Abstract:
An investigative X-ray apparatus comprises a source of X-rays emitting a cone beam centred on a beam axis, a collimator to limit the extent of the beam, and a two-dimensional detector, the apparatus being mounted on a support which is rotatable about a rotation axis, the collimator having a first state in which the collimated beam is directed towards the rotation axis and the second state in which the collimated beam is offset from the rotation axis, the two-dimensional detector being movable accordingly, the beam axis being offset from the rotation axis by a lesser amount than the collimated beam in the second state. The X-ray source is no longer directed towards the isocentre as would normally be the case; the X-ray source is not orthogonal to the collimators. This is advantageous in that the entire field of the X-ray tube can be utilised. As a result, a lesser field is required of the X-ray tube and the choice of tube designs and capacities can be widened so as to optimise the performance of the X-ray tube in other aspects.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Section 371 National Stage Application of International Application No. PCT/GB2005/002894, filed 25 Jul. 2005 and published as WO 2006/013325 on 9 Feb. 2006, in English. 
     FIELD OF THE INVENTION 
     The present invention relates to X-ray apparatus, in particular X-ray apparatus that has an investigative function. Such X-ray apparatus can be stand-alone X-ray apparatus, for purely investigative purposes, or can be integrated as part of an investigative function provided on a radiotherapeutic apparatus. 
     BACKGROUND ART 
     Computed Tomography scanning is a well known diagnostic technique and, in Its cone beam form, involves directing a wide beam of X-rays towards and through the patient and capturing the resulting two-dimensional image on a flat panel detector behind the patient. The apparatus (source and detector) is then rotated around the patient to obtain a multiplicity of images from different directions. These images are combined via a suitable computing means in order to produce a three-dimensional representation of the internal structure of the patient. 
     One limiting factor is the cost and size of the detector. Flat panel X-ray detectors are typically very expensive, and the cost increases with the dimensions of the detector. In practice this places an upper limit on the possible size of the flat panel detector. This in turn places a limit on the maximum aperture of the apparatus. 
     There are ways to increase the effective aperture of the device, within limits. Normally, the cone beam is directed along a central beam axis that coincides with the isocentre of the device, and the flat panel detector is centred on that beam axis. This will mean that each successive image taken by the flat panel detector will show a section of the patient centred on the isocentre. These can then be reconstructed in the normal way. 
     However, for particularly large patients this aperture may be insufficient. In this case, the aperture of the apparatus can be increased by moving the flat panel detector such that the central beam axis intersects near to one edge of the detector. This X-ray beam can then be collimated differently so that the cone beam is offset from the (previous) central beam axis and still covers the area of the flat panel detector. The beam will then be centred on an offset beam axis. In this case, each individual image will only show half of the relevant portion of the patient. However, after the apparatus has rotated through 180°, the other half will be brought into the image. When these images are reconstructed using a suitably reconstructed algorithm, a complete rendering of the patient will still be possible, albeit with a lower resolution reflecting the fact that each voxel of the reconstructed volume has been reconstructed using only half the amount of data. 
     SUMMARY OF THE INVENTION 
     Whilst this arrangement is potentially beneficial in that it allows a different compromise to be reached between aperture and image quality in cases that demand it, it does place some limitations on the apparatus design. In particular, the X-ray tube must be able to provide a beam that is of twice the width otherwise required. This limits the choice and specification of X-ray tubes that can be used and may impose difficulties in other areas, in that a tube that is able to provide a sufficiently wide beam may be inadequate in other ways. 
     The present invention therefore provides an investigative X-ray apparatus, comprising a source of X-rays emitting a cone beam centred on a beam axis, a collimator to limit the extent of the beam, and a two-dimensional detector, the apparatus being mounted on a support which is rotatable about a rotation axis, the collimator having a first state in which the collimated beam is directed towards the rotation axis and the second state in which the collimated beam is offset from the rotation axis, the two-dimensional detector being movable accordingly, the beam axis being offset from the rotation axis by a lesser amount than the collimated beam in the second state. 
     Thus, in effect, the X-ray source is given a permanent offset of a few degrees (such as 3-4°) such that its natural axis is halfway between the two extremes called for by the collimator. The X-ray source is no longer directed towards the isocentre as would normally be the case. 
     Thus, in an alternative aspect, the present invention provides an investigative X-ray apparatus comprising an X-ray source and a collimator set in which the beam is not orthogonal to the collimators. 
     This is advantageous in that the entire field of the X-ray tube can be utilised. X-ray tubes typically have edge effects such as tube heel, and this can be kept away from both potential images. As a result, a lesser field is required of the X-ray tube and the choice of tube designs and capacities can be widened so as to optimise the performance of the X-ray tube in other aspects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; 
         FIGS. 1 and 2  show a conventional machine in normal and offset states, respectively; 
         FIGS. 3 and 4  illustrate diagrammatically the field coverage of the normal and offset states, respectively; 
         FIG. 5  shows the apparatus according to the present invention in a first collimation; 
         FIG. 6  shows the apparatus of  FIG. 5  in a second collimation; 
         FIG. 7  shows the apparatus of  FIG. 5  in a third collimation; 
         FIG. 8  shows a typical anode for investigative X-ray apparatus; and 
         FIG. 9  shows the X-ray tube schematically. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows a typical radiotherapy machine. This has a rotatable support  10  on which is mounted a therapeutic X-ray source  12  which is able to produce a collimated beam of high energy X-rays  14  centred on a therapeutic beam axis  16 . Also mounted on the rotatable support  10  is an investigative X-ray source  18 , which produces a beam of low-energy X-rays  20  along an investigative beam axis  22 . On the opposite side of the support  10 , a flat panel detector  24  is positioned so as to intersect with the investigative beam axis  22 . 
     The rotatable support  10  is arranged to rotate about an axis which passes through the coincidence of the therapeutic beam axis  16  and the investigative beam axis  22 , and which is orthogonal to both axes. In this case, the therapeutic beam axis and the investigative beam axis are orthogonal to each other, but this is not essential and other designs are possible. The point of coincidence of the two beam axis  16 ,  22  and the rotation axis of the support  10  is referred to as the “isocentre”. A patient table  26  is located slightly below the isocentre, and a patient  28  resting on the patient table will therefore just lie at the isocentre of the apparatus. In practice, the patient table  26  is made so as to be moveable, to allow the patient to be positioned relative to the isocentre, and permit the treatment of tumours at a variety of bodily locations. 
     During treatment, the therapeutic X-ray source  12  is activated and the beam  14  is collimated so as to match the shape of the tumour. The rotatable support  10  can be used to rotate the therapeutic X-ray source  12  around the patient so as to direct the beam  14  towards the patient from a variety of directions. Provided that the tumour is at or near the isocentre, it will always be Irradiated. However, the use of a variety of irradiation directions is one factor in reducing the dosage given to healthy tissue whilst maximising the dosage given to the tumour. 
     It is of course essential to ensure that the patient is correctly positioned prior to treatment. To do so, the investigative X-ray source  18  is activated and the low energy beam  20  is passed through the patient and, after attenuation by the patient, is detected by the flat panel detector  24 . This produces a two-dimensional projection image of the patient. The rotatable support  10  is then used to rotate the investigative X-ray source  18  and the flat panel detector  24  around the patient thereby producing a collection of projected images showing the patient from every variety of directions. These can be reconstructed using known algorithms to produce a three-dimensional image of the patients adhering structure, the process known as computed tomography or CT scanning. This internal image of the patient can be used as a final check that the patient is in the correct position, and potentially, as a source of feedback to allow fine adjustment of the position of the patient table  26 . 
     In  FIG. 1 , the investigative beam  20  is shown collimated so that the image it projects covers the entire working surface of the flat panel detector  24 . As a result, the width of the beam  20  at the patient  28  is large enough to ensure that the whole of the patient  28  is included in the image obtained by the flat panel detector  24 . Problems can arise in the case of very large patients, part of whom will lie outside the beam  20 . In general, it is not possible simply to select a larger flat panel detector  24  and allow a wider beam, since the flat panel detector  24  is a high value item and larger examples cannot be procured at economic cost. 
     Accordingly, larger patients are dealt with as shown in  FIG. 2 . The same flat panel detector  24  is moved on its support to an offset position, as shown. Whilst the flat panel detector  24  still coincides with the investigative beam axis  22 , that axis  22  now crosses the flat panel detector  24  near one edge of the detector  24 . The investigative beam  20  is now collimated slightly differently so that it is no longer centred on the investigative axis  22  but extends from that axis  22  and to one side. As a result, the beam  20  produces an image of approximately one half of the patient  28 , in this case the half lying above the isocentre. However, as the apparatus is rotated around the patient  28 , after a total rotation of 180° the image will show the area of the patient below the isocentre. 
       FIGS. 3 and 4  illustrate the point schematically. In  FIG. 3 , the solid vertical line  30  shows the section of the patient which is being viewed at the start of the rotation process. This is represented as a line, whereas the images are of course projected images rather than a section, but  FIG. 3  illustrates the principle only. As the apparatus rotates, the effective image moves through an angle to the dotted line  32 , and as rotation continues further the images moves to the dotted line  34 . Thus, as rotation continues, the image taken of the patient maps out a cylindrical volume centred on the axis of rotation. 
       FIG. 4  illustrates the offset method. A solid line  36  of identical length to the solid line  30  is again rotated, but this time the axis of rotation is at one end of the solid line  36 . Thus, as the image rotates through  38  and  40  etc., a larger cylinder is mapped out. This caters for the larger patient. However, it can be seen that twice as many lines are required to map out the same cylindrical volume. Thus, the offset rotation arrangement must either spend twice as long gathering images in order to produce the same quality CT reconstruction, or must accept a lower quality CT reconstruction deriving from fewer images. This choice is however clinically useful. 
       FIG. 5  shows an improved apparatus for use in this type of diagnosis. This comprises, in general, an X-ray generating source  50  and a collimator set  52 . The investigative beam axis  22  is shown, together with the isocentre  54  and the flat panel detector  24 . 
     The X-ray source  50 , which we will described in more detail later, produces a beam  56  which is then collimated in the collimator  52 . In this design of collimator set  52 , a number of slots  58 ,  60  are provided to receive collimators and filters as required. The first slot  58  contains a beam collimator  62  to produce the investigative beam  20  from the output beam  56  of the source  50 , so that the beam  20  just covers the flat panel detector  24 . In this case, as shown, the beam collimator  62  collimates the beam  56  evenly, by reducing its width equally on both sides. 
     As shown in  FIG. 5 , the collimator  52  is aligned with the beam axis  22  and the isocentre  54 . However, the X-ray source  50  is offset by an angle  66  from being perfectly orthogonal to the investigative beam axis  220 . As a result, the X-ray source  50  and the collimator  52  are not in alignment, and the approximate centre  68  of the beam does not coincide with the isocentre  54 . However, the beam does extend across the beam axis  22  and the isocentre  58  is included within the extent of the beam  20 . 
       FIG. 6  shows the same apparatus in which an alternative beam collimator  62 ′ has been fitted, together with an alternative filter  64 ′. The second filter  64 ′ differs only in that its centre is suitably offset. The alternative beam collimator  62 ′ differs in that it collimates the beam asymmetrically with respect to the beam  56  emanating from the X-ray source  50 , but nevertheless symmetrically about the beam axis  22  and the isocentre  54 . In this way, the apparatus can be used as described with respect to  FIG. 1 . 
       FIG. 7  shows the same apparatus with a still further alternative collimator  62 ″ and filter  64 ″. In this case, the collimator  62 ″ collimates the beam  56  asymmetrically, but this time in the opposite sense to that of  FIG. 6 . Instead of returning the beam towards the isocentre  54 , the beam is offset still further from the isocentre  54  such that the beam  20  only just overlaps with the isocentre  54 . This produces an offset beam for use in the manner as described with respect to  FIG. 2  above. 
     It will be appreciated that the two extremities of collimation that are required in clinical practice, as shown in  FIG. 6  and  FIG. 7  respectively, now occupy the extremities of the usable area of the beam. The fullest available extent  56  of the beam is therefore used, by virtue of the angle  66  between the X-ray source  50  and the collimator set  52 . 
     This relieves the designer of the need to select an X-ray tube on the basis of its wide available field, and allows the optimisation of the X-ray tube based on other requirements of the device. 
       FIG. 8  shows, for information, a typical target  70  for use in an X-ray source  50 .  FIG. 9  shows the apparatus, schematically, including the target  70 . A high voltage source  72 , typically providing 150 kV is arranged to produce a potential between a hot filament  74  and the anode  70 . The anode  70  is itself mounted on a spindle  76  which is rotatable by a motor  78 . Thus, a beam of electrons  80  travels from the filament  74  towards the anode  70 . The anode  70  has a molybdenum core  82  with a generally circular face on which is mounted an annular ring  84  of tungsten/rhenium target material. The apparatus is disposed such that the electron beam  80  lands on the anode at the target material  84 . The surface is slightly bevelled so that, in respect of the surface, the electron beam  80  arrives at an angle and, as a result, an emitted beam of X-rays  86  departs the anode  70  in a direction which is roughly perpendicular to the incoming electron beam  80 . This beam  86  is then collimated by suitable beam stops  88  to produce the output beam  56  of the X-ray source. 
     The motor  78  drives the anode  70  via the spindle  76  so that the annular target  84  is constantly rotating. As a result, the point of contact of the incoming electron beam  80  is constantly moving across the anode although the rotationally symmetric design of the anode  70  means that this does not affect the output beam  56 . As a result, the anode  70  is better able to cool notwithstanding the energy absorbed from the electron beam  80 . 
     The entire apparatus of  FIG. 9  is typically enclosed within a suitable vacuum flask, which is itself suspended in a bath of flowing oil so as to assist in heat removal. 
     It will be appreciate from  FIGS. 8 and 9  that it is only possible to widen the output beam  56  within limits. The width of the output beam  56  will in practice be limited by the size of the rotating anode  70  and by the geometry of the apparatus, for example of the direction of the electron beam  80 , the degree to which the target surface  84  is bevelled, and the dimensions of the target surface  84 . Limitations such as the need to rotate the anode  70  and the requirement that the anode be adequately cooled mean that there are limits to the available width of the beam  56 . Beyond those available limits, the X-ray Intensity becomes less uniform as it eventually fades away to nothing, a phenomenon known as “tube heel”. 
     Accordingly, the invention as described with respect to  FIGS. 5 ,  6  and  7  allows the available extent of the X-ray beam to be used more efficiently, thereby relaxing the design requirements placed on this aspect of the X-ray tube and allowing it to be optimised in other respects. 
     It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention.