Description
A system for patient positioning and a method thereof
The present invention relates to a system and a method for
positioning of a patient during a medical analysis or
treatment, particularly in radiation therapy.
Accurate and precise patient positioning for stereotactic
radio-surgery and in the case of radiation therapy like
Intensity-Modulated Radiation Therapy (IMRT) is very
critical. One among the few methods practiced today, involves
using a cage like structure bolted to the patient's head. By
bolting the cage to the treatment table and tightening the
screws on the frame as required, the patient position is kept
absolutely fixed during treatment. This method can cause
significant discomfort to the patient undergoing treatment.
Another approach, proposed relies on tracking fiducials using
a pair of camera. The fiducials comprise infrared LEDs
mounted on a frame attached to a bite block. The fiducials
are tracked using a pair of ceiling mounted IR cameras. There
are two downsides to the approach however: (a) A pair of
infra-red cameras is required for tracking purposes. These
can be expensive, b) The distance of the target from the
cameras is approximately two meters. The accuracy of 3D
position estimation is inversely proportional to the distance
of the target from the camera.
It is an object of the present invention to provide a low
cost system and method for patient positioning in radiation
therapy.
The above object is achieved by a structure for patient
positioning which comprises at least one surface having a
two-dimensional pattern and a holding means to place the said

at least one surface having the two-dimensional pattern at a
fixed position with respect to the head of the patient.
The underlying idea of the present invention is to use a
novel structure, to detect the patient position. A two-
dimensional pattern in the structure is used to track the
position of the patient. The two-dimensional pattern which is
associated with a surface is placed at a fixed position with
respect to the head of the patient. The image of the two-
dimensional pattern is captured using an optical camera,
which is less expensive. The patient position is finally,
detected using the positional changes in the two-dimensional
pattern.
In a preferred embodiment of the invention, two-dimensional
pattern is a cornered pattern. The reason to pick this
pattern is that, when relying on visual camera processing,
corners with sub pixel accuracy can be obtained despite
variable illumination conditions.
In a further preferred embodiment, the cornered pattern is a
checkerboard cornered pattern. The checkerboard corner
pattern increase the accuracy of detection during patient
position.
In a further preferred embodiment, the holding means is a
mouth bite. The mouth bite could be treated as a simple and
inexpensive way to achieve the objective of placing the two-
dimensional pattern at a fixed position with respect to the
head.
In a further preferred embodiment, said mouth bite is
associated with a profile, said profile allowing said mouth
bite to be rigidly fixed in the mouth of the patient. This
allows the mouth bite to be easily and rigidly attached to
the mouth of the patient, so that any change in patient
position is reflected in the structure. The mouth bite avoids
a bolted structure which is uncomfortable to the patient.

The above object is further achieved by a system for patient
positioning comprising a structure and at least one camera
positioned to capture the two-dimensional pattern on the at
least one surface of the structure.
The underlying idea is to capture the two-dimensional pattern
associated with the structure using one or more camera and
detect the patient position by analyzing the change in
position of the two-dimensional pattern by comparing the
images captured by the said camera.
In a preferred embodiment of the invention, the system
further comprises two cameras to capture the two-dimensional
pattern associated with the two surfaces in said structure.
There is high possibility that the area of the vision
associated with the camera gets obstructed by the movement of
the gantry of the machine used for radiation therapy. So if
one camera is blocked, the other one could be used for
detecting patient position. Improvement in the accuracy could
be guaranteed further by using both the cameras
simultaneously if needed.
In an alternative embodiment, the cameras are attached to a
patient platform. In an alternative embodiment, the cameras
are positioned at a distance of less than 1 meter from the
said structure. This ensures that the cameras are placed
close to the patient to capture the images of the two-
dimensional pattern enabling accurate patient position
detection.
The above object is achieved by a method using a structure
for patient positioning, comprising the steps of:
-aligning the structure, at a fixed position with respect to
the head of the patient;
-capturing a plurality of images of the two-dimensional
pattern on at least one surface by at least one camera; and
-processing the plurality of images to determine the position
of the patient.

The idea is to capture and analyze camera images of a two-
dimensional pattern, and determine the patient position.
In a preferred embodiment of the invention, capturing a
plurality of images further comprise the steps of capturing a
reference camera image and periodic camera images. This
further enables the comparison of these images to detect a
change in the patient position.
In an alternative embodiment, processing the plurality of
images further comprises the step of detecting the shift in
position of the two-dimensional pattern in the reference
image and a periodic camera image. The shift translates to
the actual movement of the patient; hence detecting this is a
key factor in patient positioning.
In an alternative embodiment, the method further comprises
the step of finding position of the radio opaque elements in
an X-ray based imaging system. This allows in relating the
position of the patient with respect to the camera to its
corresponding position determined based in the X-ray imaging
system.
In an alternative embodiment, the method further comprises
the step of mapping the shift in position of the at least one
surface with the two-dimensional pattern in the reference
image and a periodic camera image to the X-ray based imaging
system using the offset information between the two-
dimensional pattern and the radio opaque elements in the
structure.
In an alternative embodiment, the method further comprises
the step of controlling a radiation treatment based on the
position of the patient.
In an alternative embodiment, controlling a radiation
treatment further comprise the step of stopping a radiation

In an alternative embodiment, said structure is made of
acrylic. Acrylic has got advantages of being inexpensive and
light in weight. Moreover acrylic is not radio opaque
allowing the detection of the presence of any other radio
opaque elements present in any acrylic structure in any X-ray
based images.
In an alternative embodiment, said structure further
comprises of two surfaces having said two-dimensional pattern
held by the holding means. This allows the detection of
patient position from different angles. For example by
capturing images by two cameras from two sides or any one of
the two sides. This also allows more flexibility and freedom
to the radiologist while doing any procedure or administering
any treatment.
In an alternative embodiment, said two surfaces are
positioned to form a shape of an inverted hut. Providing such
a shape helps in a balanced structure which the patient can
hold in the mouth. Further this helps in placing devices to
detect the two-dimensional pattern conveniently close to the
patient platform. In said embodiment, the pattern is located
at the lower side of the surfaces forming the two sides of
the hut. This enables to detect the two-dimensional pattern
using a camera placed at a lower elevation from the patient
head.
In an alternative embodiment, the structure further comprises
of radio opaque elements positioned at a fixed offset from
the two-dimensional pattern. This allows in determining the
position of the radio opaque elements in an X-ray or CT image
taken when the patient holds the structure and further helps
in mapping the change in the position in the two-dimensional
pattern due to the change in patient position to the X-ray or
CT imaging system.

treatment when the position of the patient has shifted more
from a set threshold value.
The present invention is further described hereinafter with
reference to illustrated embodiments shown in the
accompanying drawings, in which:
FIG 1 is an axial view of the structure used for patient
positioning along with an enlarged view of a surface with a
two dimensional pattern,
FIG 2 illustrates an embodiment of the invention showing a
system for patient positioning comprising the structure along
with camera attached to the patient platform, and
FIG 3 is a flowchart illustrating a method using a structure
for patient positioning in radiation therapy.
FIG 1 is an axial view of a structure 100 used for patient
positioning, this also shows an enlarged view of a surface
with a two dimensional pattern 120. The structure 100 is
designed in the form of an inverted hut. The corner of the
virtual roof formed by the intersection of the two surfaces
of the structure defines the origin of a coordinate system
formed by the structure. The inverted hut shape enables the
two-dimensional pattern to be located at the lower side of
the surfaces and further enabling the detection of the two-
dimensional pattern using a camera placed at a lower
elevation from the patient head.
The structure shown here comprises of surfaces 140, 150
having two-dimensional pattern 120. The surface 150 as
enlarged shows a checker board pattern and plurality of radio
opaque elements 170 positioned below said checkerboard
pattern.
The radio opaque elements could be rods (for example lead
rods). The arrangement or positioning of the radio opaque

elements 170 is not restricted below the two-dimensional
pattern 120. The said elements could be arranged anywhere in
the structure in different orientations, so as to make a
fixed offset with respect to the two-dimensional pattern. The
inverted hut shape of the structure enables capturing images
of the two-dimensional pattern for patient positioning from a
closer location (for example from a patient platform/table).
The structure 100 further comprises of a holding means 130 to
place the surfaces 140, 150 having the two-dimensional
pattern 120 at a fixed position with respect to the head of
the patient. The holding means 130 is a mouth bite, where
said mouth bite is associated with a profile 160, said
profile 160 allowing said mouth bite to be rigidly fixed in
the mouth of the patient. By doing this any change in patient
position is reflected in the two-dimensional pattern 120 of
the structure.
The two-dimensional pattern 120 is a cornered pattern. The
reason to pick this pattern is that this gives sub pixel
accuracy despite variable illumination conditions during
visual camera processing. The two-dimensional pattern can be
any type of cornered pattern, for example it might be a
triangular, square or any other polygonal structure.
Generally any two-dimensional pattern could be used whose
corners are easily detectable. FIG 1 reflects a checkerboard
pattern. In checkerboard, the corners are well defined and
distinguishable and it has got a known spacing of the
squares. The structure 100 could be made of any non radio
opaque element, for example acrylic. Acrylic is not radio
opaque allowing the detection of the presence of any other
radio opaque elements present in the said acrylic structure
in X-ray based images.
US Patent Number 5,525,883 propose an apparatus and method
for using specially designed landmarks to provide the
position and orientation in three dimensions of an autonomous
vehicle, such as a mobile robot. The landmarks are

checkerboards of rectangular cells of contrasting colors that
are arranged to create a pseudo-random pattern. The
autonomous vehicle carries a recognition and measurement
unit, for example including a camera and a digital computer
that processes the output of the camera. A map stored in the
recognition and measurement unit describes the position,
orientation and dimensions of each pseudo-random checkerboard
pattern. The recognition and measurement unit processes the
image of a pseudo-random checkerboard pattern to identify the
landmark on which the pattern is displayed and to calculate
the position and orientation of the autonomous vehicle. The
pseudo-randomness of each checkerboard pattern enables
determination of the position and orientation determination
of the autonomous vehicle even if partial errors are made in
visual interpretation of the landmark. The patent does not
refer to any aspect of or patient position detection during a
medical analysis.
FIG 2 illustrates an embodiment of the invention showing a
system 200 for patient positioning, comprising the structure
100 along with cameras 210, 220 attached to a patient
platform 230. The system can have two cameras 210, 220 to
capture the two-dimensional pattern 120 associated with the
two surfaces 140, 150 in said structure 100. Patient
positioning is generally detected by using a single camera at
a time. But it is possible to increase or endorse the
accuracy of the position detection using two cameras. For
example, the patient position could be determined
independently based on the two camera images and then
averaged to reduce the margin of error. Also one could act as
a fall back device if a problem is faced by one camera for'
capturing images.
The figure shows a patient's head 240 holding the structure
100. As shown the camera is attached to an L -shaped joint
which at the time of treatment comes from the parked position
to the operational position. The joint could be a motorized
joint. The camera is positioned to look up at an angle

towards the checkerboard. This design, positions the camera
as close to the checkerboard pattern as possible and takes
care of possible obstructions by the gantry of the treatment
system. Also, in the said arrangement the cameras are much
closer than those cameras mounted on the ceiling as discussed
in the prior art.
One of the cameras is expected to be operational at a given
time instant. The choice of which camera is to be used
depends on the treatment plan also. Specifically when a
camera obstructs the gantry the other camera is used for
position detection. For example, at the time of treatment if
the gantry is vertical or 0 degrees either of the cameras can
be used. At 90 degrees only one camera is operational. Also,
the expected distance of the camera is less than 1 meter from
the said structure. It is practically possible to reduce the
distance to even 500mm or less.
FIG 3 is a flowchart illustrating a method using a structure
for patient positioning in radiation therapy. Initially as
shown in step 310, the structure 100 is aligned, at a fixed
position with respect to the head 240 of the patient. In
radiation therapy detecting and tracking the exact position
of a volume of interest (for example a tumor in the head of a
patient) is extremely important.
In a practical scenario, the radio opaque elements in the
structure registers its offset information with the X-ray
based imaging system in a planning phase prior to the actual
treatment phase. This is done by taking an X-ray based image
of the structure, while the patient holds the structure using
the holding means 130 and is in an immobilized stage. The
structure 100 has two surfaces with the pattern (orthogonal
to each other) with radio opaque elements along axial
orientation. The orthogonal surfaces form a coordinate
system. The radio opaque elements, which could be lead rods
for example, would appear as circular sections in the axial
X-ray based image slice. Using these land marks, the offset

of the radio opaque elements and the X-ray based imaging
system is computed.
At step 320 a reference camera image and periodic camera
images of the two-dimensional pattern are taken by any one of
the camera 210, 220. It should be noted that the reference
camera image is taken when the patient is immobilized, so
that each of the other periodic images, taken at a non
immobilized state could be compared with the reference. At
step 330 the reference camera image and a periodic image is
processed to find a shift in position of the checkerboard
pattern.
Image processing steps involves a) corner extraction by
computing maxima of absolute gradient in scale space using
Harris's method and b) Filtering the extracted corners to get
a subset that corresponds only to the checkerboard.
At step 340, the shift in position of the checkerboard
pattern is mapped to the X-ray based imaging system. For this
we use the offset information between the two-dimensional
pattern 120 and the radio opaque elements 170 in the
structure 100.
It should be noted that, a change in the patient position
involves a translational and a rotation change in the
position of the checkerboard. Also it should be noted that
the two-dimensional pattern, the structure, X-ray based
imaging system all have its own coordinate systems. Since we
already determined, the offset information of the radio
opaque elements 170 and the X-ray based imaging system and as
we already know the offset information between the two-
dimensional pattern 120 and the radio opaque elements we can
easily translate the shift of the coordinate system defined
by the two-dimensional pattern 120, which are detected by the
one or more cameras, to the coordinate system defined by the
radio opaque rods, which are detected by the X-ray based

imaging system, by applying any known coordinate
transformation.
Two coordinates orthogonal to the optical axis of the camera
can be detected by the position of the pattern on the image
whereas the third coordinate, corresponding to the distance
of the pattern to the camera, can be determined based on the
appearance of an appropriate scale related to the pattern in
the image. The scale could be the known distance of squares
in the checkerboard pattern. The farther the pattern is away
from the camera the smaller the scale will appear. The
orientation of the structure and hence the patient can also
be determined based on the appearance of the pattern in the
image. The rotation of the pattern on the image reflects the
rotational orientation with respect to the optical axis of
the camera. The rotational orientation with respect to two
axis orthogonal to the optical axis can be determined based
on the relative distortion of two orthogonal scales of the
pattern in the image. These two orthogonal scales could be
the length of the squares of the two checkerboard patterns.
If one scale appears smaller than the other this means the
pattern must be tilted in the direction of the smaller scale.
At step 350, the radiation treatment is controlled based on
the shift in position of the checkerboard pattern. For
example if the said shift in position goes above a threshold
value then the radiation treatment can be stopped.
Summarizing, the present invention introduces a novel
structure, to detect the patient position. A two-dimensional
pattern in the structure is used to track the position of the
patient. The two-dimensional pattern which is associated with
a surface is placed at a fixed position with respect to the
head of the patient. The patient position is finally,
detected using the positional changes in the two-dimensional
pattern. This is done by capturing the two-dimensional
pattern associated with the structure using an optical camera

and detecting the patient position by analyzing the change in
position of the pattern by comparing the images captured.
The resulting advantages of the idea are two-fold: a) It
proposes an inverted hut shaped structure which the patient
can hold in his mouth. This is monitored by a pair of cameras
on either side of the treatment table. Each camera is at a
short distance from the plate, thereby increasing accuracy of
3D estimation, b) The use of checkerboard pattern which makes
the use of low cost visual cameras suitable as opposed to
expensive IR cameras. The checkerboard pattern permits
localization of the corner features to sub pixel accuracy as
opposed to the use of other fiducials used in previous
solutions.
Although the invention has been described with reference to
specific embodiments, this description is not meant to be
construed in a limiting sense. Various modifications of the
disclosed embodiments, as well as alternate embodiments of
the invention, will become apparent to persons skilled in the
art upon reference to the description of the invention. It is
therefore contemplated that such modifications can be made
without departing from the spirit or scope of the present
invention as defined.

Patent claims:
1. A structure (100) for patient positioning, said structure
comprising:
-at least one surface (140, 150) having a two-dimensional
pattern (120); and
-a holding means (130) to place the said at least one surface
(140, 150) having the two-dimensional pattern (120) at a
fixed position with respect to the head (240) of the patient.
2. The structure according to claim 1, wherein two-
dimensional pattern (120) is a cornered pattern.
3. The structure according to claim 2, wherein the cornered
pattern is a checkerboard cornered pattern.
4. The structure according to any of the preceding claims,
wherein the holding means (130) is a mouth bite.
5. The structure according to claim 4, wherein said mouth
bite is associated with a profile (160) , said profile (160)
allowing said mouth bite to be rigidly fixed in the mouth of
the patient.
6. The structure according to any of the preceding claims,
wherein said structure (100) is made of acrylic.
7. The structure according to any of the preceding claims,
wherein said structure (100) further comprises of two
surfaces (140, 150) having said two-dimensional pattern (120)
held by the holding means (130).
8. The structure according to claim 7, wherein said two
surfaces (140, 150) are positioned to form a shape of an
inverted hut.
9. The structure according to any of the preceding claims,
wherein the structure further comprise of radio opaque

elements (170) positioned at a fixed offset from the two-
dimensional pattern (120).
10. A system for patient positioning, comprising:
-a structure (100) as claimed in any of the claims 1 to 9;
and
-at least one camera (210, 220) positioned to capture the
two-dimensional pattern (120) on the at least one surface
(140, 150) of the structure (100).
11. The system according to claim 10, further comprises two
cameras (210, 220) to capture the two-dimensional pattern
(120) associated with the two surfaces (140, 150) in said
structure (100) .
12. The system according to claim 10 or 11, wherein the
cameras (210, 220) are attached to a patient platform (230).
13. The system according to any of the claims 10 to 12,
wherein the cameras (210, 220) are positioned at a distance
of less than 1 meter from the said structure (100).
14. A method using a structure (100) as claimed in any one of
the claims 1 to 9 for patient positioning, comprising the
steps of:
-aligning the structure (100), at a fixed position with
respect to the head (240) of the patient;
-capturing a plurality of images of the two-dimensional
pattern (120) on at least one surface (140, 150) by at least
one camera (210, 220); and
-processing the plurality of images to determine the position
of the patient.
15. The method according to claim 14, wherein capturing a
plurality of images further comprise the steps of capturing a
reference camera image and periodic camera images.

16. The method according to claim 14 or 15, wherein
processing the plurality of images further comprises the step
of detecting the shift in position of the two-dimensional
pattern in the reference image and a periodic camera image.
17. The method according to any of the claim 14 to 16,
further comprising the step of finding position of the radio
opaque elements (170) in a X-ray based imaging system.
18. The method according to any of the claim 14 to 17,
further comprising the step of mapping the shift in position
of the at least one surface with the two-dimensional pattern
(120) in the reference image and a periodic camera image to
the X-ray based imaging system using the offset information
between the two-dimensional pattern (120) and the radio
opaque elements (170) in the structure (100).
19. The method according to any of the claim 14 to 18,
further comprising the step of controlling a radiation
treatment based on the position of the patient.
20. The method according to claim 19, wherein controlling a
radiation treatment further comprise the step of stopping a
radiation treatment when the position of the patient has
shifted more from a threshold value.

The present invention introduces a novel structure (100), to
detect the patient position. A two-dimensional pattern (120)
in the structure (150) is used to track the position of the
patient. The two-dimensional pattern (120) which is
associated with a surface (150) is placed at a fixed position
with respect to the head (240) of the patient. The patient
position is finally, detected using the positional changes in
the two-dimensional pattern (120). This is done by capturing
the two-dimensional pattern (120) associated with the
structure (100) using an optical camera (210) and detecting
the patient position by analyzing the change in position of
the pattern by comparing the images captured.