GB2582588A

GB2582588A - Multi-leaf collimator camera arrangement

Info

Publication number: GB2582588A
Application number: GB1904173.0A
Authority: GB
Inventors: Furth Mark; Broad Martin
Original assignee: Elekta Ltd
Current assignee: Elekta Ltd
Priority date: 2019-03-26
Filing date: 2019-03-26
Publication date: 2020-09-30
Also published as: GB201904173D0

Abstract

A multi-leaf collimator comprises a plurality of leaves 200, 202 which are extendable into the path of a radiation beam 232, wherein each leaf is provided with a fluorescent or reflective marker 219, 218 fixedly attached thereto. A plurality of cameras 224, 226 are also provided to directly view light emitted or reflected from at least one of the markers. Each marker is provided on the tail end of the leaf, or on an extension (306, 308, Fig. 3) extending therefrom, so that light emitted or reflected by the marker can be imaged directly by the cameras at positions away from the radiation beam. The known position of each marker relative to the tip of the leaf allows the position of each leaf to be determined. The positioning of the cameras away from the radiation beam allows the use of cheaper, off-the-shelf optical equipment that does not require shielding and that can be more easily replaced.

Description

Multi-leaf collimator camera arrangement

FIELD OF THE INVENTION

The present invention relates to a machine vision system.

BACKGROUND

Radiotherapeutic apparatus involves the production of a beam of ionising radiation, usually x-rays or a beam of electrons or other sub-atomic particles. This is directed towards a cancerous region of a patient, and adversely affects the tumour cells causing an alleviation of the patient's symptoms. The beam is delimited so that the radiation dose is maximised in the tumour cells and minimised in healthy cells of the patient, as this improves the efficiency of treatment and reduces the side effects suffered by a patient.

In a radiotherapy apparatus the beam can be delimited using a beam limiting device such as a 'multi-leaf collimator' (MLC). This is a collimator which consists of a large number of elongate thin leaves arranged side to side in an array. Each leaf is moveable longitudinally so that its tip can be extended into or withdrawn from the radiation field. The array of leaf tips can thus be positioned so as to define a variable edge to the collimator.

All the leaves can be withdrawn to open the radiation field, or all the leaves can be extended so as to close it down. Alternatively, some leaves can be withdrawn and some extended so as to define any desired shape, within operational limits. A multi-leaf collimator usually consists of two banks of such arrays, each bank projecting into the radiation field from opposite sides of the collimator.

SUMMARY

There is presented a collimation apparatus having a multi-leaf collimator and one or more cameras. At least one leaf of the multi-leaf collimator has a reflective or fluorescent marker which is fixedly attached to the leaf. The one or more cameras are positioned to directly image light from the markers. The disclosed system eliminates the requirement for an optical system to reflect the light from the marker to the camera. There is also presented a radiation head comprising a source of radiation and a collimation apparatus.

There is also provided a method for determining the position of a leaf of a multi-leaf collimator, comprising: directly receiving, at a plurality of cameras, light from a marker; sending, from each of the plurality of cameras to a processor, an image of the received light; combining, at the processor, the received images; determining, from the combined images, the location of the marker; calculating, using the known distance between the marker and the tip of the leaf of the multi-leaf collimator, the position of the tip of the leaf of the multi-leaf collimator.

Aspects and features of the present invention are set out in the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments are described below by way of example only and with reference to the accompanying drawings in which: Figure 1 shows a known positional readout arrangement; and Figure 2 shows a radiation head according to one embodiment; Figure 3 shows a radiation head according to a further embodiment; Figure 4 shows a radiation head according to an embodiment having a plurality of cameras.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An MLC typically comprises two banks of leaves facing each other across the radiation field, and actuation means configured to control the location of each leaf, and in particular to control the degree to which each leaf is extended into the radiation field.

The actuation means may comprise one or more leaf guides or rails, upon which a bank of leaves is mounted and the entire bank of leaves may be moved into or out from the radiation field to the same degree, as well as means such as lead screws for individually actuating each leaf. The action of the actuation means control the position of each leaf.

Accurately knowing the position of each leaf is particularly important in the field of radiotherapy, where it is imperative to know the characteristics of the dose of radiation delivered to a patient.

One example of a known positional sensor is fluorescent markers. Figure 1 is a schematic cross-section showing an example of a known arrangement. For simplicity of illustration, some mirrors have been omitted, so straight paths for some of the optical systems are shown. A fluorescing marker is one that accepts incident light energy and emits light at a different wavelength (or frequency). This differs from simple reflection or retro-reflection, in which the reflected light is of substantially the same wavelength as that which was incident.

The MLC leaves 100, 102 are illuminated by a light source 110. Each leaf has a ruby marker 118 at its tip. These ruby markers 118 move as the leaves 110, 102 are moved. Fiducial markers (not shown) are also placed at set positions on the MLC apparatus. The fiducial markers are not placed on leaves and do not move as the leaves are moved. The light source is tuned to cause the rubies to fluoresce in a wavelength different to that of the illuminating light. When illuminated with certain wavelengths of light, ruby crystals will fluoresce in the dark red/near infrared band -nominally 695nm. Thus, the ruby markers 118 will be illuminated by for example, a 410nm monochromatic source 110. This will cause the ruby to fluoresce, emitting light in a variety of directions including upwards, and is reflected out of the path of the beam and to a camera.

Light emitted from the markers is therefore emitted from a location in, or near, the beam. A camera cannot be in the radiation beam since it would block the radiation and also damage the camera. To image this light therefore, the light must be directed away from the radiation beam.

The camera 124 receiving the light from the ruby markers is positioned outside the beam to protect it from the harmful radiation. The camera has a filter which omits the light of the illuminating source and other reflected light.

Because the fluoresced light from the rubies has a different wavelength to the light from the illuminating source, suitable filters can be used to distinguish the fluoresced light and therefore determine the position of the markers. The location of the detected markers is the approximate location of the tip of the leaf.

Figure 2 shows a radiation head according to the present disclosure. Like components are shown with like reference numerals.

Figure 2 shoes a multi-leaf collimator having two banks of leaves, including leaves 200, 202. On each leaf in the MLC is a fluorescent marker 218, 219. In this example, the fluorescent marker is a ruby marker. The rubies are positioned at the tail end of the leaf. The tail end is the opposite longitudinal end of the leaf to the tip. Illumination sources 212, 214 are positioned above the leaves and are configured to emit illumination light having an illumination wavelength.

Cameras 224, 226 are positioned above the leaves 200, 202. There is a plurality of cameras positioned above or below the leaves on each bank of leaves. The cameras are positioned side-by-side along the width of the leaf bank (i.e. in a direction out of the page on Figure 2). Each camera has a field of view which encompasses fluorescent markers from a plurality of leaves.

The cameras have an infra-red pass filter. The filter is configured to filter out light having the wavelength of the illumination light, so that light having the wavelength of the illumination light does not enter the camera.

For each leaf and marker, the relative position of the florescent marker and the tip of the leaf is known. In the arrangement in Figure 2, the length of the leaf is known. The leaves can only move forwards and backwards along a single axis. Therefore, once the position of the marker is identified, the position of the tip of the leaf can be calculated.

A radiation source, 230 is configured to emit a beam of radiotherapeutic energy 232. The tips of the MLC leaves 200, 202 project into the radiation beam 232, to delimited (collimate) the beam. The leaves are made of, for example, tungsten which attenuates the radiation beam.

In use the MLC leaves are illuminated by an illumination source 212, 214. The illumination source has a frequency which will cause the rubies to fluoresce.

The rubies fluoresce and emit light in a variety of directions, including upwards towards the cameras 224, 226. Fluoresced light from the ruby markers is imaged by the camera. The illumination light is filtered out by a filter, and light form the rubies is imaged.

The cameras 224, 226 image the light emitted from the ruby markers directly. The fluoresced light is not reflected or redirected by a mirror in the beam path.

Since the rubies are positioned at the tail end of the leaves, the cameras can be positioned outside the path of radiation and image the light emitted from the rubies directly without the need for a mirror.

The cameras receive light from the rubies but not from the illumination sources 212, 214. The camera system can create an image of the ruby markers. Each camera images the light received from each of the rubies in the field of view of that camera.

The collimation of the beam is determined as follows. The cameras send information on the light detected at the cameras to a processor (not shown in Figure 2). At the processor the images are stitched together using known methods and are used to determine the location of each of the ruby markers. Since the relative position between the ruby marker and the tip of the leaf to which it is attached is known, the position of the tip of the leaf can be determined. The positions of the tips of the leaves of the MLC determines the edge presented to the radiation field and hence the amount of collimation of the radiation beam.

The tail of the leaf (the end opposite to the tip) is positioned further away from the centre of the beam of radiation than the tip of the leaf. By including the marker on the tail of the leaf, rather than on the tip of the leaf, the fluoresced light is emitted from a point further away from the centre of the beam. This means that it can be imaged directly by cameras which are positioned outside of the beam. There is no requirement for an angled mirror to reflect the light fluoresced from the rubies to the cameras.

Figure 3 shows a further embodiment in which cameras directly image markers of the leaves of a collimator. In the radiation head illustrated in Figure 3, the illumination sources and the radiation source are not shown. Leaves of the multileaf collimator 300, 302 each have an extension attached, shown as 308 and 306 respectively. The extension extends from the tail end of the leaf, although in other examples extends in a different direction. A fluorescent marker 318, 391 is attached to each extension. This is in contrast to being attached directly to the leaf as in the embodiment of Figure 2.

There is a bank of leaves and an array of cameras above each bank of leaves. Two leaves and two cameras are shown in Figure 3.

Each camera 326 and 324 images light directly from a marker 318 and 319. The relative distance between the markers 318 and 319 and the tips of the respective leaves 302 and 300 is known. Each camera system 326 and 324 can identify the position of the marker 318 and 319 which it has imaged. From the identified position of the marker, the position of the tip of the leaf can be calculated. Therefore the collimation of the beam can be determined. This can be done using the same method as in the embedment of Figure 2.

By positioning a marker on an extension to the leaf, the camera is thus positioned even further away from the beam and images light directly from the fluorescent marker. The extension can be made of a material different to the material of the leaf. Such materials are cheaper and lighter than the material used to make an MLC leaf (e.g. tungsten).

The radiation heads illustrated in Figures 2 and 3 include cameras which directly image light from markers.

This is possible since the marker is not located on the tip of the collimator leaf. That is, the marker has been decoupled from the tip of the leaf. The known displacement between the marker and the tip of the leaf is used to determine the location of the leaf.

The light is directly imaged in that it is not reflected between being emitted from the marker to the camera. In other words, there is a line-of-sight path between the camera and the marker. Light emitted from the marker can travel along the line-ofsight path to the camera.

The radiation head having camera which directly image light from the markers removes the need for an angled mirror to reflect the light from the markers to the cameras.

In the system of the present disclosure, the required "stack height" is reduced. The stack height is the height of the treatment head, or radiation head. Reducing the stack height has multiple possible advantages.

In many radiotherapy systems the radiation head is rotated on a gantry around the patient, so that radiation can be delivered from different angles to minimise the radiation dose to healthy tissue. If the stack height of the radiation head is reduced by /, the width and height of the circle "swept out" by the radiation head can each be each reduced by 2L Therefore, by reducing the stack height, the size of the room of the radiotherapy device can be reduced.

Further, in a system with a reduced stack height, the radiation source can be brought closer to the patient and the divergence of the beam at the patient can be reduced. For example, in a linac configured to produce a therapeutic X-ray beam by accelerating an electron beam toward a tungsten target, the tungsten target may be brought closer to the tumour / target region inside the patient.

Alternatively, in a system with a reduced treatment head stack height, the radiation source can be maintained at the original position and the distance between the patient and the closest point of the treatment head to the patient is increased, thus improving options of patient positioning.

Alternatively, the space previously occupied by the angled mirror can be used for other components. Accordingly, disclosed systems make better use of available space / real estate within the treatment head.

Because the beam diverges, things further from the beam, e.g. the MLC, need to be bigger if they are to sit across the entire radiation field. Thus, removing the dichroic mirror means that the more expensive components in the radiation head can be brought closer to the point at which the beam originates / the radiation source.

By providing a line-of-sight path between the marker and the camera such that the camera received light directly from the marker, there is no requirement for an optical system to reflect the light. Optical systems can be expensive, and require accurate positioning in order to reflect light to the camera system. Therefore eliminating the need for an optical reflective system is beneficial. Further, the light traveling directly from the marker to the camera to be imaged, without being reflected, reduced any image distortion compared to using an optical system to reflect the light.

Another advantage is that off-the shelf cameras can be used. Shielding and expensive radiation hardened camera systems are needed in the case that the camera is in or near the radiation beam. By moving the cameras away from the radiation beam as in the arrangement of the present disclosure, off-the shelf optical equipment can be used. Such cameras are cheaper and easier to replace than radiation hardened camera systems.

These advantages can be achieved by having the marker fixedly attached relative to the leaf. The marker does not have to be attached directly to the leaf. By being fixedly attached, movement of the leaf causes corresponding movement of the marker. Light from the marker is imaged. Because the position of the marker relative to the leaf is known, and because the marker is fixedly attached relative to the leaf, the position of the marker imaged by the camera can be used to determine the position of the leaf.

In the embodiment in Figure 2 the fluorescent markers are positioned at the tail end of the leaf. The tip of the leaf is the part of the leaf which collimates the beam of radiation. The tail end of the leaf is the opposing end of the leaf to the tip.

In other embodiments, the marker is positioned in the tail half of the leaf (i.e. closer to the tail than to the tip). In other embodiments, the marker is positioned on the tail of the leaf (i.e. at the outermost end). In other embodiments, the marker is positioned at any point along the leaf which is displaced from the tip of the leaf.

In the above embodiments the fluorescent marker is a ruby, however other fluorescent markers could be used. In one embodiment the fluorescent markers are ruby bearings.

In the above embodiments a fluorescent marker is used which is illuminated by a light having a wavelength used to stimulate fluorescence. The fluoresced light is then imaged by the camera.

In other embodiments a different type of marker is used. For example, a reflective marker could be used to reflect light directly to a camera. In this embodiment, the illumination source emits light of a given wavelength. The light is reflected from the reflective marker and imaged by the camera system. The light emitted by the illumination source in this embodiment is not used to stimulate florescence of a marker, therefore the light could be of any wavelength (i.e. is not limited to a wavelength which will stimulate fluorescence). The camera detects light of the given wavelength.

In the radiation heads described herein any number of cameras may be used. A single camera may be used, or more than one camera may be used. There may be a single camera positioned to image each bank of leaves. In one embodiment, there is an array of cameras aligned above each bank of leaves and positioned to image each bank of leaves.

A group of cameras positioned around the beam may be used. In this latter implementation, a camera rig may be attachable to the linac treatment head accessory mount. The camera rig is configured to hold one or more cameras and is positioned below the primary collimator in the stack. Each of the one or more cameras is arranged such that they can image the MLC leaves.

The light detected by the cameras is communicated to a control unit. The control unit identifies the position of the leaves from: the detected position of the markers, the known distance between the markers and the leaves, and the known distance between the cameras and the leaves.

Determining the collimation of the beam is performed on a processor having a computer readable medium which, when processed by the processor, causes the processor to perform the method of determining the collimation.

In another embodiment the cameras image the leaves directly.

In another embodiment, non radiation-hard camera, such as CMOS or mobile phone type cameras, can be used to image the leaves. Such cameras have a limited life compared to a radiation hard camera, but are much simpler and cheaper to produce. Such cameras could be used to image the markers on the leaves or to view the leaves directly. For example, such cameras may be positioned in or close to the path of the beam.

Since non-radiation hardened-cameras will more quickly degrade due to the radiation, a housing for a plurality of cameras, such as a carousel, could be used to provide a simple way to replace a camera without requiring extended machine downtime.

Figure 4a shows a collimation system according to the present disclosure. A leaf of an mlc 310 collimates a radiation beam 332. The leaf is imaged by a camera 326a which is one of a plurality of cameras including camera located on a carousel 98. The carousel 98 is shown from above in Figure 4b.

Camera 326a is positioned to directly image the leaf or a marker on the leaf. The camera 326 is positioned near to the radiation beam 332. The radiation beam will cause cameras to degrade over time. In some embodiments the camera is positioned in the beam.

A carousel of cameras is used to provide a simple solution to handling the degrading of a camera in or close to the beam. The carousel 98 houses the plurality of cameras 326. The carousel is rotatable about an axis 98a. Camera 96a is positioned in the imaging position. The imaging position is the position at which a camera images the mlc. The imaging position is close to the radiation path and exposed to scattered radiation. The remaining cameras 96 that are not in the imaging position are positioned further away from the radiation beam and shielded from scattered radiation. The camera in imaging position will more quickly degrade from exposure to radiation than the cameras outside the imaging position.

The camera 96a images the mlc leaf positions. The camera 96a will deteriorate from exposure to radiation. When the camera has deteriorated beyond use or to a low quality, the carousel 98 can be rotated about axis 98a as shown by the arrow in Figure 9b and can move the camera 96a out of the imaging position and move the adjacent camera 96b into the imaging position. Camera 96b will not have previously been positioned in imaging position and therefore will not have experienced the deterioration of camera 96a and will be useable to image the m lc. Camera 96b can then be used to image the mlc until it too deteriorates due to exposure to the radiation.

After camera 98b has deteriorated beyond use, the carousel is rotated again to position the next camera in the imaging position. This process can be repeated until all of the cameras 96 have been used for imaging. After this time, all of the cameras will be damaged, and the carousel is removed from the system. The cameras 96 are removed from the carousel, a new set of cameras are housed in the carousel and the carousel is re-inserted into the system. Alternatively, a new carousel housing new cameras is inserted into the imager.

Cameras are increasingly low in cost and are easily replaceable. Further, the carousel and/or the detector can be replaceable, resulting in an increased lifespan of the system as a whole. The carousel provides a convenient and quick mechanism to move cameras into the imaging area.

Instead of a carousel, a different housing could be used. Some examples are given below, but any means for moving detectors into an imaging position one by one to directly image m lc leaves or markers on mlc leaves can be used.

A belt, chain or magazine comprising a plurality of detectors could be used. The belt, chain or magazine could be movable or slidable, horizontally or vertically. The belt is moved along a place again to position the next detector in the imaging position. The belt may be a continuous loop.

A bank holding the plurality of detectors could be used. The bank comprises an actuation mechanism to move the plurality of detectors in turn into the imaging position. The actuation means is may comprise an arm, for example the arm may be extendable, rotatable or otherwise moveable. The extendable arm is configured to push the respective detectors forwards into the imaging position and retract backwards to allow the detector to return into the bank.

In any of the examples, a radiation shield can be placed above the detectors not in the imaging position to help shield them from scattered radiation. The shield may have a hole overlaying the imaging position, wherein the hole is configured to allow the light to reach the detector in the imaging position. The shield is made from a high density material. Typically, the higher the density of material used the less thickness you need to absorb the radiation. Good materials for radiation shielding include cast iron, lead and tungsten.

The movement of the plurality of detectors can be triggered by a command which, when received, the first detector is moved out of the imaging position and a second detector is moved into the imaging position. This process can be repeated until all the detectors have been used for imaging.

The command may be triggered by a predetermined time interval, for example when a detector has been in the imaging position for a predetermined amount of time. The predetermined time interval may be in the region of expected lifetime of the detector or may be shorter, so the detectors can be moved more often and therefore all break at a similar time and can then be replaced together.

The command may be triggered when a detector in the imaging position has received a predetermined amount of radiation. This can be measured by detecting the radiation received at the detector. Another way to trigger the command to move the plurality of detectors is when there is a deterioration in the image quality. The command may be triggered manually by an operator.

The detectors can be replaced. The entire carousel can be ejected from the radiotherapy device and replaced, or the detectors may be replaced in the carousel which is then reinserted. The carousel can be moved to a replacement position in which each detector can be replaced in turn.

The downtime of the mlc when a detector is broken as a replacement detector can be used whilst the damaged detector (or set of detectors) is reduced. By using a plurality of detectors it is possible to use cheaper cameras. Typically, expensive radiation resistant detectors are used in a radiation environment, however, by alternating the detectors and having a supply of working detectors that haven't been damaged it is possible to use non-radiation resistant detectors. These non-radiation resistant detectors are typically cheaper and produce a better image quality. The carousel allows these cheaper detectors to be used in turn when each one breaks.

There is presented a collimation apparatus for collimating a beam of radiation. There is also presented a radiation apparatus comprising a collimation apparatus.

Features of the above aspects can be combined in any suitable manner. It will be understood that the above description is of specific embodiments by way of aspect only and that many modifications and alterations will be within the skilled person's reach and are intended to be covered by the scope of the appendant claims.

Claims

Claims 1. A collimation apparatus for collimating a beam of radiation, the collimation apparatus comprising: a multi-leaf collimator comprising a plurality of leaves being extendable into the path of the radiation beam; and at least one fluorescent or reflective marker fixedly attached with respect to one of the leaves; and a camera arranged to directly view light emitted or reflected from the marker.
2. A collimation apparatus according to claim 1 in which the marker is positioned on the tail end of the leaf.
3. A collimation apparatus according to claim 1, in which the marker is positioned on an extension which extends from the tail end of the leaf.
4. A collimation apparatus according to any preceding claim, wherein movement of the leaf causes corresponding movement of the marker.
5. A collimation apparatus according to any preceding claim wherein the camera is arranged with a line-of-sight path to the marker.
6. A collimation apparatus according to any preceding claim, wherein substantially all the leaves of the collimator have a marker.
7. A collimation apparatus according to claim 6, comprising a plurality of cameras, each camera arranged to directly view light emitted from at least one of the markers.
8. A collimation apparatus according to any preceding claim, further comprising a light source, configured to illuminate the marker.
9. A collimation apparatus according to claim 8, wherein the marker is a fluorescent marker and wherein the illumination source is configured to generate light having a first wavelength for stimulating fluorescence in the fluorescent marker.
10.A collimation apparatus according to claim 9 in which the source of illumination is a diffuse source.
11.A collimation apparatus according to claim 9 or claim 10 wherein the fluorescent marker is arranged to emit light of a wavelength longer than the incident light.
12. A collimation apparatus according to any of claims 9 to 11, wherein the camera has a filter arranged to substantially prevent light of the wavelength emitted by the source of illumination from entering the camera.
13.A collimation apparatus according to any preceding claim wherein the marker is a fluorescent marker and wherein the marker comprises ruby.
14. A radiation head comprising a source of radiation and a collimation apparatus according to any of claims 1 to 13.
15. A multi-leaf collimator comprising: a plurality of leaves being extendable into the path of the radiation beam; and at least one fluorescent or reflective marker fixedly attached with respect to one of the leaves, wherein the marker is attached to the tail portion of the leaf or to an extension which extends from the tail end of the leaf.
16. A method for determining the position of a leaf of a multi-leaf collimator, comprising: directly receiving, at a plurality of cameras, light from a marker; sending, from each of the plurality of cameras to a processor, an image of the received light; combining, at the processor, the received images; determining, from the combined images, the location of the marker; calculating, using the known distance between the marker and the tip of the leaf of the multi-leaf collimator, the position of the tip of the leaf of the multi-leaf collimator.