GB2315395A - Method and apparatus for presenting images - Google Patents

Abstract

A perceived three dimensional image of the internal structure of a subject 1 is generated from a series of two dimensional images. The method comprises directing a beam of X-rays or other penetrating radiation at the subject and capturing the image of the resultant radiation passing through, absorbed, emitted, reflected, scattered and/or diffracted by the internal structure of the subject and viewing a plurality of such images. The images are generated by directing radiation at the subject from a number of points displaced angularly from one another along a path relative to the subject using a gun 2 and a C-arm 4. The images are captured and stored as a sequence of discrete images 6. The images from the storage medium are presented, without significant reconstruction of the individual images, as a series of individual images or pairs of images to an observer, preferably in pairs as if they were stereoscopic pairs of images; and the observer selects the speed of presentation of the images and/or the relative direction within the stored sequence of the images at which the images are presented, so as to optimise the three dimensional depth perceived by the observer.

Description

TITLE: METHOD AND APPARATUS The present invention relates to a method and apparatus for use in that method, notably to a method for presenting X ray images and to an apparatus for presenting stored digitised X ray images to an observer.

BACKGROUND TO THE INVENTION: In making an X ray image, a beam of X rays from a source is directed onto a subject and the X rays are absorbed to different extents by the subject. This provides an image of the radiation which passes through the subject whose intensity varies according to the amount of radiation absorbed or scattered by the internal structure of the subject. This image is captured on a photographic plate to give a single image fixed in time, but which offers a high resolution image which can be stored for future use.

However, such an image is a purely two dimensional image and lacks any shading or other clues which would give an observer any impression of depth in the image. Thus, an observer can deduce very little about the three dimensional composition of the internal structure of the subject.

It has therefore been proposed to take a second image with the X ray source displaced at a known angle from the first to provide a pair of images. This pair of images can then be viewed together through a suitable stereoscopic viewer to give the observer a measure of depth perception. However, this requires that two images be taken, which increases the dosage of X rays to which the subject is exposed, which may be unacceptable.

In order to reduce the radiation dosage to which the subject is exposed, it has been proposed to subject the image to an intensification process, for exmple by locating the film which captures the image between two photoluminscent screens which illuminate the film when they are excited by being struck by X rays. It has also been proposed to capture the images using an array of phosphors to create a visible image which is then viewed through a photomultiplier by a camera or other image capture device. In this way a low radiation level can be used to produce an image of low intensity and the effect of the photomultiplier is to enhace the image for capture. Whilst such a system reduces the radiation dosage to which the subject is exposed, it reduces the definition of the image captured.

Furthermore, even with a single stereoscopic pair of images, the depth perception is poor and it has been proposed to increase the number of images taken so as to create a series of stereoscopic pairs of images which can be viewed in sequence so as to increase the viewers comprehension of the internal structure of the subject. However, the images are presented in the same order as that in which they are taken and at a specific speed of presentation, typically in the sequence and at the rate at which they are captured.

Several forms of such a display system are known and include ones in which the images are captured via an image intensifier and displayed on a video display screen as the images are captured. The memory unit in such a system thus typically stores the data for only one or two images at a time. Such a system requires that a number of images be taken to create the sequence of images which are observed, thus increasing the potential radiation dose administered to the subject and does not envisage the storage of the images for future use and reference.

In a modification of such a system, the images are digitised and held in a digital memory for subsequent replay as a series of images, and the image can be processed to provide other forms of image, for example subtractive images.

It has been proposed in what is known as a CAT scanner, to direct a series of pencil beams of high intensity radiation through the subject from a series of locations angularly displaced about the subject. The images from such beams are captured individually and are fed to a computer which creates a two dimensional synthetic image of the whole slice of the subject being scanned. Due to the collimated nature of the beam and the high intensity of the X ray radiation used, the images are of high definition, but are only of a specific path through the subject and require considerable computer processing to combine the data from each beam image to create the image of the slice, which is viewed at 900 to the plane of the slice image created.

Such a system provides sufficient data from which the computer can create an image of the transverse slice through the subject which can be rotated, enlarged or otherwise processed by the computer to give the observer flexibility as to how the image is examined. However, a CAT scanner exposes the subject to high radiation doses in order to achieve the requisite number of images. Furthermore, a CAT scanner does not give a broad picture of the interior of the subject and it is necessary to capture a number of images not only around the subject to create the image of a slice, but to capture further images at axially displaced positions along the subject to create the data for a series of slices from which a complete image can be created. This further increases the radiation dosage to which the subject is exposed and requires yet more computer processing power to handle the data and create the images for viewing by the observer. A CAT scanner is therefore an expensive and complex piece of equipment which cannot readily be transported from one site to another.

We have now devised a method of capturing and presenting X ray images which reduces the cost and complexity problems of a CAT scanner and does not require the high dosage rates required to produce the series of images from which an improved perception of depth can be obtained, thus enhancing the confidence with which a viewer can identify the nature and relative position of items within the internal structure of the subject.

SUMMARY OF THE INVENTION: Accordingly, the present invention provides a process for generating a perceived three dimensional image of the internal structure of a subject from a series of two dimensional images, which method comprises passing a beam of X rays through the subject and capturing the image of the resultant beam after it has passed through the subject, characterised in that: a. the X ray beam is directed at the subject from a number of points displaced angularly from one another along a path relative to the subject; b. the images of the intensity of the resultant beams after they have passed through the subject are captured for some or all of the said points from which the X rays are directed at the subject; c. the images are stored as a sequence of discrete images in a machine readable storage medium having sufficient capacity to store more than two of the images concurrently; d. the images from the storage medium are presented, without significant reconstruction of the individual images, as a series of individual images or pairs of images to an observer, preferably in pairs as if they were stereoscopic pairs of images; e. the observer selects the speed of presentation of the images and/or the position within the said sequence from which the successive images presented to him are selected so as to optimise the three dimensional depth perceived by him.

By selecting the speed of presentation of the images to the observer, we have found that the perception of the relative positions of items within the internal structure of the subject presented in two dimensions is surpringly enhanced and the onserver can deduce more that would have been expected from such a series of images than from observation of the individual images. Where the sequence of images from the storage medium is presented first in one order then in the reverse order and/or the direction of presentation is repeatedly changed, the perception of the relative position of items in the internal structure of the subject is further enhanced. If such speed and/or direction of presentation is applied to pairs of images presented as stereoscopic pairs, the perception is further enhanced; and where the observer selects images out of the strict sequence in which they are held in the storage medium, the perception of both depth and the relative position of the internal structure of the subject is enhanced yet further. Furthermore, by varying the speed, direction and relative separation of successive images presented to the observer from within the sequence as stored, the observer can optimise the perception of relative positions and depth to suit his subjective requirements.

This will increase the confidence with which he can identify the relative position of items within the subject and hence his diagnosis of the condition of the subject.

With systems proposed hitherto, the images were presented in a fixed sequence and at a predetermined speed. The observer was not given the means to select the criteria of image presentation which we have found enhance his perception.

The invention thus provides a simple and effective means for enhancing the three dimensional information which an observer can obtain from a series of two dimensional images which does not expose the subject to excessive radiation doses and does not require large computing power since the images are presented in substantially the form in which they are stored and are not subjected to dis-assembly and reassembly as is required with a CAT scanner image. The necessary hardware and software required to operate the invention can readily be connected to an existing X ray machine with little or no modification of the machine, thus converting machines which had hitherto been considered suitable only for taking individual images or low resolution pairs of images into more versatile machines capable of providing the user with more information than had hitherto been considered possible.

Since the images are stored in a machine readable form, for example in a computer memory or data disc, the observer can also freeze the images during presentation for more detailed static image inspection. The observer can also enlarge areas of the image or superimpose images to provide total flexibility in the manner in which the images are presented so as to extract maximum information therefrom in a manner best suited to his individual requirements.

It is particularly preferred to store the images in digital form in a frame store which is accessed by a computer and to provide the computer with means for selecting the order and speed at which the images are to be viewed by the observer.

We have found that the images can be held at a resolution of from 256 by 256 pixels upwards and that a typical pair of images will require about 128 kilobytes of memory for its storage. We prefer to present at least 10, notably at least 20, images or pairs of images to the observer and this will require about 1.2 megabyte of storage on a conventional 3.5 inch high density magnetic disc. The invention thus readily lends itself to operation on a PC based system connected to a known CCD array or other image capture device.

Where the images are taken at 4" intervals, then a series of images along a path extending over and arc or rotation of 88" about the subject can be accomodated. By allowing dynamic, interactive display of images from this series a better perception of depth can be achieved for this sequence than either a single high resolution 1024 x 1024 image or a stereo-pair of images of about 700 by 700 pixels.

The invention thus also provides apparatus for use in the method of the invention, which apparatus comprises: a. means for directing an X ray beam onto a subject from a number of locations around a subject; b. means for capturing a series of the individual images of the intensity of the X ray beam passing through the subject at each of at least some of the said points around the subject; c. machine readable storage means for storing the series of individual images in identified locations within a sequence of captured images; d. means for presenting images from said storage means to an observer either singly or in pairs; and e. observer controlled means for selecting the speed of presentation of the images and/or the position within the said sequence from which the successive images presented to the observer are selected from said sequence of images in said storage means.

Preferably, the X ray beam is generated by a conventional single X ray source which is caused to travel around the subject on an arcuate path centred on an axis of the subject. For example, the X ray generator is carried upon a C frame which rotates under the influence of a manual, electric, hydraulic or other drive means so that the generator follows a substantially circular about the subject. Preferably, the path extends for at least 30 to 1200 about the longitudinal axis of the subject and the images are taken at intervals of up to 100 along that path.

The optimum angular separation between the points at which images are taken depends, inter alia upon the speed of movement of the X ray generator and the manner in which the images are to be presented to the observer. In general, the faster the movement of the X ray generator, the greater the angular separation at a given time interval between generation of X ray beams. However, if the resultant images are presented to an observer as a series of individual images, the image will appear jerky and the enhancement of position perception may be reduced if the angular separation exceeds 10 . It is usually preferred to provide images which are at from 2 to 5 separation and to take those at a rate of 6 to 10 images per seond, ie. over an arc of travel of about 30 per second. Such angular separations and rates of arcuate travel can be achieved using a conventional C arm X ray device using conventional control means. A typical example of such a frame is shown in Figure 1 of the accompanying drawings. The X ray generator can provide a continuous beam of radiation and the beam is a divergent conical beam of radiation as is typically obtained from a conventional X ray source. However, it is preferred to operate the generator intermittently so as to reduce the total radiation dose to which the subject is exposed and to generate the X ray beam at each location at which an image is to be captured.

The radiation passing through the subject is captured by any suitable medium, for example by means of an image intensifier and plumbicon tube as used with a conventional fluoroscopic imaging X ray machine. Alternatively, the image can be captured using a radiation sensitive array of individual sensors, each capturing a pixel or a component of a pixel of the image to be stored in the storage medium.

Thus, for a 256 by 256 pixel frame store memory, the image capture device will comprise an array of radiation sensitive transistors or diodes on a grid pattern of 256 by 256 or more. Such image capture devices are commercially known and have been proposed for use in capturing X ray images and may be used in their commercially available forms in the present invention.

It will usually be desired to minimise the dosage to which the subject is exposed, and it has been proposed to employ a comparatively low level of radiation, ie. a low power X ray source and to enhance the image of the radiation passing through the subject by suitable means. For example, the image can be reduced in size to achieve a smaller image having a greater intensity of image so as to increase the contrast in the grey shades, typically 256 or more shades of grey, observed by the image capture device. Alternatively, the radiation passing through the subject can fall upon a screen of phosphors to create a visible image which is viewed through a photomultiplier by a suitable camera to capture the visible image created on the phosphor screen.

Such image capture and intensifier devices can be of conventional design and construction and may be used in the method and apparatus of the invention with little or no modification.

Surprisingly, we have found that, despite to reduction is resolution which is caused by the use of a low intensity radiation source, the three dimensional perception of resultant image using the method and apparatus of the invention is enhanced as compared to conventional static stereoscopic scanning of the images.

The phosphor screen and image intensifier are preferably mounted in association with the image capture device at the diametrically opposed end of the C frame carrying the X ray generation device described above.

The X ray generation device and the image intensification/ collection device are caused to precess about the subject by any suitable means so that individual images can be captured at a series of locations about the subject. The precession can be continuous or stepwise and the X ray generation and/or image collection devices operated intermittently so as to capture images at intervals of at least 2" angular separation about the subject. Preferably, the X ray generation device is located radially with respect to the subject and has its axis of rotation substantially coincident with the longitudinal axis of the subject.

However, if desired, the X ray generator can be rotated about an axis passing through a specific point within the subject which is radially off set from the longitudinal axis of the subject, or can be located at one end of the subject and directed axially with respect to the subject and rotated about an axis co-incident with or parallel to the longitudinal axis of the subject.

For convenience, the invention will be described in terms of rotating a radially orientated C frame about the longitudinal axis of the subject.

The image collection device can be a single array of sensors which travel with the X ray generator as described above.

However, it is within the scope of the present invention to provide a stationary arcuate array of sensors upon which the beam of X rays passing through the subject falls at each operation of the X ray generator upon its diametrically opposed arcuate path of travel. It is also within the scope of the present invention to provide a series of X ray generators mounted upon an arc about the subject and to operate those generators in series to provide the individual images. However, it is preferred to provide a single X ray generator which moves about the subject in opposition to a corresponding moving diametrically opposed image capture/intensification device.

The image from the image collection device will typically be in analog form and requires to be stored for future viewing by the observer. However, it may be desired to observe the images are they are generated to ensure that any abnormality in the images can be corrected, if due to incorrect set up or operation of the apparatus. Alternatively, where the abnormality is due to some feature of interest within the internal structure of the subject, the set up and operation of the apparatus can be deliberately altered to concentrate on the feature of interest, for example by altering the longitudinal position of the apparatus with respect to the subject and/or altering the arc swept around the subject by the apparatus. Such real time display of the images being generated can be achieved using any appropriate method, for example a VDU receiving its signals from the image capture device. However, in such a case the images will be displayed in the sequence in which they are captured and the observer will not have the control over the sequence and rate of display of the images which the apparatus of the invention can achieve. It may therefore be desired to feed any real time display of the images being generated from the observer controlled selection device described below so that the observer can make use of the features of the invention, whilst still effectively being able to review the images in pseudo real time.

The image may also be passed to an analog data storage device, for example a photographic film or electromagnetic recording tape, for interim storage prior to digitisation of the images for storage in the machine readable format. For convenience, the invention will be described hereinafter in terms of a method and apparatus in which the signals from the image capture device are in an analog format and are fed directly to a digital signal storage device.

Thus, the signals from the CCD array or other image capture device are fed to an analog to digital convertor in which the signals representing the shades of grey, typically 256 shades of grey, are converted to digital signals for storage, where the image capture device does not already digitise the image it has captured, as is the case with a solid state camera. Such analog to digital conversion can be carried out using conventional techniques and equipment.

If desired, the images may be subjected to initial pre or post treatment. For example, the image can be subjected to filtering to remove background noise or extraneous signals outside the frequency range which it is desired to monitor.

The resultant cleaned image can be subjected to image processing to enhance the captured image, for example to complete portions of the image which have been rendered unclear due to the use of low levels of radiation intensity resulting in loss of resolution in the image captured. It amy also be desired to incorporate means by which one image may be substracted from another to present a resultant image which shows only the changed subject matter of the two images, for example the presence of dye or opacity in a blood vessel. Such subtraction is preferably done upon the digitised image and such digital subtraction can be achieved using conventional software techniques.

Such treatments can be of conventional nature and can be carried out using known techniques and equipment. However, such treatments do not incorporate disassembly of the image and reconstruction of the image from modified data as occurs when a synthetic image is computed in a CAT scanner. For convenience, the invention will be described hereinafter in terms of the direct conversion, where necessary, of the signals from the image capture/intensification device and their storage in an electromagnetic storage device.

The storage device can be a memory chip or other device for storing digital data in a machine readable form. However, as indicated above, the memory requirement for the storage of a series of 20 images will be about 1.2 megabyte and it is preferred to use a conventional magnetic disc as the data storage medium for the present invention. Thus, it is preferred to capture the image using a conventional frame grabber device and to store the data from each frame in a frame store device to ensure that the data for each individual image is retained as a unit and can be given a unique address for its location in the series of images generated by the X ray generator/image collection devices.

Such a frame grabber and frame store devices can be of conventional design and operation.

The images from the machine readable storage medium can be called up by the user as a series of individual images. In which case the depth perception is achieved by the user recognising the clues to the relative positions of items within the two dimensional images given by the changes between successive images due to the apparent rotation of the subject as the series of images is presented. In this case it is preferred that the images are presented at such a rate that there is a visually smooth transition from one orientation of the subject to the next. As indicated above, this will not be achieved at angular separations between the points at which the images are captured of more than 10 .

Typically, it is preferred to present the images at such a rate and at such angular separation that the subject appears to rotate through 300 per second and the rotation is through about 60 to 900 over 20 to 30 images which are taken at angular separations of from 2 to 3 . As indicated above, it is also preferred to provide the observer with means for reversing the direction of the flow of images so that he can back-track through part or all of the sequence of images being presented. This abilty to travel to and fro within the sequence of images and to vary the speed at which such travel occurs enhances the perception of the relative positions of items within the structure of the subject.

However, such dynamic perception of the relative positions of items within the structure of the subject does not give a good impression of depth in the image. It is therefore preferred to present the images as pairs of images which are viewed through a suitable stereoscopic viewer to give a three dimensionsal perception of the subject. Typically, the angular separation between each image in a stereoscopic pair will be up to 8". However, we have found that at a fixed angular separation between successive images in the sequence as stored and between individual images in a pair of images, the three dimensional perception is often not sufficient to give the observer confidence that he has correctly identified the three dimensional structure of the subject. Surprisingly, we have found this if the observer can select the angual separation between images and within pairs of images, he can adjust the presentation of the images so that the three dimensional perception is enhanced for him and thus his confidence in correctly identifying the structure of the subject is enhanced.

The images may also be presented as pairs, one to each eye, by any suitable optical means. For example, one image may be tinted red, the other green and the observer wears appropriately coloured glasses. Alternatively, by imposing a delay on the presentation of the image to one eye as opposed to the other, a perception of depth may also be created. However, it is preferred to present the images as black and white images simultaneously through separate paths to each eye of the observer, for example using a suitable mirror path to achieve the desired separation of the images and to create the illusion that they are generated from a common source. Typically, the eyes of an observer require the pairs of images in a true stereo-scopic pair to be generated from positions separated by from less than 80, typically 2 to 60 angular displacement.

Where stereo-scopic pairs of images are to be viewed, it will usually be necessary to provide some sort of viewing device by which the images can be presented separately to the observer's eyes. Such a device typically takes the form of a mirror arrangement by which the user can observe separate images on a VDU screen or screens. Typically, the optical paths to each eye will be screened from one another to minimise cross-talk between the paths. Such stereoscopic viewing devices are readily available or can be made using conventional optical techniques and equipment.

In the apparatus and method of the invention, the observer can select which images are to be viewed together from the sequence in the series as stored by selecting the required locations in the addresses in the frame store so that he can fine tune the angular separation in the pairs observed by him. For example, one observer may find that the depth perception is enhanced when he views image I with image 4 in the series as stored rather than image 1 with its companion image 2. Another observer may find that viewing images 1 as a stereo pair combined with the scrolling of the series of images gives optimal depth perception.

The user can select the order or pairing of the images presented to him by any suitable means. For example, by suitable programming, the keyboard of the PC can be used to present simultaneous images (ie. image 1 with image 1) at .pressing key 0, succeeding images (ie. image 1 with image 2) on keying key 1, images separated by one position in the sequence as stored (ie. image 1 with image 3) by keying key 2 and so on. Alternatively, a joystick, touch sensitive screen, mouse or voice activated system can be used to provide the physical interface upon which the observer acts to control the selection of the order and pairing of the images.

Such programming of the selection of the images and their order of presentation can be achieved using any suitable method by those skilled in the programming art. The programming can also be used to dictate other features in the presentation of the images singly or in pairs to the observer. For example, the duration of each display and the time interval before the next display can also be controlled by the observer so that he can scroll rapidly or slowly through the selected sequence of images so as to achieve a perception of rotation of the subject being observed; the scrolling can be stopped to review a particular image; and means can be provided for identifying areas of the image of principal interest and for presenting enlarged portions of the images as stored to concentrate on those areas. Again such operations can be achieved by conventional programming techniques.

As indicated above, the apparatus of the invention preferably incorporates a conventional computer or microprocessor device operating under the control of specific software to act as the frame grabber/store and to read the data from the storage medium for manipulation by the observer prior to display upon a V sequence of images held in a computer accessible memory unit.

The invention also provides a program for controlling operation of a computer, which program includes instructions for operating the computer to select images out of sequence from a sequence of images held in a computer accessible memory unit. Preferably, the program is carried on a portable storage means. However, the program may be held in a master memory unit which is accessible to many users of the program.

The . additional apparatus, for example the frame grabber/store, the data store, the microprocessor required to operate those and the stereo-scopic scanning device together with the associated software required to operate the apparatus can readily be incorporated into existing designs of X ray generator/image capture devices during manufacture or can be readily applied to existing X ray generator/image capture devices with little or no modification of the existing device.

The invention can be applied to the generation of images from a wide range of subjects for a wide range of purposes.

Typically, the method and apparatus of the invention will be used in the analysis of X ray images of human patients, for example to locate tumours, heart disorders, bone and other fractures, blood clots and vascular or arterial blockages or malfunctions. In such cases it will usually be desired to minimise the radiation dose to which the patient is exposed and to use an image intensifier to enable a satisfactory image to be captured. Whilst such an image is of lower resolution that when a high intensity radiation source is used, the ability of the observer to adjust the speed of presentation and the order in which images are presented enables the observer to identify items and their relative positions within the subject with greater confidence than hitherto. However, the invention can also be applied to inanimate objects where the level of dosage exposure is not material. For example, the invention can be applied to X ray analysis of internal flaws or cracks in metal castings or welds in tubes. In such cases, there is no need to reduce the radiation intensity of the X ray beam and no need to use an image intensifier to enable a satisfactory image to be captured.

The invention has been described above in terms of X rays passing through a subject. Since the images which are captured represent shadow images, there are not normally any of the normal visual clues, such as surface folds, textures or the like, which an observer can use to assess the shape and structure of a subject. The invention is therefore of especial application in viewing such transmission images.

However, the invention can also be applied to other forms of image where other clues as to relative position of features and of three dimensional shape are available, for example in images from magnetic resonance imaging.

DESCRIPTION OF THE DRAWINGS: The method and apparatus of the invention will now be illustrated by way of illustration only with respect to the accompanying drawings in which Figure 1 is a diagrammatic representation of the X ray generation/image collection device; and Figure 2 is a block diagram representation of the frame store and image selection equipment for use in selecting the image to be viewed by the observer.

DESCRIPTION OF THE PREFERRED EMBODIMENT: The X ray generation and image collection device for scanning a subject 1 comprises a conventional X ray gun 2 mounted at one and of a C shaped arm 4 which is mounted generally radially with respect to the longitudinal axis of the subject 1. The C shaped arm extends over and arc of about 180O and the other, diametrically opposed, end of the arm carries a phosphor screen, a photomultiplier and CCD array camera collectively shown as 3 and referred to herefter as the camera for collecting the X ray radiation which passes through the subject and presenting an image in machine readable form. The arm 4 is rotated about the longitudinal axis of the subject by a motor 5. In this way the relative position of the X ray source 2 and the camera 3 with respect to the subject is kept substantially constant.

The output from the camera will usually be an analog electrical signal representing the intensity of the X ray image observed by the CCD array in 256 shades of grey and will typically have a resolution of 256 by 256 pixels per square inch. However, other forms of image presentation may be used, for example a full colour picture at higher resolution if desired. The analog signal requires to be converted to a digital signal, where this has not already been done within the camera circuitry, for storage in a suitable storage means 6. Such conversion may be carried out at any suitable point in the processing of the image and using conventional techniques and equipment. Typically, the analog to digital conversion is carried out within the camera circuitry to provide a digitised output from the camera. However, the analog signal may be stored, for example an electromagnetic tape, to provide an analog record of the images for display on a conventional video recorder and television, or a VDU for real time display. However, it is preferred to feed the digital signals to a frame store or other electronic memory store.

The frame store typically comprises a series of individual memory cell addresses each large enough to accommodate the data for a single image from device 3. This frame store and its individual cell addresses is schematically represented as item 10 in Figure 2 and has sufficient capacity to store from 10 to 40 images concurrently. The identity and sequence of recall of data from the individual cell addresses is selected by an observer via input device 12, for example the keyboard, touch screen panel or mouse of a computer (not shown). This controls the operation of a suitable selection and addressing device 11, for example the computer itself, which transfers the data from the selected cell address to a visual display device 13. The visual display device may be a mono-scopic device in which individual images are displayed at the desired rate and in the desired sequence according to the selection made at the input device 12. However, it is preferred to present the images as pairs of images and Figure 2 shows a stereo-scopic visual display device in which two images from the store 10 are displayed simultaneously to the observer.

In operation, the C arm 4 is caused to rotate about the subject 1 and the X ray gun 2 is operated at preset time intervals on the path of the arm to provide images which are captured by the camera 3 at angular intervals about the subject. Typically, these intervals are from 3 to 60, but may be more or less frequent. The images captured at the camera are fed as digital images to the frame store 10.

Whilst real time images may be taken directly from the output of the camera, such images will be presented in the order in which they are captured and at the rate at which they are captured. It may therefore be preferred to take the images from the data in the frame store 10 and to use the input device 12 to control the image presented. Whilst this will allow the images to be presented in real time in the order in which they are captured, it will also allow the observer to modify the presentation if this is necessary and to back track over the images captured to allow images to be retaken if a problem has been encountered or a specific area of interest within the subject identified and further images are required. This may enable the observer to take extra images whilst the subject is still in position within the C arm and avoid the need to recall the subject at a later date.

The user can select the identity of the cell addresses required to be observed, their sequence and rate of presentation and the pairing of the cell addresses required so as to optimise the depth perception created by not only the presentation of the images as stereo-scopic pairs but also by virtue of the dynamic sequence of the changing images. Typically, the images will be presented at the rate of from 5 to 15 images per second so as to give the impression of smooth rotational movement of the subject.

However, the observer can increase or slow down the rate of presentation and can increase or decrease the separation of the cell addresses within the sequence in the frame store so as to optimise the perception of depth in the observed images. Typically, the separation of the cell addresses will correspond to angular separations of the images of from plus or minus 2 to 3 .

Claims (11)

CLAIMS:

1. A process for generating a perceived three dimensional image of a subject from a series of two dimensional images, which method comprises generating a plurality of images of the subject and viewing those, characterised in that: a. the images are generated from a number of points displaced angularly from one another along a path relative to the subject; b. the images are captured for some or all of the said points; c. the images are stored as a sequence of discrete images in a machine readable storage medium having sufficient capacity to store more than two of the images concurrently; d. the images from the storage medium are presented, without significant reconstruction of the individual images, as a series of individual images or pairs of images to an observer; e. the observer selects the speed of presentation of the images and/or the position within the said sequence from which the successive images presented to him are selected so as to optimise the three dimensional depth perceived by him.

2. A process as claimed in claim 1 wherein the images are transmission images obtained by passing radiation through the subject.

3. A process as claimed in claim 1 for generating a perceived three dimensional image of the internal structure of a subject from a series of two dimensional images, which method comprises passing a beam of X rays through the subject and capturing the image of the resultant beam after it has passed through the subject, characterised in that: a. the X ray beam is directed at the subject from a number of points displaced angularly from one another along a path relative to the subject; b. the images of the intensity of the resultant beams after they have passed through the subject are captured for some or all of the said points from which the X rays are directed at the subject; c. the images are stored as a sequence of discrete images in a machine readable storage medium having sufficient capacity to store more than two of the images concurrently; d. the images from the storage medium are presented, without significant reconstruction of the individual images, as a series of individual images or pairs of images to an observer; e. the observer selects the speed of presentation of the images and/or the position within the said sequence from which the successive images presented to him are selected so as to optimise the three dimensional depth perceived by him.

4. A process as claimed in any one of the preceding claims, wherein the images are presented to the observer in pairs as if they were stereoscopic pairs of images

5. A process as claimed in any one of the preceding claims, wherein the source of radiation is carried upon a C frame which rotates under the influence of a manual, electric, hydraulic or other drive means so that the generator follows a substantially circular arcuate path about the subject.

6. A process as claimed in claim 5, wherein the path extends for at least 30 to 1200 about the longitudinal axis of the subject and the images are taken at intervals of up to 100 along that path.

7. A process as claimed in any one of the preceding claims, wherein the observer selects both the speed of presentation of the images to him and the relative position of successive images presented to him from the sequence of images in the machine readable storage medium.

8. A process as claimed in any one of the preceding claims, wherein the observer selects the speed or presentation of the images and the relative position of the successive images presented to him from the sequence in the storage medium by means of a computer means programmed to control the presentation of images from a data store operatively associated with the computer means.

9. Apparatus for use in the method of claim 1, which apparatus comprises: a. means for directing a beam of radiation, notably an X ray beam, onto a subject from a number of locations around a subject; b. means for capturing a series of the individual images of the subject, notably images of the intensity of the X ray beam passing through the subject, at each of at least some of the said points around the subject; c. machine readable storage means for storing the series of individual images in identified locations within a sequence of captured images; d. means for presenting images from said storage means to an observer either singly or in pairs; and e. observer controlled means for selecting the speed of presentation of the images and/or the position within the said sequence from which the successive images presented to the observer are selected from said sequence of images in said storage means.

10. A computer adapted to control the presentation of plurality of two dimensionsal images from a computer accessible memory unit of such images in a specified sequence, characterised in that the computer is programmed to select the speed and/or order and separation of images to be presented to an observer from that sequence of images.

11. A program for controlling operation of a computer, which program includes instructions for operating the computer to select images out of sequence from a sequence of images held in a computer accessible memory unit.