Field of the Invention
The present invention is directed generally to fluoroscopic systems and more particularly to a large field, low intensity, radiation imaging system.
U.S. Pat. No. 4,142,101 to Yin discloses a low intensity x-ray and gamma-ray imaging device. The imaging device includes a phosphor screen for converting an x-ray image into a visible light image. The resulting light is transmitted via an input fiber optic plate to a photocathode where it is converted to electrons. The electrons are accelerated through a vacuum to a microchannel plate image intensifier which multiplies the electrons. The resulting electrons are then converted to light by a second phosphor screen and the light is thereafter directed via an output fiber optic plate to a viewer. The low intensity x-ray and gamma-ray imaging device disclosed in Yin is also described in Yin et al., "The Lixiscope", Nasa Technical Memorandum 79634, September 1978.
The device disclosed in U.S. Pat. No. 4,142,101 has become known as a Lixiscope which stands for low intensity x-ray imaging scope. Lixiscopes are commercially available from HealthMate, Inc., 3000 Dundee Road, Northbrook, Ill. 60062. Lixiscopes provide substantial advantages over traditional x-ray imaging systems Lixiscopes are small, lightweight, and portable. In addition, because of the microchannel plate image intensifier, a very low level source of x-rays or gamma radiation can be used thus reducing the overall cost of the system and decreasing the exposure of the patient. Unfortunately, Lixiscopes are limited to small fields of view, typically about two inches. Thus, Lixiscopes do not compete with large format medical fluoroscopic devices.
Large field x-ray images can be produced by an x-ray image amplifier such as that disclosed in U.S. Pat. No. 2,681,868. Such x-ray image amplifiers allow a large field of view but require sizable x-ray exposure of the patient. Also, the x-ray generator must be sufficiently large to produce the required x-rays which increases the cost of the overall system. Despite the exposure of the patient to a large dosage of x-rays, it is oftentimes necessary to darken the background such that the x-ray image can be properly viewed. Thus, it is desirable to combine the large field of view of traditional fluoroscopic devices with the low radiation exposure, high gain, and low cost of the Lixiscope.
One attempt at combining the large field of view of traditional fluorscopic devices with the advantages of the Lixiscope is disclosed in U.S. patent application Ser. No. 738,616 for a Large Screen Microchannel Plate Radiation Imaging System filed May 28, 1985, and assigned to the same assignee as the present invention. In U.S. patent application Ser. No. 738,616 a method and apparatus are disclosed in which invisible radiation from a radiation source is passed through an object to form a large field invisible radiation image of the object. The invisible radiation image is converted to a visible light image. The visible light image is reduced in size using lenses, mirrors, fiber optic tapers, or some combination thereof. The reduced visible light image is then intensified and the intensified image may be projected for viewing.
When a fiber optic taper is used for reducing the size of the visible light image, optical fibers are typically subjected to an additional manufacturing step which draws the fibers such that the input ends of the optical fibers have a greater diameter than the output ends of the optical fibers. Although this does effect size reduction, the drawing of the optical fibers results in optical fibers having lower numerical apertures i.e. optical fibers which transmit less light. Also, because of radiation scattered by the object being viewed, the image of the object may be blurred. This blurring may be reduced by using a grid. However, the grid, in addition to eliminating the scattered radiation, also prevents some of the desired radiation from being transmitted which may result in degradation of the image and loss of brightness. Finally, if the grid is not oscillated during exposure of the object to the radiation, the grid pattern will be superimposed on the resulting image.
A different approach to providing a large field, low intensity, radiation imaging apparatus centers around the reduction of the electron image corresponding to the radiation image of interest rather than the reduction of the visible light image as set forth in the above-identified patent application. An apparatus directed to the approach of reducing the size of the electron image is disclosed in U.S. patent application Ser. No. 849,907 for a Large Field, Low Intensity, Radiation Imaging System filed Apr. 9, 1986, and assigned to the same assignee as the present invention. The apparatus disclosed in that patent application converts a radiation image to an electron image. Circuitry is provided for reducing the size and increasing the intensity of the electron image. Additional circuitry outputs the increased intensity electron image.
The apparatus of Ser. No. 849,907 may suffer from degradation of the output image due to scattered radiation. Although the scattered radiation may be substantially eliminated by the use of a grid, the grid will also interfere with certain of the radiation which is a portion of the image to be viewed. Thus, the grid will result in signal degradation and grid lines will be superimposed on the image as in U.S. patent application Ser. No. 738,616. Oscillation of the grid washes the grid lines from the image but also degrades the image and results in loss of brightness.
It therefore remains desirable to provide a large field, low intensity, radiation imaging system which takes advantage of the improvements provided by using a grid but which does not suffer from any degradation of output or loss of brightness as a result of using the grid.