1.  Field of the Invention
The present invention relates to a so-called cone beam X-ray computerized tomographic apparatus which scans an object to be examined with a pyramidal X-ray beam to obtain 3-D information. 
2.  Description of the Related Art
In a cone beam scan scheme, an object to be examined is scanned with an X-ray beam emitted from an X-ray tube and trimmed into a pyramidal shape by an X-ray stop.  The X-ray beam transmitted through the object is detected by a 2-D array type detector.  As an X-ray detector of this type, a detector having an array of a relatively small number of line detectors, typically four line detectors, has become widespread.  Recent years, however, have witnessed the advent of an X-ray detector having 32 or more arrays of line detectors by using solid-state detecting elements constituted by combinations of scintillator elements and photodiode elements or solid-state detecting elements made of selenium or the like which directly convert X-rays into electric charges.  The 2-D array type detector has the form of the cylinder or the plane. 
As a cone beam image reconstruction method, the FeldKamp method is generally used.  The FeldKamp method is an approximate reconstruction method based on the fan beam convolution/back projection method.  Convolution processing is performed by regarding data as a fan projection data on the premise that the cone angle is relatively small.  However, back projection processing is performed along an actual ray. 
That is, an image is reconstructed by the following procedure:                (1) assigning Z-axis-dependent weights to projection data;        (2) performing convolution for the data in (1) by using the same reconstruction function as that for a fan beam reconstruction; and        (3) performing back projection with respect to the data in (2) along an actual oblique ray having a cone angle.         
In such an image reconstruction method, however, the effective height of a field of view changes depending on the radius of the field of view.  This problem will be described in detail below. 
FIG.  1 is a side view of a field of view whose radius is set to a relatively long length RLL.  FIG.  2 is a side view of a field of view whose radius is set to a relatively short length RSS.  A radius R of the field of view is set to a length within which a region to be examined, e.g., the head, lungs, body.  Note that the “effective height” of the field of view is defined by the length of the field of view in the slice direction in which the set radius R is maintained (the length of the field of view in the body axis direction of the object). 
In the FeldKamp method, since data projected over one rotation are required, the maximum range in which image reconstruction can be done is limited to a cylindrical shape.  In this range, the effective height of the field of view within which the radius R is maintained is limited to WLL when the radius is RLL, as shown in FIG.  3A. When the radius is RSS, the effective height is limited to WSS, as shown in FIG.  3B. In this manner, the effective height of the field of view changes corresponding to the radius to which the field of view is set.