Image reconstruction method for computed tomography

In an image reconstruction method for computed tomography the reconstruction of an image from datasets is accelerated so that, with adequate image quality, dynamic studies of moving organs or monitoring of moving therapeutic instruments are possible. To this end, data segments for a sub-scan reconstruction are taken from the continuously measured and pre-processed data stream. The starting angle of the individual data segments is not restricted to a fixed grid. The grid can be arbitrarily defined according to the current computing capability of the reconstruction computer.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an image reconstruction 
method for a computed tomography apparatus, wherein the reconstruction is 
accelerated compared to known methods and wherein, with an adequate image 
quality, dynamic studies of moving organs or the monitoring of moving 
therapeutic instruments are possible. The delay between subject motion and 
the motion presented in the displayed image sequence should be as slight 
as possible. 
The above object is achieved in accordance with the principles of the 
present invention in an image reconstruction method for use in a computed 
tomography apparatus having an X-ray source and a radiation detector for 
conducting a scan of a subject by rotating around the subject to generate 
a continuous measurement data stream while irradiating said subject from 
different rotational angles, and image reconstruction computer and a 
monitor on which an image of the subject reconstructed by the image 
reconstruction computer is displayed, the image reconstruction method 
including the steps of taking successive data segments from the continuous 
measurement data stream, said data segments respectively comprising data 
generated beginning from different starting rotational angles of said 
X-ray source, using the data segments for sub-scan image 
reconstructions/in *) the image reconstruction computer, and selecting the 
respective starting rotational angles for the respective data segments 
according to a reconstruction grid for the sub-scan reconstructions which 
substantially maximizes the computing capacity of the image reconstruction 
computer. 
The basis for the inventive method is that data segments for a sub-scan 
reconstruction are taken from the continuously measured, pre-processed 
data stream. 
FNT *), i.e. images based on data acquired over a rotational angle of less then 
360.degree., 
The starting angle of the individual reconstruction therefore is not 
restricted to a fixed grid as in known methods. On the contrary, the grid 
is arbitrarily defined according to the computing capacity of the 
reconstruction unit available at the moment (current computing capacity). 
The inventive image reconstruction method thus allows a flexible 
adaptation to an existing reconstruction unit, allowing the existing 
computing capacity to be maximally utilized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 4 illustrates the inventive method by schematically showing the 
relationship among data segments which are used for respective sub-scan 
image reconstructions. The data employed in the inventive method are 
obtained in the same manner as described above using a conventional 
computed tomography system, however, the image reconstruction computer 7 
shown therein is programmed differently from the conventional manner, 
i.e., it is programmed to practice the inventive method illustrated in 
FIG. 4. 
As can be seen from FIG. 4, the data segments 9 through 12 for a sub-scan 
are not restricted to a fixed grid but are arbitrarily defined, and in 
fact overlap in the example. 
The use of a new algorithm wherein data segments for a sub-scan 
reconstruction are taken from the continuously measured and pre-processed 
data stream is necessary for processing the data in the manner shown in 
FIG. 4. The starting angle of the individual reconstruction is thereby not 
restricted to a fixed grid as in known methods. The grid can be 
arbitrarily defined according to the currently available computing 
capacity of the reconstruction computer (FIG. 4). 
Given movement of the bed 1, this method offers a higher spatial resolution 
due to the sub-scan algorithm. Without movement of the bed 1, a high time 
resolution is achieved in dynamic studies. Moreover, the waiting time 
t.sub.w,c =t.sub.w +t.sub.r +t.sub.u .multidot.F is significantly shorter. 
The factor F determines how many data points are required for the 
reconstruction of the sub-scan compared to 360.degree. reconstruction. The 
factor F varies between 0.52 and the value of 0.5 +fan angle/360.degree. 
standard for sub-scan. The reduced waiting time is especially effective 
given the start-stop operation that is standard in this mode. 
The remainder of the image reconstruction (acting on data supplied 
according to FIG. 4) can be implemented with known reconstruction methods 
such as, for example, filtered back-projection methods or Fourier methods, 
based on fan or parallel beams for computed tomography systems of the 
3.sup.rd, 4.sup.th or 5.sup.th generation (5.sup.th generation being a 
system with a number of rotating foci), and using single-line and 
multi-line, or planar detector system. The data rate can be reduced by 
interpolation over the individual detector channels, by interpolation over 
the individual projections and, possibly, over the data of different 
detector lines. Additionally, the reconstruction matrix is reduced, with 
the image presentation by interpolation ensuing with a larger matrix. The 
described data reductions can be applied individually or in combination. 
The data reduction can ensue directly on the measured data or on the 
pre-processed data. An increase in speed thus is already possible in the 
pre-processing, or can be achieved by virtue of the reconstruction ensuing 
at a higher speed. A reconstruction with high image quality of an 
individual slice or a number of slices, on the basis of stored data 
pre-processed in the described way then ensues. The measured data may be 
intermediately stored before the data reduction. 
Alternatively, however, image reconstruction can be carried out without 
data but image matrix reduction and with a correspondingly 
high-performance reconstruction computer. 
When the data pre-processed on-line are stored on disk or when a data 
buffer is present in the reconstruction unit for storing data from a 
sub-scan, or for storing the data from the most recent 360.degree. 
rotation, then a reconstruction with high image quality can ensue 
following the high-speed reconstruction. Dependent on the demands made on 
the reconstruction speed or image quality, the described data reduction 
can be disenabled individually, in combinations or entirely. In start-stop 
mode, the last measured dataset reconstructed with high quality then 
appears at the monitor. When the switchover is triggered by an 
interruption of the input data or an interruption of the X-rays, then such 
a reconstruction is possible at any time without serious interruption of 
the examination sequence. The high-quality image can then be permanently 
displayed either at a separate monitor or in a different image segment in 
the same monitor. 
The described algorithm accesses overlapping data segments. This does not 
automatically mean that the data need to be completely newly 
reconstructed. On the contrary, the disclosed algorithm allows projections 
from reconstruction n-1 to continue to be employed in reconstruction n. 
As already explained, the reconstruction grid can be flexibly adapted to 
the existing computing capacity of the reconstruction unit. If a 
data-independent reconstruction time is assumed, then an equidistant 
reconstruction grid is used. Alternatively, however, the time grid can be 
selected non-equidistant. For example, this can ensue with an external 
trigger (ECG trigger). As a result, cardiac activity triggered exposures 
are possible even given high pulse frequencies or a number of defined 
exposures are also possible within one heart period. Moreover, the 
reconstruction intervals can be adapted on-line to the current contrast 
agent concentration, for example, when tracking a contrast agent bolus. 
Although modifications and changes may be suggested by those skilled in the 
art, it is the intention of the inventor to embody within the patent 
warranted hereon all changes and modifications as reasonably and properly 
come within the scope of his contribution to the art.