In a conventional film radiography system, an x-ray source directed a divergent beam of x-rays through a patient. After passing through the patient, the x-ray beam irradiated a phosphorescent screen as well as a light and x-ray sensitive film positioned adjacent the screen. As the radiation passed through the patient, it was attenuated in accordance with the tissue through which it passed to produce a shadow image on the film. The radiologist examined the gray scale, i.e. the light and dark regions, of the film directly for diagnostic purposes.
In digital radiography, the gray scale of each pixel or incremental region of the image was represented by an electronic, digital value. These digital values were processed by various data processing and image enhancement techniques to improve the diagnostic value of the image. Various techniques have been developed for digitizing film and other shadowgraphic images to derive the electronic digital values.
In one prior art digital radiographic technique, the digital values were derived directly from photographic film using a photodensiometer, video camera, or the like. In another prior art technique, a phosphor with a long term memory temporarily recorded the shadowgraphic image. A photo-sensitive apparatus converted the phosphor luminescence into corresponding digital values on a pixel by pixel basis.
The intensity of x-ray beams traversing the patient have also been converted directly into electronic signals. An array of electronic radiation detectors was disposed opposite the patient to receive the x-rays passing therethrough. In one technique, often denoted as "scan or slit projection radiography", a thin fan beam of radiation passed through a narrow plane of the patient and impacted a linear array of x-ray detectors. The radiation fan and the detector array were moved transverse to the fan beam plane to scan a selected rectangular region of the patient.
In another direct detection technique, an "area" beam of radiation was directed through the entire rectangular region of interest simultaneously to impact upon a large rectangular detector array. The intensities detected by the detectors were digitized for appropriate data processing and display on a video monitor or the like.
The material or body tissue can be characterized by the difference in its attenuation of a high energy and a low energy x-ray beam. To improve the diagnostic value of radiographic images, images of the region of interest have been constructed from both high and low energy radiation. In KV switching, dual energy radiography, two time sequential images were taken, one with higher energy x-rays and the other with lower energy x-rays. In dual detector scanners, both images were taken simultaneously with back to back detectors, one of which was sensitive primarily to the lower energy x-rays and the other was sensitive primarily to the higher energy x-rays.
In the prior art dual energy digital radiography, both high energy and low energy electronic image representations were derived. Each representation commonly included a rectangular array of pixels, each pixel having a pixel value indicative of the degree of radiation attenuation or transmissivity through a corresponding path of the region of interest. The pixel value variations were commonly represented by gray scale variations in man-readable images. Utilizing the transform functions set forth in "Generalized Image Combinations in Dual KVP Digital Radiography," L. A. Lehmann, et al., Medical Physics, Vol. 8, No. 5, pages 659-667, September/October 1981, the high and low energy specific images were transformed into one or more material specific basis image representations. The Lehmann, et al. transform operated on the pixels of the high and low energy images that correspond to the same volumetric subregion of the region of interest to produce a corresponding pixel value of the material specific image. Most commonly, two basis images were generated--one for water or soft tissue and the other for bone or calcium. For calibration purposes, plexiglass was utilized to approximate water and soft tissue and aluminum was utilized to approximate bone and calcium. Other transformations, such as pixel by pixel subtraction of weighted high and low energy images, have also been utilized.
One of the problems with the Lehmann, et al. transformation was that it amplified the noise of the images, i.e. the basis images had a lower signal-to-noise ratio than either the high or the low energy images. The random noise degradation was particularly bad in the high x-ray attenuation or thick portions of the spine which were represented by relatively low pixel values.
Various techniques have been developed for operating on the soft tissue and bone specific basis images to reduce the effect of random noise. One technique was to convolve each pixel of the basis image with a smoothing or filter function that averaged each pixel value with a percentage of the average value of the surrounding pixels. One drawback of this technique was that it removed the apparent random noise in the dense bone regions of the image by blurring the entire image.
To avoid blurring all regions of the basis image, others have provided a region specific filter function. Random noise commonly appears as a speck of a different intensity from the surrounding areas, i.e. a pixel value that is much larger or smaller than the values of adjacent pixels. Specifically, the deviation between a pixel value and each of its surrounding neighbors was determined. If greater than a preselected deviation was observed, the blurring filter function was applied to that pixel of the basis image. If less than the preselected deviation was observed, no filtering was applied to that pixel.
One disadvantage of this selective filtering technique is that the deviation between neighboring pixel values was not always an indication of noise. Rather, the deviation may have been the result of a tissue or bone boundary, a thin blood vessel, or the like. This led to an averaging which tended to blur edges and obscure fine details of the filtered basis image. Other filtering and data processing techniques, such as edge enhancement techniques, other types of smoothing or filtering, and the like have also been performed on the resultant basis images.
In accordance with the present invention, there is provided a new and improved image enhancement method and apparatus which overcomes the above referenced problems and others.