U.S. patent application number 10/068583 was filed with the patent office on 2002-07-18 for imaging system with automatic gain control for reflectance and fluorescence endoscopy.
This patent application is currently assigned to Xillix Technologies Corp.. Invention is credited to Cline, Richard W., Dawson, Remy, Fengler, John J.P., Figley, Curtis B., Jaggi, Bruno W..
Application Number | 20020093563 10/068583 |
Document ID | / |
Family ID | 22057495 |
Filed Date | 2002-07-18 |
United States Patent
Application |
20020093563 |
Kind Code |
A1 |
Cline, Richard W. ; et
al. |
July 18, 2002 |
Imaging system with automatic gain control for reflectance and
fluorescence endoscopy
Abstract
An imaging system for white light and fluorescence endoscopy
that includes an automatic gain control circuit 30 that adjusts the
brightness of an image produced based on distribution of pixel
intensities in one or more video frames. The magnitude of the image
signals produced by a pair of high sensitivity imaging devices such
as intensified CCD transducers are compared to a number of
reference thresholds. A time-over-threshold counter (112)
determines the number of pixels in the image signals having
magnitudes greater than or less than the reference thresholds. The
distribution of pixel intensities is supplied to a decision tree
algorithm (116) that determines whether the gain of the-intensified
CCD transducers (44a, 44b) used to produced the autofluorescence
images or the intensity of the excitation light produced by a light
source (36) should be increased or decreased. In addition, a mode
switch mechanism is provided to change rapidly from the
fluorescence imaging mode to the white light imaging mode or vice
versa. This mechanism includes provisions to prevent the accidental
application of reflected white illumination light to the
image-intensified CCD transducers. Proximity switches (192, 194)
monitor the position of a light directing mechanism such as a
mirror (186) to allow light to pass to fluorescence camera head
(42) or to a color video camera head (46). The light source is not
switched to produce white light until it is known that the mirror
is in position to direct the reflected light to the color video
camera head. Finally, the present invention produces a quantitative
display of the relative intensities of the autofluorescence light
produced in a pair of spectral bands.
Inventors: |
Cline, Richard W.;
(Vancouver, CA) ; Fengler, John J.P.; (North
Vancouver, CA) ; Figley, Curtis B.; (Edmonton,
CA) ; Dawson, Remy; (Vancouver, CA) ; Jaggi,
Bruno W.; (Vancouver, CA) |
Correspondence
Address: |
CHRISTENSEN, O'CONNOR, JOHNSON, KINDNESS, PLLC
1420 FIFTH AVENUE
SUITE 2800
SEATTLE
WA
98101-2347
US
|
Assignee: |
Xillix Technologies Corp.
|
Family ID: |
22057495 |
Appl. No.: |
10/068583 |
Filed: |
February 6, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10068583 |
Feb 6, 2002 |
|
|
|
09064667 |
Apr 20, 1998 |
|
|
|
Current U.S.
Class: |
348/65 ; 348/68;
600/101; 600/109 |
Current CPC
Class: |
A61B 1/045 20130101;
A61B 1/043 20130101; A61B 5/0084 20130101; A61B 1/00009 20130101;
A61B 5/0071 20130101 |
Class at
Publication: |
348/65 ; 348/68;
600/101; 600/109 |
International
Class: |
H04N 007/18 |
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. An imaging system for fluorescence endoscopy, comprising: a
light source that produces fluorescence excitation light; an
endoscope that delivers the fluorescence excitation light to tissue
under examination in vivo and collects autofluorescence produced by
the tissue; a dual channel fluorescence camera containing a first
and second high sensitivity imaging device that receive the
autofluorescence in a first and second spectral band and produce
electronic signals that are representative of the tissue under
examination; a control center, including an image processing board
that receives the electronic signals produced by the dual channel
fluorescence camera and said control center causes an image of the
tissue under examination to be processed, stored and displayed on a
video monitor; an automatic gain control circuit that determines a
distribution of intensity levels in the electronic signals produced
by the dual channel fluorescence camera and adjusts a gain of the
first and second high sensitivity image device and/or adjusts a
light source intensity based on said distribution of the intensity
levels such that the relative gain between the two intensified CCD
transducers follows substantially a polynomial; and a video monitor
that receives the video signals produced by the image processing
board and displays an image of the tissue under examination.
2. The imaging system for fluorescence endoscopy of claim 1,
wherein the automatic gain control circuit comprises: a plurality
of time-over-threshold counters that determine an image area in one
or more video fields that have intensities above a plurality of
predetermined thresholds.
3. The imaging system for fluorescence endoscopy of claim 2,
wherein the time-over-threshold counters further comprise: a clock
signal having a frequency substantially equal to a pixel clock of
the dual channel fluorescence camera; a gating circuit that passes
the clock signal during an active portion of the electronic signals
produced by the dual channel fluorescence camera; a plurality of
counters that count pulses of the gated clock signal; a plurality
of comparators having the electronic signals produced by the dual
channel fluorescence camera connected to a first input and a
programmable reference voltage connected to another input such that
when the magnitude of the video signals exceed the reference
voltage of the comparator, the comparator produces an output which
enables one of the plurality of counters; and a processor that is
programmed to adjust a gain of the high sensitivity imaging device
and to adjust the light source intensity such that the distribution
of intensity levels in one or more electronic fields substantially
equals a desired distribution.
4. The imaging system for fluorescence endoscopy of claim 1,
wherein the light source is programmable to produce fluorescence
excitation light or white light, the system further comprising: a
color video camera coupled to receive light collected by the
endoscope; a light path directing mechanism selectively positioned
to direct light collected by the endoscope to the dual channel
fluorescence camera or to the color video camera; at least one
switch that produces a signal that is indicative of the position of
the light path directing mechanism; and a light source controller
that receives the signal from the switch and causes the light
source to produce white light after the signal produced by the
switch indicates that the light path directing mechanism is
positioned to direct the light collected by the endoscope to the
color video camera.
5. The imaging system for fluorescence endoscopy of claim 4,
wherein the light source controller causes the light source to stop
producing white light and begin producing fluorescence excitation
light before the light path directing mechanism is moved from a
position where light collected from the endoscope is directed to
the color video camera head to a position where light collected
from the endoscope is directed to the dual channel fluorescence
camera.
6. The imaging system for fluorescence endoscopy of claim 1,
wherein the first high sensitivity imaging device receives
autofluorescence in a first spectral band and the second high
sensitivity imaging device receives autofluorescence in a second
spectral band, the imaging system further comprising a central
processing unit that produces a quantitative indication of the
intensity of the autofluorescence light in the first spectral band
versus the intensity of autofluorescence light in the second
spectral band.
7. An imaging system for white light and fluorescence endoscopy,
comprising: a light source that produces white light and
fluorescence excitation light; an endoscope that delivers the light
to tissue under examination in vivo and collects reflected light or
autofluorescence light produced by the tissue sample; a
fluorescence camera containing a first and second high sensitivity
imaging device that receive the autofluorescence in a first and
second spectral band and produce electronic signals that are
representative of the tissue under examination; a color video
camera that receives the reflected illumination light collected by
the endoscope and produces electronic signals that are
representative of the tissue under examination; a control center,
including an image processing board, that receives the electronic
signals produced by the dual channel fluorescence camera or the
color video camera and said control center causes an image of the
tissue under examination to be processed, stored and displayed on a
video monitor; a two-part mode switch mechanism including, i) a
light director that is selectively positioned to direct light
collected by the endoscope to the fluorescence camera head or to
the color video camera head, and ii) a mechanism that operates to
change the light source to produce either fluorescence excitation
light or white light according to the position of the light
director; and a video monitor that receives the signals produced by
the image processing board and displays an image of the tissue
under examination.
8. The imaging system for white light and fluorescence endoscopy of
claim 7, further comprising: an automatic gain control circuit
within the control center that determines a distribution of
intensity levels in the electronic signals produced by the first
and second high sensitivity imaging devices and adjusts a gain of
the high sensitivity imaging devices or adjusts the light source
intensity based on said distribution of intensity levels such that
the relative gain between the two intensified CCDs follows
substantially a polynomial.
9. The imaging system for white light and fluorescence endoscopy of
claim 8, wherein the automatic gain control circuit comprises: a
plurality of time-over-threshold counters that determine an image
area in one or more video fields that have intensities above a
plurality of predetermined thresholds.
10. The imaging system for white light and fluorescence endoscopy
of claim 9, wherein the time-over-threshold counters further
comprise: a clock signal having a frequency substantially equal to
a pixel clock of the intensified CCD transducers; a gating circuit
that passes the clock signal during an active portion of the
electronic signals produced by the dual channel fluorescence
camera; a plurality of counters that count pulses of the gated
clock signal; a plurality of comparators having the electronic
signals produced by the dual channel fluorescence camera connected
to a first input and a programmable reference voltage connected to
another input such that when the magnitude of the electronic
signals exceed the reference voltage of the comparator, the
comparator produces an output which enables one of the plurality of
counters; and a processor that is programmed to adjust a gain of
the high sensitivity imaging devices and to adjust the light source
intensity such that the distribution of intensity levels in one or
more video fields substantially equals a desired distribution.
11. An imaging system for fluorescence endoscopy, comprising: a
light source that produces fluorescence excitation light; an
endoscope that delivers the excitation light to tissue under
examination in vivo and collects autofluorescence produced by the
tissue sample; a dual channel fluorescence camera containing a
first and second high sensitivity imaging device that receives the
autofluorescence light in a first and second spectral band and
produces electronic signals that are representative of the tissue
sample under examination; a control center including an image
processing board that receives the electronic signals produced by
the dual channel fluorescence camera that are representative of the
tissue under examination and produces an indication of a relative
intensity of the autofluorescence light in each of the first and
second spectral band that produces a portion of an image of the
tissue; and a video monitor that displays the video signals to
create an image of the tissue under examination.
12. The imaging system for fluorescence endoscopy of claim 11,
wherein the indication of the relative intensity comprises a
numeric representation of the relative intensities of the
autofluorescence light in each of the first and second spectral
bands.
13. The imaging system for fluorescence endoscopy of claim 12,
wherein the indication is displayed on a video monitor.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. Pat. No.
09,064,667, filed Apr. 20, 1998, the benefit of the filing date
being claimed under 35 U.S.C. .sctn.120.
FIELD OF THE INVENTION
[0002] The present invention relates to imaging systems for medical
endoscopy, in general and to endoscopic imaging systems for
fluorescence and reflectance endoscopy, in particular.
BACKGROUND OF THE INVENTION
[0003] One common diagnostic technique used by physicians to detect
diseases within a body cavity of a patient is white light optical
fiber endoscopy. With this technique, white light is directed into
the body cavity via a non-coherent fiber-optic illumination guide
of an endoscope. The light illuminates the tissue under examination
and the reflected illumination light is gathered and transmitted
through a coherent fiber-optic imaging guide of the endoscope. The
image formed by the reflected white light at the end of the imaging
guide may be viewed directly through the endoscope eyepiece or may
be imaged by a color video camera connected to the eyepiece. Images
transduced by the camera are then typically transmitted to an image
processing/storage device and to a video monitor where they can be
viewed by the physician.
[0004] To aid physicians performing endoscopy in detecting the
presence of cancerous or pre-cancerous tissue, the differences in
the autofluorescence (also referred to as native fluorescence)
spectrum of normal and abnormal tissue can be exploited. In
fluorescence optical fiber endoscopy, a fluorescence excitation
light is delivered into the body cavity via the illumination guide
of the endoscope. The wavelengths of this light are matched to the
absorption spectrum of the naturally occurring fluorescing
molecules (or fluorophores) present in the tissue (i.e., to blue
light). The fluorescence excitation light causes the tissue in the
body cavity to fluoresce with a green and red emission spectrum and
the resulting light is collected and transmitted through the
optical fiber imaging guide of the endoscope. The resulting image
is transduced by a camera that filters out any reflected blue light
and divides the autofluorescence into two broad (green and red)
spectral bands. The image formed by the light in each spectral band
is projected onto a separate intensified CCD (ICCD) transducer and
the resulting signal is fed into a control center for processing,
storage and, finally, for display on a video monitor. The
difference in the autofluorescence emission spectrum of normal and
abnormal tissue is presented as a difference in color on the video
monitor.
[0005] Systems for fluorescence fiber endoscopy are fully described
in U.S. Pat. Nos. 5,507,287; 5,590,660, 5,647,368 and 4,786,813
that are assigned to Xillix Technologies Corp. of Richmond, BC,
Canada, the assignee of the present invention, and are sold by
Xillix as the Xillix.RTM. LIFE-Lung Fluorescence Endoscopy
System.TM. (the "LIFE-Lung System"). Multi-center clinical trials
have shown that by using the Xillix LIFE-Lung System as an adjunct
to white light endoscopy, the physician's sensitivity in detecting
moderate dysplasia, or worse, is 2.71 times greater than the
sensitivity of a physician using white light endoscopy alone.
[0006] The current LIFE-Lung System has a number of limitations,
however. First, the current embodiment of the system requires the
physician to manually adjust the gain of the system (i.e., to
increase and decrease the camera's sensitivity to the tissue
autofluorescence). This is a cumbersome task for the physician to
perform, when he/she is simultaneously trying to maneuver the
endoscope in the patient. Although automatic gain control circuits
for video systems are widely available, they do not provide
adequate gain control for the complex scene conditions encountered
in imaging autofluorescence with ICCDs. If, for example, the
average brightness of an image is increased to an acceptable level,
there may be bright spots that can damage the ICCDs. Similarly, if
the peak brightness of an image is reduced to prevent localized
image saturation, the remainder of the image may become too dark to
be recognizable. Furthermore, commonly available average and
peak-based automatic gain control circuits do not provide images
with a good dynamic range under a variety of viewing conditions,
i.e. with an optimized contrast. In endoscopy, these viewing
conditions include situations whereby the range of fluorescence
light intensities are greater than the dynamic range of ICCDs and
the image scenes vary from complex structures (i.e. lots of
intensity variations) to flat structures (i.e. homogeneous).
[0007] A further complication with the use of an automatic gain
control circuit arises due to the fact that the gain relationship
between the two channels (green and red) of the imaging system must
follow a defined function. If the gain of each channel is varied
independently, the colors in the resulting video image will not
consistently reflect the spectral differences in the
autofluorescence of the tissue.
[0008] A second limitation of the current LIFE-Lung System becomes
evident when a physician wishes to switch between white light
(reflectance) and fluorescence imaging modes. With the current
system, the physician must switch light sources and cameras
manually (i.e., from a white light illumination source to a
fluorescence excitation light source and from an RGB color video
camera to the fluorescence camera). One technique for addressing
this time consuming process is to have all light sources and
cameras connected to the endoscope simultaneously and to utilize a
mode switching mechanism to switch from one imaging mode to the
other. However, some precaution must be taken in the implementation
of a switching mechanism since the ICCDs can be damaged if they are
subjected to the bright, reflected illumination light. Care must be
taken to ensure that the ICCDs are not energized unless the
appropriate illumination conditions exist.
[0009] A third limitation of the current LIFE-Lung System is that a
physician viewing the image displayed by the system has no way of
objectively quantifying the extent of abnormality exhibited by the
tissue under examination. The effective use of the system is
dependent on such subjective factors as the physician's ability to
distinguish color and his/her ability to interpret this color
information in the context of other image features. A means to
objectively quantify the difference in the autofluorescence spectra
of normal and abnormal tissue, or even an additional means to
subjectively differentiate these tissues based on their difference
in autofluorescence spectra could improve the clinical usability of
this system. This can be accomplished using computational
techniques using the spectral information of the emitted
fluorescence and displaying the results on the monitor together
with the images.
[0010] In summary, the operation of current fluorescence endoscopy
systems may be significantly improved by:
[0011] a) an automatic gain control circuit that will optimally
adjust the brightness of autofluorescence images and that will
maintain a defined relationship between the two channels of the
imaging system;
[0012] b) a mechanism that allows rapid switching between white
light and fluorescence imaging modes, while preventing the
accidental exposure of energized ICCDs to damaging light
intensities; and
[0013] c) a means of utilizing the differences in the
autofluorescence emission spectra of normal to abnormal tissue to
objectively quantify the degree of abnormality of the tissue.
SUMMARY OF THE INVENTION
[0014] The present invention is an imaging system for white light
and fluorescence endoscopy that includes a particular automatic
gain control (AGC) circuit in the fluorescence imaging mode. The
AGC circuit adjusts the gain of the imaging system by adjusting the
gain of two high sensitivity imaging devices such as image
intensified CCD (ICCDs) transducers in a fluorescence camera head
and by adjusting the light intensity of the excitation light
source. The video signals from a pair channels (the "green" and
"red" channel) of a fluorescence camera are supplied to a set of
counters. The counters, consisting of counters connected to a
clocking oscillator, measure the length of time each video signal
has a magnitude that exceeds a reference threshold that is
individually set for each counter. Thus, by appropriately arranging
the threshold levels, the outputs of the counters can be made to
indicate the distribution of video signal amplitudes in one or more
video fields. Based upon the outputs of the counters, a decision
tree algorithm determines if the gain of the imaging system or the
light source intensity should be increased or decreased. A gain
control equation determines the appropriate value of light source
intensity change and maps the resulting imaging system gain
increase or decrease to an individual gain change for each ICCD
transducer such that the relative gain between the two channels
remains the same.
[0015] The present invention also includes a mode switching
mechanism that allows for convenient switching between white light
and fluorescence endoscopy imaging modes. The implementation of
mode switching implies that white light and fluorescence light
sources and cameras are connected to the endoscope simultaneously
and that the appropriate combination of camera and light source are
activated when switching modes. This requires a two-part mode
switching mechanism: one switching the cameras and one switching
the light sources. The camera mode switching mechanism consists of
a light directing mechanism such as a mirror that is movable
between a first position, where the image from the endoscope is
reflected towards an RGB video camera head, and a second position,
where the image from the endoscope is allowed to pass to the
fluorescence camera head. When a physician uses the mode switch to
change from white light imaging to fluorescence imaging or vice
versa, a pair of proximity switches provide signals to the system
control center, which monitors the position of the mirror, to
ensure that the ICCDs are not energized until the appropriate light
source has been selected. The light source mode switching mechanism
consists of a filter driver that positions blue, fluorescence
excitation filters or white light filters in an illumination light
path that extends between the light source and an endoscope.
[0016] The present invention also provides a means of objectively
quantifying the spectral differences between normal and abnormal
tissue by using the relative brightness of autofluorescence in the
spectral bands being imaged (green and red). A portion of the
autofluorescence image is analyzed and the numerical value defined
by a particular mathematical function such as the ratio of the
image brightnesses of the two wavebands is displayed for the
physician.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
become better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0018] FIG. 1 is a block diagram of an imaging system for white
light and fluorescence endoscopy according to the present
invention;
[0019] FIG. 2 is a block diagram of a light source used in the
imaging system for white light and fluorescence endoscopy shown in
FIG. 1;
[0020] FIG. 3 is a block diagram of an automatic gain control
circuit in accordance with a first aspect of the present
invention;
[0021] FIG. 4 is a block diagram of a number of comparators and
time-over threshold counters that are included in the automatic
gain control circuit shown in FIG. 3;
[0022] FIG. 5 is a flowchart illustrating the steps performed by
the present invention to change the gain of the imaging system
shown in FIG. 3 or the intensity of light produced by the light
source shown in FIG. 2;
[0023] FIG. 6 is a block diagram of an imaging mode switching
mechanism located in the combination camera head in accordance with
another aspect of the present invention; and
[0024] FIG. 7 is a pictorial illustration of an autofluorescence
image that includes a quantitative indication of the relative
intensities of the autofluorescence light present in two spectral
bands in accordance with another aspect of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] The present invention is an imaging system for white light
and fluorescence endoscopy that includes an automatic gain control
(AGC) circuit in the fluorescence imaging mode. The AGC circuit
controls the image brightness in two ways, a) by adjusting the gain
of the two image-intensified CCDs (ICCDs) transducers in a
fluorescence camera head, and b) by adjusting the intensity of an
excitation light source. The input to the AGC circuit are the two
video signals (a green and red channel) produced by the
fluorescence camera. The video signals are supplied to a set of
counters that determine a total period of time during which the
video signal has a magnitude that exceeds reference threshold (set
individually for each counter). The outputs of the counters are
indicative of the distribution of video signal amplitudes in one or
more video fields. Based upon the outputs of the counters, a
decision tree algorithm determines if the gain of the imaging
system or light source intensity should be increased or decreased.
A gain control equation determines the appropriate value of light
source intensity change and maps the gain increase or decrease to
an individual gain change for each ICCD such that the relative gain
between the two channels remains the same.
[0026] FIG. 1 is a block diagram of an imaging system 10 for white
light and fluorescence endoscopy according to the present
invention. At the heart of the imaging system is a control center
20 that includes a central processing unit 22 that is programmed to
control the operation of the system including a combination light
source 36 and a combination camera head 42. An internal random
access memory (RAM), a hard disk drive and read-only memory (ROM)
24 stores a computer software program that controls the operation
of the central processing unit 22. The memory also allows the
storage of data such as acquired images, parameters and log files.
A number of controls 26 on a front panel of the control center 20,
allow an operator to adjust the operation of the imaging
system.
[0027] The control center 20 also includes an imaging board 28 that
receives analog video signals that originate from a number of
sources including a fluorescence camera head 44 and an RGB camera
head 46 that are enclosed within the combination camera head 42. A
video switch, that is part of a digital and video I/O 32, receives
and selects the fluorescence or RGB video signals to be supplied as
an input to the imaging board. The imaging board 28 digitizes the
selected video signal, then processes and converts the digitized
signals to appropriate signals to be displayed on a video monitor
54.
[0028] An automatic gain control circuit 30, included within the
control center 20, automatically adjusts the gain of the
autofluorescence camera head 44 and the intensity of the
fluorescence excitation light from the combination light source
36.
[0029] The combination light source 36 provides the white light and
fluorescence excitation light. The control center 20 is interfaced
with the light source 36 through status and control lines 102, 106,
and 108. Broadband white illumination light or fluorescence
excitation light (typically at 437 nm.+-.10 nm) is supplied from
the combination light source 36 to an illumination guide 38 of a
fiber-optic endoscope 40. Light from the illumination guide 38
illuminates an internal body cavity of a patient. Reflected white
light or autofluorescence light from the tissue under examination
is transmitted by an imaging guide of the fiber-optic endoscope 40
and is projected onto the combination camera head 42. The
combination camera head 42 also includes a mode switch mechanism 67
that directs the light received from the endoscope 40 to either the
RGB video camera head 46 or the fluorescence camera head 44. With
the fluorescence imaging mode selected, the fluorescence camera
head 44 produces electronic signals that are routed to a dual
channel fluorescence camera control unit within the control center
20 (not shown) that converts the electronic signals to standard
video signals. The video signals are then routed through the video
I/O 32 to the imaging board 28 where they are processed before
being displayed on the RGB video monitor 54. Alternatively, if the
physician desires to view a reflectance white light image, the
position of the mode switch mechanism 67 is selected to project the
reflected illumination light onto an RGB video camera head 46. The
electronic signals produced by the camera head 46 are supplied to
an RGB camera control unit 48 that is external to the control
center 20, where they are converted to RGB video signals. The white
light RGB video signals are also routed through the video I/0 32 to
the imaging board 28 and are processed before being displayed on
the RGB video monitor 54. The RGB video camera control unit 48
includes an automatic gain control circuit that also has the
capability of adjusting the intensity of the light produced by the
combination light source 36 when the system is operating in white
light mode. The automatic gain control signals for the white light
mode are transmitted to the combination light source on a lead
110.
[0030] A keyboard 52 interfaces with the control center 20 through
the digital I/O 32 on the computer motherboard and allows the
operator to enter patient data or to change the operating
parameters of the imaging system.
[0031] In order to display the white light and fluorescence images,
the RGB video monitor 54 is connected to the control center 20
through the video I/O 32. A VCR 56 may be connected so that video
images can be recorded for later review and analysis. A video
printer 58 allows a physician to print hard copies of a video
frame. Images may also be recorded by a film recorder 60 or stored
on a magneto-optical disk 62.
[0032] To allow a user to control the operation of the imaging
system, several programmable operator input devices are provided. A
footswitch 64 and three operator control switches 65 on the camera
head 42 allow the operator to remotely activate various control
center 20 functions such as freezing and storing images, selecting
different AGC modes, or to control some of the peripheral devices
such as the video printer 58, film recorder 60, or magneto optical
disk 62.
[0033] FIG. 2 illustrates in further detail the combination light
source 36 that is shown in FIG. 1. The light source includes a
metal halide lamp 80 that produces broadband white light with
mercury (Hg) peaks. Light produced by the lamp 80 is passed through
a number of filters 82. Depending on the imaging mode selected, the
light is transmitted through either a broadband white light filter
(i.e. triple notch filter to remove the Hg peaks) that eliminates
the Hg peaks and shapes the spectrum of the metal halide lamp so
that it is similar to that of a Xenon lamp. Alternatively, if
fluorescence imaging is selected, light from the lamp is passed
through a blue fluorescence excitation light filter that comprises
a blue pass band having a center frequency near the mercury peak
that occurs at 437 nanometers.
[0034] Light passing through the filters 82, also passes through an
adjustable intensity control mechanism 84, which controls the
intensity of the light delivered to an endoscope. The intensity
control 84 is preferably a metal plate with an appropriate shape to
block a variable amount of light when it is moved in and out of the
light path.
[0035] After passing through the intensity control mechanism 84,
the light passes through a shutter mechanism 86 that opens to allow
the light to enter the illumination guide of the endoscope, if the
latter is plugged in.
[0036] The operation of the combination light source 36 is
controlled by a microprocessor-based light source controller 90.
The light source controller 90 controls the operation of a metal
halide lamp ballast 92 that provides the operating voltage for the
metal halide lamp 80. In addition, the light source controller
provides control signals to a filter driver 94, that physically
moves one of the filters 82 into the light path in accordance with
time imaging mode selected.
[0037] An intensity control driver 96 receives control signals from
the light source controller 90 in order to move the intensity
control 84 in and out of the light path, and thereby varies the
intensity of the light that reaches the illumination guide of the
endoscope. The light source controller 90 also sends control
signals to a shutter driver/motor 98 that causes the shutter
mechanism 86 to open and close.
[0038] In addition to controlling the components that adjust the
intensity and wavelength of light that is provided to the
illumination guide of the endoscope, the light source controller 90
also interfaces with a number of front panel switches 100 that
allow a physician to manually adjust the operation of the light
source. Alternatively, the light source controller 90 receives
commands to control the light source from an interface to the
status and control lines 102 that is coupled to the control center
20 that controls the overall operation of the imaging system as
shown in FIG. 1.
[0039] To change the output of the combination light source 36 from
white illumination light to blue excitation light or vice versa, as
well as to control the intensity of the light produced, the light
source controller 90 also receives control signals from the control
center on lead 106 that indicate which of the filters 82 should be
placed into the light path in order to create the white light
illumination or blue excitation light. The light source controller
90 receives signals from the control center on the status and
control lines 108 that indicate whether the intensity of the
excitation light produced should be increased or decreased.
Finally, the light source controller 90 receives signals from the
RGB video camera control unit 48 on the lead 110 that adjusts the
intensity of the white illumination light produced.
[0040] To eliminate the need for a physician to manually adjust the
gain while in the fluorescence imaging mode, the imaging system of
the present invention includes a fluorescence mode automatic gain
control (AGC) circuit 30 as shown in FIG. 3. The imaging system can
also be operated under manual control as the current Xillix
LIFE-Lung Fluorescence Endoscopy System.TM.. The implementation of
the fluorescence mode AGC is as follows: As described previously,
autofluorescence light produced by the tissue under examination is
divided into a pair of spectral bands and projected onto a pair of
high sensitivity imaging devices such as a pair of electron
bombarded CCD's or image intensified CCD transducers 44a and 44b.
The transducer 44a receives the light in a wavelength band
.DELTA..lambda..sub.1, which is located in the green portion of the
visible spectrum, while the transducer 44b receives light in a
wavelength band .DELTA..lambda..sub.1, which is located in the red
portion of the visible spectrum. The electronic signals produced by
the intensified CCD transducers 44a and 44b are supplied to camera
control units (CCUs) 45a and 45b within the control center 20,
where they are converted into video signals and routed through the
video I/O 32 to the imaging board 28 and to the AGC circuit.
[0041] The video signals routed to the AGC circuit are applied to a
time-over-threshold counter circuit 112. The counter circuit also
receives a clock signal which is gated by the horizontal and
vertical sync signals from the CCUs. The counter 112 produces a
number of outputs #T1, #T2, . . . #Tn, each of which contains a
value which is proportional to the area in one or more video fields
that has an intensity level above an associated predefined
threshold intensity value. Each of the output values #T1, . . .
#Tn, may be weighted by a function a.sub.1, . . . a.sub.n 114
before being supplied to a decision tree algorithm 116. The
decision tree algorithm 116 determines if the gain of the imaging
system and/or the intensity of the light produced by the
combination light source 36 should be increased or decreased. The
output of the decision tree algorithm 116 indicates the amount by
which the gain should be increased/decreased and this signal is
supplied to a gain control equation 120. The gain control
calculates the amount by which the light source intensity and/or
the gain of the individual intensified CCD transducers 44a and 44b
of the imaging system should be adjusted to meet the gain change
determined by the decision tree algorithm, while maintaining a
predefined gain relationship between the two channels.
[0042] If the camera gain is to be increased or decreased, the gain
control equation 120 produces a pair of binary numbers whose
magnitude will result in a proportional gain change in the two
ICCDs. An increase/decrease gain control circuit 122 receives the
binary numbers from the gain control equation 120 and converts the
binary numbers received into a pair of voltage levels that are
supplied to a pair of transducer gain controls 124 and 126. The
transducer gain controls 124 and 126 adjust the absolute gain of
the intensified CCD transducers 44a and 44b respectively.
[0043] FIG. 4 illustrates in greater detail the time-over-threshold
counter 112 described above. The counter 112 operates to produce
numeric counts that are indicative of how long a threshold
intensity is exceeded in one or more video frames. These numeric
counts are proportional to the area in an image with an intensity
above a predefined value. A bank of independently programmable,
reference threshold digital-to-analog converters 140 is programmed
by the control center 20 to set a series of reference threshold
levels against which the video signals from the CCUs are compared.
The particular reference threshold levels are selected to represent
a percentage of the zero to full scale video signal that is
produced by the CCUs and their chosen values are generally
dependent on the type of tissue being examined, as will be
described below.
[0044] The reference thresholds are applied to the inverting inputs
of a number of comparators 144. For example, a voltage equal to 45%
of the full scale range of the green channel video signal is
supplied on a lead 142a to an inverting input of a comparator 144a.
Similarly, a voltage equal to 75% of the full scale range is
supplied on a lead 142b to an inverting input of a comparator 144b.
Another set of reference threshold voltages are applied to a set of
comparators that receive the video signal produced by the red
channel CCU. In the presently preferred embodiment of the
invention, one reference threshold for each channel is set at a
desired peak value while the other reference threshold is set at a
desired average intensity value.
[0045] The video signals produced by the dual channel fluorescence
CCUs are applied to the noninverting inputs of the comparator
circuits 144. When the voltage level of the video signals exceeds
the reference thresholds set by the digital-to-analog converters
140, the comparators 144 produce logic high signals. Associated
with each of the comparators 144 is a 24-bit counter 146. Each
counter has a counter enable pin coupled to the output of its
associated comparator such that when the comparator produces the
logic high signal, the counter is enabled.
[0046] As indicated above, the automatic gain control circuit 30
includes a free running clock 150 having a frequency that is
substantially equal to the pixel clock of the CCUs. A sync delay
and gating circuit 152 receives the horizontal and vertical
synchronization signals produced by the CCUs and only passes the
free running clock 150, during the active portions of the video
signals. The sync delay and gating circuit 152 also produces a
field clock pulse for each field of the video signals received. The
pulses are counted by a short counter 154 in order to keep track of
the number of field periods associated with the values contained in
the time-over-threshold counters.
[0047] When the counters 146 are enabled by their corresponding
comparator circuits 144, the counters 146 count the number of
sync-gated clock pulses that occur during the time when the video
signals produced by the red or green channel CCUs exceed the
reference threshold associated with the comparator that is
connected to the counter's enable pin.
[0048] The values in the counters 146 are read out through a
counter readout control circuit 160 that connects the counters 146
to the imaging system's data bus 130 located on a motherboard
within the control center 20. The counter readout control circuit
also receives the count held in the short counter 154. The short
counter 154 allows the software to be programmed to read out the
counters 146 at periodic intervals, such as every ten fields,
etc.
[0049] Although the presently preferred embodiment of the invention
utilizes two reference thresholds for each of the green and red
channels, additional threshold counters can be added to the
automatic gain control circuit in the manner described above if it
is desired to obtain more detailed information on the distribution
of the video signal amplitudes.
[0050] FIG. 5 illustrates the steps performed by the decision tree
algorithm 116 and the gain control equation 120 shown in FIG. 3 to
adjust the gain of the ICCDs and the light source intensity. FIG. 5
illustrates the two basic processes used to implement the automatic
gain control, namely, i) the setup of the parameters in steps 162
and 164, and ii) the running of the decision tree algorithm and
gain control equation in steps 166 to 172.
[0051] Beginning with a step 162, the peak and average reference
thresholds are set. These values are selected by the operator using
the system software. The values selected depend upon the type of
image being viewed. In an image that contains many structural
features, the thresholds are selected to ensure that all details
remain visible. For example, when viewing a body cavity containing
detailed structure such as the bronchi, the peak reference
threshold may be set at 90% of the full scale value and the average
reference threshold set at 50% of the full scale value.
Alternatively, if the body cavity being examined is relatively
homogeneous, such as the stomach, the reference threshold values
may be set such that the average intensity of the image ensures a
relatively bright image. For example, the peak reference threshold
may be set at 80% of full scale and the average reference threshold
set at 60% of full scale. Preprogrammed thresholds selected for
commonly viewed tissue samples can be selected or custom values can
be entered.
[0052] At a step 164, the automatic gain control circuit selects a
number of AGC image fill goal values. These values represent the
nominal image area for which the video signal amplitude must be
greater than or equal to a particular threshold. For example, fill
goal values may be chosen such that 2% of the image area has video
signal amplitudes greater than the peak threshold value and 55% of
the image area has video signal amplitudes greater than the average
threshold value. The automatic gain control circuit adjusts the
gain of the ICCDs and/or the intensity of the light source such
that the image intensity distribution calculated from the
time-over-threshold counter 112 achieves the best match to the
desired image fill goal values. Like the threshold values, the fill
goal values are selected by the operator of the system.
[0053] Step 166 is the first step in the actual AGC decision tree
algorithm. At a step 166, the automatic gain control circuit waits
for the last gain change to take effect and then measures the image
intensity distribution for specified number fields. At the end of
the specified number fields, the values from the counters 146 in
the time-over-threshold counter circuit 112 are read and the image
areas analyzed.
[0054] The image area having video signal amplitudes above the
higher, or "peak" threshold, and the image area having video signal
amplitudes above the lower, or "average" threshold, are applied to
the decision tree at step 168. The decision tree determines whether
the gain should be changed so that the intensity distribution will
better meet the AGC fill goal values desired. As discussed above,
the image area allowed to exceed the peak or the average threshold
may be weighted by the functions 114, in order to make the
automatic gain control circuit operate more like a peak or average
value control circuit as desired for the particular viewing
situation.
[0055] The amount of gain change determined by the decision tree
algorithm 116 is modified by well known process control techniques
at a step 170 to optimize transient behavior such as overshoot,
settling time, and oscillatory behavior. These techniques include a
leaky integrator function, deadband control, control function
mapping, proportional control, and rate and range limiting actions
on the next applied gain change. These techniques ensure gain
changes occur as quickly as possible without creating stability
problems.
[0056] At a step 172, the gain change for the green or red channel
ICCD is determined and if required, the amount of light source
intensity change. The gain change is modified by the control
techniques and is applied to the gain control equation 120. This
equation relates the gain setting of the ICCD in each of the two
channels, such that the ratio (first order polynomial) of the gains
between the two channels is maintained. The ratio of the gains
between the two channels may be selected by the system operator.
The operator may adjust the ratio, such that the resulting video
image appears more red or more green as desired. In the presently
preferred embodiment of the invention, the relative gain of the
ICCD in the red channel to the ICCD in the green channel can be
varied over a range of 0.75 to 3. In some applications, the
relationship between the gains of the two channels may be a higher
order polynomial, e.g. g.sub.1=c+ag.sub.2+bg.sub.2.sup.2+ . . .
where g.sub.1 is the gain of the red channel, g.sub.2 is the gain
of the green channel and a, b, c, are constants.
[0057] The situation may occur that the required fluorescence
camera gain falls outside of the optimal gain adjustment range of
the ICCD in one or both of the channels. If the calculated gain
setting of either channel is greater than the maximum optimal
setting or smaller than the minimum optimal setting, then the
intensity of the excitation light source is increased or decreased
by a fixed amount. The intensity of the light produced by the light
source is adjusted a sufficient amount to return the camera gain
settings to within the optimal working range. A pseudocode listing
of the decision tree algorithm 116 and gain control equation 120 is
set forth in Appendix A.
[0058] The present invention also includes a two part mode switch
mechanism (one part in the light source and one part in the
combination camera head) that allows for convenient switching
between white light and fluorescence endoscopy imaging modes. FIG.
6 is a schematic block diagram of the mode switching mechanism of
the combination camera head. The switching mode mechanism of the
light source is shown in FIG. 2. The preferred embodiment of the
mechanism requires the endoscope to be attached to the combination
light source 36 and the combination camera head 42 by means of the
endoscope connector 180. The combination light source 36 is capable
of providing white light (reflectance) illumination and blue light
(fluorescence excitation) illumination. The combination camera head
42 is capable of transducing three channel RGB reflectance images
and two channel fluorescence images.
[0059] Because the light source 36 and camera head 42 are
physically separate, the mode switching mechanism is composed of
two parts. The two parts of the mechanism are linked through
control signals via the imaging system control center 20 and the
light source system controller 90. Since the metal halide lamp 80
in the combination light source 36 is capable of providing both the
white and blue light, a light source part of the mode switch
consists of the filter driver 94 and the white light an blue light
filters 82. The filter driver 94 responds to instructions from the
light source system controller 90 and positions the appropriate
filter in the light path between the lamp and the endoscope
illumination guide. The status of the filter driver 94 is also
monitored by the light source system controller 90, which then
communicates with the control center 20 via the interface to the
status and control lines 102.
[0060] A second part of the mode switching mechanism is located in
the combination camera head 42. This part of the mode switching
mechanism 67 utilizes a movable light path directing mechanism such
as a mirror 186. When in the imaging system is in fluorescence
imaging mode, the mirror is moved out of the light path between the
endoscope eyepiece and the fluorescence camera head 44. In this way
the fluorescence light reaches the dichroic mirror 182 that
separates spectrally .DELTA..lambda..sub.1 and
.DELTA..lambda..sub.2 into their respective optical paths. When the
imaging system is in the white light imaging mode, the mirror 186
is moved into the light path. In this position, light from the
endoscope is directed to a second, fixed mirror 190, where the
light path is folded to form a periscope that redirects the light
from the endoscope eyepiece to the RGB video camera head 46.
[0061] The operation of both parts of the mode switching mechanism
is controlled by an operator input on the combination camera head
42. The operator initiates a switch 65 to change the operation of
the imaging system. This results in a signal being sent to the
control center indicating that a switch of imaging modes has been
initiated. A signal is generated by a pair of electrical or optical
proximity switches 192, 194 in the combination camera head 42 that
sense the position of the movable mirror 186. A second signal is
generated by switches 192, 194 and sent to the control center 20
when the movable mirror 186 has reached its new position.
[0062] The switches 192, 194 function as a safety mechanism for the
ICCDs in the fluorescence camera head. When energized, the ICCDs
are susceptible to damage from bright light (e.g. white light
reflectance images from the endoscope eyepiece). If the movable
mirror 186 is not completely in the fluorescence mode imaging
position, the control center 20 reacts by immediately shutting off
the power to the ICCDs, thereby protecting them from exposure to
possibly damaging illumination.
[0063] The control center 20 reacts differently to the switch
signals depending on whether the operator is switching from
fluorescence imaging mode into the white light imaging mode or from
white light imaging mode into the fluorescence imaging mode. In the
former case, the control center 20 reacts to the first switch
signal by immediately shutting off the power to the ICCDs and
stopping the display of all images. When the control center 20
receives the second switch signal, indicating that the movable
mirror 186 in the camera head 42 has reached the white light
imaging mode position, the control center sends a signal to the
light source system controller 90 instructing it to move the white
light filter into the light path. When the light source mode switch
has completed the filter change, the light source system controller
90 generates a light source status signal, which is transmitted to
the control center 20. Upon receipt of the light source status
signal, the control signal routes the video signal from the RGB
video camera control unit 48 to the RGB video monitor 54 and the
resulting white light image is displayed.
[0064] In the case of switching from white light imaging mode into
the fluorescence imaging mode, the control center 20 reacts to the
first switch signal from the combination camera head 42 by sending
a signal to the light source system controller 90 instructing it to
move the blue light filter into the light path. A light source
status signal is generated and sent the control center 20 when the
light source mode switch has completed the filter change. When the
control center also receives the second switch signal from the
combination camera head 42, indicating that the movable mirror 186
has reached the fluorescence imaging mode position, the control
center energizes the ICCDs in the fluorescence camera head 44 and
routes the video signals from the fluorescence camera control units
to the RGB video monitor 54. The resulting fluorescence image is
displayed on the RGB video monitor 54. If the incorrect light
source status signal is received by the control center 20, the
ICCDs in the fluorescence camera head will not be energized, even
if the second switch signal has been received from the combination
camera head 42.
[0065] In accordance with another aspect of the present invention,
the imaging system of the present invention quantifies the relative
brightness of the autofluorescence light produced by the tissue in
each of the spectral bands .DELTA..lambda..sub.1 and
.DELTA..lambda..sub.2, in an objective manner. FIG. 7, shows a
monitor display 200 with an image 202 of the tissue under
examination. Differences in the autofluorescence spectrum produced
by normal and abnormal tissue are shown as areas of different color
in the image. For example, abnormal tissue 204 produces
proportionally less autofluorescence light in the green portion of
the spectrum than normal tissue and is shown as a reddish area in
the displayed image.
[0066] The relative brightness of the autofluorescence light in the
green and red wavebands imaged by the system, .DELTA..lambda..sub.1
and .DELTA..lambda..sub.2, can be used as a measure of the
difference in the actual fluorescence emission spectra of normal
and abnormal tissue. A ratio (or other function relating the
.DELTA..lambda..sub.1 to the .DELTA..lambda..sub.2 waveband) of the
brightness of the tissue autofluorescence in the green and red
spectral bands is calculated and displayed to the physician. The
ratio is calculated for a small area such as a region 206 defined
in the center of the field of view. Since the color ratio can be
recalculated on a frame by frame basis in real time, the color
ratio displayed represents the average color ratio of the tissue
imaged within the bounds of area 206. Although the area 206 is
shown as a particular area located in the center of the field of
view, other locations within the field of view and larger or
smaller areas could be used.
[0067] The ratio calculation is implemented as follows: As
described above, the video signals from the fluorescence camera
control units are routed to the imaging board 28. The imaging board
28 digitizes the video signals such that the video signal
amplitudes correspond proportionally to digital grey level values.
The central processing unit 22 within the control center 20 reads
the data digitized by the imaging board 28 and sums the grey level
values of all the red channel digital data within area 206 and
divides that sum by the sum of all the green channel data within
area 206. The quotient of these two sums is shown as a
dimensionless number 208 on the monitor.
[0068] As an alternative to displaying a dimensionless number,
other non-visual cues could be used to quantify the relative
brightness of the tissue autofluorescence in spectral wavebands
.DELTA..lambda..sub.1 and .DELTA..lambda..sub.2. For example, a
tone having a frequency that is dependent upon the ratio of the
brightness of the autofluorescence in each spectral band could be
produced. Similarly, the frequency of a blinking light could be
made to change in proportion to the changing ratio.
[0069] While the preferred embodiment of the invention has been
illustrated and described in the preceding description, it will be
appreciated that various changes can be made therein without
departing from the spirit and scope of the invention. The scope of
the invention is therefore to be determined from the following
claims and equivalents thereto.
* * * * *