Note: Descriptions are shown in the official language in which they were submitted.
CA 02733415 2011-03-07
METHODS AND APPARATUS FOR IMAGING IN CONJUNCTION WITH
RADIOTHERAPY
Technical Field
100011 The invention relates to medical imaging and to radiotherapy.
Embodiments
provide imaging methods that may be performed using MeV radiation sources.
Background
100021 Cancer is a disease characterized by the rapid uncontrolled growth of
cells that
are able to invade nearby tissues, as well as metastasize to other areas of
the body.
Several methods for cancer treatment are employed today. These include
systemic
treatments such as chemotherapy, hormone therapy or biological therapy, and
local
treatments such as surgery, cryosurgery. or radiotherapy.
100031 Radiotherapy is an important treatment for many types of cancer. Recent
advances in radiotherapy have provided finer control over the distribution of
radiation
dose delivered to subjects' tissues. This fine control can be exploited to
permit
radiation to be delivered to a lesion such as a tumor while sparing normal
tissue that is
closely adjacent to the tumor. Imaging is an important adjunct to
radiotherapy.
Imaging is used to identify the extent of lesions that may be treated by
radiotherapy as
well as to determine the location of the lesion relative to other nearby
anatomical
structures. Imaging is also used to monitor the response of a subject to
treatment.
[0004] The use of radiotherapy is not limited to cancer treatment.
Radiotherapy can
also be useful in the treatment of other conditions.
100051 Image-guided radiation therapy (1GRT) is a technique that involves
acquiring
images during a course of radiation therapy. IGRT can deliver radiation with
improved accuracy by taking into account changes in the subject as revealed by
the
images. Images may be taken before or during the delivery of radiation. Some
radiation sources. such as linear accelerators are equipped with imaging
systems such
as kV X-ray imagers for acquiring images of a subject while the subject is
positioned
for the delivery of radiation.
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Summary of the Invention
[0006] The invention has a number of aspects. Some of these aspects relate to
features
that can be applied individually or in combination with other aspects. A non-
limiting
list of aspects of the invention includes: treatment planning systems for
planning
radiotherapy treatments that include functionality for planning imaging
sequences:
methods for planning radiotherapy treatments that take into account imaging
dose;
methods for acquiring images in the course of radiotherapy treatments;
radiotherapy
treatment systems that incorporate imaging functionality; and media containing
computer instructions for causing a processor to perform methods as described
herein.
[0007] One aspect provides methods for imaging comprising generating a cone X-
ray
beam by directing a megavolt electron beam at a low-atomic-number target,
shaping
the cone X-ray beam to match a shape of a volume of interest (VOI) in a
subject, and
detecting X-rays of the cone X-ray beam that have passed through the subject
at an
imaging X-ray detector. The method may be practised using a medical linear
accelerator to generate the electron beam. In some embodiments, shaping the
cone X-
ray beam comprises adjusting positions of leaves of a multi-leaf collimator
and/or
rotating a multi-leaf collimator about its axis.
100081 In some embodiments the low-atomic-number target is supported on a
gantry
that is rotatable relative to the subject and the method comprises repeating:
shaping
the cone X-ray beam to match a shape of a volume of interest in a subject; and
detecting at the imaging X-ray detector X-rays of the cone X-ray beam that
have
passed through the subject to obtain a plurality of images for a corresponding
plurality
of different angles of the gantry relative to the subject.
[0009] Another aspect provides a method for planning a radiation treatment for
delivery by a radiotherapy apparatus comprising a radiation source that is
rotatable to
different beam angles around a subject and a beam shaper configured to control
a
shape of a radiation beam emitted by the radiation source. The method
comprises
defining at least one set of imaging conditions. Each set of imaging
conditions
comprises at least a beam angle and a beam shape for exposing at least one
volume of
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interest to radiation. The method estimates a volumetric radiation dose for
the at least
one set of imaging conditions and establishes a plan for a therapeutic
radiation
treatment. The plan comprises apertures for a plurality of beam angles.
Establishing
the plan comprises optimizing the apertures to deliver a desired radiation
dose to a
target region of a subject while maintaining radiation dose to tissues outside
of the
target region below one or more thresholds. Establishing the plan comprises
taking
into account the estimated volumetric radiation dose for the at least one set
of imaging
conditions at least in a selected region outside of the target region.
[0010] In some embodiments, optimizing the apertures comprises estimating
volumetric radiation doses for the apertures and summing the volumetric
radiation
doses for the apertures together with the estimated volumetric radiation dose
for the at
least one set of imaging conditions.
100111 In some embodiments the selected region outside of the target region
corresponds to a sensitive tissue desired to be spared by the radiation
treatment and
the optimization comprises applying a cost function that values minimizing
dose to
the selected region.
10012] Another aspect provides a method for planning a radiation treatment.
The
method comprises: planning exposures of a subject to radiation to be used for
imaging; computing a contribution to dose from the imaging exposures; and
using the imaging dose contributions in generating a treatment plan. The
method may
be performed automatically by a computerized treatment planning system. The
treatment plan may comprise control signals that may be applied to control a
radiation
delivery system.
100131 Another aspect provides an imaging method comprising, for each of a
plurality
of different beam angles, controlling a beam shaper to shape a radiation beam
such
that delivery of radiation is primarily limited to paths that pass through a
plurality of
volumes of interest within a subject; obtaining images of radiation that has
passed
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through the volumes of interest; and, processing the images to obtain
volumetric
images of the plurality of volumes of interest.
[0014] Further aspects of the invention and features of specific example
embodiments
of the invention are described below.
Brief Description of the Drawings
[0015] The accompanying drawings illustrate non-limiting embodiments of the
invention.
[0016] Figure 1 is a block diagram of radiotherapy apparatus according to an
example
embodiment.
[0017] Figure 2 is a graph illustrating CNR as a function of dose for bone and
lung
tissue images for three different X-ray beams.
100181 Figure 3 is a flow chart illustrating an imaging method according to
one
embodiment.
[0019] Figure 4 is a flow chart illustrating an example method for filling
images.
[0020] Figure 5 illustrates an example image. a digitally-reconstructed
radiograph
(DRR) and a filled image.
[0021] Figure 6A is a graph showing contrast as a function of dose for CT
results
based on filled and unfilled images.
[0022] Figure 6B is a graph showing noise as a function of dose for CT results
based
on filled and unfilled images.
[0023] Figure 6C is a graph showing CNR as a function of dose for CT results
based
on filled and unfilled images.
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[0024] Figures 7A and 7B illustrate dose from an imaging exposure to cone-beam
X-
ray radiation as a function of position for 4 cm diameter and 8 cm diameter
cylindrical
volumes of interest.
[00251 Figure 8A shows a beam's-eye view of a plurality of volumes of interest
projected into the plane of a multileaf collimator.
[00261 Figure 8B is an example of an arrangement of volumes of interest viewed
from
an angle for which it is impossible to use a multileaf collimator to shape an
X-ray
beam to match the projection of the volumes of interest.
[0027] Figures 8C and 8D illustrate two different configurations of the leaves
of a
multileaf collimator.
[0028] Figure 9 is a flow chart illustrating a method for obtaining 3D image
data
covering multiple volumes of interest.
[0029] Figure 10 is a block diagram of a radiotherapy system that includes a
treatment
planning unit operating in conjunction with a radiation delivery machine.
[0030] Figure 11 illustrates one method for using a 3D imaging dose
distribution in
the optimization of a treatment plan.
Description
[0031] Throughout the following description, specific details are set forth in
order to
provide a more thorough understanding of the invention. However, the invention
may
be practiced without these particulars. In other instances, well known
elements have
not been shown or described in detail to avoid unnecessarily obscuring the
invention.
Accordingly, the specification and drawings are to be regarded in an
illustrative, rather
than a restrictive, sense.
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100321 Figure 1 is a block diagram of radiotherapy apparatus 10. Radiotherapy
apparatus 10 comprises a source 12 of radiation. In the illustrated
embodiment, source
12 comprises a linear accelerator 14 that accelerates a beam 15 of electrons
to MeV
energies and directs the electron beam 15 at a target 16 of a high atomic
number (high
-- Z) material. Target 16 may, for example, comprise tungsten or tungsten
backed with
copper. Suitable targets for generating X-rays suitable for radiotherapy in a
linear
accelerator are commercially available. Electron beam 15 interacts with target
16 to
generate a beam 17 of X-rays.
-- 100331 X-ray beam 17 is shaped by upper and lower sets of jaws 18A and 18B
and a
multileaf collimator 20 before being delivered toward a patient support 19 on
which a
subject may be supported for treatment. Apparatus 10 additionally includes a
flattening, filter 22 that increases the uniformity of the fluence of X-ray
beam 17 and
an ionization chamber 23 that can be applied to measure the fluence of X-ray
beam
-- 17.
[00341 Apparatus 10 comprises a gantry 25 that permits an angle 0 at which X-
ray
beam 17 is incident toward patient support 19 to be rotated. In some cases,
gantry 25
permits rotation through a full 360 degrees around patient support 19.
100351 An imaging detector 30 is opposed to target 16 such that some X-rays in
beam
17 originating at target 16 can pass through a subject on patient support 19
and be
detected by imaging detector 30. Imaging detector 30 may comprise, for
example, an
electronic portal imaging device.
100361 Imaging detector 30 may comprise an amorphous silicon flat panel
detector.
Such detectors are in widespread use for detecting MV photons in medical
linear
accelerators. Such detectors typically comprise a layer of copper overlying
the active
detector matrix. The copper layer increases detection efficiency for MV
photons.
-- Where imaging is performed with lower energy photons, as described below,
it is
advantageous (but not required) that imaging detector 30 not have such a
copper layer.
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100371 A treatment planning system 40 generates control parameters for
apparatus 10.
The control parameters may. for example, specify a number of units of
radiation to be
delivered to a subject for each of some number of corresponding gantry angles.
MLC
rotation angles and MLC leaf settings. The control parameters may specify
conditions
for a number of discrete radiation exposures (a step-and-shoot mode) and/or
conditions for dynamic delivery of radiation (e.g. delivery of radiation while
a
configuration of apparatus 10 is changing). Apparatus 10 may be controlled
according
to the control parameters generated by treatment system 40 to deliver
radiation to a
subject according to a treatment plan.
100381 Treatment planning system 40 may comprise a computer system executing
software that generates a treatment plan under the supervision of and/or with
the
assistance of a human operator. Treatment planning system 40 has access to a
set of
image data for a subject. The image data may, for example, comprise 3D data
such as
results of a computed tomography (CT) scan. In the illustrated embodiment,
treatment
planning system 40 has access to a data store 42 containing imaging data 43
for a
subject.
[0039] One example of a treatment planning system is the ECLIPSETM treatment
planning system available from Varian Medical Systems of Palo Alto California.
[0040] Contrast to noise ratio (CNR) is a useful indicator of image quality.
One way
to define CNR is as follows:
-
CNR = _________________________________________________________________ (1)
ab
i
where: grõ, is the average signal in the subject being imaged; 3/, is the
average signal
in the background and ob is the standard deviation of the signal in the
background.
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100411 CNR can be increased by increasing dose because, in general, the
available
means for increasing dose (e.g. increasing the number of exposures by taking
images
from more angles, increasing the X-ray fluence rate, or increasing slice size)
can all
result in reductions in quantum noise. However, it is generally considered
desirable to
keep imaging doses as small as practical.
[0042] It has been determined that when X-rays resulting from the interaction
of
electron beam 15 with low-Z target 50 are used for imaging, the resulting
images can
have significantly higher contrast to noise ratio (CNR) than images based on X-
rays
resulting from the interaction of electron beam 15 with high-Z target 16.
100431 To facilitate improved imaging, apparatus 10 includes a second target
50.
Second target 50 comprises a low-atomic-number (Low Z) material. For example,
second target 50 may comprise aluminum (atomic number 13) or beryllium (atomic
number 4) or another suitable element having an atomic number in the range of
6 to
13, for example. In some embodiments second target 50 comprises an element
having
an atomic number in the range of 8 to 20. Second target 50 is preferably
sufficiently
thick that electron beam 15 does not pass through second target 50
significantly. In
some embodiments, second target 50 has a thickness of 5mm or more or 3 mm or
more. However, in some embodiments. especially those in which the energy of
electron beam 15 is reduced, second target 50 may be thinner and still stop
essentially
all electrons of electron beam 15. It is typically advantageous to make second
target
50 no thicker than necessary to provide appropriate mechanical strength and
stop
electron beam 15.
[0044] In the illustrated embodiment, apparatus 10 comprises an actuator 52
configured to insert low-Z target into electron beam 15 while removing high-Z
target
16 from the path of electron beam 15 or vice versa. There are a number of ways
in
which switching among targets 16 and 50 may be accomplished. These include.
for
30 example, steering electron beam 15 to one or the other of targets 16 and
50; rotating
or translating a carousel or other carrier on which targets 16 and 50 are
supported;
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providing separate mechanisms for moving targets 16 and 50 into and out of the
path
of electron beam 15 and controlling those mechanisms in a coordinated manner;
manually operating a mechanism to remove target 16 and replace it with target
50; etc.
100451 A beneficial feature arising from the use of a low-Z target 50 for
generating an
imaging beam is that, for a given electron beam current. X-ray photon
generation is
less efficient that for higher-Z targets. For typical electron currents
produced by a
medical linear accelerator, precise control of radiation dose can be
maintained even at
low exposures.
100461 It has also been determined that CNR may be further improved by imaging
without a flattening filter 22. The illustrated apparatus 10 comprises an
actuator 53 for
moving flattening filter 22 into or out of the path of X-ray beam 17. In some
embodiments actuators 52 and 53 are combined or operated in a coordinated
fashion
to provide an imaging configuration - in which electron beam 15 impinges on
low-Z
target 50 and flattening filter 22 is not present. and a radiotherapy
configuration - in
which electron beam 15 impinges on high-Z target 16 and flattening filter 22
is
present in the path of X-ray beam 17.
100471 Some linear accelerators provide carousels intended for holding
flattening
filters. The carousels are rotatable to bring a desired flattening filter into
the beam.
Some embodiments exploit the carousel to hold low-Z target 50 in place of a
flattening filter. In such embodiments the linear accelerator may be placed
into an
imaging mode by removing high-Z target 16 from electron beam 15, rotating the
carousel to bring low-Z target 50 into the path of the electron beam and
setting
parameters of the electron beam (e.g. beam current and beam energy) to yield
an X-
ray beam 17 having properties that are better for imaging (e.g. providing
better
contrast) than the X-ray beam 17 resulting from impingement of the MeV
electron
beam 15 on high-Z target 16.
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,
[0048] The quality of X-ray beam 17 can be further improved by reducing the
energy
of electron beam 15. For example, some medical linear accelerators can produce
electron beams with energies of 1.75 MeV or lower. With current linear
accelerator
designs the electron current (and consequently the X-ray beam flux) falls with
decreasing electron energy. This can place a lower limit on the electron beam
energy
that it is practical to use. In some embodiments, the control of a linear
accelerator is
set to produce a lower energy electron beam 15 when switching to a low-Z
target 50
and to increase the energy of electron beam 15 when switching back to high-Z
target
16. In some embodiments the electron beam energy is set to a value of 3 MeV or
less
for generating an imaging X-ray beam 17.
[0049] Figure 2 is a graph illustrating CNR as a function of dose for bone and
lung
tissue images for three different X-ray beams. Curves 55A and 55B are for a
beam
generated by impinging a 3.5 MeV electron beam on an aluminum target. Curves
57A
and 57B are for a 6MV therapeutic radiation beam. Curves 56A and 56B are for a
beam generated by impinging a 7.0 MeV electron beam on an aluminum target. It
can
be seen from Figure 2 that, for the same CNR, using the 3.5 MeV electron beam
with
a low-Z target can reduce dose by a factor exceeding 7 as compared to images
made
using the 6 MV therapeutic beam. Figure 2 is taken from Robar et al.,
Megavoltage
cone-beam imaging with low-Z targets Medical Physics, Vol. 36, No. 9,
September
2009.
[0050] In some embodiments cone beam imaging is performed using an X-ray beam
derived from a MeV electron beam in which the beam is shaped to conform to the
beam's-eye-view profile of a volume of interest. The shape of the beam may be
controlled by a multileaf collimator on its own or in combination with
adjustable
jaws, for example. In some embodiments the X-ray beam is generated using a low-
Z
target such as an aluminum carbon or beryllium target. In some embodiments a
flattening filter is not present during the imaging.
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[00511 The techniques for imaging volumes of interest described herein are not
limited to X-ray beams generated by MV electron beams. These techniques may
also
be applied to kV on-board-imaging (OBI) systems of the type that are becoming
common on medical linear accelerators. To apply these techniques using an OBI
system one would need to equip the OBI system (kV x-ray source) with some type
of
beam shaping device such as a multileaf collimator.
100521 In some embodiments planning for imaging is performed using a treatment
planning system. Treatment planning systems typically include functions for
setting a
beam shaper to match projected contours of a target volume. Such functions may
be
applied in planning for imaging. The volume of interest may be selected to
provide
imaging information that is useful for guiding the delivery of radiotherapy.
For
example: the volume of interest may be selected to include all or a portion of
a lesion
to be treated as well as all or a portion of a sensitive structure nearby the
lesion that it
is intended to spare. As another example, the volume of interest may be
selected to
include a fiduciary marker (e.g. a feature of a bone or other object that can
be used as
a reference point for determining a position or orientation of a subject).
[00531 In some embodiments a treatment system includes data defining a three-
dimensional shape of a lesion to be treated and data defining a volume of
interest for
imaging is created by expanding the three dimensional shape such that the
imaging
volume of interest includes the lesion as well as a layer of tissues
immediately outside
of the lesion. Radiation may be delivered in sessions over the course of
several days.
Some treatments may be spread out over weeks. Over such a period a subject may
lose
weight or gain or lose fluids. A lesion being treated may shrink or grow.
Imaging of
the volume of interest may be performed before each session, for example.
100541 Obtaining images during the course of a radiation treatment can be
especially
beneficial in the case where a lesion being treated is in soft tissue and may
move
around depending upon the subject's posture or changes in the subject or cases
where
a lesion may change in shape or position during the time span over which the
treatment is delivered.
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[0055] As one example application, consider the case where a tumor or other
lesion to
be treated by radio therapy is close to a subject's spinal cord. It is desired
to spare the
spinal cord while delivering radiation to the lesion. An imaging volume of
interest
may include all or a portion of the lesion as well as all or a portion of the
part of the
spinal cord that passes near to the lesion. Images of this volume of interest
may be co-
registered with an image of a volumetric dose distribution to be delivered by
a
treatment plan and used to verify that a plan to deliver radiation to the
lesion will,
catch the entire lesion while, as much as possible sparing the spinal cord.
One can
determine from the image whether the subject's position is exactly correct
such that
the therapeutic radiation will be delivered to the lesion and avoid the spinal
cord.
[0056] If the image indicates that the volumetric dose distribution to be
delivered by
the treatment plan is not ideal for some other reason - for example where the
image
indicates that: the lesion has grown so that a portion of the lesion would not
be
adequately irradiated by executing the treatment plan; or the lesion has
shrunk so that
areas external to the lesion would receive more radiation than necessary by
executing
the treatment plan- then it may be necessary to establish a new treatment plan
based
on new imaging.
[0057] The image can also be used to verify that the subject is positioned in
such a
manner that the planned radiation dose will be delivered to the tumor. As
another
example application a small volume of interest may include the prostate, as
well as
interfaces with the rectum and bladder. Shaping an X-ray cone beam to conform
to
such a volume of interest permits imaging the prostate while largely sparing
peripheral
volumes of the pelvis from radiation exposure.
100581 An advantage of imaging using a shaped beam is that the total dose
delivered
during imaging is reduced since radiation dose is greatly reduced outside of
the
boundary of the shaped beam. However, image truncation resulting from the beam
shaping can result in severe imaging artifacts that can deleteriously affect
the
usefulness of the resulting images. Such artifacts tend to arise particularly
where a
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number of images are acquired and combined into a 3D dataset using computed
tomography (CT) imaging techniques.
100591 Some embodiments perform CT imaging using a cone beam. In such
embodiments, images are obtained for each of a plurality of different gantry
angles.
For each of the images the cone beam is shaped to conform with the projection
of a
volume of interest. The shaping is performed taking into account the geometry
of the
cone beam. The projection is a conical projection following rays of the
imaging cone
beam. Since the beam is diverging the shape imposed by a beam shaper such as a
multileaf collimator is magnified as the beam propagates to the subject.
Consequences
of this geometry are that beam shaper apertures set in a planning system to
shape the
beam to match a volume of interest need to be scaled to take into account this
magnification.
[00601 Where an imaging beam has a different beam geometry from a therapeutic
beam then this different beam geometry needs to be taken into account both for
establishing beam shaper settings to appropriately shape the imaging beam and
for
scaling acquired images to provide accurate spatial calibration.
100611 The resulting images are then combined to provide a 3D data set. In
some such
embodiments the individual images are filled outside of the projection of the
volume
of interest with image data. The image data used for the fill may be obtained
in
various ways from various sources as discussed below. Filling the individual
images
prior to combining them to yield a 3D dataset can significantly reduce
truncation
artifacts.
100621 The filled images may be combined using suitable cone-beam CT (CBCT)
techniques. CBCT imaging techniques which combine 2D images from multiple
angles to provide a 3D dataset are known to those of skill in the art. In some
embodiments, a Feldkamp-Kress-Davis (FDK) filtered back-projection algorithm
is
applied for reconstructing images from the dataset. Optionally the images are
filtered
prior to back-projection. For example. Shepp-Logan. Hamming, Cosine or Hann
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filters may be applied. Cone-beam CT imaging software for processing sets of
images
into 3D datasets and reconstructing images from the datasets is commercially
available. Such software may be applied to combine the filled individual
images and
to generate reconstructed images of the subject in desired planes.
[0063] Figure 3 is a flow chart illustrating an imaging method 60 according to
one
embodiment. In block 62, method 60 sets a MV radiotherapy source into an
imaging
mode to deliver radiation for imaging. Block 62 may include, for example.
replacing a
high-Z target with a low-Z target, removing a flattening filter and setting
the electron-
beam energy for a linear accelerator. In some embodiments. in the imaging
mode,
35% or more of the photons in the X-ray beam have energies in the range of
25keV to
150 keV. Block 62 is optional in the case that the radiotherapy source is
already
appropriately set up.
[0064] Loop 64 acquires images from a desired number of gantry angles. In
block
65A the gantry is positioned for the current image. In block 65B a beam shaper
such
as a multileaf collimator is set to shape an X-ray beam to the shape of a
projection of
the volume of interest. Block 65B may, for example comprise setting one or
both of
an angle of rotation of a multileaf collimator and leaf positions for leaves
of the
multileaf collimator. In block 65C, which is optional, beam parameters for the
exposure are set. Block 65C may be useful in the case of an off-axis volume of
interest in cases where the flux of X-ray beam 17 varies significantly across
the beam.
For example, when the volume of interest is located in a higher-flux portion
of X-ray
beam 17 electron beam current may be reduced below the current used when the
volume of interest is located in a lower flux portion of X-ray beam 17. This
can
reduce the dose delivered to the subject.
100651 In block 65D the subject is exposed to the shaped X-ray beam and an
image 66
is acquired. In block 65E the image 66 is corrected to compensate for
variations in
the fluence of the imaging beam as well as variations in the sensitivity of
the detector
used to obtain images 66.
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[0066] The corrections in block 65E may be based on previously-acquired
calibration
information that characterizes the X-ray beam. For example, prior to all image
acquisition, dark field (IDF) and flood field (IFF) images may be acquired for
image
calibration. A dark field image is obtained without applying any beam to the
imaging
panel. A dark field image may be applied to correct for any variations of dark
current
between individual detector elements.
100671 A flood field image may be acquired by exposing the entire sensitive
area of
the imaging panel to the beam and acquiring an image. A flood field image can
be
applied to correct for non-uniformities in the fluence of the imaging beam as
well as
for non-uniformity in detector response.
[0068] Any images acquired after IDF and 'FT images may be corrected in
software
by subtracting the IDF image and dividing the result by the IFF image to
produce a
corrected image. In some embodiments correction is performed by computing the
result:
/114 = ________________________________ -IDF
(2)
IFF
where IM, is the uncorrected image data 66,1M/ is the corrected image data and
IDF
and IFF are the dark field and light field images as defined above.
[0069] Block 65F determines whether more images are to be obtained. Block 68
fills
the portions of each image 66 lying outside of the volume of interest with
image data.
Block 69 combines the filled images 66 to provide a 3D dataset 70. Block 69
may
comprise back-projection of the images 66. Optionally block 69 comprises
filtering
images 66 prior to back-projecting images 66. Block 72 uses dataset 70 to
construct
and display an image. Block 72 may comprise, for example. constructing an
image for
corona', sagittal or axial slices through the volume of interest.
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100701 Method 60 may be varied many ways. For example. blocks 65E and 68 may
be
performed inside or outside of loop 64. Where blocks 65E and 68 are performed
inside loop 64 part or all of block 69 may also be performed in loop 64.
100711 For clarity of explanation method 60 is described above as operating in
a step-
and-shoot mode with discrete motions of a gantry and beam shaper between
imaging
positions. The invention is not limited to step-and-shoot modes. The skilled
reader
will understand that imaging may be performed while the gantry is rotated
continuously together with concurrent, synchronized, dynamic motion of the
beam
shaper. For example, a gantry may be driven to rotate through an arc without
stopping
while leaves of a multileaf collimator are driven to execute a dynamic
sequence in
tandem with the gantry motion. Images may be acquired at predetermined angles
throughout the gantry rotation. Acquiring each image may comprise generating
an
imaging X-ray beam and operating an imaging X-ray detector.
[0072] One source of image data for filling the truncated images is previously-
acquired CT data. It is almost always the case that a CT scan for a subject
has been
obtained for use in planning treatment for the subject prior to delivery of
the treatment
to the subject. In some embodiments image data for use in tilling images 66 is
obtained from such CT scan data (e.g. imaging data 43 - see Figure 1).
[0073] Figure 4 illustrates an example method 73 for filling images 66. In
block 74
imaging data 43 is processed to yield a digitally reconstructed radiograph
(DRR) 75
from the point of view of the beam for the gantry angle corresponding to the
current
image 66. The DRR 75 is constructed based upon the geometry of the cone beam
used
for imaging. Low-Z target 50 may be (and for linear accelerators of the type
in use in
2011 usually will not be) in the same location relative to the subject as high-
Z target
16. Where low-Z target 50 is located closer to the subject than high-Z target
16 the
low-Z imaging beam will gave greater divergence than the therapeutic beam.
Also.
photons from the low-Z target 50 will have substantially different energy
spectral
characteristics compared to a therapeutic beam. Algorithms for generating DRRs
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typically include the attenuation coefficient for photons, which depends on
the
incident spectrum (as well as the characteristics of the tissues through which
the
photons pass). Block 74 may implement an algorithm for generating DRR images
75
that takes into account these factors in order to produce DRRs 75 that closely
match
acquired low-Z images.
100741 Some embodiments trade off between the quality of DRR 75 and the
processing to generate DRR 75. A lower quality DRR 75 computed by applying a
simplified algorithm may have quality sufficient for use in filling images 66.
100751 DRR 75 may be pre-computed and stored in which case block 74 may
comprise retrieving the appropriate DRR 75 from a data store.
100761 Block 76 generates a mask 77 corresponding to the projection of the
volume of
interest in the current image 66. Block 76 may, for example, identify as
belonging to
the volume of interest all pixels of image 66 having values exceeding a
threshold or
calculate a mask 77 from data defining the volume of interest. In an example
embodiment mask 77 has the form of a binary image (pike' values are either 1
or 0).
Mask 77 may be a negative of the projected volume of interest (for example,
mask
values may be set to 1 for pixels in which the pixel value from image 66 is
less than a
threshold and 0 otherwise). In alternative embodiments mask 77 may be a
positive of
the projected volume of interest.
100771 Block 78 combines the image 66 with the corresponding DRR 75 using mask
77 to yield a filled image 67 which is the same as image 66 within the
projected
boundary of the volume of interest and is made up of image data from DRR 75
outside of the projected boundary of the volume of interest.
100781 In an example embodiment mask 77 has the form of a binary image (pixel
values are either 1 or 0). The mask 77 may be a negative of the projected
volume of
interest (for example, mask values may be set to 1 for pixels in which the
pixel value
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from image 66 is less than a threshold and 0 otherwise). DRR 75 may be
multiplied by
mask 77 and the result may be added to image 66 to obtain a filled image 67.
[0079] Figure 5 illustrates an example image 66, a DRR 75 a mask 77 and a
filled
image 67. In filled image 67 the boundary of the projected volume of interest
is
indicated by dashed line 79A, a region 79B inside boundary 79A comprises image
data from image 66, a region 79C outside boundary 79A comprises image data
from
DRR 75.
100801 Optionally pixel values in the image data used to fill truncated images
66 are
matched to pixel values in adjacent pixels within the volume of interest of
truncated
images 66. For example, where the image data used to fill truncated images 66
includes the volume of interest, a correlation in pixel values can be
established by
comparing corresponding regions in the truncated image 66 and the DRR 75 or
other
image data being used for filling. For example, one may generate a tone
mapping
curve (or 'grey level transformation') by plotting the values of pixels in the
truncated
image versus the values of corresponding pixels in the filling image data and
apply the
tone mapping curve to modify the filling image data to better match the
truncated
image 66. The tone mapping curve may be parameterized by fitting a
parameterized
curve to the plotted curve.
100811 Especially where images 66 are acquired for closely-spaced gantry
angles it is
not mandatory to calculate a separate DRR for filling every image 66.
Optionally the
same DRR may be reused to fill images 66 for two or more closely-spaced gantry
angles. For example a separate DRR may be computed for the images 66 taken
within
a range of gantry rotation angles. The range may span 2 or 4 or 5 degrees for
example
or even larger angles such as 15 or 20 degrees.
100821 Image data for use in filling images 66 may also be obtained by taking
some
images using unshaped (full-frame) X-ray beams or X-ray beams shaped to have
boundaries outside of the projected boundary of the volume of interest. For
example,
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one such images may be acquired for use in filling other images within a range
of
gantry angles. In an example embodiment N images 66 are acquired. An image 66
is
acquired for every m degrees of gantry rotation over an angular range spanning
(N-1)xm degrees. For example. an image may be acquired every 2 or 3 degrees of
gantry rotation over a suitable range (e.g. a range spanning about 180 degrees
- 180
degrees plus the angle of the X-ray cone beam is ideal).
[0083J The X-ray cone beams used to acquire images 66 may be shaped to match
the
projected boundary of a volume of interest except that every nth image 66 may
be a
full-frame image. Each image 66 requiring fill may be filled using image data
from
the nearest full-frame image. In some embodiments full-frame images are only
obtained for every 15 or 20 degrees or more of gantry rotation. In some
embodiments,
n is 5 or more or 10 or more.
[0084] In experiments done imaging a RANDOTm head phantom it was found that
the
quality of the portions of reconstructed images within the volume of interest
was quite
insensitive to the angular separation between full-frame images used for
providing
image data for fill. It was found that image quality in the region outside the
volume of
interest is compromised by sparse projection data. However, for many
applications
image quality in the region outside the volume of interest is unimportant.
100851 Although reducing artifacts is a main benefit of filling images 66,
filling may
provide some additional benefit as a result of improvement of CNR. Figure 6A
is a
graph showing contrast as a function of dose for CT results based on filled
(curve
80A) and unfilled (curve 80B) images. Figure 6B is a graph showing noise as a
function of dose for CT results based on filled (curve 81A) and unfilled
(curve 81B)
images. Figure 6C is a graph showing CNR as a function of dose for CT results
based
on filled (curve 82A) and unfilled (curve 82B) images.
100861 Other types of image reconstruction are also possible. For example,
images
may be reconstructed using pi-line reconstruction as described in Zou Y. et
al. Exact
image reconstruction on P1-lines tram minimum data in helical cone-beam CT
Phys
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Med Biol. 2004 Mar 21;49(6):941-59. Such reconstructions may be used in some
embodiments.
[0087] The data on which Figures 6A, 6B, and 6C are based was for a beam
shaped
using a 10 cm aperture. Fill was provided from full-field images having a
width of 26
cm. One full field image was obtained for each 20 degrees of gantry rotation.
Truncated images were filled using image data from the closest full-field
image.
[0088] The imaged subject was a bone object in a uniformly cylindrical water
phantom. It can be seen that filling the images beneficially increases
contrast and
decreases noise. Thus, CBCT data of a desired quality can be obtained for at
least
some subjects with lower radiation doses when CBCT images are filled than when
the images are subjected to CT processing without being filled.
[0089] Figures 7A and 7B illustrate dose from an imaging exposure to cone-beam
X-
ray radiation as a function of position for 4 cm diameter and 8 cm diameter
cylindrical
volumes of interest. The doses plotted in Figures 7A and 7B were measured
using
thermoluminescent dosimeters inside a phantom. Figure 7A shows dose as a
function
of position in an anterior-posterior direction in a saggital plane through the
volume of
interest. Curve 83A is dose for a full-field image. Curve 83B is dose for a
volume of
interest 8 cm in diameter. Curve 83C is dose for a volume of interest 4 cm in
diameter. Figure 7B shows dose as a function of position in a left-right
direction in a
coronal plane through the volume of interest. Curve 84A is dose for a full-
field image.
Curve 84B is dose for a volume of interest 8 cm in diameter. Curve 84C is dose
for a
volume of interest 4 cm in diameter.
[0090] Repositioning the leaves of a multileaf collimator takes some time.
Larger
movements typically require longer times. To reduce the time required for
obtaining
full-field images and images using X-ray beams shaped to conform to regions of
interest one can acquire a number of full-field images and then acquire a
number of
images using shaped X-ray beams. For example, one could move the gantry
through a
range of angles in one direction while obtaining full-field images and then
move the
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gantry again through the range of motion while acquiring images using shaped X-
ray
beams.
10091! In an example embodiment. the gantry acquires images 66 using shaped X-
ray
beams as it is moved through about 180 degrees. Subsequently the direction of
gantry
rotation is reversed and full-field images are acquired as the gantry is moved
back
through the angular range. In the alternative, full field images can be
acquired first and
images can then be acquired using a shaped beam. In another example embodiment
the gantry is rotated through 360 degrees and the MLC is controlled to shape
the X-
ray beam for images taken in a 1/2 rotation of the gantry and to obtain full-
frame
images in the other 1/2 rotation of the gantry.
100921 Another option for filling images 66 is to fill image areas outside of
the
volume of interest by extrapolation from image areas inside the volume of
interest.
This may be done on a line-by-line basis, for example. A fitting function such
as a
polynomial function may be fit to pixel values inside an image area
corresponding to a
volume of interest. Pixel values for image areas outside the volume of
interest can
then be set according to the fitting function. The fitting function may be
chosen to
avoid sharp discontinuity at the boundary of the part of the image
corresponding to the
volume of interest. The fitting function may be a lower order polynomial
function for
example.
100931 As a simpler alternative to shaping the X-ray beam to conform with a
volume
of interest, truncated images may be obtained by shaping the X-ray beam with a
predetermined on-axis shape that is the same for all apertures (i.e. the same
for each
image 66). The shape could, for example, be a circle, ellipse, oval or other
rounded
shape, a stripe or rectangle or the like.
100941 An advantage of VOI CBCT image acquisition is that radiation dose is
reduced not only outside of the volume of interest (i.e. in largely-shielded
patient
volumes) but also within the volume of interest itself. This advantage arises
where a
beam is shaped using apertures that are small relative to full-field
acquisition. In such
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cases the contribution to the dose delivered to the volume of interest by
photons
scattered from outside of the volume of interest is reduced. This effect of
beam
shaping is especially significant for low-Z imaging beams because in such X-
ray
beams (which have lower effective beam energies than beams generated from a
high-
Z target), the proportion of scattered photons tends to be higher relative to
primary
photons. The reduction of scattered photons can improve aspects of image
quality as
compared to full field imaging since scatter degrades both contrast and
spatial
resolution.
100951 It can be appreciated that some embodiments of the imaging apparatus
described above offer the advantage that imaging can be performed in a manner
that is
tightly integrated with the delivery of therapeutic radiation. No auxiliary
imaging
system is required. Associated overhead in terms of cost and quality assurance
are
reduced. A further advantage offered by some embodiments is that the imaging
and
therapeutic beams are coaxial and so a beam's-eye-view image for the imaging
beam
is also a beam's-eye-view image from the perspective of the therapeutic beam.
100961 In some embodiments. imaging as described herein may be performed
simultaneously for a plurality of different volumes of interest. The different
volumes
of interest may be disconnected from one another or may be contiguous or even
overlap.
100971 Depending upon the capabilities of a beam shaper (e.g. a MLC) it may be
possible to obtain images 66 using beams that are shaped to expose a plurality
of
volumes of interest while reducing or substantially eliminating exposure to
radiation
outside of the regions of interest. For example, Figure 8A shows a beam's-eye
view of
a plurality of volumes of interest 86A. 86B, and 86C projected into the plane
of a
multileaf collimator and shows positions of leaves 88 of the multileaf
collimator that
would shape an X-ray cone beam to expose the volumes of interest. Where the
plural
volumes of interest have sizes and locations such that a multileaf collimator
can be
controlled to shape the X-ray beam to expose the plural volumes of interest
then
imaging the plural volumes of interest may be performed as described above.
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(00981 In cases where the multileaf collimator or other beam shaper cannot be
controlled to shape the X-ray beam appropriately then the plural volumes of
interest
may be imaged using a plurality of apertures. Figure 8B is an example of an
arrangement of volumes of interest 87A, 87B, and 87C (collectively volumes 87)
viewed from an angle for which it is impossible to use a multileaf collimator
to shape
an X-ray beam to match the projection of the volumes of interest. This is
because to
block radiation from central area 89, at least some leaves 88 of the multileaf
collimator would need to also block radiation from reaching one or more of the
volumes of interest 87.
[00991 In such cases images 66 can be acquired using two or more X-ray beam
shapes
and then combining the resulting images. For example. Figures 8C and 8D
illustrate
two different configurations of the leaves 88 of a multileaf collimator. Two
images
obtained using X-ray beams shaped by these configurations will image the
volumes of
interest 87 but, each of the X-ray beams is shaped to avoid exposure outside
of
volumes of interest 87.
101001 In some embodiments a multileaf collimator is rotated about its own
axis to
allow leaves 88 to be adjusted to better match the contours of the boundaries
of the
projections of volumes of interest.
101011 Figure 9 illustrates a method 90 for obtaining 3D image data covering
multiple
volumes of interest. Loop 92 is repeated for a plurality of gantry angles that
are spaced
apart by a suitable angular distance. Block 94 determines whether the beam
shaper can
be configured to shape the X-ray beam to conform with the projection of the
volumes
of interest for the current angle. If so (YES result), block 95 configures the
beam
shaper to shape the X-ray beam and block 96 acquires an image 66.
101021 If block 94 determines that it is not possible to configure the beam
shaper to
shape the X-ray beam to conform with the projection of the volumes of interest
for the
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current angle (NO result) then block 97 determines a plurality of beam shaper
configurations that each shape the X-ray beam to expose a portion of the
projections
of volumes of interest. In aggregate, the volumes of interest are all exposed
by
exposures using made using the plural beam shaper configurations and radiation
is
blocked from other areas. Block 98 configures the beam shaper to shape the X-
ray
beam according to a current one of the configurations and block 99 acquires an
image
66A. Loop 100 is repeated until images 66A have been obtained using X-ray
beams
shaped by each of the configurations determined by block 97.
[01031 Block 102 combines images 66A into an image 66. Block 102 may comprise.
for example, setting all pixel values in images 66A that are below a threshold
value to
zero and then summing the images 66A.
101041 Block 106 fills images 66 as described above. Block 108 processes the
filled
images 67 to provide a 3D data structure. Block 110 recreates and displays an
image
in a plane passing through one or more of the volumes of interest based on the
3D
data structure. Block 110 optionally displays highlighting, lines or other
indicia
indicating boundaries of the volumes of interest on the displayed images.
[0105] It is not necessary that all volumes of interest be imaged in the same
image
quality. Some volumes of interest may be imaged using higher doses than other
volumes of interest. For example, a particular volume of interest (e.g. a
target volume
for radiotherapy and its immediate margin) may be imaged with a dose
sufficient to
provide a relatively high contrast-to-noise ratio while simultaneously
capturing the
external surface of the patient at lower CNR.
[0106] Imaging different volumes of interest with different doses may be
achieved in
various ways. One approach is to image different volumes of interest using
different
apertures (different beam shaping). Exposure in each aperture may then be
controlled
to achieve a desired image quality in the volume(s) of interest corresponding
to the
aperture. Another approach is to image a second volume of interest in fewer
apertures
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than a first volume of interest. This may be achieved, for example, by shaping
an
imaging beam to image the first volume of interest for a number of steps of
gantry
rotation and opening the aperture to include the second volume of interest for
only
some of the gantry rotation angles. For example, a full-field exposure could
be taken
at every nth gantry rotation step while exposures at other gantry rotation
steps may be
limited by shaping the beam to conform with the projection of the first volume
of
interest. Such full-field exposures may also be used as a source of fill image
data as
described above. These two approaches may be combined.
101071 Apparatus and methods according to some embodiments calculate
volumetric
radiation doses delivered during imaging. The imaging radiation doses may be
included in radiation dose estimates being used by a treatment planning
system. In
some embodiments a treatment planning system is configured to optimize a
radiation
treatment plan based upon dose estimates that include radiation dose delivered
during
imaging. Such embodiments may but do not necessarily apply the imaging methods
as
described above. In some embodiments doses from other imaging modes (e.g. MV
CBCT) may be included in dose estimates used in optimizing a treatment plan.
101081 Calculating dose delivered by imaging or therapeutic beams typically
requires
knowledge of the location of the external surface of the subject (e.g. the
location of
the subject's skin surface) since the dose calculation should take into
account
attenuation of the beam with depth. In some embodiments where it is desired to
estimate an imaging dose based on VOI CBCT images, or to recalculate a dose of
therapeutic radiation based on VOI CBCT images, a volume of interest that
includes
the subject's skin surface may be imaged using a relatively low dose. It may
be
sufficient to have image quality just good enough to determine the location of
the
external surface of the subject. For example, a volume of interest centered on
a target
volume can image the target volume at high quality while an outer volume that
includes the external surface of the subject can be imaged at lower quality
and dose as
described above.
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10109] In some embodiments, the external region including the subject's
external
surface is post processed to reduce noise. Image details for accurate dose
estimation
may be sourced from planning CT data and fitted to the external region by
deformable
co-registration.
10110] In embodiments where imaging is not performed using the same beam used
for
radiation therapy then the treatment planning system and radiotherapy
apparatus may
be commissioned and validated for both the radiotherapy beam and the imaging
beam.
For example, apparatus may be commissioned for both a treatment beam generated
using a high-Z target flattened using a flattening filter and an imaging beam
generated
using a low-Z target and no flattening filter. The commissioning may reflect
differences in the geometries of the imaging and treatment beams as well as
differences in the spectral makeup of the imaging and treatment beams.
101111 Figure 10 is a block diagram of a radiotherapy system 120 that includes
a
treatment planning unit 112 operating in conjunction with a radiation delivery
machine 122. Radiation delivery machine 122 may comprise, for example. a
linear
accelerator.
[0112] Radiation delivery machine 122 comprises interchangeable targets 124A
and
124B. A controller 125 is connected to configure radiation delivery machine
122 to
deliver a therapy beam using target 124A or an imaging beam using target 124B.
[0113] Treatment planning unit 112 has access to a data store 127 containing
data
128A and 128B that respectively characterize the therapy beam and the imaging
beam.
Treatment planning unit 112 has access to a data store 129 (which may be part
of data
store 127 or separate from data store 127) containing 3D imaging data 130 for
a
subject. 3D imaging data 130 may include data acquired from one or more of CT
scanning, magnetic resonance imaging (MRI), positron emission tomography
(PET).
ultrasound scans or other imaging modalities.
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[01141 An operator can work with treatment planning unit 112 to establish a
target
radiation dose distribution. For example, the desired target dose distribution
may be
essentially constant inside a tumor in the subject and zero (or as close to
zero as
possible) in normal tissues surrounding the tumor. The operator may view
images
generated from images 3D imaging data 130 for assistance in specifying the
target
dose distribution. In the illustrated embodiment target dose distribution data
132
specifies the target dose distribution calculated by treatment planning unit
112..
[01151 The operator can work with treatment planning unit 112 to establish a
treatment plan that attempts to efficiently and accurately deliver the target
radiation
dose distribution as specified by target dose distribution data 132. The
treatment plan
comprises instructions 134 that can be executed by controller 125 of radiation
delivery
machine 122 to deliver radiation to the subject. Instructions 134 may
comprise, for
example instructions identifying gantry angles, beam shaper settings (e.g.
leaf
positions and rotation angles for a multileaf collimator. jaw positions, etc.)
and beam
conditions (e.g. accelerator energy and fiuence). The instructions may specify
a step-
and-shoot mode of radiation delivery and/or dynamic modes of radiation
delivery.
[01161 A treatment plan may provide for delivery of the radiation in a number
of
fractions. The fractions may be delivered at intervals (for example one
fraction per
day, one fraction every few days or one fraction every several hours). The
interval
between fractions typically depends upon the condition being treated and the
treatment approach decided upon by the managing physician in consultation with
the
subject. Each fraction may comprise irradiation from a number of gantry angles
with
radiation that is shaped in one or more ways at each gantry angle. Some
treatment
plans involve many fractions and are designed to be executed over a period of
days or
weeks. Other treatment plans are designed to be executed over shorter periods.
Some
treatments, such as certain radiosurgery treatments can be executed by
delivering a
single fraction.
101171 Various approaches to treatment planning are known to those of skill in
the
art. Treatment planning systems for radiation therapy are commercially
available.
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One example is the ECLIPSETM treatment planning system available from Varian
Medical Systems of Palo Alto. California. Treatment planning unit 112 may
comprise
an add-on to an existing treatment planning system or a stand-alone treatment
planning system, for example.
[01181 Treatment planning unit 112 is configured to develop one or more
imaging
sequences in conjunction with a treatment plan. The imaging sequences may, for
example, be used to verify that the subject is properly positioned for the
delivery of
each fraction. It is necessary to ensure that the subject is in the proper
position relative
to radiation delivery machine 122 for each fraction. In an example embodiment,
a
treatment plan may include an imaging sequence at the beginning of each
fraction.
After one or more volumes of interest have been identified the imaging
sequence for
imaging those volumes of interest may be automatically generated and added to
the
treatment plan to be executed before each fraction or before certain
fractions.
101191 The imaging sequence may specify acquisition of images over a full
gantry
rotation, gantry rotation of about 180 degrees (e.g. 180 degrees plus the
angle of the
X-ray cone beam). Other angular ranges may also be used depending on the
application. For example, methods and apparatus as described herein may be
applied
in 'tomosynthesis', a technique in which a narrow rotational range is selected
in order
to reconstruct an image in a desired. single plane through the subject. The
small range
of angles is chosen based on the desired image plane. For example. one could
acquire
data to reconstruct just the sagittal or just the coronal plane of the subject
by
acquiring projections in a narrow range around that plane. Methods and
apparatus as
described herein may be applied, for example to provide a low-Z VOI
tomosynthesis.
101201 In some embodiments the imaging sequences comprise instructions that
can be
executed by controller 125 of radiation delivery machine 122 to place
radiation
delivery machine 122 in an imaging mode, deliver imaging radiation to the
subject,
and trigger an imaging detector to collect image data for each image. The
instructions
may comprise, for example instructions identifying gantry angles, beam shaper
settings (e.g. leaf positions and rotation angles for a multileaf collimator,
jaw
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positions, etc.) and beam conditions (e.g. accelerator energy and fluence). In
some
embodiments the instructions specify beam shapes for the acquisition of images
for
one or more volumes of interest as described above.
[01211 In some embodiments the instructions specify different beams for
imaging and
treatment. The beams may differ, for example in terms of energy spectra and/or
geometry. In some embodiments, imaging beams and treatment beams are both
generated using a MV electron beam from the same linear accelerator. Figure 10
shows imaging instructions 136. Imaging instructions 136 may be combined with
or
separate from treatment instructions 134.
[01221 Treatment planning unit 112 may be configured to estimate a 3D imaging
dose distribution 140 that will be delivered to the subject upon execution of
the
imaging sequence. 3D imaging dose distribution 140 may be used in the
optimization
or re-optimization of a treatment plan.
101231 Figure 11 illustrates one method 150 for using a 3D imaging dose
distribution
in the optimization of a treatment plan. In block 152 a treatment plan is
initialized. In
block 154 the dose distribution that would be delivered by the initial
treatment plan is
calculated using data 128A that characterizes a therapeutic beam to be used in
executing the treatment plan. In block 155 the dose distribution of block 154
is
compared to the target dose distribution 132.
[0124] Method 150 includes an optimization loop 156. In block 156A the
treatment
plan is modified. The modification may be stochastic or determined according
to
another optimization methodology.
[01251 In block 156B the dose distribution that would be delivered by the
modified
treatment plan is calculated using data 128A. In block 156C the dose
distribution of
block 156B is compared to the target dose distribution 132. Block 156D
determines
whether the treatment plan as modified in block 156A is better than the
treatment plan
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prior to modification by block 156A (e.g. the modified treatment plan
satisfies
relevant criteria and a the comparison of block 156C indicates a dose
distribution that
is closer to target dose distribution 132). Block 156D keeps the better of the
modified
treatment plan and the treatment plan prior to modification by block 156A.
Block
156E determines whether a termination condition is satisfied. If so (YES
result)
optimization loop 156 ends. Otherwise (NO result) processing continues at
block
156A. In block 158 the optimized treatment plan is stored.
[0126] Method 150 includes block 160 that calculates 3D imaging dose
distribution
140 from previously established imaging instructions 136 and data 128B
characterizing an imaging beam. Blocks 154 and 156C add all or parts of
imaging
dose distribution 140 to the dose distributions estimated for the treatment
plan.
[0127] A wide variety of computer-implemented treatment planning algorithms
are
known and described in the patent and technical literature. It is typical that
such
algorithms include optimization steps in which a dose distribution is
estimated and,
based upon the estimated dose distribution (usually based on a comparison of
the
estimated dose distribution to a target dose distribution), further
optimization steps are
performed. Many such algorithms are inverse planning algorithms which start
with a
desired radiation dose distribution and attempt to establish a treatment plan
(set of
instructions for a radiation delivery system) that will deliver the desired
radiation dose
distribution to the subject.
[0128] One non-limiting aspect of the present invention is to provide new
computer-
implemented treatment planning algorithms and systems by modifying such
existing
algorithms by: determining an imaging radiation dose, as described herein and
including that imaging radiation dose in the estimated dose distributions used
by the
treatment planning algorithm - thereby arriving at a treatment plan in which
the
imaging dose is counted as contributing to the therapeutic dose and the
treatment plan
is optimized taking into account a dose expected from imaging during delivery
of the
treatment.
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[0129] Some embodiments provide an automated treatment planning system that
receives an imaging volume from a user. The imaging volume may. for example,
be
defined relative to previously-obtained imaging data for the subject. The
previously-
obtained imaging data may comprise data from a CT scan, MRI or other imaging
modality or modalities, for example. The treatment planning system also
receives
from a user information specifying an imaging frequency (e.g. once per
fraction, once
per day, once every two days or the like). Optionally the treatment planning
system
receives from a user information specifying a required imaging quality. Based
upon
the user-supplied information and data characterizing an imaging beam (e.g. a
low-Z
beam as described above or other imaging beam) the treatment planning system
calculates an imaging dose distribution. The treatment planning system
receives from
the user definition of a target radiation dose distribution and then applies
an inverse
planning algorithm to generate a treatment plan for delivering a radiation
dose
distribution that is as close to the target radiation dose distribution as
practical. In the
inverse planning algorithm the previously-calculated imaging dose is used as a
baseline dose.
[0130] In some embodiments the incorporation of imaging dose distribution 140
in
the dose estimates used in optimizing a radiation treatment plan provides one
or more
of the following advantages: more accurate estimation of the dose that will be
delivered to a subject upon execution of a radiation treatment plan with
associated
imaging; the opportunity to obtain higher quality images by increasing imaging
doses
without increasing the overall dose delivered to the subject (since an
increase in
imaging dose at a location can be compensated for by modifying the treatment
plan to
decrease the therapeutic dose delivered at that location), and the opportunity
to
leverage the technology in existing treatment planning systems (treatment
planning
systems typically include functions for estimating dose distributions and
functions for
matching beam shapes to target regions. these functions can be modified
relatively
easily for estimating imaging dose and planning imaging beams).
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101311 At imaging machine 122 a subject may be placed in position and imaging
instructions 136 may be executed to obtain image data 70. Image data 70 may
comprise a VOI CBCT image set, for example. Imaging machine 122 may perform
automated co-registration between image data 70 and a target dose
distribution.
Imaging machine 122 may display alignment indicia indicating where features in
image data 70 ought to be located when the subject is properly positioned for
treatment. A user may view the images and alignment indicia to determine
whether it
is necessary to reposition the subject and, if so. to determine repositioning
parameters.
101321 In addition or in the alternative, image processing may be performed on
image
data 70 to locate fiducial features and to compare locations of those fiducial
features
to target locations. Non-limiting examples of fiducial features are gold seeds
that have
been implanted in the subject at known locations relative to a target volume;
a tumor
of a type that can be imaged with sufficient contrast to be detected; features
of bones
and the like. Radiation delivery machine 122 may include an imaging control
that
permits the user to view images of a target volume in various planes and/or
from
various viewpoints to check that surrounding tissues are not receiving more
radiation
dose than necessary.
101331 Since imaging and therapeutic radiation are both delivered by radiation
delivery machine 122 therapy can be done in the course of acquiring an image
set or
vice versa. This can allow, for example. images to be acquired as therapy
proceeds. In
some embodiments, images acquired in the course of delivering therapeutic
radiation
are 2-D images taken in a beam's eye view direction. Such images may be
obtained by
switching radiation delivery machine 122 into an imaging mode, obtaining an
image,
and switching radiation delivery machine 122 back into a therapy mode without
changing the gantry angle.
101341 In some embodiments a treatment planning system is configured to
automatically output instructions for acquiring 2-D images at one or more
times
during delivery of a fraction. The frequency of imaging may be selectable.
Upon
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execution of the treatment plan the 2-D images may be acquired and displayed
on a
monitor associated with a radiation delivery machine (e.g. a linear
accelerator). An
operator viewing the images can verify that the treatment being delivered
appears to
be delivering the radiation to the desired target volume.
[01351 The images may be co-registered with indicia indicating desired
alignment of
features in the images and/or a representation of the distribution of dose
being
delivered by the therapeutic radiation (or a combination of the doses from
therapeutic
radiation and imaging radiation). A user such as a radiation technician or a
physician
viewing the images can check to ensure that the radiation is being delivered
according
to plan.
101361 Certain implementations of the invention comprise computer processors
which
execute software instructions which cause the processors to perform a method
of the
invention. For example, one or more processors in a treatment planning system
or
radiation delivery machine may implement the methods of Figures 3,4, 5, 9 and
11 or
other methods described above by executing software instructions in a program
memory accessible to the processors. The invention may also be provided in the
form
of a program product. The program product may comprise any medium which
carries
a set of computer-readable signals comprising instructions which, when
executed by a
data processor, cause the data processor to execute a method of the invention.
Program products according to the invention may be in any of a wide variety of
forms.
The program product may comprise, for example, physical media such as magnetic
data storage media including floppy diskettes, hard disk drives, optical data
storage
media including CD ROMs, DVDs. electronic data storage media including ROMs,
flash RAM, or the like. The computer-readable signals on the program product
may
optionally be compressed or encrypted.
[0137] Where a component (e.g. a software module, processor, assembly, device,
circuit. etc.) is referred to above, unless otherwise indicated, reference to
that
component (including a reference to a "means") should be interpreted as
including as
equivalents of that component any component which performs the function of the
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described component (i.e., that is functionally equivalent), including
components
which are not structurally equivalent to the disclosed structure which
performs the
function in the illustrated exemplary embodiments of the invention.
[0138] As will be apparent to those skilled in the art in the light of the
foregoing
disclosure, many alterations and modifications are possible in the practice of
this
invention without departing from the spirit or scope thereof For example:
= The foregoing discussion has described radiation delivery machines of the
type
in which beam angle is set by rotating a gantry. Other mechanisms may be
used to change the beam angle. The term 'gantry angle' is used to describe the
angle from which an imaging or therapeutic beam is incident on a subject and
does not require a gantry or other specific mechanism be used to set that
angle.
= The invention is not limited to applications where therapeutic radiation
is
delivered in the form of X-rays. Imaging techniques and apparatus according
to at least some embodiments may be applied in cases where the therapeutic
radiation comprises an electron beam or other particle beam, for example.
Accordingly, the scope of the invention is to be construed in accordance with
the
substance defined by the following claims.
