DE19905975A1 - Computer tomography apparatus with multi-line detector system - Google Patents

Abstract

The apparatus has a multi-line detector system (6) in which individual columns of detector elements are displaced w.r.t. other columns. Arrays of detector elements may be combined in to detector modules, at least one of which is displaced w.r.t. an adjacent module in the rotation direction of a radiation source. The displacement may be equal to the length of a display element or a integer multiple thereof.

Description

The invention relates to a CT device with a radiation source, which is used for spiral scanning of an examination object is displaceable about a system axis and a beam of rays sends out that to one of an array of at least one Row and several columns of detector elements existing Detector system hits, with a storage device for the examination object and the radiation source in the direction the system axis are adjustable relative to each other, and the measured values obtained in this way from a large number of projects angles and az position on the system axis net and are fed to a computer, which from it Images of the examination object are calculated. The invention be also applies a method for operating such a CT Device.

CT devices are known which use a radiation source, e.g. B. an x-ray tube, which have a collimated, pyrami deniform beam through the object under examination, e.g. B. a patient, on one of several detector elements set up the detector system. The radiation source and Depending on the design of the CT device, the detector system are also on a gantry attached to the object under examination rotates. A storage facility for the examination object jekt can be moved along the system axis relative to the gantry ben or be moved. The position from which the beam of rays penetrates the object under examination, and the angle at which the beam of rays penetration object, are due to the rotation of the Gantry constantly changing. Anybody hit by the radiation Detector element of the detector system produces a signal which is a measure of the total transparency of the object under investigation for the radiation emanating from the radiation source represents their way to the detector system. The theorem of Aus  output signals of the detector elements of the detector system, the obtained for a specific position of the radiation source is called projection. One scan includes a set of projections at different buttocks positions of the gantry and / or different positions of the la were obtained. The CT device is picking up a variety of projections on a scan two-dimensional sectional view of a layer of the subs object to be able to build. With one from an array of several rows and columns of detector elements Built detector system can cover multiple layers at the same time be included.

Larger volumes of the object to be examined are becoming more common recorded by means of spiral scanning (spiral scan). Here rotates the gantry with the radiation source continuously the object under examination, while the storage device and the gantry continuously relative to each other along the sy stem axis are shifted. The radiation source describes so, based on the object under investigation, a spiral path, up to the volume determined before the examination is sampled. The spiral data obtained in this way then become images of individual layers calculated.

In conventional CT devices, the lines of the detector are located elements of the detector system perpendicular to the system axis. All detector elements of a row are on the same z position.

For spiral scanning with a multi-line detector system this detector arrangement is not optimal since the sampling points are not evenly distributed over the examination object. This can lead to artifacts in the reconstructed image. Here the maximum possible pitch (feed of the position tion device per revolution of the radiation source by 360 ° (Full circulation) around the object under investigation, based on the Er  extension of a detector element in the z direction, d. H. in Rich system axis) limited.

Certain reconstruction procedures for taking pictures with a Conventional, multi-line detector systems can also not use all data for reconstruction with a sol Chen detector system were included. This leads to min more detector system efficiency and unnecessary detector system system costs due to unused detector elements and egg ner exposure of the examination object with unused beam dose.

The object of the invention is to provide a CT device at the outset mentioned type so that its detector system verbes has serious properties. It is also the job of the inventor application of a method for operating such a CT device give.

According to the invention, the task relating to a CT device solved by a CT device with a radiation source which for spiral scanning of an examination object by one System axis is displaceable and emits a beam of rays, that to one of an array of at least one row and Detek constructed from several columns of detector elements door system meets, with a storage device for the Un test object and the radiation source in the direction of System axis are adjustable relative to each other, and so Measured values obtained from a variety of projection angles and assigned to a z position on the system axis are and fed to a computer, which Bil that of the object under investigation, and at least one of the columns of detector elements of the detector system opposite to that in the direction of rotation of the radiation source th column in the direction in which the storage device along the system axis relative to the radiation source is moved (z direction), is offset.  

Such an arrangement allows a uniform one Achieve spiral scanning of the examination object with one optimal use of all data recorded by the detector system. The appearance of artifacts in the reconstructed image, which at non-uniform scanning is prevented, and the maximum possible pitch is compared to conventional CT-Ge advise increased. You can also use a beam of rays adapted to the geometry of the inventive detector system cross section applied to the object under investigation Minimize the radiation dose, since only the areas of the Un object to be penetrated by radiation, the Samples can be used to reconstruct images.

Multi-line detector systems are usually constructed as follows:
The smallest unit is the detector element. At least one row and several columns of such detector elements are usually combined into detector groups (detector modules) for production reasons. The detector system is thus composed of at least one row and several columns of such detector groups.

Analog to the offset of the columns of detector elements according to the invention also individual columns of the detector relocate groups to neighboring detector groups. The Benefits that are achieved are the same as when offset columns of individual detector elements, with the further advantage that by the construction of the detector system from detector groups, these are relatively simple and inexpensive stig move against each other.

In principle, the offset of adjacent detector columns can or detector groups can be arbitrary. An offset around the end elongation of a detector element in the z direction or a whole however, numerous multiples of this are for the reconstruction tion process generally advantageous. The more moved  Detector columns or detector groups are formed, the more less is the case with certain reconstruction methods not usable proportion of detector elements in the total De detector system.

A conventional detector system is projected into the plane adorned, this projection has the shape of a rectangle. There against the projection of a detector system according to the Invention approximately from a parallelogram with two Rewrite sides parallel to the system axis. The construct tion of the detector system can depend on the pitch be optimized so that sampling is as uniform as possible of the object to be examined with maximum utilization at the same time tion of the detector system area is reached.

If, for example, the advance of the bearing device during a complete rotation of the gantry around the examination object is chosen so that it corresponds to the extent of a detector column in the z direction, then the angle α applies to a side of the parallelogram not parallel to the system axis and one Include a straight line lying at right angles to the system axis:

tg α = Δz / 2πR

where α is the included angle, Δz is the extension of a Detector column in the z direction and R the distance of the detector systems from the system axis.

A detector system constructed in this way is therefore suitable for one agreed pitch optimized. For different pitch values you can get even better results than with one achieve conventional detector system, which is detector system then no longer optimally adapted.

This is remedied by a variant of the invention which provides for the detector system to be designed in such a way that the offset of the detector elements or detector groups can be set as desired before each scan. This allows an optimized adaptation of the detector system to be achieved for each desired pitch. In this case, the following applies to the angle α:

tg α = nd / 2πR

where n is the pitch, d is the extent of a detector element in the z-direction and R the distance of the detector system from the Are system axis.

Those achievable with a detector system according to the invention Results become more accurate the less the shape of the in the plane projected detector system from a parallelo gram differs, d. that is, the more adjacent detector columns each because they are offset from each other and the smaller the offset fails in detail. This also makes it clear that for a optimal adaptation of the detector system to a desired one Pitch a continuous adjustability of the detector palette ten is advantageous for reasons of cost because of the simpler Feasibility and to simplify the reconstruction procedure, however, also an adjustment by integer in each case major multiples of the extent of a detector element in z Offers a good alternative.

Which relates to a method for operating a CT device Part of the task is due to the features of the claim 8 solved.

In the case of the method according to the invention for operating a So CT device is the cross section of the radiation source outgoing beam so superimposed that in essentially only that part of the subject of Strah is penetrated from which the detector system Ab sample values recorded. This reduces the burden on the patient object with radiation diagnosis that cannot be used diagnostically sis.

Further details and advantages of the invention will be made after following with reference to the embodiment shown in the drawings Examples explained.

Show it:

Fig. 1, a schematic representation of a CT apparatus with more contral detector system

Fig. 2 is a conventional schematic representation, more Backlighted detector system

Fig. 3 shows a schematic representation of a conventional, more Backlighted detector system with the partially non-usable for a specific pitch detector surface,

Fig. 4 is a multi-line detector system according to the invention with two by two detector elements staggered Detek torgruppen,

Fig. 5 is a multi-line detector system according to the invention, with continuously offset detector columns,

Fig. 6 in a schematic representation of a multi-line detector system according to the invention with the surface for a certain pitch be partially not usable Detektorflä.

In Fig. 1, a CT scanner is very schematically for Spiralabta stung an examination object 1 is shown which, for a radiation source 2,. B. has an X-ray tube with a focus 3 , from which a fade-in through a tube-side radiation aperture 4 , pyramid-shaped beam 5 emanates, which passes through the examination object 1 , for example a patient, and strikes a detector system 6 . This consists of an array of several parallel rows 7 and 8 columns of detector elements 9 . In the detector system shown, according to the invention, a single detector column is arranged offset in the manner shown from neighboring columns. The radiation source 2 and the detector system 6 form a measuring system, which can be displaced about a system axis 10 and rela tively displaceable along the system axis to the examination object 1 , so that the examination object is irradiated at different projection angles and different z positions along the system axis 10 . From the resulting output signals of the detector elements 9 of the detector system 6 , a data acquisition system 11 forms measured values which are fed to a computer 12 which calculates an image of the examination object 1 , which is displayed on a monitor 13 .

During the spiral scan, the measuring system 2 , 6 moves relative to the examination object 1 continuously on the spiral track 14 until the area to be reconstructed is completely covered. A volume data record is generated. The computer 12 uses an interpolation method to calculate a planar data set, from which the desired images can then be reconstructed.

Fig. 2 shows a schematic representation of a projected into the plane, formed from an array of several rows 7 and columns 8 of detector elements 9 , conventional detector system 6 in the form of a rectangle. For example, when scanning an examination object, the storage table for the examination object relative to the gantry, on which the radiation source and usually also the detector system 6 are mounted, during a complete rotation of the gantry around the examination object by the width Δz of the detector system 6 along the system axis 10 shifted in the z direction, then the detector elements 9 of the detector system 6 , which are located in the areas not hatched in FIG. 3, are partially not usable for certain reconstruction methods. This results from the fact that the associated areas of the examination object are scanned at two revolutions of the detector system 6 around the examination object. This further leads to the fact that the examination object is loaded with an unnecessary, ie diagnostically unusable radiation dose, that individual detector elements 9 of the detector system 6 are used inefficiently and that artifacts can arise in the reconstructed image.

The above disadvantages can be avoided with a CT device according to the invention, which has a detector system 6 of the type shown in FIG. 4. The offset of individual detector columns 8 in the manner shown is simple and inexpensive to achieve if, as shown in the example, entire detector groups 15 , 15 ', from which the detector system 6 is constructed, are offset from one another. The respective offset by the width of a detector element 9 in the z direction or an integer multiple thereof (in the example by the width of two detector elements) is a preferred variant of the invention due to the simpler technical feasibility and the simplification of the reconstruction method. In a further variant of the invention, the offset of the detector columns before a scan is adjustable. For this purpose, the adjusting means 19 schematically shown in the exemplary embodiment according to FIG. 4 are provided. Furthermore, it can be seen from FIG. 4 that the projection of the detector system 6 shown there into the plane is approximately a parallelogram 16 with two sides parallel to the z-axis. For the angle α, the one not to the system axis 10 pa rallele precisely the parallelogram 16 having an axis of system 10 perpendicular line 17 includes applies to the preferred application that positioning table at a complete revolution of the gantry around the examination object position for the Examination object moved along the system axis relative to the detector system exactly by the extent of a detector column in the z direction:

tg α = Δz / 2πR

where Δz is the extension of a detector column in the z direction and R the distance of the detector system from the system axis are.

However, an arbitrary offset of the detector columns 8, not corresponding to the extension of a detector element in the z direction or an integral multiple thereof, is possible, as shown in FIG. 5. The higher computational effort in the reconstruction then stands for a better adaptation to the ideal shape of a parallelogram 16 of the detector system 6 projected into the plane.

According to a particularly preferred variant of the invention, in a CT device according to FIG. 1 with a detector system 6 according to the invention with offset detector columns, the cross section of the beam 5 is adapted to the geometry of the inventive detector system. In the example, this is done by adjusting means 18 , which act on the radiation shield 4 on the tube side. It can thus be achieved that at least essentially only areas of the object under examination 1 are exposed to radiation doses from which detector elements 9 of the detector system 6 detect sampled values.

From the comparison with FIG. 3, the reduction of the partially unusable detector area from FIG. 6 - also not shown hatched in FIG. 6 - of a detector system 6 according to the invention at a given pitch is evident.

The invention is not based on that shown in the drawings presented embodiments limited. It also includes scanning one object under examination with another than that which is optimal for a detector system according to the invention Pitch, as well as the scanning with a detector system that before  a scan by offsetting individual detector columns or Detector groups in the sense of the invention only for a ge desired pitch can be optimized.

Furthermore, the invention is not limited to that in the examples described embodiment variant is limited, in which the Detector system together with the radiation source on one Gantry mounted around the examination object but moved is also in an analogous manner for fixed detector systems applicable.

Claims (8)

1. CT device with a radiation source ( 2 ), which can be shifted around a system axis ( 10 ) for the spiral scanning of an examination object ( 1 ) and a radiation beam ( 5 ) that detects an array of at least one line ( 7 ) and several columns ( 8 ) of detector elements ( 9 ) existing detector system ( 6 ), wherein a storage device for the object under examination and the radiation source are adjustable relative to one another in the direction of the system axis, and the measurement values obtained in this way are one of a large number of Projection angles and a z position on the system axis are assigned and are fed to a computer ( 12 ) which calculates images of the examination object therefrom, and at least one of the columns ( 8 ) of detector elements ( 9 ) of the detector system ( 6 ) compared to that in FIG Direction of rotation of the radiation source adjacent column in the direction in which the storage device along the system axis relative to the S radiation source is moved (z direction), is offset. 2. CT device with a detector system according to claim 1, in which arrays of detector elements ( 9 ) to detector groups (detector modules) ( 15 , 15 ') are combined and the detector system ( 6 ) from at least one row and several columns of detector groups ( 15 , 15 '), and at least one detector group ( 15 ') of the detector system ( 6 ) is offset in the z direction in relation to the detector group ( 15 ) adjacent in the direction of rotation of the radiation source. 3. CT device with a detector system according to claim 1 or 2, where the offset of the extent of a detector element in z direction or an integer multiple thereof speaks. 4. CT device with a detector system according to claim 1 or 2, where the offset is any, not with the extent  tion of a detector element in the z direction or an integer minor multiples of which correspond to the corresponding value. 5. CT device with a detector system according to one of claims 1 to 4, in which a projection of the detector system ( 6 ) in a plane, a parallelogram ( 16 ) with two sides parallel to the system axis is at least approximately rewritable and for the angle α, the Include a side of the parallelogram that is not parallel to the system axis and a straight line ( 17 ) that lies in the plane and is perpendicular to the system axis:
tg α = Δz / 2πR
where Δz is the extension of a detector column in the z direction and R is the distance of the detector system from the system axis. 6. CT device with a detector system according to one of the claims 1 to 6, where the offset of the columns of detector elements or detector groups depending on a scan the desired pitch is adjustable. 7. CT device with a detector system according to claim 5, in which a projection of the detector system into a plane a parallel lelogram ( 16 ) with two sides parallel to the system axis is at least approximately rewritable and for the angle α, which is not a parallel to the system axis Include the side of the parallelogram and an in-plane straight line ( 17 ) perpendicular to the system axis:
tg α = nd / 2πR
where n is the pitch, d is the extent of a detector element in the z direction and R is the distance of the detector system from the system axis. 8. A method for operating a CT device according to one of claims 1 to 7, in which the cross section of the radiation source ( 2 ) emanating from the radiation beam ( 5 ) is faded in through a tube-side radiation diaphragm ( 4 ) so that at least essentially only the area of the test object is penetrated by the radiation, from which the detector system ( 6 ) detects samples.