BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to radiographic apparatus and more particularly to apparatus for removing x-ray scatter during radiographic imaging.

2. Description of the Prior Art

When one exposes an object, such as a patient, to x-rays during a diagnostic imaging procedure, the x-ray tube assembly acts like a point source whereupon x-rays are emitted in a straight line. They then pass through the patient and impinge on a detector, which in the case of fluoroscopy, comprises electronic imaging apparatus such as an image intensifier coupled to a television camera. The x-rays that pass through the patient in a straight line are known as primary x-rays and are those desired to be detected. Other x-rays, known as scatter, in passing through the patient emerge at widely different divergent angles and are normally suppressed insofar as possible. There are a number of existing techniques for discriminating between primary and scattered radiation. The most common means is to utilize an anti-scatter grid which is placed behind or under the patient and in front of the detector. The anti-scatter grid is comprised of, for example, a series of lead or tantalum or other heavy metal strips that are laminated to form a plurality of parallel septa which permit x-rays to pass through if they are more or less parallel to the septa but x-rays which pass through at divergent angles have to pass through the lead or heavy metal and are thus attenuated. Disadvantages of an anti-scatter grid are several; first, it is far from perfect and still permits some scattered radiation to reach the detector. Second, a portion of the primary radiation also is removed, meaning that the total amount of radiation delivered to the body undergoing a radiographic procedure must be increased to compensate for that lost in the anti-scatter grid, meaning that whenever the primary radiation is attenuated or scattered radiation is detected, it is necessary to increase the x-ray dose to the body in order to obtain the same statistical information out of the image. Thus a trade off has normally existed between the loss of primary radiation, the x-ray dose and the removal of scatter.

Another known method that has been used to remove x-ray scatter employs a pair of thin slits, constructed of strongly x-ray attenuating material. One slit (foreslit) is placed between the x-ray source (tube) and the object (patient) being imaged. The second, a larger slit (aftslit) is placed behind or beneath the object (patient) and ahead of the detector. The slits are precisely aligned so that x-rays that pass in a straight line from the source and through the foreslit, also will pass through the aftslit if the x-rays are not scattered within the object (patient). X-rays that are scattered within the patient are most likely to strike the x-ray absorbing material contained within the aftslit. An image of an extended object is made by scanning the x-ray source, foreslit and aftslit in synchronism. Additionally, a plurality of paired slits have also been used. This method of scanning paired or multiple paired slits is intrinsically more efficient in rejecting scattered radiation than the use of an anti-scatter grid, but also is more complicated. A significant disadvantage of this method is that due to the relatively great weight and bulk of the aftslit, scanning and alignment of the paired slit assemblies present a substantial mechanical problem. Additional space is also required between the patient and the image receptor for the aftslit assembly, which causes undesirable magnification of the image and, usually, a specially constructed or modified patient support table is needed.

Accordingly, it is an object of the present invention to provide an improvement in apparatus for making a radiographic image.

It is another object of the invention to provide an improvement in the ratio of the primary radiation to the scatter radiation in an image generated during a diagnostic imaging procedure.

It is a further object of the invention to remove the anti-scatter grid assembly normally located between an object receiving x-ray radiation and an x-ray detector.

It is still another object of the invention to electronically collimate x-ray radiation for removing scatter during a radiographic imaging procedure.

SUMMARY

Briefly, the foregoing and other objects are achieved in accordance with electronic slit collimation apparatus comprising, among other things, means for providing a scanned narrow slit or multiple slits or apertures of other shapes between an x-ray source and object, such as a patient, undergoing a diagnostic imaging procedure. Additionally, means, including a detector, are located on the other side of the object for electronically rejecting x-ray image information that falls outside of the slit while retaining that inside of the slit. The detector, for example, includes an image intensifier tube coupled to a TV imaging system which produces a frame every 1/30th of a second as the slit moves across the object. Each frame including an image of the slit is digitized and fed to a recursive loop including a digital memory capable of storing video information of each pixel signal of an array of pixels in each frame. Successive frames are added together with the video information within each frame that lies within the shadow of the slit being accepted while rejecting all other video information outside of the slit. Since the video signal within the slit is much more intense than that outside the slit, discrimination between intensity levels is achieved electronically to elminate the scatter and thus obviate the need for the bulky aftslit assembly or the anti-scatter grid.

BRIEF DESCRIPTION OF THE DRAWING

While the present invention is defined in the claims annexed to and forming a part of this specification, a better understanding can be had by reference to the following description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a simplified diagram partially illustrative of the preferred embodiment of the invention;

FIG. 2 is a depiction of the analog video signal generated by the apparatus shown in FIG. 1;

FIG. 3 is a set of TV frames illustrative of the slit being scanned by the apparatus shown in FIG. 1; and

FIG. 4 is an electrical block diagram further illustrative of the preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and more particularly to FIG. 1, reference numeral 10 denotes a source of radiation such as an x-ray tube assembly which acts as a point source whereby x-rays 12 are emitted in a straight line toward an object 13, such as a patient undergoing a diagnostic imaging procedure. Above the patient is located a relatively flat plate 14 comprised of x-ray absorbent material such as lead, for example, and which includes a generally rectangular slit 16 which provides an aperture through which x-rays 12 pass to the patient 13. Further as shown in FIG. 1, the slit 16 is transverse to the patient's head with the plate 14 being moved in a scanning motion longitudinally from left to right.

Accordingly, x-rays 12 from the source 10 enter the object or patient 13 from the slit 16 in a relatively narrow band or fan beam 18 defined by the dimensions of the slit 16 and the relative spacing of the plate 14 between the patient and the x-ray tube 10. Typically, the beam 18 would be 20 centimeters wide by 1.0 centimeters across. The x-rays emerging from the patient's head are detected, for example, by an x-ray image intensifier 20 with the primary x-rays 21 impinging thereon to form a shadow image 22 of the slit 16 as well as scatter x-rays which are directed at widely divergent angles as shown by reference numeral 24. The image output of the image intensifier 20 is coupled to a video generator 26, such as a TV camera, by means of an optical system 28 and operates to successively form a new TV image or frame every 1/30 of a second. The video signal output of the TV camera 26 comprises an analog signal which is shown in FIG. 2 by reference numeral 30 and would appear, for example, at an output terminal 32. The analog video signal 30 of one line of a frame as shown includes a relatively large amplitude portion which comprises the video signal of the slit 16 while the signal due to scattered x-ray and ambient light comprises relatively low amplitude portions 34 and 36 on either side of the slit portion 32. It can further be seen in FIG. 3 that successive frames 1 through 7 of the analog video signal 30 will provide an image 22 of the slit during translation of the collimator member 14 from left to right as shown in the frames 38₁, 38₂ . . . 38₆, 38₇.

Whereas the prior art relied on one or more mechanical devices for removing the scattered x-rays 24 emerging from the patient 13 during a diagnostic imaging procedure, the present invention, as will now be shown, provides an electronic means for discriminating between video signal levels inside and outside of the image 22 of the slit in order to eliminate unwanted x-ray scatter as well as noise.

Referring now to FIG. 4, each TV frame is comprised of a matrix array of picture elements (pixels) of analog video information 30 appearing at the output terminal 32 of the TV camera 26. Each pixel signal is fed to an analog to digital converter 40 where the brightness level of each pixel of the video frame is digitized. Each digital signal of a pixel comprises, for example, an eight bit digital pulse signal p representative of one of a plurality (typically 256) graduations of luminance level. Thus the A/D converter 40 outputs a serial pulse train of digital signals p_n as each frame of n frames is generated by the TV camera 26.

Letting p_j (x, y) be the digital pixel value, i.e. video signal amplitude, appearing at terminal 32 during the jth frame at the spatial location (x, y) of the image and p'_j (x, y) be the pixel value fed to a memory 50 for storing an image frame following the end of the jth frame, each incoming digital pixel signal comprises the signal p_j which is commonly coupled to time delay means 42 such as a delay line as well as a logic circuit 44. The output of the delay circuit means 42 is fed to a digital multiplier 46 which applies a multiplying factor k₁ to the signal p_j in accordance with the output of the logic circuit 44, as will be explained subsequently, to provide a digital output of k₁ p_j. This signal is next applied to a digital adder 48 whose output comprises the signal P'_j which is coupled to a digital frame store memory 50. The memory 50 has a storage capacity for storing the individual digital video signal values for each pixel of an entire TV frame or raster. Thus for a matrix array of pixels having a dimension of n×n, where n is in the range of 500 to 2000, the memory 50 must include between 0.25 and 4.0 megabytes of storage. Such apparatus is well known to those in the art and is commercially available. A typical example comprises a digital frame store manufactured by Thompson CSF Broadcast, Inc. and identified as a Model FS-963155 digital video frame store.

The frame store memory 50 is part of a recursive loop 51 which in addition to the adder 48 also includes the second digital delay device 52 and a digital multiplier 54 which is operable to apply a multiplier factor k₂, as controlled by the logic circuit 44, to digital output signals p'_j following a delay introduced by the second delay circuit means 52. The two time delay devices 42 and 50 are provided to equalize processing time so that the output k₂ p'_j of the multiplier 54 when fed to the adder 48 is in synchronism with the signal k₁ p_j.

The pixel signals p'_j-1 out of the frame store memory during the jth frame 50 in addition to being fed to the delay circuit 52 and then to the digital multiplier 54, are applied to the logic circuit 44 for determining the values of k₁ and k₂ along with the signals p_j and two threshold signals T and α, which will be considered subsequently. The memory output pixel signals p'_j-1 also comprise the output signal which is reconverted to an analog video signal by a digital to analog converter 56 which is subsequently amplified by amplification circuitry 58 where it is then fed to a display device such as a TV monitor 60.

In operation, incoming pixel signal values p_j of the jth frame are weighted or multiplied by the factor k₁ and added to any previous same location pixel values p'_j-1 previously stored in memory but now weighted by the factor k₂. The combined pixel values k₂ p'_j-1 +k₁ p_j are set into memory as p'_j. The first TV frame, however, which may be, for example frame 38₁ shown in FIG. 1, is digitized and stored in a serial sequence, pixel by pixel, in the frame store memory 50 via the multiplier 46 and the adder 48 by setting k₁ =1 and k₂ =0. Any input to the multiplier 54 is thus deleted because k₂ p'_j-1 will be zero. The memory 50 has its pixel addresses (x, y) synchronized with the pixel addresses of the remaining part of the system including the logic circuit 44 by driving the addresses from a common sync and clock signal, not shown. As new subsequent frame digital pixel values p'.sub. j are read into the memory 50 from the adder 48, the pre-existing values p'_j-1 are simultaneously reapplied through the time delay circuit 52 and the multiplier 54. Thus by initially setting the k₂ multiplier value to zero any pre-existing frame store values are deleted. Accordingly, during the first frame only the p_j pixel elements multiplied by the factor k₁ are set into memory as p'_j. Thereafter, each subsequent frame is added to the preceding frame on a pixel-by-pixel basis depending upon the multiplication factors k₁ and k₂. The contents of the memory 50 are updated to p'_j+1 in each succeeding frame using the data collected during the j^th frame in accordance with the relation:

p'j ≧k1 pj +k2 pj-1        (1)

Although a position dependent logic or a brightness dependent logic may be employed, in the preferred embodiment of the invention shown in FIG. 2, the logic section 44 operates in accordance with brightness dependent logic for setting k₁ ≧0 and k₂ ≦1, where k₁ +k₂ =1, for pixel signals at the spatial locations which lie outside of the shadow 22 of the slit 16 and typically k₁ =1 and k₂ =0 when it is within the shadow of the slit.

The logic circuit 44 is responsive to the incoming pixel data signals p_j, the previously stored pixel data signals p'_j-1, and at least one amplitude threshold parameter T which comprises a pre-selected value setting an absolute minimum threshold for providing signal discrimination between brightness levels in and out of the slit 16. However, to reliably discriminate between inside of the slit versus outside of the slit pixel brightness levels, a second threshold parameter α is preferably utilized such that T is made relatively small and just above system noise levels as shown in FIG. 2 whereas α is set to some value approaching 1.0, typically 0.8. The logic 44 compares the respective pixel values p'_j-1 and p_j to determine whether or not they are changing on a frame-by-frame basis and at any time that p_j is greater than p'_j-1 and particularly,

pj ≧pj-1 +T

then that information will be stored in memory by setting k₁ =1 and k₂ =0 since it constitutes image information at the location of the slit 16. Conversely, if p_j begins to decrease, then it is indicative that the shadow 22 of the slit 16 is past that pixel point and such pixel signals are not to be stored. Accordingly, the values of k₁ and k₂ are changed so that k₁ =0 and k₂ =1.

By weighting the pixel values of p_j and P'_j-1 so that pixel brightness values above the threshold values established by T and α are stored while other values are discarded, discrimination against the signals outside of the shadow of the slit is achieved. As the shadow 22 of the slit 16 moves from left to right on a frame-by-frame basis, the pixel values p'_j-1 at the output of the delay device 52 are coupled to the digital to analog converter 56 where a video display of that frame is generated on the TV monitor 58.

While a brightness dependent logic has been shown in FIG. 2, another way of keeping track of the position of the slit is predicting where the x, y coordinates of the slit actually are and having a knowledge of where the slit should be, the address locations of the memory are programmed to keep the data and adding it to whatever is existing in the memory or the previous value can be discarded, depending upon how much noise can be tolerated into the signal processing and basically depends upon how much the new value exceeds the old value.

If a TV image is not required, the amplifier 58 can be used to drive a laser scanner, for example, and have the image printed out on a piece of film. Still one may take the contents of the video information and generate a computer printout of the image. The present approach, moreover, makes slit scanning practical and adaptable to virtually any fluoroscopy system with an image intensifier TV camera, whereas mechanical systems must be retrofitted to the particular x-ray tables, etc. Also elimination of the slit between the patient and the image receptor minimizes unnecessary air gap and magnification.

Having thus shown and described what is at present considered to be the preferred embodiment of the invention, it should be noted that the foregoing detailed description has been made by way of illustration and not limitation. Accordingly, all modifications, alterations and changes coming within the spirit and scope of the invention as defined in the appended claims are herein meant to be included.