1. Field of the Invention
The present invention is directed to an image reconstruction method of the type suitable for use in computed tomography.
2. Description of the Prior Art
Image reconstruction methods for computed tomography conducted with an apparatus having a pre-processing unit for the data signals of the detector and a following reconstruction unit for the image reconstruction, from which information is supplied to a monitor for image playback, wherein the data stream is continuously measured and pre-processed are disclosed, for example, in German OS 43 19 538 and German OS 196 25 863.
In a computed tomography apparatus, images of the examined region of the patient are reconstructed from the data supplied by the detector. The operation of a conventional computed tomography apparatus is explained in greater detail with reference to FIGS. 1 and 2.
FIG. 1 shows a patient bed 1 on a base 2 to which the gantry 3 of a computed tomography apparatus with a measurement opening 4 is allocated, a patient on the bed 1 being introducible into the measurement opening 4. FIG. 1 also shows the focus 13 of an X-ray source that emits a fan-shaped X-ray beam 14 that strikes a detector 15 composed of a row of detector elements (3.sup.rd generation). The focus 13 and the detector 15 rotate around the patient so that the patient is irradiated from different directions (projection angles). The data delivered by the detector 4 are supplied to a pre-processing unit 5, which is followed by a data memory 6 and a reconstruction unit 7. The image playback ensues on a monitor 8.
A specific reconstruction algorithm is based on detector signals that are supplied to the image computer from a segment of a revolution of the focus 13 of the X-ray source and the detector 15.
The CT image m is reconstructed with this algorithm from the segment images n.sub.a =m through n.sub.e =m-1+K, with K being the number of segments of a 60.degree. revolution. The CT image m arises by addition of CT image m-1 and the segment image from the data segment n.sub.e and subtraction of the segment image from the data segment n.sub.a -1.
The left-most image in FIG. 2 shows how six segment images are calculated and added in an initialization phase, for example from k=6 data segments a through f of 60.degree. each. The middle image in FIG. 2 shows how the seventh segment image is reconstructed from the data segment g after another 60.degree. revolution. The desired CT image is derived by addition of the segment image from the data segment g and the previous CT image as well as by subtraction of the segment image from the data segment a. The right-most image in FIG. 2 shows how the algorithm reconstructs the eighth segment image from a further 60.degree. data segment h. The third CT image arises in that it is added to the previous CT image, and the second segment image (from the data segment b) is subtracted from the result. This algorithm is then continued for the following segments.
In an alternative realization, the suitably edited data of the corresponding segments are subtracted from one another, or are added instead of the images.
The two described algorithms have basic disadvantages:
The 360.degree. revolution must be divided into a number of segments which is a whole-number, so that the calculating time of the existing reconstruction unit 7 allows an on-line processing of the measured data. An optimum usage, and thus the maximum calculating speed of the reconstruction unit, are not possible.
The delay time between the motion of a subject in the measuring field of the computed tomography apparatus and the presentation is comparatively large and, for example, complicates therapeutic applications.
Due to the basic operating principle, the measured data of the detector must be pre-processed on-line, i.e. the data of a revolution must be processed within the revolution time t.sub.u. The data are thereby delayed by a time t.sub.w. The subsequent reconstruction must likewise occur on-line, i.e. the reconstruction of, for example, a 60.degree. segment cannot last longer than the time for 1/6 revolution. The data are thereby delayed by the time t.sub.r and the addition and subtraction of the segment images lasts for the time t.sub.a. The waiting time from the start of radiation emission until the first image amounts to t.sub.w,a =t.sub.w +t.sub.r +t.sub.a +t.sub.u (FIG. 3a).
The waiting time is shorter when the pre-processed data are added or subtracted by segments (FIG. 3b). It amounts to t.sub.w,b =t.sub.w +t.sub.r +t.sub.u. The reconstruction computer 7, however, must be designed with higher performance since it must conduct the addition or subtraction of the pre-processed data segments in the same time.