Abstract:
A multislice CT scanner for imaging a patient comprising: an X-ray source that generates a cone beam of X-rays radiated from a focal spot of the X-ray source wherein the X-ray source is moveable in a rotation plane so as to rotate the focal spot about an axial direction along which the patient is moved to position the patient in a field of view of the scanner; and a detector array comprising a plurality of rows of X-ray detectors that generate signals responsive to X-rays in the cone beam, which signals are used to generate an image of the patient; wherein the cone beam is asymmetric with respect to the rotation plane.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to computerized tomography (CT) X-ray imaging, and in particular to a size and shape of X-ray cone beams and corresponding X-ray detector arrays in multislice CT scanners.  
       BACKGROUND OF THE INVENTION  
       [0002]     In CT X-ray imaging of a patient, X-rays are used to image internal structure and features of a region of interest (ROI) of the patient&#39;s body. A multislice CT scanner generally comprises an X-ray source that provides a cone shaped X-ray beam radiated from a focal spot of the X-ray source and an X-ray detector array comprising a plurality of closely spaced rows of X-ray detectors that face the X-ray source. In third generation multislice scanners both the X-ray source and detector array are mounted in a rotor of a gantry. In fourth generation multislice scanners the X-ray source is mounted to the rotor but the X-ray detector array, in which detectors comprised in the array form complete circles of detectors, is mounted to the gantry.  
         [0003]     A patient being imaged with the scanner is generally supported on a bed which is moved axially along a z-axis to position the ROI in a field of view (FOV) of the scanner located inside the rotor, between the X-ray source and detector array. The rotor is rotatable around the z-axis so as to position the X-ray source, and in third generation CT scanners the detector array, at different cone beam view angles around the patient, from which view angles the X-ray source illuminates the ROI with X-rays. Measurements of intensity of X-rays from the X-ray source that pass through the patient&#39;s body at the different view angles provide measurements of attenuation of the X-rays for different attenuation paths through the body. The attenuation measurements are used to image the ROI.  
         [0004]     As the rotor is rotated, motion of the X-ray source focal spot occurs in a plane, hereinafter referred to as a “rotation plane”, generally perpendicular to the z-axis. A vertex angle of a fan shaped cross section of the cone beam in the rotation plane is a “fan angle” of the cone beam. A vertex angle of a fan-shaped cross section of the cone beam in a plane perpendicular to the rotation plane that passes through the focal spot and the z-axis is a “cone beam angle” of the cone beam. The cone beam is substantially symmetric relative to the rotation plane and the rotation plane bisects the cone beam angle of the cone beam. To match the symmetric cone beam, the rows of the X-ray detectors in the detector array are symmetrically disposed relative to the rotation plane i.e. each row has a mirror image row in the rotation plane.  
         [0005]     For a given size fan angle, the cone beam volume and size of the detector array, and for a given size of X-ray detectors in the detector array, the number of detector rows in the array, are limited by the cone beam angle. The cone beam angle in turn is limited inter alia by an effect referred to as a heel effect.  
         [0006]     Let an angle relative to the rotation plane of a path along which X-rays radiated from the X-ray source focal spot propagate be referred to as “declination angle”. It is convenient to define declination angles to be positive on one side, a positive side, of the rotation plane and to be negative on the other side, a negative side, of the rotation plane. The heel effect hardens and decreases intensity of X-rays emanating from the X-ray source as the magnitude of the declination angle increases on one side, arbitrarily the negative side, of the rotation plane. The hardening and intensity reduction is a result of a configuration of an anode comprised in the X-ray source onto which an electron beam is focussed to generate X-rays. As a result of the configuration, for increasingly negative declination angles, as X-rays generated in the anode leave the target, they encounter on the average a greater amount of material from which the anode is formed. As the amount of anode material that the X-rays traverse increases, attenuation and hardening of the X-rays increase, generating thereby the heel effect.  
         [0007]     The hardening and drop in intensity as a function of decreasing declination angle determines a minimum declination angle, hereinafter a “heel effect angle” for X-rays below which intensities and energies of X-rays are generally not effective for CT imaging. As a result of the symmetry of the cone beam relative to the rotation plane, a maximum positive declination angle for X-rays is equal to the magnitude of the heel effect angle. The cone beam therefore has a cone beam angle equal to about twice the magnitude of the heel effect angle. The heel effect therefore limits the cone beam angle, and thereby a volume of the cone beam, a total number (for a given detector size) of detector rows in the scanner and a volume of a patient for which CT scan data can be simultaneously acquired.  
         [0008]     In order to provide faster CT imaging of patients and enable imaging of moving organs of the body with improved temporal resolution, for example cardiac imaging, there is a need to increase the volume of a patient for which data is simultaneously acquired by a multislice scanner.  
       SUMMARY OF THE INVENTION  
       [0009]     An aspect of some embodiments of the present invention relates to providing a multislice CT scanner having a cone beam for which the cone beam angle and thereby cone beam volume are larger than cone beam angles and volumes of prior art cone beams having a same fan angle.  
         [0010]     The inventors have noted that the heel effect does not substantially affect intensity and hardness of X-rays on the positive side of a CT scanner rotation plane and that X-rays having declination angles greater than the magnitude of the heel effect angle have intensities and energies useful for CT imaging. However, X-rays having declination angles greater than the magnitude of the heel effect angle are not used in prior art scanners as a result of the prior art constraint that a CT cone beam be symmetric with respect to its rotation plane.  
         [0011]     Therefore, a cone beam of a CT scanner in accordance with some embodiments of the invention is not symmetric with respect to its rotation plane but is larger on the positive side of the rotation plane than on the negative side. As a result, the cone beam angle and volume of the cone beam is not limited by its heel effect angle and are larger than a cone beam angle and (for a same fan angle) a volume of a corresponding prior art cone beam having a same heel effect angle. A matching detector array, in accordance with the present invention, is also asymmetric with respect to the rotation plane and for a same fan angle, larger than a prior art detector array. A CT scanner comprising a cone beam in accordance with the invention can therefore generally simultaneously acquire data for a larger volume of a patient than a CT scanner comprising a corresponding prior art cone beam.  
         [0012]     According to an aspect of some embodiments of the invention, on the positive side of the rotation plane, a number of X-ray detector rows in the matching detector array is greater than a number of detector rows on the negative side of the rotation plane.  
         [0013]     According to an aspect of some embodiments of the invention a width of at least one detector row on the positive side of the rotation plane is different from widths of detector rows on the negative side of the rotation plane.  
         [0014]     According to an aspect of some embodiments of the invention, widths of detector rows increases as declination angles at which the rows are located increases.  
         [0015]     In some embodiments of the present invention, row width of each detector row in at least a portion of the rows in the detector array is substantially proportional or equal to an apparent size of the X-ray source focal spot along the z-axis as seen from the row. Apparent z-axis size of the focal spot of the X-ray source increases with increasing declination angle of a location of the row. A detector array for a cone beam having a given cone beam angle can generally be produced with a smaller number of X-ray detectors without substantially compromising spatial resolution of the array if row widths are substantially proportional or equal to apparent focal spot size. The smaller number of X-ray detectors generally results in a lower production cost for the array.  
         [0016]     There is therefore provide in accordance with an embodiment of the present invention, a multislice CT scanner for imaging a patient comprising: an X-ray source that generates a cone beam of X-rays radiated from a focal spot of the X-ray source wherein the X-ray source is moveable in a rotation plane so as to rotate the focal spot about an axial direction along which the patient is moved to position the patient in a field of view of the scanner, and a detector array comprising a plurality of rows of X-ray detectors that generate signals responsive to X-rays in the cone beam, which signals are used to generate an image of the patient; wherein the cone beam is asymmetric with respect to the rotation plane.  
         [0017]     Optionally, trajectories of X-rays on a first side of the rotation plane that are incident on the detector array have declination angles relative to the rotation plane that have a first maximum magnitude and trajectories of X-rays incident on the detector array on a second side of the rotation plane have declination angles that have a second maximum magnitude greater than the first maximum.  
         [0018]     Optionally, the second maximum magnitude is greater than 1.25 times the first maximum magnitude. Alternatively, the second maximum magnitude is optionally greater than 1.5 times the first maximum magnitude. Alternatively, the second maximum magnitude is optionally greater than twice the first maximum magnitude.  
         [0019]     In some embodiments of the present invention, the first maximum angle is determined substantially by the heel effect.  
         [0020]     In some embodiments of the present invention, the detector rows are substantially parallel to the rotation plane and a width of each row in at least a portion of the rows is a function of a declination angle relative to the rotation plane of a line from the focal spot to the row.  
         [0021]     There is further provided in accordance with an embodiment of the present invention a multislice CT scanner for imaging a patient comprising: an X-ray source that generates a cone beam of X-rays radiated from a focal spot of the X-ray source wherein the X-ray source is moveable in a rotation plane so as to rotate the focal spot about an axial direction along which the patient is moved to position the patient in a field of view of the scanner; and a detector array comprising a plurality of rows of X-ray detectors that generate signals responsive to X-rays that are used to generate an image of the patient; wherein the detector rows are substantially parallel to the rotation plane and a width of each row in at least a portion of the rows is a function of a declination angle relative to the rotation plane of a line from the focal spot to the row.  
         [0022]     Additionally or alternatively, the declination angle of a row optionally increases in a direction from the first side to the second side and the width of each row in the at least portion of the rows increases as its declination angle increases.  
         [0023]     Optionally, the focal spot is tilted with respect to the rotation plane at an angle β and the width of each row in the at least portion of the rows is substantially proportional to 1/sin(β+φ) where φ is the declination angle of the row.  
         [0024]     Optionally, the width of each row in the at least portion of the rows is substantially equal to 1/Lsin(β+φ) where L is a dimension of the focal spot along the direction along which the focal spot is tilted. 
     
    
     BRIEF DESCRIPTION OF FIGURES  
       [0025]     Non-limiting examples of embodiments of the present invention are described below with reference to figures attached hereto and listed below. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale.  
         [0026]      FIG. 1  schematically shows a perspective view of a multislice CT scanner comprising an X-ray cone beam and detector array, in accordance with prior art;  
         [0027]      FIG. 2  schematically shows a cross section view of the multislice CT shown in  FIG. 1 ;  
         [0028]      FIG. 3  shows a schematic graph illustrating decrease in intensity and hardening of X-rays in a cone beam resulting from the heel effect;  
         [0029]      FIGS. 4A and 4B  schematically show perspective and cross section views respectively of a multislice CT scanner comprising an X-ray cone beam and detector array in accordance with an embodiment of the present invention; and  
         [0030]      FIG. 5  schematically shows a CT scanner comprising a detector array having rows of detectors for which the widths of at least some of the rows increase as declination angles at which they are located increase, in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0031]      FIG. 1  schematically shows a perspective view of a third generation multislice CT scanner  20  comprising an X-ray detector array  22  and an X-ray source  24  in accordance with prior art. Only features of multislice scanner  20  germane to the present discussion are shown in  FIG. 1 .  
         [0032]     X-ray detector array  22  comprises a plurality of rows  26  of X-ray detectors  28 . By way of example, in  FIG. 1  detector array  22  is shown comprising four detector rows  28 . X-ray source  24  comprises an anode  30 , formed with or mounted on a shaft  49 , and a cathode  32 . The cathode provides a beam of electrons represented by a block arrow  34 , which is focussed to a “focal spot”  36  on a surface  39  of the anode. Bremmstrahlung and fluorescent X-rays generated by electrons in the beam that are incident on anode  30  are radiated from material in a neighborhood of focal spot  36  in an X-ray cone beam outlined by lines  38 . The cone beam will hereinafter be referred to by the numeral  38  labeling the lines that outline the cone beam. X-rays in cone beam  38  illuminate X-ray detector array  22 . At least one collimator (not shown) comprised in X-ray source  24  collimates X-rays radiated from focal spot  36  so that cone beam  38  illuminates substantially all of and only detector array  22 .  
         [0033]     X-ray source  24  and detector array  22  are mounted to a rotor (not shown) comprised in the scanner. The rotor is rotatable around a z-axis of a coordinate system  40  so as to position X-ray source  24  and detector array  22  at different cone beam view angles about the z-axis. Motion of focal spot  36  during rotation of the rotor defines a rotation plane indicated by a dashed circle  42  of CT scanner  20 . Dashed lines  43  indicate a cross section of cone beam  38  in rotation plane  42 . A vertex angle Θ of cross section  43  is a fan angle of cone beam  38 . Dashed lines  44  indicate a cross section of cone beam  38  in a plane that passes through the z-axis and is perpendicular to rotation plane  42 . A vertex angle Φ of cross section  44  is a cone angle of cone beam  38 . An intersection line  46  of rotation plane  42  and the plane of cross section  44  is an axis of cone beam  38 .  
         [0034]     Cone beam  38  is symmetric with respect to rotation plane  42 , the rotation plane bisects cone beam angle Φ and a same number of detector rows  26  lie on both sides of the rotation plane. A declination angle of an X-ray radiated from focal spot  36  is an angle that a propagation path of the X-ray makes with rotation plane  42 . Declination angles are negative on the side of rotation plane  42  facing the origin of coordinate system  40  and positive on the other side. An X-ray propagation path  48  having a negative declination angle φ is shown in  FIG. 1 .  
         [0035]     X-ray source  24  is typically operated at power levels sufficiently high so that local heating of material in anode  30  by electron beam  34  would rapidly damage the anode. To reduce anode wear, the anode is rotated about an axis  50  and electron beam  34  is displaced along a radial direction relative to axis  50  so that focal spot  36  is not always located on a same region of surface  39 . In addition, focal spot  36  is elongated along the radial direction to disperse energy deposition radially. In  FIG. 1  rotation of anode  30  is indicated by curved arrow  52 .  
         [0036]     To enable focal spot  36  to be “seen” by detector array  22  and to reduce shielding of X-rays generated in anode  30  that propagate towards detector array  22  by material in the anode surface  39  is a truncated conical surface having a cone angle α. However, cone angle α is generally made relatively large so that an effective size of focal spot  36  as seen by detector array  22  is small enough so that the extended length of the focal spot does not impair resolution along the z-axis direction of CT scanner  20 .  
         [0037]     Spatial relations between the geometry of conical surface  39  focal spot  36 , cone beam  38  and detector array  22  are conveniently seen in  FIG. 2 , which schematically shows a cross section view of CT scanner  20  in the plane of cross section  44  of cone beam  38 . As a result of the cone angle α of conical surface  39 , the region of the surface on which focal spot  36  is located is tilted with respect to rotation plane  42  and has a “slope angle” β=(90°−α) with respect to the rotation plane. If L is the radial extent of focal spot  36  on surface  39  an average effective “z-axis” size, “L z ”, of focal spot  36  parallel to the z-axis as seen by detector array  22  is substantially equal to a length of a projection of the focal spot on the detector array. L z  is substantially equal to Lsinβ. Ideally, focal spot  36  approaches a point source of X-rays. Therefore as L is increased to improve heat dispersion, slope angle β is generally decreased so that L z  remains sufficiently small to meet desired resolution specifications for CT scanner  20 . However, as the cone surface angle a increases and the slope angle β of anode  30  decreases, the heel effect angle (and as a result the cone beam angle) of cone beam  38  decreases. The heel effect angle, “φ H ”, is generally equal to about (−β+Γ) where Γ is an angle that is substantially determined by a material from which the anode is made. As a result, the cone beam angle Φ, which is generally equal to about twice the heel effect angle, is equal to about 2(β−Γ). Typically, α has a value between about 80° and about 83° , and slope angle β has a corresponding value between about 10° and 7° . For Tungsten Γ is equal to about 3.5°.  
         [0038]      FIG. 3  shows a schematic graph  60  of intensity and mean energy of X-rays provided by X-ray source  30  shown in  FIGS. 1 and 2  as a function of declination angle φ for cone surface angle α equal to about 83° and slope angle β equal to about 7°. Anode  30  is assumed to be made from Tungsten so that for the anode, Γ=3.5° and X-ray source  30  has a heel effect angle, “φ H ” equal to about −3.5°. Dependence of mean energy of X-rays on, declination angle is shown by a solid curve  62  and intensity is shown by a dashed curve  64 . Units along the ordinate of graph  60  are arbitrary.  
         [0039]     In general, intensity of X-rays provided by an X-ray source such as X-ray source  24  decrease relatively rapidly to zero with declination angle as the declination angle approaches (−β). From graph  60  for example, it is seen that for slope angle β=7°, intensity of X-rays decreases rapidly for decreasing declination angle and is substantially equal to zero at a declination angle of about −7°. Mean energy of the X-rays also increases relatively rapidly as declination angle φ decreases. For declination angles φ less than or equal to the heel effect angle φ H , X-rays provided by X-ray source  24  do not have sufficient intensity and appropriate mean energy advantageous for performing CT imaging with minimal dosage to the patient. Cone beam  38  ( FIGS. 1 and 2 ) is therefore limited to an effective minimum negative declination angle of about −3.5°. Since prior art constrains CT cone beams to be symmetric with respect to their rotation planes, cone beam  38  is limited to a maximum X-ray declination angle φ + ˜|φ H |˜3.5° and a cone beam angle equal to about 7°. The heel effect angle φ H =−3.5° and corresponding maximum positive declination angle φ + =|φ H |=3.5° which determine the cone beam angle of cone beam  38  are indicated in graph  60 .  
         [0040]     The inventors have noticed that, as shown in graph  60 , whereas intensity of X-rays provided by X-ray source  24  falls off rapidly for negative declination angles, intensity remains relatively high for a substantial range of positive declination angles. In addition, rate of decrease of mean energy of the X-rays is moderated for declination angles greater than zero. It is therefore possible to provide a CT scanner having a larger cone beam and larger detector array than in prior art scanners by relaxing the prior art symmetry constraint, providing the scanner with an asymmetric cone beam and using thereby X-rays having positive declination angles greater than |φ H |. Therefore, in accordance with an embodiment of the present invention, a CT comprises an asymmetric cone beam characterized by a maximum positive declination angle φ+ that is larger than the magnitude of the heel effect angle |φ H |.  
         [0041]     It would of course also be possible to provide a CT scanner with a larger cone beam and detector array than in prior art scanners by tilting X-ray source  24  so to increase slope angle β. Whereas this would provide the scanner with a symmetric cone beam having a larger cone angle, the increased slope angle tends to reduce resolution of images provided by the scanner. The increased slope angle may also lead to undesirable loads on shaft  49  of anode  30  and bearings (not shown) that support the anode, which typically rotates at 9000 rpm.  
         [0042]      FIGS. 4A and 4B  schematically show respectively a perspective view and a cross section view of a CT scanner  80  in which X-ray source  24  is collimated to provide a cone beam  82 , in accordance with an embodiment of the present invention. Cone beam  82  has a cross section  84  in a plane that passes through the z-axis and is perpendicular to rotation plane  42 . X-rays in cone beam  82  illuminate a matching X-ray detector array  86  comprising a plurality of rows  26  of detectors  28 . Except for cone beam  82  and detector array  86 , elements and features of CT scanner  80  are, by way of example, similar to corresponding elements of prior art scanner  20  ( FIG. 1 ).  
         [0043]     Cone beam  82  is asymmetric and optionally larger than cone beam  38  comprised in CT scanner  20 . By way of example, cone beam  82  is characterized by a maximum positive declination angle φ+ that is larger than the magnitude of the heel effect angle |φ E | of X-ray source  24 . As a result, cone beam  82  has a cone beam angle Φ=(|φ H |+φ + ), which is greater than a typical prior art cone beam angle which is equal to about 2|φ H |. In some embodiments of the present invention φ+ is greater than or equal to 1.25|φ H |. In some embodiments of the present invention φ+ is greater than or equal to 1.5|φ E |. In some embodiments of the present invention φ+ is greater than or equal to 2|φ E |. By way of example, for CT scanner  80 , φ +  is equal to 2|φ E | and while cone beam  38  has a cone beam angle Φ=2|φ H |, cone beam  82  has a substantially larger cone beam angle Φ=3|φ H |.  
         [0044]     To match asymmetric cone beam  82 , detector array  86  is also asymmetric and larger than corresponding detector array  22  comprised in CT scanner  20 . In some embodiments of the present invention detector array  86  comprises a larger number of detector rows  28  on the positive declination angle side of rotation plane  42  than on the negative declination angle side of the rotation plane. By way of example, CT scanner  80  is shown having two rows  28  of detectors on the negative side of rotation plane  42  and four detector rows  28  on the positive side of the rotation plane.  
         [0045]     In some embodiments of the invention, as in  FIGS. 4A and 4B , all detectors in detector array  86  have substantially a same width in the z-axis direction and all rows  28  in detector array  86  have a same width. In some embodiments of the invention, detectors in at least one detector row of a CT scanner have z-axis widths larger than detectors in a different detector row of the scanner and at least two of the rows of detectors have different widths. In some embodiments of the present invention, width of a scanner detector row is a function of a declination angle φ at which the detector row is located. In some embodiments of the present invention a width of each detector row in at least a portion of the detector rows in the scanner increases as a declination angle φ at which the row is located increases.  
         [0046]      FIG. 5  shows a schematic cross section of a CT scanner  100  comprising a detector array  102  having rows  104  of detectors  106  in which widths of rows in the array increase with increasing declination angle, in accordance with an embodiment of the present invention.  
         [0047]     Optionally, in accordance with an embodiment of the invention, for a given radial length L and slope angle β for focal spot  36 , the width of each row in at least a portion of rows  104  is determined responsive to L z (φ)=Lsin(β+φ), where φ is the declination angle at which the row is located. By way of example, a declination angle φ, is shown for a row  104  in  FIG. 5 . The declination angle is an angle between axis  46  and a line  107  shown intercepting a detector  106  the row. L z (φ) is a z-axis length of a projection of focal spot  36  on row  104  at declination angle φ and thereby an effective z-axis length of the focal spot  36  as seen by the row. In some embodiments of the invention, the widths of rows in the at least portion of rows  104  are substantially proportional to L z (φ). In some embodiments of the invention, the widths of the rows in the at least portion of rows  104  are substantially equal to L z (φ).  
         [0048]     It is noted that z-axis resolution provided by a given row  104  of detectors  106  is a function of row width and generally improves as a width of the detector row decreases. However, while improvement is relatively rapid with decreasing width for widths greater than or equal to about L z (φ) improvement is slower with decreasing width for widths less than L z (φ). As a result, while there is incentive to reduce detector row width, for a detector row at a given declination angle φ having a corresponding L z (φ) the incentive to reduce the row width below L z (φ) is generally marginal.  
         [0049]     Therefore, a detector array for a cone beam having a given cone beam angle can generally be produced with a smaller number of X-ray detectors without substantially compromising spatial resolution of the array if row widths are substantially proportional or equal to L z (φ). The smaller number of X-ray detectors generally results in a lower production cost for the array. Cost advantages of producing X-ray detector arrays for which the row widths are proportional or equal to or L z (φ) can be advantageous in particular for relatively large detector arrays having rows located at relatively large declination angles.  
         [0050]     It is noted that whereas in the above discussion of detector row width in accordance with the present invention, detector array  102  is an asymmetric detector array, the discussion applies equally well to symmetric detector arrays.  
         [0051]     In the description and claims of the present application, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements or parts of the subject or subjects of the verb.  
         [0052]     The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons of the art. The scope of the invention is limited only by the following claims.