Abstract:
An illumination system for microscopic digital montage imaging based on a pulsed light illumination source triggered by the position of a specimen with respect to the optical axis of the microscope. The strobe illumination is used to facilitate high-speed tiled (montage) image capture of otherwise static specimens with higher throughput and significantly reduced mechanical precision requirements and cost. The invention allows for perfectly aligned montage tiles at high throughputs using standard microscope optics, having camera frame rate be the limiting factor in microscopic tiled image capture.

Description:
FIELD OF THE INVENTION 
     The present invention relates to microscopic digital imaging of complete tissue sections for medical and research use. In particular it describes a method for high throughput montage imaging of microscope slides using a standard microscope, digital video cameras, and a unique pulsed light illumination system. 
     BACKGROUND OF THE INVENTION 
     Laboratories in many biomedical specialties, such as anatomic pathology, hematology, and microbiology, examine tissue under a microscope for the presence and the nature of disease. In recent years, these laboratories have shown a growing interest in microscopic digital imaging as an adjunct to direct visual examination. Digital imaging has a number of advantages including the ability to document disease, share findings, collaborate (as in telemedicine), and analyze morphologic findings by computer. Though numerous studies have shown that digital image quality is acceptable for most clinical and research use, some aspects of microscopic digital imaging are limited in application. 
     Perhaps the most important limitation to microscopic digital imaging is a “sub-sampling” problem encountered in all single frame images. The sub-sampling problem has two components: a field of view problem and a resolution-based problem. The field of view problem occurs when an investigator looking at a single frame cannot determine what lies outside the view of an image on a slide. The resolution-based problem occurs when the investigator looking at an image is limited to the resolution of the image. The investigator cannot “zoom in” for a closer examination or “zoom out” for a bird&#39;s eye view. Significantly, the field of view and resolution-based problems are inversely related. Thus, as one increases magnification to improve resolution, one decreases the field of view. For example, as a general rule, increasing magnification by a factor of two decreases the field of view by a factor of four. 
     To get around the limitations of single frame imaging, developers have looked at two general options. The first option takes the general form of “dynamic-robotic” imaging, in which a video camera on the microscope transmits close to real time images to the investigator looking at a monitor, while the investigator operates the microscope by remote control. Though such systems have been used successfully for telepathology, they do not lend themselves to documentation, collaboration, or computer based analysis. 
     The second option being investigated to overcome the limitations inherent in single frame imaging is a montage (or “virtual slide”) approach. In this method, a robotic microscope systematically scans the entire slide, taking an image at every field. The individual images are then “knitted” together in a software application to form a very large data set with very appealing properties. The robotic microscope can span the entire slide area at a resolution limited only by the power of the optical system and camera. Software exists to display this data set at any resolution on a computer screen, allowing the user to zoom in, zoom out, and pan around the data set as if using a physical microscope. The data set can be stored for documentation, shared over the Internet, or analyzed by computer programs. 
     The “virtual slide” option has some limitations, however. One of the limitations is file size. For an average tissue section, the data generated at 0.33 um/pixel can be between two and five gigabytes uncompressed. In an extreme case, the data generated from one slide can be up to thirty-six gigabytes. 
     A much more difficult limitation with the prior systems is an image capture time problem. Given an optical primary magnification of twenty and a two-third inch CCD, the system field of view is approximately (8.8 mm×6.6 mm)/20=0.44×0.33 mm. A standard tissue section of approximately 2.25 square centimeters, therefore, requires approximately fifteen hundred fields to cover the tissue alone. 
     Field rate in montage systems is limited by three factors—camera frame rate, image processing speed, and the rate of slide motion between fields. Given today&#39;s technology, the rate of slide motion is a significant limiting factor largely because the existing imaging systems require the slide to come to a stop at the center of each field to capture a blur free image of the field. 
     For example, traditional bright field microscopic illumination systems were designed to support direct visual examination of specimen on the field and therefore depend on a continuous light source for illumination. Continuous light however, is a significant limitation for digital imaging in that the slide must be stationary with respect to the camera during CCD integration. Slide motion during integration results in a blurred image. Traditional montage systems, therefore, have had to move the slide (and stage) from field to field in a precise “move, stop, take image and move again” pattern. This pattern requires precise, expensive mechanics, and its speed is inherently limited by the inertia of the stage. 
     Thus, a system is needed to address the image capture time limitation. The system must also enable efficient and high quality imaging of a microscope slide via a high-resolution slide scanning process. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method and illumination system for imaging a specimen on a slide. The system includes a motorized stage, a pulse light illumination system, and a stage position detector. The motorized stage moves the slide while an image of the slide is captured. The pulsed light illumination system optically stops motion on the motorized stage while allowing continuous physical movement of the motorized stage and thus the slide. The stage position detector is associated with the motorized stage and the stage position detector controls firing of the pulsed light illumination system at predetermined positions of the motorized stage. 
     It is therefore an object of the invention to provide a microscopic imaging system for whole slide montage in which standard microscope optics, off the shelf cameras, a simple motorized stage, and pulse light illumination system can be used to produce perfectly aligned image tiles, and acquire these images at a speed limited by the camera frame rate. 
     The present invention uses a strobe light triggered by a direct Ronchi ruler or other stage-positioning device, to produce precisely aligned image tiles that can be made into a montage image of tissue sections on a microscope slide. Significantly, due to the short light pulse emitted by a strobe, clear images can be obtained without stopping the microscope stage. This significantly increases the image throughput while decreasing the expense and precision required in the stage mechanics. 
     In the preferred embodiment, a strobe arc is placed at the position of the lamp bulb in a standard microscope system. The camera shutter is opened and the strobe is fired, in response to the position of the stage as reported by a direct position sensor. If stray light is minimized, the camera exposure can be much longer than the strobe flash, allowing low cost cameras to be utilized. 
     It is another object of the invention to significantly increase the image throughput of a tiling image system by allowing, through the use of the strobe light, continuous motion of the slide under the microscope. The inventive system thus eliminates the need to stop the microscope stage to capture an image. 
     It is another object of the invention to reduce the demands of camera, stage, and strobe synchronization by controlling the firing of the strobe light based on direct stage position feedback, thereby, substantially reducing the mechanical specifications on the stage and camera components. 
     Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and advantages of the invention to be realized and attained by the microscopic image capture system will be pointed out in the written description and claims hereof as well as the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention that together with the description serve to explain the principles of the invention. 
     FIG. 1 illustrates a view of the system in a preferred embodiment; and 
     FIG. 2 illustrates timing diagramming for a camera, stage and strobe of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The following paragraphs describe the functionality of the inventive system and method for high throughput montage imaging of microscope slides using a standard microscope, camera, and a pulsed light illumination system. 
     FIG. 1 illustrates a microscope  100  that may be a bright field microscope in an embodiment of the invention. The microscope  100  includes a microscopic camera  104  that may be utilized to capture a high-resolution image of an object and a macroscopic camera  106  that may be utilized to capture a thumbnail image of an object, That embodiment also includes a motorized stage  102  and two stage motors  110  for moving the motorized stage  102  along X and Y-axes. Moreover, that embodiment includes a strobe light  108 . A slide is also shown in a loading position  116  a micro scanning position  114  and a thumbnail imaging position  112 . In this embodiment, a slide to be imaged is placed in the micro scanning position  114  in a slide holder on a motorized stage  102  and is scanned under microscope optics of the microscopic camera  104  To facilitate rapid imaging of the slide and to avoid the stop image reposition delays associated with traditional imaging systems a high-speed strobe light  108  is used to optically stop the motion of the stage  102 , and the slide specimen situated on the stage  102 , while allowing continuous stage motion. It should be apparent to one skilled in the art, that any pulsed light illumination system may be used in place of the high-speed strobe light  108 . 
     To eliminate overlap or missed tissue between microscope images, precise alignment of the stage  102  and the camera  104 , along with accurate stage positioning, and camera  104  and strobe  108  synchronization, are required. To reduce camera  104  specifications, a direct stage position sensor  118  is used to control the firing of strobe  108 , and thus the microscopic camera  104  exposure. In that fashion, the microscopic camera  104  can be operated with a long exposure window in comparison to a very short strobe flash, allowing lower cost components, specifically the stage  102  and the camera  104 , to be utilized 
     In the invention, a computer program controls the operation of the stage  102 , the camera  104  and the strobe  108  illumination. The actual slide scanning can be automated to image entire slides, image only a portion of the slide or use a user-interface to allow the user to select the regions to be imaged. Once a region has been selected for imaging, the program then controls the operation by communicating with a stage controller  120 , the stage position sensor  118 , the microscopic camera  104  and strobe firing circuitry for the strobe  108 . Preferably, tiling is performed by moving stepwise along the short axis and with continuous motion along the long axis. In other words, tiling is done one row at a time. For that reason, stage position is monitored and controlled differently along each stage axis. Along the short axis of the slide, the stage position is monitored and controlled, by the program, directly through the stage controller  120 . Along the long axis however, the stage position is monitored by a direct stage position sensor  118 , which can be separate or part of the overall stage control circuitry. 
     In a preferred embodiment, a Ronchi ruler attached to the stage  102  is used for the stage position sensor  118 , as illustrated in FIG.  1 . It should be obvious to those skilled in the art that any position sensor may be used in the invention. This sensor can be external to the stage controller  120  or the positional information can be acquired directly from the stage controller  120  with or without feedback. 
     For reference, a Ronchi ruler is a pattern of alternating light and dark bands, equally spaced along a substrate, typically either glass or plastic. A position sensor  118  based on the Ronchi ruler utilizes a light sensor that is mechanically isolated from the ruler. As the ruler passes under the light sensor, a series of electronic pulses is generated corresponding to the alternating light and dark bands of the ruler. Those pulses can be used to monitor the position and direction of the stage  102 . 
     Based on the magnification of the optics and the microscopic camera  104  utilized, the strobe  108  is fired whenever the position sensor  118  determines the stage  102  has moved into the neighboring field of view of the microscopic camera  104 . The system continues to capture image tiles with precise alignment, until the row is finished or the controlling program tells the system to stop. At the end of the capture process, the slide is removed and another slide can be inserted. With current technology, the rate-limiting step for image capture is the data transfer period in the microscopic camera  104 . 
     FIG. 2 illustrates the signals of the camera  104  the stage  102 , the stage position detector  118 , and the strobe  108 . Note that in FIG. 2, the signals from the stage position detector  118  represent motion of the stage  102 , so their timing will vary depending on the speed of the stage movement. Because the system is triggered by the location of the stage  102  as reported by the stage position sensor  118 , the absolute speed of the stage movement is not relevant, allowing for the use of low cost stages. 
     The system can be run in one of two modes, depending on how the microscopic camera  104  is controlled. In a preferred embodiment, the stage  102  location, as sensed by the position sensor  118 , fires both the camera  104  and the strobe  108 . In an alternate embodiment, the microscopic camera  104  is free running and only the strobe  108  is fired by stage  102  position. The alternative embodiment does not depend on uniform motion of the stage  102  over the area imaged, because the strobe pulse is much shorter than the integration time of the camera  104 . As long as the correct stage position is reached anytime within the integration time of the camera  104 , an excellent, well aligned image results. 
     At  202  of FIG. 2, actual position of the motorized stage  102  is plotted versus time during capture of an image. That plot of the actual position of the motorized stage  102  is compared to a plot of steady movement to show that actual stage motion is not steady. As shown in  206 , firing strobe  108  based on direct position information differs from the more traditional application of strobe photography, shown in  204 , where the strobe  108  and the camera  104  are synchronized in time and positional information of the objects can be inferred from the relative position within the image. When operated in the mode where the position feedback controls both the camera  104  and the strobe  108 , and the camera  104  is not free running, each camera frame corresponds to an equally spaced positional change, independent of the stage velocity (speed and time variations in the speed). In the case that the camera  104  is free running, the stage speed has to be matched to the camera frame rate only to the accuracy such that the strobe pulse does not fall outside the exposed window. The relative time within the exposure window is irrelevant. 
     As is obvious to one skilled in the art, while the present invention describes a microscopic optical arrangement, the invention can also be applied to other optical imaging, inspection and illumination systems that are used for building up an image by matching stage speed with camera speed. 
     The foregoing description has been directed to specific embodiments of this invention. It will be apparent, however, that other variations and modifications may be made to the described embodiments, with the attainment of some or all of their advantages. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.