 |
Description  |
|
|
TECHNICAL FIELD
This invention relates to optical imaging, and more particularly to the use
of phase encoding of an intensity modulated optical carrier in order to
image a one-, two- or three-dimensional object using a single optical
detector.
BACKGROUND ART
Numerous techniques and devices are known and available for imaging
structures within opaque or turbid objects, such as biological tissue.
Various examples of such prior art are provided in my copending U.S.
application, Ser. No. 07/722,823, filed Jun. 28, 1991, the contents of
which are hereby incorporated by reference.
The copending application describes a method and apparatus for imaging
turbid media using optical interference among diffusively propagating,
phase encoded, intensity modulated optical carriers, utilizing photons
which pass through the medium (such as biological tissue) by the wave
diffusion process as opposed to geometrically propagating "prompt"
photons. As disclosed therein, a method for imaging an object includes the
steps of applying modulated optical rays at a plurality of points along a
surface of the object, each of the rays characterized by an amplitude
modulated at a respective modulating frequency and by a specific phase.
The phases applied to the various rays are provided respective relative
phase-shifts which are selected to cause constructive interference among
the modulated rays at a predetermined volume element of interest, or
voxel.
Thus, the method effectively selects the predetermined voxel for imaging by
selecting the relative phase-shifts to be applied to the intensity
modulated light rays. In accordance with the method, the modulated rays
are diffusively propagated through the object and the intensity and phase
of a light ray resulting from the constructive interference at the
selected voxel are detected in order to image a characteristic of the
voxel.
In order to image a portion of the object which includes a number of
voxels, the method of the copending application may include the further
steps of repeating the selecting step for a sequence of predetermined
voxels, repeating the propagating step to diffusively propagate the
modulated rays through the object to the predetermined voxels, and
repeating the detecting step to detect rays respectively resulting from a
sequence of constructive interferences at the sequence of voxels. For such
a process, the selecting step preferably includes the steps of using a
signal responsive phase shifting device, such as a zone plate, for
applying the relative phase-shifts to the rays. Computer generated signals
are applied to the signal responsive phase shift device, thereby providing
non-mechanical scanning of the portion of the object to be imaged.
Although the above summarized invention disclosed in my copending
application solves a number of problems associated with the prior art, the
disclosed method and apparatus provides for imaging of absorption
characteristics using interference among diffusively propagating modulated
light rays. There accordingly remains a need for method and apparatus
capable of imaging attenuation and phase delay characteristics of an
object, using a simplified detector structure and not necessarily relying
on occurrence of an optical interference at the object.
In that regard, the prior art uses an optical mask which is time encoded to
permit various light rays to impinge on specific voxels of an object.
After passing through the object, the rays emerge with intensity
attenuation and phase change determined by the characteristics of the
voxels. The emergent rays are directed to be incident on corresponding
photodetectors for imaging thereby. Such a prior art arrangement is
illustrated in FIG. 8, wherein a n plurality of intensity modulated light
rays r.sub.i are transmitted through corresponding voxels V.sub.i (i=1, 2,
3, . . . , 4) of an object 10 for detection by a photodetector array 13. A
mask 11 includes a plurality of opaque and transparent regions in the
paths of light rays r.sub.i to transmit or block specific rays to the
object and to establish a specific timing sequence of rays for incidence
on the object.
Such a mask may be a computer controlled liquid crystal plate, for example,
having pixels or pixel groups which are selectively made opaque and
transparent according to a predetermined timing sequence. In the Figure
there is illustrated a single transparent region 15 in mask 11, the
remaining regions being opaque. By having a predetermined time sequencing
for the incident rays as well as for reading and processing of outputs of
the photo-detectors 13, a processor 16 obtains a distribution of the
intensity (amplitude) attenuation and phase shifting characteristics of
the various voxels of the object, thus imaging the object.
However, such a prior art approach as shown in FIG. 8 requires a complex
sequencing arrangement of the imaging rays, requires a complex
photodetecting structure utilizing a number of photo-detectors, and
requires passage of time from the application of the first imaging ray to
the last such ray, thus effectively prohibiting the system from obtaining
a real-time "snapshot" of the voxels of the object.
There is thus a need in the prior art for method and apparatus of using a
single detector cell rather than an array of cells for imaging attenuation
and phase delay characteristics of one or more elements which are arrayed
along a plurality of dimensions of an object, whether the imaging uses a
single exposure or a plurality of exposures.
There is yet a more particular need for a method and apparatus for imaging
a plurality of voxels of an object of interest, whether disposed as a
one-dimensional linear array, as a two-dimensional planar array, or as a
three-dimensional volume of the object, by a single exposure of the array
of voxels to imaging light including a plurality of phase-encoded,
modulated optical carrier frequencies.
There is yet another need in the prior art for imaging one or more voxels
of an object by applying one or more rays of light to the voxels, in one
or more exposures, using geometrically propagating light rays.
DISCLOSURE OF INVENTION
It is accordingly an object of the present invention to provide a method
and apparatus for imaging attenuation and phase delay characteristics of
an object requiring a simplified detector structure and without requiring
optical interference to occur at the object.
It is a more specific object to provide a method and apparatus for imaging
attenuation and phase delay characteristics of multiple dimensions of an
object using a single detector cell rather than an array of cells, whether
by a single exposure of a plurality of rays or by a plurality of
sequential exposures, each to a single ray.
It is a more particular object of the invention to provide a method and
apparatus for imaging a plurality of voxels of an object of interest,
whether disposed as a one-dimensional, linear array, as a two-dimensional,
planar array, or as a three-dimensional volume of the object, by exposing
the voxels in a single exposure to imaging light including a plurality of
phase-encoded, intensity modulated optical carrier frequencies.
Another object is to image one or more voxels of an object by applying one
or more rays of light to the voxels, in one or more exposures, using
geometrically and/or diffusively propagating light rays.
It is still a more specific object of the invention to provide method and
apparatus for obtaining optical attenuation and phase delay data
descriptive of optical characteristics of an object, thereby to image an
object, free of requirements for exposure of the object to ionizing
radiation and free of a requirement for administration of contrast or
tracing agents.
Still another object of the invention is to provide method and apparatus
utilizing a single detector for obtaining substantially simultaneously
imaging data descriptive of optical characteristics of an entire volume of
an object, thus simultaneously imaging the entire object.
In accordance with these and other objects of the invention, there is
provided an improvement in an apparatus for imaging attenuation and phase
shift characteristics of an object including applying means for applying
to a surface of the object a plurality of electromagnetic rays, each
having a carrier frequency in an optical frequency spectrum, each of the
rays modulated by a modulation frequency, and detecting means for
detecting the electromagnetic rays after attenuation and phase shifting by
each of a plurality of voxels (volume elements of interest). In accordance
with the invention, the imaged voxels are disposed in an array of n
dimensions where n is at least equal to 1, thus providing for imaging of a
single voxel, a linear array, a 2-dimensional array, and a volume array of
voxels. Moreover, the detecting means includes only a single optical
detecting cell for detecting electromagnetic radiation resulting at a
single point from attenuation and phase shift of the rays by the plurality
of voxels. Moreover, the invention includes a processor for processing an
output signal from the single cell to provide data identifying optical
attenuation and phase shift characteristics of each of the voxels thereby
to image the plurality of voxels.
The invention moreover provides for the amplitude modulated rays to be
applied either in a sequence of individual rays or simultaneously, as an
array of differently modulated rays. Of course, the detecting means could
be reflective or transmissive.
In an embodiment providing for simultaneous application of the rays to the
object in an array of modulated rays, the improved imaging apparatus
preferably further includes a phase array means for providing a
predetermined phase shift to each of the electromagnetic rays in the
array, thereby to phase encode the electromagnetic rays incident on the
surface of the object. The phase array means may include a plurality of
optical paths having various different path lengths, thus providing
various different phase shifts to the rays.
Thus, the phase array means may include a plurality of free space optical
paths each including means for receiving a respective ray and means
establishing a predetermined free space optical path for the respective
ray. Moreover, the means establishing may include a corner cube reflector
displaced by a predetermined distance from the means for receiving,
thereby providing a predetermined optical path and a predetermined phase
shift to a ray prior to incidence of the ray on a respective voxel to be
imaged thereby.
Alternatively, the phase array means may include a plurality of fiber
optical paths, wherein each optical path includes means for receiving a
respective ray and a predetermined length of optical fiber between the
means for receiving and the surface of the object, in order to provide a
predetermined optical path and a predetermined phase shift to a ray prior
to incidence of the ray on a respective voxel to be imaged thereby.
Additionally, the improved imaging apparatus may provide (in the phase
array means) a plurality of condition responsive optical devices for
modulating each of the electromagnetic rays and for providing
predetermined phase shifts to each of the rays modulated thereby.
Therein, the condition responsive optical devices may include a plurality
of electro-optic means. Each of these means receives an unmodulated
electromagnetic ray and is connected to receive an electrical signal from
a source of electrical signals to modulate the ray received thereby at a
predetermined frequency and phase in order to provide the predetermined
phase shifts to each of the rays.
Alternatively, the condition responsive optical devices may include a
plurality of acousto-optic means each receiving an unmodulated
electromagnetic ray and connected to receive an acoustic signal to
modulate the ray received thereby at a predetermined frequency and phase,
in order to provide the predetermined phase shifts to each of the rays.
As yet another alternative, the condition responsive optical devices may
include a plurality of opto-optic means each receiving an unmodulated
electromagnetic ray and connected to receive an optical signal, thus to
modulate the intensity of the ray received thereby at a predetermined
frequency and phase in order to provide the predetermined phase shifts to
each of the rays.
In its generic form, the applying means of the invention includes
modulating means for modulating each of the plurality of electromagnetic
rays by a respective modulation frequency, thereby producing a plurality
of electromagnetic rays modulated at respective modulation frequencies,
for application to the surface of the object.
In an embodiment providing for simultaneous application of the rays to the
object the applying means may further include phase array means for
providing a respective predetermined phase shift to each of the modulated
electromagnetic rays thereby to phase encode the electromagnetic rays
incident on the surface of the object. Thus, the phase shifting is applied
to the amplitude modulated rays.
The modulating means of this embodiment preferably operates to amplitude
modulate the beam intensity of each electromagnetic ray by a modulation
frequency less than approximately one percent of the carrier frequency.
Moreover, the modulating means may include means for amplitude modulating
the beam intensity of each of the electromagnetic rays by a different
modulation frequency.
In accordance with another aspect of the invention, the differently phase
encoded rays may be applied to the object sequentially, rather than
simultaneously. In this embodiment, the applying means includes sequencing
means for sequentially applying the plurality of electromagnetic rays to
the object.
Thus, the invention also encompasses an apparatus for imaging attenuation
and phase shift characteristics of an object including applying means for
sequentially applying to a surface of the object a sequence of intensity
modulated electromagnetic rays each having a carrier frequency in an
optical frequency spectrum, and after attenuation and phase shifting by
each of a plurality of voxels. The detecting means includes only a single
optical detecting cell for detecting electromagnetic radiation resulting
at a single point from attenuation and phase shift of each of the
sequentially applied rays by the plurality of voxels. A processing means
processes an output signal from the single cell to provide data
identifying optical attenuation and phase shift characteristics of each of
the voxel. The applying means includes modulating means for modulating
each of the electromagnetic rays, thereby producing a sequence of
intensity modulated electromagnetic rays, and for sequentially applying
the modulated electromagnetic rays to the surface of said object.
With respect to the broad aspects of the invention, a single detector may
be used to image, or provide characteristics descriptive of, a single
voxel of the object by using a single detector cell for detecting
intensity and phase of a light ray passing through or reflected by the
selected voxel, the cell thus imaging the voxel as a function of the
intensity and phase of the detected light ray.
Further, the optical rays are modulated as part of the inventive method.
Particularly, where a signal responsive device is used to modulate the
intensity of the optical rays, the signal responsive device may also be
used to provide the selected phase shifts to the various beams.
Thus, the object being imaged may be imaged by a single detector cell
receiving a plurality of separately phased rays. Where the rays are
applied simultaneously the image provides an snap-shot of the portion of
interest in the object Where the rays are applied in a timed set the
single photodetector effectively provides a non-mechanical scan of the
array of voxels. The intensity and phase information of the detected rays
provide solutions to a plurality of equations describing relationships
between the detected information and the transmitted rays, the solution
providing a distribution of the attenuation and phase delay
characteristics of the object (i.e., providing an image of the object).
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, incorporated into and forming a part of the
specification, illustrate several aspects of a preferred embodiment of the
present invention and, together with the description, serve to explain the
principles of the invention. In the drawings:
FIG. 1 illustrates a preferred embodiment of the present invention;
FIG. 2 is an illustration of a generic structure used in implementing the
embodiment of FIG. 1;
FIG. 3 shows a first specific embodiment of the structure of FIG. 2;
FIG. 4 shows a second specific embodiment of the structure of FIG. 2;
FIG. 5 shows a third specific embodiment of the structure of FIG. 2;
FIG. 6 shows a free-space phase encoding structure used in implementing the
embodiment of FIG. 1;
FIG. 7 shows an optical fiber phase encoding structure used in implementing
the embodiment of FIG. 1; and
FIG. 8 shows a prior art arrangement for imaging an object.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring now to the drawings, there is shown in FIG. 1 an embodiment of
the broad concepts of the present invention.
Prior to describing the embodiment illustrated therein, however, it should
be noted that the term optical imaging of an object as used herein relates
to detection of a light ray transmitted through, or reflected by, the
object of interest and specifically transmitted through or reflected by
various internal components of the object. Hereinafter, reference to a
light ray received from (or transmitted through) such an element should be
understood to include both transmitted and reflected rays, as explained
below with respect to possible modifications of FIG. 1.
The detected light ray contains information descriptive of various
characteristics of the object or its internal components. For example, the
information may be descriptive of the absorption characteristics of the
internal structural components of the object, or may be descriptive of
density, fluorescence, or other characteristics thereof. The present
invention is directed at apparatus for detecting specific imaging
characteristics of the internal structure of an object, and specifically
at detection of the characteristics of a specific portion of an object
which affect intensity attenuation and phase delay characteristics of
light rays passing therethrough or reflected thereby, and thus at an
ability to provide an optical image of a desired portion of an object of
interest, whether the desired portion is at the surface of the object or
deep within the object.
Further, use of the term "optical" or "light" rays generally refers to
electromagnetic radiation, in the visible frequency range of approximately
4.times.10.sup.14 -8.times.10.sup.14 Hz with wave lengths in the range of
approximately 0.75 to 0.38 micron. However, the present invention is not
necessarily limited to the visible spectrum of electromagnetic radiation
and other spectra of the electromagnetic continuum may be utilized, to the
extent practicable.
As hereinabove noted, FIG. 1 provides a schematic illustration of an
embodiment of the present invention, wherein, similarly to the prior art,
a plurality of intensity modulated light rays r.sub.i are transmitted
through corresponding voxels V.sub.i (i=1, 2, 3, . . . , 4) of an object
10 for detection by a detector 14 and for subsequent imaging.
In that regard, detecting device 14 may be any type of light sensitive
device, whether a vacuum type photomultiplier tube (PMT), a solid state
device, or the like. Photodetector 14 converts the optical radiation
incident thereon to electrical signals in a known manner. The resultant
electrical signals are then processed in a well known manner, to obtain
numerical data or to generate displays as hereinabove summarized.
Moreover, although photodetector 14 is shown as receiving the light
transmitted through object 10, and is thus on an opposite side of the
object from the incident light rays, it is also known to provide the
photodetector 14 on the same side of the object 10 as the incident light
rays, in which case reflected light rays are processed for imaging. Thus,
imaging may utilize light rays transmitted through or reflected by the
object, and specifically by internal voxels V.sub.i within the object
being imaged, in order to provide information descriptive of the internal
structure of the object.
It should also be recognized that the one dimensional array of voxels
illustrated in FIG. 1 is merely illustrative of the manner in which the
voxels of interest may be distributed in the object. That is, the portion
of the object which is of interest for imaging may be a linear portion, as
represented by the illustrative linear array of voxels. Alternatively,
however, the portion of interest may be a surface, represented by a
two-dimensional array of voxels, or a volume, represented by a
three-dimensional array. Thus, any arbitrarily shaped portion of the
object may be imaged using the concepts of the present invention as
described herein.
In accordance with the invention, the light rays passing through and
emerging from object 10 are focused by a lens 12 on a single photodetector
14, which generates an output signal 15 representative of the detected
illumination.
The output signal 15 represents the intensity of the optical illumination
incident on the photodetector 14 and thus provides a specific time
waveform to processor 16 for analysis. The amplitude of the signal
provided to the processor 16 represents the results of attenuation of the
original light rays r.sub.i by the voxels V.sub.i, while the timing of the
signal represents phase shifts imparted to the incident rays by the
voxels. Such phase shifts represent information descriptive of the
absorption coefficients, scattering coefficients, and fluorescence decay
of the voxels V.sub.i. Accordingly, a reference signal 17 is provided to
the processor representative of the phase of the incident rays r.sub.i,
for use in conjunction with the signal 15 to determine the phase shifts
imparted thereto by the voxels being imaged.
The light rays r.sub.i are each a modulated light ray, having a specific
carrier frequency (e.g., .omega..sub.o) provided with an amplitude
modulation of a given frequency (.omega..sub.j). Each voxel, depending on
its specific composition and other physical characteristics, will have a
particular attenuation coefficient (A.sub.i ) and phase delay
(.DELTA..omega..sub.j). characteristic for the frequency .omega..sub.o.
Upon determining the characteristics A.sub.i and .DELTA..omega..sub.i of
each of the voxels forming the portion of interest, an image of the
portion of interest can be generated by plotting the characteristics,
whether on a CRT, LCD, or other display or by printing or generating other
hard copy outputs representative thereof.
Upon applying to the i.sup.th voxel of the object a j.sup.th incident ray,
with modulation frequency .omega..sub.j, but without the phase shift of
the present invention, the attenuation and phase delay exerted by the
voxel will result in an exiting ray incident on photodetector 14 having an
intensity .psi..sub.j given by Equation (1):
##EQU1##
This equation reflects the intensity of the j.sup.th exiting ray as being a
summation of the illumination passing through each of the voxels
(summation over the index i) after attenuation by the attenuation
coefficient thereof and delay by the phase delay characteristic. Standard
mathematical calculations (such as a Gaussian elimination procedure
described by Bronstein) may be used to obtain a plurality of equations in
order to solve for the plural coefficients A.sub.i and .DELTA..phi..sub.i.
If the individual rays are individually applied, so that the individual
output signals 15 can be associated with individual rays, the above
equations may be solved. That is, by knowing which beam is being applied
at a particular time and which voxel is primarily responsible for the ray
impinging on the photodetecting array, it is possible to obtain
information on the attenuation coefficient and phase delay characteristic
of that voxel. Individual application of the rays, however, requires a
sequential application of light rays, and/or utilization of a plurality of
detectors to obtain a plurality of output signals. It is impossible to
solve the plurality of equations resulting from simultaneous application
of a plurality of rays together with simultaneous detection of the exiting
rays from a plurality of voxels by a single detector, because the
equations become redundant.
In accordance with the invention, and to enable simultaneous application of
a plurality of rays and simultaneous detection thereof by a single
detector, the above noted redundancy of equations is eliminated by phase
encoding each of the incident rays. That is, to each incident ray there is
provided not only a modulating frequency .omega..sub.j, but also a
specific relative phase angle .phi..sub.ij.
The term .phi..sub.ij contains the phase information of the j.sup.th ray
before impinging on the i.sup.th voxel, which may include effects of the
different path length from the source to the various voxels. Thus, rather
than Equation (1), the phase encoding provided by the present invention
results in a set of equations of the type in Equation (2):
##EQU2##
The added phase encoding term .phi..sub.ij eliminates the redundancy and
enables solution of the k vector equations in 2k unknowns (A.sub.i and
.DELTA..phi..sub.i for i=1, 2, . . . ,k) to obtain real and imaginary
components of the detected intensity, as shown in Equations (3) and (4),
respectively:
##EQU3##
The above thus results in at least 2k equations in 2k unknowns, describing
detected intensity for a plurality of geometrically propagating light rays
passing through a number of voxels. Upon slight modification, the
equations are substantially equally applicable for diffusively propagating
light rays. The plurality of equations may be solved by application of the
known Gaussian elimination procedure to obtain values for the attenuation
coefficients A.sub.i and the phase delay characteristics
.DELTA..phi..sub.i of each of the voxels of interest. Thus, the present
invention permits imaging of a portion of an object by simultaneous
application of a plurality of intensity modulated phase encoded light
rays, obtaining a real-time "snap-shot", or by obtaining a non-mechanical
scan of the object by application of a sequence of rays thereto. In either
case, only a single photodetector is required, though an array including a
plurality of detectors may be used.
It should be noted that the modulation frequencies .omega..sub.j are
related to the carrier frequency of the light ray by the relationship
.omega..sub.j <<.omega..sub.o.
Thus, by using a central modulating frequency of approximately 1 GHz, which
is approximately 2.times.10.sup.-6 (0.0002%) of the frequency of light, it
is assured that even with uses of 100 different modulation frequencies
.omega..sub.j at 100 kHz spacing, or a bandwidth of approximately .+-.10
MHz from the central modulating frequency, the attenuation coefficients
and phase delay characteristics of the voxels are relatively constant for
each of the incident phase-encoded rays.
An advantage of utilizing a modulating frequency on the order of GHz is
that very large wavelengths are involved, thus enabling measurement of the
very small phase delays associated with the voxels being imaged. The
bandwidth of modulating frequencies is deliberately kept small to avoid
errors arising from dependence of .DELTA..phi..sub.i on frequency in some
media. Thus, preferably the bandwidth is kept less than .+-.1%, as by
limiting the bandwidth to .+-.10 MHz for a central modulating frequency of
approximately 1 GHz.
Use of different modulating frequencies permits simultaneous collection of
all exiting light rays to enable the above noted parallel, rather than
sequential, solution of equations (3) and (4) and the resultant equivalent
of "flash", or "snap-shot", imaging of the object.
The following description provides details of several alternative
embodiments for generating the plural phase-encoded rays used in
conjunction with the invention.
Referring now to FIG. 2, there is shown a general illustration of a
structure for providing the plurality of phase-encoded light rays r.sub.1,
r.sub.2, . . . , r.sub.k for imaging the object of interest. A light
source (not shown), which is typically a laser, generates a light ray 20
to be directed at object 10. A plurality of beam splitters 22 direct
portions of the unmodulated light ray 20 at a plurality of controlled
phase changing devices 24. Control circuits 26 are used to control the
devices 24 to provide the desired phase angles to the rays passing
therethrough, thus to result in the incident rays r.sub.1, . . . ,
r.sub.k.
Thus, the illustrated structure includes an array of devices which
effectively provides a phased array of modulated coherent light rays. The
devices 24 used in FIG. 2 may be an array of electro-optic, acousto-optic,
or opto-optic devices, for example, which are driven by sinusoidal signals
of predetermined phase relationships provided by appropriate control
circuits 26. The devices 24, which provide phase modulation to the
portions of incoming light ray 20, may thus also be used to provide the
specific intensity modulation thereto. Thus, the embodiment of FIG. 2
effectively utilizes optical modulators, which receive input control
signals oscillating at the modulation frequency at the predetermined phase
distribution to be applied to the light rays. This embodiment provides for
both modulation and phase encoding of the coherent light rays.
In one form | | |
