US 20020089740 A1
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.
1. An illumination system for imaging a specimen, the system comprising:
a motorized stage, in an optical system, for moving the specimen while an image of the specimen is captured;
a pulsed light illumination system in the optical system, wherein the pulse light illumination system optically stops motion on the motorized stage while allowing continuous physical movement of the motorized stage and thus the specimen;
a stage position detector, associated with the motorized stage, wherein the stage position detector controls firing of the pulsed light illumination system at predetermined positions of the motorized stage;
means for eliminating overlapping or missed specimen between images by precisely aligning the motorized stage and a camera, accurately positioning the motorized stage, and synchronizing the camera and the pulsed light illumination system; and
means for capturing well aligned image tiles for creation of an image montage.
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25. A method for imaging specimen, the method comprising the steps of:
placing the specimen to be imaged in a holder on a motorized stage;
controlling a tiling process by moving the motorized stage;
capturing image tiles with precise alignment by executing a strobe illumination system whenever a stage position sensor determines that the motorized stage has moved to a neighboring field of view of a camera;
scanning each row; and
removing the specimen and inserting another specimen to be imaged.
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35. An illumination system for imaging a specimen, the system comprising:
a motorized stage, in an optical system, for moving the specimen while an image of the specimen is captured;
a pulsed light illumination system in the optical system, wherein the pulse light illumination system optically stops motion on the motorized stage while allowing continuous physical movement of the motorized stage;
means for eliminating overlapping or missed specimen between images by precisely aligning the motorized stage and a camera, accurately positioning the motorized stage, and synchronizing the camera and the pulsed light illumination system by using positional information that is calculated from a stage controller; and
means for capturing well aligned image tiles for creation of an image montage.
 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.
 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'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 inherit 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'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.
 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.
 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;
FIG. 2 illustrates timing diagramming for a camera, stage and strobe of the present invention; and
 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 pulse light illumination system.
FIG. 1 illustrates a preferred embodiment of the invention. In this embodiment, a slide 114 to be imaged is placed in a micro scanning position in a slide holder on a motorized stage 102 and is scanned under microscope optics 104. To facilitate rapid imaging of slide 104 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 stage 102, thus the slide specimen, while allowing continuous stage motion. It should be apparent to one skilled in the art, that any pulse light illumination system may be used in place of the high-speed strobe light 102.
 To eliminate overlap or missed tissue between microscope images 114, precise alignment of stage 102 and 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 is used to control the firing of strobe 108, and thus the camera exposure. In this fashion, camera 104 can be operated with a long exposure window in comparison to the very short strobe flash, allowing lower cost components, specifically stage 102 and camera 104, to be utilized.
 In the invention, a computer program controls the operation of stage 102, camera 104 and 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, a stage position sensor, camera 104 and strobe firing circuitry 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 this reason, a 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. Along the long axis, however, the stage position is monitored by a direct stage position sensor, which can be separate or part of the overall stage control circuitry.
 In a preferred embodiment, a Ronchi ruler attached to stage 102 is used for the stage position sensor. 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 or the positional information can be acquired directly from the stage controller 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 based on the Ronchi ruler utilizes a light sensor that is mechanically isolated from the ruler. As the ruler passes under the sensor, a series of electronic pulses is generated corresponding to the alternating light and dark bands of the ruler. These pulses can be used to monitor the position and direction of stage 102.
 Based on the magnification of the optics and the camera utilized, strobe 108 is fired whenever the position sensor determines stage 102 has moved into the neighboring field of view of 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 camera.
FIG. 2 illustrates the signals of camera 104, stage 102, optical position detector, and strobe 108. Note that in FIG. 2, the signals from the optical position detector represent motion of stage 102, so their timing will vary depending on the speed of the stage movement. Because the system is triggered by the location of 102 stage as reported by the optical position sensor, 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 camera is controlled. In a preferred embodiment, the stage location, as sensed by a position sensor, fires both camera 104 and the strobe 108. In an alternate embodiment, camera 104 is free running and only strobe 108 is fired by stage position. This mode does not depend on uniform motion of stage 102 over the area imaged, because the strobe pulse is much shorter than the integration time of camera 104. As long as the correct stage position is reached anytime within the integration time of camera 104, an excellent, well aligned image results.
 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 strobe 108 and 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 camera 104 and strobe 108, and 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 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 exposure 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 the stage speed with the 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.