Tomographic method of x-ray imaging

Abstract

A method is disclosed for obtaining the density distributions of three-dimensional elements that compose objects or groups of objects, by examining the objects with beams of x-rays or gamma radiation that are transmitted through the object in a plurality of approximately parallel paths and measuring the intensity of the radiation, scattered approximately perpendicular to the parallel paths, in arrays of detectors around the object. The energy of the x-rays or gamma rays is such that dominant interaction in the object is Compton scattering. The density of each element is determined from the totality of measurements by standard mathematical tomographic or relaxation techniques of data manipulation.

We claim:  
     
       1. A method for determining a three-dimensional density,  distribution among volume elements in a volume containing at least one material, the method comprising: 
       a. producing a beam of energetic photons for penetrating the volume;  
       b. scanning the beam sequentially across incremental positions of the volume in a plurality,  of paths having substantially parallel directions separated by incremental steps;  
       c. detecting scattered photons of substantially all energies scattered by the material in said volume with a detector having a spatial resolution in a direction substantially parallel to the paths of the beam;  
       d. measuring the intensity of scattered photons scattered approximately perpendicular to each substantially parallel direction of the beam at each incremental position of the beam to derive an independent measurement of intensity,  of scattered photons scattered from the volume elements in the volume;  
       e. identifying a volume element as the approximate origin of scattering along the path of the beam giving rise to the measurement of intensity of scattered photons; and  
       f. calculating an independent density for each volume element of the material in the volume.  
     
     
       2. A method according to  claim 1 , in which the size of each volume element in the volume is determined by the diameter of the beam that traverses said volume element, the step size of the incremental steps of said beam, and the spatial resolution of the detector substantially parallel to the beam path. 
     
     
       3. A method according to  claim 1 , in which the number of independent measurements of the intensity of scattered photons scattered from the volume elements in said volume is at least approximately equal to the number of volume elements. 
     
     
       4. A method according to  claim 1 , in which the intensity,  of each beam transmitted through the volume is measured together with the intensity of said scattered photons scattered by the material. 
     
     
       5. A method according to  claim 1 , in which the energies of the energetic photons in each beam are in an energy range where the interaction of the energetic photons with the material is dominated by the probability for Compton scattering. 
     
     
       6. A method according to  claim 1 , in which the step of calculating the density of each volume element of the material in the volume includes a mathematical relaxation procedure in which a first trial function in the mathematical relaxation procedure uses the independent measurements of the intensity,  of scattered photons scattered from the volume elements in the volume. 
     
     
       7. A method according to  claim 1 , in which the density distribution among the volume elements in the volume is determined by a computerized tomographic reconstruction procedure having a number of independent measurements at least equal to the number of independent densities calculated in the step of calculating. 
     
     
       8. A method according to  claim 1 , in which the step of scanning the beam sequentially across incremental position of the volume includes scanning a first beam and a second beam, the first beam having a first mean energy high enough so that the Compton interaction dominates the interaction of the first beam with the material, and the second beam having a mean energy low enough so that the photoelectric interaction makes a significant contribution to the interaction of the second beam with at least some material in the volume, such that the intensity of scattered radiation from the first and second beams are used to determine the effective atomic number of the at least one material in the volume by a mathematical reconstruction technique. 
     
     
       9. A method according to  claim 8 , wherein the mathematical reconstruction technique includes relaxation methods. 
     
     
       10. A method according to  claim 8 , wherein the mathematical reconstruction technique includes methods of computerized tomographic reconstruction. 
     
     
       11. A device for determining densities in volume elements in a material present in an assembly of objects, the device comprising: 
       a. a source for producing a beam of energetic photons having a direction for penetrating the material;  
       b. an arrangement for scanning the beam of energetic photons in a sequence of sequential beams across the assembly in a manner such that successive directions of the sequential beams are substantially parallel to each other, the sequential beams passing through every volume of the assembly;  
       c. a detector disposed substantially parallel to the direction of the beam of energetic photons for providing measurements of the intensity of scattered photons of substantially all energies scattered approximately perpendicular to the direction of the beam at each incremental position of the beam; and  
       d. a computer for determining the densities in the volume elements of the material in the assembly from a totality of measurements of the intensity of scattered photons scattered approximately perpendicular to the direction of the beam at each incremental position of the beam by a mathematical reconstruction technique.  
     
     
       12. A device according to  claim 11 , further comprising a transmission detector for detecting energetic photons transmitted through the assembly of objects. 
     
     
       13. A device according to  claim 11 , wherein the beam of energetic photons comprises energetic photons in the range where the interaction of the energetic photons with the material in the assembly is dominated by the probability for Compton scattering. 
     
     
       14. A device according to  claim 11 , wherein the detector is segmented in a direction having a component substantially parallel to the beam of energetic photons. 
     
     
       15. A device according to  claim 11 , wherein the detector is collimated with respect to directions substantially perpendicular to the beam of energetic photons. 
     
     
       16. A device according to  claim 11 , wherein the mathematical reconstruction technique includes relaxation methods. 
     
     
       17. A device according to  claim 11 , wherein the mathematical reconstruction technique includes methods of computerized tomographic reconstruction. 
     
     
       18. A tomography system for analyzing a material concealed within an enveloping surface, the system comprising: 
       
         A. at least one source of penetrating radiation for emitting a beam along a beam axis having an orientation disposed with respect to the enveloping surface;  
       
       
         B. a scanner for varying the orientation of the beam axis with respect to the material in incremental steps; and  
       
       
         C. at least one array of segmented detectors disposed along a detector axis disposed substantially parallel to the beam axis for detecting scattered radiation of substantially all energies of scattered radiation and producing signals corresponding at least to the scattered radiation. 
       
     
     
       19. The tomography system in accordance with  claim 18 , additionally comprising at least one transmission detector disposed along the beam axis for measuring penetrating radiation transmitted through the material. 
     
     
       20. The tomography system in accordance with  claim 18 , additionally comprising a conveyor for transporting the material. 
     
     
       21. The tomography system in accordance with  claim 18 , wherein the scanner arrangement includes a scanner for raster- scanning the beam axis in a plane transverse to the beam axis.   
     
     
       22. The tomography system in accordance with  claim 18 , further comprising a plurality of collimators disposed in directions substantially perpendicular to the beam axis for limiting the field of view of each segmented detector. 
     
     
       23. A method for analyzing material concealed within an enveloping surface, the method comprising: 
       
         a. illuminating the enveloping surface with penetrating radiation propagating substantially along a beam axis, the penetrating radiation characterized by a first incident energy;  
       
       
         b. measuring a profile of penetrating radiation characterized by a first incident energy that is scattered by the concealed material;  
       
       
         c. illuminating the enveloping surface with penetrating radiation propagating substantially along the beam axis, the penetrating radiation characterized by a second incident energy;  
       
       
         d. measuring a profile of penetrating radiation characterized by a first incident energy that is scattered by the concealed material; and  
       
       
         e. determining at least one of the density and atomic number associated with each of a plurality of voxels based on the profiles of penetrating radiation characterized by the first and second incident energies and scattered by the concealed material. 
       
     
     
       24. A method for analyzing a material concealed within an enveloping surface, the method comprising: 
       
         A. producing a beam of energetic photons for penetrating the volume;  
       
       
         B. scanning the beam across the material incrementally in a plurality of beam directions;  
       
       
         C. detecting scattered photons of substantially all energies scattered by the material with a detector array having a spatial resolution in a direction substantially parallel to the beam directions;  
       
       
         D. measuring the intensity of scattered photons scattered approximately perpendicular to each direction of the beam; and  
       
       
         E. characterizing the material based on the intensity of scattered photons scattered approximately perpendicular to each incremental direction of the beam. 
       
     
     
       25. The method according to  claim 24 , further including measuring penetrating radiation transmitted through the material. 
     
     
       26. The method according to  claim 24 , further including deriving an attenuation per unit length characteristic of the material. 
     
     
       27. The method according to  claim 24 , further comprising the step of raster- scanning the beam axis in a plane transverse to the beam axis.   
     
     
       28. The method according to  claim 24 , further comprising the step of conveying the enveloping surface in a direction perpendicular to the beam axis. 
     
     
       29. An apparatus for generating sequential beams of penetrating electromagnetic radiation comprising: 
       
         a. a source for producing a beam of charged particles;  
       
       
         b. a target having a surface which receives the beam of charged particles and emits electromagnetic waves in response thereto;  
       
       
         c. a beam director that directs the beam of charged particles to the plurality of specified locations on the target; and  
       
       
         d. a collimator, the collimator having an array of transmitting regions and being disposed proximal to the target such that electromagnetic waves emitted from the target pass through the collimator and emerge from the array of transmitting regions in a series of parallel beams as the beam of charged particles is directed at a plurality of specified locations on the target. 
       
     
     
       30. An apparatus according to  claim 29 , wherein the collimator includes an array of apertures. 
     
     
       31. An apparatus according to  claim 29 , wherein the collimator includes a plurality of apertures in an absorbing matrix. 
     
     
       32. An apparatus according to  claim 31 , wherein the absorbing matrix is lead. 
     
     
       33. An apparatus according to  claim 29 , wherein the penetrating electromagnetic radiation includes x- rays.   
     
     
       34. The apparatus according  claim 29 , wherein the source for producing a beam of charged particles includes an cathode ray tube. 
     
     
       35. A method for generating sequential beams of penetrating electromagnetic radiation comprising: 
       
         a. providing a source for producing charged particles;  
       
       
         b. directing the particles to a plurality of specified locations on a target having a surface to produce radiation; and  
       
       
         c. collimating the radiation. 
       
     
     
       36. A method according to  claim 35 , wherein the step of collimating the radiation includes directing the radiation through apertures in an absorbing matrix.