Patent Number: 
Section: claims

1. An X-ray CT apparatus comprising:X-ray generating means for generating X-rays;X-ray detecting means arranged opposite to the X-ray generating means for two-dimensionally detecting a dose of X-rays passing through an object to be examined;means for holding the X-ray generating means and the X-ray detecting means so that the object is located therebetween;means for adjusting a distance between the X-ray generating means and the X-ray detectinci means on the holdinci means;rotation driving means for driving the holding means to rotate around the object;image processing means for producing a tomogram of the object based on the X-ray dose detected by the X-ray detecting means;display means for displaying the tomogram; andmeans for changing an FOV size of the X-ray detecting means so as to reduce the FOV size of the X-ray detecting means to less than a maximum FOV size of the X-ray detecting means in accordance with a detected portion of the object displayed by the display means;wherein the FOV size changing means includes X-ray converting means for converting the X-rays into visible light; light receiving means for converting the visible light converted by the X-ray converting means into electrical signals; and means for changing a portion of the visible light converted by the X-ray converting means necessary for creating the tomogram. 2. An X-ray CT apparatus according to claim 1, wherein the FOV size changing means includes means for inputting a desired FOV size by moving a cursor of a pointing device onto an FOV size changing switch displayed on the display means and control means for changing an FOV size to the size input by the inputting means. 3. An X-ray CT apparatus according to claim 1, wherein the adjusting means adjusts a size of the tomogram displayed by the display means. 4. An X-ray CT apparatus according to claim 3, further including means for expanding the tomogram obtained with the FOV size changed by the FOV size changing means. 5. An X-ray CT apparatus according to claim 1, wherein the adjusting means adjusts the distance between the X-ray generating means and the X-ray detecting means by extending and retracting a part of the holding means. 6. An X-ray CT apparatus according to claim 1, wherein the holding means is a U-shaped arm which is adjustable by the adjusting means and is provided with the X-ray generating means and the X-ray detecting means respectively on the ends, and the rotation driving means rotates the holding means around an intermediate portion of the U-shaped arm being the center. 7. An X-ray CT apparatus according to claim 1, further comprising means for determining a position of the object so that an arbitrary portion of the object is located on the rotation center of the X-ray generating means and the X-ray detecting means. 8. An X-ray CT apparatus according to claim 1, wherein the maximum FOV size is based upon the detection of actual data and the FOV size changing means reduces the FOV size to less than the maximum FOV size based upon detection of actual data. 9. An X-ray CT apparatus according to claim 1, wherein the FOV range changing means reduces the FOV size to less than the maximum FOV size while maintaining the same resolution as obtained with the maximum FOV size. 10. An x-ray CT apparatus according to claim 9, wherein the FOV size changing means maintains substantially the same contrast when the FOV size is reduced as obtained with the maximum FOV size of the X-ray detection means. 11. An X-ray CT apparatus comprising:X-ray generating means for generating X-rays;X-ray detecting means arranged opposite to the X-ray generating means for two-dimensionally detecting a dose of X-rays passing through an object to be examined;means for holding the X-ray generating means and the X-ray detecting means so that the object is located therebetween;means for adjusting a distance between the X-ray generating means and the X-ray detecting means on the holding means;rotation driving means for driving the holding means to rotate around the object;image processing means for producing a tomogram of the object based on the X-ray dose detected by the X-ray detecting means;display means for displaying the tomogram; andmeans for changing an FOV size of the X-ray detecting means so as to reduce the FOV size of the X-ray detecting means to less than a maximum FOV size of the X-ray detecting means in accordance with a detected portion of the object displayed by the display means;wherein the X-ray detecting means is a two-dimensional X-ray flat sensor for converting X-rays into electrical signals, and the FOV size changing means includes means for changing a portion of the electrical signals converted by the two-dimensional X-ray flat sensor necessary for creating the tomogram. 12. An X-ray CT apparatus according to claim 3, wherein the adjusting means adjusts a size of the tomogram displayed by the display means. 13. An X-ray CT apparatus according to claim 3, wherein the adjusting means adjusts the distance between the X-ray generating means and the X-ray detecting means by extending and retracting a part of the holding means. 14. An X-ray CT apparatus comprising:X-ray generating means for generating X-rays;X-ray detecting means arranged opposite to the X-ray generating means for two-dimensionally detecting a dose of X-rays passing through an object to be examined;means for holding the X-ray generating means and the X-ray detecting means so that the object is located therebetween;means for adjusting a distance between the X-ray generating means and the X-ray detecting means on the holding means;rotation driving means for driving the holding means to rotate around the object;image processing means for producing a tomogram of the object based on the X-ray dose detected by the X-ray detecting means;display means for displaying the tomogram,means for changing an FOV size of the X-ray detecting means in accordance with a detected portion of the object displayed by the display means; andmeans for controlling conditions of X-ray generation of the X-ray generating means in accordance with the FOV size changed by the FOV size changing means. 15. An X-ray CT apparatus according to claim 14, wherein the means for controlling conditions of X-ray generation of the X-ray generating means controls conditions of an X-ray generator of the X-ray generating means in accordance with the FOV size as changed by the FOV size changing means so that each of images obtained in a different FOV size has the same contrast. 16. An X-ray CT apparatus according to claim 15, wherein the means for controlling conditions of X-ray generation of the X-ray generating means controls conditions of the X-ray generator so that an image obtained at an imaging portion of interest has the same resolution as that of an image at a portion relating to the imaging portion. 17. An X-ray CT apparatus according to claim 14, wherein the means for controlling conditions of X-ray generation of the X-ray generating means controls conditions of an X-ray generator of the X-ray generating means so that an image obtained at an imaging portion of interest has the same contrast as that of an image at a portion related to the imaging portion. 18. An X-ray CT apparatus according to claim 5, wherein the adjusting means adjusts a size of the tomogram displayed by the display means. 19. An X-ray CT apparatus according to claim 5, wherein the adjusting means adjusts the distance between the X-ray generating means and the X-ray detecting means by extending and retracting a part of the holding means.