U.S. patent application number 15/433088 was filed with the patent office on 2017-07-13 for object information acquiring apparatus and object information acquiring method.
The applicant listed for this patent is CANON KABUSHIKI KAISHA. Invention is credited to Kazuhiko Fukutani.
Application Number | 20170196461 15/433088 |
Document ID | / |
Family ID | 46052843 |
Filed Date | 2017-07-13 |
United States Patent
Application |
20170196461 |
Kind Code |
A1 |
Fukutani; Kazuhiko |
July 13, 2017 |
OBJECT INFORMATION ACQUIRING APPARATUS AND OBJECT INFORMATION
ACQUIRING METHOD
Abstract
An object information acquiring apparatus, comprising: a
plurality of detecting elements which detect an acoustic wave
generated from an object irradiated with light and convert the
acoustic wave into detection signals; a signal determining unit
which determines a detection signal detected by a detecting element
which is not in acoustic contact with the object, of the plurality
of detecting elements; a signal acquisition unit which generates a
corrected detection signal by deleting at least a region which is
not based on an acoustic wave generated from the interior of the
object, from the determined detection signal; and an image
processor which forms image data of the object from a detection
signal detected by a detecting element which is in acoustic contact
with the object and from the corrected detection signal.
Inventors: |
Fukutani; Kazuhiko;
(Kyoto-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CANON KABUSHIKI KAISHA |
Tokyo |
|
JP |
|
|
Family ID: |
46052843 |
Appl. No.: |
15/433088 |
Filed: |
February 15, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14007386 |
Sep 25, 2013 |
9615751 |
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PCT/JP2012/002465 |
Apr 9, 2012 |
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15433088 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B 5/7203 20130101;
A61B 8/429 20130101; A61B 8/0825 20130101; G01N 21/1702 20130101;
G01N 2021/1706 20130101; A61B 5/0095 20130101; A61B 8/4281
20130101; A61B 2576/00 20130101; A61B 5/7246 20130101 |
International
Class: |
A61B 5/00 20060101
A61B005/00; G01N 21/17 20060101 G01N021/17 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 12, 2011 |
JP |
2011-088312 |
Claims
1. (canceled)
2. An apparatus for determining a detecting element which is not in
acoustic contact with an object among from a plurality of detection
elements for detecting an acoustic wave generated from the object
irradiated with light, comprising: a signal processing unit
configured to: receive digital signals converted from respective
analog time-series signals obtained by detecting the acoustic wave
with each of the plurality of detecting elements, and determine
that at least one detecting element of the plurality of detection
elements is not in acoustic contact with the object, when an
unwanted digital signal is included in the received digital signals
corresponding to the at least one detecting element, wherein said
unwanted signal corresponds to an analog time-series signal having
a signal portion whose waveform is reversed relative to a waveform
of another signal portion in the analog time-series signal
corresponding to the acoustic wave generated from the object.
3. The apparatus according to claim 2, further comprising a memory
unit which stores a predetermined template signal having a signal
portion whose waveform is reversed relative to a waveform of
another signal portion in the predetermined template signal,
wherein the unwanted digital signal is a signal whose correlation
value with the predetermined template signal is equal to or greater
than a threshold value.
4. The apparatus according to claim 3, wherein said signal
processing unit is configured to adjust the threshold value.
5. The apparatus according to claim 3, wherein said memory unit
stores the predetermined template signal created based on a signal
generated corresponding to an acoustic wave reflected by a holding
unit which holds the object.
6. A method of determining a detecting element which is not in
acoustic contact with an object among from a plurality of detection
elements for detecting an acoustic wave generated from the object
irradiated with light, comprising: receiving digital signals
converted from respective analog time-series signals obtained by
detecting the acoustic wave with each of the plurality of detecting
elements; and determining that at least one detecting element of
the plurality of detection elements is not in acoustic contact with
the object, when an unwanted digital signal is included in the
received digital signals corresponding to the at least one
detecting element, wherein the unwanted signal corresponds to an
analog time-series signal having a signal portion whose waveform is
reversed relative to a waveform of another signal portion in the
analog time-series signal corresponding to the acoustic wave
generated from the object.
7. The method according to claim 6, wherein the unwanted digital
signal is a signal whose correlation value with a predetermined
template signal is equal to or greater than a threshold value, and
wherein the predetermined template signal has a signal portion
whose waveform is reversed relative to a waveform of another signal
portion in the predetermined template signal.
8. The method according to claim 7, further comprising adjusting
the threshold value.
9. The method according to claim 7, further comprising storing the
predetermined template signal created based on a signal generated
corresponding to an acoustic wave reflected by a holding unit which
holds the object.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of application Ser. No.
14/007,386, filed Sep. 25, 2013, which is a national-stage entry of
international application PCT/JP2012/002465, filed Apr. 9, 2012. It
claims benefit of that application under 35 U.S.C. .sctn.120, and
claims benefit under 35 U.S.C. .sctn.119 of Japanese Patent
Application No. 2011-088312, filed on Apr. 12, 2011. The entire
contents of each of the mentioned prior applications are
incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to an object information
acquiring apparatus and an object information acquiring method.
BACKGROUND ART
[0003] In the medical field, active progress has been made in
research of optical imaging technology that forms an image of
information from the interior of a living organism, which is
obtained on the basis of incident light when light is irradiated
onto the living organism from a light source such as a laser. One
optical imaging technology of this kind is Photoacoustic Tomography
(PAT). In photoacoustic tomography, pulsed light generated from a
light source is irradiated onto a living organism, an acoustic wave
is generated from the living tissue which has absorbed the energy
of the pulsed light that has propagated or diffused inside the
organism, and this acoustic wave (typically, an ultrasonic wave) is
detected. In other words, using the difference in absorptivity of
the light energy in an examination site, such as a tumor, and other
tissue, the elastic wave generated when the examination site
momentarily expands upon absorbing irradiated light energy is
received with an acoustic probe (also called a probe or
transducer). By analyzing this detection signal, it is possible to
obtain an image which is directly proportional to the initial
pressure distribution or the light absorption energy density
distribution (the product of the absorption coefficient
distribution and the light amount distribution) (NPL 1).
[0004] Furthermore, by performing measurement using light of
various wavelengths, this image information can be used for
quantitative measurement of specific properties of the organism,
such as the total hemoglobin density or blood oxygen saturation, or
the like. In recent years, photoacoustic tomography of this kind
has been use to make active progress in preclinical research for
creating images of blood vessels in small animals, and clinical
research which applies this principle to the diagnosis of breast
cancer, prostate cancer, carotid artery plaque, and so on.
CITATION LIST
Non Patent Literature
[0005] [NPL 1]
[0006] "Photoacoustic imaging in biomedicine", M. Xu, L. V. Wang,
REVIEW OF SCIENTIFIC INSTRUMENT, 77, 041101, 2006
SUMMARY OF INVENTION
Technical Problem
[0007] In photoacoustic tomography, if the object under examination
is not in acoustic contact with a portion of the detection surface
of the acoustic probe, then the received data obtained by the
detecting elements of the acoustic probe in that region may include
a signal other than the acoustic wave generated inside the
organism. In cases where a signal of this kind is received, if an
image is reconstructed by using all of the detection signals
obtained, then a false image (artifact) is generated apart from the
initial sound pressure distribution or absorbed light energy
density distribution inside the object, and hence there has been a
problem in that the image is markedly degraded.
[0008] The present invention was devised in view of problems of
this kind. It is an object of the present invention to provide
technology for reducing image deterioration even in cases where an
object is not in acoustic contact with the detecting elements of
the acoustic probe.
Solution to Problem
[0009] The present invention provides an object information
acquiring apparatus, comprising:
[0010] a plurality of detecting elements which detect an acoustic
wave generated from an object irradiated with light and convert the
acoustic wave into detection signals;
[0011] a signal determining unit which determines, from the
detection signals, a detection signal detected by a detecting
element which is not in acoustic contact with the object, of the
plurality of detecting elements;
[0012] a signal acquisition unit which generates a corrected
detection signal by deleting at least a region which is not based
on an acoustic wave generated from an interior of the object, from
the detection signal detected by the detecting element which is not
in acoustic contact with the object; and
[0013] an image processor which forms image data of the object from
a detection signal detected by a detecting element which is in
acoustic contact with the object and from the corrected detection
signal.
[0014] The present invention also provides an object information
acquiring method comprising:
[0015] a step in which a plurality of detecting elements detect an
acoustic wave generated from an object irradiated with light and
convert the acoustic wave into detection signals;
[0016] a step in which a signal determining unit determines, from
the detection signals, a detection signal detected by a detecting
element which is not acoustic contact with the object, of the
plurality of detecting elements;
[0017] a step in which a signal acquisition unit generates a
corrected detection signal by deleting at least a region which is
not based on an acoustic wave generated from an interior of the
object, from the detection signal detected by the detecting element
which is not in acoustic contact with the object; and
[0018] a step in which an image processor forms image data of the
object from a detection signal detected by a detecting element
which is in acoustic contact with the object and from the corrected
detection signal.
Advantageous Effects of Invention
[0019] According to the present invention, it is possible to
provide technology for reducing image deterioration even in cases
where an object is not in acoustic contact with the detecting
elements of an acoustic probe.
[0020] 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 DRAWINGS
[0021] FIG. 1 is a diagram showing a schematic view of the
composition of a photoacoustic image-forming apparatus according to
the present invention.
[0022] FIG. 2 is a flow diagram illustrating an example of
processing of a detection signal according to the present
invention.
[0023] FIGS. 3A and 3B are schematic drawings showing examples of a
detection signal from a detecting element.
[0024] FIGS. 4A to 4C are diagrams showing a measurement object and
images obtained by measurement.
[0025] FIG. 5 is a diagram showing a schematic view of the
composition of a photoacoustic image-forming apparatus according to
the present invention.
DESCRIPTION OF EMBODIMENTS
[0026] Below, the present invention is described in more details
with reference to the drawings. Constituent elements which are the
same are in principle labeled with the same reference numerals and
description thereof is omitted here.
(Photoacoustic Image-Forming Apparatus)
[0027] The composition of a photoacoustic image-forming apparatus
relating to the present embodiment is described here with reference
to FIG. 1. The photoacoustic image-forming apparatus relating to
the present embodiment is an apparatus which creates an image of
optical characteristics value information from the interior of an
object. The optical characteristics value information generally
means the initial sound pressure distribution, the absorbed light
energy density distribution, or an absorption coefficient
distribution derived from these. As described hereinafter, the
optical characteristics value information is also called object
information, and therefore the photoacoustic image-forming
apparatus according to the present invention may also be understood
as an object information acquiring apparatus.
[0028] The photoacoustic image-forming apparatus according to the
present embodiment comprises, as basic hardware, a light source 11,
an acoustic probe 17 forming an acoustic detector, and a signal
processing unit 19. Pulsed light 12 emitted from the light source
11 is obtained by processing the light to a desired shape by an
optical system (not illustrated) such as a lens, mirror, optical
fiber, diffusion plate, or the like, and is irradiated onto the
object 13, such as a living organism. When one portion of the light
energy which has propagated inside the object 13 is absorbed by a
light absorbing body, such as a blood vessel (which consequently
forms a sound source) 14, an acoustic wave (and typically an
ultrasonic wave) 15 is generated due to thermal expansion of the
light absorbing body 14. This is also called a "photoacoustic
wave". The acoustic wave 15 is detected by detecting elements 22 of
the acoustic wave probe 17, amplified and converted to digital by a
signal acquisition system 18, and then converted to image data of
the object by a signal processor 19. Moreover, the image data is
displayed as an image on the display apparatus 20.
(Light Source 11)
[0029] If the object is a living organism, light having a specific
wavelength which is absorbed by a particular component, of the
components which constitute the living organism, is irradiated from
the light source 11. The light source may be provided in an
integrated fashion with the image-forming apparatus of the present
embodiment, or may be provided separately from the image-forming
apparatus. For the light source, it is desirable to use a pulsed
light source capable of generating a pulsed light of the order of
several nanoseconds to several hundred nanoseconds. More
specifically, in order to generate a photoacoustic wave
efficiently, a pulse width of approximately 10 nanoseconds is
used.
[0030] For the light source, it is desirable to employ a laser
since a large output can be obtained, but it is also possible to
use a light-emitting diode, or the like, instead of a laser. As a
laser, it is possible to use lasers of various types, such as a
solid-state laser, a gas laser, a dye laser, a semiconductor laser,
or the like. The irradiation timing, light intensity, and the like,
are controlled by a light source control unit, which is not
illustrated. The light source control unit is generally integrated
with the light source. In the present invention, the wavelength of
the light source used is desirably a wavelength at which the light
propagates to the interior of the object. More specifically, if the
object is a living organism, then the wavelength of the light is no
less than 500 nm and no more than 1200 nm.
(Object 13 and Light Absorbing Body 14)
[0031] These elements do not constitute a portion of the
photoacoustic image-forming apparatus of the present invention, but
are described below. The main purpose of the photoacoustic
image-forming apparatus according to the present invention is for
contrast imaging of blood vessels, diagnosis of malign tumors or
vascular disease in humans and animals, follow-up observation of
chemotherapy, and the like. Therefore, it is envisaged that the
object 13 is a living organism, and more specifically, a diagnostic
site in a human or animal, such as a breast, finger, foot, or the
like. In the case of small animals such as mice, the whole of
animal is the object rather than a particular site.
[0032] The light absorbing body 14 inside the object indicates an
object having a relatively high absorption coefficient inside the
object. Although it depends on the wavelength of the light used, if
a human being is the measurement object, then the light absorbing
body 14 may correspond to oxygenated or deoxygenated hemoglobin, or
blood vessels containing a large amount of these, or a malign tumor
which includes a large number of new blood vessels. In the present
invention, "object information" refers to the acoustic wave
generating source distribution which is produced by irradiation of
light, and this means the initial sound pressure distribution
inside the organism or an absorbed light energy density
distribution or absorption coefficient distribution derived from
these. Moreover, "object information" means the density
distribution of the substances which constitute the living tissue
(and in particular, the oxygenated and reduced hemoglobin). For
example, the density distribution of the substance is the oxygen
saturation, or the like. The object information which is formed
into an image is called "image data".
(Holding Plate 16)
[0033] A contact surface on the object 13 is flattened by the
holding plate 16 in order to couple the object acoustically with
the detecting elements 22 of the acoustic wave probe 17 over a
broad range. Normally, the holding plate 16 is used in order to
hold the object 13 or to maintain same in a uniform shape. In order
to receive the acoustic wave efficiently, it is desirable to choose
a material for the holding plate which is close to the acoustic
impedance of the object. If the object is a breast, for example,
then desirably, a plate formed in polymethyl pentene is desirable.
The shape of the plate is desirably a flat plate, but it is
possible to use any shape which enables close contact between the
acoustic wave receiver and the plate installation surface. If the
plate is a flat plate, then the thinner the plate thickness, the
better, from the viewpoint of attenuation of the acoustic wave, and
so on, but the plate should desirably have a thickness which
prevents deformation of the shape of the plate. Typically, the
plate has a thickness of approximately 5 mm to 10 mm The holding
plate 16 can be omitted in the present invention, provided that it
is possible to impart a function similar to that of the holding
plate to the detection surface of the acoustic wave probe.
(Acoustic Wave Probe 17)
[0034] The acoustic wave probe 17 is a detector which detects a
photoacoustic wave generated at the surface of an object and the
interior of an object, and the like, due to pulsed light, and the
acoustic wave probe 17 detects an acoustic wave and then converts
the wave to an analog electrical signal. Hereinafter, the acoustic
wave probe 17 is also referred to simply as a probe or a
transducer. It is possible to use an acoustic wave probe of any
kind, such as a transducer based on a piezoelectric effect, a
transducer based on light resonance, or a transducer based on
change in capacitance, or the like, provided that the probe is
capable of detecting an acoustic wave signal. In the acoustic wave
probe 17 according to the present embodiment, typically, a
plurality of detecting elements 22 are arranged one-dimensionally
or two-dimensionally. By using a multi-dimensional arrangement of
elements in this way, it is possible to detect an acoustic wave in
a plurality of locations, simultaneously, and the measurement time
can be shortened. As a result of this, it is possible to reduce the
effects of vibration of the object, and the like.
(Signal Acquisition System 18)
[0035] Desirably, the photoacoustic image-forming apparatus
according to the present embodiment has a signal acquisition system
18 which amplifies an electrical signal obtained from an acoustic
wave probe 17 and converts the electrical signal from an analog
signal to a digital signal. The signal acquisition system 18 is
typically constituted by an amplifier, an A/D converter, a FPGA
(Field Programmable Gate Array) chip, and the like. Desirably, if a
plurality of detection signals are obtained from the acoustic wave
probe 17, then the signal acquisition system 18 is able to process
this plurality of signals simultaneously. By this means, it is
possible to shorten the time taken to form an image. In the present
specification, the "detection signal" is a concept which also
includes a digital signal obtained by AD conversion from an analog
signal acquired from the acoustic wave probe 17. The detection
signal may also be called a "photoacoustic signal".
(Signal Processing Unit 19)
[0036] The main role of the signal processing unit 19 is to process
a digital signal obtained from a signal acquisition system 18, and
to then perform image reconstruction to create an image of the
optical characteristics value information from the inside of the
object. Furthermore, the signal processing unit 19 according to the
present invention detects, and reduces or deletes, unwanted
acoustic wave signals received by detecting elements in a region 21
which is not in acoustic contact with the object, in the digital
signals obtained from the signal acquisition system 18, this
process being a characteristic feature of the present invention. As
a result of this, it is possible to reduce image degradation caused
by unwanted acoustic signals of this kind.
[0037] There now follows a description of the region 21 which is
not in acoustic contact with the object referred to here. More
specifically, the region 21 is a region where the object 13 and the
detection surface of the probe do not lie in physical contact, via
the holding plate 16 or another acoustic wave transmission
material, such as gel, on a vertical line drawn from the center of
the detection surface of the detecting element 22 of the acoustic
wave probe 17, as shown in FIG. 1. In other words, this means a
region where a medium which does not readily transmit an acoustic
wave, such as air, is interposed on the vertical line drawn through
the object 13 and the detecting element 22. In FIG. 1, a detecting
element which is not in acoustic contact with the object is taken
as a detecting element 22b, and a detecting element which is in
acoustic contact with the object is taken as a detecting element
22a.
[0038] Normally, a computer such as a work station is typically
used for the signal processing unit 19, and the detection signal
processing and image reconstruction processing, and the like, are
carried out by previously programmed software. The software used in
a work station includes, for example, a signal determining module
19a, a signal processing module 19b and an image reconstructing
module 19c.
[0039] The signal determining module 19a determines a detecting
element 21a which is in a region that is not in acoustic contact
with the object, on the basis of the received signal. The signal
processing module 19b generates a corrected detection signal by
correcting the signal received by a detecting element that has been
determined as a detecting element 21a in a region 21 which is not
in acoustic contact with the object. The image reconstructing
module 19c carries out image reconstruction using this corrected
signal.
[0040] Here, the signal determining module 19a, the signal
processing module 19b and the image reconstructing module 19c are
normally handled as a computer, such as a workstation and
associated software, and therefore are often treated as a single
signal processing apparatus 19, as shown in FIG. 1. In the present
invention, the signal determining module corresponds to a signal
determining unit, the signal processing module corresponds to a
signal acquisition unit and the image reconstructing module
corresponds to an image processing unit.
[0041] The basic function of the signal determining module 19a and
the signal processing module 19b is to determine the detecting
elements 22b which are in a region that is not in acoustic contact
with the object, from the digital signal obtained by the signal
acquisition system 18. Correction processing, such as unwanted
signal reduction processing, and the like, of the data received by
the detecting elements thus determined, is then carried out. The
details of this processing method are described hereinafter. The
basic function of the image reconstructing module 19c is to form
image data based on image reconstruction, by using corrected
detection signal data obtained from the signal processing module
19b. The image reconstruction algorithm used is, for example, back
projection in a time domain or Fourier domain, as commonly used in
tomography technology. If a large amount of time is available for
reconstruction, then it is possible to use an image reconstruction
method such as an iterative method, which uses repeated processing.
Various methods can be employed as an image reconstruction
technique in PAT, as described in NPL 1. Typical image
reconstruction methods are a Fourier transform method, universal
back projection method, deconvolution method, filtered back
projection method, an iterative reconstruction method, and so on.
In the present invention, it is possible to use an image
reconstruction technique of any kind.
(Display Apparatus 20)
[0042] The display apparatus 20 is an apparatus which displays
image data output by a signal processing unit 19 and typically, a
liquid crystal display, or the like, is employed. The display
apparatus may also be provided separately from the photoacoustic
image-forming device according to the present invention.
(Detection Signal Processing)
[0043] Next, one example of a correction processing method for
unwanted signals in the detection signals, which is performed by
the signal processing unit 19 and which is a characteristic feature
of the present invention, will be described with reference to FIG.
2 and FIG. 3. The step numbers given below coincide with the step
numbers in FIG. 2.
[0044] Step (1) (Step S201): A step of analyzing the detection
signals and determining detecting elements which are not in
acoustic contact with the object.
[0045] For example, in the signal determining module 19a, detecting
elements which are not in a stat of acoustic contact with the
object are determined by using the characteristics of the signals
received from detecting elements 22b which are not in acoustic
contact with the object. FIG. 3A is one example of a signal which
is received by a detecting element 22b that is not in acoustic
contact with the object, and FIG. 3B is one example of a signal
which is received by a detecting element 22a which is in acoustic
contact with the object. The horizontal axis in the drawings
represents a sample number, and in the case of 20 MHz sampling, one
measurement is performed every 50 nanoseconds. The vertical axis
represents the intensity of the received acoustic wave. In other
words, in the case of 20 MHz sampling, the horizontal axis shows
the sampling time when the sample number is multiplied by 50
nanoseconds. In the case of normal photoacoustic imaging, the
timing of light irradiation is taken to be zero seconds.
[0046] Comparing FIGS. 3A and 3B, the signal indicated by the
dotted line B in FIG. 3A is not detected in FIG. 3B. This signal is
received when a photoacoustic wave generated at the surface of the
acoustic wave probe (a signal in the region indicated by the dotted
line A) is fully reflected at the interface between the holding
plate 16 and the air, and is received again as an acoustic wave. In
other words, this is a signal which is received as a large signal,
if the detecting element is a detecting element 22b which is not in
acoustic contact with the object. Since the signal is reflected at
the interface with the air, then a characteristic feature of the
signal is that the phase is reversed (the waveform is reversed)
with respect to a signal in the region indicated by the dotted line
A.
[0047] Furthermore, if the thickness of the holding plate 16 and
the speed of sound therein are known, then it is possible to
predict, in advance, that the signal will be detected at a specific
timing, for instance, at t1 in FIG. 3A. A signal in the region
indicated by dotted line C in FIG. 3B is not measured in FIG. 3A,
and is therefore inferred to be produced by a photoacoustic wave
generated inside the object.
[0048] Next, a method of determining a detecting element 22b which
is not in acoustic contact with the object will be described.
[0049] A signal in the region indicated by the dotted line A in
FIG. 3A is a detection signal (photoacoustic signal) which is
generated due to light being irradiated directly onto the surface
of the probe. This signal may be observed in FIG. 3B, but a
comparison of FIGS. 3A and 3B shows that the intensity in FIG. 3A
is clearly greater. For example, if the detecting element is in
acoustic contact with the object, then the surface of the detecting
element covers the object, and therefore the pulsed light does not
reach the surface of the detecting element directly. For instance,
supposing that a photoacoustic wave is generated by diffused light,
or the like, then only a small signal is observed.
[0050] On the other hand, if the detecting element is not in
acoustic contact with the object, then when light is irradiated
onto the object from the opposite side to the probe, as shown in
FIG. 1, the light is irradiated directly onto the surface of the
detecting element, and a large signal is observed from the surface
of the detecting element. More specifically, a detecting element
which is not in acoustic contact with the object can be determined
by comparing the intensity of this signal. More specifically, a
detecting element showing an intensity equal to or greater than a
predetermined threshold value is determined as a detecting element
which is not in acoustic contact with the object. The predetermined
threshold value referred to here is a value which is determined
experimentally, for example, one half of the peak value of the
detection signal (photoacoustic signal) generated by irradiating
light directly onto the surface of the acoustic wave probe when no
object is present. Furthermore, since this threshold value is an
intrinsic value of the apparatus, then desirably the threshold
value is adjusted respectively for each apparatus. The timing at
which the predetermined threshold value is specified may be
immediately before the actual measurement or the threshold value
may be specified in advance, for each type of apparatus. The
predetermined threshold value thus specified can be used at any
time by storing the value in a memory (memory unit), for
example.
[0051] Furthermore, it is possible to envisage the following method
as an alternative method. The waveforms indicated by the dotted
line A and the dotted line B in FIG. 3A are characteristic of a
signal which is received by a detecting element 22b which is not in
acoustic contact with the object. Therefore, these two signals are
used as a template to obtain a correlation with the whole detection
signal. By means of this correlation, it is possible to determine a
signal which is received by a detecting element that is not in
physical contact with the object and a signal which is received by
a detecting element that is in acoustic contact with the object. In
this case, the portion indicated by the dotted line A is a
detection signal based on a first acoustic wave. More specifically,
a detecting element showing a correlation equal to or greater than
a predetermined threshold value is determined as a detecting
element 22b which is not in acoustic contact with the object. The
predetermined threshold value referred to here is 0.2, for
instance, if the correlation value is 1 when the signal is
completely matching. However, since this threshold value is an
intrinsic value of the apparatus, then desirably the threshold
value is adjusted respectively for each apparatus. The timing at
which these threshold values are specified may be immediately
before the actual measurement or the values may be specified in
advance, for each type of apparatus. The predetermined threshold
value thus specified can be used at any time by storing the value
in a memory (memory unit), for example.
[0052] The determination method described here is no more than one
example. The essence of the present invention is to extract and
determine the characteristics of a signal which is received by a
detecting element that is not in acoustic contact with the object,
and any method may be employed provided that it does not depart
from the scope of this essence.
[0053] Process (2) (step S202): A step of eliminating unwanted
signals from the signals of the detecting elements determined in
S201.
[0054] For example, in the signal processing module 19b, a new
detection signal group is created by deleting the whole of a signal
which has been received by a detecting element 22b determined as
not in acoustic contact with the object. Alternatively, a corrected
signal is generated by reducing to zero a signal received by a
detecting element 22b which is determined as not in acoustic
contact with the object. In general, when a detecting element 22 is
not in acoustic contact with the object, it is not able to receive
a photoacoustic wave generated inside the object. Consequently, all
of the data received in a region which does not make acoustic
contact with the object does not contribute to the image produced
when creating an image of the initial pressure distribution or
light energy density distribution inside the object. Therefore, it
is possible to set all of the data to zero, or to delete the
detection signal itself and assume that the detecting element is
not present. In other words, in the present step, a signal which is
separate to the digital signal obtained from the signal acquisition
system 18, in other words, a corrected digital signal, is generated
by carrying out processing such as that described above.
[0055] Process (3) (Step S203): A step of carrying out image
reconstruction by using the detection signal obtained in S202.
[0056] For example, image reconstruction is carried out by using a
corrected digital signal which is obtained in S202, whereby an
image relating to the initial pressure distribution or the light
energy density distribution of the object 15 is formed. In respect
of this processing, it is possible to use an image reconstruction
process of any kind which is normally employed in photoacoustic
tomography. For instance, a method of back projection in a time
domain or a field domain, or the like, is employed.
[0057] By carrying out the steps described above, in cases where a
portion of the detecting elements 22 are not in acoustic contact
with the object 13 as in the example in FIG. 1, it is possible to
eliminate signals which are not necessary for the formation of an
image of the interior of the object. As a result of this, it is
possible to provide a photoacoustic image-forming apparatus which
shows little image deterioration.
First Embodiment
[0058] One example of a photoacoustic image-forming apparatus which
employs photoacoustic tomography applying the present embodiment
will now be described. This image-forming apparatus is described
here with reference to the schematic drawing in FIG. 1. In the
present embodiment, a Q-switch YAG laser which generates pulsed
light of approximately 10 nanoseconds at a wavelength of 1064 nm
was used as the light source 11. The energy of the light pulse
emitted from the pulsed light 12 was 0.6 J, and the pulsed light
was broadened to a radius of approximately 2 cm using an optical
system such as a mirror and a beam expander, or the like.
Subsequently, the optical system was set up so as to be able to
irradiate the pulsed light onto the object on the opposite side to
the acoustic wave probe 17.
[0059] A phantom which simulates a breast shape was used as the
object 13. The breast-shaped phantom was prepared using urethane
rubber, titanium oxide and ink, in such a manner that the reduced
scattering coefficient and the absorption coefficient were
substantially the same as a breast. Three round bar-shaped cysts
having a diameter of 2 mm were buried in the phantom as light
absorbing bodies 14. Furthermore, the breast-shaped phantom had a
curved surface shape. Therefore, in order to flatten the shape and
achieve acoustic coupling with the acoustic wave probe 17, a
holding plate 16 formed for 10 mm-thick polymethyl pentene was
installed between the acoustic wave probe 17 and the breast-shaped
phantom 13. FIG. 4A shows a photograph of the breast-shaped phantom
taken from the side of the polymethyl pentene in this case. As FIG.
4A reveals, the breast-shaped phantom does not make close contact
in the whole of the region of the polymethyl pentene. In this
region, air is interposed between the phantom and the detecting
elements, and therefore the phantom and the detecting elements
cannot be coupled acoustically. As a result of this, the signal
from the detecting elements in this region where close contact is
not made is a cause of image degradation.
[0060] Furthermore, pulsed light 12 was irradiated onto the
breast-shaped phantom which had been set up in this way, on the
surface of the phantom opposite to the acoustic wave probe 17, as
shown in FIG. 1. The acoustic wave probe 17 employed a 2D array
type probe formed of a plurality of detecting elements 22 arranged
in a two-dimensional configuration. Furthermore, a portion of the
region measured by the 2D array type acoustic wave probe 17
included a region 21 not in acoustic contact with the breast-shaped
phantom, as shown in FIG. 1.
[0061] Next, the generated photoacoustic wave was received by the
plurality of detecting elements 22 of the 2D array type acoustic
wave probe 17. The detection signals of these elements were
obtained as a digital signal of photoacoustic signal by using a
signal acquisition system 18 comprising an amplifier, an A/D
converter and an FPGA. Thereupon, the obtained digital signal was
transferred to a work station (WS) forming a signal processor 19,
and was saved in the WS. Thereupon, the digital signal was analyzed
by the signal determining module 19a and the signal processing
module 19b, which are software programs inside the WS.
[0062] In the present embodiment, signal determination was carried
out by using the reception intensity of the photoacoustic wave
generated at the surface of the detecting elements. The
determination method was as follows. More specifically, in the
received acoustic wave signal, the maximum value of the
photoacoustic wave generated at the detecting element surfaces as
observed during an initial measurement period indicated by the
region A in FIG. 3 is detected. If this maximum value is greater
than a certain value, then it is taken to be a signal received from
a detecting element 22b which is not in acoustic contact with the
breast-shaped phantom. The signal in this region is sufficiently
large in comparison with noise, and therefore the signal can be
determined in a stable fashion. In the present embodiment, if the
intensity of the signal was 200 or above, then the signal was
judged to be one from a detecting element 22b which was not in
acoustic contact with the breast-shaped phantom, and if the
intensity was less than 200, then the signal was judged to be from
a detecting element 22a which was in acoustic contact with the
breast-shaped phantom. Furthermore, a corrected signal was formed
in which signals received by detecting elements 22b not in acoustic
contact with the breast-shaped phantom were all set to zero.
[0063] Thereupon, image reconstruction was carried out in the
reconstruction module 19c, which is a software program in the WS,
using the corrected signal created in this way. Here,
three-dimensional volume data was created by using a universal back
projection method, which is a time domain method, from among the
plurality of image reconstruction techniques. FIG. 4C shows one
example of an image obtained in this case. FIG. 4C shows a MIP
(Maximum Intensity Projection) image obtained by projecting the
maximum brightness in the direction in which all of the absorbing
bodies can be imaged, in the three-dimensional image data.
Thereupon, an image was calculated by the image reconstruction
technique employed as described above, using an uncorrected digital
signal which had been saved in the WS. FIG. 4B shows one example of
an image obtained in this case. FIG. 4B is an MIP image in which
the maximum brightness in the direction where all of the absorbing
bodies can be imaged is projected from the three-dimensional image
data.
[0064] FIGS. 4B and 4C will now be compared. In a signal which is
received from a detecting element that is not in acoustic contact
with the breast-shaped phantom, a reflected wave of the
photoacoustic wave generated at the surface of the probe is
observed as shown by the dotted line B in FIG. 3A, and therefore
the image produced by this signal appears as the region B in FIG.
4B. The region of the dotted line B in FIG. 3 is an artifact which
is present horizontally at a certain depth, and this is caused by a
reflected wave of the photoacoustic wave which is generated at the
surface of the probe as described above. On the other hand, in FIG.
4C, the unnecessary signal of this kind is deleted, and therefore
an image C caused by the photoacoustic wave generated inside the
phantom, such as that indicated by the dotted line C in FIG. 3B,
appears only. When measuring body tissue, such as a breast, it is
not possible to distinguish between an image produced by an
acoustic wave generated inside the breast and other images, and
therefore an unnecessary image A of this kind can lead to
misdiagnosis. In other words, the image in FIG. 4C can be regarded
as superior in terms of a diagnostic image.
[0065] From the foregoing, by eliminating signals which are not
necessary for image formation, from the detection signals of
detecting elements that are not in acoustic contact with the
object, it is possible to provide a photoacoustic image-forming
apparatus capable of generating an image with little degradation
which is superior as a diagnostic image to the prior art.
Second Embodiment
[0066] One example of a photoacoustic image-forming apparatus which
employs photoacoustic tomography applying the present embodiment
will now be described with reference to FIG. 5. In the present
embodiment, a phantom and a measurement system substantially the
same as those of the first embodiment were used. However, in
contrast to the schematic drawing of the apparatus in FIG. 1, here,
the light 12 was irradiated in the direction of the breast-shaped
phantom from the side of the acoustic wave probe 17, as shown in
FIG. 5. Furthermore, a scanning action of the acoustic wave probe
17 and the light 12 was performed in order to form an image of the
whole of the breast-shaped phantom.
[0067] In an apparatus of this kind, in the present embodiment,
similarly to the first embodiment, after acquiring a digital signal
of the generated photoacoustic wave, the obtained digital signal
was transferred to a workstation (WS) forming a signal processor 19
and was saved in the WS. Thereupon, the digital signal was analyzed
by the signal determining module 19a and the signal processing
module 19b, which are software programs inside the WS.
[0068] In the present embodiment, the method of determining the
detecting elements 22b which were not in acoustic contact with the
object employed a method such as that described below. When pulsed
light 12 is irradiated from the side of the acoustic wave probe,
then a photoacoustic wave is generated due to the light irradiated
onto the probe surface, similarly to the first embodiment.
Furthermore, since the photoacoustic wave is fully reflected at the
interface between the holding plate 16 and the air, then a
photo-acoustic wave having a reversed phase is received again.
Since the difference in the reception times of these two acoustic
waves is uniform, as indicated by the relationship between the
regions A and B in FIG. 3A, and since their shapes are also unique,
then it is possible to determine these waves by using this
shape.
[0069] The specific determination method employed will now be
described. Firstly, in a composition where there is no phantom, the
signal received when light is irradiated directly onto the surface
of the probe is measured, and the photoacoustic wave generated at
the surface of the probe and the signal which is received again
when this acoustic wave is reflected at the holding plate are
extracted from the measured signal. The extracted signal is saved
as a template in the workstation (WS) which forms the signal
processor 19. Thereupon, the correlations between this template and
all of the received signals are calculated, and a detection signal
having a high correlation value is determined to be a signal
received by a detecting element which is not making physical
contact with the object. Furthermore, the detection signals from
the detector which are determined in this way are not treated as
detection signals, and a new corrected signal which excludes these
signals is created.
[0070] Thereupon, image reconstruction was carried out in the
reconstruction module 19c, which is a software program in the WS,
using the corrected signal created in this way. Here, in contrast
to the first embodiment, three-dimensional volume data was created
by using a Fourier domain. The image of the breast-shaped phantom
obtained by a method of this kind was similar to that in FIG. 4C,
and a clearer image than FIG. 4B, which is an image acquired by
conventional technology, was obtained.
[0071] From the foregoing, it is possible to determine detecting
elements which are not in acoustic contact with the object, by
determining the shape of the detection signal, and it is possible
to provide a photoacoustic image-forming apparatus having little
image degradation, by eliminating signals which are not necessary
for image formation, from the detection signal.
Third Embodiment
[0072] One example of a photoacoustic image-forming apparatus which
employs photoacoustic tomography applying the present embodiment
will now be described. In the present embodiment, the same phantom
and measurement system as the first embodiment were used, with the
exception that the holding plate 16 was not present. Firstly,
similarly to the first embodiment, after acquiring a digital signal
of a photoacoustic wave generated by irradiation of pulsed light
12, the obtained digital signal was transferred to a workstation
(WS) forming a signal processor 19 and saved in the WS. Thereupon,
the digital signal was analyzed in the signal determination and
signal processing modules which are software programs in the WS. In
the present embodiment, detecting elements which were not in
acoustic contact with the phantom were determined by using the
reception intensity of the photoacoustic wave generated at the
surface of the probe, similarly to the first embodiment. Thereupon,
all of the data from the detecting elements determined as not in
acoustic contact with the phantom was deleted, and image
reconstruction was carried out using only the detection signals
from detecting elements which were in acoustic contact with the
phantom. The image obtained by a method of this kind was similar to
that in FIG. 4C, and a clearer image than FIG. 4B, which is an
image obtained by conventional technology, was obtained.
[0073] From the foregoing, even if there is no holding plate, by
analyzing the intensity of the detection signal, it is possible to
determine detecting elements which are not in acoustic contact with
the object and by eliminating the detection signals of these
elements, it is possible to provide a photoacoustic image-forming
apparatus which has little image degradation.
[0074] 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.
[0075] This application claims the benefit of Japanese Patent
Application No. 2011-88312, filed on Apr. 12, 2011, which is hereby
incorporated by reference herein in its entirety.
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