Abstract:
A multi-leaf collimator comprises an elongate leaf moveable in a longitudinal direction, and having an associated toothed rack driven by a pinion, wherein the rack is carried on an elongate actuator section, having a transversely extending link section, the leaf being connected to the link section and thereby being spaced from the actuator section.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a multi-leaf collimator. 
   BACKGROUND ART 
   Multi-leaf collimators (MLC) are used (principally) in the field of radiotherapy. A beam of radiation is directed toward a patient and must be collimated to fit the shape of the area to be treated. It is important to ensure that the dose in the areas outside that shape is as low as possible, but also that the whole area is treated. If areas are left untreated then the likelihood of recurrence is increased, whereas if non-treatment regions are irradiated then damage will be caused to healthy tissue resulting in greater side effects and longer recovery times after treatment. 
   As the treatment area is rarely rectilinear, multi-leaf collimators are employed. These comprise an array of finger-shaped leaves of a radiation-absorbing material, each disposed in a parallel relationship and each able to move longitudinally relative to the others. By moving each leaf to a selected position, a collimator is provided which can exhibit a non-linear edge. In general, one such array (or “bank”) will be provided on each side of the beam. 
   Previously, the leaves have been driven by various means. One involves a threaded shaft extending rearwardly away from the leaf; this can be supported in a threaded bore or connected to the leaf via a threaded socket. In either case, as the shaft or bore is rotated the leaf will be forced to move. An example is shown at U.S. Pat. No. 4,868,844 in which motors are placed to one side and linked to the threaded shafts by a flexible shaft. Activation of the motor under microprocessor control forces the threaded shaft to rotate and move through the threaded bore in which it is held. This then urges the leaf in the appropriate direction. The use of flexible shafts allows the motors to be spatially separated from the leaves and allows for the facts that the motors are significantly wider than the leaves. 
   A further example is shown in  FIG. 9 . The leaf  200  is supported on its lower edge by a roller bearing  202 , and is guided along its upper edge by a pair of roller bearings  204 ,  206 . A longitudinal slot  208  is cut into the leaf, extending from the rear edge  210  towards the front edge  212 . At the start of the slot  208 , proximate the rear edge  210 , a threaded nut  214  is fixed in place, in line with the slot. A leadscrew  216  is then threaded through the nut  214  and sits in the slot  208 . 
   A motor and gearbox  218  are positioned behind the leaf and drive the leadscrew  216  via a shaft support and coupling  220 . Thus, as the leadscrew is driven, the nut  214  and hence the leaf  200  will be driven rearwardly or forwardly, depending on the direction of rotation. 
   Other designs use a rack and pinion system, where a toothed rack is cut into the edges of the leaves, and motors are mounted outboard of the leaves with a drive shaft that extends perpendicular to the leaves, across the bank. Each shaft carries a pinion at the relevant location so as to engage with the rack of the appropriate leaf. 
   Multi leaf collimators are now being designed with smaller resolution leaves which are therefore thinner and more numerous. A significant problem in doing so is the need to drive the leaves, i.e. provide a means of physically moving them to the required degree of accuracy, and the impact of this on the length of a leafbank, the length of a leaf, and its the complexity and serviceability. This in turn increases the size of the treatment head and can restrict the patient treatment access. 
   If the leaves are driven from one end, then the motors are outboard and undesirably add length to the assembly. Likewise, reductions in the leaf width mean that it becomes increasingly difficult to embed a leadscrew. 
   The existing rack &amp; pinion methods also become increasingly difficult with a larger number of leaves, and usually result in extending the leaf length in order to accommodate the necessary number of motor drives. 
   SUMMARY OF THE INVENTION 
   The present invention therefore provides a multi-leaf collimator, comprising an elongate leaf moveable in a longitudinal direction, and having an associated toothed rack driven by a pinion, wherein the rack is carried on an elongate actuator section, having a transversely extending link section, the leaf being connected to the link section and thereby being spaced from the actuator section. 
   The actuator section can be formed integrally with the leaf, but we prefer it to be joined to the leaf, particularly if joined in a manner that is detachable. 
   The multi-leaf collimator will generally comprise a plurality of such leaves arranged in an array. We then prefer that at least one leaf of the plurality has a rack formed on an edge of the respective actuator section proximate the respective leaf. The pinion which drives that leaf will then be located between the actuator section and the leaf. The pinion can thus be mounted on a shaft which is disposed transversely to the leaves of the array, and which passes between those leaves and their respective actuators. 
   That or other leaves can also have a rack formed on an edge of the actuator section distal the respective leaf. Thus, some leaves can be driven via pinions mounted on shafts passing over the array, and other leaves can be driven via pinions mounted on shafts passing within the gaps between leaves and actuators. This means that twice the number of drive shafts can be fitted into the same length of array, thus either shortening the array or permitting more leaves and hence greater resolution. 
   The drive mechanism can also be made as a separate unit. Thus, the invention further provides a drive mechanism for a multi-leaf collimator, comprising an elongate actuator section for each leaf of the collimator, the actuator being moveable in a longitudinal direction and having a toothed rack driven by a pinion, and a transversely extending link section, the link section have an engagement means for connection with a leaf, thereby to space the leaf from the actuator section. 
   A separate drive unit is advantageous in manufacturing as it allows the drive unit to be produced, built and tested as a separate assembly in a specialist environment. Service replacement of the entire gearbox in the event of a fault will also be swifter, with minimal re-setting. It is likely that the replacement of such a gearbox will take considerably less time than the replacement of the sum of individual component parts that constitute existing drive technologies. 
   Further, the gearbox will be an enclosed unit, with integral lubrication and shielded from environmental dust and debris. 
   The invention also offers a major advantage in the leaf length is no longer dictated by the length of the rack. The leaf can be made smaller, saving material and reducing its weight. That reduction in weight will make movement of the leaf easier, improving both accuracy and reliability. 

   
     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; 
       FIG. 1  is a perspective view of a multi-leaf collimator according to the present invention; 
       FIG. 2  shows in detail part of the drive mechanism of  FIG. 1 ; 
       FIGS. 3 and 4  show sections through the drive mechanism; 
       FIG. 5  shows the drive unit from beneath; 
       FIG. 6  shows an alternative embodiment, in use; 
       FIGS. 7 and 8  show the drive mechanisms of  FIG. 7  in detail; and 
       FIG. 9 , described above, shows an existing design of collimator leaf. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1  shows a multi-leaf collimator  10 . A housing  12  contains two opposing arrays of elongate leaves  14 , a selection of which are shown in  FIG. 1 . These leaves  14  are each moveable longitudinally within the array so that they can each project by a greater or lesser distance into the open space  16  disposed generally in the middle of the housing  12 . In use, a beam of radiation is directed through that open space  16  and its extent is collimated by the leaves  14 . The leaves are relatively thin so as to allow a high resolution to be obtained, but they are relatively deep in the direction of the beam in order to render them fully opaque at X-ray energies. The leaves  14  are relatively elongate so as to allow them to adopt a wide range of positions. 
   For each array of leaves  14 , there is a drive unit  18 . This has arrays of motors  20  on either side, each of which is associated with an individual leaf. A suitable micro-processor will typically be provided (not shown) which will provide power to the required motors in order to move the appropriate leaf or leaves and provide the required collimation. However, this requires that the motors be operatively connected to the relevant leaf. 
     FIG. 2  shows general how this is achieved. Each motor  20  has an in-line gear box  22  which is mounted to the side of the drive housing  24 . The drive housing  24  has a series of slots  26  in which are disposed actuator sections  28  in a slideable manner. That is, the actuator section  28  can slide longitudinally within the slot  26 . Transverse channels  30  are formed in the drive housing  24 , running perpendicularly to the slots  26  albeit somewhat shallower. Drive shafts  32  are located in the transverse channels  30  and are driven by the gearboxes  22 . Thus, each motor  20  drives an in-line gearbox  22 , which drives an in-line drive shaft  32  lying in a channel  30 . On each drive shaft  32  there is a single pinion  34  which thus lies within a slot  26 . The pinion  34  has teeth which engage with a rack cut into one edge of the actuator section  28 . Thus, each motor  20  drives a particular actuator shaft  28 . 
   It will seem from  FIG. 2  that the motors  20  are arranged in two banks. An upper bank powers drive shafts  32  that are located in shallow transverse channels  30 . Pinions  34  located on these shafts  32  drive an actuator section  28  that is disposed beneath the pinion, and which therefore has a rack formed on its upper surface. 
   The remaining motors  20 A are disposed beneath the above-described motors  20 , in a staggered configuration. These power drive shafts  32  located in relatively deeper transverse channels  30 , and drive pinions  34  that are located beneath the relevant actuator section  28 . Thus, such actuator sections  28 A have a rack that is cut into their lower edge. 
   This can be seen more clearly in  FIGS. 3 and 4 . Both are sections through the drive unit at a slot  26 , but  FIG. 3  is a section at a slot containing an actuator section driven from beneath whereas  FIG. 4  is a section at a slot containing an actuator section driven from above. 
   Referring to  FIG. 3 , this shows a deeper transverse channel  30   a  in which lies a drive shaft on which is mounted a pinion  34   a . The pinion  34   a  sits in a recess  36  created at the foot of the transverse channel  30   a  to allow the pinion  34   a  to rotate. The actuator section  28   a  is placed in the slot  26 , above the pinion  34   a , and has a rack  38   a  on its lower edge. Thus, as the motor  20   a  drives the relevant gear box and drive shaft  32   a , the pinion  34   a  rotates and the actuator section  28   a  moves linearly and in a longitudinal direction. 
   Likewise, as can be seen from  FIG. 4 , the adjacent motor  20  drives a shaft  32  in a transverse recess  30  that is somewhat more shallow. A pinion  34  is mounted on that drive shaft  30  and engages with a rack  38  formed on the upper surface of an actuator section  28 . That actuator section sits in a slot  26  that is the same depth as the slot  26  illustrated in  FIG. 3 , and therefore all the actuated sections line up. However, the lesser depth of the transverse channel  30  in which the drive shaft  32  is placed means that that pinion  34  correctly engages with the rack  38  on the upper surface of the actuator section  28 . 
   In this way, by staggering the motors and pinions  34 , a greater number of drives can be incorporated into the same length of housing  24 . This therefore means that the housing  24  can be made shorter, thereby reducing the overall size of the MLC drive, or it means that a greater number of leaves can be fitted into the same size drive thereby increasing resolution. 
   As can be seen in  FIGS. 3 and 4  each actuator section  28  has an associated link section  40 . This is formed integrally with the actuator section  28  and extends downwardly towards the array of leaves  14 . At its tip there is an engagement section  42  which fits within a corresponding formation on the leaf  14 . Thus, the gearbox can be fitted in place over a leaf array and the relevant link sections will engage with the appropriate leaf. Thus, as the actuator section is moved linearly, the link section  40 ,  42  will drag the relevant leaf  14  with it. 
     FIG. 5  shows the drive mechanism  18  from below. The structure described with reference to  FIGS. 2 ,  3  and  4  is duplicated on either side of the device, thereby still further minimizing the necessary length of the unit. For clarity, only a minority of the link sections  40  are shown. 
     FIG. 6  shows an alternative device. In this arrangement, each of the two opposing banks of leaves  114  is associated with a pair of gear boxes  118 ,  119 . These are mounted on the respective upper and lower faces of the housing  112 , and each drive unit powers one half of the leaves  114  (of which a selection are illustrated only, for clarity). Thus, this allows the necessary length of the device to be reduced still further, or (alternatively), permits a greater number of leaves to be incorporated for the same size of device. In a further alternative, this allows the actuator sections  128  and pinions  134  etc. to be constructed of a heaver gage material and therefore made somewhat more robust. Each actuator section  128  on (for example) the upper drive section  118  can drive alternate leaves  114  of the array. The remaining leaves of  114  can be driven by the drive section  119  located on the lower section of the housing  112 . 
     FIGS. 7 and 8  show such an alternative construction for the actuated sections  128  etc. A channel section  144  extends in line with and above the relevant leaf  114 , supported at one or both ends by a cross member  146 . The drive shaft  132  extends from the gear box  122  and drives the pinion  134  via a universal joint  148 , to allow for production tolerances. The pinion  134  is housed within a sleeve  150  attached to the channel section  144  and depending downwardly thereof. This envelopes the pinion  134  and includes a suitably sized rectangular bore  152  through which the actuated section  128  passes. Thus, the pinion  134  meshes with the rack  138  formed on the edge of the actuated section  128  and the motor  120  is able to drive the actuator section  128 . 
   A corresponding but inverted mechanism is provided for those actuated sections  128  that are driven from beneath. 
   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.