Patent Publication Number: US-6698887-B2

Title: Stereo scanning laser ophthalmoscope

Description:
This is a continuation of application Ser. No. 09/555,242 May 26, 2000 now U.S. Pat. No. 6,379,006. 
    
    
     The present invention relates to a method and apparatus for the stereoscopic examination of the fundus of the eye. This invention has applications in the investigation and diagnosis of diseases that affect the posterior chamber of the eyeball. The invention will be described in reference to the above application, however, it is possible and envisioned that the apparatus and technique of the present invention may be used for stereoscopic imaging in other medical processes. 
     Visualisation of the ocular fundus can provide important information about the state of the eye and of the body. Knowledge regarding ocular and systemic diseases, such as glaucoma, macula degeneration, diabetes or hypertension can be gained from examination of the posterior pole of the eye. In the past, imagining of the ocular fundus has been performed through the use of an ophthalmoscope, in which a direct view of the retina may be obtained. Other methods include the use of fundus cameras to obtain photographic images. However, these techniques usually require the use of mydriatic dilating drugs. The amount of light required to illuminate the fundus may also be uncomfortable for the patient. 
     Recent developments have resulted in the emergence of a new imaging tool for the ophthalmologist, in which an image of the eye may be observed in real-time and captured on a television monitor or screen, during procedures such as fluorescein angiography. This instrument, known as a scanning laser ophthalmoscope (SLO), first described in U.S. Pat. No. 4,213,678, is currently used to produce representations of the ocular fundus in two dimensions. U.S. Pat. Nos. 4,765,730, 5,268,711 and 5,430,509 describe different embodiments of the scanning laser ophthalmoscope. All utilise a laser beam or light source that is directed through the pupil and onto the retina by way of two, directional, scanning mechanisms. The light from the laser is reflected off the retinal wall towards a photosensitive detection device. Electro-optical circuitry is employed to convert the light into synchronized signals, so that it is possible to display an image of the fundus on a television screen or monitor. 
     U.S. Pat. No. 5,430,509 describes a different embodiment of the scanning laser ophthalmoscope in which three or more scanning mechanisms are employed to develop a video image of the fundus. Scanning occurs in three directions, horizontal, vertical and in the direction of the optical axis. The inventors suggest that the use of three scanning devices, each with a different scanning frequency, will reduce the demands and wear on the scanner&#39;s bearings. However, despite the three scanning mechanisms, only a two dimensional image can be reconstructed on screen in real time. 
     The prior SLO technology described herein above does not permit stereoscopic viewing of the ocular fundus. Nevertheless, the optic disc region and retinal layers have a three dimensional structure. Stereoscopic images of the ocular fundus can impart valuable information that cannot otherwise be derived from a two dimensional representation, especially in relation to the diagnosis of glaucoma. Efforts have therefore been made to create a device capable of producing three dimensional fundus images, while improving on the contrast and resolution of conventional SLO images. 
     Frambach, Dacey &amp; Sadun (1992, 1993) describe a method of producing a three dimensional fundus picture during fluorescein angiography, using a modified SLO. To obtain stereoscopic data the SLO was manually moved from side to side during angiogram proceedings, much like a fundus camera is moved to enable viewing from two different positions. Individual frames from the video tape were chosen from left and right perspectives to provide a three dimensional image. An alternative approach employed by the authors involved the use of a modified Allen separator. A piece of flat glass was attached to an extended rod, coupled to the Allen separator, so that the glass was interposed between the eye and the SLO. The glass was then rapidly rotated to provide the left and right perspectives. The resulting frames were digitized by computer and viewed directly on a video screen. Superimposed images were formed by breaking a stereo pair down into corresponding fields and recombining them to form a single frame. LCD glasses were then used to view the left and right fields with the corresponding eye. 
     Frambach et al (1993) illustrate that achieving a stereoscopic image from a conventional scanning laser ophthalmoscope is possible. However, the methods involved exhibit a number of disadvantages. Frambach et al&#39;s first method of shifting the SLO involved awkward and confusing adjustments, resulting in poor stereoscopic image quality. In the second method, interference due to unwanted back reflections from the Allen separator would hinder in the transmission of stereoscopic information. Unwanted scattered light would impinge on the photodetecting element, causing a decrease in the contrast and resolution of the images. 
     Improvements in SLO image resolution and contrast are possible if the detector receives light only from the plane of interest and not scattered light from the media of the eye. A scanning laser ophthalmoscope that could provide high resolution, high contrast images of an ocular fundus was realised with the invention of the confocal scanning laser ophthalmoscope (cSLO), such as that described in U.S. Pat. Nos. 5,170,276 and 5,071,246. The confocal SLO utilises a pinhole or slit aperture to focus the light reflected from the fundus onto a photodetecting element. The aperture is located at a plane in which the opening is conjugate with the plane of the fundus of the eye. In this way, only the light reflected from the plane of interest impinges on the photodetecting element and any light scattered or reflected from out-of-focus planes is prevented from degrading the image. 
     A further technique to produce three dimensional images of the ocular fundus, known as scanning laser triangulation, is described by Milbocker &amp; Reznichenko (1991). Triangulation is a method commonly used for measuring distances. Combined with a confocal aperture, this method involves synchronized scanning of a pixel of light across the fundus by way of two mirrors. The illumination and detection paths are arranged symmetrically and are defined by the two mirrors. The axial distance is measured by the displacement of the illuminated spot in the confocal plane, enabling calculation of the depth. (That is, the points above and below the ‘average’ position of the retinal wall). Disadvantages of this method include impractical computational speeds for clinical practice and an inflexible stereo base; large pupil and numerical apertures cannot be used with this technique. 
     Confocal scanning laser ophthalmoscopes are also currently used to provide three dimensional information concerning the ocular fundus. The confocal aperture of the cSLO allows the user to precisely focus on specific layers of the retina. By adjusting the focal plane of the aperture, images can be captured at different levels in the fundus, to reproduce desired depth characteristics. In this way a number of “optical sections” can be produced. A computer can then be used to extract depth information, through the process of “stacking” a selection of the optical sections taken at different levels of the retina. Information regarding the third dimension can therefore be interpolated. 
     Nevertheless, stereoscopic imaging via the method described is a time consuming process. A large number of images must be acquired at different focal planes. Interference from patient head and eye movement is likely to influence the resultant three dimensional image. Computer processing times must also be taken into account. These factors-make confocal sectioning impractical in a clinical setting, where real time images are required for fast diagnosis or treatment. 
     U.S. Pat. No. 4,900,144 (also see Optics Communications: 87(1,2): 9-14) describes a scanning laser ophthalmoscope that utilises an alternative confocal focussing arrangement. The invention is capable of producing a three dimensional representation of an object that displays multiple reflectivity characteristics (such as the ocular fundus) through a method slightly different from the conventional confocal depth production described above. This US patent teaches the use of two separate confocal slit apertures and photodetecting units. The detection slits are orientated parallel to the direction in which the light, reflected from fundus, is scanned. However, both slits are slightly displaced from the normal position: the apertures are not conjugate with the fundus of the eye. One is positioned slightly to the front of the conjugate plane, while the other is placed to the rear. Because of the positioning of the confocal apertures, the output signals from the photodetectors vary in intensity according to the unevenness of the fundus. The resultant output signals are processed electronically by division calculations, detailed in U.S. Pat. No. 4,900,144, to obtain a three dimensional profile. The resultant real-time image displays the topography of the fundus through different shade levels, reflecting different retinal depth levels. 
     Software may also be used to create three dimensional graphic patterns. However, as in the above mentioned case, this method requires a high level of computer power to construct the three dimensional image. An increase in computer processing time is the inevitable result. It also relies on the incorrect assumption of homogeneity of the retina structures. 
     The above methods make use of the depth discrimination property, axial resolution, of the confocal system. Unlike the lateral resolution, the axial resolution is strongly limited by two factors. Firstly, the shape and size of the laser light focus which is scanned over the retina may suffer from deformations and distortions, particularly in the direction of the optical axis. This is due to the limited useful numerical aperture of the eye and its&#39; optical imperfections. Furthermore, the size of the detection pinhole or slit may constrain the axial resolution of a confocal SLO. Due to intensity limitations on the living eye it is necessary to provide a detector aperture size which is larger than the optimal confocal pinhole in order to maintain a sufficient signal to noise ratio. As a result of these two factors the axial resolution of a confocal system is typically thirty times less than the lateral resolution. 
     Accordingly, there remains a need to provide a scanning laser ophthalmoscope capable of displaying true stereopsis, which is not restricted by any of the aforementioned limitations. 
     It is therefore an object of the present invention to provide an improved method and apparatus for producing a high contrast, real time, three dimensional representation of a scanned object. 
     It is a further object of the present invention to provide such an apparatus and method with the use of an additional scanning mechanism to gain information about the third dimension. 
     According, therefore, to the present invention, there is provided an apparatus for producing an image of a surface, including: 
     a laser source for producing a beam of light, 
     beam modifying means for modifying the properties of the beam, 
     beam splitting means for splitting the beam, 
     focussing means for focussing the beam, 
     first and second scanning means for scanning the beam in first and second different directions, 
     stereo base producing means for obtaining stereoscopic information concerning the surface by impinging the beam onto the surface from two different positions, and 
     reflecting means, wherein a beam from the laser source may be directed along an input path including being modified with the modifying means and split by the beam splitting means, directed onto the first scanning means to scan the beam in the first direction, directed through the second scanning means to scan the beam in the second direction, directed through the stereo base producing means, and then directed off the reflecting means and onto the surface, whereby reflected light from the surface traverses an output path identical to the input path, towards the beam splitting means, and whereby some of the reflected light is directed towards the photodetecting means coupled to the signal processing means and display means to produce an image for viewing with visualising means in real time. 
     Preferably the apparatus further includes confocal aperture means whereby the some of the reflected light is directed through the confocal aperture means towards the photodetecting means. The confocal aperture means may include a slit, diaphragm or a pinhole. 
     Preferably the apparatus includes beam guiding means for guiding the beam before the beam is directed through the second scanning means, and preferably the beam modifying means includes a beam expander and/or a pinhole aperture. 
     Thus, although a beam guiding means is unnecessary in some configurations, it may be desirable in certain embodiments. 
     Preferably the apparatus includes a plurality of mirrors for directing the beam onto the first scanning means. 
     Thus, although a single mirror and/or lens may be employed, preferably the apparatus includes a plurality of such mirrors and/or lenses. 
     Preferably the first and second directions are perpendicular to one another, and more preferably one of the first and second directions is substantially horizontal and the other substantially vertical. 
     Preferably the surface is the ocular fundus. 
     The laser source is preferably a helium-neon laser source, a Ti:Sapphire laser source, an argon-ion laser source or a laser diode source. Preferably the laser source is one of a plurality laser sources. 
     The beam splitting means may be coated or non-coated. 
     Preferably the focussing means includes a plurality of mirrors and/or lenses. 
     Preferably the first scanning means includes a rotating polygon mirror. Alternatively the first scanning means includes an acousto-optic deflector or a mirror on a resonant scanner or on a galvanometer motor. 
     Preferably the second scanning means includes a mirror mounted on a scanning galvanometer motor, a resonant scanner or rotating polygon mirror. 
     Preferably the stereo base producing means is operable to obtain stereoscopic information by directing light onto the surface from a different position every alternate frame, and more preferably these two different positions are left and right positions. More preferably the stereo base producing means includes a pair of toggling mirrors for toggling every alternate frame to image the surface from two slightly different positions, with substantially overlapping imaging areas, to create a stereo pair. 
     Preferably the reflecting means is a large, curved mirror. 
     Preferably the photodetecting means includes a photomultiplier tube or an avalanche photodiode. 
     The signal processing means is preferably a computer with a video signal capture facility, while the display means is preferably a computer monitor or any other suitable display apparatus. 
     Preferably the visualising means is a pair of Liquid Crystal Display (LCD) goggles or a pair of goggles with a different coloured filter for each eye. 
     The present invention also provides a method for scanning a surface with a laser beam to thereby produce an image of the surface, including directing a laser beam along an input path including: 
     1) modifying and then splitting the laser beam, 
     2) focussing the beam, 
     3) scanning the beam in a first direction, 
     4) scanning the beam in a second direction, 
     5) directing the beam through stereo base producing means to provide stereoscopic information by impinging the beam onto the surface from two different positions, and 
     6) reflecting the beam onto the surface, whereby reflected light from the surface traverses an output path identical to the input path, 
     splitting the beam and directing a portion of the split beam towards photodetecting means coupled to signal processing means and display means, whereby a resultant image may be viewed with visualising means in real time. 
     The method preferably includes directing the portion of the split beam through a confocal aperture means towards photodetecting means. The confocal aperture means include a slit, diaphragm or a pinhole. 
     Preferably the method includes guiding the beam before scanning the beam in the second direction. The beam guiding is preferably by means of beam guiding means including a curved mirror. 
     Preferably the first and second directions are perpendicular to one another. More preferably one of the first and second directions is substantially horizontal and the other substantially vertical. 
     Preferably the surface is an ocular fundus. 
     Preferably the beam modifying is performed by beam modifying optics including a beam expander and a pinhole aperture. 
     Preferably the beam modifying optics include a plurality of mirrors and/or lenses. 
     Preferably the method includes providing the laser beam by means of laser source means including a helium-neon laser source, a Ti:Sapphire laser, an argon-ion laser, a frequency doubled Nd:YAG laser source, a frequency doubled laser diode, or a laser diode. 
     Preferably the scanning the beam in a first direction is by means of first scanning means including a rotating polygon mirror. 
     Alternatively the first scanning means includes an acousto-optic deflector or a galvanometer mirror or a spinning multifaceted mirror, with the number of facets being a factor of the numbers of lines in the video standard, or a resonant scanner. 
     Preferably the scanning the beam in a second direction is by means of second scanning means including a mirror mounted on a scanning galvanometer motor, a resonant scanner or rotating polygon mirror. 
     Preferably the stereo base producing means is operable to obtain stereoscopic information by directing light onto the surface from a different position every alternate frame or half frame. 
     The stereo base producing means preferably includes a pair of toggling mirrors for toggling every alternate frame to image the surface from two slightly different positions, with substantially overlapping imaging areas, to create a stereo pair. Alternatively, it may include a toggling mirror and be operable to obtain stereoscopic information by adjusting the phase of the first scanning means. The adjusting may be effected mechanically or electronically. 
     Preferably the two positions are left and right positions. 
     Preferably the reflecting the beam onto the surface is performed by means of reflecting means including a large, curved mirror. 
     Preferably the photodetecting means includes a photomultiplier tube or an avalanche photodiode. 
     Preferably the signal processing means is a computer with a video signal capture facility. 
     Preferably the display means is a computer monitor or any other suitable display apparatus. 
     Preferably the visualising means is a pair of LCD goggles or a pair of goggles with a different coloured filter for each eye. 
     The present invention also provides an apparatus for visualizing human or animal tissue including: 
     laser source means for producing a beam of substantially collimated light; 
     beam modifying means for modifying the beam and/or controlling characteristics of the beam; 
     beam splitting means for splitting the beam; 
     focussing means for focussing the beam; 
     first scanning means for scanning the beam in a first direction; 
     second scanning means for scanning the beam in a second direction substantially perpendicular to the first direction, whereby the beam can be converted into a raster pattern; 
     means for adjusting beam paths so scanned laser beam or beams approach tissue from different positions; 
     reflecting means for reflecting the raster pattern; 
     electro-optical means for controlling the rate of scanning in the first and second directions and the rate of depth scanning; 
     photodetecting means for detecting the light reflected from the tissue; 
     signal processing means for converting the signals into television lines; 
     display means for displaying an image; software means for converting the image into an interlaced image; and 
     visualising means for viewing the image in three dimensions in real time. 
     Preferably the apparatus includes confocal aperture means for substantially eliminating scattered, unwanted light or out of focus light. The confocal aperture means is preferably a slit, diaphragm or pinhole. 
     Preferably the apparatus includes beam guiding means for guiding the beam onto the second scanning means. The beam guiding means preferably includes a curved mirror. Alternatively the beam guiding means may include lenses and/or mirrors. 
     Preferably the laser source means is a helium-neon laser source, a Ti:Sapphire laser source, an argon-ion laser source, a frequency doubled Nd:YAG laser source, a frequency doubled laser diode, a laser diode source or any other monochromatic light source. 
     Preferably the laser source means includes two or more laser sources. 
     Preferably the beam modifying means includes a beam expander and/or a pinhole. 
     Preferably the second focussing optics includes a plurality of mirrors and/or lenses. 
     Preferably the first scanning means includes an acousto-optic deflector or a resonant or galvanometer mirror or a spinning multifaceted mirror, with the number of facets being a factor of the numbers of lines in the video standard (e.g. 5 or 25 for PAL), or a resonant scanner. 
     Preferably the second scanning means includes a galvanometer mounted mirror, or a resonant scanner or a rotating polygon mirror. 
     Preferably the stereo base producing means is operable to provide stereoscopic information about the surface by directing light onto the surface from two different positions, alternating every alternate frame, and more preferably the stereo base producing means includes a pair of toggling mirrors that toggle every alternate frame to image the surface from slightly different positions, with substantially overlapping imaging areas, such that a stereo pair can be created. 
     Alternatively the one or both of the pair of toggling mirrors may be replaced by electronic circuitry that can vary the phase of the first scanning means, or a galvanometer for turning the whole first scanning means, a prism or glass plate or any other means that is capable of directing light onto the surface from disparate positions every alternate frame. 
     Preferably the reflecting means is a large, curved mirror. 
     Preferably the electro-optic means is a plurality of electric circuits with optical or electronic feedback on the scan position of the mirrors. 
     Preferably the photodetecting means includes a photomultiplier tube, or an avalanche photodiode. 
     Preferably the signal processing means is a computer with a video signal capture facility. 
     Preferably the display means is a computer monitor or any other suitable display monitor. Preferably the software means is an image processing program that is capable of producing an interlaced image in real time. 
     Preferably the visualizing means includes a pair of LCD goggles or a pair of goggles with a different coloured filter for each eye. 
     The tissue may be ocular, which may be the fundus of an eye, in the optic nerve head region, or any other ocular feature of interest. 
     Preferably the first direction is substantially horizontal. 
    
    
     In order that the invention be more fully ascertained, some preferred embodiments will be described, by way of example, with reference to the accompanying drawing in which: 
     FIG. 1 is a schematic diagram of a first preferred embodiment of the present invention; 
     FIG. 2 illustrates the process of image production; and 
     FIG. 3 is a schematic diagram illustrating an electrical configuration of one preferred embodiment of the present invention. 
    
    
     Referring to FIG. 1, a preferred embodiment of the present invention utilises a Helium-Neon laser source  1 , or any other suitable source of substantially collimated light, which emits a laser beam  2 . Alternatively, two or more laser sources may be utilised to produce beam  2 . This beam is directed through a focussing system including beam expanders  3  and  5  (in the form of lenses), which expand the beam to a specified size, and a pinhole  4  between beam expanders  3  and  5 . The beam  2  is then reflected from a mirror  6  towards beam splitting means in the form of beam splitter  7 . Part of the light passes through the beam splitter  7  into a light trap  8 . The rest of the light is reflected towards a number of focussing mirrors  9 . Alternatively a lens or combination of lens and mirrors may be placed at this point. The focussing mirrors  9  guide the beam onto a resonant scanner or a rapidly rotating multiple facet mirror  10 , which acts as a horizontal scanner, and subsequently onto a small curved mirror  11 , which shapes the beam into a horizontal line. The beam travels to a vertical scanner  12 , in this case a galvanometer controlled mirror, and is preferably directed onto a pair of toggling mirrors  13  and  14 . These mirrors are positioned so that each directs the beam  2  onto the surface-to-be-imaged from two slightly different positions, with substantially overlapping imaging areas. They preferably toggle every alternate frame, such that visual information is received from the right and left perspectives in alternate frames. A second preferred embodiment would position toggling mirrors  13  and/or  14  before the horizontal scanning mechanism  10 . Alternatively, the two toggling mirrors could be substituted with a single mirror that can change position preferably every second frame or half frame, or a galvanometer mounted prism or glass plate that is capable of imaging from the left and right perspective every half frame. Further embodiments include replacing toggling mirrors  13  or  14  with electronics that can vary the phase of the horizontal scanner or with a galvanometer that can turn the whole horizontal scanner. 
     The toggling mirrors  13  and  14 , or one of the various alternatives, rapidly direct the beam through the pupil of eye  16  from two slightly different positions. The light, now a raster, is reflected off a large curved mirror  15  before entering the eye. The light that enters the eye through the pupil  16  passes through the eye&#39;s internal structure to reach the retina at the back of the fundus. The light is reflected off the retinal layers and travels to the exterior through the pupil. The reflected beam  17  traverses the same output path as the incident beam  2 . Upon reaching beam splitter  7 , most of the light continues through the beam splitter towards focussing means in the form of focussing optic  18 . The reflected light is impinged off mirror  19  towards a confocal aperture  20 , preferably a slit, although a pinhole configuration is also suitable. The beam then enters a photodetector  21 , by preference a photomultiplier tube or an avalanche photodiode. The output of the photosensor is then preferably converted to a standard television signal pattern through signal processing means  22 . 
     Electro-optic signals are received by an imaging board in computer  23  and appropriate electronic hardware and software converts the information sent from toggling mirrors  13  and  14  into first and second images. An interlaced image of half frames is then constructed on a monitor  24 , preferably a computer monitor, in which every line or second line is captured from both images. Referring to FIG. 2, image A and image B, either of which may be the left or right image, are combined to form a single image C containing stereoscopic information. The top line of the interlaced image C is line  1 A from, in this example, image A, while the next line down of image C is line  2 B from image B. The process of capturing alternate lines from the two images continues until the completion of an interlaced image. A three dimensional view can then be perceived by an observer with the use of a pair of liquid crystal display (LCD) goggles  25  coupled to. display means  24  (see FIG.  1 ). As the software writes the information from image A which may be, for example, the left image, the LCD goggles momentarily shutter the observer&#39;s right eye. A complementary process occurs with image B. In this way, the left eye views the left perspective, and the right eye views the right perspective, enabling the observer to detect a stereoscopic or three dimensional image of the ocular fundus in real time. Alternatively, an interlaced image can be written so that the lines from image A are one colour, (e.g. green) and the lines from image B are another (e.g. red). Wearing goggles with matching colour filters for each lens (e.g. red and green) allows each eye to perceive one image only, and thus view a stereoscopic image. 
     FIG. 3 illustrates the signal processing involved in controlling the scanning mechanisms. The focussing optics, beam splitter, photodetector and viewing apparatus have been omitted for simplicity. Corresponding reference numerals are used for like or corresponding parts from the several diagrams. 
     Electro-optic circuitry is employed to detect the beginning and the end of each raster and the end of each image frame. Laser source  1  produces a beam of monochromatic light  2  which is directed onto horizontal scanning mechanism  10 . A driver  30 , such as a 35000 RPM motor, drives the polygon mirror in circles to generate a horizontal line scan. Alternatively, a resonant scanner is driven in a sinusoidal pattern to generate the horizontal line scan. The line is reflected off mirror  11 , and onto another mirror  32 , which is driven in a sawtooth pattern perpendicular to the motion of the line. The action of  10  and  32  together produce a rectangular raster pattern. The timing of the start of the sawtooth pattern is set by the detection of the end or the start of the line at mirror  11 . A detector  31  senses the end or start of the line, and this signal is conditioned into a sawtooth by signal processor  22 . The signal processor  22  counts the number of lines in a raster and generates a frame synch. signal  40 . A line synch. signal  42  is also generated. Both of these signals are sent to the SLO computer system  23 . After the laser beam  2  has been converted into a raster, it is further deflected by the depth producing apparatus, which in this case is shown as prism  34 . The signal processor  22  also sends a signal to the prism controller  33  to tell it when to move from one position (image A) to another (image B). The timing of this movement can be varied, but typically it is at the end of every frame or half frame. The signal processor typically sends a square wave. to the controller. The laser beam is then impinged off mirror  15  and directed through the pupil onto the retina (not shown). The reflected beam traverses the output path described in FIG. 1 to the photodetector not shown). 
     Signal processing means  22 , coupled to the photodetector, sends electro-optic signals to imaging board  35 , in computer  23 , where appropriate hardware and software interprets them and constructs image A and image B as left and right perspectives. An interlaced image, is then created by specialized software and displayed on display means  24  (not shown) in real time. LCD goggles or other appropriate viewing means, can then be used to visualise the three dimensional interlaced image. The computer could also use the angle between the two images to calculate the real three dimensional shape. 
     Modification within the spirit and scope of the aforementioned invention may be readily effected by a person skilled in the art. Other embodiments would involve the use of a galvanometer mounted prism or glass plate in place of toggling mirrors to produce images from two different perspectives. Electronics that vary the phase of the horizontal scanner or a galvanometer that turns the whole horizontal scanner may also be utilised. It is to be understood, therefore, that this invention is not limited to the particular embodiments described by way of example hereinabove.