U.S. patent application number 14/820310 was filed with the patent office on 2015-11-26 for method and apparatus for analysing images.
The applicant listed for this patent is RENISHAW (IRELAND) LIMITED. Invention is credited to Steven Streatfield GILL.
Application Number | 20150335390 14/820310 |
Document ID | / |
Family ID | 43877224 |
Filed Date | 2015-11-26 |
United States Patent
Application |
20150335390 |
Kind Code |
A1 |
GILL; Steven Streatfield |
November 26, 2015 |
METHOD AND APPARATUS FOR ANALYSING IMAGES
Abstract
A method is described for determining brain shift, such as the
brain shift that occurs following a neurosurgical intervention. The
method comprises taking a first image of the brain (4) of a
subject, the first image showing the position of blood vessels (20)
in the brain relative to a reference position (30). A second image
of the brain (4), that has been acquired following an intervention
on the subject is also taken. This second image has been acquired
using computed tomography (CT) imaging apparatus, The second image
shows the position of blood vessels (20') in the brain relative to
the reference position (30). Brain shift is then determined from
the shift in position of the blood vessels in at least one region
of interest (32) of the brain, with respect to the reference
position (30), between the first and second images. Corresponding
apparatus is also described.
Inventors: |
GILL; Steven Streatfield;
(Bristol, GB) |
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Applicant: |
Name |
City |
State |
Country |
Type |
RENISHAW (IRELAND) LIMITED |
Swords |
|
IE |
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|
Family ID: |
43877224 |
Appl. No.: |
14/820310 |
Filed: |
August 6, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13994459 |
Jun 14, 2013 |
|
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PCT/GB2011/001754 |
Dec 21, 2011 |
|
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14820310 |
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Current U.S.
Class: |
600/424 |
Current CPC
Class: |
A61B 34/20 20160201;
G06T 2200/04 20130101; G06T 2207/30101 20130101; A61B 2090/3966
20160201; A61B 2090/3983 20160201; A61B 90/37 20160201; A61B
2034/107 20160201; G06T 7/0014 20130101; G06T 7/337 20170101; A61B
2034/303 20160201; G06T 2207/10081 20130101; A61G 13/00 20130101;
A61B 34/30 20160201; A61B 2090/3762 20160201; A61B 2090/363
20160201; G06T 7/74 20170101; G06T 2207/30016 20130101; A61B
2034/2065 20160201; A61B 2017/00694 20130101; A61B 2090/374
20160201; A61B 2090/364 20160201; A61B 90/39 20160201; A61B 6/032
20130101; G06T 2210/41 20130101; A61B 6/501 20130101 |
International
Class: |
A61B 19/00 20060101
A61B019/00; A61B 6/00 20060101 A61B006/00; A61G 13/00 20060101
A61G013/00; G06T 7/00 20060101 G06T007/00; A61B 6/03 20060101
A61B006/03 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 21, 2010 |
EP |
10252178.8 |
Claims
1. A system for performing surgery, comprising: a surgical robot
for performing surgery on a patient, the surgical robot comprising:
an articulated arm having a known position in a robot coordinate
system, and a fiducial marker unit that is attached or configured
to be attached to the articulated arm, the fiducial marker unit
having a reference structure that has a known position in the robot
coordinate system when it is attached to the articulated arm; and a
medical imaging apparatus for acquiring intra-operative images of
the patient, positions in the acquired intra-operative images being
defined in an imaging coordinate system, wherein, when the fiducial
marker unit is attached to the articulated arm, the reference
structure is configured to be positioned so that the reference
structure is visible in images acquired by the medical imaging
apparatus so that a positional relationship can be established
between the robot coordinate system and the imaging coordinate
system.
2. The system according to claim 1, further comprising a table that
is physically linked to the surgical robot and supports a part of
the patient.
3. The system according to claim 1, wherein the fiducial marker
unit is configured to be attached to the articulated arm in a
repeatable position.
4. The system according to claim 3, wherein the articulated arm
comprises a mount and the fiducial marker unit is configured to be
attached to the mount.
5. The system according to claim 4, wherein the fiducial marker
unit has a known position relative to the mount when the fiducial
marker unit is attached to the mount.
6. The system according to claim 5, wherein the fiducial marker
unit and the mount provide a kinematic attachment mechanism for
releasably attaching the mount to the articulated arm in the known
position.
7. The system according to claim 1, wherein: a tool holder is
provided at a distal end of the articulated arm, and the tool
holder is configured to hold the fiducial marker unit so that the
fiducial marker unit is attached to the articulated arm when it is
held by the tool holder.
8. The system according to claim 7, wherein the tool holder is
further configured to hold at least one surgical tool, the at least
one surgical tool being separate from the fiducial marker unit.
9. The system according to claim 1, wherein the medical imaging
apparatus comprises a computerised tomography (CT) scanner and the
reference structure comprises a plurality of computerised
tomography (CT) visible markers.
10. The system according to claim 1, wherein the surgical robot
comprises a computer to control motion of the articulated arm.
11. A fiducial marker unit for a surgical robot, comprising: a
reference structure comprising one or more computerised tomography
(CT) visible markers; and an attachment portion configured to
enable the fiducial marker unit to be releasably attached to the
articulated arm of a surgical robot such that the reference
structure has a known position in a robot coordinate system of the
surgical robot.
12. The fiducial marker unit of claim 11, wherein the attachment
portion is configured to enable the fiducial marker unit to be
releasably attached to the articulated arm in a repeatable
position.
13. The fiducial marker unit of claim 11, wherein the attachment
portion is configured to enable the fiducial marker unit to be
releasably attached to the articulated arm in the known position
using a kinematic attachment mechanism.
14. A method for registering a robot with a medical imaging
apparatus, wherein: the medical imaging apparatus is configured to
acquire images from an imaging volume and one or more positions in
the acquired images are known in an imaging coordinate system, and
the robot comprises an articulated arm carrying a fiducial marker
unit having a reference structure that is visible in images
acquired by the medical imaging apparatus, the method comprising
the steps of: (i) placing the reference structure at a position
within the imaging volume of the of the medical imaging apparatus,
the position of the reference structure being known in a robot
coordinate system of the robot; (ii) acquiring an image of the
reference structure using the medical imaging apparatus; (iii)
determining a position of the reference structure in the imaging
coordinate system from the image acquired in step (ii); and (iv)
determining a difference between the position of the reference
structure in the robot coordinate system and the position of the
reference structure in the imaging coordinate system.
15. The method according to claim 14, further comprising the step
of calculating a coordinate transformation between the imaging
coordinate system and the robot coordinate system based on the
difference between the position of the reference structure in the
robot coordinate system and the position of the reference structure
in the imaging coordinate system.
16. The method according to claim 15, further comprising the step
of converting positions measured in the imaging coordinate system
into positions in the robot coordinate system using the calculated
coordinate transformation.
17. The method according to claim 15, further comprising the steps
of: acquiring at least one further image using the medical imaging
apparatus, and converting positions identified in the at least one
further image into positions in the robot coordinate system using
the calculated coordinate transformation.
18. The method according to claim 17, wherein the at least one
further image comprises an image of at least a part of a
patient.
19. The method according to claim 14, wherein at least a part of a
patient is also located within the imaging volume in step (i) such
that the image acquired in step (ii) includes both the reference
structure and said at least a part of the patient.
20. The method according to claim 14, comprising a step of
attaching the fiducial marker unit to the articulating arm of the
robot.
21. The method according to claim 14, wherein the medical imaging
apparatus comprises a CT scanner and the reference structure
comprises a plurality of CT visible markers.
22. The method according to claim 15, comprising the further step
of performing a surgical procedure on a patient using the
robot.
23. The method according to claim 22, comprising a step of
detaching the fiducial marker unit from the articulated arm of the
robot prior to the step of performing the surgical procedure on the
patient.
24. The method according to claim 22, wherein the surgical
procedure performed using the robot comprises a neurosurgical
procedure that includes inserting an electrode or catheter along a
trajectory to a target position within the brain of the patient,
wherein calculation of at least one of the target position and the
trajectory is based on the calculated coordinate
transformation.
25. An apparatus for performing surgery, comprising: a robot for
performing surgery on a patient, the robot comprising: an
articulated arm having a known position in a robot coordinate
system, and a fiducial marker unit that is attached or configured
to be attached to the robot, the fiducial marker unit having a
reference structure that the reference structure has a known
position in the robot coordinate system when it is attached to the
robot, wherein, when the fiducial marker unit is attached to the
robot, the fiducial marker unit is configured to be positioned so
that the reference structure is visible in images acquired by an
imaging apparatus.
Description
[0001] The present invention is a continuation application of U.S.
application Ser. No. 13/994,459, filed Jun. 14, 2013, which is a
371 of International Application No. PCT/GB2011/001754, filed Dec.
21, 2011, the contents of which are incorporated herein by
reference.
[0002] The present invention relates to an apparatus and method for
analysing images of the brain and in particular to a computer
implemented technique for determining brain shift from images of
the brain in which blood vessels are visible.
[0003] A variety of neourosurgical techniques have been developed
over the years that involve inserting neurosurgical instruments,
such as catheters or electrodes, to targets within the brain
parenchyma. These may be guided to a position with sub-millimetre
accuracy using stereotactic techniques of the type described in,
for example, EP1509153.
[0004] Although stereotactic apparatus is available that can
precisely guide neurosurgical instruments to a predetermined
location, there is no guarantee that such instruments actually
reach the target structure within the brain that has been
identified by the surgeon from pre-operative MRI scans or the like.
For example, the accuracy with which a brain structure can be
targeted may be affected by so-called "brain shift" that often
occurs following the MRI scan that was used to identify the brain
target. There are various reasons why such brain shift may occur.
For example, it may arise from any changes in the orientation of
the patient's head between image acquisition and placement on the
operating table. Leakage of cerebrospinal fluid (CSF) and the
ingress of air into the skull during surgery can also cause the
brain to sink. Furthermore, there may be displacement or swelling
of the brain within the skull as neurosurgical instruments are
forced through the brain tissue.
[0005] To date, the existence of brain shift has been noted by
surgeons and surgical procedures have been devised and adapted to
minimise its effect. This has been attempted by, for example,
always placing a patient's head in a certain orientation during
surgery (although such an orientation may be sub-optimal for
surgical access purposes) to minimise brain shift. Similarly,
neurosurgeons often compensate for brain shift by adding an offset
to the target position identified in an MRI image based on
experience gained from previous surgical procedures about the
magnitude of brain shift that is likely to occur. Although such
known techniques can compensate for the effects of brain shift in
most cases, they are highly subjective and rely totally on the
skill and judgement of the neurosurgeon. The existence of brain
shift also means it is typically not possible to tell if the
desired target has been reached by an implanted neurosurgical
instrument (e.g. electrode, catheter etc) during the surgical
procedure itself; this is only established subsequently by a
follow-up MRI scan.
[0006] Various techniques have been developed previously for
image-guided neurosurgery that attempt to measure or predict brain
shift. For example, WO2005/025404 describes a cortical surface
registration technique. In particular, WO2005/025404 describes
using a line stripe laser scanner to intra-operatively measure the
three dimensional profile of the cortical surface. This surface
profile is then registered to previously acquired MRI data.
WO2010/037850 describes how brain shift may be estimated using a
biomechanical model that analyses pre-operatively acquired MRI
images and intra-operatively acquired Doppler ultrasound images. A
biomechanical model for estimating brain shift is also described in
the publication "Framework for a low-cost intra-operative
image-guided neuronavigator including brain shift compensation" by
M Bucki et al., 2007 Annual international conference of the IEEE
engineering in medicine and biology society, Lyon, France, 22-26
Aug. 2007, pages 872-875 (ISBN:987-1-4244-0787-3). WO2006/028474
describes generating a statistical atlas of brain displacements
using intra-operative images of the brain surface. None of these
techniques, however, provide a sufficiently accurate and reliable
measure of brain shift that can be used to assess and/or improve
the accuracy of device placement in complex (e.g. deep brain)
neurosurgical procedures.
[0007] According to the present invention, there is provided a
computer implemented method for determining brain shift by
analysing images of the brain, the method comprising the steps of:
[0008] (i) taking a first image of the brain of a subject, the
first image showing the position of blood vessels in the brain
relative to a reference position, [0009] (ii) taking a second image
of the brain that was acquired following an intervention on the
subject, the second image having been acquired by Computed
Tomography (CT) imaging apparatus and showing the position of blood
vessels in the brain relative to the reference position, and [0010]
(iii) calculating brain shift from the shift in position of the
blood vessels in at least one region of interest of the brain, with
respect to the reference position, between the first and second
images.
[0011] The present invention thus comprises a computer implemented
method for establishing brain shift from first and second images of
the brain of a subject. These images show the position of blood
vessels in the brain of the subject relative to a certain reference
position before and after an intervention. The second image of the
brain is acquired using CT imaging apparatus (e.g. intra-operative
CT Angiogram imaging apparatus) and can thus be assumed to provide
a spatially accurate representation of brain anatomy. In
particular, the use of CT imaging apparatus provides accurate
spatial information about regions deep within the brain. The images
may show the whole brain or just a selected part of the brain. The
method comprises assessing the movement of the blood vessels
relative to a fixed reference position (e.g. the skull or a point
in a common coordinate system) between the first and second images
thereby allowing the amount of brain shift to be quantified. The
amount of blood vessel movement may be established using all, or
preferably just some, of the blood vessels present in the first and
second images. As explained below, step (iii) may comprise directly
calculating brain shift (e.g. establishing a vector or scalar brain
shift value) or indirectly determining brain shift (e.g. by
establishing a correction or adjustment to a coordinate system in
which positions within the brain are defined or by modifying one or
more target positions and/or trajectories within such a coordinate
system).
[0012] The method of the present invention has the advantage that
it can be performed completely in-theatre (e.g. using first and
second CT angiogram images acquired using intra-operative CT
apparatus) in order to provide a measure of brain shift at one or
more locations within the brain at one or more instances during a
neurosurgical procedure. Such a measure of brain shift may be used
to aid the surgeon when targeting structures in the brain (e.g. by
allowing the settings of manual stereotactic apparatus to be
adjusted) or to update the target coordinates of a surgical robot
system. In this manner, targeting accuracy and hence treatment
efficacy can be improved. Alternatively, a pre-operatively acquired
first image may be compared to an intra-operatively (or
post-operatively) acquired second image.
[0013] Advantageously, step (iii) comprises using a co-registration
process to determine the shift in position of blood vessels in said
at least one region of interest. In other words, a co-registration
procedure is performed to find a transformation that best maps
blood vessels (in said at least one region of interest) from the
first image onto the second image, or vice versa.
[0014] Preferably, such a registration procedure is performed using
an intensity based registration technique. The registration
procedure may comprise a non-linear registration process. Such a
non-linear registration procedure may comprise a non-deformable
registration procedure. The non-deformable registration procedure
may comprise an affine registration process.
[0015] Preferably, the registration procedure comprises a rigid
registration procedure. Such a rigid registration procedure may
comprise using a rigid transformation in which one image is
translated, rotated and/or scaled to find the best correspondence
between the blood vessels present in said at least one region of
interest. In a preferred embodiment, the second image may be left
unaltered and the first image may be at least one of translated,
rotated and scaled to find the best correspondence. The use of a
rigid registration procedure is particularly preferred when both
the first and second images are acquired using CT imaging
apparatus; the images can then be assumed to be spatially accurate
(e.g. substantially distortion-free). As a common reference
position is defined in both the first and second image, the
transformation provides a measure of blood vessel movement (and
hence brain shift) in the region of interest.
[0016] Step (iii) may comprise determining brain shift in one
region of interest in the brain. Alternatively, step (iii) may
comprise establishing brain shift for a plurality of regions of
interest in the brain. If a plurality of regions of interest are
used, such regions may be overlapping or mutually exclusive. Brain
shift may be determined in region(s) of interest in the vicinity of
a target site or sites within the brain and/or in region(s) of
interest along a trajectory or trajectories through the brain to
reach such a target site or sites. A region of interest may
comprise a deep brain target. It is preferred that the volume of
each region of interest within the brain is smaller than the volume
of the whole brain. Preferably, the volume of each region of
interest is no larger than 50% of the entire brain volume, more
preferably no larger than 25% of the entire brain volume and more
preferably no larger than 10% of the entire brain volume. In
absolute terms, each region of interest is preferably less than 200
cm.sup.3, less than 100 cm.sup.3 and more preferably less than 50
cm.sup.3. Using such a sub-region of the brain as a region of
interest ensures that localised brain shift is determined in that
region without being affected by other regions of the brain (e.g.
near the skull) where blood vessel movement may be substantially
less. This is especially advantageous when establishing brain shift
for deep targets, where the amount of brain shift may be markedly
different to that present near the cortical surface.
[0017] The blood vessels visible in the first and second images may
comprise arteries and/or veins. Although the majority of arteries
in the brain move with the brain tissue through which they pass, it
should be noted that certain veins may exhibit less movement; e.g.
peripheral veins may be anchored, to some extent, to the inside of
the skull. It is therefore advantageous, but by no means essential,
that the first and second images show blood vessels consisting of
arteries. It is also preferred, but not essential, that any regions
of interest exclude areas of the brain that include blood vessels
(such as peripheral veins that are immobilised to some extent
relative to the skull) that tend not to move with brain tissue.
[0018] Conveniently, the reference position common to both the
first and second images is established using a reference structure
that is visible in those images. In such a case, two registration
processes may be performed. A first registration process may be
performed to establish a first transformation between the reference
structure visible in each image. This first registration process
may be based on a comparison of the whole of the first and second
images, or a comparison of a selected portion of those images where
the reference structure is visible. A second registration process
may then be performed to establish a second transformation between
the blood vessels that are present in the selected regions of
interest of each image. The difference between the first and second
transformations can then provide a measure of brain shift (as
evidenced by blood vessel movement) relative to the reference
structure.
[0019] Any suitable reference structure may be used that is visible
in the first and second images. Advantageously, the reference
structure comprises an anatomical feature of the subject that has a
substantially invariant position within the body. For example, the
anatomical feature may comprise the skull, or a portion of the
skull, of the subject. Alternatively, the reference structure may
comprise a fiducial marker set that is immobilised relative to the
skull of the subject. For example, a fiducial marker set may be
attached (e.g. screwed or glued) to the skull bone.
[0020] Instead of providing a reference structure that is visible
in the images, the first and second images of the brain may be
acquired by the same imaging apparatus with the head of the subject
placed in the same, fixed, location relative to said imaging
apparatus. In such an embodiment, the reference position may
comprise a positional reference point or origin of said imaging
apparatus. In other words, the reference position may be an
internal origin or some known reference point defined by the
imaging system of the imaging apparatus.
[0021] As outlined above, the imaging apparatus used to acquire the
second image comprises computerised/computed tomography (CT)
imaging apparatus, such as x-ray computed tomography (CT) imaging
apparatus. The imaging apparatus used to acquire the first image
may comprise any suitable medical imaging apparatus. For example,
it may comprise a tomograph, computed tomography (CT), x-ray
computed tomography, x-ray, magnetic resonance imaging (MRI),
positron emission tomography (PET), ultrasound etc. Preferably, the
medical imaging apparatus provides a three dimensional or
volumetric image or such a volumetric image can be constructed from
data collected using that apparatus. Different imaging apparatus,
including different types of imaging apparatus, may be used to
collect the first and second images. Preferably, the same imaging
apparatus is used to collect the first and second images. In a
preferred embodiment, in-theatre computerised tomography angiogram
(CTA) imaging apparatus is used to acquire the first and second
images. In such an example, the first and second images comprise
first and second CTA images. Such CTA apparatus may conveniently
comprise a CT scanner device that can be used in-theatre, such as
an O-arm.RTM. surgical (CT) imaging system as sold by Medtronics,
Inc. The use of in-theatre medical imaging apparatus has the
advantage of allowing the first and second images that are analysed
using the method of the present invention to be collected whilst
the patient is undergoing a surgical procedure. Brain shift
measurements can thus be performed quickly (e.g. in seconds or
minutes) and the results used to help a surgeon optimise subsequent
surgical actions.
[0022] Advantageously, the method comprises the step of defining at
least a first target site in the first image. This step may, for
example, comprise a surgeon reviewing the first image and
identifying where one or more neurosurgical instruments (e.g.
catheters, electrodes etc) need to be placed. The site or sites
identified by the surgeon can then be defined directly on the first
image; e.g. a point may be (electronically) marked at a certain
position on the first image. A plurality of such target sites may
be defined as necessary.
[0023] Advantageously, the first target site (and any additional
target sites) may be defined in the first image by importing target
information from at least one further image. The further image may
be taken using further imaging apparatus; this may be a different
type of imaging apparatus than that used to acquire the first and
second images. The step of mapping target(s) onto the first image
from the further image is preferably performed using a registration
technique (e.g. of the type described above).
[0024] Conveniently, the further image comprises an image acquired
using an MRI scanner. Advantageously, the further image shows blood
vessels within the brain. A number of different MRI acquisition
protocols may be used to acquire MR images in which blood vessels
are visible. For example, magnetic resonance angiography (MRA)
allows the blood flowing through vessels to be distinguished from
stationary tissue. Blood vessels can also be seen on "T1" MRI scans
and the contrast of such images can optionally be improved by
injecting a contrast agent into the patient's blood stream
(providing a so-called contrast MRA image). The registration
procedure may then be based on the comparison (e.g. registration)
of blood vessels visible in the first image and the further image.
Preferably, the registration process is performed using data
showing at least one region of interest within the brain. Such a
region of interest may be a volume surrounding a certain target
site or volume of interest and may be the same, or different, to
the region of interest used when comparing the first and second
images.
[0025] Advantageously, the co-registration process described above
provides the necessary positional link or transformation that
allows positions to be mapped between the further image and the
first image. The further image itself may be used by the surgeon to
identify target sites or trajectories for instruments that are to
be inserted into the brain of the subject. Alternatively, target
information may be imported from analysis of one or more other
images that have a known positional relationship with respect to
the further image. For example, the step of co-registering the
first image with a further image advantageously permits a surgical
planning image (or a plurality of such surgical planning images)
also acquired using the further imaging apparatus (i.e. the
apparatus used to acquire the further image used in the
registration process) to be registered with the first image. The
surgical planning image may include a target or targets identified
by a neurosurgeon or by using appropriate planning software. Such a
surgical planning image may, for example, comprise a
pre-operatively acquired image (e.g. taken using MRI apparatus
operating in a T2 acquisition mode to show relevant brain
structures) that a surgical team have had time to analyse in great
detail over the course of several days or weeks.
[0026] Although any suitable medical imaging apparatus may be used,
it is preferred that the further image comprises an angiogram taken
using further apparatus that comprises an MRI scanner.
Advantageously, the surgical planning image is acquired
pre-operatively using the same MRI scanner (e.g. operating in T2
mode). The subject may be maintained in the same location within
the MRI scanner when the further image and the surgical planning
image are acquired. In this manner, positions in the further image
will correspond to positions in the surgical planning image. If
necessary, MR visible fiducials may be used during MR image
acquisition to allow all the acquired MRI images to be tied to a
common reference position; this can also account for any movement
of the subject between scans. As MRI data can be subject to
distortions, it also possible for the registration procedure used
to generate the transformation between the further (MRI) image and
the first image to be elastic. The further (MRI) image may then be
morphed (i.e. stretched/squashed and not just translated/rotated)
to achieve the best fit with the first image. In this manner, the
effects of MRI distortions on targeting accuracy can be reduced or
substantially removed. Alternatively, a rigid registration
procedure may be used.
[0027] The second image analysed using the method of the present
invention is acquired after an intervention on the patient. The
intervention that occurred may comprise any event or interaction
with the subject that may cause the brain to shift within the
skull. The intervention may comprise a surgical intervention (e.g.
cutting or inserting an instrument into the brain of the subject)
or a non-surgical intervention (e.g. re-orientating the subject).
It should be noted that a surgical intervention does not comprise a
part of the method of the first aspect of the present invention
described herein. Advantageously, the second image (i.e. as defined
in step (ii)) has been acquired after an intervention that
comprises inserting a neurosurgical instrument to said first target
site. The second image may thus show the inserted neurosurgical
instrument. The intervention may also have comprised inserting
further neurosurgical instruments to further target sites. Step
(iii) may then further comprise the step of assessing the location
of the inserted neurosurgical instrument relative to the first
target site. In addition to measuring brain shift, this also allows
any positioning errors (e.g. related to unexpected instrument
deflections etc.) to be measured.
[0028] Step (iii) may comprise directly or indirectly establishing
brain shift. For example, step (iii) may comprise directly
determining brain shift by calculating a scalar quantity that
defines the magnitude of brain shift. Advantageously, step (iii)
comprises directly determining brain shift by calculating a brain
shift vector that describes both the magnitude and direction (e.g.
with respect to the reference position) of brain shift. The brain
shift determined in step (iii) may advantageously be used to update
target coordinates, for example as used by a surgical robot or a
stereotactic system. Step (iii) may alternatively comprise
indirectly establishing brain shift. For example, step (iii) may
comprise calculating new or updated target coordinates that take
into account brain shift or establishing an offset to a co-ordinate
system to account for brain shift. Again, such indirect measures of
brain shift may be advantageously used for providing target
coordinates (e.g. of a surgical robot or a stereotactic system) in
which the effects of brain shift have been taken into account. It
should also be noted that the ability to compensate for the effect
of brain shift during a surgical procedure is particularly
advantageous.
[0029] It should be noted that the term "image" is used herein to
refer to data collected from imaging apparatus (e.g. from a
volumetric medical imager or tomograph) and does not mean that such
data must be somehow displayed visually. Similarly, references to
structures being "visible" in images is intended to mean that
information relating to such structures is present in an image and
the term visible should not be read as requiring such images or
structures to be displayed graphically. The phrase "taking an
image" is intended to mean herein retrieving an image that has been
acquired (e.g. by loading the image into a computer) and/or
acquiring the image using appropriate imaging apparatus.
[0030] The above described method is implemented using a computer.
The present invention therefore also encompasses a computer that is
programmed to implement the above method. Computer software that,
when run on a suitable computer, implements the above method is
also envisaged. Apparatus including a processor configured to
implement the above described method is also encompassed by the
present invention.
[0031] Apparatus for measuring brain shift is also described
herein, the apparatus comprising: means for taking a first image of
the brain of a subject, the first image showing the position of
blood vessels in the brain relative to a reference position, means
for taking a second image of the brain acquired following an
intervention on the subject, the second image showing the position
of blood vessels in the brain relative to the reference position,
and means for determining brain shift from the shift in position of
the blood vessels in at least one region of interest of the brain,
with respect to the reference position, between the first and
second images. The apparatus may include any of the features
described above in respect of the associated method. The apparatus
for measuring brain shift may be used in combination with a
surgical robot, the target coordinates and/or coordinate system
used by the surgical robot being updated to account for the brain
shift as determined by the means for determining brain shift.
[0032] A surgical robot system is also described herein that
comprises; a robot for guiding a surgical instrument to at least
one target site located within the brain parenchyma of a subject, a
medical imaging apparatus (e.g. CT imaging apparatus) for imaging
at least a part of the head of a subject, the medical imaging
apparatus being suitable for collecting a series of images of the
head that show the blood vessels within the brain parenchyma, and a
computer controller for updating the co-ordinates of said at least
one target site as used by the robot to account for brain shift,
said brain shift being determined from a plurality of the images
acquired by the medical imaging apparatus.
[0033] A further aspect of the invention provides apparatus for
performing robotic neurosurgery on a patient. The apparatus
preferably comprises a motorised neurosurgical robot for moving a
medical instrument to positions defined in a robot coordinate
system. The robot may be computer controlled.
[0034] The apparatus advantageously also provides imaging apparatus
for acquiring intra-operative images of the brain of a patient. For
example, the apparatus may include CT imaging apparatus (e.g. an
O-arm). At least one position in each intra-operative image is
preferably known in the robot coordinate system. For example, a map
or link is preferably provided between the robot coordinate system
and the co-ordinates of the intra-operative images.
[0035] Targeting apparatus is also provided for calculating a
target location within the patient's brain to which the medical
instrument is to be advanced. The targeting apparatus preferably
takes at least one previously (e.g. pre-operatively) acquired image
of the patient in which a target has been identified and calculates
the corresponding target location in the robot coordinate system by
registering said at least one previously acquired image with at
least one intra-operative image taken using the imaging apparatus.
The at least one previously acquired image may have been acquired
pre-operatively (e.g. using MRI apparatus) or at an earlier point
in the surgical procedure (e.g. by the imaging apparatus used to
acquire the intra-operative image). In a preferred embodiment, the
targeting apparatus registers the at least one previously acquired
image with the at least one intra-operative image using blood
vessels visible in the images. Blood vessels in a certain region of
interest may be registered, as described in detail herein, to
provide the target location in the robot coordinate system.
[0036] There is also described herein a method for registering CT
and MRI images, the method comprising the steps of: (a) taking a
first image of the brain of a subject acquired using CT apparatus,
the first image showing the position of blood vessels in the brain,
(b) taking a second image of the brain acquired using MRI
apparatus, the second image showing the position of blood vessels
in the brain and (c) registering the first and second images using
the blood vessels that are visible therein. Any of the above
described registration techniques may be used in step (c). For
example, a rigid registration may be used. Preferably, the step (c)
of registering the first and second images comprises elastically
distorting the second image so as to overlay blood vessels visible
in the second image on the corresponding blood vessels visible in
the first image. Such an elastic distortion of the second (MRI)
image removes any distortions that are inherently present in images
acquired using such MRI apparatus. Conveniently, the step (c) of
registering the first and second images comprises performing a
registration in at least one region of interest within the first
and second images. Preferably, the volume of each region of
interest is no larger than 50% of the entire brain volume, more
preferably no larger than 25% of the entire brain volume and more
preferably no larger than 10% of the entire brain volume. Targets
defined in the MRI coordinate system may then be mapped into the CT
coordinate system.
[0037] Also described herein is surgical apparatus that comprises:
a surgical robot for performing surgery on a patient, the surgical
robot comprising an articulated arm having a known position in a
robot coordinate system, and medical imaging apparatus for
acquiring intra-operative images of the patient, positions in the
images acquired by the medical imaging apparatus being defined in
an imaging coordinate system, wherein the surgical apparatus
comprises a fiducial marker unit that can be held by the
articulated arm such that is has a known position in the robot
coordinate system, the fiducial marker unit including a reference
structure that is visible in images acquired by the medical imaging
apparatus. In this manner, the positional relationship between the
coordinate systems of the robot and medical imaging apparatus can
be found without needing to directly attach (e.g. screw) a fiducial
marker to the skull of the patient. This removes the need for minor
surgery to attach markers or frames to the skull of the patient
prior to performance of the neurosurgical procedure. Preferably,
the fiducial marker unit is provided in the form of a tool that can
be retained in a tool holder of the articulated arm of the surgical
robot. Preferably, the fiducial marker unit can be attached to the
articulated arm in a repeatable position (e.g. using a kinematic or
pseudo-kinematic attachment mechanism).
[0038] There is also described herein a method for determining
brain shift, the method comprising the steps of: (i) taking a first
image of the brain of a subject, the first image showing the
position of blood vessels in the brain relative to a reference
position, (ii) taking a second image of the brain acquired
following an intervention on the subject, the second image showing
the position of blood vessels in the brain relative to the
reference position, and (iii) determining brain shift from the
shift in position of the blood vessels in at least one region of
interest of the brain, with respect to the reference position,
between the first and second images. The method may also include
any one or more of the steps or features described herein.
[0039] The present invention will now be described, by way of
example only, with reference to the accompanying drawings in
which;
[0040] FIG. 1 illustrates pre-operatively acquired MRI (T2)
data,
[0041] FIG. 2 illustrates pre-operatively acquired MRA data,
[0042] FIG. 3 illustrates a CT angiogram acquired with the patient
"in-theatre" prior to neurosurgery,
[0043] FIG. 4 illustrates a CT angiogram acquired with the patient
in-theatre after a catheter has been surgically implanted,
[0044] FIG. 5 shows a surgical robot and an in-theatre CT
scanner,
[0045] FIG. 6 is a flow chart outlining a suitable registration
process,
[0046] FIG. 7 is a flow chart s outlining a process for the
registration of pre-operative and post-operative CT data, and
[0047] FIG. 8 is a flow chart outlining a process for the
registration of MRA and CTA data.
[0048] The present invention, which relates to a method of
measuring brain shift, will now be described in the context of
surgical procedures that could introduce such a brain shift. It
should be noted that the computer implemented method for analysing
images that is described herein is related to analysing the medical
images taken during such a surgical procedure and is not intended
to include any of the surgical steps outlined below. These surgical
steps are described below purely to aid understanding of how the
computer implemented analysis method of the present invention can
be used.
[0049] Referring to FIG. 1, a pre-operatively acquired (T2) MRI
image is illustrated in very general terms. The image shows the
skull 2 and brain 4 of a patient. Such a T2 MRI image would also
show in detail the different structures within the brain, although
such structures are omitted from FIG. 1 for ease of
understanding.
[0050] As is traditional in the art of neurosurgery, such an MRI
image is used by surgeons to plan surgical procedures and is
usually acquired pre-operatively (e.g. days or weeks in advance).
One example of a surgical procedure that may be planned using such
an MRI image is the placement of a catheter to deliver a
therapeutic agent to a specific structure within the brain. FIG. 1
also illustrates a target region or target site 6 and the
trajectory 8 along which a catheter is to be inserted to reach the
target located at the identified brain structure; these are
typically planned manually by the neurosurgeon. Positions within
the image shown in FIG. 1 are defined in an MRI co-ordinate system.
Such an MRI co-ordinate system may, for example, be defined with
reference to MR visible fiducial markers that are secured to the
patient head prior to imaging (but not shown in FIG. 1).
[0051] Referring next to FIG. 2, there is shown an illustration of
a (T1) MRI image taken after an MRI contrast agent has been
injected into the patient's blood stream. This image, which can be
referred to as a contrast MR angiogram or "contrast MRA" image,
shows the various blood vessels 10 within the brain but it does not
show in any detail the various structures within the brain; this
image is therefore typically unsuitable for target identification
and detailed surgical planning purposes. It should be noted that,
instead of a contrast MRA, a standard MRA (in which the flow of
blood within vessels is distinguished from the stationary tissue
surrounding those vessels) could be acquired.
[0052] The contrast MRA image shown in FIG. 2 is of the same
patient as the MRI image of FIG. 1 and all positions are described
in the same MRI co-ordinate system. This may be achieved, for
example, by reference to MR visible fiducial markers that have a
fixed position relative to the skull. Alternatively, the patient's
head may be maintained in the same position between scans thereby
removing the need for fiducial markers. The acquisition of the
contrast MRA image in the same co-ordinate system as the MRI image
allows the target site 6 and trajectory 8 from the MRI image shown
in FIG. 1 to be mapped onto the contrast MRA image of FIG. 2 in
which the blood vessels are visible.
[0053] Referring to FIG. 3, there is shown a first CT angiogram
(CTA) image of the patient. This CTA image is acquired using
in-theatre CT apparatus (which is preferable mobile) whilst the
patient is located on the operating table with their head secured
to that table in the desired position for the subsequent
neurosurgical procedure. The first CTA image, which is acquired
after the injection of a suitable CT contrast agent into the
patient's bloodstream, shows the location of the blood vessels 20
within the brain in the CT co-ordinate system. In this example, the
CT co-ordinate system is defined relative to a reference structure
in the form of the skull 30 that is also visible in the first CTA
image.
[0054] A registration procedure is then performed to register the
contrast MRA image of FIG. 2 (which includes the target site and
trajectory information) with the first CTA image of FIG. 3. This
registration process involves defining a region of interest 32
around the target site 6 and then performing a co-registration
process to find the best positional match between the CTA and
contrast MRA images in this region of interest. This registration
process is preferably intensity based and makes use of the blood
vessels that are visible in both the CTA and contrast MRA images.
To account for MRI distortions, the registration process may be
fully or partly elastic to allow the contrast MRA image to be
morphed (compressed/stretched) to fit the vessel structure of the
CTA image. Further details of the preferred co-registration
procedure are outlined in more detail below, but the basic result
is a mathematical transformation that allows positions measured in
the MRI co-ordinate system to be mapped into the CT co-ordinate
system. This registration process therefore allows the target site
6 and trajectory 8 that were defined in the MR co-ordinate system
to be mapped into the CT co-ordinate system, as shown in FIG.
3.
[0055] The above described process thus permits target and
trajectory information that is based on pre-operatively acquired
MRI data to be mapped into the CT co-ordinate system that is
established "in-theatre" on the day of surgery. Importantly, this
can be done without the need for fiducial markers to remain affixed
to the patient between the pre-operative MRI scans and the surgical
procedure. As will be described in more detail below, the CT
co-ordinate system may also be linked to a robotic or stereotactic
coordinate system that allows external equipment, such a
stereotactic frame or a surgical robot, to insert surgical
instruments to desired targets within the brain that have been
described in CT based co-ordinates.
[0056] Referring to FIG. 4, a second CTA image is schematically
illustrated that is taken after insertion of a catheter 40. This
second CTA image is taken with the same in-theatre CT apparatus
that was used to acquire the first CTA image of FIG. 3 and with the
patient's head still secured to the operating table (noting some
head movement may have occurred during surgery). As can be seen
from FIG. 4, the catheter has been accurately inserted along the
trajectory shown in FIG. 3 and has its tip precisely located at the
target co-ordinates that were defined using the first CTA image
shown in FIG. 3. The brain has, however, shifted relative to the
skull at some point during the surgical procedure (noting that this
brain shift is greatly exaggerated in FIG. 4). This brain shift
(which results in the blood vessels adopting the positions shown as
20' in FIG. 4) means that the tip of the catheter is not actually
placed at the target site within the brain where delivery of the
therapeutic agent is required. In other words, there has been brain
shift defined by the vector B shown in FIG. 4 that has caused the
tip of the catheter to deviate from its desired target within the
brain parenchyma.
[0057] It should be noted that in FIG. 4 it is assumed that the
catheter is located precisely where it was originally intended and
that all the error arises from brain shift. It is, of course,
possible that there may be some other error in catheter placement
or that the catheter may deflect away from the desired trajectory
due to variations in the density of brain tissue. In such a case,
it is also possible to establish from the second CTA image any
variation in the actual catheter position from the position that
was planned.
[0058] Normally, brain shift of the type described above would only
be identified post-operatively (e.g. during a subsequent MRI scan)
or when the treatment failed. The acquisition of a second CTA
image, however, permits any brain shift to be measured
perioperatively in the following manner. Firstly, a registration
procedure is performed to establish a reference transformation that
describes the spatial relationship between the skull (or one or
more predefined regions of the skull) shown in the first and second
CTA images. The first CTA image is then registered with the second
CTA in the region of interest 32 of the brain that surrounds the
target site; this provides a brain (blood vessel) transformation.
The difference between the reference transformation and the brain
(blood vessel) transformation describes how much the region of
interest 32 within the brain has moved relative to the skull
between the first and second CTA images. In other words, a vector
describing brain shift in the region of interest 32 can be
established using the skull as a fixed position reference
feature.
[0059] This manner of measuring brain shift can be used purely as a
tool to quantify the amount of brain shift that accompanies a
certain neurosurgical procedure. If the brain shift exceeds a
certain amount, the procedure may be reversed (e.g. by explanting a
catheter) and performed again (e.g. by implanting a catheter to the
shifted target site). Alternatively, brain shift may be measured
multiple times during a surgical procedure and if any significant
brain shift is measured an appropriate correction to the target
co-ordinates may be applied. For example, brain shift could be
measured after boring an opening the skull and piercing the dura.
The measured brain shift could then be used to update the target
co-ordinates that are to be used during a subsequent stereotactic
or robotic procedure to insert a neurosurgical instrument.
Measurement of brain shift using the above method could also be
performed periodically during the implantation of a neurosurgical
instrument, and the target coordinates or the origin of the
surgical coordinate system updated accordingly.
[0060] A technique will now be described with reference to FIG. 5
for relating the coordinate system of a neurosurgical stereotactic
robot, such a Neurolmate robot system 50 produced by Renishaw
Mayfield, to the coordinate system of an associated CT apparatus
52. In particular, FIG. 5 shows a Neurolmate robot system 50
arranged to perform surgery on a patient 54 whose head is held
rigidly in place on a table 56 linked to the robot 50. A mobile,
in-theatre, CT scanner 52 is located around the patient's head and
arranged to acquire a CT image in a CT imaging volume 58 that
encompasses the head. The robot system 50 also comprises an
articulated robotic arm 60 having a mount 62 at its distal end to
which neurosurgical instruments and the like can be attached. In
FIG. 5, the articulated arm 60 carries a CT visible marker
arrangement 64 that has a known (pre-calibrated) position relative
to the mount 62; i.e. the CT visible marker arrangement 64 has a
known position within the robot coordinate system.
[0061] In order to establish a positional link between the robot
coordinate system and the CT coordinate system, the articulated arm
60 places the CT visible marker arrangement 64 in the CT imaging
volume 58 and the mobile CT scanner 52 performs a CT scan. A
suitable position for the CT visible marker arrangement 64 during
such a scan is shown as 64' in FIG. 5. The resulting CT image
(which may be a CT angiogram image as described above) thus shows
positions within the patients head (e.g. positions of blood vessels
and target sites) relative to the CT visible marker arrangement 64.
Analysis of the CT image thus allows a link or transformation to be
established between the CT coordinate system and the robot
coordinate system, thereby allowing target positions and
trajectories that have been defined in the CT coordinate system
(which may have been mapped into the CT coordinate system from an
MRI coordinate system as described above) to be mapped into the
robot coordinate system. The marker arrangement 64 may then be
detached from the mount and replaced with a surgical instrument
that can be guided along the desired trajectory to the target
coordinates. The marker arrangement 64 may be re-attached to the
mount (in the same position) and re-imaged as often as required;
for example, if the map between the CT coordinate system and the
robot coordinate system needs to be re-established or verified.
[0062] It should be noted that it is also possible to establish the
map between the CT coordinate system and the robot coordinate
system prior to the patient being present. For example, the CT
visible marker arrangement could be held by the arm of the robot in
various different locations within the imaging volume of the CT
scanner prior to the patient being placed on the table 56. This
does, however, assume that the relative position of the CT scanner
and surgical robot does not change when the patient is introduced.
Alternatively, the patient 54 could have the CT visible marker
arrangement affixed to their skull, for example as part of a head
frame or by direct (e.g. screw thread) attachment. Such an
attachment may comprise a feature to engage with an instrument held
by the arm 60 of the robot 50 to enable the position of the CT
visible marker to be established in the robot coordinate
system.
[0063] Once the transformation between the CT coordinate system and
the robot coordinate system has been established, it is possible to
update target positions defined in the robot coordinate system if
those target positions alter (e.g. due to brain shift). For
example, the amount of brain shift may be measured in the manner
described above (i.e. by looking for such brain shift relative to
the skull). Assuming the patient's skull does not move with respect
to the robot, the measured brain shift may then be directly used to
correct the target coordinates in the robot coordinate system. It
should be noted that, because brain shift is measured relative to
the skull in the above described method, the CT scanner may be
moved relative to the patient between measurements. Furthermore,
such corrections for brain shift may be determined periodically or
at certain points in the procedure. The target position(s) in the
robot coordinate system can then be changed accordingly during the
procedure to account for any brain shift. It should also be noted
that the origin of the robot coordinate system may be adjusted to
account for brain shift; this has the same effect as altering the
target positions.
[0064] Although the above examples refer to assessing the brain
shift in the region of a target, it should be noted that brain
shift may be measured for a plurality of regions of interest within
the brain. These regions of interest may correspond to multiple
target sites (e.g. if implanting a plurality of neurosurgical
instruments) and/or multiple points along an insertion trajectory
(e.g. to ensure the inserted instrument does not damage sensitive
tissue or puncture any large blood vessels). It is also important
to note that although the above examples show two-dimensional
images, these are merely slices through three-dimensional or
volumetric images. The two-dimensional illustrations described
above are simply provided for ease of understanding and the skilled
person would fully appreciate that, in most practical situations,
the coordinate systems and any brain shift would be defined and
measured in three-dimensions.
[0065] The registration of images is described above in general
terms, but more details of suitable registration processes will now
be described with reference to FIGS. 6 to 8.
[0066] The co-registration of two data sets that show blood vessels
(e.g. Angio-MR, Angio-CT, vessel models or vessel atlas data sets)
can be performed using a variety of known registration techniques.
For example, suitable registration methods include any one of the
following three groups of techniques. Firstly, it is possible to
use intensity based registration methods. If the 3D vessels data
comprises a vessel model or atlas, it is preferably converted into
3D volume data prior to registration. In such techniques, the
transformation is determined by maximising the intensity
similarity. Secondly, it is possible to use feature based
registration methods. These methods typically extract certain
features from both data sets. Suitable features may include vessels
junction and/or vessels shapes. The optimum transformation can be
estimated by minimising the distance between identified features of
the different sets. Finally, hybrid techniques based on both
intensity and feature registration methods can be used. Such hybrid
methods include the so-called active appearance model and particle
system methods.
[0067] Referring now to FIG. 6, a flow chart for an intensity based
registration process is shown. As can be seen from FIG. 6, step 70
comprises taking so called moving data (such as the second CTA
image mentioned above) and step 72 comprises taking fixed data
(such as the first CTA image mentioned above). A region of interest
is defined for the fixed and moving data, such as a small region
surrounding a target site defined in the brain. A step 74 is
performed which warps the moving data (in the region of interest)
using a transform (T.sub.moving.fwdarw.fixed). The intensities of
the warped moving data and fixed data are compared in step 76 by
calculation of a cost function. A step 78 then attempts to select
improved parameters for the values used in the transform
(T.sub.moving.fwdarw.fixed) which are fed back into step 74. This
process is iteratively repeated until the correlation between the
warped moving data and the fixed data can be improved no further.
The final, optimised, value of the transform
(T.sub.moving.fwdarw.fixed) is then returned in step 80.
[0068] This registration process can also be viewed as a
superposition of two basic parts. The first part is the selection
of the transformation model and the second part is the estimation
of the transformation parameters. Moreover, most image registration
problems can be treated as optimisation problems that can be
described in general mathematical terms as follows:
arg max T moving .fwdarw. fixed ( f cost ( I fixed , W ( I moving |
T moving .fwdarw. fixed ) ) ) , ( 1 ) ##EQU00001##
[0069] where T.sub.moving and T.sub.fixed are the moving (to be
warped) and fixed (target) data, T.sub.moving.fwdarw.fixed is a
parameter vector in the selected transformation model,
W(T.sub.moving|T.sub.moving.fwdarw.fixed) is warped moving data
under the transformation model with the parameter
T.sub.moving.fwdarw.fixed, and f.sub.cost() is a cost function to
measure the intensity similarity between the warped moving data and
the fixed data.
[0070] Referring next to FIG. 7, a flow-chart describing the use of
a registration technique to measure brain shift between
pre-operative and post-operative CTA images is shown. The
registration technique uses two region based rigid registrations (3
translations and 3 rotation angles) using the pre-operative and
post-operative CTA data. The method thus estimates the brain shift
by measuring the spatial changes with respect to the skull at the
point where the brain shift is to be measured (e.g. at the tip of a
stylette). This gives a vector indicating the magnitude and
direction of brain shift with respect to the skull. The
registration is based on the volumetric CTA data and it minimises
slice distance and slice thickness issues. As it is CT based, it
also avoids any MRI distortion.
[0071] Referring to FIG. 7 in more detail, steps 90 and 92 comprise
taking pre-operative and post-operative CTA images respectively.
Both images show a reference structure (e.g. the skull) and blood
vessels within the brain. A step 94 is performed that comprises a
rigid registration between the pre-operative and post-operative CTA
images based on the reference structure (e.g. the skull); this
produces the transformation T.sub.fixed2moving.sup.skull. A step 96
is also performed that comprises co-registering the blood vessels
in a predefined region of interest in the pre-operative and
post-operative CTA images; this produces the transformation
T.sub.fixed2moving.sup.vessel. The size of this region of interest
may, for example, be 50.times.40.times.15 mm.
[0072] Brain shift is then calculated, as follows, at two points
where the tips of two stylettes (left and right stylettes) are
located after insertion. Firstly, the tip position of each stylette
is determined in the and post-operative CTA image in the UVW
coordinates of the CT machine; these positions are defined as (a)
r.sub.fixed and (b) l.sub.fixed, The positions of the stylette tips
are then calculated in the pre-operative CTA using the
T.sub.fixed2moving.sup.skull thereby giving the positions (a)
r.sub.moving.sup.skull and (b) l.sub.moving.sup.skull. The
positions of the stylette tips are then calculated in the
pre-operative CTA using T.sub.fixed2moving.sup.vessel, which
provides the positions (a) r.sub.moving.sup.vessel and (b)
l.sub.moving.sup.vessel.
[0073] This allows the brain shift (S.sub.l and S.sub.r) at the
position of each stylette to be calculated from the
expressions:
S.sub.l=r.sub.moving.sup.skull-r.sub.moving.sup.vessel (2a)
S.sub.r=l.sub.moving.sup.skull-l.sub.moving.sup.vessel (2b)
Referring to FIG. 8, a process for registering MRA data with CTA
data is illustrated. Steps 100 and 102 comprise taking (e.g.
retrieving or acquiring) CTA data and MRA data respectively. A step
104 is then performed of defining a region of interest in an area
where blood vessels are visible. This region of interest may be
located around a target structure or site within the brain to which
a neurosurgical instrument is to be inserted. A step 106 is then
performed to co-register the two (MRA and CTA) data sets by
determining the optimal transformation to describe the
correspondence between the data. Information from the MRA data
(e.g. relating to the position of target structures) can then be
mapped onto the CTA data.
[0074] The techniques described above are preferably implemented
using an appropriately programmed computer. It should also be
remembered that the above examples are non-limiting and provided
merely to aid understanding of the invention. The skilled person
would be fully aware of the many variations to the above examples
that would be possible in accordance with the general teachings
that are outlined above.
* * * * *