Helical scanning computed tomography apparatus

#This invention relates to computed tomography ("CT") systems where the patient is moved continuously during the scanning and more specifically to a CT system employing multiple rows of detector elements displaced along the axis of patient movement.

#Referring to FIG. 1, in a prior art x-ray computed tomography system, an x-ray source 10 is collimated to form a generally planar fan beam 12, along a beam axis 13. The fan beam is orientated to lie within the x-y plane of a Cartesian coordinate system, termed the "imaging plane", and diverges about the beam axis 13 within the imaging plane with a defined fan beam angle .gamma..

#The fan beam 12 projects through an imaged object 14 to an x-ray detector array 16 also lying within the imaging plane. The detector array 16 is comprised of a large number of adjacent detector elements 18' which each measure the intensity of transmitted radiation along a different ray extending between the x-ray source 10 and that particular detector element 18. The detector elements 18 can be organized along an arc each to intercept x-rays from the x-ray source 10 along a different ray of the fan beam 12.

#FIG. 1 is a pictorial representation of a prior art CT gantry with an imaged object showing the helical scanning of a fan beam and the relative angles and axes associated therewith as discussed in the Background of the Invention above;

#FIG. 2(a) is an exaggerated cross-section of the fan beam of FIG. 1 showing a penumbra caused by a collimator;

#FIG. 2(b) is a graphical representation of the slice profile produced by the fan beam of FIG. 2(a);

#Referring to FIG. 3, a CT system for use with the present invention includes a gantry 20 supporting an x-ray source 10. The x-ray source 10 is oriented to project a cone beam 40 of x-rays along a beam axis 41 through a patient 42 to a supported and opposed detector array 44. The gantry 20 rotates to swing the beam axis 41 within a gantry plane 38 defining the x-y plane of a Cartesian coordinate system. Rotation of the gantry 20 is measured by angle .beta. from an arbitrary reference position within the gantry plane 38.

#The patient 42 rests on a table 46 that may be moved along a translation axis 48 aligned with the z-axis of the Cartesian coordinate system. The table 46 may move across the gantry plane 38 and is radiotranslucent so as not to interfere with the imaging process.

#The x-rays of the cone beam 40 diverge from the beam axis 41 and the gantry plane 38, along the translation axis 48, and also diverge from the beam axis 41 along the gantry plane 38 and along a transverse axis 50 generally orthogonal to both the beam axis 41 and the translation axis 48.