WO2007110793A1

WO2007110793A1 - Scanning unit, tomography apparatus and tomography method

Info

Publication number: WO2007110793A1
Application number: PCT/IB2007/050876
Authority: WO
Inventors: Thomas Köhler; Peter Forthmann
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2006-03-28
Filing date: 2007-03-14
Publication date: 2007-10-04
Also published as: TW200800109A

Abstract

According to an exemplary embodiment scanning unit for a tomography apparatus, the scanning unit comprising a plurality of detection units, wherein each detection unit comprises a plurality of detection elements arranged in columns and rows, wherein the detection units are shifted in respect to each other in such a way that the detection elements of one row of a first detection unit have an offset of a predetermined fraction of one detection element in respect to a row of a second detection unit.

Description

Scanning unit, tomography apparatus and tomography method

The invention relates to a scanning unit, a tomography apparatus and a tomography method, in particular to a scanning unit for a computer tomography apparatus, a computer tomography apparatus and a computer tomography method.

Computed tomography (CT) is a process of using digital processing to generate a three-dimensional image of the internal of an object under investigation (object of interest, object under examination) from a series of two-dimensional x-ray images taken around a single axis of rotation. The reconstruction of CT images can be done by applying appropriate algorithms.

A basic principle of CT imaging is that projection data of an object under examination are taken by detectors of a CT system. The projection data represent information of the object passed by radiation beams. To generate an image out of the projection data these projection data (line integrals) can be back-projected leading to a two-dimensional image, i.e. representing a disc. Out of a plurality of such two- dimensional images a so-called voxel representation, i.e. a representation of three dimensional pixels, can be reconstructed. In case that the detectors are already arranged in form of a plane, two-dimensional projection data are achieved and the result of the back-projection is a three-dimensional voxel. This processing can be performed using two-dimensional helical reconstruction methods, where different parts of the detector data of one projection are backprojected into planes at different position, which may even have a different orientation. In order to effect a substantial reduction in the time taken for examining ways for scanning several parallel layers were introduced. From US 4 303 830 a computer tomography system is known comprising several sources of radiation and a number of radiation receivers equal to the number of radiation sources. Each radiation receiver consist of a row of detectors, and the radiation receivers are arranged offset in the direction of the rotational axis so that a plurality of layers are scanned simultaneously, i.e. from the data taken by each single radiation receiver a layer representing the scanned object can be reconstructed.

In modern, more sophisticated so-called "cone-beam" CT and reconstruction methods the projection data of two-dimensional detectors, i.e. detectors having a plurality of detecting elements arranged in form of a matrix, are directly backprojected into a three-dimensional distribution of voxels in one single reconstruction step. The current trend in computer tomography goes into the direction of increasing the number of detector rows and thus toward a very large coverage of the object under examination. One important application of the computer tomography is the so-called cardiac computer tomography, which is related to the reconstruction of a three- dimensional image of a beating heart. Systems with large detectors, i.e. systems comprising two-dimensional detection units, will allow to cover the entire heart using a circular or almost circular trajectory, i.e. no helical reconstruction is necessary. In order to increase the temporal resolution of these type of scanners, arrangements with multiple radiation sources, also-called tubes, and detectors on a gantry of the CT have been proposed in the past. Two or three tubes and detectors, comprising a matrix of detection elements, can be mounted on the gantry. When the detectors are placed in the right relation to each other a complete set of projections can be acquired in only a half or a third of the usual time. In the ideal situation of such a cardiac CT scan, data over an angular range of 180 degrees (or 360 degrees) are acquired such, that this angular range is partitioned into non-overlapping segments, wherein each segment is provided by one detector. However, the corresponding arrangement of the detectors provides in the range, where data from several detectors are used, identical samples. This increases the sensitivity of the system to aliasing artifacts.

It may be desirable to provide an alternative scanning unit for a tomography apparatus, a tomography apparatus, and a tomography method which may be less prone to non-optimal sampling of line-integrals or may reduce the occurrence of artefacts in the reconstructed image . This need may be met by a scanning unit for a tomography apparatus, a tomography apparatus, and a tomography method according to the independent claims. According to an exemplary embodiment a scanning unit for a tomography apparatus is provided, the scanning unit comprising a plurality of detection units, wherein each detection unit comprises a plurality of detection elements arranged in columns and rows. The detection units are shifted in respect to each other in such a way that the detection elements of one row of a first detection unit have an offset of a predetermined fraction of one detection element in respect to a row of a second detection unit.

According to an exemplary embodiment of a tomography apparatus a tomography apparatus comprises a scanning unit, wherein the scanning unit comprises a plurality of detection units, wherein each detection unit comprises a plurality of detection elements arranged in columns and rows. The detection units are shifted in respect to each other in such a way that the detection elements of one row of a first detection unit have an offset of a predetermined fraction of one detection element in respect to a row of a second detection unit. According to an exemplary embodiment a tomography method for reconstructing images of an object under examination, using a tomography apparatus according to an exemplary embodiment of the invention, wherein the method comprises emitting at least one radiation beam, detecting the at least one radiation beam after its passage through the object under examination with a plurality of detection units and providing signals indicative for the object under examination by the detection units. Preferably, the image of the object under examination is reconstructed out of these signals or data.

It may be seen as the gist of an exemplary embodiment of the present invention that a scanning unit for a tomography apparatus, e.g. a computer tomography apparatus, is provided, wherein the scanning unit comprises a plurality of detection units. Each of these detection units comprises a number of detection elements which are arranged in the form of a matrix, i.e. in columns and rows. In particular the detection units are arranged in respect to each other in a predetermined way so that each row of detection elements of a first detection unit is shifted in respect to rows of detection elements of a second detection unit. In other words the rows of one detection unit are not aligned with the rows of another detection unit but do exhibit a mismatch in respect to each other.

This shift or offset may lead to the fact that the sampling patterns of the different detector units interleave. This interleaving may lead to an improved reconstruction quality of tomography images, in particular the resolution of the reconstructed image may be increased due to the fact that a slightly mismatch between the data is archived. In typical case segments of a scanning unit, wherein each segment is provided by a single detection unit, do overlap, which, in scanning units according to the prior art, may lead to non-optimal sampling of line-integrals, when identical placement, i.e. no shifting of rows of detection elements between different detection units, of the detection units is used. This may be in particular the case if a rotation speed is too fast or so-called Dual-Focal-Spot sampling (DFS sampling) or so-called z-direction Focal-Spot sampling (zFS sampling), i.e. sampling methods in which the positions of the focus of the radiation sources are switched between alternate positions, is used. According to an exemplary embodiment a non identical placement of the detection units is used which may lead to the fact that overlapping segments are acquired using different detection units and that redundant data, generated by the overlapping segments, are captured with different sampling patterns. In the following reconstruction, this may automatically lead to a suppression of artefacts in a reconstructed image, e.g. of so-called windmill artefacts.

Another positive side effect may become apparent if cardiac computer tomography data are later used for a so-called non-gated reconstruction. Such a non- gated reconstruction may be done in order to perform diagnosis on the lungs, for example. In this case high spatial resolution is mandatory. Such a high resolution may be achieved by interleaving the data from all n detector units, wherein n is the number of detection units arranged in the scanning unit, into one data set with a n-times better sampling in the direction of a rotation axis than the individual data sets, resulting in an improved resolution in the images. In the following, further exemplary embodiments of the scanning unit will be described. However, these embodiments apply also for the tomography apparatus and the tomography method.

According to another exemplary embodiment of the scanning unit between each pair of adjacent detection units an equal offset is provided. In particular the offset may be in the opposite direction. That is, in case a row of a first detection unit is defined as the center line, the corresponding row of the proximate second detection unit is shifted by a given offset, e.g. one third of the dimension of a row, while the corresponding row of the proximate third detection unit is shifted by the same given offset in respect to the row of the first detection unit but in the opposite direction. Preferably, the predetermined fraction is one divided by the number of detection units which are arranged in the scanning unit. Other shifts are also possible, in particular the shift is not restricted to one divided by a whole number, i.e. to rational numbers.

Such an equidistant spacing between proximate detection units may provide an improved sampling of line-integrals.

According to another exemplary embodiment of the scanning unit the scanning unit is adapted to rotate around a rotation axis and the offset is in the direction of the rotation axis.

The rotation axis is commonly called z-direction, thus it may be said that the shift of the individual detection units to each other is in the z-direction of the scanning unit. However, the invention is not limited to a shift in this z-direction. The individual detection elements can be shifted also in another direction. For example, the detection elements can be shifted also in the angular direction, i.e. in the circular direction of the rotation, by a fraction of a width of one detection element. Such an displacement in the angular direction may lead to the fact that a radiation source unit associated to a detection unit may be off-centered with respect to the detection element. Such an off-centering may lead also to an interleaving of the data acquired by one single detection unit after a rotation of 180 degrees. That is, due to the off-centering the data of a single detection element taken at 0 degree are not identical to the data of the same detection element taken at 180 degree. Such an interleaving in the z-direction and/or an interleaving in the other direction, i.e. the circular direction, i.e. substantially perpendicular to the z-direction, may improve the in-plane resolution.

According to another exemplary embodiment of the scanning unit further comprises a radiation source, wherein the radiation source comprises a plurality of radiation source units. Preferably, the number of radiation source units equals the number of detection units, i.e. for each detection unit one radiation source unit is provided, which are associated to each other. According to another exemplary embodiment of the scanning unit the number of detection units is three. A number of three detection units may be a good compromise between simplicity of the scanning unit and time required for taking an image of the object under examination. As the number of detection units, and preferably also of the radiation source units, may increase the complexity of the scanning unit may increase On the other hand the time to take an image may decrease since more data can be acquired in one rotation of the scanning unit around the object under examination. According to another exemplary embodiment of the scanning unit the scanning unit is formed as a cone-beam scanning unit. That is, the detection units are adapted in such a way that signals are providable which are indicative for a whole object under examination, i.e. the whole object under examination is illuminated by the radiation source of the scanning unit. Thus the scanning unit does not have to be moved along the rotation axis in order to achieve signals representing the whole object. This may simplify the complexity of the scanning unit in particular of the mounting since no movement along the z-axis is necessary. In the following, a further exemplary embodiment of the tomography apparatus will be described. However, this embodiment apply also for the scanning unit and the tomography method.

According to another exemplary embodiment the tomography apparatus further comprises a reconstruction unit, wherein the reconstruction unit is adapted to reconstruct an image representing an object under examination out of signals provided by the detection units.

The examination of the object of interest, e.g. the analysis and reconstruction of cardiac computer tomography data taken by a scanning unit and/or a tomography apparatus according to the invention, may be realized by a computer program, i.e. by software, or by using one or more special electronic optimization circuits, i.e. in hardware, or in hybrid form, i.e. by software components and hardware components. The computer program may be written in any suitable programming language, such as, for example, C++ and may be stored on a computer-readable medium, such as a CD-ROM. Also, the computer program may be available from a network, such as the Worldwide Web, from which it may be downloaded into image processing units or processors, or any suitable computers.

It should be noted in this context, that the present invention is not limited to computer tomography, but may include C-arm based 3D rotational X-ray imaging, magnetic resonance imaging, positron emission tomography or the like as well. It should also be noted that this technique may in particular be useful for medical imaging like diagnosis of the heart or lungs of a patient.

It may be seen as the gist of an exemplary embodiment of the present invention that a scanning unit for a computer tomography apparatus comprises a plurality of radiation source units and detection units which are two dimensional, i.e. having detection elements arranged in columns and rows. A preferred number of detection units and radiation source units may be three. The different detection units are placed such that their sampling patterns interleave. In an embodiment having three detection units one detection unit may be placed centered, while the other two are shifted by one third of a pixel height up and down, respectively. That is, one detection unit is shifted in one direction along the z-axis and the other detection unit is shifted in the opposite direction along the z-axis. If overlapping segments are acquired using different detector units, these redundant data may be captured with different sampling patterns. In a following reconstruction, this may automatically lead to a suppression of artefacts, e.g. so-called windmill artefacts. A positive side effect may become apparent if cardiac CT data are later used for non-gated reconstruction. This is usually done in order to perform diagnosis on the lungs. In this reconstruction high spatial resolution is mandatory. In this case the data from all three detection units (detectors) can be interleaved into one data set with three times better sampling in z-direction than the individual data sets, resulting in an improved z-resolution of the images. A similar advantage with respect to the in-plane resolution may be achieved by shifting the detectors in the other direction by a third of a pixel width, i.e. a third of the width of one detection element, or a sixth of a pixel widths if a Double-Focal-Spot sampling acquisition is used. In this context the term "the other direction" means along the rows of the detection elements, i.e. substantially perpendicular to the z-direction. The scanning unit according to an exemplary embodiment may be in particular advantageous when using Double-Focal-Spot sampling either Double-Focal-Spot sampling in the direction of the rotation of the scanning unit (DFS sampling) or in the direction substantially perpendicular to this direction, i.e. the z-direction (zFS sampling).

These and other aspects of the present invention will become apparent from and elucidated with reference to the embodiment described hereinafter.

An exemplary embodiment of the present invention will be described in the following, with reference to the following drawings.

Fig. 1 shows a simplified schematic representation of a computer tomography system according to an exemplary embodiment of the present invention. Fig. 2 shows a schematic representation of the relative placement of different detection units according to an exemplary embodiment.

The illustration in the drawings is schematically. In different drawings, similar or identical elements are provided with the similar or identical reference signs.

Fig. 1 shows a simplified schematic representation of an exemplary embodiment of a computed tomography scanner system in which a scanning unit according an embodiment of the invention can be used.

The computer tomography apparatus 100 depicted in Fig. 1 is a cone- beam CT scanner. However, the invention may also be carried out with a fan-beam geometry. The CT scanner depicted in Fig. 1 comprises a gantry 101, which is rotatable around a rotational axis 102. The gantry 101 is driven by means of a motor 103.

Reference numeral 105 designates a source of radiation such as an X-ray source, which emits polychromatic or monochromatic radiation. Reference numeral 106 designates an aperture system which forms the radiation beam emitted from the radiation source unit to a cone-shaped radiation beam 107. The cone-beam 107 is directed such that it penetrates an object of interest 110 arranged in the center of the gantry 101, i.e. in an examination region of the CT scanner, and impinges onto the detector 115 (detection unit). As may be taken from Fig. 1, the detector 115 is arranged on the gantry 101 opposite to the radiation source unit 105, such that the surface of the detector 115 is covered by the cone beam 107. The detector 115 depicted in Fig. 1 comprises a plurality of detection elements 115a each capable of detecting X-rays which have been scattered by, attenuated by or passed through the object of interest 110. The detector 115 schematically shown in Fig. 1 is a two- dimensional detector, i.e. the individual detector elements are arranged in a plane. Such detectors are used in so-called cone-beam tomography. For clarity reasons only one radiation source unit and one detection unit are depicted in Fig. 1. According to the exemplary embodiment of the present invention at least two detection units and a corresponding number of radiation source units are arranged on the gantry 101. Preferably, three detection units and three radiation source units are used, but any suitable number of units can be used, e.g. four, five or six, depending on the needs. As the number of units increases more data will be sampled in a single revolution of the gantry, while on the other hand the system become more and more complex. The detection units and the radiation source units are placed equidistant on the gantry 101, i.e. in the case of three detection units, the units are placed every 120 degree. During scanning the object of interest 110, the radiation source unit 105, the aperture system 106 and the detector 115 are rotated along the gantry 101 in the direction indicated by an arrow 117. For rotation of the gantry 101 with the radiation source unit 105, the aperture system 106 and the detector 115, the motor 103 is connected to a motor control unit 120, which is connected to a control unit 125 (which might also be denoted as a calculation, reconstruction or determination unit).

In Fig. 1, the object of interest 110 is a human being which is disposed on an operation table 112. During the scan of a head 110a or a heart of the human being 110, while the gantry 101 rotates around the human being 110, the operation table 112 may displace the human being 110 along a direction parallel to the rotational axis 102 of the gantry 101. This displacement may be done by a motor 113. By this, the head 110a is scanned along a helical scan path. The operation table 112 may also be stopped during the scans to thereby measure data using a so-called axial or circular scan. However, it should be noted that preferably in all of the described cases a circular scan is performed, where there is no displacement in a direction parallel to the rotational axis 102, but only the rotation of the gantry 101 around the rotational axis 102. Optionally, an electrocardiogram device can be provided which measures an electrocardiogram of the heart of the human being 110 while X-rays attenuated by passing the heart are detected by detector 115. The data related to the measured electrocardiogram are transmitted to the control unit 125.

The detector 115 is connected to the control unit 125. The control unit 125 receives the detection result, i.e. the read-outs from the detection elements 115a of the detector 115 and determines a scanning result on the basis of these read-outs. Furthermore, the control unit 125 communicates with the motor control unit 120 in order to coordinate the movement of the gantry 101 with motors 103 and 113 with the operation table 112. The control unit 125 may be adapted for reconstructing an image from read-outs of the detector 115. A reconstructed image generated by the control unit 125 may be output to a display (not shown in Fig. 1) via an interface.

The control unit 125 may be realized by a data processor to process readouts from the detector elements 115a of the detector 115. The computer tomography apparatus shown in Fig. 1 may capture multicycle cardiac computer tomography data of the heart. In other words, when the gantry 101 rotates and when the operation table 112 is shifted linearly, then a helical scan is performed by the X-ray source 105 and the detector 115 with respect to the heart. During this helical scan, the heart may beat a plurality of times and multiple RR-cycles are covered. During these beats, a plurality of cardiac computer tomography data are acquired. Simultaneously, an electrocardiogram may be measured by the electrocardiogram unit. After having acquired these data, the data are transferred to the control unit 125, and the measured data may be analyzed retrospectively.

The measured data, namely the cardiac computer tomography data and the electrocardiogram data are processed by the control unit 125 which may be further controlled via a graphical user-interface (GUI). It should be noted, however, that the present invention is not limited to this specific data acquisition and reconstruction.

Fig. 2 schematically shows a schematic representation of a scanning unit according to an exemplary embodiment, i.e. the relative placement of different detection units. The scanning unit 200 comprises three radiation source units which are schematically depicted as the dots 201, 202, 203. The radiation source units are placed circularly on a gantry which is not shown in Fig. 2. In Fig. 2 the path on which the radiation source units rotate is depicted schematically by the black line 207. Opposite to each radiation source unit, a detection unit is arranged, i.e. opposite to the first radiation source unit 201 a first detection unit 204 is arranged, opposite to the second radiation source unit 202 a second detection unit 205 is arranged, and opposite to the third radiation source unit 203 a third detection unit 206 is arranged. Each detection unit comprises a number of detection elements which are arranged in form of a matrix, i.e. each detection unit is a two-dimensional detection unit. Schematically a matrix of eight rows 208, 209, 210,211, 212, 213, 214, and 215 and 24 columns is shown, but any suitable number of rows and columns can be used. According to the exemplary embodiment the first detection unit 204 is centered in respect to the z-direction which is indicated in the Fig. 2 by an arrow 216, i.e. the black line207 passes between row 210 and 211. The second detection unit 205 is shifted one third of a pixel height down in respect to the first detection unit 204, while the third detection unit 206 is shifted one third of a pixel height up in respect to the first detection unit 204. These shift is schematically illustrated by the placement of the black line 207 relative to the rows of the detection elements.

According to the present invention it is also possible to shift the detector units in the other direction of the two-dimensional detector unit, i.e. substantially perpendicular to the z-direction. Preferably, this shift is also depending on the number of detector units, i.e. in the shown exemplary embodiment it is preferably also one third of a pixel width, or a sixth of a pixel width in case so-called Double-Focal-Spot sampling is used. Such a shift means that a line defined by the center point of a radiation source unit, e.g. of the second radiation source 202, and the center of the rotation, i.e. the center point of the circumferential black line 207, does not intersect with the center of the two- dimensional plane of the second detector unit 205, but intersects this plane at a point shifted one third of a pixel width (one sixth in case of DFS sampling). This will lead to a so-called interleaving of the data taken by the respecting detection units, since the data (line integral) taken when the gantry is at 0 degree will not match to the data (line integral) taken when the gantry is at 180 degree. There will be a small mismatch (interleaving) of the data leading to an improved in-plane resolution.

Summarizing it may be seen as one aspect of the present invention that two-dimensional detection units of a scanning unit of a computer tomography apparatus are arranged in the scanning unit in such a way that they exhibit a predetermined mismatch in the z-direction which respect to each other. This mismatch may lead to an interleaving of the data taken from an object under examination which may lead to an improved reconstruction quality of the images taken from the object under examination.

It should be noted that the term "comprising" does not exclude other elements or steps and the "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

CLAIMS:

1. A scanning unit for a tomography apparatus, the scanning unit comprising: a plurality of detection units, wherein each detection unit comprises a plurality of detection elements arranged in columns and rows, wherein the detection units are shifted in respect to each other in such a way that the detection elements of one row of a first detection unit have an offset of a predetermined fraction of one detection element in respect to a row of a second detection unit.

2. The scanning unit according claim 1, wherein between each pair of adjacent detection units an equal offset is provided.

3. The scanning unit according claim 1 or 2, wherein the predetermined fraction is one divided by the number of detection units.

4. The scanning unit according to anyone of the claims 1 to 3, wherein the scanning unit is adapted to rotate around a rotation axis; and wherein the offset is in the direction of the rotation axis.

5. The scanning unit according to anyone of the claims 1 to 4, further comprising: a radiation source, wherein the radiation source comprises a plurality of radiation source units.

6. The scanning unit according to anyone of the claims 1 to 5, wherein the number of detection units is three.

7. The scanning unit according to anyone of the claims 1 to 6, wherein the scanning unit is formed as a cone-beam scanning unit.

8. A tomography apparatus comprising: a scanning unit according to anyone of the claims 1 to 7.

9. The tomography apparatus according claim 8, further comprising: a reconstruction unit, wherein the reconstruction unit is adapted to reconstruct an image representing an object under examination out of signals provided by the detection units.

10. A tomography method for reconstructing images of an object under examination, using a tomography apparatus according to claim 8 or 9, the method comprising: emitting at least one radiation beam; detecting the at least one radiation beam after its passage through the object under examination with a plurality of detection units; providing signals indicative for the object under examination by the detection units.