, the
above subtraction result may be configured to be divided by the
autocorrelation component of the reference light. Still
furthermore, in calculating a component of the light, the above
division result may be configured to be divided by wavelength
dispersion in the optical amplifier used for amplifying the
returning light. The wavelength spectrum data obtained from the
acquired returning light, the reference light and the combined
light may be used to form an image without having depth
resolution.
[0074] According to the optical tomographic imaging apparatus and
the method of imaging an optical tomographic image according to the
present embodiment, measurement can be performed while noises
caused by the autocorrelation components of the reference light and
the returning light are sequentially being removed according to a
position of an object to be detected, providing a high-resolution
tomographic image.
[Storage Medium and Program]
[0075] As another embodiment, the above imaging method according to
the present embodiment may be stored in a computer-readable storage
medium (for example, a flexible disk, a hard disk, an optical disk,
a magneto-optical disk, a CD-ROM, a CD-R, a magnetic tape, a
nonvolatile memory card, a ROM, an EEPROM, a blue ray disk and the
like) as a program to be executed by a computer. As further another
embodiment, there may be a program for causing a computer to
execute the above imaging method.
EXAMPLES
[0076] Examples of the present invention are described below.
Example 1
Michelson Type Interferometer
[0077] There is described an example of an imaging apparatus (or an
optical tomographic imaging apparatus) using optical coherence
tomography according to the example 1 with reference to FIG. 2A.
The optical tomographic imaging apparatus of the present example
forms a Michelson type interferometer as a whole and uses a
mechanical mechanism in a part of controlling light.
[0078] Light emitted from a light source 101 passes through a lens
102 and is split into the measuring light 112 and the reference
light 114 by a beam splitter 103. The measuring light 112 reaches
the object 106 to be detected 106 through the XY scanner 104 and an
object lens 105. A light transmissive film is arranged on the
object 106 to be detected. The returning light 113 scattered and
reflected by the surface and interface of the film returns through
the object lens 105, the XY scanner 104 and the beam splitter 103
in this order and reaches a spectroscope 108 through an imaging
lens 107. On the other hand, the reference light 114 is reflected
by the reference mirror 115. The reference mirror 115 can adjust
optical path length with the aid of a position adjusting device
116. The reference light 114 is added to the returning light by the
beam splitter 103. The reference light 114 and the returning light
113 can be cut off by rotary shutters 117-1 and 117-2 respectively.
The shutters 117-1 and 117-2 can continuously control the
transmittance and cutoff of light by the use of a control unit (not
shown). Of course the shutter does not need to be of a rotary type.
A slide shutter may be movably arranged on the light path.
[0079] The light source 101 uses a super luminescent diode (SLD)
which is a typical low-coherent light source. The center wavelength
thereof is 830 nm, for example, and the band width is 50 nm. The
band width is an important factor because it affects the resolution
in the optical axis of the tomographic image to be obtained.
Although the SLD has been selected as the light source, any others
that can emit low-coherent light may be used, e.g. an amplified
spontaneous emission (ASE) light source may be used. Of course
other light sources such as a halogen lamp and the like may be used
depending upon the contents of the object to be detected. However,
the wavelength affects the resolution in the lateral direction of
the tomographic image to be obtained, so that it is desirable to
use a short wavelength in attaching importance to lateral
resolution.
[0080] The spectroscope 108 includes the prism 109 and the imaging
device 110 and disperses the returning light 113, the reference
light 114 and the combined light, respectively. The dispersed light
is captured by the imaging device in the spectroscope 108 as the
spectrum data of wavelength. The spectrum data of wavelength imaged
by the imaging device is analyzed by a computer 111. The computer
111 has functions not only to analyze the data, but also to issue
instructions for storing data, displaying an image and measuring
data. The XY scanner 104 raster-scans the measuring light 112 over
the object 106 to be detected in the direction perpendicular to the
optical axis with the aid of computer control to obtain the cross
section image of the object 106 to be detected.
[0081] A time profile of transmittance of the reference light 114
and the returning light 113 in the present example is described
below with reference to FIGS. 3A and 3B. FIGS. 3A and 3B are timing
charts for describing examples of time profiles of transmittance of
the reference light 114 and the returning light 113 respectively,
where transmittance is 0% or 100% in the examples.
[0082] For the rotary shutter, a disk in the same circumference of
which three holes are made is used and a blocking member is
arranged in one hole. The rotation of the disk at constant speed
provides such a profile. The reference light, the returning light
and the combined light are obtained by intervals 301, 302 and 303
illustrated in FIG. 3A respectively. Of course, the computer 111
controls the spectroscope 108 and the XY scanner 104 based on the
set control profile.
[0083] Transmittance does not need to be 100% and may be changed in
the case where only the reference light is measured and where the
combined light is measured. For the rotary shutter, for example,
the use of a ND filter in the hole of the disk enables
transmittance to be changed. If the use of the ND filter changes
optical path length, a glass member which is the same in optical
path length is arranged in the hole of the other disk. The time
intervals 301, 302 and 303 do not need to be equal to one another.
Since the reference light is large in amount, the time intervals
can be short. The use of a slide shutter allows optionally changing
the time interval of the shutter. If the wavelength spectrum of the
light source is temporally stable, a profile for continuously
obtaining the returning light and the combined light can be
accepted. In this case, the wavelength spectrum of the light source
needs to be previously obtained.
[0084] Processing behavior in the present example is described
below with reference to FIG. 4. FIG. 4 is a flow chart for
describing the processing behavior in the present example.
[0085] In step S1, measurement is started.
[0086] In step S2, the reference light 114 is obtained by the
imaging device 110 while the measuring light 112 is cutoff by the
shutter 117-2 and stored in a memory.
[0087] Intensity I.sub.r(k) being the autocorrelation component of
the reference light 114 is expressed by an equation 1 with a wave
number k and the electric field of the reference light as
E.sub.r(k).
I.sub.r(k)={E.sub.r(k)}.sup.2 (Equation 1)
[0088] Of course, a general spectroscope acquires the spectrum of a
wavelength .lamda..
[0089] The spectrum of the wavelength .lamda. is converted to the
spectrum of a wave number by using the relationship of the wave
number k to the wavelength .lamda. being k=2.pi./.lamda.. It is
desirable to resample a spectrum equally spaced in the wave number
in consideration of Fourier transformation in the later
process.
[0090] Herein, the spectrum is considered to be equally spaced in
the wave number. If the spectrum of the reference light is not
temporally changed, the reference light 114 may be previously
obtained and stored in the memory.
[0091] In step S3, the returning light 113 is obtained by the
imaging device 110 while the reference light 114 is cutoff by the
shutter 117-1.
[0092] There is determined the relationship among the intensity
I.sub.s(k) of the returning light 113, the electric field
E.sub.s(k) of the returning light 113 and the electric field
E.sub.r(k) of the reference light 114.
[0093] As illustrated in FIG. 5, the spatial distance of i-th
interface away from a position 401 of an equivalent reference
mirror 401 is taken as z.sub.i, a reflectivity of the interface is
taken as R(z.sub.i) and a round-trip transmittance between the
reference mirror and the position z.sub.i is taken as
T(z.sub.i).
[0094] The use of an average refractive index to the position
z.sub.i taken as n(z.sub.i) allows representing an intensity
I.sub.s(k) being the autocorrelation component of the returning
light 113 by an equation 2.
[0095] Incidentally, herein, it is assumed that the reference light
114 and the measuring light 112 are equally split and the spectra
thereof are equal to each other. An imaginary unit is denoted by
"j".
I s ( k ) = { E s ( k ) } 2 = { z i T ( z i ) R ( z i ) - j2 kn ( z
i ) z i E r ( k ) } 2 ( Equation 2 ) ##EQU00001##
[0096] Thus, the spectrum of the returning light 113 has
information in the depth direction.
[0097] The depth information varies with the XY coordinate of the
object to be detected and measured, so that it is required for
obtaining two or three dimensional image.
[0098] Incidentally, the integration of the equation 2 with respect
to a desired wave number provides the intensity signal of the
entire returning light 113 without depth resolution.
[0099] This can be used as a confocal laser scanning ophthalmoscopy
image. The equation 2 may be imaged only by a specific wave number
without integration.
[0100] There is described below the reflectivity R(z.sub.i) and the
round-trip transmittance T(z.sub.i) between the reference mirror
located in an optically equivalent position and the position
z.sub.i.
[0101] A reflectivity and a transmittance at an interface in the
case where light is incident on a substance with a refractive index
n.sub.t from a substance with a refractive index n, can be
represented by equations 3 and 4 respectively.
[0102] When the refractive index is not changed, the reflectivity
is 0 and the transmittance is 1
[0103] The phase is reversed at a negative reflectivity.
[0104] For the sake of simplicity, there is neglected the influence
of absorption and multiple reflection in a medium.
r ( z st ) = n s - n i n s + n i ( Equation 3 ) t ( z st ) = 2 n s
n s + n i ( Equation 4 ) ##EQU00002##
[0105] The examples of the reflectivity R(z.sub.i) and the
round-trip transmittance T(z.sub.i) are illustrated by using
tomogram with structure as illustrated in FIG. 5.
[0106] The tomogram in this case is formed of a first interface
402, a second interface 403 and a third interface 404. In addition,
the reference mirror 401 is illustrated in an optically equivalent
position.
[0107] The refractive index of the reference mirror located in an
optically equivalent position and the first interface is taken as
N.sub.o, and the reflectivity and the transmittance of light
incident on the first interface from the side of the reference
mirror are taken as r.sub.01 and t.sub.01 respectively.
[0108] On the other hand, the transmittance of light incident on
the first interface from the side of the second interface is taken
as t.sub.10.
[0109] Similarly, as illustrated in FIG. 5, the refractive index of
the first interface and the second interface is taken as N.sub.1,
the reflectivity and the transmittance of light incident on the
second interface from the side of first interface are taken as
r.sub.12 and t.sub.12 respectively and the transmittance of light
incident on the second interface from the side of the third
interface is taken as t.sub.21.
[0110] The refractive index of the second interface and the third
interface is taken as N.sub.2, the reflectivity and the
transmittance of light incident on the third interface from the
side of the second interface are taken as r.sub.23 and t.sub.23
respectively and, on the other hand, the transmittance of light
incident on the third interface from the side of the fourth
interface is taken as t.sub.32. The refractive index of the third
interface and the fourth interface is taken as N.sub.3.
[0111] With the use of the above symbols, For example, the
reflectivity of light incident on the third interface from the side
of the reference mirror can be expressed by an equation 5.
R ( z 3 ) = r 23 n 2 - n 3 n 2 + n 3 ( Equation 5 )
##EQU00003##
[0112] On the other hand, the round-trip transmittance can be
obtained by multiplying together the transmittances of the
interfaces through which light passes and represented by an
equation 6.
T ( z 3 ) = t 0 1 t 12 t 21 t 10 = 2 n 0 n 0 + n 1 2 n 1 n 1 + n 2
2 n 2 n 2 + n 1 2 n 1 n 1 + n 0 ( Equation 6 ) ##EQU00004##
[0113] In step S4, the combined light I.sub.add (k) of the
reference light 114 and the returning light 113 is obtained by the
imaging device 110 while the shutter 117-1 and the shutter 117-2
does not cutoff the light. At this point, a coherence component
I.sub.rs (k) as well as the autocorrelation components of the
reference light 114 and the returning light 113 appears. The
combined light I.sub.add (k) can be represented by an equation
7.
I.sub.add(k)=I.sub.r(k)=I.sub.rs(k)+I.sub.s(k)={E.sub.r(k)+E.sub.s(k)}.s-
up.2 (Equation 7)
[0114] The subtraction of the equations 1 and 2 from the equation 7
provides the coherence component I.sub.rs (k). This can be
expressed by the following equation 8.
I rs ( k ) = E r ( k ) E s * ( k ) + E r * ( k ) E s ( k ) = z i 2
R ( z i ) T ( z i ) cos { 2 kn ( z i ) z i } E r 2 ( k ) ( Equation
8 ) ##EQU00005##
[0115] The division of the subtraction result of the equation 8 by
the autocorrelation component of the reference light 114 (equation
1) provides a standardized spectrum S.sub.rs(k) represented by the
following equation 9. This corresponds to the elimination of
influence of wavelength dispersion of the light source 101 and the
spectroscope 108.
S rs ( k ) = z i 2 R ( z i ) T ( z i ) cos { 2 kn ( z i ) z i } (
Equation 9 ) ##EQU00006##
[0116] The Fourier transformation of the equation 9 causes a signal
corresponding to R(z.sub.i)T(z.sub.i) to appear in a position of an
optical distance n(z.sub.i)z.sub.i, providing a tomographic image.
The equations 8 and 9 may be calculated after each of the desired
returning light 113, reference light 114 and combined light have
been obtained.
[0117] The results of Fourier transformation in the equation 9 are
accumulated to be used as a confocal laser scanning ophthalmoscopy
image. That is to say, actually, light returning from the object
106 is weak, so that the returning light 113 expressed by the
equation 2 sometimes cannot be measured. In such case, the
multiplication of the reference light 114 by the returning light as
expressed by the equation 7 allows the detection of the component
of the returning light 113.
[0118] In step S5, a judgment is made as to whether a desired
region has been measured. If the measurement is finished, the
process proceeds to step S10.
[0119] If the measurement is not finished, the process proceeds to
step S6. The term "desired region" refers to, for example, 512
points at a 20-.mu.m step in the X direction or 512 points at a
20-1 .mu.m step in the Y direction on the object to be detected. Of
course the distance and the number of points may be different with
an object to be detected and an apparatus.
[0120] In step S6, a judgment is made as to whether movement is
performed in the X direction.
[0121] If the movement is performed in the X direction, the process
proceeds to step S7. If not, the process proceeds to step S8.
[0122] In step S7, the movement is performed by a desired distance
in the X direction and the process proceeds to step S8.
[0123] The term "desired distance" is 20 .mu.m, for example.
[0124] In step S8, a judgment is made as to whether movement is
performed in the Y direction.
[0125] If the movement is performed in the Y direction, the process
proceeds to step S9. If not, the process proceeds to step S2.
[0126] In step S9, the movement is performed by a desired distance
in the Y direction and the process proceeds to step S2.
[0127] The term "desired distance" is 20 .mu.m, for example.
[0128] Finally, in step S10, measurement is finished.
[0129] Of course, as being an ideal measurement result, the
interface in FIG. 5 is displayed as a tomographic image.
[0130] There is described below the effect of removing the
autocorrelation components.
[0131] The combined light is expressed by the equation 7 and the
required components are expressed by the equation 8. The
autocorrelation components of the reference light 114 and the
returning light 113 are noises, so that the components need to be
removed.
[0132] In the case where an SLD is used as a general light source,
the autocorrelation component of the reference light, as an image
which gradually attenuates from a peak as its original point to a
position away from the original point, overlaps with the
tomographic image. In addition, the autocorrelation component is
folded, as shown in the equation 8, distorting the tomographic
image.
[0133] On the other hand, in the example of FIG. 5 to be described
below, the autocorrelation components of the returning light 113
interfere with each other at the first, second and third
interfaces.
[0134] Those autocorrelation components overlap the tomographic
image to produce noises. In other words, an image appears in a
position of N.sub.1 (z.sub.2-z.sub.1), N.sub.2(z.sub.3-z.sub.2),
N.sub.1 (z.sub.2-z.sub.1)+N.sub.2(z.sub.3-z.sub.2) on the image
formed at n(z.sub.i)z.sub.i.
[0135] In general, the autocorrelation component of the returning
light is more extensive than the component attributed to the
reference light.
[0136] When the autocorrelation component of the returning light is
not removed, the distance between the reference mirror located in
an optically equivalent position and the first mirror needs to be
greater than the thickness of layer of the object to be detected.
Thereby, the tomographic component can be separated from the
autocorrelation component even if a layer is displayed as if to lie
between the reference mirror 401 located in an optically equivalent
position and the first mirror 402.
[0137] The removal of the autocorrelation components of the
reference light 114 and the returning light 113 allows the
reference mirror 401 located in an optically equivalent position to
be arranged in a position near the first interface.
[0138] In general, the arrangement of the reference mirror 401
located in an optically equivalent position in a position near the
first interface enables a high sensitive measurement, which is
especially effective for the object to be detected that is low in
reflectivity like the eyes.
[0139] The division with the spectrum of the reference light 114
eliminates the distortion of a tomographic image to provide a high
resolution image.
[0140] There is conceived a method of measuring the autocorrelation
components of the reference light 114 and the returning light 113
using a plurality of spectroscopes. According to the present
example, the method using a single spectroscope is realized,
resulting in enabling with a low cost. There is no need for
considering the individual difference between spectroscopes.
Example 2
Mach-Zehnder Type Interferometer
[0141] There is described an example of an optical tomographic
imaging apparatus according to the example 2 with reference to FIG.
2B. FIG. 2B is a schematic diagram for describing an optical system
for the optical tomographic imaging apparatus in the present
example. The optical tomographic imaging apparatus of the present
example forms a Mach-Zehnder type interferometer as a whole and
uses an electrical mechanism in a part of controlling light.
[0142] In the present example, the object to be detected is the
eyes, so that the returning light is small in amount. For this
reason, the use of the optical amplifier 517 for amplifying the
returning light enables a high-speed light control.
[0143] Light emitted from a light source 501 is conducted to a lens
511-1 to 511-3 through a single mode fiber 512-1 and split into the
reference light 505 and the measuring light 506 by a beam splitter
503-1 and 503-2. The measuring light 506 is reflected or scattered
by the eye as an object to be detected 507 being the object to be
detected and returned as returning light 508. The reference light
and the returning light are incident on a spectroscope 518 through
a fiber coupler 521. Data such as wavelength spectrum obtained by
the spectroscope is captured in a computer 519. The light source
501 uses a super luminescent diode (SLD) being a typical
low-coherent light source. In consideration of measuring the eyes,
near infrared light is suitable as a wavelength.
[0144] The light path for the reference light 505 is described
below.
[0145] The reference light 505 split by the beam splitter 503-1 is
sequentially incident on mirrors 514-1, 514-2 and 514-3 to be
changed in its direction, converged by a lens 511-3 and incident on
an optical switch 516-1. There is used a directivity-coupler
optical switch in which light is switched by changing a refractive
index as the optical switch. Of course, there may be used a
Mach-Zehnder type interferometer optical switch, a gate-type
optical switch using an optical gate element capable of controlling
the transmittance of light and a total internal reflection type
optical switch using a semiconductor.
[0146] A dispersion compensation glass 515 is L1 in length, which
desirably doubles the depth of a normal eye. The dispersion
compensation glass 515 compensates dispersion of the measuring
light 506 traveling to and from the eye 507 for the reference light
505.
[0147] The length L is taken as 46 mm, which doubles the diameter
of an average Japanese eyeball of 23 mm. An electric stage 513 can
be moved in the direction indicated by an arrow in the figure and
adjust and control the light path length of the reference light
505.
[0148] The light path for the measuring light 506 is described
below.
[0149] The measuring light 506 split by the beam splitter 503-1 is
reflected by the beam splitter 503-2 and then incident on the
mirror of an XY scanner 504.
[0150] The XY scanner 504 performs a raster-scan on a retina 510 in
the direction perpendicular to the optical axis.
[0151] The center of the measuring light 506 is adjusted to
coincide with the rotation center of the mirror of the XY scanner
504.
[0152] The lenses 520-1 and 520-2 is an optical system for scanning
the retina 510 and plays a role to scan the measuring light 506
over the retina 510 with the vicinity of the cornea 509 as
fulcrum.
[0153] The lenses 520-1 and 520-2 are 50 mm and 50 mm in focal
distance respectively. The incidence of the measuring light 506 on
the eye 507 produces the returning light 508 by reflection and/or
scatter from the retina 510.
[0154] The returning light 508 passes through the optical amplifier
517, is converged by a lens 511-2 and led to the spectroscope 518
through an optical switch 516-2 and the fiber coupler 521.
[0155] The optical amplifier uses a semiconductor amplifier. Some
optical amplifiers can be used as a gate element for changing
transmittance.
[0156] The use of this as the gate-type optical switch allows the
elimination of the optical amplifier 517 and the integration of the
optical switch 516 and the fiber coupler 521.
[0157] A time profile of transmittance of the reference light and
the returning light in the present example is described below with
reference to FIGS. 3C and 3D. FIGS. 3C and 3D are profiles of the
reference light and the returning light respectively. Transmittance
is desirably 0% or 100% but a waveguide light path sometimes
generates a little loss and leakage. The reference light, the
returning light and the combined light are obtained by intervals
301, 302 and 303 illustrated in FIG. 3C respectively.
[0158] Signal processing in the present example is described below
with reference to FIG. 6. FIG. 6 is a flow chart for describing the
signal processing in the present example.
[0159] In step S1, measurement is started.
[0160] In step S2-1, a judgment is made as to whether the reference
light is obtained.
[0161] The judgment as to whether the reference light is obtained
is made based on a control profile.
[0162] If the reference light is obtained, the process proceeds to
step S2-2, and if no, the process proceeds to step S3-1.
[0163] In step S2-2, the reference light 505 is obtained by a
spectroscope 518 while the returning light 508 is cutoff by the
optical switch 516-2 and stored in a memory. Intensity I.sub.r(k)
being the autocorrelation component of the reference light is
expressed by the equation 1.
[0164] In step S3-1, a judgment is made as to whether the returning
light is obtained.
[0165] The judgment as to whether the returning light is obtained
is made based on the control profile.
[0166] If the returning light is obtained, the process proceeds to
step S3-2, and if no, the process proceeds to step S4-1.
[0167] In step S3-2, the returning light 508 is obtained by a
spectroscope 518 while the reference light 505 is cutoff by the
optical switch 516-1.
[0168] The intensity I.sub.s(k) being the autocorrelation component
of the returning light is different from that of the example 1 and
can be expressed by an equation 10 in which the equation 2 is
multiplied by a wavelength dispersion G(k) of the optical
amplifier.
I s ( k ) = { E s ( k ) } 2 = { z i R ( z i ) T ( z i ) - j 2 kn (
z i ) z i G ( k ) E r ( k ) } 2 ( Equation 10 ) ##EQU00007##
[0169] If wavelength dispersion generated due to a difference
between devices in the light path is included in G(k), a range may
be increased.
[0170] In step S4-1, a judgment is made as to whether the combined
light is obtained. The judgment as to whether the combined light is
obtained is made based on the control profile.
[0171] If the combined light is obtained, the process proceeds to
step S4-2, and if no, the process proceeds to step S5.
[0172] In step S4-2, the combined light I.sub.add(k) is obtained by
a spectroscope 518 while the reference light 505 and the returning
light 508 is cutoff by the optical switch 516-1 and 516-2,
respectively. At this point, a coherence component I.sub.rs (k) as
well as the autocorrelation components of the reference light and
the returning light appears. The subtraction of the equations 1 and
10 from the combined light I.sub.add(k) provides an equation
11.
I rs ( k ) = I rs ( k ) = E r ( k ) E s * ( k ) + E r * ( k ) E s (
k ) = z i 2 R ( z i ) T ( z i ) cos { 2 kn ( z i ) z i } G ( k ) E
r 2 ( k ) ( Equation 11 ) ##EQU00008##
[0173] The division of the equation 11 by the autocorrelation
component of the reference light (equation 1) provides a
standardized spectrum S.sub.rs(k) represented by an equation
12.
S rs ( k ) = z i 2 R ( z i ) T ( z i ) cos { 2 kn ( z i ) z i } G (
k ) ( Equation 12 ) ##EQU00009##
[0174] Since the equation 12 is different from the equation 9 in
that the equation 12 is multiplied by the wavelength dispersion
G(k) of the optical amplifier, the division of the equation 12 by
G(k) provides the equation 9 in the example 1. Incidentally, the
wavelength dispersion of the optical amplifier is previously
obtained and stored in a memory.
[0175] Specifically, a mirror, for example, is arranged instead of
the eyes, the spectra of the reference light and the returning
light are obtained and calculated to obtain the wavelength
dispersion of the optical amplifier.
[0176] In step S5, a judgment is made as to whether measurement is
finished.
[0177] If the measurement is to be finished, the process proceeds
to step S10. If no, the process proceeds to step S6.
[0178] In step S6, a judgment is made as to whether movement is
performed in the X direction.
[0179] If the movement is performed in the X direction, the process
proceeds to step S7. If not, the process proceeds to step S8.
[0180] In step S7, the movement is performed by a desired distance
in the X direction and the process proceeds to step S8.
[0181] In step S8, a judgment is made as to whether movement is
performed in the Y direction.
[0182] If the movement is performed in the Y direction, the process
proceeds to step S9. If not, the process proceeds to step S2-1.
[0183] In step S9, the movement is performed by a desired distance
in the Y direction and the process proceeds to step S2-1. If it is
finally determined in step S5 through the above steps that
measurement is finished, the process proceeds to step S10 to finish
the measurement.
[0184] In the present example, the use of an electrical optical
switch allows a high speed measurement. Furthermore, the use of the
optical amplifier enables even weak light to be measured. Still
furthermore, the calculation provides a high resolution tomographic
image at a high speed.
Example 3
Shutter
[0185] In the example 3, there is described an example of an
ophthalmic OCT apparatus using a safety shutter with reference to
FIG. 7. FIG. 7 is a schematic diagram for describing an optical
system for the optical tomographic imaging apparatus in the present
example. The same reference number refers the same component as in
FIGS. 2A and 2B, and the points different from the examples 1 and 2
are described below.
[0186] The shutter prevents the measuring light to radiate out of
the imaging apparatus when the light quantity of the measuring
light is different from the predetermined light quantity (or is out
of the range of the predetermined light quantity). A case when the
light quantity of the measuring is different from the predetermined
light quantity naturally includes a case that the light quantity of
the measuring light is smaller or greater than the predetermined
light value.
[0187] A shutter 801 opens and closes the light path through an
electrical circuit in response to instructions from the computer
111. A beam splitter 802 splits the measuring light 112 into a
light for a detector 803 and a measuring light for an object to be
measured. A detector 803 detects the quantity of light and inputs a
signal thereof to an electrical circuit 804. The detector is a
photo diode, for example, and a current-voltage converter therein
converts current to voltage and the voltage signal is input to the
electrical circuit.
[0188] The shutter includes electrical-optical, magneto-optic and
mechanical shutters. The electrical-optical shutter is formed such
that a prism electrode is arranged on an optical deflection element
of PLZT:(Pb, La) (Zr, Ti)O.sub.3. The application of voltage to the
prism electrode changes the refractive index in a prism shape to
allow the beam to be bent. Transmittance and cutoff can be switched
by the angle of the beam. The response speed ranges from several
nanoseconds to several hundred nanoseconds. The magneto-optic
shutter is formed such that a magneto-optic element wrapped with a
magnetic coil, for example, is arranged between polarizers
rectangular to each other. Current flowing into the magnetic coil
rotates the polarization plane of the magneto-optic element to
control the transmittance and cutoff of light. The response speed
ranges from several microseconds to several hundred microseconds.
The mechanical shutter changes the angle of the mirror using a MEMS
device, for example, enabling switching the transmittance and
cutoff of light. The response speed ranges from several hundred
microseconds to several milliseconds. Moving a blocking object to
and from the light path by means of a magnetic coil allows
switching the transmittance and cutoff of light. The response speed
ranges from several tens of milliseconds to several hundred
milliseconds.
[0189] A flow chart of the example 3 is described below with
reference to FIG. 8.
[0190] The process starts in step A1 and proceeds to steps A2 and
M1.
[0191] In step M1, the quantity of light is detected by the
detector 803 and monitored by the electric circuit 804. The step A2
is stand-by time until the monitor of quantity of light is surely
started and the stand-by time is several tens of milliseconds, for
example.
[0192] In step M2, a confirmation is made as to whether the
quantity of light is within a specified range and measurement is
not finished. If the above conditions are satisfied, the process
returns to step M1 (801). Note that the specified quantity of light
is determined by the standard such as ANSI, e.g. 700 .mu.W. If the
quantity of light is not within a specified range, e.g. not within
680-700 .mu.m, the process proceeds to step M3 (802). If
measurement is finished, the process proceeds to step A10 and ends
(803).
[0193] In step M3, if the shutter is opened at this moment, the
shutter is closed. Thereafter, the process proceeds to step M4 to
process error handling. The processing of error handling means that
the acquisition of data by a spectroscope is stopped by a computer
to return a scanner and a reference mirror to an initial position,
and so on. In addition, an error message is output on a screen of
the computer.
[0194] In step A3, the shutter is opened. Opening the shutter
causes the measuring light to reach the eye to be examined to cause
the returning light to reach the spectroscope, enabling the
combined light to be measured. The quantity of light is monitored
once or more in step M1 during the stand-by time in step A2.
[0195] In step A4, the scanner is moved to a desired position. The
movement of the scanner refers to scan in the X and Y directions.
The scanner is moved in the X and Y directions on the assumption
that 3D measurement is performed to obtain data of 512
points.times.512 points in the XY plane. The X direction is taken
as Fast-Axis in which a round-trip scanning is performed at a high
speed. The Y direction is taken as Slow-Axis in which a
unidirectional scanning is performed at a low speed.
[0196] In step A5, a judgment is made as to whether the state of
the shutter is changed. The change of state of the shutter changes
the state of transmittance or cutoff of light. If the change of
state is required, the process proceeds to step A6 to change state
of the shutter. If the change of state is not required, the process
proceeds to step A7.
[0197] In step A7, measurement is performed by a spectroscope 108.
When the shutter is closed, the reference light can be measured
because the returning light does not exist. The intensity of the
reference light corresponds to the equation 1. On the other hand,
when the shutter is opened, the combined light can be measured
because the returning light exists. The intensity of the combined
light corresponds to the equation 7. In general, the
autocorrelation component of the returning light from the eye to be
examined is very weak and the intensity of the returning light
(equation 2) can be regarded as zero. For this reason, the
reference light is subtracted from the combined light and the
result is divided by the autocorrelation component of the reference
light provides the equation 9. The Fourier transformation of the
equation 9 provides a tomographic image. Of course, the above
signal processing may be collectively performed after the
measurement has been finished.
[0198] In step A8, a judgment is made as to whether the desired
region has been measured. FIG. 3E illustrates the profile of the
measuring light controlled by the shutter. The figure illustrates
the profile obtained by repeating the steps A4 to A8 three times.
The reference light is measured in the interval 301. The stage is
moved in the Y direction during the interval 301, and moved back in
the X direction by a distance moved during the interval 302. The
reference light is measured once during the interval. The combined
light is measured in the interval 302. The stage is moved in the X
direction at a constant speed during the interval 302 and not moved
in the Y direction. The combined light is measured 512 times during
the interval.
[0199] In step A9, the shutter is closed. When the shutter is
originally closed, the shutter is kept as it is.
[0200] In step A10, the measurement is finished.
[0201] As seen above, in this example, a shutter to shut off the
measuring light is located on the measuring light path and the
shutter is used while reference light is detected, thereby the
stability of the measuring light during the detection is enabled
with much simple circuit configuration.
[0202] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all such modifications and
equivalent structures and functions.
[0203] This application claims priorities from Japanese Patent
Applications No. 2008-177158, filed Jul. 7, 2008 and No.
2009-151483, filed Jun. 25, 2009, which are hereby incorporated by
reference herein.
* * * * *