WO2004066215A1 - Procede de tomographie assistee par ordinateur a rayons coherents disperses et tomographie assistee par ordinateur - Google Patents

Procede de tomographie assistee par ordinateur a rayons coherents disperses et tomographie assistee par ordinateur Download PDF

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Publication number
WO2004066215A1
WO2004066215A1 PCT/IB2004/000110 IB2004000110W WO2004066215A1 WO 2004066215 A1 WO2004066215 A1 WO 2004066215A1 IB 2004000110 W IB2004000110 W IB 2004000110W WO 2004066215 A1 WO2004066215 A1 WO 2004066215A1
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WIPO (PCT)
Prior art keywords
measured values
rotation
examination zone
radiation source
axis
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Application number
PCT/IB2004/000110
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English (en)
Inventor
Michael Grass
Thomas Köhler
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Philips Intellectual Property & Standards Gmbh
Koninklijke Philips Electronics N.V.
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Publication of WO2004066215A1 publication Critical patent/WO2004066215A1/fr

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Classifications

    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/48—Diagnostic techniques
    • A61B6/483—Diagnostic techniques involving scattered radiation
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
    • A61B6/03—Computed tomography [CT]
    • A61B6/032—Transmission computed tomography [CT]
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/40—Arrangements for generating radiation specially adapted for radiation diagnosis
    • A61B6/4064—Arrangements for generating radiation specially adapted for radiation diagnosis specially adapted for producing a particular type of beam
    • A61B6/4085—Cone-beams
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T11/00—2D [Two Dimensional] image generation
    • G06T11/003—Reconstruction from projections, e.g. tomography
    • G06T11/005—Specific pre-processing for tomographic reconstruction, e.g. calibration, source positioning, rebinning, scatter correction, retrospective gating
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
    • A61B6/027—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis characterised by the use of a particular data acquisition trajectory, e.g. helical or spiral
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/42—Arrangements for detecting radiation specially adapted for radiation diagnosis
    • A61B6/4291—Arrangements for detecting radiation specially adapted for radiation diagnosis the detector being combined with a grid or grating

Definitions

  • the invention relates to a computed tomography method in which an examination zone is irradiated by a fan-beam and a detector unit detects coherent scattered rays in the examination zone.
  • the invention also relates to a computed tomograph for carrying out this method, and to a computer program for controlling the computed tomograph.
  • the examination zone is irradiated along a circular trajectory.
  • Coherent scattered rays are measured in the examination zone by a detector unit having a measurement surface, and the spatial profile or distribution of the scattering intensity or strength in the examination zone is reconstructed from these measured values.
  • the reconstruction is usually effected by means of iterative methods based on algebraic reconstruction techniques (ART) or by means of two- or three-dimensional filtered back-projection.
  • ART algebraic reconstruction techniques
  • These methods are disadvantageous in that in each case only one slice of the examination zone is irradiated and subsequently also only the distribution of the scattering intensity in this one slice is reconstructed.
  • ART algebraic reconstruction techniques
  • this object is achieved according to the invention by a computed tomography method having the steps: a) generation of a fan beam (41) using a radiation source (S), said fan beam (41) passing through an examination zone (13) or an object located therein, b) generation of a relative movement between the radiation source on the one hand and the examination zone or the object on the other, which movement comprises a rotation about an axis of rotation and a displacement parallel to the axis of rotation and runs in the form of a helix, c) acquisition of measured values which are dependent on the intensity of the radiation, by means of a detector unit (16) which detects radiation that is scattered coherently in the examination zone (13) or over the object, during the relative movement, d) reconstruction of a distribution of the scattering intensity in the examination zone (13) from the measured values.
  • a computed tomography method having the steps: a) generation of a fan beam (41) using a radiation source (S), said fan beam (41) passing through an examination zone (13) or an object located therein, b)
  • the radiation source moves relative to the examination zone along a helix-like trajectory. This permits more rapid acquisition of the measured values of the entire examination zone and reduces the imaging errors that are caused, for example, by movements of the patient.
  • Claim 2 describes an interpolation of various measured values, which permits subsequent reconstruction in a reduced amount of time.
  • the scattering intensity depends not only on the material but also on the scattering angle and the wavelength of the radiation.
  • the back-projection in a volume which is defined by two linearly independent vectors of the rotation plane and a wave vector transfer, as claimed in claim 3 has the advantage that the scattering intensity parameterized in this way is now only dependent on the scattering material. This is because the wave vector transfer, as is known, is proportional to the product of the inverse wavelength and the sine of half the scattering angle.
  • the scattering angle is in this case the angle which encloses the course of the scattered ray with the course which the ray would have followed if there had not been the scattering process.
  • the scattered rays have a curved shape. Taking this curved shape of the scattered rays into account in the back-projection leads to an improved quality of the reconstructed distribution of the scattering intensity.
  • Claims 4 and 5 in each case describe a preferred reconstruction method having computing expenditure that is lower than that of other methods and leads to a good image quality.
  • a computed tomograph for carrying out the method according to the invention is described in claim 6.
  • Claim 7 defines a computer program for controlling a computed tomograph as claimed in claim 6.
  • Fig. 1 shows a computed tomograph which can be used to implement the method according to the invention.
  • Fig. 2 shows a schematic cross-sectional view of the computed tomograph in the direction of the axis of rotation.
  • Fig. 3 shows a flowchart of the method according to the invention.
  • Fig. 4 shows a plan view of a column of detector elements.
  • Fig. 5 shows a schematic perspective view of a helix-like trajectory and of a slice in the examination zone.
  • Fig. 6 shows a schematic illustration of the arrangement of virtual radiation sources.
  • Fig. 7 shows a sectional zone through the rays of the virtual radiation sources.
  • Fig. 8 shows a schematic sectional view of the irradiated examination zone and of the detector in the direction of the rotation plane.
  • Fig. 9 shows the dependence of the value of the wave vector transfer on the distance of a scattering center in the examination zone from the base of the detector (point where the primary ray strikes the detector unit).
  • the computed tomograph shown in Fig. 1 comprises a gantry 1 which can rotate about an axis of rotation 14.
  • the gantry 1 is driven by a motor 2 at a preferably constant, but adjustable, angular velocity.
  • a radiation source S for example an X- ray radiator, is fitted on the gantry 1.
  • An aperture arrangement 31 determines a fan beam 41 used for the examination, said fan beam 41 being shown by solid lines in Fig. 1.
  • the fan beam 41 runs perpendicular to the axis of rotation 14 and in the direction thereof has small dimensions, for example 1 mm.
  • a second aperture arrangement 32 which screens out a cone- shaped bundle of rays 42 from the radiation generated by the radiation source S.
  • the cone- shaped bundle of rays 42 which would arise without aperture arrangement 31 is shown by dashed lines.
  • the fan beam 41 penetrates a cylindrical examination zone 13 in which an object, e.g. a patient on a patient table (both not shown) or else a technical object, may be located.
  • the fan beam 41 strikes a detector unit 16 fitted to the gantry 1, said detector unit 16 having a measurement surface which comprises a large number of detector elements arranged in the form of a matrix.
  • the detector elements are arranged in rows and columns.
  • the detector columns run parallel to the axis of rotation 14.
  • the detector rows are located in planes perpendicular to the axis of rotation, preferably on an arc of a circle about the radiation source S. However, they may also be of a different shape, e.g. describe an arc of a circle about the axis of rotation 14 or be rectilinear.
  • the detector unit 16 detects not only the coherent scattered radiation but also the primary radiation and the incoherent scattered radiation.
  • the fan beam 41, the examination zone 13 and the detector unit 16 are adapted to one another. In a plane perpendicular to the axis of rotation 14, the dimensions of the fan beam 41 are selected such that the examination zone 13 is completely irradiated, and the length of the detector unit 16 is to be precisely dimensioned such that the fan beam 41 can be detected in its entirety.
  • the fan beam strikes the central detector row(s).
  • the object may be rotated during an examination, while the radiation source S and the detector unit 16 remain stationary.
  • the examination zone 13 - or the object or patient table - may be displaced parallel to the axis of rotation 14 by means of a motor 5.
  • the gantry 1 could also be moved in this direction in an equivalent manner.
  • Fig. 2 shows that, between the examination zone 13 and the detector unit 16, there is a coUimator arrangement 6 which comprises a large number of planar lamellae 60.
  • the lamellae 60 are made of a material that is highly absorbent for X-ray radiation and lie in planes which run parallel to the axis of rotation 14 and intersect at the focus of the radiation source S. They may be spaced apart by e.g. 1 cm, and each lamella 60 may have a dimension of e.g. 20 cm in the plane of the drawing.
  • the fan beam 41 is thus divided into a number of mutually adjacent sections, so that a column of detector elements is essentially struck by only primary or scattered rays from a section.
  • the measured values acquired by the detector unit 16 are fed to an image processing computer 10 which is connected to the detector unit 16, for example, via a contactless slip ring (not shown).
  • the image processing computer 10 reconstructs the distribution of the scattering intensity in the examination zone 13 and outputs it, for example, on a monitor 11.
  • the two motors 2 and 5, the image processing computer 10, the radiation source S and the transfer of the measured values from the detector unit 16 to the image processing computer 10 are controlled by a control unit 7.
  • the acquired measured values may be fed for reconstruction purposes initially to one or more reconstruction computers which pass the reconstructed data to the image processing computer via a glass fiber cable for example.
  • Fig. 3 shows the sequence of an embodiment of a measurement and reconstruction method which can be carried out using the computed tomograph shown in Fig. 1.
  • the gantry 1 rotates at a constant angular velocity.
  • the examination zone or the object or the patient table is displaced parallel to the axis of rotation and the radiation of the radiation source S is switched on, so that the detector unit 16 can detect the radiation from a large number of angular positions.
  • the detector element(s) in the center of each detector column essentially detect the primary radiation, while the scattered radiation (secondary radiation) is detected by the detector elements lying further out in a column.
  • Fig. 4 shows a plan view of a column of detector elements.
  • the detector elements 161 which detect the scattered radiation are shown simply by hatching, while the detector element 160 in the center, which detects the primary radiation, is marked with a cross.
  • the detector element 160 in the center which detects the primary radiation
  • On both sides of this central detector element there are, on account of the finite dimensions of the focus of the radiation source, detector elements which are struck by scattered radiation but also by a (reduced) primary radiation.
  • only those rays which are measured by the detector elements shown by hatching in the drawing are regarded as scattered rays.
  • the scattered rays comprise, as mentioned above, coherent and incoherent radiation, where, as is known, only the coherent radiation plays a part in the reconstruction of the distribution of the scattering intensity in the examination zone.
  • the scattering intensity is dependent, inter alia, on the energy of the scattered X-ray quantum. Therefore, the energy of the scattered X-ray quantum either must be measured, which requires that the detector elements are able to measure energy in a resolving manner, or else X-ray radiation with quantum energies from a range that is as small as possible (ideally monochromatic X-ray radiation) must be used. There are various possibilities for minimizing the energy difference of the X-ray quanta relative to their energy as far as possible:
  • the voltage of an X-ray tube can be optimized relative to the selected filter.
  • step 105 the measured values of the scattered rays are standardized. For this, the measured values of each radiation source position of the scattered rays are divided by the measured values of those primary rays which have caused the scattered rays.
  • the measured values are reinterpolated.
  • the measured values are reinterpolated as if the examination zone had been irradiated slice by slice by rays of a radiation source which moves along a circular trajectory.
  • the slice 21 intersects the helix-like trajectory 18 of the radiation source S at a point 23, so that the slice 21 alone has been irradiated from this interface position 23.
  • Fictitious rays of the virtual radiation source which is moving along a circular path said fictitious rays irradiating the slice 21 from other angular positions, or the corresponding measured values, are obtained by interpolation.
  • the associated measured value is determined by interpolation of the next adjacent value measured in the z direction at the same projection angle and by the same detector element.
  • P(z, ⁇ ) refers to a measured value of a detector element at the position z at the projection angle ⁇ .
  • the interpolation is carried out for each detector element, for each projection angle and for each slice of the irradiated examination zone. In other embodiments, the interpolations could also be carried out only for all slices that are to be reconstructed.
  • This type of interpolation is generally known as "360° interpolation” and has been published inter alia in "Bild sacrificede Systeme furrare Diagnostik” ["Imaging systems for medical diagnosis”], H. Morneburg, Er GmbH: Siemens Publicis MCD Verlag, 1995, to which reference is hereby made.
  • a rebinning of the interpolated measured values may take place.
  • each measured value is assigned a line from the detector element at which the measured value has been detected to the radiation source position. It is therefore assumed that rays of the fictitious, cone-shaped bundle of rays 42 have caused the measured values without the rays having been scattered.
  • the measured values are now resorted as if they had been measured with a different radiation source (a radiation source shaped like the arc of a circle, which can emit fans of rays that are in each case parallel to one another) and with a different detector (a planar, rectangular virtual detector). This is explained in more detail with reference to Fig. 6.
  • the reference 17 refers to the circular trajectory from which the radiation source irradiates the examination zone.
  • the reference 413 refers to a fan-like bundle of rays which starts from the radiation source position So and the rays of which run in a plane comprising the axis of rotation 14. It is possible to regard the cone-shaped bundle of rays, which is emitted from the radiation source position So, as being made up of a large number of planar fans of rays which are located in planes parallel to the axis of rotation 14, said planes intersecting one another at the radiation source position.
  • Fig. 6 shows only a single one of these fans of rays, namely the fan beam 413. Moreover, Fig.
  • FIG. 6 also shows other fans of rays 411, 412 and 414, 415, which lie parallel to the fan beam 413 and in planes that lie parallel to one another and to the axis of rotation 14.
  • the associated radiation source positions S. 2 , S.i and Si, S are assumed by the radiation source S before and after it has reached the radiation source position So, respectively.
  • AU rays in the fans of rays 411 to 415 have the same projection angle.
  • the projection angle is the angle which the planes of the fans of rays enclose with a reference plane that is parallel to the axis of rotation 14.
  • the fans of rays 411 to 415 define a bundle of rays 410 having a tent-like shape.
  • Figs. 6 and 7 show the sectional zone 420 which results when the bundle of rays 410 is sectioned by a plane that comprises the axis of rotation 14 and is perpendicular to the planes of the radiation fans 411 to 415.
  • the two edges of the sectional zone 420 that intersect the axis of rotation are curved. This curvature can be attributed to the fact that the radiation source positions in the center (e.g. So) are further away from the sectional plane than those at the edge (e.g. S 2 or S -2 ) and that the fans all have the same opening angle.
  • a rectangular virtual detector 170 is defined in the planar sectional zone 420, the edges 171 and 172 of which detector 170 are given by the dimensions of the outer fans of rays 411 and 415, respectively, in the planar sectional zone.
  • Fig. 7 also shows - marked by round dots - the penetration points of some rays contained in the fans of rays 411...415 through this virtual detector.
  • crosses show the support points of a regular Cartesian grid. The penetration points and the support points never coincide. The measured values at the equidistant support points within the virtual detector 170 must therefore be determined from the measured values for the penetration points.
  • step 111 a one-dimensional filtering using a ramp-like transmission factor increasing with the spatial frequency is applied to the measured values resulting from the rebinning.
  • successive values in a direction parallel to the rotation plane, that is to say along a row of the virtual detector, are used for this purpose. This filtering is carried out along each row of the virtual detector for all projection angles.
  • the rebinning could be omitted.
  • the filtered measured values are then used to reconstruct the distribution of the scattering intensity in the examination zone by means of back-projection.
  • the back-projection in this case takes place in a volume which is defined by the vectors x , y and q , where the unit vectors x and y lie in the rotation plane and are oriented horizontally or vertically and the wave vector transfer q is oriented parallel to the axis of rotation.
  • two different, linearly independent vectors of the rotation plane may be used instead of the vectors x and y .
  • the value of the wave vector transfer q is, as already mentioned above, proportional to the product of the inverse wavelength ⁇ of the scattered X-ray quanta and of the sine of half the scattering angle ⁇ :
  • the scattering angle ⁇ can be determined using the arrangement comprising the examination zone 15 irradiated by the fan beam 41 and the detector unit 16, which arrangement is shown in Fig. 7.
  • a detector element D r detects scattered rays which have been scattered at various scattering angles ⁇ .
  • d is the distance of a scattering center S
  • a is the distance of the detector element D, from the base 12 of the detector.
  • the detector element D detects rays which are scattered at the angles ⁇ 2 in the examination zone 15 irradiated by the fan beam 41.
  • the value of the wave vector transfer q as a function of the distance d of a scattering center from the base of the detector thus has a hyperbolic and thus nonlinear course (shown in Fig. 8). From this it can be seen that the originally straight course of the scattered rays is curved in the (x,y,q) space. The back-projection is thus effected along rays curved in a hyperbolic manner.
  • a voxel V(x,y,q) is determined within a predefinable (x,y) region
  • step 115 the filtered values are multiplied by a weight factor which corresponds to the reciprocal value of the cosine of the scattering angle.
  • a weight factor which corresponds to the reciprocal value of the cosine of the scattering angle.
  • V(x,y,q) are now used. If no ray of a radiation source position exactly matches the center of the voxel, then the associated value must be determined by interpolation of the measured values of adjacent rays. The measured value which can be assigned to the ray matching the voxel or the measured value obtained by interpolation is accumulated at the voxel V(x,y,q). Once the values for the relevant voxel have been accumulated in this way for all radiation source positions, in step 119 a check is made as to whether all voxels are irradiated in the (x,y,q) zone which is to be reconstructed. If this is not the case, the flowchart branches to step 113.
  • the distribution of the scattering intensity has been determined for all voxels in the FOV and the reconstruction method is terminated (step 121).
  • the back-projection can be earned out in the volume defined by two linearly independent vectors of the rotation plane and by the wave vector transfer, as an approximation along straight rays.
  • the distribution of the scattering intensity in the examination zone can be reconstructed from the interpolated measured values by known iterative methods based on algebraic reconstruction techniques (ART) or by known two- dimensional back-projections.
  • ART algebraic reconstruction techniques
  • the projections for various projection angles of a slice can be obtained by means of interpolation. This slice is then reconstructed in accordance with one of the abovementioned methods. Projections for various projection angles of another slice, e.g. an adjacent slice, are then generated by means of interpolation. This second slice is also reconstructed in accordance with one of the abovementioned methods. In this way the distribution of the scattering intensity in the entire examination zone can be reconstructed slice by slice.

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Abstract

L'invention concerne un procédé de tomographie assistée par ordinateur dans lequel une zone d'examen est irradiée le long d'une trajectoire du type hélice au moyen d'un faisceau de rayons du type éventail. On mesure, dans la zone d'examen, un rayonnement cohérent dispersé au moyen d'une unité de détection, le profil spatial de l'intensité de la dispersion dans la zone d'examen étant reconstruit à partir des valeurs mesurées. La reconstruction peut être effectuée par rétroprojection dans un volume défini par deux vecteurs linéairement indépendant du plan de rotation et par transfert de vecteur.
PCT/IB2004/000110 2003-01-21 2004-01-13 Procede de tomographie assistee par ordinateur a rayons coherents disperses et tomographie assistee par ordinateur WO2004066215A1 (fr)

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EP03100120 2003-01-21
EP03100120.9 2003-01-21

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