GB2210533A - Highlighting subtle contrast in graphical images - Google Patents

Description

%J 2 2 '10 b --- 15-UL2972 METHOD OF HIGHLIGHTING SUBTLE CONTRAST IN

GRAPHICAL IMAGES The present invention relates in general to controlling an electronic graphical display, and more specifically to a highcontrast color overlay controlled according to a histogram of the graphical image, especially a medical diagnostic image.

The various applications of video displays and of hard-copy presentation of images have increased as computer graphics capabilities have improved. In an interactive graphics system, a processor creates a display file according to instructions from an operator. In one common arrangement, a displayed image consists of a plurality of pixels arranged in columns and rows. The display file consists of a brightness magnitude (and possibly a color) for each pixel. In displaying an image, the operator can specify certain image parameters, such as image brightness andlor contrast.

Adjustment of such parameters may be critical in many applications to best visualize certain aspects of the data in the image. For example, in medical diagnostic imaging modalities, such as ultrasonography. X-ray, nuclear medicine, computed tomography (CT), and nuclear magnetic resonance (RM), an image may contain subtle tissue contrasts which are difficult to visualize. Furthermore, optimum brightness and contrast typically vary according to the objective or specific parameters of an imaging experiment.

Medical diagnostic systems usually employ gray-scale (i.e., monochrome) images, so that the adjustable characteristics of interest in those applications are brightness (i.e., level) and contrast (i.e., 2 window or maxio- deviation from level). Color overlays have been employed to convey information in addition to the main measured quantity of a medical Image, such as in Doppler ultrasound in which a flow direction in indicated by color superimposed over a gray-scale ultrasound echo image.

Nevertheless, low contrast of clinically significant tissue features continue to reduce the effectiveness of many types of quantitative image modalities. This is because spatial localization of tissue areas with nearly identical characteristics can be crucial in detection of pathologies, while the difference in appearance between the pathological area and "normal" areas might not be readily apparent. For example, in ultrasonic backscatter imaging of the heart, signal contrast between normal myocardium and severly ischemic myocardium is typically about 5 dB in the detected backscatter signal. A problem in real-time imaging is that signal contrast changes in real time as the tissue moves. It is desirable to translate this small signal difference and its changes in real time into an image in which the tissue types are more readily distinguishable.

Accordingly, one aspect of the present invention aims to improve the visibility of low-contrast features in graphical images in an interactive manner.

Another aspect of the invention aims to enhance spatial localization of tissue characteristics in medical diagnostic images.

Further aspects aim to provide a method, and apparatus, to highlight tissue contrasts in images that are changing in real time.

According to said one aspect, in a graphical display system image pixels are assigned respective intensities according to an image parameter. Subtle contrast between pixels in highlighted by adding a cue, such as color, to Image pixels having an Intensity within a specified range. The range is specified in an interactive manner using a histogram to assist In identifying variations in the image parameter. A viewer can conveniently designate a window (e.g.. upper and lower bounds) on the histogram to specify the range of pixels for receiving the cue.

In the further aspect of the invention, real-time images are displayed, as are the histograms corresponding to each frame of the real-time image. Real-time variations of the histogram of medical diagnostic images provided clues to a diagnostician leading to tissue characterization via cued pixels falling within the histogram window.

The invention both as to its organization and method of operation, may better be understood by reference to the following illustrative description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a block diagram of a graphics system suitable for practicing the present Invention.

FIGURE 2 is a diagram of a display having a plurality of pixels.

FIGURE 3 is an example of a histogram of an image display file.

FIGURE 4 is a flow chart illustrating one embodiment of the method of the present invention.

FIGURE 5 represents a display apparatus having an ultrasonic sector scan image of a heart and employing the present Invention to enhance visualization of a lesion.

Turning now to FIGURE 1, a graphics system 10 includes data acquisition apparatus 11 and a processor 12. An operator interface 13 is coupled to processor 12 to allow an operator to control the operation of system 10 (such as the level and width of the histogram window). Coupled to the output of processor 12 are a video display 1,4_and a filming device 15. In medical diagnostic applications, data acquisition apparatus 11 may comprise a well-known ultrasound system, MR apparatus or CT scanner. These systems provide information about an object which can be processed to form an image.

Video display 14 is preferably comprised of a cathode-ray tube (CRT) and associated electronics to display an image corresponding to an output signal f rom processor 12. Filming device 15 could be constructed to record images projected by video display 14 or alternatively could be gonstructed to produce hard-copy images.

In a preferred embodiment of the invention. the images displayed by video display 14 and f ilming device 15 are comprised of a plurality of pixels 21 arranged in a matrix 20 having- a plurality of rows and columns, as shown in FIG. 2. In response to data acquisition measurements f rom apparatus 11, processor 12 generates image data corresponding to an Imaging experiment such that each pixel in the image has a magnitude associated with it. For example, In ultrasonography, each pixel can be assigned a magnitude in direct proportion to the backscatter cross section of a respective tissue volume in response to ultrasonic interrogating pulses. In M imaging, each magnitude may represent a nuclei spin density in a particular volume which may be weighted according to a particular relaxation property of the nuclei. In CT, each magnitude may represent the X-ray attenuation of a particular part of a body.

The pixels of video display 14 or filming device 15 could produce an image with each pixel assuming a value directly proportional to these pixel magnitudes. However, better results are obtained When the operator conducting the experiment Is allowed to adjust the characteristics of the displayed image so as to best visualize a structure of interest within the image. But, even allowing for such operator adjustment, tissue features can remain obscured due to low contrast in the original pixel magnitudes.

In reducing the above-mentioned problem, the present invention provides a viewer feedback mechanism allowing the viewer to survey the image and select portions of the image for more detailed contrast data. In operation, the preferred embodiment employs an image ixel histogram, an example of Which is shown In FIG. 3. The histogram In FIG. 3 graphically represents the total number of pixels in the image data for a particular image having each possible value of pixel magnitude (referred to as bins). Thus, for each pixel magnitude, there are a number of pixels with that magnitude, which can be plotted as a line 30. The use of an image pixel histogram facilitates identification of low-contrast tissue populations according to histogram features such as bulge 31. The viewer adjusts the level and width of a window 32 via the operator interface. The window can be placed around a special feature such as bulge 31 or the viewer way alternatively sweep the histogram by sliding the window across the histogram While watching the displayed image for enhanced tissue features. The histogram window is preferably displayed along with the histogram.

As shown in FIG. 4, a preferred method of the invention involves the static or real-time display of the histogram (depending on whether the main image Is static or real time) in step 40. In step 41, the histogram window position and width are set, according to some feature of interest in the histogram or in a systematic search through the image. Again taking the example of an ultrasound backscatter system used for imaging the heart, features in the histogram can appear because (1) a lesion in the image might be characterized by a local pocket of uniform backscatter values tha differ from surrounding tissues, and (2) both ischemic and infarcted myocardium give a stronger backscatter signal than normal myocardium. The present invention further contemplates that the viewer can specify a region of interest in the main image and then the histogram is constructed for only that region so that only local features are seen.

In step 42, a cue is added to all pixels having a magnitude falling within the range specified by the histogram window. In a pref erred embodiment, the cue is provided by a high-contrast color overlay or a large variation in brightness, for example. Thus, localized pockets of an image that exhibit the same characteristic different from a globally defined no= can be easily viewed.

In real-time imaging, the display of the histogram in real time corresponding to each frame of the real-time image is particularly advantageous in helping to identify certain time-dependent tissue characteristics. For example, a cyclical variation in backscatter signal in ultrasonic examination of a normal heart can disappear When examining diseased heart tissue. By placing the histogram window over an unexpectedly static area of the histogram will highlight the corresponding diseased tissue.

Turning now to FIG. 5, a preferred embodiment for an interactive display apparatus 50 comprises a main image display 51, a histogram and window display 52 and a window input device 53. By way of example, main display 51 shows an ultrasound sector scan image of a heart 54. A viewer can Interactively highlight portions of the image by means of the histogram on display 52 and input device 53 Which can be a trackball, for example. by setting the histogram window to a particular portion of the histogram, a lesion 55 in heart 54 Is highlighted by a color overlay. Trackball 53 can also be used to define a region of Interest within image 51 to limit the histogram and cuing to that region.

The foregoing invention improves visibility of low-contrast features in graphical Images. In medical imaging applications, localized regions of tissue are detected in either static or real-time images.

While preferred embodiments of the invention have been shown and described herein, It will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.

j 7

Claims (14)

- 7 CLAIRS

1. A graphical display system comprising: video display means for displaying an image including a plurality of pixels. said image being def ined by assigning respective intensities to said pixels; histogram display means for displaying a histogram of said image While said image is being displayed; window setting means for controlling the width and level of a window to define a range within said histogram; and cuing means coupled to said video display means and said window setting means for adding a cue to each of the pixels in said image corresponding to said range.

2. The display system of Claim 1 wherein said histogram display means further displays the current width and level of said window.

3. The display system of Claim 1 or 2 wherein said image andsaid histogram are time varying according to successively displayed frames.

4. The display system of Claim 1, 2, or 3 wherein said cue is provided by a color overlay superimposed on said cued pixels.

5. The display system of Claim 1, 2, 3 or 4 wherein said window setting means includes a trackball.

6. A method for improving the visibility of low-contrast features in a graphical display system comprising the steps of: displaying an image including a plurality of pixels each having a respective assigned intensity; displaying a histogram corresponding to at least a portion of said image; C setting the position and width of a window to def ine a range within said histogram; and adding a cue to -said image at the locations of each of said pixels corresponding to said range.

7. The method of Claim 6 further coWrising the step of displaying aid window superimposed on said histogram.

8. The method of Claim 6 Wherein said window Is swept across said histogram.

9. - The method of Claim 6, 7 or 8 wherein said image and said histogram are time varying according to successively displayed frames.

10. The method of Claim 6, 7, 8 or 9 wherein said cue includes a color overlay superimposed on said pixels corresponding to said range.

11. The method of Claim 6, 7, 8, 9 or 10 further comprising the step of defining a region of interest within said image and limiting said histogram and said added cue to said region of interest.

12. A graphical display system substantially as hereinbefore described with reference to the accompanying drawings.

13. A medical diagnostic Imaging system comprising a display system according to any one of Claims 1 to Sand 12.

14. A method of improving the visibility of low-contrast features in an image substantially as hereinbefore described with reference to the accompanying drawings Published 19M at The Patent Office, 9;9t'llouse. 68.171 Holborn. London WC1R 4TP. Purther copies may be obtained from The Patent Office, Wes Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Mumpiex techniques itd, st mary Cray, Kent. Con. 1187.