(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`(19) World Intellectual Property Organization
`International Bureau
`
`11111111111111111111111111111111111111111111111111111111111111111111111111111111
`
`(43) International Publication Date
`23 August 2001 (23.08.2001)
`
`PCT
`
`(10) International Publication Number
`WO 01/60236 A2
`
`(51) International Patent Classification7:
`
`A61B
`
`(21) International Application Number: PCT/US01/05077
`
`(22) International Filing Date: 16 February 2001 (16.02.2001)
`
`(25) Filing Language:
`
`(26) Publication Language:
`
`English
`
`English
`
`(30) Priority Data:
`60/183,590
`
`18 February 2000 (18.02.2000) US
`
`(71) Applicant: WILLIAM BEAUMONT HOSPITAL
`[US/US]; 3601 West Thirteen Mile Road, Royal Oak, M1
`48073-6769 (US).
`
`(72) Inventors: JAFFREY, David, A.; 2476 Lincoln Road,
`Windsor, Ontario N8W 2R7 (CA). WONG, John, W.; 726
`Tennyson Downs Court, Bloomfield Hills, MI 48304 (US).
`SIEWERDSEN, Jeffrey, H.; 2383 Timbercrest Court,
`Ann Arbor, M148105 (US).
`
`(81) Designated States (national): AE, AG, AL, AM, AT, AU,
`AZ, BA, BB, BG, BR, BY, BZ, CA, CH, CN, CR, CU, CZ,
`DE, DK, DM, DZ, EE, ES, Fl, GB, GD, GE, GH, GM, HR,
`HU, ID, IL, IN, IS, JP, KE, KG, KP, KR, KZ, LC, LK, LR,
`LS, LT, LU, LV, MA, MD, MG, MK, MN, MW, MX, MZ,
`NO, NZ, PL, PT, RO, RU, SD, SE, SG, SI, SK, SL, TJ, TM,
`TR, TT, TZ, UA, UG, UZ, VN, YU, ZA, ZW.
`
`(84) Designated States (regional): ARIPO patent (GH, GM,
`KE, LS, MW, MZ, SD, SL, SZ, TZ, UG, ZW), Eurasian
`patent (AM, AZ, BY, KG, KZ, MD, RU, TJ, TM), European
`patent (AT, BE, CH, CY, DE, DK, ES, Fl, FR, GB, GR, IE,
`IT, LU, MC, NL, PT, SE, TR), OAPI patent (BF, BJ, CF,
`CG, CI, CM, GA, GN, GW, ML, MR, NE, SN, TD, TG).
`
`Published:
`without international search report and to be republished
`upon receipt of that report
`
`For two-letter codes and other abbreviations, refer to the "Guid(cid:173)
`ance Notes on Codes and Abbreviations" appearing at the begin(cid:173)
`ning of each regular issue of the PCT Gazette.
`
`iiiiiiii
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`(74) Agent: FREEMAN, John, C.; Brinks Hofer Gilson &
`Lione, P.O. Box 10087, Chicago, IL 60610 (US).
`~ ------------------------------------------------------------------------------------------
`(54) Title: CONE-BEAM COMPUTERIZED TOMOGRAPHY WITH A FLAT-PANEL IMAGER
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`-..... (57) Abstract: A radiation therapy system that includes a radiation source that moves about a path and directs a beam of radiation
`
`source that emits an x-ray beam in a cone-beam form towards an object to be imaged and an amorphous silicon flat-panel imager
`
`S towards an object and a cone-beam computer tomography system. The cone-beam computer tomography system includes an x-ray
`0 receiving x-rays after they pass through the object, the imager providing an image of the object. A computer is connected to the
`> radiation source and the cone beam computerized tomography system, wherein the computer receives the image of the object and
`~ based on the image sends a signal to the radiation source that controls the path of the radiation source.
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`CONE-BEAM COMPUTERIZED TOMOGRAPHY WITH A FLAT(cid:173)
`PANEL IMAGER
`
`Applicants claim, under 35 U.S.C. § 119(e), the benefit of priority of the
`filing date of February 18, 2000, of U.S. Provisional Patent Application Serial
`Number 60/183,590, filed on the aforementioned date, the entire contents of
`which are incorporated herein by reference.
`
`BACKGROUND OF THE INVENTION
`
`Field of the Invention
`
`The present invention relates generally to a cone-beam computed
`tomography system and, more particularly, to a cone-beam computed
`tomography system that employs an amorphous silicon flat-panel imager for
`use in radiotherapy applications where the images of the patient are acquired
`with the patient in the treatment position on the treatment table.
`
`Discussion of the Related Art
`
`Radiotherapy involves delivering a prescribed tumorcidal radiation
`dose to a specific geometrically defined target or target volume. Typically,
`this treatment is delivered to a patient in one or more therapy sessions
`(termed fractions). It is not uncommon for a treatment schedule to involve
`twenty to forty fractions, with five fractions delivered per week. While
`radiotherapy has proven successful in managing various.types and stages of
`cancer, the potential exists for increased tumor control through increased
`dose. Unfortunately, delivery of increased dose is limited by the presence of
`adjacent normal structures and the precision of beam delivery. In some sites,
`the diseased target is directly adjacent to radiosensitive normal structures.
`For example, in the treatment of prostate cancer, the prostate and rectum are
`directly adjacent. In this situation, the prostate is the targeted volume and the
`maximum deliverable dose is limited by the wall of the rectum.
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`In order to reduce the dosage encountered by radiosensitive normal
`structures, the location of the target volume relative to the radiation therapy
`source must be known precisely in each treatment session in order to
`accurately deliver a tumorcidal dose while minimizing complications in normal
`tissues. Traditionally, a radiation therapy treatment plan is formed based on
`the location and orientation of the lesion and surrounding structures in an
`initial computerized tomography or magnetic resonance image. However, the
`location and orientation of the lesion may vary during the course of treatment
`from that used to form the radiation therapy treatment plan. For example, in
`each treatment session, systematic and/or random variations in patient setup
`(termed interfraction setup errors) and in the location of the lesion relative to
`surrounding anatomy (termed interfraction organ motion errors) can each
`change the location and orientation of the lesion at the time of treatment
`compared to that assumed in the radiation therapy treatment plan.
`Furthermore, the location and orientation of the lesion can vary during a single
`treatment session (resulting in intrafraction errors) due to normal biological
`processes, such as breathing, peristalsis, etc. In the case of radiation
`treatment of a patient's prostate, it is necessary to irradiate a volume that is
`enlarged by a margin to guarantee that the prostate always receives a
`prescribed dose due to uncertainties in patient positioning and daily
`movement of the prostate within the patient. Significant dose escalation may
`be possible if these uncertainties could be reduced from current levels
`(-10mm) to 2-3mm.
`Applying large margins necessarily increases the volume of normal
`tissue that is irradiated, thereby limiting the maximum dose that can be
`delivered to the lesion without resulting in complication in normal structures.
`There is strong reason to believe that increasing the dose delivered to the
`lesion can result in more efficacious treatment. However, it is often the case
`that the maximum dose that can be safely delivered to the target volume is
`limited by the associated dose to surrounding normal structures incurred
`through the use of margins. Therefore, if one's knowledge of the location and
`orientation of the lesion at the time of treatment can be increased, then
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`margins can be reduced, and the dose to the target volume can be increased
`without increasing the risk of complication in normal tissues.
`A number of techniques have been developed to reduce uncertainty
`associated with systematic and/or random variations in lesion location
`resulting from interfraction and intrafraction errors. These include patient
`immobilization techniques (e.g., masks, body casts, bite blocks, etc.), off-line
`review processes (e.g., weekly port films, population-based or individual(cid:173)
`based statistical approaches, repeat computerized tomography scans, etc.),
`and on-line correction strategies (e.g., pre-ports, MV or kV radiographic or
`fluoroscopic monitoring, video monitoring, etc.).
`It is believed that the optimum methodology for reducing uncertainties
`associated with systematic and/or random variations in lesion location can
`only be achieved through using an on-line correction strategy that involves
`employing both on-line imaging and guidance system capable of detecting the
`target volume, such as the prostate, and surrounding structures with high
`spatial accuracy.
`An on-line imaging system providing suitable guidance has several
`requirements if it is to be applied in radiotherapy of this type. These
`requirements include contrast sensitivity sufficient to discern soft-tissue; high
`spatial resolution and low geometric distortion for precise localization of soft(cid:173)
`tissue boundaries; operation within the environment of a radiation treatment
`machine; large field-of-view (FOV) capable of imaging patients up to 40cm in
`diameter; rapid image acquisition (within a few minutes); negligible harm to
`the patient from the imaging procedure (e.g., dose much less than the
`treatment dose); and compatibility with integration into an external beam
`radiotherapy treatment machine.
`s.everal examples of known on-line imaging systems are described
`below. For example, strategies employing x-ray projections of the patient
`(e.g., film, electronic portal imaging devices, kV radiography/fluoroscopy, etc.)
`typically show only the location of bony anatomy and not soft-tissue
`structures. Hence, the location of a soft-tissue target volume must be inferred
`from the location of bony landmarks. This obvious shortcoming can be
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`alleviated by implanting radio-opaque markers on the lesion; however, this
`technique is invasive and is not applicable to all treatment sites. Tomographic
`imaging modalities (e.g., computerized tomography, magnetic resonance, and
`ultrasound), on the other hand, can provide information regarding the location
`of soft-tissue target volumes. Acquiring computerized tomography images at
`the time of treatment is possible, for example, by incorporating a
`computerized tomography scanner into the radiation therapy environment
`(e.g., with the treatment table translated between the computerized
`tomography scanner gantry and the radiation therapy gantry along rails) or by
`modifying the treatment machine to allow computerized tomography scanning.
`The former approach is a fairly expensive solution, requiring the installation of
`a dedicated computerized tomography scanner in the treatment room. The
`latter approach is possible, for example, by modifying a computer tomography
`scanner gantry to include mechanisms for radiation treatment delivery, as in
`systems for tomotherapy. Finally, soft-tissue visualization of the target
`volume can in some instances be accomplished by means of an ultrasound
`imaging system attached in a well-defined geometry to the radiation therapy
`machine. Although this approach is not applicable to all treatment sites, it is
`fairly cost-effective and has been used to illustrate the benefit of on-line
`therapy guidance.
`As illustrated in FIGS. 1 (a)-( c), a typical radiation therapy system 100
`incorporates a 4-25 MV medical linear accelerator 1 02, a collimator 1 04 for
`collimating and shaping the radiation field 1 06 that is directed onto a patient
`1 08 who is supported on a treatment table 11 0 in a given treatment position.
`Treatment involves irradiation of a lesion 112 located within a target volume
`with a radiation beam 114 directed at the lesion from one or more angles
`about the patient 108. An imaging device 116 may be employed to image the
`radiation field 118 transmitted through the patient 108 during treatment. The
`imaging device 116 for imaging the radiation field 118 can be used to verify
`patient setup prior to treatment and/or to record images of the actual radiation
`fields delivered during treatment. Typically, such images suffer from poor
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`contrast resolution and provide, at most, visualization of bony landmarks
`relative to the field edges.
`Another example of a known on-line imaging system used for reducing
`uncertainties associated with systematic and/or random variations in lesion
`location is an X-ray cone-beam computerized tomography system.
`Mechanical operation of a cone beam computerized tomography system is
`similar to that of a conventional computerized tomography system, with the
`exception that an entire volumetric image is acquired through a single rotation
`of the source and detector. This is made possible by the use of a two-
`dimensional (2-D) detector, as opposed to the 1-D detectors used in
`conventional computerized tomography. There are constraints associated
`with image reconstruction under a cone-beam geometry. However, these
`constraints can typically be addressed through innovative source and detector
`trajectories that are well known to one of ordinary skill in the art.
`As mentioned above, a cone beam computerized tomography system
`reconstructs three-dimensional (3-D) images from a plurality of two(cid:173)
`dimensional (2-D) projection images acquired at various angles about the
`subject. The method by which the 3-D image is reconstructed from the 2-D
`projections is distinct from the method employed in conventional
`computerized tomography systems. In conventional computerized
`tomography systems, one or more 2-D slices are reconstructed from one(cid:173)
`dimensional (1-D) projections of the patient, and these slices may be
`"stacked" to form a 3-D image of the patient. In cone beam computerized
`tomography, a fully 3-D image is reconstructed from a plurality of 2-D
`projections. Cone beam computerized tomography offers a number of
`advantageous characteristics, including: formation of a 3-D image of the
`patient from a single rotation about the patient (whereas conventional
`computerized tomography typically requires a rotation for each slice); spatial
`resolution that is largely isotropic (whereas in conventional computerized
`tomography the spatial resolution in the longitudinal direction is typically
`limited by slice thickness); and considerable flexibility in the imaging
`geometry. Such technology has been employed in applications such as
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`micro-computerized tomography, for example, using a kV x-ray tube and an x(cid:173)
`ray image intensifier tube to acquire 2-D projections as the object to be
`imaged is rotated, e.g., through 180° or 360°. Furthermore, cone beam
`computerized tomography has been used successfully in medical applications
`such as computerized tomography angiography, using a kV x-ray tube and an
`x-ray image intensifier tube mounted on a rotating C-arm.
`The development of a kV cone-beam computerized tomography
`imaging system for on-line tomographic guidance has been reported. The
`system consists of a kV x-ray tube and a radiographic detector mounted on
`the gantry of a medical linear accelerator. The imaging detector is based on a
`low-noise charge-coupled device (CCD) optically coupled to a phosphor
`screen. The poor optical coupling efficiency (-1 o-4) between the phosphor and
`the CCD significantly reduces the detective quantum efficiency (DQE) of the
`system. While this system is capable of producing cone beam computerized
`tomography images of sufficient quality to visualize soft tissues relevant to
`radiotherapy of the prostate, the low DQE requires imaging doses that are a
`factor of 3-4 times larger than would be required for a system with an efficient
`coupling (e.g. -SO% or better) between the screen and detector.
`Another example of a known auxiliary cone beam computerized
`·tomography imaging system is shown in FIG. 2. The auxiliary cone beam
`computerized tomography imaging system 200 replaces the CCD-based
`imager of FIGS. 1 (a)-( c) with a flat-panel imager. In particular, the imaging
`system 200 consists of a kilovoltage x-ray tube 202 and a flat panel imager
`204 having an array of amorphous silicon detectors that are incorporated into
`the geometry of a radiation therapy delivery system 206 that includes an MV
`x-ray source 208. A second flat panel imager 210 may optionally be used in
`the radiation therapy delivery system 206. Such an imaging system 200 could
`provide projection radiographs and/or continuous fluoroscopy of the lesion
`212 within the target volume as the patient 214 lies on the treatment table 216
`in the treatment position. If the geometry of the imaging system 200 relative
`to the system 206 is known, then the resulting kV projection images could be
`used to modify patient setup and improve somewhat the precision of radiation
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`treatment. However, such a system 200 still would not likely provide
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`adequate visualization of soft-tissue structures and hence be limited in the
`
`degree to which it could reduce errors resulting from organ motion.
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`Accordingly, it is an object of the present invention to generate KV
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`projection images in a cone beam computerized tomography system that
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`provide adequate visualization of soft-tissue structures so as to reduce errors
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`in radiation treatment resulting from organ motion.
`
`BRIEF SUMMARY OF THE INVENTION
`
`One aspect of the present invention regards a radiation therapy system
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`that includes a radiation source that moves about a path and directs a beam
`
`of radiation towards an object and a cone-beam computer tomography
`
`system. The cone-beam computer tomography system includes an x-ray
`
`source that emits an x-ray beam in a cone-beam form towards an object to be
`
`imaged and an amorphous silicon flat-panel imager receiving x-rays after they
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`pass through the object, the imager providing an image of the object. A
`
`computer is connected to the radiation source and the cone beam
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`computerized tomography system, wherein the computer receives the image
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`of the object and based on the image sends a signal to the radiation source
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`that controls the path of the radiation source.
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`A second aspect of the present invention regards a method of treating
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`an object with radiation that includes moving a radiation source about a path,
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`directing a beam of radiation from the radiation source towards an object and
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`emitting an x-ray beam in a cone beam form towards the object. The method
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`further includes detecting x-rays that pass through the object due to the
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`emitting an x-ray beam with an amorphous silicon flat-panel imager,
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`generating an image of the object from the detected x-rays and controlling the
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`path of the radiation source based on the image.
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`Each aspect of the present invention provides the advantage of
`
`generating KV projection images in a cone beam computerized tomography
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`system that provide adequate visualization of soft-tissue structures so as to
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`reduce errors in radiation treatment resulting from organ motion.
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`Each aspect of the present invention provides an apparatus and
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`method for improving the precision of radiation therapy by incorporating a
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`cone beam computerized tomography imaging system in the treatment room,
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`the 3-D images from which are used to modify current and subsequent
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`treatment plans.
`
`Each aspect of the present invention represents a significant shift in the
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`practice of radiation therapy. Not only does the high-precision, image-guided
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`system for radiation therapy address the immediate need to improve the
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`probability of cure through dose escalation, but it also provides opportunity for
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`broad innovation in clinical practice.
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`Each aspect of the present invention may permit alternative
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`fractionation schemes, permitting shorter courses of therapy and allowing
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`improved integration in adjuvant therapy models.
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`Each aspect of the present invention provides valuable imaging
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`information for directing radiation therapy also provides an explicit 3-D record
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`of intervention against which the success or failure of treatment can be
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`evaluated, offering new insight into the means by which disease is managed.
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`Additional objects, advantages and features of the present invention
`
`will become apparent from the following description and the appended claims
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`when taken in conjunction with the accompanying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIGS. 1 (a)-( c) schematically show the geometry and operation of a
`
`conventional radiation therapy apparatus;
`
`FIG. 2 schematically shows a perspective view of a known radiation
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`therapy apparatus including an auxiliary apparatus for cone beam
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`computerized tomography imaging;
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`FIG. 3 is a diagrammatic view of a bench-top cone beam computerized
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`tomography system employing a flat-panel imager, according to a first
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`embodiment of the present invention;
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`FIG. 4 is a schematic illustration of the geometry and procedures of the
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`cone beam computerized tomography system shown in FIG. 3;
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`FIGS. 5(a)- 5(d) are graphs depicting the fundamental performance
`characteristics of the flat-panel imager used in the cone beam computerized
`tomography system of FIG. 3;
`FIGS. 6(a)- 6(d) show various objects used in tests to investigate the
`performance of the cone beam computerized tomography system of the
`present invention, including a uniform water cylinder, six low-contrast inserts
`in a water bath, a steel wire under tension with a water bath, and an
`euthanized rat, respectively;
`FIGS. 7(a)- 7(d) depict uniformity of response of the cone beam
`computerized tomography system of the present invention, including axial and
`sagittal slices through volume images of a uniform water bath, radial profiles,
`and a vertical signal profile, respectively;
`FIGS. 8(a)- 8(d) illustrate the noise characteristics of the cone beam
`computerized tomography system of the present invention, including axial and
`sagittal noise images from volume reconstructions of a uniform water bath,
`radial noise profiles, and vertical nose profiles, respectively;
`FIGS. 9(a)- 9(b) depict response linearity and voxel noise,
`respectively, for the cone beam computerized tomography system of the
`present invention and a conventional computerized tomography scanner;
`FIGS. 1 O(a) - 1 O(c) depict the noise-power spectrum from the cone
`beam computerized tomography system of the present invention, including a
`gray scale plot of the axial noise-power spectrum, the noise-power spectrum
`measured at various exposures, and the noise-power spectrum for the cone
`beam computerized tomography system compared to a conventional
`computerized tomography scanner, respectively;
`FIGS. 11 (a)- 11 (b) depict the spatial resolution of the cone beam
`computerized tomography system of the present invention, including the
`surface plot of an axial slice image of the thin steel wire shown in FIG. 6(c)
`and the modulation transfer function measured for the cone beam
`computerized tomography system and for a conventional computerized
`tomography scanner, respectively;
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`FIGS. 12(a)- 12(b) show images of a low-contrast phantom obtained
`from the cone beam computerized tomography system of the present
`invention and a conventional computerized tomography scanner, respectively;
`FIGS. 13(a)- 13(i) show cone beam computerized tomography images
`of the euthanized rat shown in FIG. 6(d), including regions of the lungs (FIGS.
`13(a)- 13(c)), the kidneys (FIGS. 13(d)-13(f)), and the lower spine (FIGS.
`13(g)- 13(i));
`FIGS. 14(a)- 14(d) show volume renderings of cone beam
`computerized tomography images of the euthanized rat shown in FIG. 6(d)
`illustrating the degree of spatial resolution achieved in delineating structures
`of the vertebra, including volume renderings with axial and sagittal cut planes
`showing the skeletal anatomy along with soft-tissue structures of the
`abdomen, volume renderings with axial and sagittal cut planes, window to
`show skeletal features only, a magnified view of a region of the spine and ribs
`of the rat, and a magnified view of a part of two vertebra, respectively;
`FIGS. 15(a) - 15(b) depict the axial images of euthanized rat shown in
`FIG. 6(d) obtained from the cone beam computerized tomography system of
`the present invention and a conventional computerized tomography scanner,
`respectively;
`FIG. 16 is a graph showing detected quantum efficiency calculated as
`a function of exposure for an existing and hypothetical flat-panel imager
`configuration;
`FIGS. 17(a)-(e) are diagrammatic views of several angular orientations
`of a wall-mounted cone beam computerized tomography system employing a
`flat-panel imager, according to a second embodiment of the present invention;
`FIG. 18 shows a side view of the cone beam computerized tomography
`system of FIG. 17 when employing a first embodiment of a support for a flat(cid:173)
`panel imager according to the present invention;
`FIG. 19(a) shows a perspective exploded view of a mounting to be
`used with the support for a flat-panel imager of FIG. 18;
`FIG. 19(b) shows a perspective exploded view of a rotational coupling
`to be used with the mounting of FIG. 19(a};
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`FIGS. 20(a)-(b) schematically shows a front view of the wall-mounted
`cone beam computerized tomography system of FIG. 17 when employing a
`second embodiment of a support for a flat-panel imager according to the
`present invention;
`FIGS. 21 (a)-(b) schematically shows a front view of the wall-mounted
`cone beam computerized tomography system of FIG. 17 when employing a
`third embodiment of a support for a flat-panel imager according to the present
`invention;
`FIG. 22 is a diagrammatic view of a portable cone beam computerized
`tomography system empl~ying a flat-panel imager according to fifth
`embodiment of the present invention;
`FIGS. 23(a)-(d) are diagrammatic sketches illustrating the geometry
`and operation of the cone beam computerized tomography imaging systems
`of FIGS. 17-22;
`FIG. 24 is a flow-chart showing an embodiment of the processes
`involved in acquiring a cone beam computerized image for the cone beam
`computerized tomography imaging systems of FIGS. 17-22;
`FIG. 25 is a perspective drawing illustrating an embodiment of a
`method for geometric calibration of the imaging and treatment delivery
`systems of FIGS. 17-22; and
`FIG. 26 is a flow-chart showing an embodiment of the processes
`involved in the image-guided radiation therapy systems of HGS. 17-22, based
`on cone beam computerized tomography imaging of a patient, on-line
`correction of setup errors and organ motion, and off-line modification of
`subsequent treatment plans.
`
`PREFERRED EMBODIMENTS OF THE INVENTION
`A bench-top cone beam computerized tomography (CBCT) system 300
`is shown in FIG. 3, according to an embodiment of the present invention. The
`CBCT system 300 was constructed to mimic the geometry of the CBCT
`scanner currently installed on a linear accelerator, with a source-to-axis
`distance of 1 000 mm and a source-detector distance of 1600 mm. The
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`primary components of the system 300 include an x-ray tube 302, a rotation
`stage 304 and flat-panel imager (FPI) 306. These components are rigidly
`mounted to an optical bench 308. The relative position of these components
`is controlled by three translation stages, including an xobject stage 310, a
`yobject stage 312 and a yimage stage 314, which are used dur!ng initial setup
`to accurately determine and control the imaging geometry. The cone beam
`computerized tomography system 300 generates images of an object 316,
`identified throughout as a phantom, mounted on the rotation stage 304. Each
`stage 310, 312 and 314 contains a home or limit switch, and the imaging
`geometry is referenced to the location of these switches with a reproducibility
`of ±0.01 mm. The specific geometries used in the discussion herein are
`shown in FIG. 4, and are set to simulate the imaging geometry that would be
`implemented for a cone beam computerized tomography system incorporated
`on a radiotherapy treatment machine. Table 1 below shows the parameters
`of the system 300.
`A set of alignment lasers 318 allow visualization of the axis of rotation
`320 and the source plane perpendicular to the axis of rotation 320 and
`intersects focal spot 322 of the x-ray source or tube 302. The axis of
`rotation 320 is positioned such that it intersects the central ray 324 between
`the focal spot 322 and the detector plane 326 (+0.01 mm). The flat plane
`imager 326 is positioned such that the piercing point (i.e., the intersection of
`the central ray and the image plane) is centered on the imaging array (i.e.,
`between columns #256 and #257, ±0.01 mm), with a quarter-pixel offset
`applied to give improved view sampling for cone beam computerized
`tomography acquisitions in which the object 316 is rotated through 360°. The
`stage 310 is controlled manually by means of a positioning micrometer. The
`source-to-object (SOD) and source-to-image (SID) distances were measured
`to within ±0.5 mm and give an objection magnification of 1.60, equal to that of
`the imaging system on the linear accelerator. The cone angle for this
`geometry is -7.1 .
`Radiographic exposures used in the acquisition procedure are
`produced under computer control with a 300 kHU x-ray tube 302, such as
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`General Electric Maxi-ray 75 and a 100 kW generator, such as the General
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`Electric MSI-800. The tube 302 has a total minimum filtration of 2.5 mm A 1,
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`with an additional filtration of 0.127 mm Cu to further harden the beam, and a
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`nominal focal spot size of 0.6 mm. The 100 kV beam is characterized by first
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`and second HVLs of 5.9 and 13.4 mm A 1, respectively. The accelerating
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`potential of the generator was monitored over a one-week period and was
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`found to be stable to within ±1 %. All exposures were measured using an
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`x-ray multimeter, such as the RTI Electronics, Model PMX-111 with silicon
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`diode detector.
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`The exposures for the cone beam computerized tomography
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`acquisitions are reported in terms of exposure to air at the axis of rotation 320
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`in the absence of the object 316. The same method of reporting exposure
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`can be used for the images acquired on the conventional scanner. For the
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`conventional scanner, the exposure per unit charge is measured with the
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`gantry rotation disabled and the collimators set for a 1 0 mm slice thickness,
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`thereby guaranteeing complete coverage of the silicon diode. The exposure
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`per unit charge at 100 kVp was 9.9 mR/mAs and 14.9 mR/mAs for the
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`bench-top and conventional scanners, respectively.
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`The flat panel imager 306 can be the EG&G Heimann Optoelectronics
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`(RID 512-400 AO) that incorporates a 512 x 512 array of a-Si:H photodiodes
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`and thin-film transistors. The electro-mechanical characteristics of the imager
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`are shown in Table 1. The flat plane imager 306 is read-out at one of eight
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`present frame rates (up to 5 frames per second) and operates asynchronously
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`of the host computer 328 schematically shown in FIG. 4. The analog signal
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`from each pixel is integrated by ASIC amplifiers featuring correlated
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`double-sampling noise reduction circuitry. Digitization is performed at 16 bit
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`resolution. The values are transferred via an RS-422 bus to a hardware
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`buffer in the host computer 328. The processor in the host computer 328 is
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`interrupted when a complete frame is ready for transfer to host memory.
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`Table 1
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`CBCT Characteristic
`
`Acquisition Geometry
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`Source-axis-distance (SAo)
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`Source-imager-distance (S10)
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`Cone angle
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`Maximum angular rotation rate
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`Field of view (FOV)
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`X-ray Beam/Exposure Characteristics
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`Beam energy
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`Added filtration
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`Beam quality
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`Value
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`103.3 em
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`165.0 em
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`7.1°
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`0.5°/sec
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`12.8 em
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`100 kVp
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`1.5mm A1 + 0.129 mm Cu
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`HVL1 = 5.9 MM A1
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`H