Patent Application
20100080357
This disclosure relates generally to an x-ray tube and a CT system with multiple target surfaces.
Typically, in a computed tomography system or CT system, an x-ray tube emits a fan-shaped x-ray beam or a cone-beam shaped x-ray beam toward a subject or object positioned on a table. The beam, after being attenuated by the subject, impinges upon a detector assembly. The intensity of the attenuated x-ray beam received at the detector assembly is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector assembly produces a separate electrical signal indicative of the attenuated x-ray beam received.
In known third generation CT systems, the x-ray source and the detector assembly are rotated on a gantry around the object to be imaged so that a gantry angle at which the fan-shaped or cone-shaped x-ray beam intersects the object constantly changes. The table supporting the subject may be advanced while the gantry is rotating around the object being imaged. Data representing the strength of the received x-ray beam at each of the detector elements is collected across a range of gantry angles. The data are ultimately reconstructed to form an image of the object.
For third generation CT systems, it is advantageous to have a large field-of-view for certain procedures. For example, a large field-of-view allows for the collection of data in fewer gantry revolutions, which leads to a quicker acquisition time. Typically, manufacturers of CT systems have increased the size of the field-of-view in a z-direction by increasing the width of the detector assembly. However, a conventional CT system with a single x-ray source and a wide detector assembly may have to overcome limitations caused by a cone-beam artifact for wide detector assemblies. Also, the width of the field-of-view is typically significantly narrower than the width of the detector assembly, which may lead to exposing the subject to x-ray dose that does not contribute to the formation of the image. Additionally, a wide detector represents a significant increase in the cost of the CT system. For these and other reasons, an alternate solution for providing a wider field-of-view in a CT system is desired.
The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification.
In an embodiment, an x-ray tube includes an anode assembly adapted to rotate generally about a rotational axis. The anode assembly includes a first target surface at least partially disposed at a first angle greater than 70 degrees with respect to the rotational axis and a second target surface at least partially disposed at a second angle greater than 70 degrees with respect to the rotational axis. The first target surface is adapted to emit a first x-ray beam and the second target surface is adapted to emit a second x-ray beam.
In an embodiment, a CT system includes a gantry, a detector assembly mounted to the gantry, and an x-ray tube mounted to the gantry generally across from the detector assembly. The x-ray tube includes an anode assembly adapted to rotate generally about a rotational axis. The anode assembly includes a first target surface at least partially disposed at a first angle between 70 and 88 degrees with respect to the rotational axis and a second target surface at least partially disposed at a second angle between 70 and 88 degrees with respect to the rotational axis. The first target surface is adapted to emit a first x-ray beam and the second target surface is adapted to emit a second x-ray beam.
Various other features, objects, and advantages of the invention will be made apparent to those skilled in the art from the accompanying drawings and detailed description thereof.
In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken as limiting the scope of the invention.
Referring to
The first anode 36 and the second anode 38 are made from a material designed to emit x-rays when bombarded with electrons. One such material is tungsten, but many other materials may be used as is well-known by those skilled in the art. The first anode 36 is shaped to define a first target surface 42 that is designed to be hit by electrons in order to emit a plurality of x-rays. According to an embodiment, the first anode 36 is shaped so the first target surface 42 is at a first angle a with respect to the rotational axis 40 as indicated by a first dashed line 44 that is tangential to the first target surface 42.
The second anode 38 is shaped to define a second target surface 46 that is also designed to be hit by electrons in order to emit a plurality of x-rays. The second anode 38 is displaced in the z-direction from the first anode 36 as indicated by a coordinate axis 31. In a manner similar to the first anode 36, the second anode 38 is shaped so the second target surface 46 is at a second angle β with respect to the rotational axis 40 as indicated by a second dashed line 48 that is tangential to the second target surface 46.
According to other embodiments, a first anode could be shaped so that a first target surface is disposed at plurality of angles with respect to a rotational axis and/or a second anode could be shaped so that a second target surface is disposed at a plurality of angles with respect to the rotational axis. For embodiments where the first target surface is disposed at a plurality of angles with respect to the rotational axis, at least a portion of the first target surface may be disposed at a first angle greater than 70 degrees with respect to a rotational axis. Likewise, for embodiments where the second target surface is disposed at a plurality of angles with respect to the rotational axis, at least a portion of the second target surface may be disposed at a first angle greater than 70 degrees with respect to a rotational axis. According to other embodiments, a first anode may be tapered in a generally linear manner or in both a generally curved manner and a generally linear manner to define a first target surface and/or a second anode may be tapered in generally linear manner or in both a generally curved manner and a generally linear manner.
According to the embodiment shown in
Still referring to
According to an embodiment, the electron source 30 may be configured to emit the electron beam 54 at multiple kinetic energy levels. For example, the electron source 30 may emit the electron beam 54 at a first kinetic energy level during a portion of a scan and at a second kinetic energy level during a different portion of the scan. The energy level of the x-rays produced when the electron beam 54 contacts either the first target surface 42 or the second target surface 46 depends on the kinetic energy level of the electron beam 54. For example, when the electron beam 54 is at a first kinetic energy level, it will produce x-rays of a first energy level. Likewise, when the electron beam 54 is at a second kinetic energy level, it will produce x-rays of a second energy level. By acquiring data with x-rays at both the first x-ray energy level and the second x-ray energy level, it is possible to get additional insight into the materials of the object 26 (shown in
The electro-magnet 34 is positioned between the electron source 30 and the target surfaces 42, 46, and the electromagnet 34 is configured to generate an electromagnetic field when energized with an electrical current. The electron beam 54 generated by the electron source 30 travels through the electromagnetic field created by the electro-magnet 34. By adjusting the electrical current traveling through the electro-magnet 34, the path of the electron beam 54 can be adjusted as is well-known by those skilled in the art. For example, the electromagnet 34 is configured to cause the electron beam 54 to change direction and follow a first path 56 so the electron beam 54 contacts the first target surface 42. A percentage of the electrons in the electron beam 54 will interact with the first target surface 42, forming a first x-ray beam 58 that is emitted toward the detector assembly 20 (shown in
Still referring to
When the electron beam 54 follows the first path 56, the first x-ray beam 58 is generated. When the electron beam 54 follows the second path 60, the second x-ray beam 62 is generated. The electromagnet 34 may cause the electron beam 54 to oscillate between the first target surface 42 and the second target surface 46 hundreds or thousands of times during a single scan. By alternating between acquiring data with the first x-ray beam 58 and acquiring data with the second x-ray beam 62, it is possible to acquire data corresponding to a field-of-view that is wider in the z-direction. It should be appreciated that the electromagnet 34 may cause the electron beam 54 to transition from the first target surface 42 to the second target surface 46 according to a different control scheme. For example, according to an embodiment, the electron beam 54 may spend a different amount of time on either the first target surface 42 or the second target surface 46. Additionally, the electron beam 54 may transition between the first target surface 42 and the second target surface 46 in either more or less time than 5 μS.
According to another embodiment, the electro-magnet 34 may be configured to move the electron beam 54 so that it oscillates between contacting a first position 63 and a second position 64 on the first target surface 42. The first x-ray beam 58 will originate from the position where the electron beam 54 contacts the first target surface 42. Since the first position 63 is displaced from the second position 64 in the z-direction, causing the electron beam 54 to oscillate between the first position 63 and the second position 64 on the first target surface 42 may permit the acquisition of CT data with higher resolution in the z-direction. This technique is sometimes referred to as z-wobbling. According to additional embodiments, it would also be possible to perform z-wobbling when the electron beam 54 is contacting the second target surface 46 of the second anode 38 in a similar manner to that described above for when the electron beam 54 is contacting first target surface 42.
Still referring to
There is a desire to have the electron beam 54 contact a larger portion of the first target surface 42 in order to avoid overheating the first anode 36. Since the first target surface 42 is disposed at the first angle α with respect to the rotational axis 40, it is possible to have the electron beam 54 contact an area of the first target surface 42 that is longer than the width of a focal spot of the x-ray beam 58 in the z-direction. However, it may not be desirable for the first angle α to be too close to 90 degrees because the heel effect may cause the first x-ray beam 58 to vary in intensity in the z-direction. It should be appreciated that while the this paragraph described the first angle α, the same logic may be applied to the second angle β of the second target surface 46.
Therefore, it has been determined that having a first target surface 42 and a second target surface 46 each at least partially disposed at an angle of 70 to 88 degrees, or more specifically, 75 to 85 degrees with respect to an axis of rotation may yield an effective compromise between the need to spread an electron beam over a larger portion of a target surface and the need to minimize the heel effect. An embodiment includes a first target surface and a second target surface each disposed at an angle greater than 70 degrees with respect to an axis of rotation. An embodiment includes a first target surface and a second target surface each disposed at an angle greater than 75 degrees with respect to an axis of rotation. An embodiment includes a first target surface and a second target surface each disposed at an angle greater than 80 degrees with respect to an axis of rotation. An embodiment includes a first target surface and a second target surface each disposed at an angle between 70 degrees and 88 degrees with respect to an axis of rotation. An embodiment includes a first target surface and a second target surface each disposed at an angle that is between 75 degrees and 85 degrees with respect to an axis of rotation.
Referring to
The second anode 80 is tapered in a generally curved manner to define a second target surface 94 that is designed to emit a second x-ray beam 95 upon being struck by a second plurality of electrons 96 from the second electron source 72. The second anode 80 is adapted to rotate about the rotational axis 84. A second dashed line 97 is tangential to the second target surface 94. The second dashed line 97 makes a second angle δ with respect to the rotational axis 84. The second angle δ of the second target surface 94 with respect to the rotational axis 84 varies based on position in the z-direction. For example, according to the embodiment illustrated in
According to an embodiment, the first electron source 70 comprises a first filament 98, a first current source 100, and a first control grid 102. The first filament 98 is indirectly heated by the first current source 100, causing it to emit the first plurality of electrons 90. The first high-voltage power supply 74 creates a potential difference between the first filament 98 and the first target surface 86, causing the first plurality of electrons 90 to be accelerated toward the first target surface 86. The first control grid 102 partially surrounds the first filament 98 and is connected to the first high-voltage power supply 74. The first control grid 102 is used to control or limit the flow of the first plurality of electrons 90 from the first filament 98. For example, if the first control grid 102 is kept at a high enough negative potential, all of the first plurality of electrons 90 are prevented from being accelerated towards the first target surface 86. According to an embodiment, a first plurality of electrons may form an electron beam.
According to an embodiment, the second electron source 72 comprises a second filament 104, a second current source 106, and a second control grid 108. The second electron source 72 is connected to the second high-voltage power supply 76 and functions in a manner similar to that of the first electron source 70 described previously.
The first electron source 70 and the second electron source 72 may be configured to be alternately activated. For example, at times when the first control grid 102 allows the first plurality of electrons 90 to contact the first target surface 86, the second control gird 108 is keep at a potential that does not allow any of the second plurality of electrons 96 to contact the second target surface 94. Likewise, at times when the second control grid 108 allows the second plurality of electrons 96 to contact the second target surface 94, the first control grid 102 is kept at a potential that does not allow any of the first plurality of electrons 90 to contact the first target surface 86. According to an embodiment, a separate circuit (not shown) may be attached to the first control grid 102 and the second control grid 108 in order to accurately control the potentials of the control grids 102, 108 in order to facilitate the rapid switching between the first x-ray beam 88 and the second x-ray beam 95. For example, according to an embodiment, the separate circuit may be configured to switch back and forth between activating the first x-ray beam 88 and activating the second x-ray beam 95 more than one thousand times per second.
According to an embodiment, the first electron source 70 may be configured to emit the first plurality of electrons 90 at two or more kinetic energy levels. If the first plurality of electrons 90 comprises electrons at a lower kinetic energy level, then the first x-ray beam 88 will comprise lower energy x-rays. Likewise, if the first plurality of electrons 90 comprises electrons at a higher kinetic energy level, then the first x-ray beam 88 will comprise higher energy x-rays. By acquiring data with x-rays at two or more energy levels, it is possible to get additional insight into an object being scanned. The first electron source 70 may be configured to rapidly switch between emitting the first plurality of electrons 90 at the lower kinetic energy level and emitting the first plurality of electrons 90 at the higher kinetic energy level many times during one gantry rotation. According to another embodiment, the first electron source 70 may be configured to emit the first plurality of electrons 90 at the lower kinetic energy level while one dataset is acquired and then emitting the first plurality of electrons 90 at the higher kinetic energy level while another dataset is acquired. It should be appreciated that the second x-ray source 72 may also be configured to emit the second plurality of electrons 96 at two or more kinetic energy levels in a manner similar to that described for the first electron source 70.
The spacing between a first target surface and a second target surface may depend upon the geometry of a particular CT system. For example, an embodiment may have a first target surface displaced more than 3 cm away from a second target surface in a z-direction. An embodiment may have a first target surface displaced more than 6 cm away from a second target surface in a z-direction. An embodiment may have a first target surface displaced between 4 cm and 30 cm from a second target surface in a z-direction. An embodiment may have a first target surface displaced between 6 cm and 12 cm from a second target surface in a z-direction.
According to other embodiments, an anode may be shaped to define both a first target surface and a second target surface. The anode would be rotatable about a rotational axis and the first and second target surfaces would be spaced apart in a z-direction. Each target surface may be disposed at a generally constant angle with respect to the rotational axis, or each target surfaces may be disposed at a plurality of angles with respect to the rotational axis.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.
