US 7846091 B2
A system and method for imaging tissue autofluorescence through a video endoscope is described, comprising a light source capable of providing both ultraviolet light capable of inducing tissue autofluorescence and visible light which induces little or no autofluorescence, an optical system to deliver both wavelength bands to the tissue with the same apparent spatial and angular intensity distribution, a means for digitally acquiring the resulting, visible fluorescence and visible reflectance images using a single imaging detector at the distal tip of the endoscope and a means for digitally processing said images to generate a final, false-color image for display which indicates regions of tissue dysplasia. This system can either be added on to an existing video endoscope or integrated into its structure. The combined system can be electronically switched between normal white light imaging and fluorescence imaging.
1. A fluorescence imaging endoscope system comprising:
a diode laser light source for producing excitation light having a wavelength in a range of 380 nm to 420 nm that induces visible autofluorescence in tissue and a second light source for producing a reference light including red, green and blue wavelength bands, the diode laser light source and second light source being operative in response to control signals from a control system;
an optical combiner that optically couples said excitation light and said reference light onto a common optical path, said excitation light and reference light being coupled into an optical guide that delivers the light to the tissue through an endoscope;
a single image detector at a distal end of the endoscope that detects an autofluorescence image having blue, green and red light components and a reference image of the tissue; and
a data processor that processes the autofluorescence image and said reference image to produce a processed output image of the tissue.
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17. A fluorescence imaging endoscope system comprising:
a gallium nitride diode laser for producing excitation light having a wavelength in the range of 380 to 420 nm that induces visible autofluorescence in tissue and a second light source for producing a reference light including red, green and blue wavelength bands, the diode laser light source and second light source being operative in response to control signals from a control system;
an optical combiner that optically couples said excitation light and said reference light onto a common optical path, said excitation light and reference light being coupled into an optical fiber delivery system extending through the endoscope system;
a single image detector at a distal end of the endoscope that detects an autofluorescence image having blue, green and red light components and a reference image of the tissue; and
a data processor that processes the autofluorescence image and said reference image to produce a processed output image of the tissue.
18. A fluorescence imaging endoscope system comprising:
a diode laser light source for producing excitation light having a wavelength in the range of 380 to 420 nm that induces visible autofluorescence in tissue and a second light source for producing a color image;
an optical combiner that optically couples said excitation light and said light from the second light source onto a common optical path, said combined light being coupled into an optical guide that delivers the combined light to the tissue through an endoscope;
a single image detector at a distal end of the endoscope that detects an autofluorescence image having blue, green and red light components and a color image of the tissue; and
a data processor that processes the autofluorescence image and said color image to produce a processed output image of the tissue.
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This is a continuation of U.S. application Ser. No. 09/362,806 filed on Jul. 28, 1999, now U.S. Pat. No. 6,364,829, issued Apr. 2, 2002, which is a continuation-in-part of U.S. application Ser. No. 09/238,664 filed Jan. 26, 1999, now U.S. Pat. No. 6,537,211, issued Mar. 25, 2003, the entire contents of which are incorporated herein by reference.
This invention was supported in whole or in part by a grant from the National Institute of Health, Grant No. R44CA72626, “Imaging Spectrofluorimeter for Colonic Dysplasia”. The Government has certain rights in the invention.
Advanced stage cancers are difficult to treat effectively, so it is important to detect them in their premalignant stages, such as dysplasia or carcinoma in situ. Currently, the most widely used method for early detection uses visual inspection trough endoscopes, which relies upon the recognition of the gross architectural changes associated with dysplasia. Visual inspection is less effective in detecting the superficial lesions of flat dysplasia, such as ulcerative colitis and Barrett's esophagus. In these cases, surveillance requires the selection of representative sites for biopsy and subsequent histological analysis. Only a very small fraction of a large surface, such as the colon, can be surveyed in this manner and small areas of dysplasia may go undetected. More efficient methods for detecting flat dysplasia would provide a significant means of reducing cancer morbidity, mortality, and costs.
A promising technique for detecting dysplasia during endoscopy involves illuminating the tissue with light of an appropriate wavelength and observing the resulting fluorescence. Tissue fluorescence occurs at longer wavelengths than the excitation illumination and is typically much weaker, so that spectroscopic techniques are generally required for its detection. Diagnostic methods which use fluorescence information can generally be divided into two groups. The first group of methods observes fluorescence from medications administered to the patient which have accumulated in tumor tissues. The second group of methods observes endogenous fluorescence or autofluorescence arising from substances natural to the tissue itself which change their relative concentrations when the tissue becomes dysplastic. Of the two general fluorescence methods, the first method, requiring a prior application of a drug, is the more invasive. The application of these drugs takes additional time and has the potential of causing adverse side effects. Methods based on autofluorescence detection are less invasive and more suited to endoscopy for screening purposes.
Normal colon tissue, for example, when illuminated with ultraviolet light at 370 nm, exhibits a broad, blue fluorescence with a peak at 450 nm, as shown in
Diagnostic instruments which detect autofluorescence can be divided into two general groups. The first subdivision includes instruments which utilize fiberoptic probes to perform essentially point measurements on the tissue. The second subdivision includes instruments which produce detailed, two dimensional images. Point detection instruments have the advantage of providing more complete spectral information on the tissue, but are too slow for routine screening of large tissue areas and may miss small regions of dysplasia. Fluorescence imaging endoscopes are more appropriate for screening large tissue areas such as the colon.
Fluorescence imaging systems designed to sense concentrations of fluorescent markers that have been applied to the region of interest are optimized for the measurement of relatively high fluorescence levels, do not describe the additional instrumental features required to measure inherently weak autofluorescence. In particular, they do not describe a method for providing sufficient out-of-band filtering for the excitation illumination that would allow the effective measurement of autofluorescence, which typically is reduced in intensity, compared to the excitation intensity, by a factor of 1000 or more.
Fluorescence imaging endoscopes which are specifically designed to measure autofluorescence can be further divided into groups based on the excitation wavelength chosen and the method for quantifying the degree to which the autofluorescence is reduced. These design choices have a direct bearing on commercial considerations for the instrument since they affect, for example, the number of imaging devices required, the opto-mechanical complexity of the instrument, and the handling characteristics of the instrument in actual use.
Existing fluorescence imaging endoscope systems use visible blue light near 440 nm for the excitation wavelength, resulting in fluorescence peaks around 500 nm. These instruments use Helium-Cadmium lasers at 442 nm as a bright, easily controlled source for the excitation light. The high cost of Helium-Cadmium lasers makes them impractical as sources for commercial instruments.
Multiple cameras and mechanically switched optical components and/or filters require the use of endoscopes based on coherent imaging fiber bundles, so that the cameras and filters can be located at the proximal end of the endoscope where there is room for them. The coherent fiber imaging bundles introduce significant light losses and do not provide as sharp an image as those now available with video endoscopes.
The present invention relates to imaging endoscopes and in particular to an endoscope system and method for imaging autofluorescence from epithelial tissue to highlight regions of dysplasia. The system for detecting dysplastic tissue uses the autofluorescence of mucosal tissue such as that found in the colon, the esophagus, the oral cavity, the cervix and the lung. The fluorescence imaging apparatus for an endoscope of the present invention utilizes a selected range of excitation light wavelengths and a fluorescence normalization method. These choices provide for an improved endoscope which requires one, non-intensified, imaging detector at the distal tip of a video endoscope for both white light imaging and fluorescence imaging. The imaging detector can be a pixellated integrated circuit device such as a CMOS imaging device, charge coupled device (CCD) or other small two dimensional imaging sensors that can detect in the visible and infrared ranges.
The system of the present invention has the ability to switch back and forth between white light and fluorescence visualization methods electronically, requiring no moving parts within the endoscope itself The elimination of cameras and imaging optics at the proximal end of the endoscope significantly improves its handling characteristics. A computer-based imaging system allows quantitative images of the tissue to be displayed, at refresh rates of up to 10 Hz or higher, for example. A preferred embodiment uses easily seen, false-color overlays which indicate the likely areas of tissue dysplasia. By “false color” we mean that a color value is assigned to a particular level of fluorescence intensity for each pixel. A data processing system can be programmed to provide a color system suitable for imaging a given type of tissue condition. The system can be used either with color wheel (e.g., using a monochrome CCD) video endoscopes or with color imaging sensor endoscopes.
In a preferred embodiment, near ultraviolet light is chosen as the excitation wavelength, as described in Wang, et al., U.S. Provisional Application No. 60/072,455, Jan. 26, 1998. This choice in the range of 300-420 nm reduces or eliminates the need for additional filters between the tissue and the imaging detector due to the fact that standard, electronic imaging sensors, as used in video endoscopes, are insensitive to the excitation light. Visible red light, to which the imaging detector is very sensitive, is chosen to illuminate the tissue for the purpose of acquiring a reference image. This reference light is passed through the same optical guide as the excitation light, and illuminates the tissue with the same normalized spatial distribution and angular distribution as the excitation light. By correlating the spatial intensity and angular distribution of light used for the reflected image and the light for the fluorescence image, a more accurate and diagnostically useful imaging system is provided. This system of the present invention allows the reference image to be used to normalize the fluorescence image so that local reductions in fluorescence intensity can be accurately quantified. In another preferred embodiment, using color wheel (monochrome CCD) video endoscopes, the reference light and excitation light are applied sequentially. In another preferred embodiment, using color CCD video endoscopes, the fluorescence excitation light and reference light are applied simultaneously.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
The present invention relates to systems and methods for imaging autofluorescence from epithelial tissue to highlight regions of dysplasia. The tissue autofluorescence imaging system in accordance with the present invention improves upon the previous state of the art in that, in its simplest form, it can be added on to an existing video endoscope system with no modifications to the endoscope itself, and with only the addition of a shutter in the optical path of the endoscope's visible light source. The handling characteristics of the endoscope are thus not adversely affected by the addition of image intensifiers and external cameras at the proximal end as required by fluorescence imaging systems currently on the market. Switching between normal visible light imaging, in full color, and fluorescence imaging is accomplished by an electronic switch rather than a physical manipulation by the clinician, also required by current systems. The resulting fluorescence video image is processed by a computer so that the diagnostic image seen by the clinician consists of a familiar visible light image (in gray scale) with a false color overlay indicating those areas in the image where the fluorescence from the tissue is reduced in comparison to the fluorescence from normal tissue. This image is much easier to interpret than the combined red/green raw fluorescence images provided by current systems, particularly for red/green color-blind clinicians.
The system of the present invention uses only a single imaging detector at the distal end of the endoscope for acquiring normal color images, fluorescence images and visible reference images. The use of a camera at the distal end is made possible by using fluorescence excitation light at ultraviolet to deep violet wavelengths to which the CCD camera is insensitive or can be made insensitive using a fixed filter. This allows broadband collection of the resulting tissue autofluorescence from blue to red wavelengths, resulting in sufficient light for effective video signals without the need for additional image intensification. Fluorescence imaging in this fashion has been described in-vivo by Wang, et al. as described in U.S. application Ser. No. 09/238,664, entitled “Fluorescence Imaging Endoscope” filed on Jan. 26, 1999, now U.S. Pat. No. 6,537,211, issued Mar. 25, 2003, the entire contents of which is incorporated herein by reference.
Previous autofluorescence imaging systems have depended upon imaging very weak fluorescence at red wavelengths to provide an image to compare with the fluorescence image taken at blue-green wavelengths. Additional image intensification is particularly necessary to provide a usable red fluorescence image. The autofluorescence imaging system and method of the present invention avoids this expense and difficulty by supplying additional visible red light illumination to the tissue for the purpose of obtaining a reference image. To be effective, however, the UV excitation light and the visible reference light must be delivered to the tissue through a common optical guide and exit the same illumination aperture with the same angular distribution. This requires a careful design of the excitation and reference light source optics. The processing of the reference image includes other features, such as histogram analysis, to eliminate artifacts such as visible specular reflections which do not occur in fluorescence images.
The combination of the system features in accordance with the present invention can be realized in several different ways, depending upon the endoscope with which the autofluorescence imaging system is combined. Some of these features can also be built into the endoscope itself in stages as the technology is accepted by clinicians.
In a preferred embodiment, the excitation illumination and reference illumination are generated by the same arc lamp source and delivered to the tissue through the same fiberoptic probe passed through a biopsy channel of the endoscope. In other embodiments the excitation light can be generated with a stand-alone source but delivered to the tissue through UV-transmissive illumination bundles built into the endoscopes as replacements to the standard, UV-absorbing glass bundles. The reference illumination is derived from the normal white light illumination source by switching in a red-pass filter that absorbs light at blue and green wavelengths which can interfere with imaging the predominately blue fluorescence. A deep red light source or near infrared (e.g., at 670 nm or above) can be used without affecting the normal visible illumination and can be combined with a dichroic element for delivery along a common optical path. In further embodiments, the excitation light source can be combined with the standard, white light illumination source and delivered to the tissue through an endoscope with UV-transmissive fiberoptic illumination bundles. A rotating wheel light source can also be modified to produce four colors, ultraviolet, blue, green and red, to perform both the fluorescence imaging and the visual imaging.
The autofluorescence imaging system demonstrated by Wang, et al. used an argon-ion laser as a UV excitation source. Other laser sources can be used including solid state lasers, such as gallium nitride laser diodes, operating at wavelengths in the range of 380 nm to 420 nm, which have smaller size and low power operation. The systems in accordance with the present invention uses a mercury arc lamp as a source of UV excitation with a spectral band around the 365 nm mercury line. The mercury arc source is smaller, and less expensive than the argon-ion laser, requires relatively little power and is air-cooled. For the autofluorescence imaging systems used with color wheel (monochrome CCD) video endoscopes, the current to the arc lamp can be pulsed. In the color wheel video systems, the normal light source provides red, green and blue light pulses sequentially during a 33 ms video frame which are combined with the video processor to provide a color image. In the autofluorescence imaging mode these systems replace the normal blue light pulse with a UV pulse and the green light pulse with a (nominally red) reference light pulse transmitted through the same optical fiber as the UV pulse. By pulsing the arc lamp current for the 8 ms period that the blue light is normally on, the UV source can provide as much or more excitation fluence to the tissue than it could if the CW lamp intensity was integrated over the full 33 ms video frame period.
With both color wheel video systems and color CCD video systems the autofluorescence imaging mode is initiated with a footswitch which controls complementary shutters on the excitation/reference light source and the normal endoscope white light source. Shutting off the normal endoscope illumination is required in order to image the weak fluorescence from the tissue. Two different autofluorescence imaging modes can be used. In one mode, a single frame of the false color overlay from the processed autofluorescence image is combined with the immediately preceding (or following) color image and frozen on the computer screen. In the second mode the processed fluorescence image is updated continuously (at about 7.5 Hz to 10 Hz allowing for the image processing time) for as long as the footswitch is depressed. In the continuous operation mode the visible image is shown as a gray scale (since it is taken with a monochrome reference illumination) with false color overlays showing regions of likely dysplasia.
The normal illumination light is carried by a fiberoptic bundle 312 which extends through the length of the endoscope and bifurcates near its distal tip, terminating at apertures 308 and 310. A shutter 314 between the illumination source and the fiberoptic bundle, controlled by a digital signal 315, allows the white light illumination to be turned off without turning off the source lamp 316. In the type of endoscope shown, the normal color image is obtained by combining three, successive images taken with red green and blue light pulses provided by a rotating filter wheel 318. The CCD detector in this type of endoscope is sensitive to all wavelengths between 400 nm and 700 nm, but is insensitive to UV excitation wavelengths around 365 nm which are used to excite the autofluorescence. This is due both to the design of the silicon sensor array and to the choice of optical materials used to physically protect the surface of the array. The CCD detector continuously integrates all of the light which falls on its surface so the illumination must be shuttered off while the CCD rows are shifted down to the readout electronics or else a streaking effect will be seen on the image. The red, green and blue light pulses have a duration of about 6 ms followed by a dark period of 5 ms during which the camera pixels are read out, yielding a total video frame period of about 33 ms or 29.97 frames per second to meet the NTSC standard. The analog readout signal form the CCD camera is carried through a cable 320 to the endoscope video processor 322.
The three successive monochrome images are digitized and combined into a standard color video signal at the end of the video frame. The processor has two groups of standard red, green, blue (RGB) plus synchronization outputs. One output group of color signals 324 goes to the endoscope's video monitor 326 to display the normal color image of the tissue 328. Another group of color signals 330 goes to a video framegrabber 332 in the fluorescence imaging computer system 334 which will acquire and process the autofluorescence and reference images. A standard composite color signal output 336 from the video processor goes to a synchronization circuit 338, based on a National LM1881 Video Sync Separator. This synchronization circuit 338 determines when the interlaced even and odd fields occur in the video signal and outputs a binary digital signal 340 which is high during the odd field and low during the even field. This signal 340 is used throughout the fluorescence imaging system to synchronize its functions to the timing set by the endoscope's video processor.
To engage the autofluorescence imaging mode of operation, the clinician depresses footswitch 341 which sends a signal on cable 342 to the computer 334. At the appropriate time, synchronized to the next opportunity as determined by the synchronization signal 340, a signal is sent out on the shutter trigger line 315 to shutter 314 on the normal illumination source and the shutter 343 on the excitation/reference light source. These shutters are complementary, so that the signal on line 315 opens shutter 343 and simultaneously shuts shutter 314. The excitation and reference light pulses, generated by the rotating shutter wheels in the optical group 350, then pass into the fiberoptic probe 344. Probe 344 is inserted into a biopsy channel opening 345 and slid down the channel until its end window 346 is either at or just beyond the distal tip of the endoscope 300. The excitation and reference light pulses illuminate the central portion of the endoscope's visual field. The angular extent of the illumination depends on optical elements 350, the fiber numerical aperture, and the optical characteristics of the fiberoptic probe end window 346.
The excitation and reference light pulses must occur during two of the three normal illumination pulse periods for the autofluorescence and reference images to be properly acquired. These images appear on the next video output frame on two of the three video output channels from the endoscope video processor. The appropriate timing is accomplished by rotating shutters 352 and 354 in the excitation (UV) and reference (RED) light paths, respectively, of the optical group 350. The shutters are driven by direct current (DC) motors 356 and 358 which rotate at a speed which can be controlled by varying their supply voltage. A fiducial hole near the rim of the shutters, combined with an optical source and detector, generates a reference pulse for each which marks its phase as it rotates. A phase-locked loop (PLL) 362 and 360 for each motor, 358 and 356, respectively, adjusts the motor's voltage so that the reference pulse for each matches the rising edge of the synchronization pulse 340 marking the beginning of the odd video field.
By positioning the hole in the shutters appropriately the excitation light pulse and the reference light pulse can be timed to the exposure periods of the camera. The excitation pulse is timed to match the normal blue exposure since this exposure period is somewhat longer (8.1 ms) than the others. The reference light pulse is timed to match the normal green light exposure (5 ms) since it is the next longest of the normal exposure periods. The normal red exposure period, which immediately follows the normal blue exposure period, is not currently used, but can be employed to obtain additional autofluorescence or reference images or an additional visible reflectance image for other spectroscopic analyses. During the excitation exposure the current to the mercury lamp can be boosted to a higher level to increase the excitation light output. The lamp power supply utilizes a DC current section 366 to maintain the idle current and start the lamp.
A computer-controlled, pulsed current section 368, connected in parallel with the DC section 366, can rapidly switch in multiple, parallel-coupled, constant-current sources to vary the output power of the lamp as required by the imaging system. The current pulses are synchronized to the video system using the same synchronization pulse 340 that serves to lock the rotating shutters. The computer digital input/output (I/O) section 370 outputs a digital pulse 372 which is combined with the timing pulse 340 to boost the lamp current during the excitation exposure. The number of constant current sections which are triggered in parallel can be varied by a set of digital control lines 374 as required. If the tissue is sufficiently close to the excitation/reference delivery probe window 346 then no boost may be required. If the peak value of the autofluorescence image drops to a minimum acceptable level, as determined by the computer image analysis program 376 as monitored by the overall control program 378, then additional current boost sections are enabled as required for the next exposure.
Once the autofluorescence image and the reference image have been acquired by the digital framegrabber in the computer then the analysis can begin. A reflectance (non-fluorescing) image taken with an endoscope camera system measures the brightness of the tissue surface in its field of view. To the extent that the tissue surface is a Lambertian (non-specular) reflector, the reflectance image signal (as digitized by the video framegrabber for each discrete pixel in the video image) is proportional to distance of the tissue from a single illumination source (or a weighted distance from multiple sources), and to the integrated energy of the excitation illumination during that video frame. The end window 346 of the excitation/reference delivery probe is not in the direct line-of-sight from the camera lens to the tissue so there will be visible shadows. A reflectance image can thus be used to measure the excitation illumination at the tissue surface visible to the camera, including the presence of shadows in the autofluorescence image. Note that this is only true if the excitation illumination and the visible illumination emanate from the same aperture with the same transverse intensity profile and angular divergence as provided by the design of the source as described hereinabove. A visible reflectance image taken with light from the illumination bundles of a standard endoscope, for instance, is not acceptable for determining the excitation illumination from a separate optical fiber passed through a biopsy channel of that endoscope. Note also, however, that the same aperture/same divergence condition can be satisfied by passing both the excitation and the reference light pulses through the illumination bundle of the endoscope.
The sequence of steps for obtaining a false color indication of dysplasia probability using the visible reference image along with the autofluorescence image proceeds as follows. The two images are first corrected for the gamma factor applied to the video signal by the video processor which is generally connected to a video monitor rather than a framegrabber. This insures that the digitized images acquired by the framegrabber in the computer are linear functions of the illumination fluence (time-integrated intensity). The two images are then normalized to their peaks, which is generally a region of non-dysplastic tissue somewhere in the visual field. There are a few pixels in the reference image which are saturated due to specular reflections of the reference illumination. These are effectively eliminated by producing a histogram of the reference image and normalizing the image to a peak value. This generally includes about 99% of the pixels.
All normalized reference image pixels above a value of 1 (the specular reflections) are then reset to a value of 1. There are no specular reflections in the autofluorescence image so such a histogram-based normalization is not necessary. On a pixel-by-pixel basis the autofluorescence image value is then divided by the corrected reference image value to produce a ratio image. This division is performed only when the autofluorescence and reference image pixel values are above a minimum threshold condition to insure that the analysis is not attempted with too little illumination to provide a reliable measurement.
If a ratio image pixel value falls below a predetermined value (typically one-half to one-third) then that pixel represents a region of reduced fluorescence on the tissue surface which is indicative of dysplasia. The corresponding pixel in the processed output image can then be set to a false color state to indicate the relative probability of dysplasia. If the ratio image pixel value is below ⅓ the red value of the corresponding processed output image pixel it is set to the value of the reference image and the green and blue values of that pixel are set to zero (it is a shaded red, indicating a high probability of dysplasia). If the ratio image pixel value is ½ to ⅓, then the green value of the processed output image pixel it is set to the reference image value and the red and blue values are set to zero (it is a shaded green, indicating a moderate probability of dysplasia). If the ratio image pixel value is above ½ then the red, green and blue values of the processed output image pixel are all set to the reference image value (it is a shaded gray, indicating the probability that the tissue is normal). This processed output image 386 is displayed on an LCD monitor 384 attached to the system computer 334.
Separation into two paths is required so that two rotating shutters, 808 and 810, can produce UV excitation and visible reference pulses at different times during the video frame as described above. A UV-reflective surface coating on mirror 812 immediately after the dichroic mirror reflects nearly 100% of the desired UV light in that path and dumps much of the undesired visible light reflected from the dichroic into its substrate. Additional UV filters represented by element 814 may include absorptive filters such as Schott UG-1 glass as well as multilayer dielectric bandpass filters centered on the 365 nm line of mercury. The UV path rejects visible light to a high degree since the efficiency of the 460 nm tissue fluorescence is only about 0.1%. Leakage of visible source light during the UV exposure period reduces the contrast of the autofluorescence image. Some correction for such leakage is possible during the digital image processing but corrections always add a small amount of noise to the result.
The first element in the reference path is a weak lens 816 to correct for the lower refractive index of the fused silica collimating lens 804 at visible wavelengths as compared to the UV. Note that lens 816 would not be necessary if a Schwarzschild objective is used at the position of lens 804 since such a design uses only mirrors and is thus totally achromatic. An attenuator can optionally be moved into the reference path at position 817 to prevent saturation of the reference image at close inspection distances. This attenuator must be uniform across the area that the reference beam passes through in order to maintain the uniform angular intensity distribution required of the reference illumination. This attenuation can be accomplished with variable, crossed polarizers, differentially-sliding, linear attenuation wedges or electromechanically-switched, fixed-value attenuators. The visible filter 818 in the reference beam path is less critical than the UV filter when sequential excitation/reference beams are used such as in the system shown. The reference beam wavelength is chosen to avoid hemoglobin absorption bands since a significant absorption introduces an error into the analysis which assumes that the reference image is equivalent to a measurement of the excitation illumination intensity. The rotating shutters 808 and 810 follow the filters. Continuing down the visible path after shutter 808 is turning mirror 820 with a broadband visible reflective coating. A second dichroic mirror 822, identical to dichroic mirror 806, recombines the UV excitation beam and the visible reference beam onto a common path. One additional tuning mirror 824 which reflects both UV and visible light directs the two beams towards the focusing optics 826 which couples them into the delivery fiber 828. The turning mirror 824 makes the number of reflections in both the UV and visible paths of the system equal. With equal numbers of reflections, any change in the position of the mercury arc 802 relative to the position of the collection optics 804 results in the same angular deviation for both the excitation and reference beams. Equal deviation angles preserves the overlap of the excitation and reference beams on the tissue. The system as shown in
The aperture stop 830 in
Mining the moment of inertia simplifies locking the shutter's rotation rate with the PLL. Being opaque is important to prevent light leakage. The cutouts 838 around the central hub 840 of the wheels leave thin blade flexures 842 which allow the wheel to flex slightly as it rotates to stay perpendicular to the rotation axis of the motor, minimizing vibration and stress on the motor bearings. The continuous outer edge of the shutter serves to carry the timing fiducial at the maximum distance from the axis where it is most sensitive and serves a safety function as well in the case of an inadvertent touch during assembly and testing. The aperture in the visible reference shutter 844 is positioned to match the normal green or red period of the video timing sequence. This instrument can either provide a single false color image for each footswitch signal or a series of processed images at a submultiple of the 30 Hz video frame rate.
A standard personal computer system can process the acquired data and update the false color image at 10 frames per second which is fast enough to preserve the impression of smooth motion and slow enough to allow a significant increase in the pulsed lamp current. Operation at lower submultiples of 30 Hz (7.5 or 6 Hz) still provides adequate real-time feedback and permits higher pulsed currents (and thus brighter WV illumination) without exceeding the nominal 100W average power dissipation in the lamp.
The fibers can be made with multiple, small-diameter fused silica fibers, but the preferred system uses a single, UV-transmitting acrylic fiber with a diameter of from 1.5 to 2 mm. One particular acrylic fiber that is suitable is the Raytela Polymer Optical Fiber made by Toray Industries, Inc. The single fiber system increases coupling efficiency by eliminating the packing fraction losses of multifiber bundles. Eliminating the multiplicity of internal cavities also increases the reliability of the disinfection process between procedures.
A window 902 at the tip of the probe is held on with a sleeve 904 and low-fluorescence epoxy in the gaps 906, 908 and 910 as shown in
In a preferred embodiment, constructing the above probe from a UV-transmissive plastic will reduce the cost, compared to a fused silica construction, to the point where the entire probe can be disposed after a single use.
For autofluorescence detection the complementary shutters 1010 and 1024 are triggered as before, causing the tissue to be illuminated with both the excitation light and reference light simultaneously. The UV-induced autofluorescence (primarily at 460 nm) is then detected by the blue-responsive pixels in the CCD camera. The red reference reflectance image is simultaneously detected by the red-responsive pixels. Color CCD cameras typically use electronic shuttering so that they don't need a dark period for their readout. In this embodiment, the rotating shutters in the excitation/reference light source 1018 are either stopped in an open position with a detent, or eliminated altogether if the autofluorescence system is to be used only with color video endoscopes. The UV illumination thus lasts for the full 33 ms of the frame, increasing the integrated visible fluorescence and reference signals and eliminating the need for a pulsed current supply. The lamp current supply 1022 could still be pulsed occasionally for single images without overheating the lamp. The analysis would proceed as before with the reference image appearing on the red channel of the RGB NTSC signal and the autofluorescence image appearing on the blue channel.
In the fluorescence image endoscope in accordance with the present invention, both the UV-excitation light pulse and the visible-reference light pulse are delivered to the tissue through an optical probe inserted through a biopsy channel of a standard endoscope. The excitation and reference light could alternatively be passed through the illumination bundle of an endoscope if that bundle were to be modified to transmit UV as well as visible wavelengths. The requirement would remain that the excitation light and the reference light, even if generated by different sources, have the same angular distribution as they exit the distal tip of the endoscope.
The imaging optics set is detailed in
In this embodiment the reference illumination is derived from the normal red illumination light from the visible light source. Filter 1130 is made of red absorbing glass which strongly attenuates wavelengths UV, blue and green wavelengths. Filter 1130 is mounted on a sliding base 1134, shown in
For the endoscope illumination bundle 1136 to be both UV-transmissive, flexible and long-lasting it must be made of fused silica optical fibers rather than regular glass fibers. The primary optical difference between the two materials, other than transparency, is that fused silica fibers generally have a lower numerical aperture than glass fibers. This means that light is collected and emitted at smaller angles from the axis in fused silica fibers. The illumination bundle bifurcates at point 1138 and exits the optical ports 1140 and 1142 at the distal tip of the endoscope. The lens elements in the ports 1140 and 1142 must also be modified (made more negative) to maintain the same illumination angle for existing, glass fiber based endoscopes.
The video imaging detection system 1144 at the distal tip of the endoscope transmits its signals back down the electrical wires 1146 within the endoscope to the electrical connector 1147 on the connector plug 1104. A mating connector on the adapter box 1100 collects these signals which are transferred across the adapter 1100 to a connector plug 1148 identical to plug 1147 which completes the electrical connections into the video processor. Analysis of the autofluorescence and reference images then proceeds as before.
Once the UV-capable endoscope is available, the excitation source can be built into the endoscope light source and video processor so that the entire autofluorescence imaging capability is included in the endoscope system itself. This preferred embodiment is shown schematically in
While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.