U.S. patent application number 12/362664 was filed with the patent office on 2009-08-06 for machine vision system.
This patent application is currently assigned to Elekta AB (publ). Invention is credited to Shakil Ahmed Awan, Adrian Maxwell Smith.
Application Number | 20090196401 12/362664 |
Document ID | / |
Family ID | 39204093 |
Filed Date | 2009-08-06 |
United States Patent
Application |
20090196401 |
Kind Code |
A1 |
Awan; Shakil Ahmed ; et
al. |
August 6, 2009 |
Machine vision system
Abstract
A fluorescing marker is used in order to mark (for example) a
leaf of a multi-leaf collimator and/or the reference points within
the field of view. The markers are illuminated with light tuned to
cause the markers to fluoresce at a wavelength different to that of
the illuminating light. The fluorescence is then detected by a
camera. This method allows the image to be captured by the camera
with increased contrast. Accordingly, the present invention
provides a multi-leaf collimator for a radiotherapeutic apparatus,
comprising at least one leaf having a fluorescent marker. The
fluorescent marker will usually emit light of a wavelength longer
than the incident light, allowing suitable filters to be provided
in order to distinguish the light emitted by the markers. A
suitable material for use in the fluorescent markers is ruby. The
present invention also provides a radiotherapeutic apparatus
comprising a multi-leaf collimator as defined above, and a camera
arranged to view the fluorescent markers. A source of illumination
for the fluorescent markers is ideally monochromatic, or nearly so.
The camera can have a filter arranged to substantially prevent
light of the wavelength emitted by the source of illumination from
entering the camera, thereby improving the contrast of the image.
The radiotherapeutic apparatus can also comprise a source of
illumination that is optically co-located with a radiation source,
to allow the radiation field that will be emitted to be checked
visually by an operator. The co-located source is preferably
substantially monochromatic, emitting substantially no light at the
wavelength of the fluorescent markers. A filter can then be placed
over an output of the radiotherapeutic apparatus, for blocking
light of the wavelength of the fluorescent markers and thereby
enhancing the contrast of the image that is taken of the
fluorescent markers.
Inventors: |
Awan; Shakil Ahmed;
(Crawley, GB) ; Smith; Adrian Maxwell; (London,
GB) |
Correspondence
Address: |
WESTMAN CHAMPLIN & KELLY, P.A.
SUITE 1400, 900 SECOND AVENUE SOUTH
MINNEAPOLIS
MN
55402
US
|
Assignee: |
Elekta AB (publ)
Stockholm
SE
|
Family ID: |
39204093 |
Appl. No.: |
12/362664 |
Filed: |
January 30, 2009 |
Current U.S.
Class: |
378/150 |
Current CPC
Class: |
A61N 5/1042 20130101;
A61B 2090/3941 20160201; A61N 5/1049 20130101; G21K 1/04 20130101;
G06T 2207/30204 20130101; H04N 5/2256 20130101; A61N 5/1045
20130101; G06T 7/0012 20130101; A61N 2005/1059 20130101; A61N
5/1048 20130101; G21K 1/046 20130101; G06T 2207/10064 20130101 |
Class at
Publication: |
378/150 |
International
Class: |
G21K 1/04 20060101
G21K001/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 1, 2008 |
GB |
0801895.4 |
Claims
1. A leaf for a multi-leaf collimator for a radiotherapeutic
apparatus, having a fluorescent marker.
2. A multi-leaf collimator for a radiotherapeutic apparatus,
comprising at least one leaf having a fluorescent marker.
3. The multi-leaf collimator according to claim 2 in which
substantially all the leaves of the collimator have a fluorescent
marker.
4. The multi-leaf collimator according to claim 2 in which at least
one leaf has a plurality of markers.
5. The multi-leaf collimator according to claim 4 in which a
plurality of leaves have a plurality of markers, each leaf carrying
the markers in a configuration that is different to the
configuration of the other leaves.
6. The multi-leaf collimator according to claim 2 in which the
leaves are mounted on a frame, and the frame has at least one
fluorescent marker.
7. The multi-leaf collimator according to claim 6 in which the
frame has a plurality of fluorescent markers which collectively
indicate a maximum field of view of the collimator.
8. The multi-leaf collimator according to claim 2 in which the
fluorescent marker is arranged to emit light of a wavelength longer
than the incident light.
9. The multi-leaf collimator according to claim 2 in which the
fluorescent marker comprises ruby.
10. The multi-leaf collimator according to claim 2 in which the
fluorescent marker is spherical.
11. The multi-leaf collimator according to claim 2 in which the
fluorescent marker is cylindrical.
12. A radiotherapeutic apparatus comprising the multi-leaf
collimator according to claim 2, the apparatus further comprising a
camera arranged to view the fluorescent markers.
13. The radiotherapeutic apparatus according to claim 12 further
comprising a source of illumination for the fluorescent
markers.
14. The radiotherapeutic apparatus according to claim 13 in which
the camera has a filter arranged to substantially prevent light of
the wavelength emitted by the source of illumination from entering
the camera.
15. The radiotherapeutic apparatus according to claim 13, further
comprising a radiation source, wherein the source of illumination
is optically co-located with the radiation source.
16. The radiotherapeutic apparatus according to claim 15 in which
the co-located source of illumination is substantially
monochromatic.
17. The radiotherapeutic apparatus according to claim 15 in which
the co-located source of illumination is a point source.
18. The radiotherapeutic apparatus according to claim 15 in which
the co-located source emits substantially no light at the
wavelength of the fluorescent markers.
19. The radiotherapeutic apparatus according to claim 15 comprising
a filter over an output thereof for blocking light of the
wavelength of the fluorescent markers.
20. An apparatus comprising at least one moveable element to which
is attached a fluorescent marker, a source of illumination at an
excitation frequency for the fluorescent marker, and a camera
arranged to view the moveable element and capable of sensing light
at the fluorescence frequency of the marker.
21. The apparatus according to claim 20 in which the fluorescent
marker comprises ruby.
22. The apparatus according to claim 20 in which the camera has a
filter arranged to substantially prevent light of the wavelength
emitted by the source of illumination from entering the camera.
23. The apparatus according to claim 20 in which the source is
substantially monochromatic.
24. The apparatus according to claim 23 in which the source emits
substantially no light at the wavelength of the fluorescent
markers.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a machine vision system. It
provides a solution to problems that arise in machine vision
applications in hostile or otherwise difficult environments, such
as the interior of a radiotherapeutic apparatus.
BACKGROUND ART
[0002] 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 the patient, and adversely affects the tumour cells
causing an alleviation of the patient's symptoms. Generally, it is
preferred to delimit the radiation beam so that the 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. A variety of methods of
doing so have evolved.
[0003] One principal component in delimiting the radiation dose is
the so-called "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.
[0004] It will of course be necessary to monitor the current actual
position of the leaves in order to provide feedback and allow their
position to be adjusted accurately. To date, two main methodologies
have been employed in order to do so, namely; [0005] Optical vision
position sensing [0006] Traditional positional sensing, for example
potentiometers, encoders etc
[0007] The approach to the problem of accurate leaf positional
readout adopted to date by the applicant is outlined in FIG. 1. The
solution is based on a vision system whereby a camera system
"views" 84 different reflectors. 4 of these reflectors are
reference markers, one in each corner of the viewable area, and 80
of these mark the individual position of each leaf of the two
opposing banks of 40 leaves. The position of each leaf can
therefore be calculated. The reflector is one having
retro-reflective properties, i.e. light is reflected back along the
same path as the incident light.
[0008] Thus, a camera 10 views the collimator leaves 12, 14, via a
pair of tilt-adjustable mirrors 16, 18 which permit the camera to
be located out of the radiation beam. A beam splitter 20 is placed
in the optical path (between the two mirrors 16, 18 so that it is
also out of the radiation beam) to allow a light projector 22 to
illuminate the collimator leaves 12, 14 along the same optical
path. A further mirror or mirrors 24 can be provided so as to
locate the light projector (and/or other elements) in convenient
locations.
[0009] Others tend to utilise traditional measurement
methodologies. This involves measuring the position of each leaf by
an individual sensor. A common design requirement is fault
tolerance, or "single fault tolerance", which implies that in order
to assure the correct position of a single leaf, a secondary or
back-up sensor must be used. This therefore doubles the number of
position sensors required.
[0010] Various problems exist with both approaches. Optical
methodologies and other current machine vision solutions require a
high uniformity of illumination, which leads to difficulty in the
recognition of valid reflectors. The leaf reflector material has
only a limited lifetime, due to dirt and surface damage. The
reflector material must be mounted with a high degree of accuracy.
Stray light and/or stray reflections from internal reflections
and/or reflections off a treatment table top can confuse the
system. Finally, the retro-reflective properties of the leaf
markers require the light source and the camera position to be in
an optically identical location, to very tight tolerances,
otherwise the shape and apparent brightness of the marker
changes.
[0011] Traditional position measurement methodologies also suffer
from serious difficulties. In particular, a very large number of
sensors is required--a minimum two sensors per leaf to provide one
primary readback and one backup readback. The degree of accuracy
required and the quantity of sensors used conspire to mean that the
system as a whole has a generally low degree of reliability.
Further, there are difficulties in packaging the required quantity
of sensors in a sufficiently compact design, and the sensors suffer
from potentially poor reliability due to the radiation damage that
inevitably results from their field of use.
SUMMARY OF THE INVENTION
[0012] The invention involves the use of a fluorescing marker in
order to mark a leaf and/or the reference points within the field
of view. The markers are illuminated with light tuned to cause the
markers to fluoresce at a wavelength different to that of the
illuminating light. The fluorescence is then detected by a camera.
This method allows the image to be captured by the camera with
increased contrast.
[0013] 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.
[0014] Accordingly, the present invention provides a multi-leaf
collimator for a radiotherapeutic apparatus, comprising at least
one leaf having a fluorescent marker.
[0015] It is naturally preferred that substantially all the leaves
of the collimator have a fluorescent marker. However, it is not
inconceivable that a mixed system could be provided.
[0016] The leaves will typically be mounted in a frame of some
sort. It will usually be advantageous for the frame itself to have
one or more fluorescent markers, preferably several so as to
collectively indicate a maximum field of view of the collimator and
provide a frame reference.
[0017] The fluorescent marker will usually emit light of a
wavelength longer than the incident light, allowing suitable
filters to be provided in order to distinguish the light emitted by
the markers. A suitable material for use in the fluorescent markers
is ruby. This can be in a spherical or a cylindrical shape, or
another suitable shape. Several markers could of course be provided
on a single leaf or leaves, ideally in different configurations so
as to achieve greater accuracy and/or robustness of
identification.
[0018] The present invention also provides a radiotherapeutic
apparatus comprising a multi-leaf collimator as defined above, and
a camera arranged to view the fluorescent markers. A source of
illumination for the fluorescent markers will also be useful and is
ideally monochromatic or nearly so. The camera can have a filter
arranged to substantially prevent light of the wavelength emitted
by the source of illumination from entering the camera, thereby
improving the contrast of the image.
[0019] The radiotherapeutic apparatus can also comprise a source of
illumination that is optically co-located with a radiation source,
to allow the radiation field that is being emitted or will be
emitted to be checked visually by an operator. The co-located
source is preferably substantially monochromatic, emitting
substantially no light at the wavelength of the fluorescent
markers. A filter can then be placed over an output of the
radiotherapeutic apparatus, for blocking light of the wavelength of
the fluorescent markers and thereby enhancing the contrast of the
image that is taken of the fluorescent markers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] An embodiment of the present invention will now be described
by way of example, with reference to the accompanying figures in
which;
[0021] FIG. 1, described above, shows a known positional readout
arrangement; and
[0022] FIG. 2 shows a positional readout arrangement according to
the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0023] The present invention will be described in relation to
sensing the position of a large number of MLC leaves within a
radiotherapeutic apparatus. As noted above, these leaves operate
within a harsh environment in terms of the ionising radiation that
is deliberately created within the radiotherapy head and into which
they must project in order to carry out their function. This
harshness presents difficulties in the provision of a machine
vision solution to leaf monitoring that is stable and reliable in
the long term. The solution adopted in relation to the MLC leaves
is of course applicable in other situations, particularly (but not
exclusively) environments that are harsh or which present
difficulties in distinguishing illuminated markers.
[0024] FIG. 2 shows an example of the present invention. For
simplicity of illustration, the mylar mirror 18 of FIG. 1 has been
omitted, so straight paths for some of the optical systems are
shown. In practice, this mirror will of course be provided so that
sensitive components are removed from the radiation beam and so
that components can be placed in convenient locations in the
radiation head. The mirrors 16, 24 of FIG. 1 are no longer needed,
which leads to a simpler, cheaper and potentially more accurate
solution.
[0025] Thus, the MLC leaves 100, 102 are illuminated by two
difference sources. The first is a green field light projector 104
which emits a beam of green light 106 that covers the entire field,
in the form of a point source to allow sharp leaf definition. This
light is collimated by the leaves 100, 102 (and any other
collimators that may be present) and is allowed out of the head by
a green pass mylar screen 108. This light therefore falls onto the
treatment table or a patient thereon and allows the operation of
the multi-leaf collimator to be verified by an operator.
[0026] The second light source is a 410 nm monochromatic source
110, in the form of a diffuse source to provide good machine vision
characteristics. In this example, it comprises an array of 410 nm
light-emitting diodes 112, 114 which provide a diffuse high
intensity light source 116. This light is blocked by the green pass
filter 108 and therefore does not fall on the treatment table or
patient; as a result it does not confuse the shape of the projected
light field and the source therefore does not need to be optically
co-located with the radiation source. It can therefore be disposed
so as to permit a bright and even illumination of the leaves 100,
102.
[0027] Each leaf has at least one ruby marker 118. When illuminated
with certain wavelengths of light, Ruby crystals will fluoresce in
the dark red\near infra red band--nominally 695 nm. There are a
number of so-called "pump" wavelengths which can be used to
stimulate this behaviour, specifically 525 nm in the green band and
410 nm in the violet/near ultra violet band.
[0028] Thus, the ruby markers 118 will be illuminated by the 410 nm
monochromatic source 110, and also by the green field light
projector 104 which, at 530 nm or so, may be sufficiently close to
the 525 nm excitation wavelength of the ruby material. This will
cause the ruby to fluoresce, emitting light in a variety of
directions including upwards at 120 and onto a dichroic
beamsplitter 122 which diverts a proportion of the light to a
camera 124 protected by an infra-red pass filter 126. This IR pass
filter 126 will limit the light incident on the camera 124 to that
emitted by the fluorescent ruby markers 118, provided that there
are no other sources of light at this wavelength in the radiation
head.
[0029] Fluorescence can be increased by coating the rear surface of
the ruby marker with a mirror. The mirror will reflect transmitted
light back through the ruby, causing it to fluoresce more.
[0030] A particular advantage of this invention is the elimination
of reflectors. In the confined and harsh environment of an MLC
head, these impose limitations on the performance and longevity of
an optical sensing system. Accumulation of dust and dirt requires
the markers to be cleaned regularly to maintain correct function.
However, regular cleaning can itself degrade the optical
performance of the reflectors, while operating conditions can
reduce the effectiveness of the adhesive that holds them in place
leading to the loss of reflectors.
[0031] Also, the use of retro-reflectors imposes further
restrictions on the positioning of the light source and camera. In
confined spaces the problems caused by this restriction can be
considerable. As noted above, the present invention avoids such
limitations.
[0032] Using a fluorescing marker such as ruby can help overcome
these problems. Firstly, ruby is a member of the corundum family
and is therefore very hard and easily able to withstand heavy
industrial environments. Regular cleaning will not impact the
performance of the markers. Secondly, the use of one excitation
wavelength but the detection of another allows extraneous noise to
be filtered out of the system.
[0033] Thirdly, the position of the light source and camera can now
be independent of each other, and (for the MLC) the patient
illumination and machine vision illumination are now also
independent of each other.
[0034] So, by illuminating the leaves marked with ruby reflectors
with a 410 nm or 525 nm rich light source the ruby markers will
radiate red light which may be extracted from the background
illumination by utilising a near infra red filter on a video
camera. In addition, by filtering external light exiting the head
and entering the head, the effect of external lighting effects may
be minimised.
[0035] The leaves may be marked utilising ruby bearings. Such
bearings are readily available commercially and have extremely good
dimensional accuracy. This dimensional accuracy, consistency and
stability allows the bearings to be mounted accurately by counter
sinking or counter boring a hole smaller than the size of the
bearing. The tolerance of mounting is therefore potentially the
tolerance of the counter bore machining process, which compares
favourably to reflectors attached via adhesive.
[0036] The system configuration is similar to our original optical
system, albeit with several major differences:
[0037] 1) The retro reflective material would be replaced with the
ruby marker, placed using an accurate counter bore.
[0038] 2) The light projector would be modified to provide two
light sources; a 525-530 nm source for patient setup, and a 410 nm
source to stimulate fluorescence in the ruby markers. This light
source may also be supplemented with a red stop optical filter to
remove contamination in the band being sensed, i.e. >600 nm.
[0039] 3) The camera would be fitted with an infra red pass filter
which ideally passes the 695 nm light produced by the fluorescence.
This may be further optimised by the use of a band pass filter
centred on 695 nm.
[0040] 4) The external Mylar screen is optically band pass filtered
to allow only the wavelength of light through that is being used
for patient illumination. This should ideally pass no light over
600 nm, for maximum signal to noise ratio.
[0041] Potential advantages include an increase in leaf positioning
accuracy, an increase in reflector life (in that the marker should
be easier to clean and maintain), a decrease in assembly time due
to the easier and more precise placement of the reflector, an
increased tolerance of internal stray reflections since the light
being sensed is a different wavelength to the illumination, and an
increased tolerance to external light interference. Further, should
it prove necessary to replace a marker, it might be feasible to do
so with no recalibration due to the precise mounting method of the
marker.
[0042] 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.
* * * * *