DE19953613A1 - Computer tomography apparatus - Google Patents

Abstract

The CT apparatus includes a radiation source (1) which is displaced about a system axis (8) to scan an object (3). A beam (S) emerges from a focus (F) and hits a detector system (4). During the scanning, the focus (F) periodically assumes at least two different z-positions (F(Z1),F(Z2)) along the system axis relative to the apparatus housing, thus producing projections that are displaced in the z-direction. The focus may be moved abruptly from one z-position (F(Z1)) to another z-position (F(Z2)).

Description

The invention relates to a CT device with a radiation source, which is used to scan an examination object by one sy stem axis is displaceable and has a focus from which a beam of rays emanates from a detector system meets. The invention also relates to a method for loading drove such a CT device.

CT devices are known which have a radiation source, e.g. B. an x-ray tube that is a collimated, pyramid shaped beam of rays through the object under examination, e.g. B. a patient, on one of several detector elements Straighten built detector system. The radiation source and each depending on the design of the CT device, the detector system are also on attached to a gantry, which ro animals. A storage device for the examination object can be moved along the system axis relative to the gantry or be moved. The position from which the Beam penetrates the object under examination, and the Angle at which the beam of rays examines object penetrates due to the rotation of the gantry constantly changing. Every detection hit by the radiation Gate element of the detector system produces a signal that Measure of the total transparency of the examination object for the radiation emanating from the radiation source on its way to the Detector system represents. The set of output signals from the Detector elements of the detector system designed for a specific Position of the radiation source is obtained as Projection called. A scan comprises one Set of projections in different positions of the Gantry and / or various positions of the storage direction were won. The CT scanner picks up during a Scans a variety of projections to a two-dim sional sectional view of a layer of the object under examination  to be able to build. With one out of an array of several Rows and columns of detector elements constructed by Detek door system can take several layers at the same time become.

A CT device with an X-ray tube is known from US Pat. No. 4,637,040 known, which has a rotating anode and whose focus periodically relative to the patient's scan X-ray tube housing in the circumferential direction of the rotating anode or tangential to the circumferential direction from an initial in can move an end position. Due to the periodic movement focus can be used to calculate an image double the data available to a body layer and thus achieve an improvement in the image quality.

Furthermore, EP 0 062 219 A2 is a rotating anode X-ray tube known which has a plurality of cathodes. Everyone a focal spot on the anode is assigned to the cathode. The individual cathodes can be used for the respective purpose X-ray tubes can be activated selectively accordingly. A Displacement of one or more focal spots in the way that there is a periodic movement of the respective focal spot from a start to an end position is not before seen.

The object of the present invention is that with a CT Device of the type mentioned attainable image quality improve. It is also an object of the invention to provide a method to operate such a CT device.

According to the invention, the task relating to the CT device solved by the features of claim 1. Die Ver driving problem is solved by the features of Claim 14.

With the CT device according to the invention, the Increase resolution in the z direction. The focus rela  tiv ei to the housing of the radiation source during the scan periodically different z-posi tion along the system axis. With two different ones A CT device according to the invention takes z positions during egg ner complete rotation of the radiation source around the lower Search object by 360 °, for example twice as many pros injections like a conventional CT scanner. In addition to the ver better resolution will continue to reduce scan artifacts graces. In addition, combines with the invention ß CT device the advantage that in an X-ray tube, the allows changing focus positions, through the differ focal positions verify the focal path area on the anode is larger. This increases the thermal resilience the X-ray tube, which advantageously ver lengthened or the tube output can be increased. The he The invention differs from known x-rays sources where the focus position on the anode to educate slightly increased thermal resilience are adjustable, this adjustability also a com Component can include in the z direction. A periodic ver position of the focus position so that projections from under different directions are recordable, is with these be X-ray tubes were not intended.

According to one embodiment of the invention, the focus changes abruptly from a first to a second z position. In the focus remains at the different z positions for a certain time interval. Starting from the last one occupied z-position, the focus can be further z-positions take or return to the starting position.

According to a further embodiment of the invention is pre see that the focus is in relation to the housing of the rays source continuously, preferably at constant speed from a first z position to a second z position tion is moved. As a result of the "blurring" that then occurs particularly good suppression of artifacts is possible.  

For moving the focus back to the first z position, this movement preferably occurs during a substantial period shorter period than the movement from the first to the second z position, the radiation generation need not necessarily sometimes interrupted. As a rule, however, this is the case Case.

According to a preferred embodiment of the invention in the case of a radiation source, the distance between two in the z direction offset focus positions selected so that the out rays on the system axis by half Layer thickness are offset in the direction of the system axis. This corresponds to a CT device with a multi-line detector in about an offset of the rays by half of the Er Extension of a detector element in the z direction. At this Configuration, all recorded layers have the same Layer thickness and the same distance from each other, whichever positive for the resolution and the computing effort for loading calculation of images of the object to be examined. Ge compared to conventional CT devices, in which the recorded Layers usually adjoin each other, grasp that invent tical CT device between two adjacent layers third layer, which the two neighboring layers each half covered. This means a doubling of the Ab Duty cycle in the z direction.

The distance between two focus positions within the radiation source and the tube-side diaphragm so set that the resulting beams on the Sy stem axis are offset by a layer width (the scanned adjacent layers), so at a complete rotation of the radiation source by 360 ° around the Examination object recorded two layers at the same time. The periodic alternation between the two layers during scanning only requires one detector line. So can with a CT scanner with a single-line detector, for example two shifts can be recorded simultaneously per revolution. In particular  this leads to a shortening, especially in the case of sequence scanning the sampling time or an improved resolution. Analogous this procedure can also be carried out within the scope of the invention three or more shifted focus positions and the corresponding one Extend the number of detectable layers.

Should be advantageous in a CT device according to the invention In addition to the resolution in the z direction, also the resolution in ϕ direction can be increased, so the focus can be relative periodically different to the housing of the radiation source positions in the direction of rotation of the radiation source (ϕ-Rich tung). Combined with the adjustment in the z direction the focus takes on the radiation source during periodically sampling at least four different Po sitions within the radiation source. About the resolution to optimize and minimize the computational effort are at a focus with four different focus positions advantageously lie in the corner points of a rectangle. Purpose the distance between different ones is also moderate here so focus positions in the z direction or in the ϕ direction chooses that the outgoing beams on the Sy stem axis by half the extent of a detector element are offset in the z direction or in the ϕ direction. Prince However, the focus positions can also take the form of a Trapezoid, a sine line or freely arranged.

For the sake of simple feasibility, the fiction shows According to the CT device, an X-ray tube is used as the radiation source at least one electron beam source, a rotating anode and electrical and / or magnetic means for deflection kung of the electron beam. The area of the rotating anode, from which the X-rays emanate from is below a certain one Angle inclined to the system axis. To move the The focus relative to the housing of the X-ray tube is the electric deflected. The change from a focus position in another takes place suddenly. Between the periodic change of focus position leaves the Fo  kus for a certain time interval in the respective position tion.

In a further advantageous embodiment of the inventions According to the CT device, the radiation source has several elec on radiation sources, the generated by them Electron beams to different positions on the Hit the rotating anode. In this way too, they can be separated which create spaced focus positions. Anytime essentially only one electron beam source is active or directed at the rotating anode. The focus is shifting by switching between the individual electron beams swell or by deflecting individual ones generated by them Electron beams reachable.

In an advantageous embodiment, the rotating anode is graduated in the radial direction. This allows a size Ren distance of the focus positions in the z direction. Besides, will which with the anode material for generating radiation, for. B. X-ray radiation, target area of the anode redu adorns, which reduces the manufacturing costs.

According to a further embodiment of the invention lights up a beam of rays emanating from a focus position Detector system only partially. This makes a non-di area of the detector hit directly by the beam systems used advantageously for measuring scattered radiation bar. The signal measured in this way can advantageously be used with the cor rectification of the Si generated by the other detector elements pull gnale. Scattered radiation artifacts are thus ver avoidable or at least reducible. In addition, the Area of the detector not directly hit by the beam torsystems for determining the focus position relative to the Ge housing of the radiation source can be used. this makes possible an exact regulation of the focus position. Is the ray geo metry set so that the superimposed beam is completely detected by the detector system, but not too  the entire detector area is used at any time, then is on the one hand a relatively large detector area che required, on the other hand, the applied radiation dose well usable. Alternatively, the detector system is selected partially overexposed during the scan, which means the entire Detector area can be used at any time. Then however, is not all of the radiation that the subs Object penetrated, detectable by the detector system.

The invention is illustrated below with reference to the beige added drawings. Show it:

Fig. 1 shows an inventive CT apparatus, in partial block diagram, of

Fig. 2, the measurement system of a CT apparatus according to Fig. 1,

Fig. 3 shows a detail of a X-ray tube with rotating anode,

Fig. 4 shows a detail of a X-ray tube with rotating anode, in which the focus position in the z direction, and φ-direction is adjustable ver,

Fig. 5 shows a detail of a X-ray tube with graduated rotary anode,

Fig. 6 shows a measuring system for the simultaneous recording of two layers and

Fig. 7 shows a measuring system, the detector system is not fully illuminated dig.

In Fig. 1, a CT device according to the invention is shown roughly schematically, which a radiation source 1 , z. B. has an X-ray tube, with a focus F, from which a faded in through a tube-side radiation diaphragm 2 , pyramid-shaped beam of rays S, which passes through a test object 3 , for example a patient, and strikes a detector system 4 . This has an array of a plurality of mutually parallel rows 5 and a plurality of mutually parallel columns 6 of detector elements 7 . The radiation source 1 and the detector system 4 form a measuring system 1 , 4 which can be displaced about a system axis 8 . The measuring system 1 , 4 and the examination object 3 can be displaced relative to one another along the system axis, so that the examination object 3 is radiated through at different projection angles α and different z positions along the system axis 8 . From the resulting output signals of the detector elements 7 of the detector system 4 , a data acquisition system 9 forms measured values which are fed to a computer 10 , which calculates an image of the examination object 3 , which is displayed on a monitor 11 .

According to the invention, the focus F is periodically assigned to the housing of the radiation source 1 periodically under different z positions along the system axis during the scanning. From the foci F (Z1), F (Z2) at different z positions, one beam S (Z1), S (Z2) is emitted, so that the measuring system 1 , 4 is independent of the relative movement between the radiation source 1 and the Examination object 3 displaced in the z direction during the scan, but takes up body layers lying close to or overlapping one another.

The X-ray CT device according to FIG. 1 can be used both for sequence scanning and for spiral scanning. During the sequence scanning, the examination object 3 is scanned in layers. The radiation source 1 is bezüg Lich the system axis Gert 8 verla about the examination object 3, and the measuring system 1, 4 assumes a plurality of functions on projek to be able to build a two-dimensional sectional image of a slice of the examination object. 3 Between the scanning of successive slices, the examination object 3 is moved relative to the measuring system 1 , 4 in each case into a new z position. This process is repeated until all layers, which enclose the area to be reconstructed, are covered.

During the spiral scanning, the measuring system 1 , 4 moves continuously on a spiral path relative to the examination object 3 until the area to be reconstructed is completely covered. A volume data record is generated. From this, the computer 10 uses an interpolation method to calculate a planar data set, from which the desired images can then be reconstructed as in the sequence scanning.

In Fig. 2, the measuring system 1 , 4 with two ver in the z-direction set foci F (Z1) and F (Z2) is shown, from which the beams S (Z1) and S (Z2) emanate and on the detector system 4th to meet. From the drawing, only one of several columns 6 of detector elements 7 of a five-line detector system 4 can be seen. During the scanning, the focus F (Z1) is the starting point of the X-ray radiation for a specific time interval. The X-rays then emanate from the focus F (Z2) for a specific time interval. During the scanning is reciprocated periodically between the two foci and hergeschaltet so that when a complete Flip Cellphone ray tube hung by 360 ° around the examination object 3 at the same time ten layers of the object 3 to be detachable, which overlap partially.

It when the distance between the two foci in the z direction is selected so that the out based on the two foci F (Z1) and F (Z2) each by half a layer thickness d in the z direction are offset from one another. The one doubled in this way Sampling rate leads to an increased resolution in the z direction and a reduction in artifacts.

Fig. 3 shows a variant schematically shows the radiation source 1 shown in Figs. 1 and 2 with a cathode 12 for generating an electron beam 13, means 14 for Ablen effect of the electron beam and a rotating anode 15. This arrangement offers a first possibility for adjusting the focus F in the z direction. For this purpose, the cathode 12 emits the electron beam 13 , which is directed onto the plate-shaped rotating anode 15 . The rotary anode 15 rotates continuously around the axis 16 during scanning. The impact surface 17 of the electron beam 13 on the rotating anode 15 is inclined at a certain angle with respect to the system axis. If the electron beam 13 is deflected by the means 14 for deflecting the electron beam in the manner shown, this results in a shift in the z position of the focus F within the X-ray tube. For example, by periodically switching the means 14 off and on, the focus can jump periodically between the focus positions F (Z1) and F (Z2). In addition to the possibility of increasing the scanning rate by adjusting the focus, this has the further advantage that the effective anode area is increased as a result. This leads to an increased thermal resilience of the X-ray tube and thus enables longer scanning times or higher tube outputs. The means 14 for deflecting the electron beam are, in the exemplary embodiment, two plates arranged in parallel, which are connected to different poles of a voltage source (not shown) and can therefore be charged differently. The voltage source is controlled by means of a computer (not shown).

From Fig. 4 is a rotating anode assembly is shown, in which the outgoing of the cathode 12 the electron beam 13 couples by means of two perpendicular to each other arranged panels 14 and 14 'deflected. Compared to FIG. 3, the plate pair 14 'essentially causes an additional deflection of the focus F in the ϕ direction. The electron beam 13 can freely position within a range of the impingement surface 17 . In the figure, this is illustrated by the positions F (Z1, ϕ1), F (Z1, ϕ2), F (Z2, ϕ1) and F (Z2, ϕ2). The change between two positions can be abrupt or continuous, with the electron beam 13 in principle any functions (e.g. sine function) on the contact surface 17 can be described. The resolution of the images generated with a CT device that is equipped with such an x-ray tube can thus be increased both in the z direction and in the ϕ direction.

Fig. 5 shows another embodiment of the rotary anode 15th In this case, the impingement surface 17 is stepped in the radial direction. As a result, a relatively large adjustment of the focus in the z direction can be achieved even with a slight deflection of an electron beam by appropriate means. From the comparison with FIG. 3, a further embodiment variant of the X-ray tube becomes clear from FIG. 5. In the embodiment of FIG. 5, namely two electron beam sources 12 and 12 ', one of which the electron beams 13 and 13' go out that are directed to a respective stage of the rotary anode 15. The means 15 for deflecting the electron beam according to FIG. 3 are thus dispensed with. By periodically switching between the electron beam sources 12 and 12 ', for example controlled by a computer (not shown), the z position of the focus with respect to the housing of the X-ray tube can also be changed. In FIG. 5, the two beams S (Z1) and S (Z2) emanating from the focus positions F (Z1) and F (Z2) are also indicated.

The invention is in terms of the configuration of the beam lenquelle and the adjustment of the focus not on the shown th embodiments limited. Rather, there are still one A number of other design options are conceivable. At for example, an x-ray tube can hold more than two electrons have radiation sources that point to different positions an anode. By switching periodically between the electron beam sources there is also the Po sition of focus changeable. An X-ray can also be used  Gen tube with several electron beam sources Have deflection of the electron beams. The adjustable speed can also do more than two possible focus points sitions, in which the focus is for one be correct time interval lingers. Even with the design the cathode and the anode there are a variety of other possibilities similarities compared to known arrangements are to be expanded that in these the adjustment of the focus is given in the z direction. So the anode can also be disc be shaped or cylindrical jacket. Also must it is not necessarily a rotating anode.

From Fig. 6, a further advantageous embodiment of the inventive egg nes CT apparatus can be seen. In this, the distance between two focal positions F (Z1) and F (Z2) offset in the z direction and the beam diaphragm 2 are set so that the two beams S (Z1) and S (Z2) on the system axis 8 directly adjoin one another. In order to be added at every order of 360 ° around the examination object 3 at the same time two adjacent layers of the Untersuchungsob jekts 3 4 run of the measuring system 1,. This is particularly advantageous in the case of sequence scanning if, due to the adjustability of the measuring system 1 , 4 in the z direction following two simultaneously scanned layers, the two subsequent layers are scanned. The result is an increased scanning speed or resolution. As can also be seen from FIG. 6, the illustrated detector system 4 has only one row of detector elements (therefore, only one detector element 7 can be seen from the perspective chosen). Another advantage of this arrangement can be seen in the fact that despite the detector system 4 , which according to the exemplary embodiment has only one detector line, two layers of the examination object can be recorded simultaneously by alternating projections from layers offset in the z direction.

In the example shown in Fig. 7, CT device according to the invention with a measuring system 1, 4 two focus positions F are visible (Z1) and F (Z2), of which the on-screen by the radiation diaphragm 2 beam S (Z1) or S (Z2 ) going out. However, these only partially illuminate the detector system 4 . Then a not directly illuminated area of the detector system 4 can be used to detect and measure the scattered radiation on the one hand and on the other hand to determine the exact position of the focus. The last position can be used to regulate the focus position. For this purpose, a computer (not shown) detects which detector elements are hit by the beam S at a given time and which are not hit. Due to the geometrical arrangement of radiation source 1 , beam aperture 2 and detector system 4 , the computer determines the current position of focus F and corrects it if necessary, for example by influencing means 14 for deflecting the electron beam, as illustrated in FIG. 3.

In the case of the exemplary embodiments described above are third generation CT devices, i.e. H. the X-ray source and the detector rotate during the Imaging together around the system axis. The invention can also be used with fourth generation CT devices, de only the x-ray source rotates and with a solid standing detector ring interacts, find use.

The exemplary embodiments described above relate to the medical application of CT devices according to the invention. However, the invention can also be used outside of medicine for example, when checking baggage or with material sub search, find application.

Claims (14)

1. CT device with a radiation source ( 1 ), which can be shifted around a system axis ( 8 ) for scanning an examination object ( 3 ) and has a focus (F), from which a beam of rays (S) emanates, which points to a detector system ( 4 ) hits, during the scanning the focus (F) relative to the housing of the radiation source ( 1 ) periodically occupies at least two un different z positions (F (Z1), F (Z2)) along the system axis ( 8 ) Generation of projections offset in the z direction. 2. CT device according to claim 1, wherein the focus (F) relative to the housing of the radiation source ( 1 ) abruptly from a first z position (F (Z1)) in a second z position (F (Z2)) feasible is. 3. CT device according to claim 1, wherein the focus (F) relative to the housing of the radiation source ( 1 ) in a continuous movement from a first z position (F (Z1)) to a second z position (F (Z2 )) is transferable. 4. CT device according to one of claims 1 to 3 for the layer-wise scanning of an examination object ( 3 ), wherein the outgoing from the focus (F) at two different z positions (F (Z1), F (Z2)) beams (S (Z1), S (Z2)) on the system stem axis ( 8 ) are offset at least substantially by half a layer thickness (d) in the direction of the system axis (z direction) or a multiple thereof. 5. CT device according to one of claims 1 to 4, wherein during the scanning of the focus (F) relative to the housing of the radiation source ( 1 ) periodically at least two different positions (F (Z1, ϕ1), F (Z1, ϕ2 )) in the direction of rotation of the radiation source (ϕ direction). 6. CT device according to claim 5, wherein the positions of the in a Plane projected focus a rectangle, a trapezoid or a Describe sinus functions. 7. CT device according to claim 5 or 6, wherein the outgoing from the focus at two relative to the housing of the radiation source different ϕ-positions (F (Z1, ϕ1), F (Z1, ϕ2)) radiation beam (S (Z1), S (Z2)) on the system axis ( 8 ) are at least substantially offset by half the extent of a detector element ( 7 ) in the direction of rotation of the radiation source. 8. CT device according to one of claims 1 to 7 with an X-ray tube as a radiation source, the at least one electron radiation source ( 12 ), a rotating anode ( 15 ) and means ( 14 ) for deflecting the electron beam ( 13 ) to shift the focus ( F) relative to the housing of the radiation source ( 1 ) in un different positions (F (Z1), F (Z2)) along the system axis ( 8 ). 9. CT device according to claim 8 with at least two electron radiation sources ( 12 , 12 '), said generated by these electron rays ( 13 , 13 ') relative to the housing of the x-ray tube in different z-positions along the system axis ( 8 ) on the Hit the rotating anode ( 15 ). 10. CT device according to claim 9, wherein at any time in essence only the electron beam ( 13 , 13 ') of an electron beam source ( 12 , 12 ') strikes the rotating anode ( 15 ) and further electron beam sources are switched off or by these generated electron beams are deflected. 11. CT device according to one of claims 8 to 10, wherein the rotating anode ( 15 ) is stepped in the radial direction and the focus (F) by the impingement of electron beams ( 13 , 13 ') on different stages of the rotating anode ( 15 ) Varying z positions along the system axis ( 8 ) relative to the housing of the X-ray tube. 12. CT device according to one or more of claims 1 to 11, wherein one of at least one focus (F) emitting beam bundle (S) only partially illuminates the detector system ( 4 ) and not hit directly by the beam (S) ner area of the detector system for measuring stray radiation can be used. 13. CT device according to one or more of claims 1 to 12, wherein one of at least one focus (F) outgoing beam bundle (S) only partially illuminates the detector system ( 4 ) and not hit directly by the beam bundle (S) ner area of the detector system for determining the focal position can be used relative to the housing of the radiation source ( 1 ). 14. A method for scanning an examination object by means of a CT device with a radiation source ( 1 ) which can be shifted about a system axis ( 8 ) and has a focus (F), from which a beam of rays (S) emanates Detector system ( 4 ), wherein during the scanning the focus (F) relative to the housing of the radiation source ( 1 ) is periodically shifted between at least two different z positions along the system axis.