1. Field of the Invention
The present invention relates to a coded aperture imaging system, and more particularly relates to a coded aperture imaging system for generating and displaying an image signal which is representative of an image of a source of non-focusable radiation, such as a gamma ray or an x-ray emitting source. More particularly, the invention relates to a uniformly redundant array coded aperture used in a coded aperture imaging system, and a method of generating the coded aperture.
2. Description of the Prior Art
Nonfocusable radiation sources, such as gamma ray and x-ray emitting sources, are most commonly found in nuclear power plants, medical radiological laboratories, and other nuclear material handling facilities. Thus, routine monitoring of potential highly radioactive areas must be performed during nuclear power plant operation. Environmental restoration and waste management of radiation contaminated sites also require detecting and locating contaminants to assure restoration of the environment to a safe level. Additionally, in order to perform repairs and maintenance within nuclear power plants, the area to be worked in must be scanned by health physics personnel to determine an amount of exposure the radiation workers will encounter while performing their job. Worker radiation exposure is closely monitored via personally worn dosimetry and radiation workers who have reached government specified radiation exposure limits are prevented from further exposure and can no longer perform their jobs for a specified period of time. Thus, it is extremely beneficial, and it is the goal of all nuclear power plant operators, to minimize personal radiation exposure so that radiation exposure limits are not reached.
The health physics personnel responsible for scanning a potentially highly radioactive area often wear protective clothing and a respirator to protect them against airborne radiation. The scanning of suspected high radiation areas with a well known device called a Geiger-Mueller counter is necessary to locate sources of high radiation. Geiger-Mueller counters are relatively small and carried by the worker performing the scanning process. In some instances, the Geiger-Mueller counter is attached to a pole so that the worker can maintain a safe distance from radioactive equipment when scanning. The worker manually scans the entire area to determine "hot spots" or areas of high radiation.
Upon completion of the scanning process, the worker creates a map of the radiation hazards which have been identified by the scanning process using the Geiger-Mueller counter. Appropriate shields are placed around those high radiation sources to protect the workers. Based upon the results of the scanning process, radiation workers performing repairs and maintenance are instructed how long they can remain in the area and what equipment should be avoided due to high levels of radiation being emitted. Furthermore, the scanning process is time consuming because the sweep rate, i.e., the rate at which the Geiger-Mueller counter handpiece is moved over a radioactive source, is relatively slow. In some instances, the worker must get close to the gamma ray emitting source to obtain proper readings since typical Geiger-Mueller counters are not very sensitive.
The two most important factors in limiting radiation exposure are time of exposure and distance from the source of radiation. Presently available scanning techniques using a Geiger-Mueller counter are inefficient in minimizing time of radiation exposure and maximizing distance from the source of radiation. Additionally, the scanning process using a Geiger-Mueller counter provides crude "hot spot" location.
Prior art system designs utilizing the technique of coded aperture imaging have attempted to improve upon the disadvantages of this early scanning technique. Generally, by providing a system which generates a visual representation of the non-focusable radiation source, coded aperture imaging systems allow detection and location of such sources while minimizing, or even eliminating, harmful exposure by personnel. In particular, coded aperture imaging, besides providing the above identified advantage, also yields accurate images of sources of non-focusable radiation. Coded aperture imaging is itself a well known technique involving the use of an aperture composed of transparent and opaque cells. A shadow is cast through these cells on a position sensitive detector in response to the aperture being exposed to gamma or x-ray radiation sources. An image of the source is reconstructed from the information obtained during this exposure process.
However, such prior art coded aperture imaging systems have historically exhibited severe limitations. For example, very often, position sensitive detectors of coded aperture imaging systems are plagued by substantial instrument noise. In addition, the systems often operate in a high background radiation environment, and high energy photons emanating from outside the instrument field of view are detected by the position sensitive detector without first passing through, or being modulated by, the coded aperture. Thus, detected photons not modulated by the coded aperture and inherent non-modulated instrument noise degrade the signal-to-noise ratio of the instrument, resulting in poor image quality and low sensitivity.
Some prior art coded aperture imaging systems utilize an active shield technique to minimize the detrimental effect of non-modulated photons. Under this approach, if the incoming high energy photon flux is low enough to allow the instrument to count individual photon arrival, an active shield and an active coded aperture can be used to provide a means for coincidence/anti-coincidence identification. Thus, non-modulated photons can be individually identified and then excluded, or vetoed, during image reconstruction. However, if the photon flux increases such that many photons arrive at the detector simultaneously, accurate arrival time of each photon cannot be established thereby making separation of non-modulated photons from the desired modulated photons very difficult.
Other prior art coded aperture imaging systems have overcome this limitation by utilizing a normal/complementary coded aperture approach. Under this approach, the radiation source is first modulated by a coded aperture, called a normal aperture, and then by the complement of the coded aperture, called a complementary aperture. The complementary aperture is created by exchanging the location of the cells on the normal aperture. In other words, all cells that were transparent on the normal aperture are opaque on the complementary aperture, and all cells that were opaque on the normal aperture are transparent on the complementary aperture. If each aperture is exposed to the source for an equal time duration and a cancelling process is performed between the images modulated by each aperture, the detrimental effects of non-modulated photons and instrument background noise can be significantly reduced. This result is obtained even in the presence of a high energy photon flux. Coded aperture imaging systems that use this above approach have employed coded apertures formed from either rectangular or hexagonal uniformly redundant arrays (URA).
Rectangular URA coded apertures, such as those disclosed in U.S. Pat. No. 4,209,780 to Fenimore, et al. and U.S. Pat. No. 5,036,546 to Gottesman, et al., have been used for coded aperture imaging. However, use of these types of apertures requires the physical implementation of two separate apertures, one functioning as the normal aperture and the other as the complementary aperture. Systems utilizing rectangular URAs must include means for switching between apertures during the imaging process. This switching process can lead to system inaccuracies, namely, alignment inaccuracy which will negatively affect the image reproduction performance of the system. Also, if the aperture switching is accomplished manually, one of the major benefits that a coded aperture imaging system has over a Geiger-Mueller counter, namely, protection of personnel from radiation exposure, is completely lost. Further, manual aperture switching is time-consuming and detracts from the additional benefit of being able to quickly identify and contain dangerous sources of radiation.
A hexagonal URA was subsequently developed to overcome the need for two separate apertures, as disclosed by M. H. Finger and T. A. Prince, in an article entitled "Hexagonal Uniformly Redundant Arrays for Coded Aperture Imaging", Proceedings of the 19th International Cosmic Ray Conference, OG 9.2-1, pages 295-298 (1985). A single hexagonal URA (HURA) coded aperture, which is nearly anti-symmetric about its hexagonal axis, can function both as a normal aperture and a complementary aperture upon 60.degree. rotation. A HURA coded aperture is congruous for a hexagonal or circular position sensitive detector. However, a severe disadvantage exists in the use of such an aperture with a Cartesian-configured position sensitive detector. Since matching of the hexagonal geometry of the aperture with the Cartesian geometry of the detector requires significant geometric manipulation, considerable data formatting must be performed by the imaging system. Because most state-of-the-art detectors are of a Cartesian configuration, the use of a HURA coded aperture is limited.
Accordingly, there is a need for a portable, reliable, coded aperture imaging system which can automatically map out "hot spots" in an area to be monitored. Furthermore, it would be advantageous to have a device which can be permanently mounted in an area to provide a map of the radiation environment in an area with no radiation exposure to human beings. Still further, there is a need for an imaging system which incorporates a uniformly redundant array coded aperture, having an automatically complementary property, which can easily be interfaced with a readily available Cartesian-configured (rectangular or square) position sensitive detector.