U.S. patent number RE41,949 [Application Number 11/525,188] was granted by the patent office on 2010-11-23 for system and method for tomographic imaging of dynamic properties of a scattering medium.
This patent grant is currently assigned to N/A, The United States of America as represented by the Department of Health and Human Services. Invention is credited to Randall L. Barbour, Christoph H. Schmitz.
United States Patent |
RE41,949 |
Barbour , et al. |
November 23, 2010 |
**Please see images for:
( Certificate of Correction ) ** |
System and method for tomographic imaging of dynamic properties of
a scattering medium
Abstract
A system and method for the detection and three dimensional
imaging of absorption and scattering properties of a medium such as
human tissue is described. According to one embodiment of the
invention, the system directs optical energy toward a turbid medium
from at least one source and detects optical energy emerging from
the turbid medium at a plurality of locations using at least one
detector. The optical energy emerging from the medium and entering
the detector originates from the source is scattered by the medium.
The system then generates an image representing interior structure
of the turbid medium based on the detected optical energy emerging
from the medium. Generating the image includes a time-series
analysis.
Inventors: |
Barbour; Randall L. (Glenhead,
NY), Schmitz; Christoph H. (Berlin, DE) |
Assignee: |
The United States of America as
represented by the Department of Health and Human Services
(Washington, DC)
N/A (N/A)
|
Family
ID: |
26850998 |
Appl.
No.: |
11/525,188 |
Filed: |
September 14, 2000 |
PCT
Filed: |
September 14, 2000 |
PCT No.: |
PCT/US00/25155 |
371(c)(1),(2),(4) Date: |
August 29, 2002 |
PCT
Pub. No.: |
WO01/20306 |
PCT
Pub. Date: |
March 22, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60154099 |
Sep 15, 1999 |
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60153926 |
Sep 14, 1999 |
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Reissue of: |
10088254 |
Aug 29, 2002 |
06795195 |
Sep 21, 2004 |
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Current U.S.
Class: |
356/446 |
Current CPC
Class: |
G01N
21/4795 (20130101) |
Current International
Class: |
G01N
21/47 (20060101) |
Field of
Search: |
;356/445-448 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Stafira; Michael P
Attorney, Agent or Firm: Scully, Scott, Murphy &
Presser, P.C.
Parent Case Text
This application claims the benefit under 35 U.S.C. .sctn. 120 of
prior U.S. Provisional Patent Application Serial Nos. 60/153,926
filed Sep. 14, 1999, entitled DYNAMIC TOMOGRAPHY IN A SCATTERING
MEDIUM and 60/154,099 filed Sep. 15, 1999, entitled DYNAMIC
TOMOGRAPHY IN A SCATTERING MEDIUM.
This application is related to copending application Ser. No.
PCT/US00/25136 filed on the same date as this application, entitled
"METHOD AND SYSTEM FOR IMAGING THE DYNAMICS OF SCATTERING MEDIUM"
by inventor R. Barbour is hereby incorporated by reference
(hereinafter the "Barbour 4147PC2 application"). The counterpart
U.S. patent application is app. Ser. No. 10/088,190, filed Mar. 14,
2002.
This application is related to copending application Ser. No.
PCT/US00/25157 filed on the same date as this application, entitled
"METHOD AND SYSTEM FOR ENHANCED IMAGING OF A SCATTERING MEDIUM" by
inventors R. Barbour and Y. Pei and is hereby incorporated by
reference (hereinafter the "Barbour 4149PC1 application"). The
counterpart U.S. patent application is app. Ser. No. 10/088,185,
filed Mar. 14, 2002.
This application is also related to copending application Ser. No.
PCT/US00/2515, filed on the same date as this application, entitled
"IMAGING OF SCATTERING MEDIA USING RELATIVE DETECTOR VALUES" by
inventor R. Barbour and is hereby incorporated by reference
(hereinafter the "Barbour 4149PC2 application"). The counterpart
U.S. patent application is app. Ser. No. 10/088,192, filed Mar. 14
2002.
This invention was made with U.S. Government support under contract
number CA-RO166184-02A, awarded by the National Cancer institute.
The U.S. Government has certain rights in the invention.
Claims
What is claimed is:
1. A system for use in tomographic imaging of a scattering medium,
comprising.[.:.]. .Iadd.an imaging head including source fiber
bundles in a first patterned array and receiver fiber bundles in a
second patterned array, each fiber in said receiver fiber bundles
not being identical with any of the source fiber bundles, wherein
the source fiber bundles function as .Iaddend.a plurality of energy
sources .Iadd.in an illumination array.Iaddend., each energy source
emitting a respective signal for imaging the scattering
medium.[.;.]. .Iadd.onto a different illumination area on a surface
of said scattering target medium, .Iaddend.wherein .Iadd.more than
one of .Iaddend.the plurality of energy sources .[.emit.].
.Iadd.emits .Iaddend.their respective signals .[.sequentially.].
.Iadd.simultaneously .Iaddend.and the respective signals are
scattered by the scattering medium and emerge from the scattering
medium.[.;.]. .Iadd., .Iaddend.and .Iadd.wherein the plurality of
receiver fiber bundles function as .Iaddend.a plurality of
detectors for detecting the respective signals that emerge from the
scattering medium for use in measuring dynamic properties of the
scattering medium in a time series of images using optical
tomography.
.[.2. The system of claim 1, further comprising: an imaging head on
which the energy sources and the detectors are arranged; wherein
the energy sources and the detectors are arranged in a plurality of
linear arrays to enable reconstruction of a corresponding plurality
of 2-D images of the scattering medium..].
3. The system of claim 1, further comprising means for adjusting a
gain of at least one of the detectors, when the at least one of the
detectors detects the respective signal from one of the energy
sources, according to a position of the one of the energy
sources.
4. The system of claim 1, further comprising at least one
sample-and-hold circuit for freezing the respective signals
detected by the detectors to enable a simultaneous readout of the
respective signals detected by the detectors.
5. The system of clam 1, wherein the energy sources include at
least one of a non-laser optical source LED, high-pressure
incandescent lamp, laser diode, solid state laser,
titanium-sapphire laser, ruby laser, dye laser, electro-magnetic
source acoustic energy source, acoustic energy produced by optical
energy, optical energy, and combinations thereof.
6. The system of claim 1, wherein data acquisition from the
detectors is at a rate of about 100 Hz.
7. The system of claim 1, wherein the energy sources include near
infra red laser diodes that transmit multiple wavelengths.
8. The system of claim 1, wherein the detectors include at least
one of a photo-diode, PIN diode, Avalanche photodiode, charge
coupled device, charge inductive device, photo-multiplier tube,
multi-channel plate, acoustic transducer, and any combinations
thereof.
9. The system of claim 3, further including a sample-and-hold
circuit coupled to the means for adjusting that allows simultaneous
readout of the respective signals detected by the detectors.
10. A system for use in optical tomographic imaging of a scattering
medium comprising: at least one energy transmissive fiber bundle
coupled to at least one energy source; the at least one energy
transmissive fiber bundle emitting energy from the at least one
energy source, and detecting the energy after it is scattered by
the scattering medium; an imaging head for holding the at least one
energy transmissive fiber bundle; and a detection system for
collecting data regarding the optical dynamic properties of the
scattering medium from the detected energy; wherein the imaging
head undergoes uniform expansion and contraction to accommodate
different size scattering mediums.
11. The system of claim 10, wherein the at least one energy
transmissive fiber bundle is bifurcated to both emit and detect
energy.
12. The system of claim 10, wherein the imaging head comprises a
folding sphere or polygon.
13. The system of claim 10, wherein the at least one energy
transmissive fiber bundle comprises a plurality of energy
transmissive fiber bundles disposed about the imaging head.
14. A method of imaging a scattering medium using optical
tomographic imaging, comprising: (a) .Iadd.placing on the
scattering medium an imaging head including source fiber bundles in
a first patterned array and receiver fiber bundles in a second
patterned array, each fiber in said receiver fiber bundles being
different from any of the source fiber bundles; (b)
.Iaddend.exposing the scattering medium to energy from .[.a
plurality of energy sources that sequentially emit.]. .Iadd.the
source fiber bundles, wherein more than one of the source fiber
bundles simultaneously emits .Iaddend.the energy .Iadd.onto a
different illumination area on a surface of said scattering target
medium.Iaddend.; and .[.(b).]. .Iadd.(c) .Iaddend.detecting the
energy.[., via a plurality of detectors,.]. .Iadd.transmitted
through the receiver fiber bundles .Iaddend.after the energy has
been scattered by the scattering medium for use in measuring
dynamic properties of the scattering medium in a time series of
images using optical tomography.
15. The method of claim 14, wherein the scattering medium comprises
vascular tissue.
16. The system of claim 1, wherein the respective signals emitted
by the energy sources comprise optical energy of at least two
different intensity modulated wavelengths of energy.
17. The system of claim 16, further comprising a filter for
separating signals corresponding to a wavelength of the intensity
modulated energy.
.[.18. The system of claim 1, wherein the respective detectors
comprise respective fibers coupled to respective optical energy
detectors..].
19. An imaging head, comprising: a pad; a plurality of source means
for delivering optical energy to a medium; and a plurality of
detector means for detecting optical energy emerging from the
medium; wherein: the source means and detector means are attached
to the pad in a plurality of rows and columns wherein the plurality
of source means are arranged to form at least two unique imaging
planes, an imaging plane being between defined by a plane
substantially perpendicular to the pad and passing through at least
two source means and one detector means; and the source means and
detector means are arranged in first and second patterns in
alternating rows, the first pattern comprising one source means
followed by three detector means followed by one source means
followed by three detector means, and the second pattern comprising
a shifted version of the first pattern.
20. The imaging head of claim 19, wherein the source means comprise
fibers coupled to an optical energy source.
21. The imaging head of claim 19, wherein the source means comprise
optical energy sources.
22. The imaging head of claim 19, wherein the source means comprise
laser diodes.
23. The imaging head of claim 19, wherein the detector means
comprise fibers coupled to optical energy detectors.
24. The imaging head of claim 19 wherein the detector means
comprise optical energy detectors.
25. The imaging head of claim 19 wherein the detector means
comprise photodiodes.
26. The system of claim 1, wherein the energy sources and the
detectors are arranged in an illumination array that is configured
to minimize subsequent numerical effort required for data analysis
and maximizing source density covered by the illumination
array.
.[.27. The system of claim 26, wherein the energy sources and the
detectors are arranged in the illumination array to enable three
dimensional images to be computed from super positioning of two
dimensional images..].
.[.28. The detection system of claim 1, wherein the detectors
further detect fluorescence radiation excited by the energy
sources..].
.[.29. The detection system of claim 1, wherein the detectors
further detect acoustic energy produced in the scattering medium by
the energy sources..].
30. The system of claim 10, wherein the at least one energy
transmissive fiber bundle terminates inside the scattering
medium.
31. The method of claim 14, further including the step of
evaluating the dynamics in an industrial mixing process for at
least one of a gas and a liquid according to the detected
energy.
32. The method of claim 14, further including evaluating dynamics
in a foggy atmosphere according to the detected energy.
33. The method of claim 14, further including evaluating dynamics
in oceans or water masses according to the detected energy.
34. The system of claim 1, further comprising means for adjusting a
gain of at least one of the detectors according to respective
positions of the energy sources.
35. The system of claim 1, further comprising means for adjusting
respective gains of the detectors according to respective positions
of the energy sources.
36. The system of claim 1, wherein distances between
source-detector pairs of the energy sources and the detectors vary
over a distance of at least about 5 cm.
37. The system of claim 1, wherein the scattering medium comprises
a large tissue structure.
38. The system of claim 1, further comprising a data acquisition
unit for reconstructing the time series of images of the scattering
medium based on the respective signals detected by the
detectors.
.[.39. The system of claim 2, wherein there are varying numbers of
pairs of the energy sources and the detectors in the linear
arrays..].
40. The system of claim 3, wherein the means for adjusting
comprises a programmable gain amplifier.
41. The system of claim 10, wherein the imaging head undergoes
uniform expansion and contraction while preserving a hemispherical
geometry to accommodate different size scattering mediums.
42. The system of claim 10, wherein the imaging head includes a
target volume through which the scattering medium enters the
imaging head.
43. The system of claim 10, wherein detector fibers of the at least
one energy transmissive fiber bundle are located on an inner aspect
of the imaging head.
44. The system of claim 13, wherein the imaging head comprises a
Hoberman sphere, about which the plurality of energy transmissive
fiber bundles are disposed.
45. The system of claim 13, wherein the plurality of energy
transmissive fiber bundles are attached to vertices of a hemisphere
of the imaging head.
46. The system of claim 13, wherein the plurality of energy
transmissive fiber bundles are attached to interlocking joints of
the imaging head.
47. The method of claim 14, further comprising adjusting respective
gains by which the energy is detected by the detectors according to
respective positions of the energy sources.
48. The method of claim 14, wherein the energy comprises near
infra-red light.
49. The method of claim 14, wherein distances between
source-detector pairs of the sources and the detectors vary over a
distance of at least about 5 cm.
50. The method of claim 14, wherein the scattering medium comprises
a large tissue structure.
.[.51. The system of claim 1, further comprising: an imaging head,
on which the energy sources and the detectors are arranged; wherein
the energy sources and the detectors are arranged in a plurality of
linear arrays to enable reconstruction of a 3-D image of the
scattering medium..].
.Iadd.52. The system of claim 26, wherein the source fiber bundles
and the receiver fiber bundles are arranged to enable three
dimensional images to be computed from super positioning of two
dimensional images..Iaddend.
.Iadd.53. The system of claim 26, wherein the source fiber bundles
and the receiver fiber bundles detect fluorescence radiation
excited by the energy sources..Iaddend.
.Iadd.54. The system of claim 26, wherein the source fiber bundles
and the receiver fiber bundles detectors detect acoustic energy
produced in the scattering medium by the energy
sources..Iaddend.
.Iadd.55. The system of claim 1, wherein the imaging head includes
a deformable array of the source fiber bundles and the receiver
fiber bundles, wherein the deformable array conforms to a surface
of a curved medium..Iaddend.
.Iadd.56. The system of claim 1, wherein the imaging head is a
folding structure in a hemispherical geometry..Iaddend.
.Iadd.57. The system of claim 1, wherein the source fiber bundles
and the receiver fiber bundles are arranged in a geometry of a
plurality of linear arrays to enable reconstruction of a
corresponding plurality of 2-D images of the scattering
medium..Iaddend.
.Iadd.58. The system of claim 57, wherein linear arrays are
configured to have a varying number of source-detector fiber
bundles..Iaddend.
.Iadd.59. A system for use in tomographic imaging of a scattering
medium, comprising an imaging head including source fiber bundles
in a first patterned array and receiver fiber bundles in a second,
patterned array, wherein the imaging head undergoes uniform
expansion and contraction to accommodate different size scattering
mediums, wherein the source fiber bundles function as a plurality
of energy sources in an illumination array, each energy source
emitting a respective signal for imaging the scattering medium onto
a different illumination area on a surface of said scattering
target medium, wherein more than one of the plurality of energy
sources emit their respective signals and the respective signals
are scattered by the scattering medium and emerge from the
scattering medium, and wherein the receiver fiber bundles function
as a plurality of detectors for detecting the respective signals
that emerge from the scattering medium for use in measuring dynamic
properties of the scattering medium in a time series of images
using optical tomography..Iaddend.
.Iadd.60. The system of claim 59, wherein the source fiber bundles
and the receiver fiber bundles are arranged in a geometry of a
plurality of linear arrays to enable reconstruction of a
corresponding plurality of 2-D images of the scattering
medium..Iaddend.
.Iadd.61. A method of imaging a scattering medium using optical
tomographic imaging, comprising: (a) placing on the scattering
medium an imaging head including fiber bundles in a patterned
array, wherein the fiber bundles includes source fiber bundles in
an illumination array and receiver fiber bundles in another array,
wherein the imaging head undergoes uniform expansion and
contraction to accommodate different size scattering mediums; (b)
exposing the scattering medium to energy from the source fiber
bundles, each energy source emitting a respective signal for
imaging the scattering target medium onto a different illumination
area on a surface of said scattering target medium; and (c)
detecting the energy, via the receiver fiber bundles, after the
energy has been scattered by the scattering medium for use in
measuring dynamic properties of the scattering medium in a time
series of images using optical tomography..Iaddend.
.Iadd.62. A system for use in tomographic imaging of a scattering
medium, comprising an imaging head including fiber bundles in a
patterned array, wherein the fiber bundles includes a plurality of
energy sources and a plurality of detectors, wherein the imaging
head undergoes uniform expansion and contraction to accommodate
different size scattering mediums, each energy source emitting a
respective signal for imaging the scattering target medium onto a
different illumination area on a surface of said scattering target
medium, wherein more than one of the plurality of energy sources
emit their respective signals and the respective signals are
scattered by the scattering medium and emerge from the scattering
medium, and wherein the plurality of detectors detect the
respective signals that emerge from the scattering medium for use
in measuring dynamic properties of the scattering medium in a time
series of images using optical tomography..Iaddend.
.Iadd.63. A method of imaging a scattering medium using optical
tomographic imaging, comprising: (a) placing on the scattering
medium an imaging head including fiber bundles in a patterned
array, wherein the fiber bundles includes a plurality of energy
sources in an illumination array and a plurality of detectors,
wherein the imaging head undergoes uniform expansion and
contraction to accommodate different size scattering mediums; (b)
exposing the scattering medium to energy from the plurality of
energy sources, each energy source emits a respective signal for
imaging the scattering target medium onto a different illumination
area on a surface of said scattering target medium; and (c)
detecting the energy, via the plurality of detectors, after the
energy has been scattered by the scattering medium for use in
measuring dynamic properties of the scattering medium in a time
series of images using optical tomography..Iaddend.
.Iadd.64. The system of claim 59, wherein each fiber in said
receiver fiber bundles is not identical with any of the source
fiber bundles..Iaddend.
.Iadd.65. The method of claim 61, wherein each fiber in said
receiver fiber bundles is not identical with any of the source
fiber bundles..Iaddend.
.Iadd.66. The system of claim 62, wherein each detector in said
fiber bundles is not identical with any of the plurality of energy
sources in said fiber bundles..Iaddend.
.Iadd.67. The method of claim 63, wherein each detector in said
fiber bundles is not identical with any of the plurality of energy
sources in said fiber bundles..Iaddend.
Description
FIELD OF THE INVENTION
The invention relates to a system and method for tomographic
imaging of dynamic properties of a scattering medium, which may
have special application to medical imaging, and in particular to
systems and methods for tomographic imaging using near infrared
energy to image time variations in the optical properties of
tissue.
BACKGROUND OF THE INVENTION
Contrary to imaging methods relying on the use of ionizing
radiation and/or toxic/radioactive contrast agents, near infra-red
(NIR)-imaging methods bear no known risk of causing harm to the
patient. The dose of optical intensity used remains far below the
threshold of thermal damage and is therefore safe. In the regime of
wavelength/intensity/power used, there are no effects on patient
tissue that accumulate with increasing NIR dose due to over-all
irradiation time.
The general technology involved in optical tomography is developed
and understood, so that, compared to other cross-sectional imaging
techniques such as MRI, X-ray CT, and the like, only moderate costs
and relatively small-sized devices are required. Optical tomography
especially gains from the development of small, economical, yet
powerful semiconductor lasers (laser diodes) and the availability
of highly integrated, economical off-the-shelf data processing
electronics suitable for the application. Moreover, the
availability of powerful yet inexpensive computers contributes to
the attractiveness of optical tomography since a significant
computational effort may be necessary for both image reconstruction
and data analysis.
Optical tomography yields insights into anatomy and physiology that
are unavailable from other imaging methods, since the underlying
biochemical activities of physiological processes almost always
leads to changes in tissue optical properties. For example, imaging
blood content and oxygenation is of interest. Blood shows prominent
absorption spectra in the NIR region and vascular dynamics and
blood oxygenation play a major role in physiology/pathology.
However, cross-sectional or volumetric imaging of dynamic features
in large tissue structures is not extractable with current optical
imaging methods. At present, whereas a variety of methods involving
imaging and non-imaging modalities are available for assessing
specific features of the vasculature, none of these assess dynamic
properties based on measures of hemoglobin states. For instance,
detailed images of the vascular architecture involving larger
vessels (>1 mm dia.) can be provided using x-ray enhanced
contrast imaging or MR angiography. These methods however are
insensitive to hemoglobin states and only indirectly provide
measures of altered blood flow. The latter is well accomplished, in
the case of larger vessels, using Doppler ultrasound, and for
near-surface microvessels by laser Doppler measurements, but each
is insensitive to variations in tissue blood volume or blood
oxygenation. Ultrasound measurements are also limited by their
ability to penetrate bone. Other methods are available, (e.g.,
pulse volume recording, magnetic resonance (MR) BOLD method,
radioscintigraphic methods), and each is able to sample, either
directly or indirectly, only a portion of the indicated desired
measures.
Thus, there is a need for a system and method of data collection
providing cross-sectional or volumetric imaging of dynamic features
in large tissue structures
SUMMARY OF THE INVENTION
The present invention provides a system and method for generating
an image of dynamic properties in a scattering medium. The system
includes an energy source, such as a NIR emitting source, and a
detection system to measure received energy. In an exemplary
embodiment, the detection system has at least one photo-detector
such as a photodiode, a means for rapid adjustment of signal gain,
and a device for retaining a measured response in order to
investigate the dynamic variations in the optical properties of
tissues. Depending on the implementation, the detection system
further may also include at least one means for separating a
plurality of signals from the photo-receiver when multiple energy
sources are used simultaneously. This simultaneous use of multiple
energy sources allows the use of different wavelengths and/or
different source locations at the same time.
In one implementation using optical tomographic imaging, a specimen
is exposed to NIR light emitted from at least one laser diode.
Furthermore an imaging head may be utilized that contains means for
positioning at least one source location and / or at least one
detector location with respect to the medium. The energy detector
may use an energy collecting element, such as an optical fiber to
transmit the received energy. The energy detector is responsive to
the energy or light emerging from the specimen. In accordance with
the invention, the signal from the detector is selectively enhanced
in gain to increase the dynamic measurement range. The method may
further include separating via at least one lock-in amplifier a
plurality of signals generated by multiple energy sources. In
addition, the method allows simultaneous measurements of signals
produced by the NIR light by means of a sample-and-hold circuit
when more than one detector fiber is used.
BRIEF DESCRIPTION OF THE FIGURES
For a better understanding of the invention, together with the
various features and advantages thereof, reference should be made
to the following detailed description of the preferred embodiments
and to the accompanying drawings wherein:
FIG. 1 is a block diagram of one embodiment of a system according
to the invention,
FIG. 2 is a block diagram illustrating one implementation of the
system in FIG. 1;
FIG. 3 is a perspective view of a servo-motor apparatus useful in
this invention to illuminate a number of fiber bundles with a
single energy source;
FIG. 4 is a schematic illustration of the disposition for examining
human tissue such as a human breast;
FIG. 5 is a schematic illustration of a planar imaging head useful
in one embodiment of the invention;
FIG. 6 is one embodiment for the source detector arrangement on the
imaging head shown in FIG. 5;
FIG. 7 is an illustration of a spherical imaging head useful in
practicing the invention;
FIG. 8 is a block diagram of a detector channel useful in
practicing the invention;
FIG. 9 is a graphical representation of one implementation of a
timing scheme used in the system of FIG. 1;
FIG. 10 is a diagram illustrating the sequence of certain events in
a multiple channel embodiment of the invention,
FIG. 11 is a schematic illustration of the physical arrangement of
multiple detector channels used in a preferred embodiment of the
invention;
FIG. 12 is a circuit diagram of one detector channel used in FIG.
11; and
FIG. 13 is a circuit diagram of one implementation of the lock-in
module used in FIG. 12.
DETAILED DESCRIPTION OF THE INVENTION
The objective of the invention is to provide a system and method
capable to extract dynamics in properties of a scattering medium.
The use of the invention's system and method has several
applications including, but not limited to, medical imaging
applications. Although the methods described herein focus on
tomographic imaging the dynamic properties of hemoglobin states and
tissue using optical tomography, with an imaging source generating
multiple wavelengths in the NIR region, it is appreciated that the
invention is applicable to any medium that is able to scatter the
propagating energy from any energy source, including external
energy sources such as those sources located outside the medium
and/or internal sources such as those energy sources located inside
the medium. For example, other media includes, but are not limited
to, medium from mammals, botanical life, aquatic life, or
invertebrates; oceans or water masses; foggy or
gaseous-atmospheres; earth strata; industrial materials; man-made
or naturally occurring chemicals and the like. Energy sources
include, but are not limited to, non-laser optical sources like LED
and high-pressure incandescent lamps and lasers sources such as
laser diodes, solid state lasers such as titanium-sapphire laser
and ruby laser, dye laser and other electromagnetic sources,
acoustic energy, acoustic energy produced by optical energy,
optical energy, and any combinations thereof
Similarly the means to detect the signal produced by the energy
source is not limited to photodiode implementation discussed in one
of the preferred embodiments further described herein. Other
detectors can be used with the principles of the present invention
for the purpose of tomographic imaging the dynamic properties of a
medium. Such detectors include for example, but are not limited to,
photodiodes, PIN diodes (PIN), Avalanche Photodiodes (APD), charge
couple device (CCD), charge inductive device (CID),
photo-multiplier tubes (PMT), multi-channel plate (MCP), acoustic
transducers and the like.
The present invention builds upon previous disclosures in U.S. Pat.
Nos. 5,137,355 ("the '355 patent") entitled "Method of Imaging a
Random Medium" ("the '355 patent") and U.S. Pat. No. 6,081,322
("the '322 patent") entitled "NIR Clinical Opti-Scan System", the
disclosures of both the '355 and '322 patents are incorporated
herein by reference. Disclosed in these patents is an approach to
optical tomography, and the instrumentation required to accomplish
the tomography. The modifications in the present invention provide
fast data acquisition, and new imaging head designs. Fast data
acquisition allows accurate sampling of dynamic features. The
modification in the imaging head allows accommodation of different
size targets (e.g., breast); the stabilization of the target
against motion artifacts; conforming the target to a simple
well-defined geometry; and knowledge of source and detector
positioning on or about the target. All of the enumerated features
listed above for the imaging head is crucial for accurate image
reconstruction.
Additionally, the present invention uses detector circuitry that
allows quick adaptation of the measurement range to the signal
strength thereby increasing the over-all dynamic range. "Dynamic
range" for the purposes of this description means the ratio between
the highest and lowest detectable signal. This makes the circuitry
suitable for use with source-detector distances that can vary
significantly during the data collection, thereby allowing fast
data acquisition over wide viewing angles. For instance, we are
aware that dynamic features of dense scattering media may be
extractable from measurements using a single source and single
detector at a fixed distance between each other. Depending on the
implementation, such an arrangement could be made using a detector
of relatively small dynamic range. Although we are aware of the
possible usefulness of such a measurement, our invention allows the
measurement of dynamics in optical properties of dense scattering
media using source-detector pairs over a wide range of distances
(e.g., greater than or about 5 cm). Such fall tomographic
measurements allow for improved accuracy in image
reconstruction.
Depending upon the implementation, it is within the scope of the
present invention to include those embodiments using a restricted
source detector distance and therefore not requiring fast gain
adjustment. For example, in one embodiment, the system of the
present invention can also be operated using detector channels of
low-dynamic range (e.g., 1:10.00) when detector fibers of a fixed
distance from the source are being used for the measurement (e.g.,
the detector opposite the source).
The data collection scheme of the present invention disclosed
herein provides time-series of raw data sets that provide useful
information about dynamic properties of the scattering medium
without any further image reconstruction. For example, by
displaying the raw data in a color mapping format, features can be
extracted by sole visual inspection. In addition to that, analysis
algorithms of various types such as, but not limited to, linear and
non-linear time-series analysis or pattern recognition methods can
be applied to the series of raw data. The advantage of using these
analytical methods is the improved capability to reveal dynamic
signatures in the signals.
In another implementation, image reconstruction methods may be
applied to the sets of raw data thereby providing time series of
cross-sectional images of the scattering medium. For these
implementations, analysis methods of various types such as, but not
limited to, linear and non-linear time-series analysis, filtering,
or pattern recognition methods can be applied. The advantage of
using such analysis is the improved extraction of dynamic features
and cross-sectional view, thereby increasing diagnostic sensitivity
and specificity. These methods are explained in detail in the '355
and '322 patents, which were previously described and incorporated
in as reference.
The invention reveals measurements of real-time spatiotemporal
dynamics. Depending on the implementation, an image of dynamic
optical properties of scattering medium such as, but not limited
to, the vasculature of the human body in a cross-sectional view is
provided. The technology employs low cost, compact instrumentation
that uses non-damaging near infrared optical sources and features
several alternate imaging heads to permit investigation of a broad
range of anatomical sites.
In another implementation, the principles of the present invention
can be used in conjunction with contrast agents such as absorbing
and fluorescent agents. In another variant, the present invention
allows tie cross-sectional measurements of changes in optical
properties due to variations in temperature. The advantage of this
variant is seen, but not restricted to, the use of monitoring
cryosurgery.
A system using the modified instrumentation and described methods
of the instant invention is capable of producing cross-sectional
images of real-time events associated with vascular reactivity in a
variety of tissue structures (e.g., limbs, breast, head and neck).
Such measurements permit an in-depth analysis of local hemodynamic
states that can be influenced by a variety of physiological
manipulations, pharmacological agents or pathological conditions.
Measurable physiological parameters include identification of local
dynamic variations in tissue blood volume, blood oxygenation,
estimates of flow rates, and tissue oxygen consumption. It is
specifically noted that measurements of several locations on the
same medium can be taken. For example, measurements may be taken of
the leg and arm areas of a patient at the same time. Correlation,
of data between the different locations is available using the
methods described herein.
The invention also provides both linear and non-linear time series
analysis to reveal site specific functionality of the various
components of the vascular tree. Thus the response characteristics
of the major veins, arteries and structures associated with the
microcirculation can be evaluated in response to a range of
stimuli.
Fast data collection methods are particularly helpful because there
are many disease states with specific influences on the
spatial-dynamic properties of vascular responses. Accordingly, it
is understood that significantly greater contrast mechanisms are
definable, with much greater diagnostic sensitivity. This is
accomplished by collecting and evaluating data in the time domain.
These results are not available by performing static imaging
studies.
The importance of dynamic properties follows directly from an
understanding of the well known physiological reactivity of the
vascular system. Control of the peripheral vasculature is mediated
by neural, humoral and metabolic factors. Neural control is
principally through autonomic activity. The details of these
properties are well known to many, and can be found in any one of
several medical physiology texts. Loss of autonomic control occurs
in a variety of disease processes, especially in diabetes.
Invariably, this loss of control will adversely influence local
perfusion states. The current invention has the capacity to
directly evaluate the concept known as vascular sufficiency. This
tern takes into account the fact that, among its many roles, the
vasculature is uniquely responsible for the delivery of essential
nutrients to tissue, in particular, oxygen, and for the removal of
metabolic waste products. Imbalances between supply and demand lead
to relative hypoxic states, which often are clinically
significant.
FIG. 1 illustrates one embodiment of the invention. Shown is a
system 100 comprising medium 102. The medium can be any medium in
which the propagation of the used source energy is strongly
affected by scattering.
From a source module 101 energy is directed to the medium 102 from
which the exiting energy is measured by means of detector 106,
further discussed below. As previously discussed, there is a
variety of sources, media, and detectors that may be used with the
principles of the present invention. The following is a discussion
of a sampling of such elements with the intention to describe how
the invention is realized. In no way are these examples meant, nor
do they intend to limit the invention to these implementations. A
variation of elements as described herein may also utilize the
principles of the present invention.
In one implementation, measurements of dynamics in the optical
properties of the medium is accomplished by using optical source
energy and performing rapid detection of the acoustic energy
created by absorption processes in the medium. This can be
implemented using both pulsed and harmonic modulated light sources,
the latter allowing for lock-in detection. Detectors can be, but
are not limited to, piezo-electric transducers such as PZT crystals
or PVDF foils.
In another variant, a timing and control facility 104 is used to
coordinate source and detector operation. This coordination is
further described below. A device 116 provides acquisition and
storage of the data measured by the detector 106. Depending on the
implementation, control and timing of the system's components is
provided by a computer, which includes a central processor unit
(CPU), volatile and non-volatile memory, data input and output
ports, data and program code storage on fixed and removable media
and the like. Each main component is described in greater detail
below.
FIG. 2 illustrates another implementation of a preferred embodiment
of the present invention. Shown is a system and method that
incorporates at least one wavelength measurement. Depending upon
the implementation, this measurement is accomplished by alternately
coupling light from diode lasers into transmitting fibers arranged
in a circular geometry.
Referring again to FIG. 2, a system 200 includes an energy source,
which in this implementation includes one or more laser 101. A
reference detector 202 is used to monitor the actual output power
of laser 101 and is coupled to a data acquisition unit 116. Such
laser may be a laser diode in the NIR region. The laser is
intensity modulated by a modulation means 203 for providing means
of separation of background energy sources such as daylight. The
modulation signal is also sent to a phase shifter 204 whose purpose
is described further below. The light energy generated by the laser
101 is directed into an optical de-multiplexing device 300 further
discussed in detail below. Using a rotating mirror 305, the light
is directed into one of several optical source fiber bundles 306
that are used to deliver the optical energy to the medium 102. To
provide good optical contact and measurement fidelity, one of
several possible imaging heads 206 as described further below is
used. A motor controller 201 is coupled to the de-multiplexing
device 300 for controlling the motion of the rotating mirror 305.
The motor controller 201 is also in communication with a timing
control 104 for controlling the timing of the motion of mirror
305.
The measuring head 206 comprises the common end of a bifurcated
optical fiber bundle, whose split ends are formed by the source
fiber bundle 306 and detector fiber bundle 207. Source fiber bundle
306 and detector fiber bundle 207 form a bulls eye geometry at the
common end with the source fiber bundle in the center. In other
embodiments, source and detector bundles are arranged differently
at the common end (e.g., reversed geometry or arbitrary arrangement
of the bundle filaments). The common end of a bifurcated optical
fiber bundle, preferably comes in contact with the medium, however,
this embodiment is not limited to contact with the medium. For
example, the common ends may simply be disposed about the medium.
The signal is transmitted from the detector fiber bundle 207 to a
detector unit 106 that comprises at least one detector channel 205
further described herein. The detector channel 205 is coupled to
the data acquisition unit 116 and the timing control unit 104.
Depending on the implementation, a phase shifter 204 may or may not
be used, and is coupled to the detector unit 106 for the purposes
of providing a reference signal for the purposes of filtering the
signal received from bundle 207.
Depending on the implementation, illustrated in FIG. 3 is a device
for the measurement of the dynamic properties of a scattering
medium. This measurement is performed by sequentially reflecting
light 302 off of a rotatable front surface mirror 306, mounted at a
45 degree angle to the incident source, into source fibers 306
arranged in a circular geometry about the rotating optic. The
rotation is done by a motor 308 with a shaft 307 to which the
mirror is attached. This embodiment has an advantage of enabling
fast switching among the transmitting fibers. In particular, it
provides the ability to introduce beam shaping optics between the
reflective mirror and transmitting fibers thereby allowing fine
adjustment of the illumination area available for coupling into the
fibers. This is useful because it allows independent adjustment of
the rotation speed of the reflective optic (i.e., switching speed),
and the illumination time allowed for each transmitting fiber
bundle. Thus, a range of illumination frequencies can be employed
while allowing fine adjustment of the illumination time at each
source position to permit collection of data having a suitable
signal-to-noise ratio.
Light from laser 101 is transmitted to unit 300 by means of
transmitting optics 303 including, but not limited to, fiber optics
and free propagating beams. Further beam shaping optics 301 may be
used to optimize in coupling efficiency into the transmitting
fibers. Units 303 and 301 are under mechanical fine adjustment in
their position with respect to the mirror 309.
Motor 308 is operated under control of motion control 201 to allow
for precise positioning and timing. By this means, it is possible
to operate the motor under complex motion protocols such as in a
start-stop fashion where the motor stops at a desired location
thereby allowing the stable coupling of light into a transmitting
fiber bundle. After the measurement at this source location is
performed, the motor moves on to the next transmitting fiber.
Motion control is in two-way communication with the timing control
104 thereby allowing precise timing of this procedure. Motion
control allows the assignment of relative and/or absolute mirror
positions allowing for precise alignment of the mirror with respect
to the physical location of the fiber bundle. The mirror 306 is
surrounded by a cylindrical shroud 309 in order to shield off stray
light to prevent cross-talk. The shroud comprises an aperture 310
through which the light beam 302 passes toward the transmitting
fiber. It is recognized and incorporated herein other schemes which
may be used,(e.g., use of a fiber-optic switching device) to
sequentially couple light into the transmitting fibers.
In an equivalent embodiment, fast switching of source positions is
accomplished by using a number of light sources, each coupled into
one of the transmitting fibers 306 which can be turned on and of
each independently by electronic means.
The device employs the servo-motor control system 308 in FIG. 3
with beam steering optics, described above, to sequentially direct
optical energy emerging from the source optics onto about 1 mm
diameter optical fiber bundles 306, which are mounted in a circular
array in the multiplexing input coupler 300. The transmitting
optical fiber bundles 306, which are typically 2-3 meters in length
are arranged in the form of an umbilical and terminate in the
imaging head 206.
Depending on the implementation, the apparatus of the present
invention required for time-series imaging, employs the value of
using a geometrically adaptive measurement head or imaging head.
The imaging head of the present invention provides features that
include, but are not limited to, 1) accommodating different size
targets (e.g., breast); 2) stabilizing the target against motion
artifacts; 3) conforming the target to well-defined geometry; and
4) to provide exact knowledge of locations for sources and
detectors. Stability and a known geometry both contribute to the
use of efficient numerical analysis schemes.
There are several different embodiments of the imaging head for
data collection that may utilize the principles of the present
invention. For example the use of an iris imaging head previously
disclosed in the '322 and '355 patents , which are incorporated by
reference in this disclosure, may be used with the principles of
the present invention.
Described below are two exemplary imaging heads with the
understanding that the invention may or may not use any type of
imaging head, and if an imaging head is used, it would provide the
features previously described.
As illustrated in FIG. 4, the iris unit can be employed as a
parallel array of irises 402, 404, 406 enabling volume imaging
studies. FIG. 4 illustrates how this can be configured for studying
a medium 410, in this example a human breast, using an imaging head
408. As described previously, the medium used in the present
invention can be any medium, which allows scattering of energy.
In one implementation, the imaging head illustrated in FIG. 5 is a
flexible pad configuration. This planar imaging unit functions as a
deformable array and is well suited to investigate body structures
too large to permit transmission measurements (e.g., head and neck,
torso, and the like). Using this type of imaging head, optical
measurements are made in a back-reflection mode. Optical fiber
bundles 502 originating from the optical multiplexing input coupler
112 (described elsewhere) terminate at the deformable array or
flexible pad 500. The pad can be made of any flexible material such
as black rubber or the like. The optical fiber bundles may be
bifurcated and have ends 504 that both transmit and receive light.
More than one pad may or may not be used, although an additional
pad is not necessary for the purpose of the present invention, or
for measurement application to other portions of the medium or to
the same medium. For example, in the case of a breast exam, both
pads maybe applied to the same breast having one pad above and one
pad below the breast. In addition, one pad maybe applied to the
right breast by having the pad deformed around the breast.
Similarly, the other pad may be applied to the left breast. This
configuration would allow both breasts to be examined at the same
time. In addition, information may be correlation between the data
collected from the two different members of the body. Again, the
invention can be applied to other media and is not limited to
portions of the human body. Thus, correlation between different
media may be collected using this technique.
As further shown in FIG. 5, the additional pad would have similar
functions as the pad previously described and would have optical
fiber bundles 503, flexible pad 505, and bifurcated optical fiber
bundle ends 501 similar to the previous pad described. The array
itself can be deformed to conform to the surface of a curved medium
to be imaged (e.g. portion of the torso). The deformable array
optical energy source and receiver design includes, depending on
the implementation, a 7.times.9 array (63 total bundles) of optical
fiber bundles as illustrated in FIG. 6. In one variant, each bundle
is typically 3 mm in diameter. Depending on the implementation,
eighteen (18) of the sixty-three (63) fiber bundles may be arranged
in an array to serve as both optical energy sources or energy
transmitters, and receivers to sequentially deliver light to a
designated target and receive emerging optical energy. In this
implementation, the remaining forty-five (45) fiber bundles act
only as receivers of the emerging optical energy.
The geometry of the illumination array is not arbitrary. The design
shown in FIG. 6 as an exemplary illustration has been configured,
as have other implementations, to minimize the subsequent numerical
effort required for data analysis while maximizing the
source-density covered by the array. The fiber bundles are arranged
in an alternating pattern as described by FIG. 6 and shown here
with the symbols "X" and "0". In one implementation, a pattern of
00X000X00, X000X000X can be used on the imaging head. `X` denotes a
source/receiver fiber bundle, and `0` is a receiver only. FIG. 6
indicates 2D imaging planes formed by multiple source/detector
positions along a line that can be used with this particular
pattern. The labels refer to the numbers of sources/detectors found
along those lines of optical fiber ends on the pad using the
following nomenclature: "S" followed by a number indicates the
number of source positions along that line; "D" followed by a
number indicates the number of detection points along that line.
For instance; "S3-D3" indicates an imaging plane formed by three
source positions and three detection points. Basically, the design
allows for the independent solution of two dimensional (2-D) image
recovery problems from an eighteen (18) point source measurement.
As a result, a composite three dimensional (3-D) image can be
computed from super-position of the array of 2-D images oriented
perpendicular to the target surface. Another advantage of this
geometry is that it readily permits the use of parallel
computational strategies without having to consider the entire
volume under examination.
The advantage of this geometry is that each reconstruction data set
is derived from a single linear array of source-detector fibers,
thereby enabling solution of a 2-D problem without imposing undue
physical approximations. The number of source-detector fibers
belonging to an array can be varied. Scan speeds attainable with
the 2-D array illustrated in FIG. 6 are the same as for other
imaging heads with 2-D arrays since the scan speed depends only on
the properties of the input coupler. Thus, faster scan speed are
available for the creation of a 3-D image.
In another implementation, illustrated in FIG. 7, is an imaging
head based on a "Hoberman" sphere geometry. In a Hoberman
structure, the geometry is based on the intersection of a cube and
an octahedron, which makes a folding polyhedron called a
trapezoidal icosatetrahedron. This structure has been modified and
implemented in a form of an imaging head of a hemispherical
geometry. For many purposes of the instant invention, it is
appropriate to use design features of smoothly varying surfaces
based on the Hoberman concept of expanding structures. Depending on
the implementation, other polygonal or spherical-type shapes may
also be used with the principles of the present invention for other
imaging head designs. Adjustment of the device in FIG. 7 causes
uniform expansion or contraction, thereby always preserving a
hemispherical geometry. Imaging head 700 illustrates one example of
modification to the "Hoberman" geometry. A receptacle for the fiber
bundles 701 is disposed about imaging head 700. Target volume 702
is where the medium would enter the imaging head in this
implementation. This geometry is well suited for the investigation
of certain tissues such as the female breast or the head. Depending
on the implementation, attachment of optical fibers to the vertices
of the hemisphere allows for up a seventeen (17) source by
seventeen (17) detector measurement. The folding structure can be
extended to accommodate a more "tear drop" or "bullet" shape of the
target medium by attaching additional circular iris-like structures
on top that expand and contract with the hemisphere. FIG. 7 shows
the combination of the hemisphere with one top iris comprising
receptacles for 8 additional fiber bundles leading to an overall
number of 25 source by 25 detector positions at the main vertices
for this configuration. More than one iris can be attached to the
top of the hemisphere. The diameter of the additional top irises
may or may not differ from the hemisphere diameter. The detectors
or energy receivers may be disposed about the imaging head and the
detectors are located on the inner aspect of the expanding imaging
head. Additional fiber bundles can be attached to the interlocking
joints, permitting up to a 49 source by 49 detector measurement for
the hemisphere only and up to 16 source/detector positions per
added iris.
Depending on the implementation, light collected from the target
medium is measured by using any of a number of optical detection
schemes. One embodiment uses a fiber-taper, which is bonded to a
charged coupled detector (CCD) array. The front end of the fiber
taper serves to receive light exiting from the collection fibers.
These fibers are preferably optical fibers, but can be any means
that allows the transmission and reception of signals. The back end
of the fiber taper is bonded to a 2-D charge-coupled-detector (CCD)
array. In practice, use of this approach generally will require an
additional signal attenuation module.
An alternate detection scheme employs an array of discrete photo
detectors, one for each fiber bundle. This unit can be operated in
a phase lock mode thereby allowing for improved rejection of
ambient light signals and the discrimination of multiple
simultaneously operated energy sources.
In another embodiment, in order to fulfill the demands posed by the
desired physiological studies on the instrument, the following
features characterize the detector system: scalable multi-channel
design (up to 32 detector channels per unit); high detection
sensitivity (below 10 pW); large dynamic range (1:10.sup.6
minimum); multi-wavelength operation; ambient light immunity; and
fast data acquisition (order of 100 Hz all-channel simultaneous
capture rate).
To achieve this, the detector system uses photodiodes and a signal
recovering technique involving electronic gain switching and phase
sensitive detection (lock-in amplification) for each detector fiber
(in the following referred to as detection or detector channels) to
ensure a large dynamic range at the desired data acquisition rate.
The phase sensitive signal recovery scheme not only suppresses
electronic noise to a desired level but also eliminates
disturbances given by background light and allows simultaneous use
of more than one energy source. Separation of signals from
simultaneously operating sources can be achieved, as long as the
different signals are encoded in sufficiently separated modulation
frequencies. Since noise reduction techniques are based on the
reduction of detection bandwidth, the system is designed to
maintain the desired rate of measurements. In order to achieve a
timing scheme that allows simultaneous readout of the channels, a
sample-and-hold circuit (S/H) is used for each detection channel
output. The analog signals provided by the detector channels are
sampled, digitized and stored using the data acquisition system
116. One aspect is the flexibility and scalability of the detection
instrument. Not only are the detector channels organized in single,
identical modules, but also the phase detection stages, each
containing two lock-in amplifiers, are added as cards. In this way,
an existing setup can easily be upgraded in either the number of
detector channels and/or the number of wavelengths used (up to
four) by cloning parts of the existing hardware.
FIG. 8 shows the block diagram of one implementation of a detector
channel. In this implementation, two energy sources are used. After
detecting the light at the optical input 801 by a photo detector
802 the signal is fed to a transimpedance amplifier 803. The
transimpedance value of 803 is externally settable by means of
digital signals 813 (PTA=Programmable Transimpedance Amplifier).
This allows the adaptation to various signal levels thereby
increasing the dynamic range of the detector channel. The signal is
subsequently amplified by a Programmable Gain Amplifier (PGA} whose
gain can be set externally by means of digital signals 814. This
allows for additional gain for the lowest signal levels (e. g., in
one implementation pW-nW) thereby increasing the dynamic range of
the detector channel.
In one embodiment, at least one energy source is used and the
signal is sent to at least one of lock-in amplifiers (LIA) 805,
809. Each lock-in amplifier comprises an input 808,812 for the
reference signal generated by phase shifter 204 from FIG. 2. After
lock-in detection, the demodulated signal is appropriately boosted
in gain by means of a programmable gain amplifier (PGA) 806, 810 in
order to maximize noise immunity during further signal transmission
and to improve digital resolution when being digitized. The gain of
PGA 806, 810 is set by digital signals 815.
At each output, a sample-and-hold circuit (S/H) 807, 811 is used
for freezing the signal under digital timing by means of signal 816
for purposes described herein.
In one embodiment, the signal 815 is sent to PGA 806,810 in
parallel. In one embodiment, the signal 816 is sent to 807,811 in
parallel.
As previously illustrated in FIG. 1, the analog signal provided by
each of the channel outputs is sampled by a data acquisition system
116. In one embodiment, PC extension boards might be used for this
purpose. PC extension boards also provide the digital outputs that
control the timing of functions such as gain settings and
sample-and-hold.
As previously noted, timing is crucial in order to provide the
desired image capture rate and to avoid false readings due to
detector-to-detector time skew. FIG. 9 shows one improvement of the
invention over other timing schemes. With systems, not comprising
fast adaptable gain settings (such as some CCD based systems), a
schedule according to 905 has to be implemented. The implementation
in FIG. 9 illustrates one use of a silicon photo-diode in process
904, which can be replaced by various detectors previously
mentioned. A time series of data is acquired for a fixed source
position. After finishing this task, the source is moved 902 with
respect to the target 901 and another series of data is collected.
Measurements are performed in this fashion for all source
positions. Every image 903 of the resulting time series of
reconstructed images is reconstructed from data sets merged
together from the data for each source position. This schedule does
not allow real-time capture of all physiologic processes in the
medium and therefore only applies to certain modes of
investigation. Although we are aware of the use of such schemes,
e.g., when monitoring responses on repeatable maneuvers, the timing
scheme for the invention very much improves on this situation.
Because the invention allows for fast source switching and large
dynamic range and high data acquisition rates, a schedule indicated
by 904 is performed. Here, the source position is switched fast
compared to the dynamic features of interest and instantaneous
multi-channel detection is performed at each source position.
Images 903 are then reconstructed from data sets, which represent
an instant state of the dynamic properties of the medium. Only one
time series of full data sets (i. e., all source positions and all
detector positions) is being recorded. Real time measurement of
fast dynamics (e.g., faster than I Hz) of the medium is provided by
the invention.
FIG. 10 shows one embodiment of a detailed schedule and sequence of
the system tasks 1001 involved in collecting data at a source
position and the proceeding of this process in time 1002. Task 1003
is the setting of the optical de-multiplexer to a destined source
position and setting the detectors to the appropriate gain-
settings. The source position is illuminated for a period of time
1004, during which the lock-in amplifiers settle 1005. After the
time it takes the S/H to sample the signal 1006, the signal is held
for a period of time 1007, during which all channels are read out
by the data acquisition. It is worthwhile noticing that during
reading out the S/H, other tasks, like moving the optical source,
setting the detector gains for the new source position, and
settling of the lock-in, are being scheduled. This increases
greatly the achievable data acquisition rate of the instrument.
This concept of a modular system is further illustrated in FIG. 11.
Up to thirty-two (32) detector modules 1100 (each with 2 lock-in
modules each for two modulation frequencies) are arranged using an
enclosure 1102. The cabinet also can carry up to two phase shifting
modules 1104, 1106, each containing two digital phase shifter under
computer control. The ability to adjust the reference phase with
respect to the signal becomes necessary since unavoidable phase
shifts in the signal may lead to non-optimum lock-in detection or
can even result in a vanishing output signal. Organization of data,
power supply and signal lines is provided by means of two back
planes 1108, 1110
Depending on the implementation, the detector system design
illustrated in FIG. 8 allows one cabinet to operate at a capacity
of 32 detectors with four different sources requiring 128 analog to
digital circuit (ADC)-board input channels. The upper 1108 and the
lower 1110 back plane are of identical layout and have to be linked
in order to provide the appropriate distribution of supply-,
control- and signal voltages. This is achieved using a 6U-module
fitting both planes from the backside, that provides the necessary
electric linking paths, and interfaces for control- and signal
lines.
FIG. 12 shows the schematic of one implementation of a channel
module. In this implementation, a silicon photo-diode 1206 is used
as the photo-detector. A Programmable Transimpedance Amplifier
(PTA) 1201 is formed by an operational amplifier 1204, resistors
1201 and 1202 and an electronic switch 1205, the latter of which is
realized using a miniature relay. Other forms of electronic
switches such as analog switches might be used. Relay 1205 is used
to connect or disconnect 1203 from the circuit thereby changing the
transimpedance value of 1201. A high-pass filter (R2, C5) is used
to AC-couple the subsequent programmable gain instrumentation
amplifier IC2 (Burr Brown PGA202) in order to remove DC offset. The
board-to-board connectors for the two lock-in-modules are labeled
as "slot A" 1210 and "slot B" 1212. The main connector to the
backplane is a 96-pole DIN plug 1220.
FIG. 13, illustrates the electric circuit of the lock in modules
1210, 1212. The signal is subdivided and passed to two identical
lock-in-amplifiers, each of which gets one particular reference
signal according to the sources used in the experiment. The signal
is first buffered IC1, IC7 (AD LF 111) and then demodulated using
an AD630 double-balanced mixer IC2, IC8.
In order to remove undesired AC components, the demodulated signal
passes through an active 4-pole Bessel-type filter IC3, IC4, IC9,
IC10 (Burr Brown UAF42). A Bessel-type filter has been chosen in
order to provide fastest settling of the lock-in amplifier for a
given bandwidth. Since a Bessel-filter shows only slow
stopband-transition, a 4-pole filter is being used to guarantee
sufficient suppression of cross talk between signals generated by
different sources (i.e. of different modulation frequency). The
filter has its 3 dB point at 140 Hz, resulting in 6 ms settling
time for a step response (<1% deviation of actual value). The
isolation of frequencies separated by 1 kHz is 54 dB. The filters
are followed by a programmable gain amplifier IC5, IC 11, whose
general function has been described above. The last stage is formed
by a sample-and-hold chip (S/H) IC6, IC12 (National LF398).
In another implementation, the phase sensitive detection can be
achieved with digital methods using digital signal processing (DSP)
components and algorithms. The advantage of using DSP with the
principles of the present invention is improved electronic
performance and enhanced system flexibility.
In another implementation, an analog-to-digital converter is used
for each detector channel thereby improving noise immunity of the
signals.
Although illustrative embodiments have been described herein in
detail, those skilled in the art will appreciate that variations
may be made without departing from the spirit and scope of this
invention. Moreover, unless otherwise specifically stated, the
terms and expressions used herein are terms of description and not
terms of limitation, and are not intended to exclude any
equivalents of the system and methods set forth in the following
claims.
* * * * *