X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
2001-11-07
2004-07-27
McCall, Eric S. (Department: 2855)
X-ray or gamma ray systems or devices
Specific application
Absorption
Reexamination Certificate
active
06768784
ABSTRACT:
BACKGROUND
The present invention relates to the medical imaging arts. It finds application in conjunction with x-ray systems and will be described with particular reference to fluoroscopic imaging systems. However, it should be appreciated that the present invention may also find application in conjunction with other types of imaging systems and applications.
X-ray systems are often used to perform medical imaging and examinations. Examples of x-ray systems suitable for such applications include analog and digital fluoroscopy, spot imaging, and planar tomography. In such systems, an x-ray source is disposed on one side of a patient and an x-ray detector is disposed on the other side of the patient. The x-ray detector converts x-rays which have passed through the patient into secondary carriers (e.g. visible light) which are subsequently converted to signals for image processing.
With particular reference to fluoroscopic imaging systems, these systems can produce substantially instantaneous and continuous (i.e. real-time) images that are usefwil for guiding a procedure, searching through a body section, or observing a dynamic function.
Analog fluoroscopic x-ray systems typically include an x-ray source, an image aintensifier tube, an optical image distribution system, and a video system containing a camera, monitor, and associated electronics. The x-ray source directs a continuous beam of radiation through a subject under examination. A pattern of radiation emerges from the subject and passes through the image intensifier tube which converts the radiation into a pattern of light representing an image of the subject. The pattern of light then passes through the distribution system, is directed to the video camera, and is subsequently shown on the monitor.
Digital fluoroscopic systems typically include a flat panel image detector which includes a scintillator layer and an addressable silicon detector array. Each of the elements in the array convert the light detected by it into an electrical charge. This charge is converted into a corresponding digital signal for further processing and storage. One such image detector is disclosed in U.S. Pat. No. 5,117,114. Other digital fluoroscopic image detectors do not include a scintillator layer, the detectors convert incident radiation into an electrical charge using a selenium photoconductor layer on top of a microcapacitor matrix. Such detectors are disclosed, for example in U.S. Pat. Nos. 5,331,179 and 5,319,206. Still other image detectors are known in the art and are readily available.
Regardless of the type of imaging system, in order for an x-ray system to produce images, the x-ray source must produce x-ray radiation at a level such that at least a portion of the x-rays pass through the subject and are received by the x-ray detector. As the x-rays enter the subject, they are attenuated by the tissues of the subject. The more dense the tissue, the greater the attenuation of the x-rays. The x-rays are attenuated during this penetrating process as a function of the density of the tissue through which they pass by the process of absorption of energy by the tissues. As the x-rays emerge from the other side of the subject, the energy that is still retained is, therefore, a function of the density of the tissues that were penetrated as well as the initial energy of the x-rays generated by the x-ray source. Accordingly, increasing the energy of the x-ray source increases the ability of the x-rays to penetrate the subject and subsequently produce x-ray images. In cases where the quality of the images is poor due to excessive x-ray penetration, the energy can be reduced to a desired level.
One difficulty associated with producing x-ray images is that x-rays incident on the x-ray detector are either of an intensity level which is too high or too low, thereby resulting in reduced image quality. Therefore, during x-ray imaging, an intensity level of x-rays reaching the x-ray detector can be monitored to assure that the overall intensity of x-rays received by the x-ray detector is satisfactory to produce an image of diagnostic quality. The intensity of the x-rays received by the x-ray detector can be measured in terms of exposure. If it is determined that the x-ray detector is not receiving satisfactory exposure to x-rays, a signal can be sent to the x-ray source to adjust either one, or both, of the number of x-rays and the energy of the x-rays produced by the x-ray source.
In addition, despite the control of the x-ray source, it is possible that desired quality of the images cannot be achieved. This may be due to inadequate brightness or contrast within a region of interest. More specifically, image quality is substantially based on the ability to see contrast between certain types of anatomy. Typically, data from an x-ray detector are mapped, pixel by pixel, to a display scale consisting of, for example, a gray scale of 256 steps from black to white. Detected x-rays which are mapped into a central portion of the 256 steps will typically provide sufficient contrast to readily distinguish among different anatomy within a region of interest whereas pixels mapped within a small range of the gray scale or near the upper or lower bounds of the gray scale will typically fail to exhibit enough image contrast to properly distinguish and interpret such portions of the image. This can occur when the x-ray detector pixels cause there to be a wide range of x-ray data mapped in most of the gray scale steps thereby causing the range of pixels holding the image data of the region of interest to be mapped to a region having lower overall gray scale contrast.
Manipulating the x-ray intensity and processing the images to achieve high quality images can be difficult, however. For example, it is difficult to know whether, and to what extent, to control the radiation energy, radiation dose, and image processing parameters. Even if the user is skilled, this process can be time consuming and difficult. It is therefore desirable to automate such controls.
U.S. Pat. No. 5,574,764 obtains images by adjusting the x-ray intensity and image display brightness level. This is accomplished by feeding back an average pixel brightness value within a region of interest to the x-ray source to control the x-ray exposure. The average pixel value is also used to determine an image processing scaling factor to maintain a desired brightness level in situations where the necessary x-ray exposure cannot be attained.
The utility of this approach is limited where a region of interest within the field of view of the x-ray system is too dark or bright and adjusting the x-ray source controls and the image processing parameters is insufficient to adequately display the region of interest. In U.S. Pat. No. 5,574,764, the region of interest is fixed in both location and size and cannot be changed interactively by the user during image acquisition. This is problematic when, for example, the portion of a subject's anatomy that the user wishes to view falls outside the initial region of interest or is too small with respect to the size of the region of interest to distinguish.
It is therefore desirable to have automated x-ray source and image processing controls that respond to the image characteristics of an operator-defined region of interest. It is also accordingly desirable that the operator can select such a region of interest interactively during real-time imaging.
SUMMARY
Embodiments of the present invention address these matters, and others.
In accordance with one aspect of the present invention, a fluoroscopic imaging apparatus is provided. The imaging apparatus includes an x-ray source for projecting x-rays through a subject, the x-ray source having a voltage and a current associated therewith and an x-ray detector for detecting radiation which has passed through the subject. The apparatus also includes a monitor for displaying an image indicative of the detected radiation and the image defining a field of view. In addition, the apparatus includes an operator interface for selecti
Green Donald T.
Molter Kurtis M.
Roos Pieter Gerhard
Lundin Thomas M.
McCall Eric S.
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