Abstract:
X-ray positioning apparatus is described for aligning an x-ray imaging device ( 16 ) relative to a patient ( 10 ) to permit the operator to select and obtain an x-ray image from a selected anatomical region. A plurality of light reflective markers ( 14 ) is located on the patient&#39;s body to define certain anatomical landmarks. The apparatus comprises an x-ray source ( 20 ) and a separate light source ( 30 ) and an image sensor ( 33 ). Light reflected from the markers are detected by the image sensor ( 33 ). The image data is processed by an image processing system ( 46 ) which provides a variety of functions including determining the actual position the image device should be located to obtain the desired image. Embodiments of the invention are described.

Description:
FIELD OF THE INVENTION 
     The present invention relates to medical imaging systems and in particular to a method and apparatus for aligning an imaging device relative to a patient&#39;s body. The invention is particularly, but not exclusively, applicable to the positioning of x-ray apparatus. 
     BACKGROUND OF THE INVENTION 
     When taking an x-ray of a patient, it is important to correctly position the x-ray source relative to the patient&#39;s body. Typically, the x-ray film is placed beneath a table on which the patient lies, with an ionization chamber positioned between the patient and the film. In order to ensure that the image formed on the film is in focus, the vertical distance of the x-ray source above the patient is set by the operator. Similarly, in order to ensure that the x-ray beam is incident upon the appropriate region of the patient&#39;s body, the x-ray source can also be moved by the operator in a horizontal plane. 
     Given the known dangers of x-ray exposure, it is important to confine the x-ray beam to that region of the body which is of particular interest. However, by utilising a smaller beam area the risk of missing the anatomical region of interest is increased. Failure to correctly capture an image of the area of interest will result in the need to take a further x-ray image, in addition to the wastage of x-ray film. Misalignment of the x-ray source in the vertical direction may also result in an out of focus image requiring that the x-ray procedure be repeated. 
     A known system for enabling accurate positioning of the x-ray source in the horizontal plane involves the projection of a beam of light from a light source onto a patient&#39;s body. The light source is positioned adjacent to the x-ray source and is fixed relative thereto. Typically, a x-ray radiation translucent mirror is used to direct the light beam onto the patient&#39;s body along the path taken by the x-ray beam. Prior to taking the x-ray, the operator positions the x-ray source using the light beam as a guide. Whilst in theory this system improves x-ray source alignment, in practice, regular checks must be carried out on the x-ray apparatus to ensure that the light source does not become misaligned relative to the x-ray source. Misalignment is common due to the strain placed upon the x-ray apparatus by constant movement. In addition, positioning systems of this type rely upon the expert knowledge and perception of a human operator to correctly position the light beam. X-ray images can be required for any part of the body and the operator may find difficulty in exactly centering the x-ray source to obtain a focused exposure. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to obviate or mitigate at least one of the disadvantages of the existing system. 
     According to a first aspect of the present invention there is provided apparatus for aligning an imaging device relative to a human or animal body to enable an image of an operator selected anatomical region to be obtained, the apparatus comprising: 
     a plurality of light reflective markers for attachment to respective anatomical landmarks of the body, which landmarks mark out said anatomical region; 
     an image sensor arranged in use to view at least a portion of the body including attached reflective markers; 
     a light source arranged in use to illuminate attached reflective markers; and 
     an image processing computer for receiving image data from the image sensor, the computer being arranged: 
     a) to store or access a database of standard patient records including, for each of a plurality of anatomical regions, a locating position for the imaging device relative to the positions of said anatomical landmarks; 
     b) to select an appropriate record from the database using operator input data for the patient to be imaged; 
     c) to select from the selected record a relative locating position for the imaging device using operator input data identifying the anatomical region to be imaged; 
     d) to determine from said image data the position of the reflective markers relative to the imaging device, and 
     e) to determine using the selected relative locating position and the determined positions, the actual position at which the imaging device should be located in order to obtain the desired image. 
     The number of reflective markers required depends upon the particular area to be imaged. Typically however, a relatively small number of markers are used, e.g. between two and five. 
     Preferably, the image sensor comprises at least one CCD camera. Alternatively, the image sensor comprises a pair of CCD cameras which are spaced apart so as to enable a three dimensional image of a patient&#39;s body to be obtained. The resulting three dimensional image can be used in order to provide position information in three dimensions. The computer may be arranged to store focus information defining a preferred focal distance for each anatomical region. The three dimensional position information may be used to manually, or automatically set the distance of the imaging device from the patient&#39;s body. Preferably, three dimensional rendering techniques can be applied to allow a 3-D visual image of the patient to be presented to an operator, maximising the information available to the operator. 
     Preferably said image sensor is positioned adjacent to said light source and the light reflective markers are retro-reflective markers which tend to reflect light in the direction of the light source and the image sensor. 
     Preferably, the image sensor is arranged to generate image data in which areas without reflective markers are identified as black with the reflective markers being identified as light spots. This may be achieved by appropriately setting the aperture of the image sensor or by shuttering the image sensor at an appropriately high speed. Alternatively, or in addition, image data generated by the image sensor may be applied to a variable gain amplifier. 
     Preferably, the light source comprises a flash lamp which may be operated in synchronisation with an image sensor shutter. 
     Preferably, the imaging device which is aligned by the present invention is a x-ray imaging system. 
     Preferably, the image sensor is rigidly secured to the x-ray generating apparatus so that alignment of the image sensor relative to the x-ray tube can be accurately maintained. 
     In one embodiment of the invention, the image processing computer comprises a video display unit. The computer may be arranged to superimpose the determined positions of the light reflective markers on the display. The locating position of the imaging device may also be displayed. 
     The apparatus of the above first aspect of the present invention may comprise positioning means, coupled to said computer, for automatically positioning the imaging means The image processing computer may also be arranged to locate the imaging device at the correct focal distance, e.g. using said three-dimensional position information and using a calibration frame containing permanent markers positioned strategically on the x-ray table. 
     According to a second aspect of the present invention there is provided a method of aligning an imaging device relative to a human or animal body to enable an image of a selected anatomical region to be obtained, the method comprises the steps of: 
     attaching a plurality of light reflective markers to anatomical landmarks of the body; 
     illuminating the body and the reflective markers with light; 
     detecting light reflected from the reflective markers with an image sensor; 
     selecting from a database of standard patient records, a record appropriate to the patient or animal to be imaged, each record including, for each of a plurality of anatomical regions, a locating position for the imaging device relative to the positions of said anatomical landmarks; 
     selecting from the selected record a set of relative positions using operator input data identifying the anatomical region to be imaged; 
     using the selected relative position data and the determined positions of the reflective markers to determine the actual position at which the imaging device should be located in order to obtain a desired image; and 
     positioning the imaging device at said determined position. 
     According to a further aspect of the invention there is provided an imaging aligning system for aligning an object to be imaged with a source of imaging radiation to which the object is exposed for creating an image of the object, said image aligning system comprising: 
     a first radiation source for generating an incident alignment radiation signal, 
     a plurality of reflective markers adapted to be located at predetermined locations on the object to be imaged, 
     reflected alignment radiation means for receiving reflected alignment radiation from the reflective markers disposed on said object, 
     processing means coupled to the radiation source mans for providing positional information of said reflective markers relative to said object and to said source of imaging radiation, said processing means being coupled to a database of object data identifying the position for locating said source of imaging radiation relative to said object for centering said source relative to said reflective markers for exposing said object to said source of imaging information. 
     According to yet a further aspect of the invention there is provided apparatus for obtaining a desired image of an object, said apparatus having a source of imaging radiation to which the object is exposed and imaging means for obtaining an image of the object exposed to said radiation source, said object to be exposed having a plurality of markers defining an area of said object to be imaged, said apparatus including an aligning system stated in the above further aspect of the invention for facilitating alignment of the source of imaging radiation and said markers in order to obtain said desired image. 
     According to yet a further aspect of the invention there is provided a method of obtaining a desired image of an object using apparatus having a source of radiation for exposed the object to the radiating and imaging means for obtaining an image of said exposed object, said method comprising the steps of: 
     disposing locating reflective markers on predetermined locations of said object, 
     transmitting a first incident signal from a signal source towards said object, 
     collecting reflected signals from said markers, said reflected signals being indicative of the position of said markers, 
     processing said reflected signals by comparing said marker location information with predetermined stored information from a database providing a relative position of the object to be imaged, 
     determining the position of said reflective markers relative to said source of radiation, and 
     aligning the source with the reflective markers in which imaging of said object produces said desired image. 
     For a better understanding of the present invention and in order to show how the same may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a bird&#39;s-eye view of a supine patient to be x-rayed with reflective anatomical landmarks disposed at certain positions for imaging the anterio-posterior lumbar spine; 
     FIG. 2 is a perspective view of the patient of FIG. 1; 
     FIG. 3 is a perspective view of the patient with three reflective anatomical markers disposed for assisting in imaging a lateral view of the lumbar spine; 
     FIG. 4 illustrates a x-ray apparatus, incorporating a positioning system according to an embodiment of the present invention; and 
     FIG. 5 is a block diagram of the system of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     There is shown in FIG. 1 a bird&#39;s-eye view of a patient  10  lying on the table  12  of a x-ray system. FIG. 2 shows a perspective view of the patient. Three retro-reflective markers  14  are located on specific parts of the patient&#39;s body by an operator, usually a radiographer. The exact location of these markers depends upon the anatomical region to be imaged. In FIG. 1 where the anterio-posterior lumbar spine is to be imaged, so the markers are positioned on the sternal notch and both anterior superior iliac spine. When it is desired to obtain a lateral image of the lumbar spine, the markers are positioned, as shown in FIG. 3, on the axilla, the anterior superior iliac spine, and the posterior superior iliac spine. The required positions of the markers are readily located by an experienced radiographer. 
     Each of the retro-reflective markers  14  comprise a mirrored surface coated with glass micro-spheres (manufactured by 3M Corporation) which act as prisms to reflect incident light back towards its source. The markers reflect light received from a light source mainly back towards that light source and thus are highly efficient where a light detector, used to detect reflected light, is placed adjacent to the light source. 
     FIG. 4 illustrates a combined x-ray apparatus and positioning system, generally indicated by reference numeral  16 , which is arranged to use positional information from the reflecting markers  14  to accurately align the x-ray tube apparatus, generally indicated by reference numeral  18 , to the selected anatomical region of the patient&#39;s body. X-ray radiation is produced by a x-ray source  20  (shown in broken outline) contained within a lead-lined housing  22 . X-rays from the source  20  are collimated within a channel  24  through which the x-ray radiation beam exits. The x-ray apparatus and positioning system  16  is adjustably mounted on a support  26  such that the height of the apparatus  16  above the patient  10  can be varied. The mounting arrangement also allows the x-ray apparatus  16  to be moved in a horizontal plane for alignment with the patient. 
     Contained within the x-ray apparatus channel  24  is a x-radiation translucent mirror  28 , oriented at an angle of 45° to the vertical. The mirror  28  allows x-ray radiation to pass freely therethrough towards the patient and the film but presents a reflecting surface to reflected light. 
     Positioned inside the x-ray apparatus  16  is an infra-red strobe  30  which can generate relatively short pulses of light (i.e. 50 pulses per second at a wavelength of 980 mm). Light from strobe  30  is reflected from the patient  10  and from the reflective markers  14  is reflected towards the x-ray apparatus  14  and the strobe  30  due to the retro-reflective nature of the markers  14 . Reflected light entering the channel  24  is reflected by the mirror  28  towards a side-wall  32  of the channel  24 . A charge coupled camera  33  having camera elements  34 , 36  are mounted in this side wall for receiving the reflected radiation. Cameras elements  34 , 36  are spaced apart in the vertical direction such that they receive reflective light beams  38 , 40  from markers  14  located horizontally spaced apart positions on the patient  10 . 
     Positioned beneath the table  12  is an ionisation chamber  42  which forms part of a known type automatic exposure device (AED). As is well known, the AED monitors the x-ray beam transmitted through the area of the patient&#39;s body under examination. Using ionisation chamber timers, the AED terminates the exposure when a sufficient quantity of radiation has been received to produce a radiographic image of the required density. The AED exercises accurate control on the quantity of radiation provided that the positioning of the patient is accurate in relation to the site of the ionisation chamber. The film is placed beneath the ionisation chamber in a mount  44  and receives the radiation which has been transmitted through the patient and AED. 
     FIG. 5 depicts the block diagram which shows schematically the image processing system, generally indicated by reference numeral  46 , which is used to analyse the outputs of the two CCD camera elements  34 , 36  to determine alignment information for the x-ray apparatus  16 . The image processing system  46  includes a video processor  48  for capturing visual information and a computer for analysing the captured information. After a predetermined time interval following activation of the strobe  30 , the video processor  48  captures an image frame from each of the elements  34 , 36 . The video processor  48  looks for transitions from dark to light in these frames and identifies any such transitions as the presence of light reflecting markers  14 . The video processor scans each horizontal line of the captured image frames from left to right and identifies transitions from dark to light. Providing that transitions are located at approximately the same position on two consecutive horizontal lines, these transitions are grouped together and are identified as the site of a reflecting marker. 
     In order to optimise this search process, the elements  34 , 36  are provided with respective shutters (not shown) which open for a very short time interval during the strobe exposure. Typically, the shutters open for 1/1250 of a second. The output of each CCD element is also applied to a variable gain amplifier (not shown) within the video processor  48  the gain of which is adjusted so that the intensity produced by the reflective markers  14  is set as a ‘normal’ intensity. This set-up generates image data where the reflective markers  14  show up as bright spots while remaining areas are black. The system may be further optimised by minimising the background light levels in the room containing the x-ray apparatus. 
     The resulting position data, which defines the absolute position of the retro-reflective markers  14  relative to the patient  10  and the x-ray apparatus  16 , is delivered from the video processor to the computer  50 . The computer  508  contains a database of standard patient records. The patients selected for the database cover a range of heights and weights, and each record contains anatomical position data for each of a number of anatomical regions. This anatomical data defines the position at which the x-ray apparatus  16  should be positioned for the X-ray to be centred upon the required anatomical region, relative to the positions of the reflecting markers  14  on the anatomical landmarks. 
     The radiographer uses a keyboard  52  to enter the height, weight and identification of the patient  10  to be imaged. The computer  50  then selects the appropriate record from the stored database. The radiographer then also selects the anatomical region of interest and the appropriate data is extracted from the selected record. From a knowledge of the actual positions of the reflective markers  14 , and the locating position of the x-ray apparatus  16  relative to these markers, the computer is able to calculate the actual locating position. This positional information can either be displayed to the operator who can then manually position the x-ray apparatus or the positional information can be fed to automatic x-ray apparatus adjustment means for automatically repositioning the apparatus  16 . 
     The system  46  optionally contains a display monitor  54  for displaying image data generated by one of the CCD cameras  34 , 36 . The cameras  34 , 36  may be arranged to generate normal continuous image data for display on the monitor  54 , the cameras only being shuttered for a relatively short time period to obtain reflective marker position information. Once this position information has been obtained, it can be overlaid on the image of the patient&#39;s body on the display. This allows the operator to visually confirm the correct identification of the reflective markers  14  before automatic alignment of the x-ray apparatus occurs. Similarly, the computer may be arranged to overlay the determined locating position of the x-ray apparatus on the display. 
     As another example, to obtain an anterio-posterior view of the knee joint a first marker would be placed 2.5 cm above the superior aspect of the patella, a second marker 2.5 cm below the tibial tuberosity, and third and fourth markers on the lateral and medial borders adjacent to the apex of the patella. To obtain a lateral view of the knee joint, a first marker would be placed 2.5 cm above the upper border of the knee on the medial side, and a second marker placed 2.5 cm below the apex of the patella on the medial side. 
     In addition to anatomical positioning, patient movement can be monitored visually, thus allowing the operator to correct patient position relative to any movement. This function therefore provides an assessment of the correctness of positioning before the x-ray exposure has been made. Monitoring of patient movement continues until the exposure has been completed. 
     Various modifications may be made to the embodiments hereinbefore described without departing from the scope of the invention. The x-ray system and positioning apparatus can be used to image an animal body and the light source used could be replaced by an infra-red source with an infra-red camera. The positioning system may be used with any other suitable medical imaging system where alignment of the source of radiation with the part of the patient to be imaged is required.