WO2008075267A2

WO2008075267A2 - Device and method for imaging an object

Info

Publication number: WO2008075267A2
Application number: PCT/IB2007/055083
Authority: WO
Inventors: Gilad Shechter; Shmuel Glasberg; Udo Van Stevendaal; Peter Forthmann; Thomas Koehler; Claas Bontus
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2006-12-19
Filing date: 2007-12-13
Publication date: 2008-06-26
Also published as: WO2008075267A3

Abstract

An imaging device (100,200) for imaging a volume of interest (5,45) of a subject (4,44) comprises a radiation source (1) for emitting radiation (6), which is arranged for rotational movement around an axis (8) of the subject (4,44), a collimator (3) for collimating the radiation (6) at least in the axial direction of the subject (4,44) before traversing through the subject (4,44), a detector (2) for receiving the collimated radiation (11,12,13,14,15,16) that has traversed through the subject (4,44), and a control unit (7) for controlling the collimator (3) based on at least one geometry parameter that defines a geometrical relationship between the volume of interest (5,45) and the imaging device (100,200). The imaging device (100,200) provides for a reduced dose of radiation applied to the subject (4,44).

Description

Device and method for imaging an object

FIELD OF THE INVENTION

The invention relates to an imaging device for irradiating objects with beams of radiation that have a collimator for forming the beam, and more particularly the invention concerns the control of such a collimator in dependence on a volume of interest of the object. The invention also relates to a method for imaging an object by irradiating the object with beams of collimated radiation, and more particularly the invention concerns the control of the collimation in dependence on a volume of interest of the object.

BACKGROUND OF THE INVENTION Devices for imaging an object that have a collimator for forming a beam of radiation that is emitted from a radiation source are known. Particularly, such devices are Computed Tomography (CT) scanners where the utilized radiation is X-ray radiation. Today's CT scanners have an axial coverage of 4 - 8 cm or even more as today's CT detectors consist of a plurality of rows of detector elements that are axially assembled and hence allow for spatially resolved volume imaging. To scan and reconstruct volumes that are larger than the coverage of a given detector two techniques may be applied. Firstly, in the step-and-shoot technique several circular scans are made with different axial positions relative to the volume of interest so that a large size volume is covered. Secondly, using a helical trajectory where each rotational position is assigned to a slightly displaced axial position, a large sized volume of interest can be scanned without interruptions.

US patent No. 6,501,828 B2 describes an apparatus for influencing X-rays in a fan-beam path, which influence is dynamically adjustable during radiological exposure. The X-rays emanate from an X-ray source in the direction of a subject and the X-rays source is arranged to rotate around the subject. The apparatus further comprises a collimator that has independently controllable X-ray absorbing elements that can be adjusted to set the size and position of a radiation window in the fan direction, which is in a plane perpendicular to the axis of rotation of the X-ray source. The adjustment of the collimator is made such that the X-ray beam penetrates essentially only the region or volume of interest of the subject to be examined, such as for example the heart of a patient. This leads to a reduction of X-ray dose applied to diagnostically uninteresting tissue. Disadvantages of the known apparatus are that it is not suitable for the step-and-shoot technique and the helical scan and that in the axial direction, which is parallel to the axis of rotation, parts of the object that do not relate to the volume of interest are irradiated and hence receive unnecessary X-ray dose.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an imaging device and an imaging method that allow for a reduction of radiation outside the volume of interest of a subject in the axial direction. The invention is defined by the independent claims. Advantageous embodiments are defined by the dependent claims.

This object is achieved by an imaging device for imaging a volume of interest of a subject comprising: a radiation source for emitting radiation and arranged for rotational movement around an axis of the subject; - a collimator for collimating the radiation at least in the axial direction of the subject before traversing through the subject; a detector for receiving the collimated radiation that has traversed through the subject; and a control unit for controlling the collimator based on at least one geometry parameter that defines a geometrical relationship between the volume of interest and the imaging device.

In this way the radiation can be adjusted with the collimator in the axial direction of the subject thereby enabling a reduction of the radiation outside the volume of interest of the subject in the axial direction, which parts of the subject do not need to be diagnosed. The adjustment of the radiation with the collimator depends on the geometrical relationship between the volume of interest of the subject and the imaging device.

This object is also achieved by a method for imaging a volume of interest of a subject with an imaging device that includes the steps of: determining at least one geometry parameter that defines a geometrical relationship between the volume of interest and the imaging device; emitting radiation from a plurality of rotational positions around an axis of the subject; controlling a collimation of the radiation at least in the axial direction of the subject based on the at least one geometry parameter; irradiating at least the volume of interest of the subject with the collimated radiation from the plurality of rotational positions; and detecting the collimated radiation after it has traversed through the subject. In a preferred embodiment of the imaging device according to the invention, transport means are provided for moving the subject in the axial direction of the subject with respect to the imaging device either in succession to or during a rotational movement of the radiation source. This advantageously provides for an imaging device in which a large sized volume of interest can be scanned.

In the imaging device according to the invention, the control unit is arranged to control the collimator such that a part of the radiation, that would not traverse through the volume of interest, is at least partially blocked.

In an embodiment of the imaging device according to the invention, the control unit is adapted to control the collimator such that the radiation traversing through each volume element of the volume of interest fulfills a reconstruction criterion. In this way an adjustment of the radiation with the collimator is provided via the control unit in which the adjustment depends, amongst others, on the reconstruction criterion. This results in an optimization of the radiation dose as a function of the required reconstruction of the image as defined by the reconstruction criterion thereby advantageously safeguarding the quality of the image, such as for example the image resolution. In an embodiment of the imaging device according to the invention, the imaging device further comprises a processor unit, coupled to the control unit, for computing and storing control data from planned rotational movements of the radiation source and planned axial movements of the subject. In this way the planned rotational and axial movements control, via the processor unit and the control unit, the collimator. In an embodiment of the imaging device according to the invention, the collimated radiation covers at least a part of the volume of interest for each axial position of the subject, and the control unit is arranged to control the collimator such that the collimated radiation for at least one axial position of the subject is aligned with an axial boundary side of the volume of interest. This advantageously reduces the dose of the radiation in a region that is outside and next to the axial boundary side of the volume of interest of the subject, which is a region that is not used in the diagnosis of the image.

In an embodiment of the method according to the invention, the collimation is controlled such that radiation, that would not traverse through the volume of interest, is at least partially blocked. This advantageously reduces unwanted radiation of parts of the subject that do not need to be diagnosed.

In an embodiment of the method according to the invention, the method further comprises the step of moving the subject in the axial direction of the subject with respect to the imaging device such that each of the plurality of rotational positions is assigned to one of at least two different axial positions of the subject. In this way an increased volume of interest of the subject is irradiated.

In an embodiment of the method according to the invention, the method further comprises the steps of: - computing control data from the plurality of rotational positions and the at least two axial positions of the subject to be used in the irradiating step; and using the control data in the step of controlling the collimation of the radiation.

In this way the rotational positions of the radiation and the axial positions of the subject control how the radiation is collimated. In an embodiment of the method according to the invention, the step of controlling the collimation of the radiation aligns the collimated radiation with an axial boundary side of the volume of interest for at least one axial position of the subject. This advantageously reduces the dose of the radiation in a region that is outside and next to the axial boundary side of the volume of interest of the subject, which is a region that is not used in the diagnosis of the image.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be further elucidated and described with reference to the drawings, in which: Fig. 1 is a prespective view of a known imaging device;

Figs. 2A-E are schematic cross-sectional views of different stages of irradiating a subject with an embodiment of an imaging device according to the invention;

Figs. 3 is a schematic cross-sectional views showing a rotational movement around a subject of an embodiment of an imaging device according to the invention Figs. 4A-C are schematic cross-sectional views of different stages of irradiating a subject with another embodiment of an imaging device according to the invention;

The Figures are not drawn to scale. In general, identical components are denoted by the same reference numerals in the figures. DETAILED DESCRIPTION OF EMBODIMENTS

Fig.1 schematically shows a perspective view of a known X-ray computed tomography (CT) scanner comprising a gantry 10. The gantry 10 has a radiation source 1 for emitting X-rays towards a detector 2, which is on the opposite side of the gantry 10. The detector 2 receives and senses the projected X-rays that pass through a subject, such as, for example, a patient (not shown). The detector 2 produces electrical signals that represent the intensity of impinging X-rays and hence the attenuation of the X-rays as they pass through the patient. During a scan the gantry 10, and the components mounted thereon, such as the radiation source 1 and the detector 2, rotate around a common rotation axis 8. The patient is located on a support 9, which is movable into an axial direction parallel to the rotation axis 8. When applying a step-and-shoot technique, a circular scan is made for each different axial position of the movable support 9, resulting in a plurality of circular scans of the patient at different axial positions wherein a volume of interest of the patient is completely irradiated by the radiation source 1. In another irradiation technique, called a helical CT-scan, the support 9 moves in the axial direction while the radiation source 1 and the detector 2 rotate, and each rotational position of the radiation source 1 and the detector 2 is assigned to a slightly displaced axial position of the movable support 9, and hence of the patient. This provides for the scanning of a large sized volume of interest without interruptions. In this case the X-rays that are emitted from the radiation source 1 are collimated by a pre-patient collimating device (not shown), or collimator, before passing through the patient. The collimator influences the beam profile and/or the intensity profile of the X-rays and comprises X-ray absorbing material within an aperture therein for restricting the profile of the X-ray beam. Figs. 2A-E are schematic cross-sectional views along a plane parallel to the common rotation axis 8, showing different stages of irradiating a subject 4 with an imaging device 100 according to the invention. The radiation source 1 emits a cone-shaped X-ray beam 6 towards the detector 2. Fig. 2A shows a cross-section of the X-ray beam 6 in a plane parallel to the direction of movement of the movable support 9 and also parallel to the axis of rotation 8 of the radiation source 1 and the detector 2. Furthermore, Fig. 2A shows that the X- ray beam 6 is absorbed by a collimator 3, comprising, in this example, a first collimator blade 21 and a second collimator blade 22, which are both movable to create an aperture through which part of the X-ray beam 6 traverses. The movable support 9 is at a first axial position and carries the subject 4 having a volume of interest 5 that has to be irradiated by the X-ray beam 6 to provide for a medical diagnosis of the volume of interest 5. The parts of the subject 4 that are outside the volume of interest 5 do not need to be diagnosed and hence do not need to be irradiated by the X-ray beam 6, thereby reducing the X-ray dose that is applied outside the volume of interest 5 of the subject 4 as compared to the situation in which no collimation of the X-ray beam 6 is applied. A controller 7 controls the movement of the first collimator blade 21 and of the second collimator blade 22 based, in this example, on the relative position of, or geometrical relationship between, the volume of interest 5 with respect to the radiation source 1, the detector 2, the emitted X-ray beam 6, the first collimator blade 21 and the second collimator blade 22. The controller 7 is synchronized with the axial motion of the subject 4.

The movable support 9 is translated in a Z-direction, which is parallel to the rotation axis 8 common to the radiation source 1 and the detector 2, to a second axial position, as is shown in Fig. 2B. The controller 7 has moved and positioned the second collimator blade 22 such that a first aperture 51 is created resulting in a collimated X-ray beam 11 that irradiates a first part of the volume of interest 5 of the subject 4. As is obvious from Fig. 2B, a part of the subject 4 has not been irradiated because of the collimation of the X-ray beam 6. In this position of the volume of interest 5, a first edge of the volume of interest 5 coincides with a first edge of the collimated X-ray beam 11, thereby preventing that a part of the subject 4 adjacent to the first edge of the volume of interest 5 is irradiated by the X-ray beam 6.

Then, as is shown in Fig.2C, the movable support 9 is translated in the Z- direction, which is parallel to the rotation axis 8 common to the radiation source 1 and the detector 2, to a third axial position. The controller 7 has moved and positioned the second collimator blade 22 and, in this case, also the first collimator 21 such that a second aperture 52 is created resulting in that a collimated X-ray beam 12 irradiates, in this case, the whole volume of interest 5 of the subject 5. Note that also in this position of the subject 4, the X-ray dose applied outside the volume of interest 5 of the subject 4 is reduced compared to a device without the collimator 3.

Fig. 2D shows a fourth axial position of the movable support 9 in which the controller 7 has created a third aperture 53, by moving and positioning of the first collimator blade 21 and the second collimator blade 22, thereby resulting in that a collimated X-ray beam 13 irradiates a second part of the volume of interest 5. In this position of the volume of interest 5, a second edge of the volume of interest 5 coincides with a second edge of the collimated X-ray beam 13, thereby preventing that a part of the subject 4 adjacent to the second edge of the volume of interest 5 is irradiated by the X-ray beam 6.

Fig. 2E shows a fifth axial position of the movable support 9 in which the controller 7 has created a situation in which there is no aperture through which the X-ray beam 6 can traverse. In this case there is no X-ray beam 6 that expands in the z-direction, thereby preventing that a part of the part of the subject 4 adjacent to the second edge of the volume of interest 5 is irradiated by the X-ray beam 6.

In this example, the radiation source 1 and the detector 2 assume a different rotational position at each axial position of the movable support 9, and hence also of the volume of interest 5 of the subject 4. In this helical CT-scan the radiation source 1 and the detector 2 rotate around a common rotation axis 8, which is parallel to the Z-direction. Fig. 3 illustrates a rotation R of the radiation source 1 and the detector 2 around their common axis 8 in a plane that is perpendicular to the rotation axis 8, further showing the X-ray beam 6, which is, in this plane, fan-shaped. As is shown in Fig. 3, the collimator 3 in this example does not collimate the fan-shaped X-ray beam 6. However, collimating the beam in the Z- direction, which is perpendicular to the plane of Fig.3, such as is illustrated in Figs.2A-E, in combination with, simultaneously, collimating the fan- shaped X-ray beam 6 in the plane perpendicular to the Z-direction is another embodiment that will further reduce the X-ray dose applied outside the volume of interest 5 of the subject 4. The helical CT-scan with the collimator 3, while adjusting the X-ray beam 6 to reduce the applied X-ray dose outside the volume of interest 5, also advantageously replaces the use of a variable speed of the support 9. Presently, a variation of the speed of movement of the support 9 is used to ensure that the volume of interest 5 is measured accurately enough, while the remaining parts of the subject 4 are covered in a shorter time. A disadvantage of this method is that a variable speed of movement of the support 9 results in undesired movement of the subject 4 and increases the complexity of the reconstruction of the X-ray image. By varying the collimation of the X-ray beam 6, instead of varying the speed of movement of the support 9, these disadvantages are overcome.

The achieved reduction of the X-ray dose applied outside the volume of interest 5 of the subject 4 depends on several factors, such as for example the cone angle of the X-ray beam 6 and the size of the volume of interest 5. For example, for reconstructing a heart of a patient, the applied X-ray dose can be reduced by up to approximately 30%.

The helical CT-scan creates several images of the subject 4 from different rotational positions of the radiation source 1 and the detector 2 and corresponding axial positions of the movable support 9. A reconstruction algorithm is used to reconstruct a 3- dimensional image of a part of the subject 4. The applied reconstruction algorithm may determine that radiation has to be applied in a part of the subject 4 that is outside the volume of interest 5 of the subject 4. In that case, the first collimator blade 21 and the second collimator blade 22 have to be positioned accordingly to reduce the radiation that is applied outside the volume of interest 5. Also the reconstruction algorithm may still provide for an acceptable resolution of the reconstructed image when the radiation applied to a part of the volume of interest 5 is reduced or partly blocked. This can be achieved by positioning the first collimator blade 21 and the second collimator blade 22 accordingly. In this way the applied X-ray dose is reduced while still having an acceptable resolution of the reconstructed image.

Figs. 4A-C are schematic cross-sectional views along a plane parallel to the common rotation axis 8, showing different stages of irradiating a subject 44 with an imaging device 200 according to the invention. In this case a step-and-shoot technique is applied in which a circular scan is made for each different axial position of the movable support 9, resulting in a plurality of circular scans wherein a volume of interest 45, which is in this case cylindrical, of the subject 44 is completely irradiated by the radiation source 1. The common rotation axis 8 is parallel to the Z-direction of the movement of the subject 44.

Fig. 4A shows a first axial position of the subject 44 at which a first circular scan is made. The radiation source 1 emits the cone-shaped X-ray beam 6 towards the detector 2 and the controller 7 has moved and positioned the first collimator blade 21 and the second collimator blade 22 such that a collimated X-ray beam 14 irradiates a part of the volume of interest 45 of the subject 44. In this first axial position of the volume of interest 45, a first edge 31 of the volume of interest 45 coincides with a first edge of the collimated X-ray beam 14, thereby preventing that a part of the subject 44 adjacent to the first edge 31 of the volume of interest 45 is irradiated by the X-ray beam 6 and hence reducing the X-ray dose applied outside the volume of interest 45 of the subject 44.

Fig. 4B shows a second axial position of the subject 44 at which a second circular scan is made. In this second axial position of the subject 44 the controller 7 has moved and positioned the first collimator blade 21 and the second collimator blade 22 such that a collimated X-ray beam 15 irradiates the whole volume of interest 45 of the subject 44. Fig. 4C shows a third and final axial position of the subject 44 at which a third and final circular scan is made. The controller 7 has moved and positioned the first collimator blade 21 and the second collimator blade 22 such that a collimated X-ray beam 16 irradiates a part of the volume of interest 45 of the subject 44. In this third axial position of the volume of interest 45, a second edge 32 of the volume of interest 45 coincides with a second edge of the collimated X-ray beam 16, thereby preventing that a part of the subject 44 adjacent to the second edge 32 of the volume of interest 45 is irradiated by the X-ray beam 6 and hence reducing the X-ray dose applied outside the volume of interest 45 of the subject 44.

When the volume of interest 45 of the subject 44 is known, the number of required circular scans can be determined in combination with an optimum collimation.

The movement of the collimator blades 21, 22 is controlled by the controller 7 and depends on the geometrical relationship between the volume of interest 5,45 and the imaging device 100,200. For example, this geometrical relationship comprises the relative position of the volume of interest 5,45 with respect to the movable support 9 combined with the relative position of the movable support 9 with respect to the collimator 3, which has a fixed relative position with respect to the radiation source 1 and the detector 2. Based on planned rotational movements of the radiation source 1 and the detector 2 and on a planned axial position of the movable support 9, a processor unit computes control data that are input to the controller 7 to provide for an appropriate movement of the collimator blades 21, 22 resulting in an appropriate collimation of the X-ray beam 6. The processor unit, such as a computer, may also be coupled to a further controller, which controls the movement of the movable support 9. Furthermore, the processor unit may also control the radiation source 1, the detector 2 and the rotational movements of the radiation source 1 and the detector 2. In summary, the invention provides for an imaging device for imaging a volume of interest of a subject, the device comprising a radiation source for emitting radiation, which is arranged for rotational movement around an axis of the subject, a collimator for collimating the radiation at least in the axial direction of the subject before traversing through the subject, a detector for receiving the collimated radiation that has traversed through the subject, and a control unit for controlling the collimator based on at least one geometry parameter that defines a geometrical relationship between the volume of interest and the imaging device. The imaging device provides for a reduced dose of radiation applied to the subject. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. Imaging device (100,200) for imaging a volume of interest (5,45) of a subject

(4.44) comprising: a radiation source (1) for emitting radiation (6) and arranged for rotational movement around an axis (8) of the subject (4,44); - a collimator (3) for collimating the radiation (6) at least in the axial direction of the subject (4,44) before traversing through the subject (4,44); a detector (2) for receiving the collimated radiation (11,12,13,14,15,16) that has traversed through the subject (4,44); and a control unit (7) for controlling the collimator (3) based on at least one geometry parameter that defines a geometrical relationship between the volume of interest

(5.45) and the imaging device (100,200).

2. Imaging device according to claim 1, wherein transport means (9) are provided for moving the subject (4,44) in the axial direction of the subject (4,44) with respect to the imaging device (100,200) either in succession to or during a rotational movement of the radiation source (1).

3. Imaging device according to claim 1, wherein the control unit (7) is arranged to control the collimator (3) such that a part of the radiation (6), that would not traverse through the volume of interest (5,45), is at least partially blocked.

4. Imaging device according to claim 1, wherein the control unit (7) is adapted to control the collimator (3) such that the radiation (11,12,13,14,15,16) traversing through each volume element of the volume of interest (5,45) fulfills a reconstruction criterion.

5. Imaging device according to claim 2, wherein the imaging device (100,200) further comprises a processor unit, coupled to the control unit (7), for computing and storing control data from planned rotational movements of the radiation source (1) and planned axial movements of the subject (4,44).

6. Imaging device according to claim 2, wherein the collimated radiation (11,12,13,14,15,16) covers at least a part of the volume of interest (5,45) for each axial position of the subject (4,44), and wherein the control unit (7) is arranged to control the collimator (3) such that the collimated radiation (11,12,13,14,15,16) for at least one axial position of the subject (4,44) is aligned with an axial boundary side (31,32) of the volume of interest (5,45).

7. Method for imaging a volume of interest (5,45) of a subject (4,44) with an imaging device (100,200) that includes the steps of: determining at least one geometry parameter that defines a geometrical relationship between the volume of interest (5,45) and the imaging device (100,200); emitting radiation (6) from a plurality of rotational positions around an axis (8) of the subject (4,44); - controlling a collimation of the radiation (6) at least in the axial direction of the subject (4,44) based on the at least one geometry parameter; irradiating at least the volume of interest (5,45) of the subject with the collimated radiation (11,12,13,14,15,16) from the plurality of rotational positions; and detecting the collimated radiation (11,12,13,14,15,16) after it has traversed through the subject (4,44).

8. Method according to claim 7, wherein the collimation is controlled such that radiation (6), that would not traverse through the volume of interest (5,45), is at least partially blocked.

9. Method according to claim 7, further comprising the step of moving the subject (4,44) in the axial direction of the subject (4,44) with respect to the imaging device (100,200) such that each of the plurality of rotational positions is assigned to one of at least two different axial positions of the subject (4,44).

10. Method according to claim 9, further comprising the steps of : computing control data from the plurality of rotational positions and the at least two axial positions of the subject (4,44) to be used in the irradiating step; and using the control data in the step of controlling the collimation of the radiation (6).

11. Method according to claim 9, wherein the step of controlling the collimation of the radiation (6) aligns the collimated radiation (11,12,13,14,15,16) with an axial boundary side (31,32) of the volume of interest (5,45) for at least one axial position of the subject (4,44).