WO2013127005A1 - Reduced dose x-ray imaging - Google Patents

Abstract

An X-ray imaging system has a beam shaper comprising a variable aperture mounted for translation transverse to an X-ray beam. The beam shaper may be controlled to attenuate X-rays outside of a projection of a volume of interest. The beam shaper may be very compact. In an example embodiment the variable aperture comprises an iris and a control system that controls opening and closing the iris. The volume of interest is not constrained to lie on an axis of gantry rotation.

Description

REDUCED DOSE X-RAY IMAGING

Technical Field

[0001] This invention relates to X-ray imaging. The invention relates to imaging modalities in which an X-ray beam is attenuated outside of a volume of interest (VOI). Example embodiments of the invention provide apparatus and methods for cone beam computed tomography (CBCT) imaging.

Background

[0002] X-ray imaging involves detecting X-rays that have passed through a subject from an X-ray source. In cone beam X-ray imaging, the X-ray source approximates a point source from which X-rays emanate. X-rays passing through the subject, therefore diverge. [0003] Computed tomography (CT) imaging involves building a 3-D image from images taken from a number of angles. For CT imaging, an X-ray source and detector are typically mounted on a gantry which can rotate around a subject. X-ray images can be acquired for multiple different gantry angles. Ideally, the range of gantry angles for acquiring a CT image is at least 180 degrees plus the cone angle of the X-ray beam. Typically CT images may be acquired at various gantry angles within a range of approximately 200 degrees. For example, an image may be acquired for every 1 degree of rotation within the range. A CT scan can therefore involve hundreds of individual exposures. [0004] A concern with X-ray imaging in general is the dose of X-rays delivered to a patient during imaging. Large cumulative doses of X-rays can have adverse impacts on health. Consequently, there is a desire to reduce dose in X-ray imaging. This is a particular concern for CT imaging since each CT scan can involve the acquisition of a significant number of X-ray images.

[0005] The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.

Summary

[0006] This invention has a range of different aspects. Those of skill in the art will understand that these aspects may be advantageously used together but that certain have applications standing on their own or in sub-combinations with other aspects described herein or in combination with apparatus and methods known in the art. Example non-limiting aspects of the invention include:

• X-ray beam-shaping apparatus comprising a translatable variable aperture.

Such apparatus has application, for example, in VOI imaging.

· Methods for reconstructing CT images from VOI images based on known beam shaper geometries.

• Variable apertures for use in X-ray imaging comprising members arranged in an iris configuration.

• Methods for acquiring VOI images for multiple different VOIs.

· Non-transitory media containing stored computer-readable instructions

which, when executed by a data processor cause the data processor to execute a method as described herein.

• Add-on beam shapers for use with X-ray imaging machines.

• Replacement components for beam shapers (e.g. X-ray attenuating leaves configured for use in a beam shaper as described herein).

• Methods for tracking patient position during treatments.

• Apparatus for tracking patient position during treatments.

[0007] Technology as described herein may be applied in special purpose imaging applications, for example, such technology may be integrated into a CBCT platform for X-ray imaging, breast imaging, angiography, dental CBCT imaging, or the like.

[0008] Application of the technology described herein is not limited to imaging human subjects. The technology may also be applied to acquire images of animal subjects or to acquire images of inanimate subjects.

[0009] Further aspects and example embodiments are illustrated in the

accompanying drawings and/or described in the following description. Brief Description of the Drawings

[0010] The accompanying drawings illustrate non-limiting example embodiments of the invention.

[0011] Figure 1 is a schematic view showing an X-ray system according to an example embodiment of the invention. [0012] Figure 2 is a plan schematic view showing a beam shaper according to an example embodiment.

[0013] Figure 3 is a flow chart for a method for acquiring projections for VOI CT imaging according to an example embodiment.

[0014] Figure 3A is a schematic depiction of a volume of interest and illustrates how a beam shaper may limit an X-ray beam to expose the volume of interest.

[0015] Figure 4 is a schematic view of an X-ray system with a movable detector.

[0016] Figure 5 is a schematic view of an iris-type variable aperture.

[0017] Figure 5A is a photograph of a prototype iris-type variable aperture.

[0018] Figure 6 is a plan view of a guide plate for the variable aperture of Figure 5.

[0019] Figure 6A is a photograph of the prototype iris-type variable aperture of Figure 5 A with a guide plate installed.

[0020] Figure 7 is a schematic illustration showing operation of leaves of an example iris type variable aperture.

[0021] Figure 7A is a photograph showing prototype iris leaves.

[0022] Figure 7B is a schematic cross-section of an interface between two iris leaves showing a tongue and groove engagement thereof.

[0023] Figure 8 is a cross section view of a beam shaper comprising an iris-type variable aperture and a two-dimensional translation stage.

[0024] Figure 8A is a photograph of a prototype beam shaper similar to the beam shaper of Figure 8.

[0025] Figures 9A to 9F illustrate various imaging protocols. [0026] Figures 10A and 10B illustrate two example arrangements for obtaining projection images for a plurality of volumes of interest.

[0027] Figure 11 illustrates an alternative variable aperture.

[0028] Figure 12 schematically illustrates X-ray apparatus comprising a pair of variable apertures, one on each side of a subject.

Description

[0029] Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.

[0030] Figure 1 illustrates schematically a first aspect of the invention. An X-ray machine 10 comprises an X-ray source 12 and an X-ray detector 14 mounted to a rotatable gantry 16. Gantry 16 permits rotation about an axis of rotation 18. A table 20 supports a patient P so that a portion of patient P which it is desired to image is located between X-ray source 12 and X-ray detector 14. In the illustrated embodiment, X-ray source 12 is a cone beam source. X-rays can be seen to be diverging from source 12.

[0031] According to this aspect of the invention, a beam shaper 25 is provided on X-ray machine 10. As discussed in more detail below, beam shaper 25 may be controlled by the same control system that controls X-ray machine 10 or may have a separate parallel control system. Beam shaper 25 may be applied to attenuate X- rays from X-ray source 12 outside of a projection of the volume of interest V within patient P.

[0032] The location and shape in three dimensions of the volume of interest will generally be known in advance (e.g. from information received from a radiation treatment planning system in the case of application to imaging during a course of radiation therapy).

[0033] Volume of interest V may, for example, be a volume comprising an organ, lesion, or other structure which it is desired to image. One example situation in which X-ray imaging is frequently performed is in cancer treatment. In cancer treatment it is common to wish to obtain X-ray images of tumours or suspected tumours for purposes of diagnosis and for purposes of planning and monitoring treatment. Example applications in the field of cancer treatment are given below but it should be clearly understood that the application of the technology described herein is not limited to the field of cancer treatment.

[0034] In some embodiments, the methods and apparatus described herein are applied to image fiducial markers (which may be anatomical features of a patient and/or markers that are attached to or implanted in a subject's body). Images of such fiducial markers may be used to monitor the location and orientation of a subject. In some embodiments, knowledge of the locations of fiducial markers derived from such images is applied in controlling a treatment process (for example by terminating the treatment process if the locations of the one or more fiducial markers do not agree with expected locations to within a desired tolerance or by adjusting the treatment process in response to the locations of the fiducial markers as obtained from the images). Adjusting the treatment process may, for example, comprise moving the subject relative to a treatment beam, moving the treatment beam relative to the subject and/or reshaping the treatment beam (e.g. by adjusting leaves of a multileaf collimator). In such embodiments, beam shaper 25 may optionally be controlled to shape the X-ray radiation into a tight beam which is only as large as necessary to obtain sufficiently accurate information regarding the location(s) of one or more fiducial markers. The beams may approach pencil beams in some cases.

[0035] Exposure of structures within patient P to radiation can be significantly reduced by operating beam shaper 25 to block X-ray radiation which is outside of the volume of interest V to be imaged. [0036] Figure 2 shows schematically beam shaping apparatus 25 from a viewpoint at radiation source 12 looking toward patient P. Beam shaping apparatus 25 comprises an adjustable aperture 30 which is mounted on a stage for translation in at least one, preferably two, dimensions. In the illustrated embodiment, stage 32 comprises a x, y stage which permits positioning adjustable aperture 30 independently in two orthogonal directions 34X and 34Y. In an example embodiment, direction 34X is a direction in the plane of gantry rotation and direction 34Y is aligned with the craniocaudal axis. In the illustrated embodiment, variable aperture 30 can be positioned by stage 32 at any location in an X-ray field 31. X-rays falling outside of the window of variable aperture 30 are attenuated an X-ray attenuating structure such as one or more plates or the like (not shown in Figure 2) [0037] Aperture 30 is controllable to adjust the size and/or shape of an opening or X-ray transmitting window 35 through which X-rays from source 12 may pass to patient P. Outside of window 35, X-rays must pass through one or more layers of an X-ray attenuating material such that fluence of the X-ray beam outside of opening 35 is significantly reduced.

[0038] Figure 1 shows a schematic cross-section through beam shaping apparatus 25 and a portion of patient P comprising a volume of interest V. It can be seen that variable aperture 30 may be translated and adjusted so that X-rays 37 from source 12 are attenuated outside of the boundary of volume of interest V. The shape of variable aperture V need not exactly match to the boundary of the projection of volume of interest V.

[0039] Figure 2 illustrates schematically an example embodiment of beam shaping apparatus 25 which comprises a controller 38 which controls three actuators 39X, 39Y, and 40. Actuator 39X controls the position of variable aperture 30 along axis 34X. Actuator 39Y controls the position of variable aperture 30 along axis 34Y. Actuator 40 controls the opening of window 35 of adjustable aperture 30.

[0040] In the embodiment shown in Figure 2, adjustable aperture 30 has a single degree of freedom and is controlled by a single actuator 40. In alternative embodiments, adjustable aperture 30 may have two or more degrees of freedom and may be controlled by multiple actuators. For example, adjustable aperture 30 may comprise one or more of the following:

• a multi-leaf collimator;

· a rotatable multi-leaf collimator;

• an iris;

• a jaw or pair of jaws defining an opening which may, for example, be

rectangular or diamond shaped or rectangular with rounded corners;

• or the like.

[0041] The illustrated embodiment allows a relatively small variable aperture 30 to be used to limit exposure of areas of a patient outside of a volume of interest to X- rays even where the volume of interest is not located on the axis of rotation 18 of gantry 16.

[0042] Ideally, transmission of X-rays outside of the opening of variable aperture 30 is attenuated such that there is at least a 95% reduction in X-ray fluence outside the area irradiated through the variable aperture and its penumbra. In some

embodiments the attenuation is at least 98 % , at least 99% , or at least 99.5% outside of this central area. [0043] In an example embodiment, a projection image is acquired every one degree of gantry rotation through a range of 180 degrees or more. Various protocols are possible. In some embodiments, images are acquired every 0.5 to 2.5 degrees of gantry rotation. In some embodiments acquisition of images is controlled according to radiation exposure. For example, each image may be acquired after an incremental exposure of a suitable radiation dose (e.g. 0.1 mGy (e.g.) per image). In such embodiments the angular increment between sequential images is not fixed if either the gantry rotation speed or dose rate is not constant.

[0044] In some embodiments images are acquired continuously at a desired rate (e.g. at a rate of 5-10 Hz). In such embodiment image acquisition may be independent of gantry angle. Image acquisition at a desired rate may proceed for any number of fixed beam angles for example.

[0045] Methods and apparatus as described herein may be applied to acquire Images may be acquired throughout delivery of a treatment (such as radiation treatment) or during a portion of a treatment. In some embodiments, images are acquired prior to commencement of treatment and/or after completion of treatment. For example, for a brief period prior to patient treatment (e.g. 60 to 180 degrees of gantry rotation over a few seconds). As the images are acquired, beam shaping apparatus 25 may be operated to keep X-ray transmitting window 35 sized and positioned to limit delivery of X-rays outside of the volume of interest. Locations of fiducial markers seen in the images may be used in the delivery of a treatment (e.g. a radiation treatment). In some embodiments X-ray transmitting window is controlled to be as small as practical while still permitting sufficiently accurate determination of the locations of one or more fiducial markers. [0046] In some embodiments a treatment (such as a radiation treatment) may require more precise positional control for some gantry angles than for others. For example, at some gantry angles the treatment beam may be very close to critical tissues that should be spared whereas at other gantry angles the critical tissues may be well outside of the radiation beam. In such embodiments, more imaging may be carried out at those gantry angles requiring more accurate position control of the treatment beam and less or no imaging may be carried out at other gantry angles. In some embodiments, the size of transmitting window 35 may be automatically increased somewhat for gantry angles for which more precise position information is required and the size of transmitting window 35 may be automatically decreased somewhat for gantry angles at which less precise control is required.

[0047] Projections (two-dimensional images acquired at different gantry rotation angles) may be provided as input to a reconstruction algorithm that generates a three dimensional image of the interior of the volume of interest within the patient.

[0048] Figure 3 illustrates a method 40 for operating apparatus of a type, for example, described above, to obtain images of a region of interest in a patient. In block 42, a gantry of an imaging machine is moved to a desired gantry angle. In block 44 (which may occur prior to, during, or subsequent to block 42), a projection of a boundary of the region of interest into a plane of variable aperture 30 is determined). This projection may be determined from the known three- dimensional geometry of the volume of interest, the known geometry of the X-ray machine, the known position of the patient relative to the X-ray machine and the gantry angle.

[0049] The projection of the volume of interest may be determined in any suitable manner. One way is to identify an outer periphery of the volume of interest in a plane within the patient that is parallel to the plane in which variable aperture 30 is located. A mathematical transform may be applied to determine a corresponding area within the plane in which the variable aperture 30 is located. In general, the transformation may provide translation and demagnification of the area. Another way to visualize the construction of the projection of region of interest into the plane of variable aperture 30 is to consider all rays extending from X-ray source 12 to any point within the volume of interest V, identify the set of points that represents the intersection of those rays with the plane of variable aperture 30, and draw a boundary of that set of points. [0050] In some embodiments information from acquired images is applied to update the current location of the volume of interest. For example, positions of fiducial markers or anatomical fiducial features in images may be determined and these positions may be used to determine changes in the location of the volume of interest that may occur during treatment. In some embodiments, beam shaping apparatus 25 is controlled using such updated locations for the volume of interest. For example, in radiation treatment beam shaping apparatus 25 may be controlled to adapt to the fiducial or anatomical surrogate being monitored at each couch and gantry angle.

[0051] Figure 3A shows an off-axis irregular VOI and corresponding positions and sizes for variable aperture 30 for three different gantry angles.

[0052] In block 46, a desired x-y position for variable aperture 30 is determined. Block 46 may, for example, comprise determining a center of the projection in each of x and y directions and determining a position for variable aperture 30 such that a center of variable aperture 30 coincides with the center of the projection.

[0053] In block 48, a desired size for variable aperture 30 is determined. In the example case where the size of variable aperture 30 is determined by a single degree of freedom, block 48 may comprise adjusting the size of variable aperture 30 such that the entire projection of the VOI lies within the window 35 of variable aperture 30. In block 49, control signals are applied to the actuators of apparatus 25 to move variable aperture 30 to the desired location and to open aperture 30 to the desired size. In block 50 an X-ray exposure is taken. X-rays in the X-ray beam lying outside of the open area of variable aperture 35 are strongly attenuated. Thus, dose to the patient outside the region of interest is significantly reduced in comparison to a full-field X-ray exposure. In block 51 the next gantry angle is selected. [0054] Blocks 44 through 48 may be performed in advance. In embodiments where this is done, block 49 may comprise reading stored commands indicating the predetermined position and size of the variable aperture for the current gantry angle.

[0055] As discussed in some more detail below, selection of the position and size of variable aperture 30 may be optimized in certain instances, depending upon the particular structure of variable aperture 30 and the degree to which the size and/or shape of variable aperture 30 may be controlled. [0056] Two advantages that may be obtained by shaping a beam to reduce X-ray fluence outside of a volume of interest are reduction of imaging dose to a patient by limiting the beam to expose only the volume of interest rather than the patient's entire body; and, an improvement in image quality that may be obtained due to an increase in the ratio of primary-to-scattered photons. This is a particular issue for X- ray beams having energy in the range normally used for X-ray diagnostic procedures. For example, scattering of X-rays in a 80 kVp X-ray beam can significantly reduce the contrast to noise ratio (CNR).

[0057] Optionally, the imaging protocol may be configured to close variable aperture 30 for certain gantry rotation angles. This may be done to protect certain internal structures of the patient from exposure to X-rays. For example, if a particular nerve or other sensitive structure crosses the projection of the volume of interest from the point of view of certain gantry angles then, for some of those gantry angles, the variable aperture may be fully closed, thereby reducing exposure of the nerve or other sensitive structure to X-rays.

[0058] In the embodiment of Figure 1 an X-ray exposure is being taken through apparatus 25 with variable aperture 30 set, as described above. Radiation from radiation source 12 impinges on detector 14 in a region 52 which corresponds to the current location and size of window 35 of variable aperture 30. Since areas of detector 14 outside of region 52 do not receive any significant amounts of radiation, in some embodiments data acquired in regions of detector 14 outside of region 52 is not used for imaging purposes. This introduces the possibility of using a smaller, moving detector to obtain X-ray images in place of a full-field detector.

[0059] Figure 4 illustrates an alternative embodiment in which a small detector 55 (sized to detect all X-rays that pass through window 35 of variable aperture 30 when fully opened at any x-y position of variable aperture 30), is moved by a positioning system 56. Positioning system 56 is configured to translate detector 55 in one or two dimensions. In an example embodiment, positioning system 56 comprises an X-Y positioning system. [0060] Positioning system 56 is controlled to move detector 55 to a location corresponding to the location of variable aperture 30. For example, a controller may control positioning system 56 to move detector 55 to a position such that an X-ray from X-ray source 12, passing through a center of window 35 of variable aperture 30, will, when extended to the plane of detector 55, pass through the center of detector 55 also. In such embodiments the position of detector 55 may be determined by the position of variable aperture 30.

[0061] An advantage of providing a detector configured to move in tandem with a variable aperture is that a smaller -area detector may be used. This is particularly advantageous for detectors of types for which detector cost increases dramatically with area - and this is certainly the case for newer, high efficiency detectors. A second advantage is possible extension, compared to one or more dimensions of the detector, of the maximum possible dimension of the region imaged. In this application, for example, a CBCT scan may be performed with the aperture/detector axis located through a particular axial plane. Thereafter a second scan could be performed after shifting the aperture/detector pair superiorly or inferiorly, by the super ioinferior detector dimension (thus doubling the scan dimension.)

[0062] High efficiency (e.g. segmented scintillating crystal) detectors have high sensitivities and can consequently be operated to acquire images with lower doses delivered to the patient. However, these detectors are currently substantially more expensive than typical detectors. In some embodiments, the movable detector 55 is a high efficiency detector such as a segmented scintillating crystal detector.

[0063] Figure 5 shows a variable aperture comprising an iris structure 60 according to one example embodiment. Figure 5A is a photograph of a prototype iris having a construction like that shown in Figure 5. Variable aperture 60 comprises a plurality of leaves 62 which are slidable adjacent with one another to provide an iris opening 63 (See Fig. 7). The size of iris opening 63 may be altered by changing the configuration of leaves 62 as described below. In the illustrated embodiment there are eight leaves 62 but the number of leaves 62 may be more or fewer than 8.

[0064] Leaves 62 are mounted on a cam plate 66 formed with guiding features 68. In the illustrated embodiment, guiding features 68 are slots or grooves into which are engaged pins 67 that project from leaves 62. However, many variations in the particular construction of cam plate 66 and guiding features 68 are possible. By rotating cam plate 66 relative to leaves 62, leaves 62 can be made to slide relative to one another into a new configuration in which the size of central opening 63 is larger or smaller (depending upon the direction of rotation of cam plate 66). Figure 7 illustrates this schematically.

[0065] Figure 6 shows a guiding plate 69 having linear guiding features 70 which constrain the motion of leaves 62 such that in the various configurations of leaves 62 the leaves remain in abutting sliding relationship to one another (without leaving significant gaps between adjacent leaves 62 through which X-rays could pass). In the illustrated embodiment, pins 67 and 67A engage guiding features 70 of guide plate 69.

[0066] Leaves 62 may be made of any suitable radiation absorbing material.

Example materials include tungsten, copper, high-density, high-z alloys, or the like. Leaves 62 may optionally be coated with a low-friction material, such as Teflon, to facilitate smooth adjustment of the size of aperture 63.

[0067] The material of leaves 62 may be selected based in part on the beam energy to be used for X-ray generation. For example, several millimetres of tungsten or lead may be required to attenuate a 120 kVp X-ray beam. [0068] As seen in Figures 7A and 7B, leaves 62 may comprise projections which overlap with adjacent ones of leaves 62. In the illustrated embodiment, each leaf 62 comprises a tongue 77 projecting along one side of the leaf 62. The adjacent leaf 62 includes a groove or recess 78 dimensioned to receive the tongue 77. This overlap ensures that there will not be any gaps between the abutting sections of adjacent leaves 62 through which X-rays can freely pass to irradiate the patient outside of the region of interest.

[0069] Leaves 62 may be sized to allow opening to a desired maximum diameter while still allowing sufficient portions of leaves 62 to continue to be engaged with one another for smooth operation of iris 60. Various alterations in the illustrated design are possible. For example, cam wheel 66 is shown as having a central opening. This central opening is not required, especially where cam wheel 66 is made of a material that is not significant attenuating to X-rays. For example, cam wheel 66 may comprise a circular plastic plate or have a central region filled with plastic or the like. Where this is done, any material in the central region of cam plate 66 is advantageously made thin to minimize scatter of X-rays. [0070] In the illustrated embodiment, cam plate 66 and guide plate 69 are mounted to plates 71 A and 7 IB such that leaves 62 are sandwiched between plates 71 A and 7 IB (see Fig. 8). Top and bottom plates 71A and 71B have central apertures 72A and 72B through which X-rays may pass. Top and bottom plates 71A and 71B help to attenuate X-rays outside of apertures 72 A and 72B.

[0071] Advantageously, a number of components of iris assemblies 60 as illustrated, for example, in Figures 5 to 8 may be fabricated from a low-friction material, such as Teflon, polyethylene, or another low-friction material that is suitably radiation hard for use in an X-ray beam. For example, cam plate 66 and/or guide plate 69 may be fabricated from a low friction material.

[0072] Iris assembly 60 is mounted to x-y positioning assembly 74 which, in the illustrated embodiment, comprises linear guides 76X and 76Y which respectively guide translation of iris assembly 60 in Y and Y directions. The linear guides may, for example, comprise linear bearings or the like. Suitable actuators (not shown in Figure 8) may be applied to move iris assembly 60 to a desired x-y position. The entire apparatus including iris assembly 60 and x-y positioning assembly 74 may include a mounting plate 79 configured for suitable mounting to an X-ray machine. Details of construction of mounting plate 79 may vary depending upon the configuration of the X-ray machine to which it is desirable to attach the mounting plate 79.

[0073] In some embodiments, mounting plate 79 has features to interface with a mechanical or electrical interlock system provided by the X-ray machine. For example, the mounting plate or an attachment to the mounting plate may include electrical contacts that interface to electrical contacts on the X-ray machine, mechanical notches, holes, or other features that allow the mounting plate to fit onto the X-ray machine and/or actuate switches or the like that are part of an interlock system of the X-ray machine and/or openings, light sources, or other optical features that interface with optical sensors of the X-ray machine to permit the X-ray machine to operate with beam-shaping apparatus 25 in place.

[0074] A position control system may include one or more encoders for encoding the current position of actuators or components of beam shaping apparatus and feeding those current positions back to the controller for improved positioning accuracy. For example, linear position encoders may be provided to sense the X- and Y-positions of an iris assembly 60 or other variable aperture 30. The linear position sensors may be located to the side where they are out of the exposure to the X-ray beam.

[0075] Any suitable arrangements may be provided for actuating the positioning and opening of variable aperture 30. For example, actuators for moving the variable aperture 30 in X- and Y- directions may include commercially-available linear actuators, linear motors, rack and pinion drives, worm gear drives, screw drives, cable drives, and the like. In some embodiments the actuators comprise servo- controlled actuators.

[0076] The configuration of actuators for adjusting the size of variable aperture 30 will depend upon the construction of variable aperture 30. In the illustrated embodiment, where the size of variable iris 60 is controlled by turning cam plate 66 a motor or other rotary drive may be coupled to rotate cam plate 66 by means of a gear, belt drive, or the like. In the alternative, rotation of cam plate 66 may be achieved by use of a linear actuator connected to move a lever or arm coupled to cam plate 66 or coupled to a point on cam plate 66 directly.

[0077] Features of some embodiments like those illustrated in Figures 5 to 8 that may be advantageous in some applications include the possibility of a very simple drive and control system only requiring three actuators (two actuators if the apparatus only permits translation in one direction).

[0078] Another advantageous feature of the embodiment illustrated in Figure 8 is that the design may permit leaves 62 to be easily replaceable. Replacement of leaves 62 may be desired for various imaging modes. For example, it is generally desirable to use thinner leaves 62 where possible. Thinner leaves 62 are advantageous for reduced weight and also can provide a reduced penumbra region for X-rays detected near the boundary of the projection of opening 63. On the other hand, leaves 62 should be thick enough to provide a desired degree of attenuation of the X-ray beam outside of the volume of interest. Since some X-ray imaging modalities require greater X-ray energies than others, the optimum leaf thickness may differ for different imaging modalities. A design as illustrated may permits ready replacement of leaves, or even ready replacement of the entire iris assembly 60 with a different iris assembly 60 having leaves of a different thickness and/or different material or having other different characteristics. [0079] Another advantage of the illustrated embodiment is that it can be made to be compact. For example, a compact beam shaping apparatus can be advantageous where it is desired to provide a beam shaping system on an existing X-ray machine having a cavity which is designed to accept a filter, such as a "bow-tie" filter.

[0080] The ability to fit a beam shaping device, as described herein, into a concavity or other recess provided in an existing X-ray machine, is especially advantageous since minimizing features that stick out from the X-ray machine can minimize the risk of collisions of such projections with a patient, an operator, a table, or other apparatus in the vicinity of the X-ray machine.

[0081] The simplicity of iris assembly 60 as compared, for example, to a multi-leaf collimator which requires a large number of independent actuators permits such an iris assembly to be made relatively compact.

[0082] Another advantage of the designs illustrated in Figures 5 to 8A is that the iris design permits a relatively large range of aperture sizes with a relatively small mass of leaves 62. This can be advantageous in the case of an X-ray machine which has a structure that may be deflected by the weight of apparatus supported on the X-ray machine. This, in turn, minimizes the need to make geometric corrections to images prior to use of those images for reconstructing a three-dimensional view of the region of interest or other application.

[0083] As noted above, in use, variable aperture 30 may be moved to different x-y locations and opened to different sizes, depending upon the current gantry angle. A control system for beam shaping apparatus 25 should therefore have some knowledge of the current gantry angle. Various control architectures are possible. In one architecture, a control system which controls the X-ray machine with which beam shaping apparatus is used is configured to also control beam shaping apparatus 25. The same sensors and routines which are used to control the gantry angle of the X- ray machine may also be used to appropriately set beam shaping apparatus 25 to shape the X-ray beam appropriately for the current gantry angle.

[0084] In an alternative control architecture, beam shaping apparatus 25 is provided as an add-on to an X-ray machine and may require little or no interfacing to the control system of the X-ray machine itself. In some embodiments, beam shaping apparatus 25 has an independent control system. The independent control system may receive an output from the control system of the X-ray machine indicating the current gantry angle and may then independently control the configuration of beam shaping apparatus 25 for the current gantry angle. [0085] In another embodiment, a control system for beam shaping apparatus 25 has a sensor for determining the gantry angle. The sensor may take a number of different forms. In some embodiments, the sensor comprises an encoder, potentiometer, or other sensor that directly or indirectly detects a current gantry angle. In another example embodiment, an inclinometer is mounted on the gantry. An output of the inclinometer may be used as a direct measure of the gantry angle. In some embodiments the inclinometer is mounted to beam shaping apparatus 25. The inclinometer may measure the angle of inclination of beam shaping apparatus 25 to the vertical and consequently allows determination of the current gantry angle. The inclinometer may, for example, comprise an accelerometer. Provision of an independent sensor for detecting gantry angle facilitates portability of beam shaping apparatus across different imaging platforms. An inclinometer 80 is illustrated in Figure 8.

[0086] In embodiments which use an accelerometer to monitor gantry angle, a control system for a beam shaping device 25 may be at least somewhat predictive. In such embodiments, an average rotational velocity of the gantry may be estimated in advance. The inclinometer reading may be used to adjust a current configuration of beam shaping device 25 as required. [0087] In some embodiments, the control system of the X-ray system is set to require a communication from beam shaping device 25 (or a controller for beam shaping device 25) in the course of acquiring a set of X-ray images. For example, gantry rotation could be slowed or stopped, or imaging could be temporarily paused in the event of an asynchronicity between a desired configuration of one or more elements of the beam shaping system 25 and the current gantry angle.

[0088] In some embodiments, the configurations of one or more elements of beam shaping system 25 are varied dynamically as a gantry angle is changed (as opposed to operating in a "step and shoot" fashion). Dynamic relationships may be provided relating parameters specifying the configuration of beam shaping device 25 and gantry angle. Example parameters are X representing aperture X position (which may be, for example a direction parallel to axis of gantry rotation 18); Y representing aperture Y position (which may, for example, be a position in a direction perpendicular to the axis of gantry rotation 18, and one or more parameters D representing aperture dimension(s). Each of these parameters may have a corresponding desired trajectory Χ(θ), Υ(θ), and D(0) where Θ is the gantry angle. These relationships may be specified in advance, for example in a table that fully describes a desired trajectory for each of these parameters during image acquisition.

[0089] In some embodiments the trajectories for different parameters of the beam shaping device are determined with the assistance of digitally reconstructed radiographs (DRRs) reconstructed from previously-acquired CT scan data. The DRRs may show the expected locations of fiducial markers or other volumes of interest in images taken from different imaging angles. This procedure may be automated or human-guided. [0090] In some embodiments a radiation treatment planning software has functions for generating reconstructed images corresponding to different viewing directions and such reconstructed images may be used to determine settings for the parameters of a beam shaping apparatus 25 to image one or more fiducial markers or other points of interest from different directions. In some embodiments the radiation treatment planning software has functions for contouring objects represented in CT data. A user may operate the radiation treatment planning software to contour a volume of interest. The software may determine a beam's eye view of the contoured object from various beam directions and automatically select aperture sizes and positions for a beam shaper 25 for the various beam directions.

[0091] In some cases several fiducial markers may be available for imaging. As a non-limiting example of this, it is not uncommon to implant from 4 to 7 seeds into an organ such as the prostate in preparation for delivering radiation treatment to the prostate or other organ. Some such fiducial markers may be obscured by other structures (e.g. dense bones such as the hip or other fiducial markers or apparatus to be used in the treatment) from some viewing directions. An imaging system including a beam shaping device 25 may be controlled in various ways that may be advantageous. [0092] The following are some non-limiting example ways in which beam shaping device 25 may be controlled in cases where multiple fiducial markers are present. Beam shaping device 25 may be positioned and sized to generate a beam that images a first fiducial marker for some gantry angles and shifts in position and/or size to image a second fiducial marker for other gantry angles. The beam shaper 25 may be positioned and/or sized to image both the first and second fiducial markers for at least some gantry angles. The first and/or second fiducial markers may be obscured and/or aligned with tissues that it is desired to spare for some other gantry angles. These gantry angles may be determined in advance using DRR and/or functions of a radiation treatment planning system and this information may be used in turn to determine the angles for imaging the first and second fiducial markers. The images of the first and second fiducial markers may optionally also include one or more fiducial markers.

[0093] As another possibility, beam shaping device 25 may be positioned and sized to image a constellation of two or more fiducial markers for at least some gantry angles. As another possibility, beam shaping device 25 may be positioned and sized to image a single fiducial marker or small group of fiducial markers for some gantry angles and may be opened up to image a larger constellation of two or more fiducial markers for at least some other gantry angles.

[0094] It can be appreciated that, in some embodiments, effective imaging is facilitated by using a radiation treatment planning system to show the expected appearance of patient X-ray images from various directions and, based on these images, to select fiducial markers to be imaged. Different fiducial markers may be selected for different angles. The treatment planning system may be configured to automatically or semi-automatically with human assistance generate trajectories for parameters of beam shaper 25. The parameters may, for example, be X-position, Y- position and aperture size. Preparing an imaging trajectory may be done in advance at the same time as planning a radiation therapy treatment, for example.

[0095] Various optimizations are possible as options to further optimize imaging and/or to further reduce dose during imaging. One such option is to control the intensity (fluence) of the X-ray beam as a function of gantry angle. To form a three- dimensional CT image, one needs to have images acquired through a sufficient angular range. As noted above, this angular range is ideally at least 180 degrees plus the cone angle of the X-ray beam source. Typically, CT images are obtained through a range of gantry angles spanning approximately 200 degrees. In some embodiments, fluence may be increased or decreased as a function of gantry angle. For example, fluence may be made larger for selected gantry angles at which the beam shaping apparatus 25 can produce a good match to the projection of the volume of interest, Fluence may be reduced relatively speaking at other gantry angles. The images may be re-normalized prior to combining them into a 3D image. [0096] In some embodiments, such as the embodiment illustrated in Figure 7 which is only an example, variable aperture 30 has a specific shape. In such embodiments the variable aperture can be controlled to adjust the size of the shape but the shape remains constant across the operation (or at least the shape cannot be varied for any particular aperture size). In such embodiments, optimization may be possible to provide a best match between the opening of the aperture and the projection of the volume of interest at the current gantry angle. For example, in the case where the aperture is in the form of an octagon, there may be cases where a reduction in the size of the octagon, coupled with a corresponding rotation of the octagon, may still permit the entire projection of the volume of interest to be fit within the octagon even though the projection of the region of interest may not fit or may just barely fit when the aperture is set to have a larger area (with a different corresponding rotation of the octagon). In some embodiments, a system for establishing an appropriate setting for a size of an aperture for different gantry angles may include optimization to identify a smallest size of aperture that will work for the current gantry angle. This may be done, for example, by identifying a first aperture size that is a fairly close match to the projection of the volume of interest and then modeling the aperture for one or more smaller sizes to see if the region of interest may also fit adequately into one of the smaller sizes. [0097] Various imaging modes are possible. The nature of the imaging modes may depend upon the location of the volume of interest (i.e. whether the volume of interest is on or off an axis 18 of gantry rotation) as well as the desired output (i.e. whether the required output is a three-dimensional CT scan of the volume of interest) or whether an image on a plane through the volume of interest is all that is required for current purposes. Different imaging modes may also be used in cases where there is more than one volume of interest to be imaged.

[0098] Several different acquisition modes are envisaged to maximize utility of the system. Examples of these are illustrated schematically in Figures 9A to 9F.

[0099] Single VOI on axis of rotation (Figure 9A). In this mode, variable aperture 30 can be centered on the beam axis and the aperture size D of variable aperture 30 is set as required. In cases where the VOI is symmetrical the variable aperture may be maintained at a fixed X,Y position and be set at a fixed size D throughout image acquisition. [0100] Single VOI off of axis of rotation (Figure 9B). Here, a single, pre-defined VOI is captured during acquisition. Variables Y (travel along a dimension perpendicular to the axis of gantry rotation) and D may be varied with gantry angle Θ to track the VOI during rotation and to account for beam divergence. [0101] Multiple, separate VOIs contained within same axial planes (Figure 9C) The X -ray beam can be shaped to reduce exposure outside of the VOI without varying variable Y. Variables X and D may be varied with gantry angle Θ to track the VOIs during rotation and to account for beam divergence. Separate VOIs may be acquired at different dose levels/image qualities.

[0102] Multiple, separate VOIs on different axial planes. (Figure 9D) In this mode, X, Y and D may be varied with Θ. Separate VOIs may be acquired at different dose levels /image qualities. [0103] Nested VOIs. (Figure 9E) Nested VOIs are a special case of multiple, separate VOIs contained within same axial planes where an inner VOI is contained within an outer VOI. Clinical scenarios where this can be useful include the acquisition of a high-quality image of a target e.g, to allow alignment based on soft-tissue detail, with a lower quality (and higher noise) image of immediately surrounding organs at risk. Alternatively, the dimension of the outer VOI could be set sufficiently large to capture the full external surface of the patient. In this case, the outer VOI could be associated with comparatively lower dose.

[0104] Region-of-interest tomosynthesis. This is a method which involves acquiring image data for reconstructing an image along a single, chosen plane through the patient rather than a volume. The chosen plane may coronal, sagittal, or oblique as required. Compared to CBCT, the range of Θ required for tomosynthesis is small (e.g. , 20-30 degrees) and therefore acquisition times can be very short, e.g. 10% of the time required for CBCT. Depending on the anatomy and image guidance task, tomosynthesis could be useful and it may also be applicable to patients for whom it is advantageous to minimize the duration of the procedure. [0105] Various options exist for obtaining sets of projections suitable for

reconstructing CT images of multiple VOIs. Nominally, the range of gantry rotation, for a single VOI, will be 180 degrees plus the cone angle. For typical imaging platforms, this equals approximately 200 degrees. As illustrated in Figure 10A, one approach to acquiring projections for two VOIs is to assign arc segments of a full rotation, e.g. , to each the VOIs, where each segment is 200 degrees in length. As the diagram suggests, this means that both VOIs must be sampled across an overlap region of 40 degrees, e.g. , in two 20 degree arc segments. For simplicity it may be possible to eliminate this overlap region, i.e. such that only 180 degrees are allocated to each of the two VOIs, with some compromise in angular sampling for each VOI. The advantage of this "sequential" sampling method is simplicity of the aperture trajectory, but the disadvantage is that the total arc length increases with number of VOIs. [0106] Another option is illustrated in Figure 10B. Here, the full arc is divided into "control points" at which projection data are acquired and correspond, in sequence, to VOI 1 , VOI 2, etc. , to VOI n, where total n is the number of VOIs. At each control point the aperture (X,Y,D) variables are set to be specific to the VOI being imaged. Moreover, if the nominal angular increment between projections for a single VOI is ΔΘ, it may be possible to reduce this to Δθ/η in this approach, such that the angular sampling period for a particular VOI is maintained at ΔΘ. The advantage of this technique is that it is "scalable" : in principle, projection data for many VOIs may be acquired in a single rotation. The technique is also feasible with regard to dynamics, in that the maximum required translation of the aperture is small, even when widely separated VOIs are to be imaged. An additional advantage is that the sampling can be made symmetric for all VOIs.

[0107] To obtain high quality images in the case where a beam shaping system is used (which essentially truncates the images at or near the boundary of the projection of a volume of interest) it is typically necessary adjust the images to correct for truncation artifacts. Truncation artefacts can take the form of boundary rings and "cupping" , for example. One way to reduce or avoid the creation of truncation artifacts is to modify the acquired images by filling in areas outside of the beam as shaped by beam shaping apparatus 25 with an estimate of the surrounding image data. Such filling is described, for example, in United States patent Application No. 13/042162 which is hereby incorporated herein by reference for all purposes. [0108] Other approaches to ameliorate truncation artifacts are by simple

extrapolation or apodization of the truncated data. These techniques typically require knowledge of the location of the boundary of the truncated region in the projection image. In some embodiments, the boundary may be identified by image processing. Pixels outside of the boundary typically will have a low exposures while typical pixels inside the boundary will on average have higher exposures. Image processing may therefore be applied to estimate the location of the boundary.

[0109] Especially in the case where variable aperture 30 has a known shape, location, and orientation, the location of the boundary in the image may be predicted from the known shape, location, and orientation of variable aperture 30 (to which a transform may be applied to locate the projection of the edges of variable aperture 30 in the imaging plane). This permits creation of image masks which may be applied in the creation of composite images which include detected image data inside the shaped X-ray beams and fill data in peripheral areas of the composite images outside of the shaped X-ray beams.

[0110] Knowing where the edge of the projection of variable aperture 30 will be located in an image in advance is beneficial for providing a more robust

reconstruction process especially where the images are acquired a low X-ray intensities. Furthermore, avoiding image processing to locate beam boundaries can make reconstruction of images faster.

[0111] Suitable fill data may be obtained from various sources. Fill data may be taken, for example from a previously-obtained image (for example an image from a CT image set obtained previously for treatment planning). In the alternative, one of a number of extended-field projection images may be acquired for the purpose of filling. In some embodiments, the filling is performed by morphing previously- acquired images to match the acquired image along the boundary of the shaped X-ray beam. Where nested VOIs are being imaged, image data acquired to image an outer VOI may be used to fill projections obtained to image a VOI nested within the outer VOL

[0112] Occasional full-field images may be acquired by opening variable aperture 30 fully for selected gantry angles. It is advantageous from a dose point of view to minimize the number of full-field projections acquired. However, suitable images for filling may be obtained even if one such image is taken for every 30 degrees or so of gantry rotation. Therefore, only a few full-field images are typically required to make a useful estimate of missing data for reconstruction purposes. This filling is described more thoroughly in the MSC Thesis of Alexander Owen MacDonald entitled INVESTIGATION OF VOLUME-OF-INTEREST MEGA VOLTAGE CONE-BEAM COMPUTED TOMOGRAPHY, Dalhousie University

Halifax, Nova Scotia, August 18, 2010 which is hereby incorporated herein by reference for all purposes.

[0113] As noted above, in many applications a volume of interest is known in advance. For example, in many applications in image guidance for radiation therapy or radiosurgery, the location and three dimensional shape of the VOI will be known beforehand, and this information can be used to generate a dynamic trajectory of the aperture during VOI CBCT. However, in other applications, e.g. , VOI CBCT in a diagnostic setting, it may be required to choose a VOI in situ, i.e. with the patient positioned in the X-ray machine. Some embodiments permit a user to define a volume of interest. This may be done, for example, using 2D "Scout" images that may have been obtained for some other purpose or may be obtained specifically for identifying a VOI. [0114] In an example embodiment, two or more projection images are acquired prior to the CBCT scan. These images may, for example, comprise anterior -posterior and lateral views. While these views are convenient, other views may be used in addition of in the alternative. Based on these 2-D projection images, the user may define locations and dimensions of one or more VOI. The full dynamic trajectory for a beam shaper may then be determined, for example as described above. This trajectory may be used in the subsequent CBCT scan.

[0115] In an example embodiment, a user uses a graphical interface which displays the 2-D projection images in order to select a volume of interest. In some embodiments, a selection tool is pre-configured to have the shape of variable aperture 30 (or to have a shape achievable by variable aperture 30 in the case that variable aperture 30 has a variable shape). The user may place the shape around an area in the image that is of interest and adjust the size of the shape to arrange the control to indicate a setting for the beam shaping device that will illuminate the area of interest in each projection while minimizing exposure to other areas. The system may then create a volume of interest that matches the projection in the two (or more) 2D projections being viewed by the user. [0116] A similar approach may be used to define a trajectory for the parameters of a beam shaping device 25 that is to be used to image fiducial markers. A user may use a user interface on which 2D images of a volume of the patient taken from different angles may be displayed. These images may, for example, comprise DRRs. A user may identify fiducial marks in the images. The user may place a control on the image to indicate the position and size of the window of beam shaper 25 to be used to image the fiducial markers at that angle. A computer system may interpolate between the window sizes and positions to generate trajectories for the parameters of beam shaper 25. The computer system may be configured to play back simulated images showing where and how big the window of beam shaper 25 will be and what structures should be inside the window (from the perspective of the X-ray beam) when beam shaper 25 is controlled according to the trajectories. [0117] In an alternative embodiment, a user may select objects or areas in an image and the computer system may be configured to automatically determine a suitable position and size of the window of beam shaper 25 for use in imaging the selected objects or areas. [0118] For example, where the control is octagonal (matching the shape of an iris as illustrated in Figure 5 for example), the system may allow the user to place octagons optimally on the views of the areas of interest in the 2D projections. The system may determine a three-dimensional volume of interest by extruding the octagons perpendicular to their planes of the projection images to create a three-dimensional volume of interest. From this volume of interest, one can find a center point. In some embodiments, the volume of interest is formed by interpolating between the octagon diameters for different angles or taking average of the octagon sides or the like. [0119] In some embodiments, image acquisition is planned in a similar manner to the delivery of therapeutic radiation in the sense that a software module is provided to optimize the acquisition of the image data. An arc segment for image acquisition may be selected based on minimizing the exposure to X-rays of certain organs identified as being at risk. Such a system may be built into a radiation treatment planning system, for example. [0120] The presence of an aperture that is able to block the X-ray beam completely may be useful in order to minimize the imaging dose to particular structures. For example, in performing VOI CBCT in the brain, it would be desirable to limit the dose to lenses of the patient's eyes. With appropriate software functionality, the dynamic trajectory could specify closing the aperture where entrance or exit beams impinge on the lenses. The same software functionality could also allow

customization of the start/stop angles of the scanning protocol. In the example above, to minimize lens dose, if the rotation range is 200 degrees, one could chose an arc segment centred on the posterior, rather than the anterior of the patient.

[0121] Some embodiments are configured to calculate imaging dose distribution. It may be useful to calculate the dose distribution due to the VOI CBCT imaging. Predictions of the magnitude and distribution of dose may be used to optimize the imaging protocol to minimize dose to particular regions of anatomy. Dose calculation may be performed within a treatment planning system or other (e.g. stand-alone) software. Dose estimations may also be used to track cumulative exposure of a patient to radiation.

[0122] An image obtained by a full CT scan can be much more accurate for dose calculation than an image acquired for the case where X-ray beams have been shaped to image a volume of interest. Accurate dose calculation requires imaging of all structures to the surface of a patient. There may be an existing image of these structures (e.g. a planning CT image set). However, a patient's body may change over time (for example over the course of a treatment for cancer). Changes may be long term or short term. For example, imaging may have been obtained when the patient's bladder was empty and treatment may be delivered when the patient's bladder is full. The patient may lose or gain weight over a course of treatment. The patient's lungs may have fluid in them when a treatment planning CT scan is performed and may be empty of fluid or have a reduced amount of fluid later when a treatment is delivered or imaging is performed. The patient's organs may move around relative to one another to some degree.

[0123] In some embodiments, a VOI image for a volume of interest within a patient is used to correct a previously obtained three-dimensional image (e.g. a planning CT scan image set) to more closely resemble the current state of a patient such that the corrected image can be used to more accurately perform dose calculations than would be possible using the VOI image only or the original planning CT image only. [0124] In an example embodiment, imaging as described herein is performed for a larger and a smaller VOI. The larger VOI captures the external surface of the patient. The outer VOI may be acquired at very low dose (and thus high noise). Accurate dose calculations may be achieved by co-registering a previously-acquired high quality image (e.g. a planning CT or previous diagnostic CT) to the VOI CBCT set. The co-registration may be either rigid affine, or deformable as required. The co-registration establishes a direct mapping of the previous high quality data to the new VOI CBCT set. If the previous data is more accurate with regard to HU accuracy, as it may be, e.g. , if it were acquired on a fan-beam CT scanner, the previous data could be used to correct HU units in the VOI CBCT scan data. In turn, this facilitates, for example, dose calculation on the just-acquired VOI CBCT set, allowing visualization of a dose distribution based on the current, imaged anatomy.

[0125] It is often necessary or desirable to co-register acquired VOI CBCT data to a reference CT image set. This may be necessary, for example, to provide information (shift and rotation) allowing patient alignment. Co-registration may be facilitated by automatically identifying a portion of the reference CT image in the vicinity of the VOI. Where this is done, only data corresponding generally to the VOI in the reference set is considered. For example, if the VOI is the prostate, only data within the region of the prostate in the reference CT set would be considered during the co-registration process. [0126] Variable apertures are not limited to the structures described above and illustrated in the drawings. An alternative construction for a variable aperture is a set of two or four independently-translatable collimator plates defining a rectangular or diamond shaped aperture. Another alternative configuration for a variable aperture 30 is a structure which provides a number of plates or other members that have different sizes and/or shapes of apertures that can be selectably positioned to collimate X-rays from the X-ray source. For example, a rotatable carousel as shown in Figure 11 that includes a plurality of apertures may be provided. A strip or other linear array having a plurality of apertures that may be moved into the X-ray beam to collimate the X-ray beam may be provided.

[0127] In applications involving monitoring positions of one or more fiducial markers during delivery of a treatment, it may be desirable to know the positions of the fiducial marker(s) in three dimensions. In some embodiments, two imaging systems as described herein may be used to acquire images of the same fiducial marker(s) more or less simultaneously from different directions. Although this doubles the number of X-ray images acquired the imaging X-ray dose may be kept relatively small by tightly constraining the X-ray beams in each of the imaging systems as described above. Where two imaging systems are used in conjunction with a treatment system that has a rotatable gantry for delivery of a treatment the imaging systems may be mounted to rotate with the gantry or fixed (e.g. room- mounted).

[0128] One issue that can occur when taking X-ray images of a volume within a subject at the same time that the subject is being exposed to a treatment beam (e.g. a MV beam) is that scattering from the MV beam can reduce contrast in the X-ray images. Sometimes this contrast reduction is bad enough that fiducial markers or other structures of interest cannot be seen in the resulting X-ray images. This problem may be addressed in a number of ways. One approach that takes advantage of the X-ray dose-reducing effect of beam shaper 25 is to increase the fluence of the X-ray beam to obtain better contrast. Even though X-ray dose increases with increasing fluence the overall dose may still be much smaller than in conventional cone-beam imaging. Another approach that may be used alone or combined with the first approach is to attenuate, turn off, or completely block the X-ray beam to obtain an image due entirely or substantially entirely to scattering from the MV treatment beam. This 'scattering image' may then be subtracted from an image taken with the X-ray beam on to improve contrast in the X-ray image. Another approach that may be used is to add control periods (typically called 'control points') to the treatment protocol during which the MV treatment beam is off and to acquire images free of scattering in the control periods. A disadvantage of this third approach is that it can increase the overall time required to deliver a treatment. [0129] Figure 12 illustrates apparatus which facilitates another approach to reducing the effect of scattering from a MV treatment beam that may be applied on its own or in combination with any of the above approaches. Apparatus 100 as shown in Figure 12 is the same as apparatus 10 of Figure 1 with the addition of a second beam shaping device 25 A located between patient P and detector 14. Second beam shaping device may be constructed in the manner described above for beam shaper 25

(although it is not necessary that beam shaper 25 and second beam shaping device 25 A have the identical construction). [0130] A control system (not shown in Fig 1) controls second beam shaping device 25A to provide a window that is aligned (from the perspective of X-ray source 12) with the window of beam shaper 25. The effectiveness of second beam shaping device 25 A at blocking scattered radiation from reaching detector 14 will tend to increase as the size of the window provided by second beam shaping device 25A is made smaller and as second beam shaping device 25A is positioned closer to patient P. [0131] The controller causes both beam shaper 25 and second beam shaping device 25A to position and size their respective windows to match a projection of a volume of interest or an area of interest. The window provided by second beam shaping device 25A will generally be larger than that provided by beam shaper 25 by a magnification factor determined by the geometry of beam 37 (i.e. the relative distances between beam shaper 25 and second beam shaping device 25 A and X-ray source 12). Where second beam shaping device 25 A comprises an iris or other opening that is translated (e.g. in X and Y directions) the controller may cause second beam shaping device 25A to move through distances that are greater by the same magnification factor than the distances by which the window of beam shaper 25 is moved.

[0132] In any embodiment one could optionally control beam energy of the X-ray beam as a function of gantry angle. For example, the beam voltage (kV) may be increased to provide increased penetration for gantry angles corresponding to longer path lengths through a patent's body.

INTERPRETATION OF TERMS

[0133] Unless the context clearly requires otherwise, throughout the description and the claims:

· "comprise, " "comprising," and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to" .

• "connected," "coupled," or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof. "herein," "above, " "below, " and words of similar import, when used to describe this specification shall refer to this specification as a whole and not to any particular portions of this specification.

"or," in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list,

the singular forms "a", "an" and "the" also include the meaning of any appropriate plural forms. [0134] Words that indicate directions such as "vertical" , "transverse" , "horizontal", "upward" , "downward", "forward" , "backward" , "inward" , "outward" , "vertical" , "transverse", "left" , "right" , "front", "back" , "top", "bottom", "below" , "above", "under" , and the like, used in this description and any accompanying claims (where present) depend on the specific orientation of the apparatus described and illustrated. The subject matter described herein may assume various alternative orientations. Accordingly, these directional terms are not strictly defined and should not be interpreted narrowly.

[0135] Embodiments of the invention may be implemented using specifically designed hardware, configurable hardware, programmable data processors configured by the provision of software (which may optionally comprise 'firmware') capable of executing on the data processors, special purpose computers or data processors that are specifically programmed, configured, or constructed to perform one or more steps in a method as explained in detail herein and/or combinations of two or more of these. Examples of specifically designed hardware are: logic circuits, application-specific integrated circuits ("ASICs"), large scale integrated circuits ("LSIs"), very large scale integrated circuits ("VLSIs") and the like. Examples of configurable hardware are: one or more programmable logic devices such as programmable array logic ("PALs"), programmable logic arrays ("PLAs") and field programmable gate arrays ("FPGAs") ). Examples of programmable data processors are: microprocessors, digital signal processors ("DSPs"), embedded processors, graphics processors, math co-processors, general purpose computers, server computers, cloud computers, mainframe computers, computer workstations, and the like. For example, one or more data processors in a control circuit for a device may implement methods as described herein by executing software instructions in a program memory accessible to the processors. [0136] Processing may be centralized or distributed. Where processing is distributed, information including software and/or data may be kept centrally or distributed. Such information may be exchanged between different functional units by way of a communications network, such as a Local Area Network (LAN), Wide Area

Network (WAN), or the Internet, wired or wireless data links, electromagnetic signals, or other data communication channel.

[0137] For example, while processes or blocks are presented in a given order, alternative examples may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified to provide alternative or

subcombinations. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed in parallel, or may be performed at different times.

[0138] In addition, while elements are at times shown as being performed sequentially, they may instead be performed simultaneously or in different sequences. It is therefore intended that the following claims are interpreted to include all such variations as are within their intended scope.

[0139] Software and other modules may reside on servers, workstations, personal computers, treatment planning systems, image processing systems, X-ray machine control systems, CT reconstruction systems, and other devices suitable for the purposes described herein. Those skilled in the relevant art will appreciate that aspects of the system can be practised with other communications, data processing, or computer system configurations, including: Internet appliances, hand-held devices (including personal digital assistants (PDAs)), multi-processor systems, and the like. [0140] The invention may also be provided in the form of a program product. The program product may comprise any non-transitory medium which carries a set of computer-readable 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, non-transitory 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, EPROMs, hardwired or preprogrammed chips (e.g. , EEPROM semiconductor chips), nanotechnology memory, or the like. The computer-readable signals on the program product may optionally be compressed or encrypted.

In some embodiments, the invention may be implemented in software. For greater clarity, "software" includes any instructions executed on a processor, and may include (but is not limited to) firmware, resident software, microcode, and the like. Both processing hardware and software may be centralized or distributed (or a combination thereof), in whole or in part, as known to those skilled in the art. [0141] 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 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.

[0142] Specific examples of systems, methods and apparatus have been described herein for purposes of illustration. These are only examples. The technology provided herein can be applied to systems other than the example systems described above. Many alterations, modifications, additions, omissions and permutations are possible within the practice of this invention. This invention includes variations on described embodiments that would be apparent to the skilled addressee, including variations obtained by: replacing features, elements and/or acts with equivalent features, elements and/or acts; mixing and matching of features, elements and/or acts from different embodiments; combining features, elements and/or acts from embodiments as described herein with features, elements and/or acts of other technology; and/or omitting combining features, elements and/or acts from described embodiments.

[0143] It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions, omissions and sub-combinations as may reasonably be inferred. The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole. [0144] While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.

Claims

WHAT IS CLAIMED IS:

1. X-ray imaging apparatus comprising:

an X-ray source;

an X-ray detector; and

a first X-ray beam shaper between the X-ray source and the X-ray detector, the X-ray beam shaper comprising:

a variable aperture mounted to a translation stage, the variable aperture having an X-ray transmitting window having a controllable area, the translation stage configured to translate the variable aperture in at least one first direction transverse to an X-ray beam from the X- ray source.

2. Apparatus according to claim 1 wherein the translation stage is configured to translate the variable aperture in two directions transverse to the X-ray source.

3. Apparatus according to claim 1 or claim 2 wherein the variable aperture comprises an actuator connected to control a cross-sectional area of the X-ray transmitting window defined by the variable aperture.

4. Apparatus according to claim 3 wherein the X-ray transmitting window has a controllably variable shape.

5. Apparatus according to claim 3 wherein the variable aperture comprises a plurality of abutting X-ray attenuating members slidably movable with respect to one another, the X-ray attenuating members surrounding the X-ray transmitting window and movable to vary the cross-sectional area of the X-ray transmitting window.

6. Apparatus according to claim 5 wherein the X-ray attenuating members are arranged to form an iris defining a polygonal X-ray transmitting window.

7. Apparatus according to claim 5 or 6 wherein the X-ray attenuating members each comprise a projecting tongue arranged to overlap with an adjacent one of the X- ray attenuating members along a side where the X-ray attenuating member adjoins the adjacent one of the X-ray attenuating members.

8. Apparatus according to claim 7 comprising an inclinometer mounted to determine a gantry angle of a gantry supporting the X-ray source.

9. Apparatus according to claim 8 wherein the inclinometer is mounted to the first X-ray beam shaper.

10. Apparatus according to any one of claims 1 to 9 wherein the detector is mounted to a second translation stage configured to translate the detector transverse to the X-ray beam parallel to the at least one first direction.

11. Apparatus according to any one of claims 1 to 10 comprising a control system, the control system configured to determine a desired position for the variable aperture and a desired area for the variable aperture for a gantry angle such that a projection of a volume of interest into a plane of the variable aperture lies within the X-ray transmitting window.

12. Apparatus according to any one of claims 1 to 10 comprising an additional X- ray beam shaper between the first X-ray beam shaper and the X-ray detector.

13. Apparatus according to claim 12 wherein the additional X-ray beam shaper comprises a variable aperture mounted to a translation stage, the variable aperture having an X-ray transmitting window having a controllable area, the translation stage configured to translate the variable aperture in at least one first direction transverse to the X-ray beam from the X-ray source.

14. Apparatus according to claim 12 or 13 comprising a control system, the control system configured to determine a desired position for the variable aperture of the first X-ray beam shaper and a desired area for the variable aperture of the first X- ray beam shaper for a gantry angle such that a projection of a volume of interest into a plane of the variable aperture of the first X-ray beam shaper lies within the X-ray transmitting window of the first X-ray beam shaper.

15. Apparatus according to claim 14 wherein the control system is configured to determine a desired position for the variable aperture of the additional X-ray beam shaper and a desired area for the variable aperture of the additional X-ray beam shaper for a gantry angle such that a projection of a volume of interest into a plane of the variable aperture of the additional X-ray beam shaper lies within the X-ray transmitting window of the additional X-ray beam shaper.

16. Apparatus according to claim 14 or 15 wherein the control system is configured to operate the X-ray imaging apparatus to acquire an image for each of a predetermined set of gantry rotation angles.

17. Apparatus according to claim 14 or 15 wherein the control system is configured to operate the X-ray imaging apparatus to acquire a series of images at a defined rate.

18. Apparatus according to claim 17 wherein the defined rate is in the range of 5 to 10 Hz.

19. Apparatus according to claim 14 or 15 wherein the control system is configured to operate the X-ray imaging apparatus to acquire a series of images and to control acquisition of the series of images based on an accumulated radiation dose from a treatment beam.

20. Apparatus according to any one of claims 1 to 19 wherein the X-ray detector is a high efficiency detector.

21. Apparatus according to claim 20 wherein the X-ray detector comprises a segmented scintillating crystal detector.

22. An X-ray imaging method comprising:

providing a beam shaper comprising a first variable aperture between an X- ray source and an X-ray detector;

aligning the first variable aperture with a volume of interest by translating the first variable aperture;

operating the X-ray source to generate X-rays and detecting X-rays that pass through the first variable aperture and the volume of interest.

23. An X-ray imaging method according to claim 22 comprising adjusting a size of the first variable aperture based on a size of the volume of interest and a distance of the volume of interest from the X-ray source.

24. An X-ray imaging method according to claim 23 wherein adjusting the size of the first variable aperture comprises maintaining a cross-sectional shape of the first variable aperture constant.

25. An X-ray imaging method according to any one of claims 22 to 24 comprising providing a second variable aperture on a side of the volume of interest opposed to the first variable aperture; and aligning the second variable aperture with the volume of interest by translating the second variable aperture.

26. An X-ray imaging method according to claim 25 comprising adjusting a size of the second variable aperture.

27. An X-ray imaging method according to claim 26 wherein adjusting the size of the second variable aperture comprises maintaining a cross-sectional shape of the second variable aperture constant.

28. An X-ray imaging method according to claim 26 or 27 wherein adjusting the size of the second variable aperture comprises maintaining a fixed ratio between sizes of the first and second variable apertures.

29. An X-ray imaging method according to any one of claims 22 to 28 wherein detecting the X-rays is performed by an X-ray detector panel and the method comprises moving the X-ray detector panel to intercept the X-rays that pass through the first variable aperture and the volume of interest.

30. An X-ray imaging method according to any one of claims 22 to 29 wherein the first variable aperture comprises a plurality of abutting X-ray attenuating members slidably movable with respect to one another.

31. An X-ray imaging method according to claim 30 wherein the X-ray attenuating members are arranged to form an iris defining a polygonal X-ray transmitting window.

32. An X-ray imaging method according to any one of claims 22 to 31 comprising providing a gantry with an adjustable gantry angle, the gantry supporting the X-ray source.

33. An X-ray imaging method according to claim 32 comprising rotating the gantry though a plurality of angles and performing the method to obtain an image for each of a plurality of the gantry angles.

34. An X-ray imaging method according to according to any one of claims 22 to 33 comprising performing the method to acquire X-ray images at a rate of between 5 and 10 Hz.

35. An X-ray imaging method according to any one of claims 32 to 34 comprising adjusting the size of the first variable aperture to block the X-rays outside of a narrow beam located to illuminate a fiducial marker.

36. An X-ray imaging method according to any one of claims 32 to 34 comprising increasing the size of the first variable aperture at a first gantry angle and decreasing the size of the first variable aperture at a second gantry angle.

37. An X-ray imaging method according to any one of claims 32 to 34 comprising maintaining a fixed position and size of the first variable aperture throughout an image acquisition process.

38. An X-ray imaging method according to any one of claims 32 to 34 comprising maintaining the position of the first variable aperture constant in at least a first direction throughout an image acquisition process.

39. An X-ray imaging method according to any one of claims 32 to 34 comprising predetermining a position and size of the first variable aperture for each of a plurality of gantry angles.

40. An X-ray imaging method according to claim 32 to 39 comprising closing the first variable aperture at at least one gantry angle.

41. An X-ray imaging method according to claim 40 comprising selecting the at least one gantry angle in order to protect an internal structure of a patient from exposure to X-rays.

42. An X-ray imaging method according to any one of claims 22 to 41 comprising adjusting the intensity of the X-rays that pass through the first variable aperture as a function of the gantry angle.

43. An X-ray imaging method according to any one of claims 22 to 42 comprising opening the first variable aperture fully for at least one gantry angle.

44. An X-ray imaging method according to claim 43 comprising determining the volume of interest based at least in part on an image obtained with the first variable aperture fully opened.