SHADOWGRAPH IMAGING

Abstract: A method and apparatus for obtaining shadowgraph images suitable for use in computed tomography scanner apparatus for subject localization. A source (20) of radiation that orbits the subject during a normal computed tomography scan is fixed relative an array of radiation detectors (28). The subject is then moved in a direction generally perpendicular to the plane of source (20) and array (28) to obtain a first shadowgraph data set. The source (20) is then orbited a small amount and the subject is again moved relative the source (20) and detector array (28) to obtain a second shadow­ graph set of data. The two sets of data are then inter­ leaved to obtain a shadowgraph image having higher resolution than either the first or second shadowgraph.

Description: This invention relates to methods and apparatus for use in obtaining shadowgraph images and more especially to such methods and apparatus for use in locating a tomographic plane to be scanned using a computed tomography (CT) apparatus. In computed tomography (CT), a subject is irradiated from a number of directions and the intensity of radiation passing through the subject is detected. This procedure provides information sufficient to generate a cross-­ sectional mapping of attenuation within the subject. The procedure for generating this mapping is known within the computed tomography art as filtered back projection. Various methods of reconstruction imaging have been developed as the computer tomography art has matured. In most all, if not all, commercial CT scanners presently known to applicants, x-ray attenuation data are fourier transformed, filtered, and then retransformed prior to back projection. Computed tomography scanners have evolved in stages. An initial scanner used a single x-ray source and single detector which traversed back and forth in unison obtaining intensity information for use in the back projection process. Second and third generation scanners employed detector arrays rather than individual detectors. In second generation scanners these arrays traversed and orbited while in third generation they simply orbit with the x-ray source. A fourth generation scanner employs a circular array of detectors circumscribing a subject region of interest. The x-ray source orbits with respect to this circular detector array to irradiate the subject from a plurality of positions. One example of a fourth generation medical computed tomography scanner is marketed under the designation Synerview 1200 by Picker International, Inc. of Cleveland, Ohio. In this scanner, the detector array includes 1200 closely spaced detectors forming a circle whose diameter is greater than the diameter of the circular path followed by the x-ray tube as it orbits the patient. Stated another way, x-radiation from the tube travels a shorter distance between tube and patient than between the patient and detector array after traversing the patient. One requirement of computed tomography scanners is a procedure for accurately positioning the subject prior to conducting a computed tomography scan. For example, if a diagnosing physician is interested in viewing the internal structure of a particular organ, it is inefficient and undesirable to scan the entire body. Instead, only a region of interest including the organ or other body part of concern is scanned. To accomplish this positioning the x-ray source can be locked in a stationary position relative to the detector array and the subject moved past the array as the source irradiates the subject. X-ray intensity data from a group of detectors is sensed sequentially at each of a series of data collection positions. At each data collection position a new row of intensity information is gathered. After the subject has moved past the detector array an image similar to a conventional digital x-ray shadowgraph image is created. Such an image does not represent a cross-sectional image but instead, depicts the transmissivity of the subject along paths from the fixed x-ray source to individual elements of the detector array. While not providing the resolution and/or information of a computed tomography scan, such a localization scan can be used to locate structure within the subject in anticipation of positioning that subject for a complete CT scan. It is an object of the present invention to provide an improved method and apparatus suitable for obtaining such shadowgraph images for localization purposes prior to a complete CT scan. Accordingly the invention provides a method of obtaining shadowgraph image data relating to a subject wherein: radiation from a radiation source is directed through the subject to an array of radiation detectors; and data is obtained representing the radiation reaching said array through said subject as said subject is moved relative to said source and said array; characterised in that data is obtained during two corresponding movements of said subject relative to said source and said array; and said data for the two movements are combined to produce said image data. In one particular method according to the invention the said radiation is directed along trajectories through said subject during one said movement which are different from the trajectories during the other said movment; and said data for the two movements are combined by interleaving so as to provide image data of greater resolution than that given by the data for either one movement alone. In another particular method according to the invention said data for the two movements are subtracted one from the other to obtain contrast image data. The invention also provides an apparatus for producing a shadowgraph image of a subject comprising: a radiation source; an array of radiation detectors; means positioning said source and array so as to direct radiation through said subject onto said array; means for moving said subject relative to said source and said array; means for producing signals representing the radiation reaching said array from the source through said subject as said subject is moved relative to said source and said array; and means for utilising said signals to construct said image characterised in that said means for moving comprises means for carrying out two corresponding movements of said subject relative to said source and said array; and said means for utilising combines the said signals produced during both said movements. In one particular apparatus in accordance with the invention said means for positioning includes means for moving said source relative to said subject and said array between said movements so that the said radiation is directed along trajectories through said subject during one said movement which are different from the trajectories during the other said movement; and said means for utilising combines the signals produced during said two movements by interleaving so as to construct an image of greater resolution than that given by the signals produced during either one said movement alone. In another particular apparatus in accordance with the invention said means for utilising comprises means for subtracting the said signals produced during one said movement from the said signals produced during the other said movement so as to construct a contrast image of said subject. It will be appreciated that in a method and apparatus according to the invention said source and detector array may be the source and detector array of a CT scanner apparatus. One method and apparatus in accordance with the invention will now be described by way of example with reference to the accompanying drawing in which:- Figure 1 is a perspective view of a medical computed tomography scanner having a motorized couch for moving a patient relative to a scanner x-ray tube and stationary detector array; Figure 2 is a side elevation view of the patient couch having a motor for moving the patient longitudinally into and out of a scanner patient aperture; Figure 3 is an end elevation view of the Figure 2 couch; Figure 3A is an enlarged partially sectioned, view of the couch top and its support; Figure 4 is a top plan view of the couch frame showing a drive for moving a patient support into and out of a scanner aperture; Figure 5 is a side elevation view, partly in section, of the Figure 4 drive; Figure 6 is a schematic showing a relationship between the scanner x-ray tube and a portion of the scanner detector array to illustrate an interleaving of shadow­ graph image data; and Figure 7 is a mapping of interleaved intensity data showing increased image resolution through practice of the invention; Turning now to the drawings, and in particular Figure 1, a computed tomography scanner 10 and a patient couch 12 are illustrated. The scanner 10 is illustrated with a front panel 14 pivoted away from its in use position to show the internal components of the scanner 10. The scanner 10 includes a rotatable x-ray source 20 mounted on a rotatable frame 22. The frame 22 is journalled for rotation within a gantry 24. Coupled to and supported by the gantry 24 are a plurality of x-radiation detectors forming a detector array 28. During operation radiation from the source 20 passes across a patient aperture 30 in a plane 32 to the detector array 28. During computed tomography scanning, the x-ray source 20 is rotated by a motor (not shown) in a circular orbit around a patient aperture 30. A generally planar and spread beam of radiation from the source 20 impinges upon a group of detectors forming a part of the detector array 28. As the source rotates, different detectors are irradiated and intensity information obtained from these detectors. Output signals from the detector array 28 are processed by components that convert the radiation into visible light, to an analog electric signal, to a frequency, and then to digital attenuation values. A computer 40 stores this data derived from these values and uses it for reconstruction processing. As seen most clearly in Figures 2 and 3, the patient couch 12 includes a base 52 supporting a movable frame 54 which supports a top 56. The top 56 and frame 54 are bounded on either side by retractable handles 58 and an arm rest 60. To position a patient in a patient examination position in the patient aperture for computed tomog­ graphy scanning, a frame lift motor (not shown) raises the frame 54, top 56 and handles 58 from the position shown in solid in Figure 2 to the position shown in phantom. In this raised position, the patient can be moved into and out of the patient aperture 30. A longitudinal drive mechanism for the couch frame 54 and top 56 is seen in Figure 5. One of two pedestal supports 62, is shown supporting the frame 54. This support 62 is coupled to a second pedestal (not shown) by a shaft and rack and pinion drive so that the two pedestal supports move in unison as the frame is raised and lowered. A motor driven screw jack 74 is coupled to a mounting block 75 which engages a bottom surface 54b of the frame 54. Longitudinal movement of the top 56 with respect to the couch frame 54 is provided by a D.C. motor 50 mounted within the frame 54. A drive pulley 82 mounted to a motor drive shaft is rotated in either of two dir­ ections. A pulley 84 (Figure 4) mounted to a shaft 85 journaled in the frame 54 is connected to the drive pulley 82 by a belt 87 and rotates in response to ener­ gization of the motor 50. A gear 86 mounted on the shaft 85 engages an elongated gear belt 88 coupled to the movable top 56 so that rotation of the gear 86 exerts a force on the moveable top 56. Controlled energization of the motor 50 drives the gear belt and attached top 56 into and out of the aperture 30. The top 56 and frame 54 coact along two bearing rails 90 (only one of which is seen in Figure 3A). Each rail is mounted on a shaft support block 92 (one of which is shown in Figure 3A). The rails 90 each engage bearings 93 running the length of the moveable top 56. Directly beneath the rail 90 (Figure 3A) are side rails 94 which support the arm rests 60 (Figure 3). As has been indicated the patient couch 12 is motor­ ized so that a patient placed on the couch can be moved into the scanner aperture 30. The patient is moved into the aperture 30 so that radiation from the tube 20 passes through the patient cross-section 32 (Figure 2). In preparation for cross-sectional scanning, a shadowgraph patient image is obtained. The x-ray tube 20 is fixed with respect to the gantry 24 and a beam of x-radiation is directed through the patient to a segment of adjacent detectors of the detector array 28. The x-­ ray tube 20 remains energized as the motor 50 continuously moves the patient axially of the aperutre 30 and through the beam of radiation. X-radiation data from the energized x-ray detector segment is sensed and stored for the time period in which the patient is driven with respect to the scanner through a first longitudinal scan. During this first scan, movement of the top 56 is co-ordinated with radiation attentuation sensing so the data stored in the computer is organized in a rectangular array of pixels of attenuation data. In one embodiment of the invention for use with a 600 detector scanner, the radia­ tion output from 128 adjacent detectors is sampled every millimeter at 128 different patient/scanner positons to obtain a 128 by 128 pixel shadowgraph data set. After the first longitudinal scan is conducted, the source 20 is orbitally indexed a small amount to provide a second shadowgraph data set. In a 600 detec­ tor scanner, where the spacing between detectors is 0.6 degrees, the source is moved .3 degrees from its original position. The motor 50 is then energized to reverse the longitudinal scanning direction. The top 56 then moves in an opposite direction as a second set of shadow­ graph data is obtained. The pixel data for each of the two shadowgraph images is then interleaved by the computer 40 to obtain a resultant shadowgraph of 256 by 128 pixels having twice the data set resolution of either individual longitudinal scan. A grid like mapping (Figure 7) of data from the 128 detectors is created. Along the longitudinal direc­ tion the center-to-center spacing between adjacent data regions is 1 mm. Without multiple scans the detector spacing is approximately 4 mm but with the interleaving of data from two scans with the source orbited between scans the separation between data regions is about 2 mm. Turning now to Figure 6, a geometric examination of shadowgraph data interleaving is presented. In Figure 6, two source positions S1 and S2 are seen in relation to a detector array having representative detectors D n , D n+1 , and D n-1 . The angle γ represents the angle by which the scanning frame is rotated between longitudinal scans. Depicted in a central scanning plane are two points X₁, X₂. The following two equations represent the distance of these points X₁, X₂ from the origin O as a function of the Figure 6 parameters. These equations are derived from the law of sines and co-sines for the geometry of the Figure 6 representation. A tabulation of these values for X₁, X₂ for representa­ tion detectors of a 600 detector scanner is summarized in Table I. That table shows the distance along the central plane from the origin to the locations X₁ and X₂ for various adjacent detectors for two source posi­ tions .3° degrees apart. By way of example, the designa­ tion X₂ (0,3) means the distance from X₂ to the origin in the central plane for the ray passing from the source at position S₂ to the third detector in the detector array. To obtain contrast data image information instead of enhanced resolution in a patient localization scan, the source position is left unchanged and two consecutive scans are conducted wherein the difference between shadow­ graph data is caused by radio-opaque flow within the subject. In this application, contrast image information is obtained using digital subtraction techniques from digital radiography. It will be appreciated that when obtaining contrast information, a radiation contrast agent is normally injected into the patient after the first and before the second of the scans. Digital subtraction angiography techniques are then utilized to image contrast agent flow within the patient. This contrast study approximates the image produced by a digital x-ray unit with perhaps less resolution. While continual use of a CT scanner for such digital subtraction or other digital studies would be inefficient, an occasional such use in a hospital with no digital x-ray unit available for such a study is contemplated. Several passes over a patient can be conducted to produce a sequence of images. If one image used as a subtraction mask, a series of images showing contrast changes results. The ability to use a multiple pass patient imaging system may require registration of the images which are to be combined. Patient movement may require interactive real time registration to minimize motion artifacts in both contrast studies and interleaved scans. An article entitled "A Digital Video Image Processor for Real-Time X-ray Subtraction Imaging" to Kruger et al, Optical Engineering Vol. 17 No. 6, Nov-Dec 1978 discusses both digital subtraction and real time imaging and is incorporated herein by reference. While the invention has been described with a degree of particularity, it is the intent that the invention include all modifications and/or alterations falling within the spirit or scope of the appended claims.

Claims: 1. A method of obtaining shadowgraph image data relating to a subject wherein: radiaton from a radiaton source (20) is directed through the subject to an array of radiation detectors (28); and data is obtained representing the radiation reaching said array (28) through said subject as said subject is moved relative to said source (20) and said array (28); characterised in that data is obtained during two corresponding movements of said subject relative to said source (20) and said array (25); and said data for the two movements are combined to produce said image data. 2. A method according to Claim 1 wherein said two movements are movements in opposite directions along the same path. 3. A method according to Claim 1 or Claim 2 wherein the said radiation is directed along trajectories through said subject during one said movement which are different from the trajectories during the other said movement; and said data for the two movements are combined by interleaving so as to provide image data of greater resolution than that given by the data for either one movement alone. 4. A method according to Claim 3 wherein said different trajectories are obtained by positioning said source (20) at a different angular position (S1, S2) about the direction of said movements during each of said two movements. 5. A method according to Claim 4 wherein said array (28) is a circular array and said different angular positions (S1, S2) lie on a circular path having the same centre as said array (28). 6. A method according to Claim 5 wherein the angular position (S1, S2) of said source (20) during said two movements differ by an angle (γ) approximately one half of the angular spacing between adjacent detectors in said array. 7. A method according to Claim 1 wherein said data for the two movements are subtracted one from the other to obtain contrast image data. 8. A method according to any one of the preceding claims wherein said source (20) and said array (28) are positioned around an aperture (30) and said movements are in a direction axial of said aperture (30). 9. A method according to any one of the preceding claims wherein during each said movement data is obtained for each of a number of discrete positions of said subject relative to said source (20) and said array (28). 10. A method according to any one of the preceding claims wherein said movements are effected by moving said subject whilst said array (28) and said source (20) remain fixed in position. 11. A method according to any one of the preceding claims wherein said source (20) is an X-ray source. 12. A method according to any one of the preceding claims wherein said method is carried out using a computed tomography scanner apparatus (10), said source (20) and said array (28) being a source and array used to produce computed tomography scans in use of said apparatus (10). 13. An apparatus for producing a shadowgraph image of a subject comprising: a radiation source (20); an array of radiation detectors (28); means (22, 24) positioning said source (20) and array (28) so as to direct radiation through said subject onto said array (28); means (12, 24, 50 to 62, 82 to 94) for moving said subject relative to said source (20) and said array (28); means (40) for producing signals representing the radiation reaching said array (28) from the source (20) through said subject as said subject is moved relative to said source (20) and said array (28); and means (40) for utilising said signals to construct said image; characterised in that said means (12, 24, 50 to 62, 82 to 94) for moving comprises means for carrying out two corresponding movements of said subject relative to said source (20) and said array (28); and said means (40) for utilising combines the said signals produced during both said movements. 14. An apparatus according to Claim 13 wherein said means (22,24) for positioning includes means (22) for moving said source (20) relative to said subject and said array (28) between said movements so that the said radiation is directed along trajectories through said subject during one said movement which are different from the trajectories during the other said movement; and said means (40) for utilising combines the signals produced during said two movements by interleaving so as to construct an image of greater resolution than that given by the signals produced during either one said movement alone. 15. An apparatus according to Claim 14 wherein said means (22) for moving said source (20) comprises means (22) for moving said source (20) to different positions (S1, S2) along a circular path around the direction of said movements. 16. An apparatus according to Claim 15 wherein said array (28) is a circular array having the same centre as said circular path. 17. An apparatus according to Claim 13 wherein said means (40) for utilising comprises means (40) for subtracting the said signals produced during one said movement from the said signals produced during the other said movement so as to construct a contrast image of said subject. 18. An apparatus according to any one of Claims 13 to 17 wherein said source (20) and said array (28) are positioned around a circular aperture (30) and said movements are in a direction axial of said aperture (30). 19. An apparatus according to any one of Claims 13 to 18 wherein said source (20) and said array (28) are a radiation source and detector array of a computed tomography scanner apparatus (10).

Inventors: DEMEESTER, GORDON D., MATTSON, RODNEY A.
Applicants: PICKER INTERNATIONAL, INC.
Application year: 1986
Main classification: A61B6
IPC codes: A61B6, G01N23