JPH067338A - Fan-shaped X-ray fan beam CT system - Google Patents

Abstract

(57) [Summary] [Objective] To provide a fan-shaped X-ray fan beam CT apparatus having the above-described shape so that high image quality can be achieved without scanning scan artifacts in a short imaging time. It is the purpose of the invention. [Structure] Detector element (7) of detector array (4)
Are subdivided into groups for data collection and the data acquisition system is arranged to be controlled for collection of the output signal of the detector element (7). Here, the integration and reading are controlled so that they are performed simultaneously within one group, but with a time lag in each group that is successively continuous.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a third generation fan X-ray fan beam C having a measuring unit which rotates about a system axis consisting of an X-ray beam source and a detector array.
Regarding the T device.

[0002]

2. Description of the Prior Art In the case of a third-generation fan beam or fan-beam with a continuous beam, the arrangement of the detector elements and the scanning detection should be performed as follows: scanning. Artifacts on the (scan) are avoided, and as much image quality (local- and contrast resolution) as possible under a given total number of measurement data, or a given image quality, a small beam on the patient. Achieved with load,
Or, as large a volume as possible for a given shooting time
Tomography (tomographic scanning, scanning) should be performed so as to be achieved.

According to the prior art, the means for reducing scanning scan artifacts (false images) is the static quarter offset of the detector device. Avoidance of scanning scan artifacts and simultaneous increase of local resolution are achieved by the jump focus technique,
In this technique, of course, the number of measurement data is multiplied (multiplied).

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a fan-shaped X-ray fan beam CT apparatus of the above-indicated shape so that high image quality can be achieved without scanning scan artifacts in a short imaging time. Is to provide.

[0005]

SUMMARY OF THE INVENTION The above-mentioned problems are solved according to the invention as follows: a third generation fan X-ray having a measuring unit which rotates around a system axis consisting of an X-ray beam source and a detector array. In the fan-beam CT device, the detector elements of the detector array are subdivided into groups for data collection, and the data acquisition system is configured to be controlled for collection of the output signals of the detector elements, where: , The integration and the reading are performed simultaneously within one group, but with a time lag in each successive group, wherein the time lag is the measuring unit between successive projection directions. It is designed to correspond precisely to the time for the rotation of the.

The above-mentioned data acquisition fulfills the preconditions for applying the modified and modified convolution-backprojection algorithm for image construction, which corresponds to the state of the art in mathematics. (F. Na
tterer: “Sampling in fan b
eam tomography ", Westfaeli
sche Wilhelms-Universitae
t Muenster, Institut fuel
numerische und instru-men
telle Mathematic, 19.12.19
91, Published in SIAM J. Appl. Math.
1992).

In summary, unlike the conventional data acquisition (collection) according to the prior art, the number of data for each projection direction is reduced, but the number of projection directions is increased, in which case the measurement field in question is measured beam. The purpose is to cover as effectively as possible. The effective covering of the measuring field by the beam can be interpreted as the so-called good lattice mathematical theory (= good lattice points for the numerical integration of multidimensional functions). (S.K.Z
aremba (ed.): “Applications
of Number Theory to Nume
local Analysis ”, New York
and London, Academic Pres
s, 1972).

The present invention is based on the common number of detector elements in the third generation CT apparatus. For the collection of data, the detector elements are subdivided into groups and divided into groups of compression of the data used for each projection by a factor M. In that case, the above M groups can be synthesized, for example, as follows: Group: Elements No K ', K' + M, K '+ 2 * M ,. ． ． Etc. K. 〃: 〃 O ', O' + M, O '+ 2 * M ,. ． ． Etc. (where (O ', l', ..., (M-1) 'can be permutations or permutations of the order (O, l, ..., M-1)) Third Generation Because of the increase in the number of measurement data,
It is known to dynamically shift fan beams or fan beams to improve image quality (jump focus-technology). In the present invention, the dynamic shifting of the fan-shaped beam is equivalently performed by the division of the detector array into each group and the time shift during data acquisition. A combination of jump focus and technique is then possible (may be done).

The present invention will be described in detail with reference to the embodiment shown in the drawings.

[0010]

1 shows a focal circle 1 in which the focal point 2 of the X-ray beam source rotates. The fan beam or fan beam 3 emanating from the focal point 2 impinges on a detector array 4, which consists of a series of detector elements and rotates with the focal point 2. Detector array 4
The output signal of the detector element of the is supplied to a calculator which then images the layer under test of the measuring object located in the measuring field 5 or the volume of the measuring object.
Computes an image of and renders it on a visible device.

FIG. 1 shows a measuring beam 3, for example a fan beam or fan beam 3, which impinges on a detector element 7. The angle between the measuring beam 6 and the beam extending from the focus 2 to the system axis 8 is indicated by psi (psi), and the focus 2 to the system axis 8 described above is used.
The angle between the beam extending to and the abscissa of the coordinate system x, y is denoted by phi phi (φ). The angle φ changes continuously during rotation of the focal point 2 on the focal circle 1 and the detector array 4, and the respective projections (in this projection, the signals of the detector elements of the detector array 4, more precisely the sub-arrays of the detector array 4) are The signal of the detector element of the group is detected).

Continuous beam of the X-ray beam source and continuous rotation of the measuring unit at a constant angular velocity w (X-ray beam source and detector array 4 about a common system axis 8).
Rotation) is assumed. In that case, one predetermined projection angle φ = w * t is uniquely associated with each time point t during system rotation, and vice versa.

A data point or a measured value is represented by the two angular coordinates ψ and φ of one measuring beam. 1
One measurement beam is defined by a line connecting the center of gravity of the detector element and the focus 2 at a fixed time t = φ / w. The individual detector elements-measuring signals are respectively collected and read over an integration time interval dt. Then at the data point, the center of the integration interval or the corresponding projection angle φ
Are associated with. Data that is grouped under one common projection angle is called a (fan beam) projection.

The diagram based on the coordinates φ and ψ is called a fan beam-sinogram (Sinogram). The display of the data points in this sinogram visualizes the scanning scan raster (= scan pattern). When the scan pattern (shown statically) is further supplemented with information about the time course of the data acquisition (collection), the expression scan scaner (scan schematic pattern) is used.

In FIG. 2, the following method is shown. The detector numbers are indicated by K × M, K · M + 1, etc. toward the left, and the detector group is indicated by 0, 1, 2, 0 toward the upper side.

1. The conventional number of detector elements is not reduced (or much less). The detector elements are divided into groups for collecting data, eg M− for compression of the data used for each projection by a factor M−.
And the M-groups are synthesized as follows, for example. Group: Element No. O ', O' + M, O '+ 2 *
M ,. ． ． Etc. K. Group: Element No. K ', K' + M, K '+ 2 *
M ,. ． ． Etc. However, (O ′, l ′, ..., (M−1) ′ is the sequence (O,
l ,. ． ． , M-1) permutation or permutation. In FIG. 2, M = 3, and the order is not permuted or replaced.

2. The data acquisition system should be controlled as follows: integration or readout of each signal within one group at the same time, but with staggered time in each successive group. The time shift in question then corresponds exactly to the rotation of the system between successive projection directions. Figure 2 shows the deviation of DAS-start point for each group
It is indicated by a middle arrow, and the arrow indicates the integration interval for the tedictor group. In order to utilize the beam penetrating the patient without loss, the signal integration time in the M detector group spans M consecutive projections. The integration time corresponds to the conventional scanning schematic pattern in the example of FIG.

In short, the dynamic shift of the fan beam or fan beam 3 in the prior art can be equivalently performed due to the natural spatial shift of the detector group inside the detector array and the time shift during data acquisition. It may be used in combination with it.

In the embodiment of FIG. 2—compared with the prior art—the detector packing density is reduced by a factor of 2 (reduced to 1/2). The means shown in FIG. 3 is proposed in order to make full use of the beam and not to lose the spatial resolution due to the double width detector element.

Complete packing density, ie FIG.
Twice as many detector elements and data points at are used. The scan pattern of FIG. 3 is performed by superimposing the same (equivalent) scan pattern, which is obtained by a mirror image on the φ-axis and a time offset of 1/2 projection increment. At that time, the superimposed pattern is indicated by a cross (cross mark) in FIG. The above new scanning pattern is also the type described in the above item 1 (here, with a detector group of M = 6). Each of the two superimposed sub-scan patterns is significant as an actual mathematical relationship. For image reproduction, it is advantageous to process the data of each sub-scanning pattern in parallel.

Variable fan beam offset is achieved without dynamic mechanical shifts and without jump focus. The beam is fully utilized, augmenting the parallel processing of data for reconstruction. The jump focus can be omitted, but need not be omitted. Alternatively, it is possible to have conventional data acquisition on the one hand and a switch to the active mode on the other hand. Furthermore, in the case of the jump focus-technique combination, a relatively great flexibility is achievable in different selective effective modes.

[0022]

According to the present invention, it is possible to realize a fan-shaped X-ray fan beam CT apparatus having the above-described shape so that a high image quality can be achieved in a short imaging time without scanning scan artifacts. Played.

[Brief description of drawings]

FIG. 1 is a third-generation fan beam C for explaining the present invention.
It is a conceptual diagram of T device.

2 is a characteristic diagram of a scanning pattern of a detector element of the CT apparatus shown in FIG. 1 according to the present invention.

FIG. 3 is a relevant characteristic diagram of a characteristic pattern different from that of FIG.

[Explanation of symbols]

 1 focus circle 2 focus 3 fan beam 4 detector array 5 measurement field

Claims (1)

[Claims] 1. X-ray beam source and detector array (4)
Generation fan X-ray fan beam CT with a measuring unit rotating around a system axis (8) consisting of
In the device, the detector elements (7) of the detector array (4) are subdivided into groups for data collection, and the data acquisition system comprises the detector elements (7).
Are controlled for the acquisition of the output signals of the above, wherein the integration and the reading are performed simultaneously within one group, but with a time offset in each successive group. A fan-shaped X-ray fan beam CT apparatus, characterized in that it is configured in a controlled manner, wherein the said time offset corresponds precisely to the time for the rotation of the measuring unit between successive projection directions.