Patent Number: 050900374
Section: claims

1. A method of acquiring tomographic projection data of an imaged object comprising: supporting and translating the imaged object concurrently along a translation axis;  projecting a beam of x-rays from an x-ray generator through the imaged object and alternately sweeping the beam along the translation axis in a first direction with translation of the imaged object during a first period, and in a second direction along the translation axis but counter to the translation of the imaged object during a second period;  receiving the beam from the x-ray generator with an x-ray detector array after it passes through the imaged object; and  holding the x-ray generator and x-ray detector in opposition around the imaged body and concurrently rotating the same around a center of rotation and the imaged object, in an gantry plane substantially perpendicular to the translation axis.  a fixed x-ray source; and  a collimator having an aperture movable along the translation axis.  C.sub.z is the distance along the translation axis between the center of the collimator and the gantry plane;  l.sub.1 is the distance between the x-ray source and the collimator; and  l.sub.2 is the distance between the collimator and the translation axis.  V.sub.z is the distance along the translation axis between the predetermined volume element and the gantry plane;  C.sub.z is the distance along the translation axis between the center of the collimator and the gantry plane;  .alpha. is the angle, with respect to the center of rotation, between the volume of interest and gantry angle .theta.=O;  .DELTA. is the distance between the volume of interest and the center of gantry rotation;  l.sub.1 is the distance between the x-ray source and the collimator; and  l.sub.2 is the distance between the collimator and the translation axis.  F.sub.z is the distance along the translation axis between x-ray source and the gantry plane;  C.sub.z is the distance along the translation axis between the center of the collimator and the gantry plane;  l.sub.1 is the distance between the x-ray source and the collimator;  l.sub.2 is the distance between the collimator and the translation axis; and  l.sub.3 is the distance between the translation axis and the x-ray detector.  and where .theta. is the gantry angle;  .alpha. is the angle, with respect to the center of rotation, between the volume of interest and gantry angle .theta.=O;  .DELTA. is the distance between the volume of interest and the center of gantry rotation;  V.sub.z is the distance along the translation axis between the predetermined volume element and the gantry plane;  F.sub.z is the distance along the translation axis between x-ray source and the gantry plane;  C.sub.z is the distance along the translation axis between the center of the collimator and the gantry plane;  l.sub.1 is the distance between the x-ray source and the collimator;  l.sub.2 is the distance between the collimator and the translation axis; and  l.sub.3 is the distance between the translation axis and the x-ray detector.  where V.sub.z is the distance along the translation axis between the predetermined volume element and the gantry plane;  F.sub.z is the distance along the translation axis between x-ray source and the gantry plane; and  C.sub.z is the distance along the translation axis between the center of the collimator and the gantry plane.  V.sub.z is the distance along the translation axis between the predetermined volume element and the gantry plane;  l.sub.1 is the distance between the x-ray source and the collimator; and  l.sub.2 is the distance between the collimator and the translation axis.  and where .theta. is the gantry angle;  .alpha. is the angle, with respect to the center of rotation, between the volume of interest and gantry angle .theta.=O;  .DELTA. is the distance between the volume of interest and the center of gantry rotation;  F.sub.z is the distance along the translation axis between x-ray source and the gantry plane;  V.sub.z is the distance along the translation axis between the predetermined volume element and the gantry plane;  l.sub.1 is the distance between the x-ray source and the collimator; and  l.sub.2 is the distance between the collimator and the translation axis. 2. The method as recited in claim 1 wherein the x-ray exposure of the imaged object by the x-ray beam during the first period is greater than the x-ray exposure of the imaged object by the x-ray beam during the second period. 3. The method as recited in claim 2 wherein the intensity of the x-ray beam is reduced during the second period. 4. The method as recited in claim 1 wherein the sweeping of the x-ray beam during the first period is such as to maintain the beam centered on a predetermined volume element on the translation axis. 5. The method recited in claim 4 wherein the x-ray generator comprises: 6. The method recited in claim 5 wherein the aperture of the collimator is controlled according to the following equation: ##EQU5## where V.sub.z is the distance along the translation axis between the predetermined volume element and the gantry plane; 7. The method recited in claim 5 wherein the aperture of the collimator is controlled according to the following equation gantry angle .theta. as follows: ##EQU6## where EQU l.sub.2 '=l.sub. -cos(.theta.+.alpha.) (.DELTA.) 8. The method recited in claim 4 wherein the x-ray generator comprises an x-ray source having a focal point movable along the translation axis and collimator having a aperture movable along the translation axis. 9. The method recited in claim 8 wherein the focal point and aperture of the collimator are controlled according to the following equations: ##EQU7## where V.sub.z is the distance along the translation axis between the predetermined volume element and the gantry plane; 10. The method recited in claim 8 wherein the focal point and the aperture of the collimator are controlled according to the following equations: ##EQU8## where EQU l.sub.2 '=l.sub.2 -cos(74 +.alpha.) (.DELTA.) EQU l.sub.3 '=l.sub.3 +cos(.theta.+.alpha.) (.DELTA.) 11. The method recited in claim 8 wherein the focal point and the aperture of the collimator are controlled according to the following equation: EQU F.sub.z =C.sub.z =V.sub.z 12. The method recited in claim 4 wherein the x-ray generator comprises an x-ray source having a focal point movable along translation axis and a fixed collimator aperture. 13. The method recited in claim 12 wherein the focal point is controlled according to the following equation: ##EQU9## where F.sub.z is the distance along the translation axis between x-ray source and the gantry plane; 14. The method recited in claim 10 wherein the focal point is controlled according to the following equation: ##EQU10## where EQU l.sub.2 '=l.sub.2 -cos(.theta.+.alpha.) (.DELTA.)