CN1362868A - CCD array as a multiple-detector in an optical imaging apparatus - Google Patents

Abstract

A scanner assembly for a laser imaging apparatus comprises a plurality of collimators disposed in an arc around an opening in which an object to be scanned is disposed; a plurality of fiber optic cables, each having one end being associated with a respective collimator; a light tight enclosure; and a CCD array disposed within the enclosure and disposed at the other ends of the fiber optic cables to detect light picked up by the collimators and transmitted by the fiber optic cables.

Description

CCD array as multi-detector in optical imaging device

technical field

This invention relates to optical imaging devices, and more particularly to imaging devices using charge coupled devices (CCDs).

Background of the invention

A charge-coupled device (CCD) may be a single device, or more typically, individual cells or pixels arranged in an array (either as a linear array or as an X-Y matrix). With an appropriate optical lens, a CCD is usually used as a sensor of an image camera. CCDs have a spectral response in the visible region, 400-700nm, and generally extend into the near-infrared region, 700-1100nm.

Traditional use has centered on the use of CCDs as the sensitive element of an image camera (generating an electronic image of the field of view). The time interval between restoring the CCD array and sampling the accumulated charge is sometimes referred to as the integration time interval. In typical image formats, the integration interval is a result of the vertical frame rate, which is typically 50 or 60 times per second for integration intervals of 20 and 16.7 milliseconds (ms), respectively.

In some image situations, the available light level is very low. Here are some devices that convert radiant energy into electrical energy. Traditional sensors for low light levels are photomultiplier tubes (PMTs) and microchannel plates (MCPs), which provide amplification through secondary emission of electrons. In a PMT, a photosensitive cathode releases electrons when photons impinge on it, ie, photoelectron emission. A positive bias applied to the elements of the PMT, the dynode, attracts the negatively charged electrons emitted by the photocathode, e ⁻ . When electrons strike the dynode electrodes, they cause a secondary release of additional electrons, ie, secondary emission. These additional electrons then hit the dynode, which has a higher positive bias and release additional electrons. This collapse effect continues until the last dynode is involved. This collapse causes amplification of a single photon striking the photocathode. Typically, a 12-stage photomultiplier has a magnification factor of about 17 megabits. A single photon results in the release of about 17 trillion electrons. The pulse has a duration of about 5 nanoseconds resulting in a peak current flow of about 1 milliampere.

Microchannel plates amplify single photon events in a similar fashion. Micro-diameter holes are constructed in the plates, which are then coated with a conductive material. The effect is similar to a PMT. Electrons enter the hole and expel other electrons, which in turn expel more electrons. The overall result is that a single electron initiates electron collapse and signal amplification occurs. When used alone or in cascade, a signal gain of 10 ⁴ to 10 ⁷ can be obtained with a time resolution of approximately 100 picoseconds. Spatial resolution is limited by the channel spacing, typically 9 μm diameter channels on 10 μm centers. The resolution of this design is about 16 line pairs/mm. The inside of the glass envelope is coated with an anode of the MCP and the anode is fabricated with a photoemissive material. When accelerated photons strike the coating, a flash of light is emitted.

In an MCP CCD array, a CCD element is coupled to the anode terminal of the MCP and light flashes are directed to pixels in the CCD array. A signal as small as a single photon can lead to a detectable event. In some designs, two MCPs are cascaded to more than double the sensitivity of the CCD system. The CCD array is constructed with parallel conduction strips in one direction and the P-type channels stop at the correct angle. Electron hole pairs are created when light is incident on the P ⁺ type silicon gate. The charge representing the pixel signal of the picture is stored in the potential well under the biased lossy electrode, ie, the picture element (pixel) in the array. Charge is transferred by applying a positive pulse to adjacent electrodes. The entire image is transferred to a storage raster. Each horizontal line is sequentially read from the storage raster to provide an output signal. Signals can also be read out from individual pixels, and this may be desirable in certain circumstances.

The rate at which data is read from a CCD array has significant ramification. If the CCD is not read for a long time and the incident light is very low, the stored charge will grow over time and a usable image can be obtained.

While it is possible to construct a PMT array to emulate the detection capabilities of an enhanced CCD, there are physical reasons that make this approach insufficient. An advantage of an enhanced CCD is the small size required for the detector area. A large number of pixels can fit in an area of less than one square centimeter. If a bundle of fiber optic cables is in front of the MCP input surface and a suitable lens is used to focus the light emitted by the fiber optic cables, a large number of individual fiber optic cables can be monitored with as little as a single pixel for each fiber in the array. Because there are several hundred pixels here—512x512 pixels are unusual—a significant number of individual fiber optic cables can be coupled to a single detector array.

Thus, the present invention describes an apparatus and method for using a single CCD array as a detector for a single photoelectric event seen through a single input fiber optic cable or for significantly more than one photoelectric event that can be monitored by the array.

Contents of the invention

It is an object of the present invention to use a CCD or an image intensifying cooled CCD array as a detector for photoelectric events occurring in the visible and near infrared part of the spectrum.

Another object of the present invention is to use a lens to couple light formed from a bundle of fiber optic cables onto the input surface of an image intensifier device located in front of a cooled CCD array.

Another object of the invention is to use a single image intensifying cooled CCD array as one detector for one or more detectors for each event.

Another object of the present invention is to use a single image intensifying cooled CCD array as an integrating detector so that temporally separated photoelectric events can be obtained by a single detector array.

In general, the present invention provides a scanner assembly for a laser imaging device, comprising a plurality of collimators arranged in an arcuate manner around an opening in which an object to be scanned is placed; fiber optic cables, each associated with a respective collimator; and a CCD array disposed at the other end of the fiber optic cable to detect light acquired by the collimator and transmitted through the fiber optic cable.

These and other objects of the invention will become apparent from the following detailed description.

Description of drawings

Fig. 1 is a schematic perspective view of a detector assembly according to the present invention, showing a CCD array with N*N pixel matrix, a focusing lens, an optical filter, a manifold for supporting fiber optic bundles and a bundle of fiber optic cables.

Fig. 2 is a schematic perspective view showing a portion of a CCD array illustrated by light rays.

Figure 3 is a schematic block diagram of a CCD camera including an electronic circuit for controlling and reading the CCD array.

Figure 4 is a schematic diagram of a data acquisition system that uses one or more fiber optic cables to couple light collected by individual collimators positioned as an array around the chest in an optical scanning application.

Fig. 5 is the configuration of the terminals of the fiber optic cables at the CCD array.

best practice

In FIG. 1 is disclosed an assembly R which uses a CCD array 2 as a photodetector in a computed tomography laser chest radiography apparatus. Light collected from the scanning cavity is emitted through a plurality of fiber optic cables 4 joined together by appropriate branch tubes 6 to position the fiber optic cables 4 in front of the CCD array 2 in a specific pattern. The branch tube 6 is a support comprising a matrix of holes 7 each suitable for receiving and supporting one fiber optic cable 4 therein. Light rays 9 emitted from the terminals of the fiber optic cable 4 are directed to the CCD array 2 . An optical filter 8 can be used to selectively tune the wavelengths emitted by the fibers (before they impinge on the CCD array 2). A single filter 8 can be used for an entire fiber optic cable bundle instead of one filter per fiber. Filter 8 may be a bandpass filter passing only the desired wavelength range. The filter 8 may also be a cut-off filter which transmits only the desired wavelength range which is larger or smaller than the cut-off wavelength. Filter 8 is used in fluorescence imaging as disclosed in US Patent No. 5,952,664. The lens 10 can be used to focus the light from the optical fiber on the CCD array 2 . The lens 10 can be eliminated when the fiber optic bundle is placed in close proximity to or in contact with a microchannel plate (MCP) 12 used in conjunction with the CCD array 2 .

The CCD array 2 includes individual cells or pixels 14 arranged in an array (either a linear array or an X-Y matrix). Referring to Figure 2, the rays from fiber 4 represented by trace 13 represent four pixels. The number of pixels illustrated can be controlled through the use of lens 10 . Changing the lens 10 can reduce or expand the number of pixels illustrated. The choice of lens 10 takes into account an area as small as a few pixels, or as large as the area covered by all pixels to illustrate. Each element, or pixel, of the CCD array acts as an individual sensor, responding to light emitted from the fiber optic cable. The intensity of light seen by the sensor is read out and used to reconstruct an image of the scanned object.

A standard CCD camera 15 used in the present invention is disclosed in FIG. 3 . Incident light 16 emitted by fiber optic cable 4 is directed onto optical lens 18 , which distributes the incident light to CCD array 2 . The shutter 20 can be used to control the light radiated on the CCD array 2 . Window seal 22 provides a light tight enclosure. Camera control logic and power supply 24 are used to control CCD array 2 . A DC voltage 26 , a serial clock drive signal 28 and a parallel clock drive signal 30 are connected to the CCD array 2 . A low noise preamplifier 32 is used to amplify the CCD pixel signal. Module 34 provides an analog processing and analog-to-digital conversion. The pixel output data 36 is coupled to a computer interface circuit 38, which is a high precision image frame grabber such as the Pico Star High Resolution-Product Imager from the company La Vision Gmbh in Goettingen, Germany. Camera control logic and power supply 24 provides a camera status signal 40 . Computer interface circuitry 38 provides camera commands 42 . Circuitry 38 provides an interface between computer 44 and CCD array 2 . A computer 44 is used to manipulate the information to produce images of the chest seen by the CCD array. Computer 44 can also extract information from uniquely selected pixels within the CCD array. The CCD array 2 is cooled by a thermoelectric cooler 46 or other suitable cooler. A Charge Enhanced Device (CID) or Ceramic Metal Oxide Semiconductor (CMOS) array, cooled or intensified, may be used in place of the CCD array.

Referring to FIG. 4 , in a typical chest scanning application, the chest (not shown) is to be suspended within the opening 50 . The laser scanner 52 includes a laser beam source 54 for striking the chest. The light exiting the chest is picked up by a collimator directed to the chest area from which the light exits. The collimator is arranged in an arcuate fashion around the chest and is supported by a structure 57 that rotates 360 degrees about a centrally-centred axis of the opening 50 . The fiber optic cables are of equal length and provide enough slack to allow the structure 57 to rotate. Each collimator is coupled to a fiber optic cable. The scanner comprising collimator 56 and laser beam 54 is rotated as a unit around the chest through equal angular displacements until a full circle has been traversed. At each angular position, the collimator captures any light emerging from the chest and couples the light to the respective fiber optic cable. Groupings of fiber optic cables 4 are held by branch tubes 6 . The fiber optic cable 4 guides the light to the microchannel plate 12 of the CCD array 2, forming the input of the MCP CCD camera 15. The computer 44 processes the light detected by the camera to produce an image of the breast.

The fiber optic cable 4 is arranged in the branch tube 6 so that adjacent fiber optic terminals at the collimator are not adjacent to other terminals at the CCD array. This configuration is designed to provide minimal crosstalk between fibers. In a laser imaging setup using 84 detectors, each associated with a respective collimator and fiber optic cable, where collimator 56 is designated detector 1 and collimator 60 is detector 84, an example is : The configuration of the terminals of the fiber optic cables at the CCD array is disclosed in FIG. 5 . Note that adjacent fibers at the collimator terminal are not adjacent at branch tube 6.

The use of collimators in laser imaging devices is described in Application No. 08/963,760, filed November 4, 1997, hereby incorporated by reference. The use of fiber optic cables to transmit light captured by the collimator to remote detectors is described in application number 09/199,440 filed November 25, 1998, thus incorporated by reference, an example of a laser imaging machine is disclosed in the U.S. Patent 5,692,511 and Application 09/199,440, hereby incorporated by reference. Methods for reconstructing images from data provided by an array of detectors are described in application 08/979,624, filed November 28, 1997, hereby incorporated by reference. The determination of the circumference of the scanned chest for use in image reconstruction is described in the 08/965,148 and 06/965,149 applications, hereby incorporated by reference.

Although the invention has been described as having a preferred design, it is understood that, in general, the principles of the invention can be further modified, used and/or adapted, and include those known or conventional in the art to which the invention pertains. Deviations from practice, as they arise, may also be applied to the stated essential characteristics and are not within the scope of the invention or the limitations of the appended claims.

Claims (20)

1. A scanner assembly for a laser imaging device, comprising: a) a plurality of collimators arranged in an arc around an opening in which the object to be scanned is arranged; b) a plurality of fiber optic cables, each having a terminal associated with a respective collimator; c) an optically tight envelope; and d) A CCD array arranged within said housing and on the other terminal of said fiber optic cable for detecting light picked up by said collimator and transmitted through said fiber optic cable. 2. The scanner assembly of claim 1, wherein: a) The CCD array includes a microchannel plate. 3. The scanner assembly of claim 1, further comprising: a) A bracket for positioning said further terminal of said fiber optic cable in front of said CCD array in a pattern. 4. The scanner assembly of claim 3, wherein: a) The support is a branch tube comprising a matrix of holes to accommodate the respective fiber optic cables therein. 5. The scanner assembly of claim 3, wherein: a) Arranging said further terminal of said fiber optic cable in such a way that adjacent terminals at said collimator are not adjacent at said bracket. 6. The scanner assembly of claim 1, further comprising: a) a lens arranged between said CCD array and said further terminal of said fiber optic cable to focus light from said further terminal of said fiber optic cable onto said CCD array. 7. The scanner assembly of claim 1, further comprising; a) a lens disposed between said CCD array and said further terminal of said fiber optic cable to focus light from said further terminal of said fiber optic cable onto a selected portion of said CCD array. 8. The scanner assembly of claim 1, further comprising: a) A filter arranged between said CCD array and said further terminal of said fiber optic cable. 9. The scanner assembly of claim 8, wherein: a) The filter is a bandpass filter. 10. The scanner assembly of claim 8, wherein: a) The filter is a cutoff filter. 11. A scanner assembly for a laser imaging device, comprising: a) a plurality of collimators arranged in an arc around an opening in which the object to be scanned is arranged; b) a plurality of fiber optic cables, each associated with a respective collimator; and c) A CCD camera operatively arranged at the further terminal of said fiber optic cable to detect light picked up by said collimator and transmitted through said fiber optic cable. 12. The scanner assembly of claim 11, further comprising: a) A bracket for positioning said further terminal of said fiber optic cable in a pattern in the lens of said CCD camera. 13. The scanner assembly of claim 12, wherein: a) The support is a branch tube comprising a matrix of holes to accommodate the respective fiber optic cables therein. 14. A scanner assembly for optical imaging comprising: a) a plurality of collimators arranged in an arc around an opening in which the object to be scanned is arranged; b) a plurality of fiber optic cables, each having one end associated with a respective collimator; c) a bracket for securing opposite terminals of said fiber optic cable in a pattern, and d) A CCD camera operatively arranged at the further terminal of said fiber optic cable to detect light picked up by said collimator and transmitted through said fiber optic cable. 15. The scanner assembly of claim 14, wherein: a) Arranging said further terminal of said fiber optic cable in such a way that adjacent terminals at said collimator are not adjacent at said bracket. 16. A method for collecting data for use in image reconstruction of an object scanned with a laser beam, comprising: a) providing a laser beam directed at the object being scanned; b) providing a plurality of collimators arranged in an arc around an object to be scanned in order to collect the light radiated from the object; c) providing a plurality of fiber optic cables, each having one end associated with a respective collimator; d) Bundling the opposite terminals of the fiber optic cables together; e) providing a CCD array to generate an image of the bundled terminal of the fiber optic cable; and f) reconstructing an image of the object using the image on the CCD array; 17. The method of claim 16, wherein: a) The bundling is performed in such a way that adjacent fiber optic cables at the collimator are not adjacent at the other terminal. 18. The method of claim 16, wherein: The bundling is achieved using a branch tube to secure further terminals of the fiber optic cables in a pattern. 19. The method of claim 16, wherein: a) Fiber optic cables are of equal length. 20. The method of claim 16, wherein: a) The CCD array includes a portion of the CCD camera.

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