Method and apparatus for digital control of scanning x-ray imaging systems

We claim:

1. X-ray image data generating apparatus having an x-ray source which includes an anode plate, means for directing an electron beam to said plate to produce x-rays at an x-ray origin point on said plate and means for traveling said x-ray origin point in a raster scanning motion within a raster scan area on said plate in response to an x-axis sweep frequency signal and a y-axis sweep frequency signal, said apparatus further having an x-ray detector which produces a detector signal that is indicative of variations of x-ray intensity at a detection point that is spaced apart from said anode plate,

means for producing a first sequence of digital data bytes which encode successive values indicative of variations in the magnitude of said x-sweep frequency signal that are to occur during the course of the raster scanning motion at said raster area,

means for producing a second sequence of digital data bytes which encode successive values indicative of variations in the magnitude of said y-sweep frequency signal that are to occur during the course of said raster scanning motion at said raster area,

means for producing said x-sweep frequency signal and said y-sweep frequency signal during the course of said raster scanning at said raster scan area by conversion of the values encoded by successive data bytes of said first and second sequences thereof into analog signals wherein the improvement comprises:

digital data processing means for producing any of a plurality of different digital control signals for said x-ray image data generating apparatus including digitized sweep frequency control signals, and

scanning control means for modifying at least one characteristic of at least one of said x-sweep frequency signal and said y-sweep frequency in response to said digitized sweep frequency control signals.

2. The apparatus of claim 1 further including a computer memory, means for producing a third sequence of digital data bytes which encode values indicative of variations of said detector signal and means for storing said first, second and third sequences of digital data bytes in said computer memory.

3. The apparatus of claim 1 wherein said scanning control means enables scan speed control by varying the repetition rate of said data bytes of said said first and second sequences thereof.

4. The apparatus of claim 1 wherein said scanning control means enables resolution control by varying the number of said data bytes in said first and second sequences thereof.

5. The apparatus of claim 1 further including means for producing zoom signal data bytes which encode values indicative of a selected size for said raster scan area, and wherein said scanning control means varies the amplitude ranges of said x-sweep frequency signal and said y-sweep frequency signal in response to changes of said values encoded by said zoom signal data bytes.

6. The apparatus of claim 1 wherein said scanning control means enables variation of the number of data bytes in said first and second sequences thereof whereby the aspect ratio of said raster scan area may be varied.

7. The apparatus of claim 1 further including means for producing a third sequence of data bytes which encode values indicative of variations of said detector signal, means for producing a contrast control signal data byte which encodes a value indicative of a selected degree of contrast which is to be exhibited by said image, means for increasing and decreasing differences between the values encoded by successive ones of said data bytes of said third sequence thereof in response to changes in the value encoded by said contrast control signal data byte.

8. The apparatus of claim 1 wherein said means for directing an electron beam to said anode plate of said x-ray source includes an electron gun having a filament, an electron emissive cathode that is heated by said filament, a control grid and a focusing electrode disposed in spaced apart relationship to generate and control said electron beam and wherein a negative high voltage supply is connected to said cathode, further including:

means for producing electron gun control signals in the form of digital data bytes which encode values for filament current, cathode voltage and control grid voltage and wherein each of said values may be varied, and

electron gun control means for applying current to said filament and voltages to said cathode and control grid that are determined by said values which are encoded by said digital electron gun control signals.

9. The apparatus of claim 1 further including means for producing area of interest digital values which encode the location of a selected portion of said raster scan area and wherein said scanning control means reduces the size of said raster scan area at said anode plate in response to a zoom signal and positions the reduced raster scan area at a location on said anode plate that corresponds to the location that is encoded by said area of interest digital values.

10. The apparatus of claim 1 further including means for producing a sweep frequency voltage error signal in response to an absence of either or both of said x-axis and y-axis sweep frequency signals and means for suppressing application of said high voltage to said anode plate in response to said error signal.

11. The apparatus of claim 1 further including:

an analog to digital signal converter having an input which receives said detector signal and an output which transmits a sequence of digital data bytes encoding values indicative of changes in the magnitude of said detector signal and wherein said converter can produce a range of values that is bounded by a maximum value and a minimum value,

means for detecting the highest and lowest values encoded by said sequence of data bytes during a first raster scan of said anode plate, and

means for adjusting the amplitude range of said detector signal to cause said highest value to produce said maximum value at said converter output during a rescanning of said anode plate and to cause said lowest value to produce said minimum value at said converter output during said rescanning whereby contrast in said image is automatically optimized.

12. A method for producing radiographic image data by x-ray scanning of a subject which includes the steps of:

scanning an electron beam in a first raster pattern on an anode plate to produce a moving x-ray origin point,

generating said radiographic image data by detecting x-rays at a detection point situated at the opposite side of said subject from said anode plate and by producing a detector output signal that vary in accordance with variations of x-ray intensity at said detection point,

selecting an area within said first raster pattern for magnification,

encoding the location of the selected area of said first raster pattern in digital signals and initiating a zoom signal,

reducing the size of said first raster pattern in response to said zoom signal to provide a smaller second raster pattern, and

positioning the smaller second raster pattern at a location on said anode plate that corresponds to the location that is encoded in the digital signals.

13. A method of obtaining radiographic image data by x-ray scanning of a subject which includes the steps of:

producing x-rays at an x-ray origin point on an anode plate of an x-ray tube by directing an electron beam to said anode plate,

traveling said x-ray origin point in a raster scanning motion within a raster scan area on said anode plate by applying an x-axis sweep frequency signal and a y-axis sweep frequency signal to said x-ray tube,

generating said radiographic image data by detecting x-rays at a detection point situated at the opposite side of said subject from said x-ray origin point and producing a detector signal that is indicative of variations of x-ray intensity at said detection point as said x-ray origin point moves to successive locations in said raster scan area,

producing a first sequence of digital data bytes which encode successive values indicative of variations in the magnitude of said x-sweep frequency signal that are to occur during the course of said raster scanning motion at said raster scan area,

producing a second sequence of digital data bytes which encode successive values indicative of variations in the magnitude of said y-sweep frequency signal that are to occur during the course of said raster scanning motion at said raster scan area,

modulating the magnitudes of said x-sweep frequency signal and said y-sweep frequency signal during the course of said raster scanning at said first raster scan area by reference to the values encoded by successive data bytes of said first and second sequences thereof, and

modifying at least one of said x-sweep frequency signal and said y-sweep frequency signal by modifying the values that are encoded by at least one of said first and second sequences of data bytes.

Description Submit all comments and votes

TECHNICAL FIELD

This invention relates to radiography. More particularly the invention relates to scanning x-ray imaging systems in which the subject is situated between an electronic x-ray detector and an x-ray source at which a moving x-ray origin point is swept in a raster pattern and in which the image may be displayed at the screen of a display monitor.

BACKGROUND OF THE INVENTION

Use of photographic film for obtaining x-ray images has several disadvantages. The image is not immediately available because of the need to develop the film. Radiation exposure of the subject is high and exposure time is prolonged as a majority of the x-rays do not react with the film. Fluoroscopic screens enable instant viewing of an image but are otherwise subject to many of the disadvantages of film.

Efforts to resolve the problems associated with older x-ray imaging techniques have included use of an image intensifier and video camera imaging chain to generate a visible image on the screen of a display monitor. This produces a third generation image which tends to be degraded by electronic noise. The first generation image appears on a fluorescent a screen at the input of the image intensifier and the second generation image appears at another fluorescent screen at the output of the intensifier. The third generation image is produced by a video camera that views the image intensifier output. In order to improve image quality, the electronic signal generated by the image intensifier has been digitized to enable computerized image enhancement but this produces only marginal improvement.

In some more recent systems, the image intensifier system is replaced with an array of minute electronic x-ray detectors such as charge coupled devices. Data for constructing the image is read out of the array on a pixel by pixel basis to provide an image which may be displayed at the screen of a video display monitor. Primary disadvantages of these systems include high cost and complexity and an undesirably small field of view.

All of the prior x-ray imaging systems discussed above use what may be termed conventional geometry. That is, the x-rays diverge from a small fixed point and are detected at a large area detector such as the film, screen or detector array. My prior U.S. Pat. No. 3,949,229 issued Apr. 6, 1976 and entitled "X-ray Scanning Method and Apparatus" discloses an advantageous imaging system having a reversed geometry. The system of that prior patent uses an x-ray source having an extensive anode plate which is raster scanned by an electron beam to provide a moving x-ray origin point. X-rays emitted from different successive locations on the large anode plate in the course of a raster scan converge at an electronic detector which has a relatively small x-ray sensitive area. A moving light origin point at the screen of a display monitor undergoes a similar raster scan and is modulated by the detector output signal to provide the x-ray image at an analog X-Y storage cathode ray tube component of the monitor.

The reversed geometry provides a number of advantages. Radiation exposure of the subject may be greatly reduced as the electronic detector responds to incoming x-rays much more efficiently than film or a fluoroscopic screen. Collimators of the type disclosed in my prior U.S. Pat. No. 4,465,540 issued Aug. 14, 1984 and entitled "Method of Manufacture of Laminate Radiation Collimator" may be used to suppress x-rays that are not directed towards the small detector and which are therefore incapable of contributing to the desired image. The system can also be relatively uncomplicated and inexpensive in comparison with other forms of x-ray scanning equipment.

The reverse geometry also enables magnification of an area of the image that is of particular interest without relative movement of the subject, x-ray source and detection means. This is accomplished by reducing the size of the raster pattern at the anode plate of the x-ray source without making a corresponding reduction in the size of the raster pattern at the image display monitor. Conventional geometry systems require repositioning of the subject and/or the source and detector in order to accomplish a similar result. Magnification without such repositioning in a conventional geometry system reduces resolution in the image.

Initiating such magnification in the reverse geometry system of prior U.S. Pat. No. 3,949,229 is a somewhat time consuming and involved operation as a series of different controls must be manually adjusted and operator coordination of the adjustments with each other is necessary. Varying other characteristics of the image and changing operating parameters of the scanning x-ray source also require operator coordination of various manual controls and can be time consuming and somewhat taxing. Analog controls of this kind do not enable a number of highly advantageous modes of operation that will hereinafter be described.

Reducing the size of the raster scan area at the x.-ray source to obtain a magnified image concentrates electron beam heating at a limited area of the anode plate. Avoiding heat damage to the x-ray source requires careful attention by the operator and still more control adjustments.

My prior U.S. Pat. No. 4,259,582, issued Mar. 31, 1981 and entitled "Plural Image Signal System for Scanning x-Ray Apparatus", discloses reverse geometry scanning x-ray apparatus of the above discussed kind which as an option enables digitizing of the detector output and sweep frequency signals and digital storage of data values from which the detector output voltage and the raster scan sweep frequency voltages can be reconstructed in order to reproduce the x-ray image at a later time. The system further enables certain forms of digital processing of the data to change characteristics of the image. This includes magnification of a selected area of the image but does not provide for increased resolution or definition in the magnified region of the image. Control of the x-ray source and scan raster parameters continues to require time consuming adjustments and coordination of various analog voltage controls on the part of the operator.

The present invention is directed to overcoming one or more of the problems discussed above.

SUMMARY OF THE INVENTION

In one aspect, the invention provides x-ray imaging apparatus having an x-ray source which includes an anode plate, means for directing an electron beam to the plate to produce x-rays at an x-ray origin point on the plate, and means for traveling the x-ray origin point in a raster scanning motion within a first raster scan area on the plate in response to an x-axis sweep frequency signal and a y-axis sweep frequency signal. An x-ray detector produces a detector signal that is indicative of variations of x-ray intensity at a detection point that is spaced apart from the anode plate. A monitor has an image display screen and means for moving a visible light origin point in a raster scanning motion within a second raster scan area at the screen. The intensity of the light origin point is modulated during the course of the raster scanning motion at the second raster scan area by the variations of the detector signal which occur during the course of the raster scanning at the first raster scan area. The apparatus further includes means for producing a first sequence of digital data bytes which encode successive values indicative of variations in the magnitude of the x-sweep frequency signal that are to occur during the course of the raster scanning at the first raster area, means for producing a second sequence of digital data bytes which encode successive values indicative of variations in the magnitude of the y-sweep frequency signal that are to occur during the course of the raster scanning at the first raster area, and means for producing the x-sweep frequency signal and the y-sweep frequency signal during the course of the raster scanning at the first raster scan area by conversion of the values encoded by successive data bytes of the first and second sequences into analog signals.

In another aspect, the invention provides X-ray imaging apparatus having an x-ray source which includes an anode plate and means for directing an electron beam to the plate to produce x-rays at an x-ray origin point on the plate and means for traveling the x-ray origin point in a raster scanning motion within a first raster scan area on the plate in response to x and y axis sweep frequency signals. An x-ray detector produces a detector signal indicative of variations of x-ray intensity at a detection point that is spaced apart from the anode plate. A monitor has an image display screen and means for moving a visible light origin point in a raster scanning motion within a second raster scan area at the screen. The intensity of the light origin point is modulated by the variations of the detector signal which occur during the course of the raster scanning motion at the first raster scan area. Means are provided for producing and storing digital signals which encode the location of a selected area of the image in response to area of interest selection controls. Further components include means for reducing the size of the first raster pattern at the anode plate in response to a zoom signal and means for positioning the reduced first raster pattern at a location on the anode that corresponds to the selected location on the image display screen that is encoded by the digital signals.

In another aspect, the invention provides a method for creating a radiographic image of a subject which includes the step of scanning an electron beam in a first raster pattern on an anode plate to produce a moving x-ray origin point. X-rays are detected at a detection point situated at the opposite side of the subject from the anode plate and a detector output voltage is produced in response to the detected x-rays. Further steps include sweeping a light origin point on a display screen in a second raster pattern and varying the intensity of the light origin point at successive points in the second raster pattern in accordance with variations of the detector output voltage at corresponding points in the first raster pattern, selecting an area of the image at the display screen for magnification, encoding the location of the selected area in digital signals and initiating a zoom signal. Still further steps in the method include reducing the size of the first raster pattern in response to the zoom signal and positioning the reduced first raster pattern at a location on the anode plate that corresponds to the location in the image that is encoded in the digital signals.

In still another aspect, the invention provides a method of obtaining a radiographic image of a subject which includes the steps of producing x-rays at an x-ray origin point on an anode plate of an x-ray tube by directing an electron beam to the plate, traveling the x-ray origin point in a raster scanning motion within a first raster scan area on the anode plate by applying an x-axis sweep frequency signal and a y-axis sweep frequency signal to the x-ray tube and detecting x-rays at a detection point situated at the opposite side of said subject from the x-ray origin point. Further steps include producing a detector signal that is indicative of variations of x-ray intensity at the detection point as the x-ray origin point moves to successive locations in the first raster scan area, producing a radiographic image by moving a visible light origin point at a display screen in a raster scanning motion within a second raster scan area at the screen and using the detector signal to produce variations of the intensity of the light origin point at successive locations in the second raster scan area. Still further steps in the method include producing a first sequence of digital data bytes which encode successive values indicative of variations in the magnitude of the x-sweep frequency signal that are to occur during the course of said raster scanning motion at the first raster scan area, producing a second sequence of digital data bytes which encode successive values indicative of variations in the magnitude of the y-sweep frequency signal that are .to occur during the course of the raster scanning motion at the first raster scan area and modulating the magnitudes of the x-sweep frequency signal and the y-sweep frequency signal during the course of the raster scanning at the first raster scan area by reference to the values encoded by successive data bytes of the first and second sequences thereof.

The invention enables faster operation of reversed geometry scanning x-ray systems, simplifies the operator's control manipulations and expands the capabilities of the system with respect to producing images of different types by enabling digital data processor control of the scanning x-ray source and image characteristics. The operator may, for example, zoom in to magnify one or more areas of the image that are of particular interest by simple actuations of one or more standard computer input devices. In the preferred form of the invention, high resolution scanning of the subject can be limited to selected regions which are of interest thereby reducing scanning time and minimizing radiation exposure of the subject. Magnified high definition images of selected regions of a subject can be acquired, stored, digitally enhanced in any of various ways and then be displayed sequentially or simultaneously. In the preferred form, the system can produce an unblurred image of a moving subject by automatically shifting the location of the raster scan at the anode plate of the x-ray source as necessary to track the movement of the subject. The preferred form of the invention also automatically adjusts the voltages and currents that are applied to components of the scanning x-ray source during different modes of operation to avoid overheating of the anode component. In the preferred form, the invention enables variation of the aspect ratio or height to width ratio of the image in response to digital signals to facilitate imaging of differently shaped subjects or, in the case of a moving subject, to compensate for an image distortion which can otherwise result from the motion of the subject.

The invention, together with further aspects and advantages thereof, may be further understood by reference to the following description of the preferred embodiment and by reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is in part a perspective view of a scanning x-ray source and x-ray detector and in part a block diagram showing major components of the preferred embodiment of the invention.

FIG. 2 depicts the screen of a video display monitor and certain other components of the apparatus of FIG. 1 and diagramatically depicts operations which are involved in the process of acquiring magnified high resolution images of regions of a lower resolution image that are of particular interest.

FIG. 3 depicts the face of the x-ray source of FIG. 2 at later stages in the process of acquiring high resolution images of selected areas of interest.

FIG. 4 depicts the display screen of FIG. 2 during simultaneous presentation of a plurality of the high resolution images of areas of interest.

FIG. 5 is a diagram showing how FIGS. 6A, 6B and 6C may be disposed in side by side relationship to form a single continuous circuit diagram.

FIGS. 6A, 6B and 6C are jointly a circuit diagram showing the apparatus of the preceding figures in greater detail.

FIG. 7 is a circuit diagram depicting counter components of the circuit of FIG. 6A in still greater detail.

FIG. 8 is a program flowchart of computer operations which take place during the process of obtaining and storing data for enabling display of magnified, high resolution images of areas of particular interest that have been selected in a wider angle, lower resolution image.

FIG. 9 is a program flowchart of computer operations involved in acquiring digitized image data of areas in an image at which a grey scale transition of selected magnitude occurs and which may be used to produce an unblurred image of a moving object without physical movement of the x-ray source and/or the detector.

FIG. 10 is a program flowchart of computer operations involved in automatically searching an image to locate grey scale transitions of the type that are tracked by the operations shown in FIG. 9.

FIG. 11 is a circuit diagram of a sweep frequency error detection circuit which is depicted in block form in FIG. 6B.

FIG. 12 depicts voltage variations as a function of time that occur at certain points in the circuit of FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring initially to FIG. 1 of the drawings, an x-ray imaging system 11 in accordance with this embodiment of the invention includes a scanning x-ray source or tube 12 and an x-ray detector 13 which components may be similar to those described in my hereinbefore discussed U.S. Pat. No. 3,949,229.

The scanning x-ray source 12 has an electron gun 14, situated in an evacuated envelope 16, which directs an electron beam 17 towards an electrically conductive anode plate 18 that forms the front face of the envelope. Anode plate 18 is grounded and a tube voltage supply circuit 19 applies a high negative voltage to the electron gun 14. The voltage difference accelerates electron beam 17 and the impact of the high energy electrons on anode plate 18 results in emission of x-rays at an x-ray origin point 21 situated at the point of impact of the beam on the plate.

The x-ray origin point 21 is swept in a first raster pattern 22 on anode plate 18 by beam deflection means 23 which receives beam deflection signals from an x-axis sweep frequency generator 24 and a y-axis sweep frequency generator 26. X-axis sweep frequency generator 24 produces a voltage having a sawtooth waveform that exhibits repetitive rises separated by abrupt drops while y-axis generator 26 produces a similar waveform that rises and drops at a lower frequency. Consequently, x-ray origin point 21 scans anode plate 18 along a series of substantially parallel scan lines 27 that jointly define the first raster pattern 22. As will hereinafter be described in more detail, sweep frequency generators 24 and 26 adjust the output voltages as needed to compensate for pincushion distortion and to accommodate to changes of electron beam energy.

The beam deflection means 23 in this embodiment includes a magnetic deflection yoke 28 of the known form although it is also possible to make use of known forms of electrostatic beam deflector.

X-ray detector 13 is spaced apart from the x-ray source 12 and the subject 29 which is to be imaged is situated between the source and detector. The detector 13 may be of one of the known types which has a small, radiation sensitive area 31 and which produces an output signal voltage that varies in accordance with variations of x-ray intensity at the sensitive area. The detector 13 may, for example, be a scintillation detector or small ionization detector although other forms of detector may also be used.

The x-ray image is displayed at the screen 32 of a video display monitor 33 which may be of the known type in which a light origin point 34 at the screen is scanned in a raster pattern 36 in response to x and y axis sweep frequency voltages of the hereinbefore described kind and in which the intensity of the light origin point is modulated in the course of the raster scan in response to a z-axis or intensity signal. The x-ray image may be produced by establishing a raster pattern 36 at monitor 33 that is similar to the raster pattern 22 that occurs at the x-ray source 12 and by modulating the intensity of light origin point 34 in the course of the raster scan in accordance with variations of the detector 13 output voltage that occur in the course of a raster 22 scan at the x-ray source. This produces a radiographic image as the detector output voltage at any given instant is determined by the x-ray absorbency of the region of the subject 29 that lies on a line extending from the momentary position of the x-ray origin point 21 to the relatively small x-ray sensitive area 31 of the detector 13. Thus variations of x-ray absorbency at successive stages of the scan cause corresponding variations in the brightness of the image at display screen 32.

In the original x-ray imaging systems of this general kind, the x and y sweep frequency generators were analog circuits and sweep frequency voltages corresponding to the output of the generators were simultaneously applied to the x-ray source and display monitor to synchronize the raster scans. Voltage variations at the output of the x-ray detector were also processed in analog form and were applied to the z or intensity signal terminal of the display monitor. The present invention greatly facilitates control of the system 11 and enables novel modes of operation by employing digital data processing techniques to control the beam deflection means 23, the electron gun 14 and characteristics of the image at display monitor 33 and by digitizing the detector 13 output signals. A computer central processing unit 38 and standard operator input devices may then be used to initiate different modes of operation of the system 11 and also to automatically adjust operating voltages and currents as needed to accommodate to the different modes of operation. The input devices of this particular embodiment are a keyboard 41 and track ball 42 although other known forms of operator input device may be substituted or used in conjunction with such inputs.

For example, the operator may select one or more particular areas of an initial full sized image at the display monitor screen 32 for rescanning at higher resolution and for presentation as a magnified image. The magnified images, which may have selectable sizes and ratios of height to width, may then be displayed in sequence or simultaneously. Simple input manipulations at keyboard 41 enable the operator to change the size, shape and location of the raster scan pattern 22 at the tube anode plate 18 in order to inspect different regions of the subject 29 or these parameters may be automatically varied by programming of the central processing unit 38. Resolution and scan speed at the x-ray source 12 may also be varied by the operator or in response to programming. The program of this embodiment of the invention also enables operator initiation of standard forms of image processing including colorizing of the image based on different gray scale levels in the image, edge enhancement, field flattening, stretching or compression of the image, image subtraction and histogram equalization.

A detector circuit 43 generates a sequence of serial data bytes which encode values indicative of changes of x-ray intensity at detection point 31 during the course of each raster scan at the tube 12. In one mode of operation the data bytes are transmitted to the buffer storage 44 of a video board 46 through a computer interface 47 and are stored at x-y addresses in the buffer storage that correspond to successive points in the raster scan at tube 12. Alternately, in instances where high precision image processing or high resolution scanning are to be performed, interface 47 first transmits the detector signal data bytes to the memory 48 of the central processing unit 38 and the processed data is then transmitted to buffer storage 44. The video board 46, which may be of known form, sequentially converts the stored x and y addresses and digital detector signal values to analog voltages which are transmitted to monitor 33 to cause the raster scan 36 and radiographic image display.

A magnified, high resolution image of an area 49 of the subject 29 that is of particular interest is produced by reducing the size of the raster pattern 22 at tube 12 as depicted by dashed line 22a and by shifting the location of the reduced raster pattern on anode plate 18, if necessary, to cause x-rays which travel from the reduced raster pattern to detection point 31 to pass through the area of interest 49. As the raster pattern 36 at monitor 33 remains full sized, an enlarged image of the area 49 is produced at screen 32. A relatively dense inclusion 51 in the subject 29 that appears at an off center location at screen 32 during a full sized raster scan at tube 12 appears at a more centered location 51a on the screen in the subsequent magnified image if the reduced raster pattern 22a has been shifted to be centered on the inclusion.

Referring to FIG. 2, central processing unit 38 has a cursor control circuit 52 of the known form which controls the movement of a small visible cursor symbol 53 at display screen 32 in response to the operator's manipulations of the track ball 42 or cursor controls at keyboard 41. Upon inspection of a full size image at screen 32, the operator may select a localized area 49 of interest for magnification by initially traveling cursor 53 to the upper left corner of the area 49. An initial actuation of the track ball switch 54 signals the central processor to store the x and y axis raster address of that corner in memory 48. The operator than moves cursor 53 to the lower right corner of area 49 and a second actuation of track ball switch 54 results in digital storage of the raster address of that corner.

CPU 38 interprets the second actuation of track ball switch 54 as a zoom signal and initiates a rescanning at x-ray source 12 within a reduced raster pattern 22a. Utilizing the stored area of interest raster addresses, CPU 38 determines and initiates changes in the x and y sweep frequency waveforms that are needed to confine the reduced raster pattern 22a to the portion of the original full sized raster pattern that begins at an address corresponding to the first stored raster address and ends at the address which corresponds to the second stored raster address.

Referring jointly to FIGS. 2 and 3, the reduction and relocation of the x-ray tube raster pattern enables production of a magnified, high resolution image at screen 32 in the manner previously described.

To expedite x-ray inspection of subjects that may have a number of areas 49 of particular interest, central processing unit 38 is programmed to store a plurality of sets of tube raster scan addresses which are selected by the operator in the above described manner, three such areas 49, 49a and 49b being depicted in FIG. 2. Actuation of a keyboard key 56 instructs central processing unit 38 to execute the corresponding three reduced and repositioned raster scans 22a, 22b and 22c in sequence and to store the resulting image data as three separate images that can be read out to produce video signals for transmission to the display monitor 33 in the manner previously described. The data is stored in the buffer storage 44 of video board 46 unless high precision image processing or very high resolution scanning is performed in which cases the data is temporarily stored in CPU memory 48.

By actuating different ones of the keyboard keys 56, the operator may optionally initiate a full sized display of a selected one of the areas of interest 49, 49a and 49b or a simultaneous display as shown in FIG. 4 in which each image appears at a separate quadrant of the display screen 32. Images having a resolution that is greater than the resolution of the display screen 32 are stored in CPU memor