The present invention relates to a radiation tomography method and apparatus for sequentially measuring projection data of a subject in a plurality of view directions around the subject by radiations transmitting through a plurality of paths, and producing a tomographic image of the subject based on the projection data.
Radiation tomography apparatuses include the x-ray CT (computed tomography) apparatus, for example. In the x-ray CT apparatus, x-rays are employed as radiation. The apparatus is configured to scan a subject by a radiation emission/detection apparatus, i.e., an x-ray emission/detection apparatus rotating around the subject to measure x-ray projection data of the subject respectively in a plurality of view directions around the subject, and produce (i.e., reconstruct) a tomographic image based on the projection data.
Due to speedup of the x-ray CT apparatus, a scan can be completed within about 0.8 sec. Accordingly, by scanning the subject synchronously with respiratory monitoring signals from the subject (i.e., patient), and in conformity with a time phase at which the body motion is slow such as the maximum inspiration or expiration time, a tomographic image for the lung field, abdomen etc. which is not much affected by body motion is produced.
When tomography is performed on moving internal organs such as the lung or heart, it is desired to produce a tomographic image close to a stationary image by reducing the required imaging time span to as short as possible. One approach to achieve this is the half-scan technique. The half-scan technique comprises reconstructing an image from image information acquired by a half rotation of radiation beam generation means.
For the heart etc. which move faster than the respiratory motion, since the magnitude of the motion is large within a scan time even as short as 0.8 sec., conventional scanning techniques do not provide satisfactory imaging. Thus, an imaging method employing a heart-gate scan technique is practiced.
The technique comprises: continuously scanning a subject a plurality of times over multiple cardiac strokes while monitoring ECG (electrocardiographic) signals; sorting the acquired projection data by phase based on the ECG signal; and reconstructing a tomographic image of the heart at each phase based on the sorted projection data.
However, the half-scan technique cannot provide high enough image quality because of the low accuracy of the acquired data. Moreover, in computed tomography, an appropriate estimating calculation (e.g., interpolation) is conventionally performed on view data which are opposite each other obtained by opposite radiation beams to improve the image quality, but the opposite view data is not acquired with the half-scan technique, and it is impossible to perform such a processing for improving the image quality.
Furthermore, when the scan is performed synchronously with the respiration, it is difficult to completely avoid the body motion effect with a scan time of 0.8 sec. or slightly less, and therefore a tomographic image having a satisfactory image quality is not always produced. Also, the heart-gate scan technique requires a scan over multiple cardiac strokes, resulting in an increased scan duration and hence an increased x-ray exposure on the subject.