US 20060182223 A1
A computed tomography imaging system includes an x ray tube (12, 212) that injects an x ray conebeam into an examination region (14). The x ray tube (12, 212) includes a rotating cylindrical anode (30, 230, 330, 430) having a target outer surface region. The cylindrical anode (30, 230, 330, 430) rotates about a longitudinally aligned cylinder axis (32). Electrons are accelerated toward a selected spot on the target outer surface region of the cylindrical anode (30, 230, 330, 430). Electrostatic or electromagnetic deflectors (64, 68) sweep the selected spot back and forth across the target outer surface region of the cylindrical anode (30, 330, 430). The imaging system further includes a rotating gantry (22) that revolves the x ray tube (12, 212) about the examination region (14) around a rotation axis that is parallel to the cylindrical axis, and an x-ray detector (16) arranged to detect x rays after said x rays pass through the examination region (14).
1. An x-ray tube that injects an x-ray conebeam into an examination region, the x-ray tube including:
a rotating cylindrical anode having a target outer surface region, the cylindrical anode rotating about a longitudinally aligned cylinder axis;
an electron accelerating means for accelerating electrons toward at least one selected spot on the target outer surface region of the cylindrical anode to generate x-rays; and
a sweep means for relatively longitudinally sweeping the at least one selected spot across the target outer surface region of the cylindrical anode.
2. The x-ray tube as set forth in
a central supporting cylinder; and
a metallic layer at least a portion of which defines the target outer surface region.
3. The x-ray tube as set forth in
an outer shell defining a hollow cylinder core; and
at least one structural support member disposed in the hollow cylinder core, the at least one structural support member mechanically coupled to an associated rotating shaft.
4. The x-ray tube as set forth in
5. The x-ray tube as set forth in
a substantially solid metallic cylinder at least a portion of an outer surface of said solid metallic cylinder defining the target outer surface region of the cylindrical anode.
6. The x-ray tube as set forth in
a substantially hollow outer cylindrical shell; and
at least one structural support member disposed in the substantially hollow outer cylindrical shell, the at least one structural support member mechanically coupled to an associated rotating shaft.
7. The x-ray tube as set forth in
an electron deflector that selectively deflects the electron beam to sweep the at least one selected spot across the target outer surface region of the cylindrical anode.
8. The x-ray tube as set forth in
a rotating cylindrical helical-slot collimator having a helical collimating slot formed therein, the collimator surrounding the rotating cylindrical anode and rotating about a collimator axis parallel to the cylinder axis, a helical pitch of the helical collimating slot and a rotation rate of the collimator being selected relative to the sweep of the at least one selected spot such that the at least one selected spot coincides with the helical-slot during the sweeping.
9. The x-ray tube as set forth in
an evacuated frame that surrounds the rotating cylindrical anode the rotating cylindrical helical-slot collimator being arranged outside of the evacuated frame.
10. The x-ray tube as set forth in
an outer cylindrical shell surrounding the rotating cylindrical anode and aligned with the collimator axis, the outer cylindrical shell having a first helical slot defined therein; and
an inner cylindrical shell surrounding the rotating cylindrical anode and disposed inside the outer cylindrical shell, the inner cylindrical shell being aligned with the collimator axis and having a second helical slot defined therein that aligns with the first helical slot, the first and second helical slots cooperatively defining the helical collimating slot.
11. The x-ray tube as set forth in
12. The x-ray tube as set forth in
a fixed axially limiting collimator that axially limits the x-rays.
13. The x-ray tube as set forth in
a longitudinal reciprocating mechanism longitudinally reciprocating the cylindrical anode to effect a longitudinal reciprocating sweep of the at least one selected spot across the target outer surface region of the cylindrical anode.
14. A CT scanner including:
a rotating gantry which rotates around an examination region and an axis of revolution, the x-ray tube of
an x-ray detector arranged to detect x-rays after the x-rays pass through the examination region; and
a reconstruction processor for reconstructing output signals from the x-ray detector into an image representation.
15. The computed tomography imaging system as set forth in
a synchronization circuit that synchronizes the sweep with rotation of the rotating gantry.
16. The computed tomography imaging system as set forth in
a rotating cylindrical helical-slot collimator Shaving a helical collimating slot formed therein, the collimator surrounding the rotating cylindrical anode and rotating about a collimator axis that is parallel to the cylinder axis, a helical pitch of the helical collimating slot and a rotation rate of the collimator being selected relative to the sweep of the at least one selected spot such that the at least one selected spot coincides with the helical-slot during the sweeping.
17. A method of generating x-rays including:
rotating a cylindrical anode about a cylinder axis, the cylindrical anode having a cylindrical target outer surface region;
accelerating electrons toward at least one selected spot on the target outer surface region of the cylindrical anode to generate x-rays; and
relatively sweeping the at least one selected spot continuously across the target outer surface region of the cylindrical anode along a beam trajectory substantially parallel to the cylinder axis.
18. The method as set forth in
steering at least one electron beam defined by the accelerated electrons longitudinally across the cylindrical anode.
19. The method as set forth in
fast-retracing the at least one electron beam to return to a longitudinal sweep starting point subsequent after each longitudinal sweep across the cylindrical anode.
20. The method as set forth in
longitudinally reciprocating the cylindrical anode to effect longitudinal reciprocating sweeping of the at least one selected spot on the target outer surface region of the cylindrical anode.
21. The method as set forth in
rotating a helical-slot collimator around a collimator axis that is parallel to the cylinder axis; and
sweeping the at least one selected spot in coordination with rotating the helical-slot collimator.
22. The method as set forth in
23. The method as set forth in
24. The method as set forth in
rotating the cylindrical anode around an axis of rotation, the axis of rotation being parallel to the cylindrical axis;
sweeping the at least one selected spot in coordination with the rotating;
detecting the x-rays which have passed through a subject along the axis of rotation;
converting the detected x-rays into an image of the subject.
The following relates to the radiation generation arts. It finds particular application in x-ray computed tomography imaging, and will be described with particular reference thereto. However, it also finds application in other arts that employ x-ray tubes or other radiation sources.
In transmission computed tomography, an x-ray tube mounted on a rotating gantry injects an x-ray beam into an examination region defined by the rotating gantry. After passing through the examination region and being partially absorbed with an imaging subject disposed therein, the absorption-attenuated x-ray beam is measured by an x-ray detector array. The rotating gantry rotates to acquire angular views of the imaging subject angularly spanning about 180° or more. In helical scanning, the rotating gantry rotates continuously as the subject is moved or reciprocated longitudinally. An image reconstruction processor employs filtered backprojection or another reconstruction technique to produce a reconstructed volume image based on the acquired x-ray measurements of the slab defined by the longitudinal reciprocation, typically 2-20 cm.
Characteristics of the x-ray tube can limit the performance of the computed tomography imaging scanner in various ways. Gantry rotation speed is limited by x-ray intensity. The rotation should be slow enough to provide adequate time-integrated signal intensity over each angular viewing interval. Hence, higher x-ray output intensity can translate to faster gantry rotation rates and improved spatial and temporal resolution.
The x-ray intensity generated by an x-ray tube is typically thermally limited. A peak temperature is reached at a spot where accelerating electrons strike the anode surface. The x-ray tube anode is generally disk-shaped and rotated to distribute heating across a target track near an outer diameter of the anode disk. Anode thermal characteristics are suitably quantified in terms of a peak temperature at the x-ray generation spot (this spot moves around the target track as the anode rotates) and a base temperature corresponding to an elevated temperature of the anode as a whole due to thermal dissipation into the anode. Anode rotation and other thermal design techniques provide some control of anode heating; however, anode heating still commonly constrains the x-ray output intensity.
In conebeam computed tomography, the cone angle is also generally limited by the x-ray tube. The target track along which the x-ray generation spot travels is beveled or otherwise arranged at a shallow anode target angle respective to the incident electron beam. The anode target angle is about 7°-10° in present x-ray tubes. Shallower angles are impractical due to the heel effect. The target angle limitation, in turn, imposes a limit on the maximum cone angle. This cone angle limitation is particularly problematic in cone beam computed tomography scanners for which a larger cone angle enables greater coverage and faster volumetric imaging.
Resolution uniformity is also limited by the x-ray tube. The heel effect limits the anode target angle to about 7°-10°, which in turn introduces substantial resolution anisotropy due to an elongated electron beam footprint on the disk anode.
The present invention contemplates an improved apparatus and method that overcomes the aforementioned limitations and others.
According to one aspect, a computed tomography imaging system includes an x-ray tube that injects an x-ray conebeam into an examination region. The x-ray tube includes a rotating cylindrical anode having a target outer surface region. The cylindrical anode rotates about a longitudinally aligned cylinder axis. An electron accelerating means accelerates electrons toward at least one selected spot on the target outer surface region of the cylindrical anode. A sweep means relatively longitudinally sweeps the at least one selected spot across the target outer surface region of the cylindrical anode. The imaging system further includes a revolving means for revolving the x-ray tube about the examination region, and an x-ray detector arranged to detect x-rays after said x-rays pass through the examination region.
According to another aspect, a computed tomography imaging method is provided. An x-ray tube is revolved about an examination region. The x-ray tube is operated to inject an x-ray beam into the examination region, The operating of the x-ray tube includes: rotating a cylindrical anode about a cylinder axis, the cylindrical anode having a target outer surface region; accelerating electrons toward at least one selected spot on the target outer surface region of the cylindrical anode; and relatively sweeping the at least one selected spot across the target outer surface region of the cylindrical anode along a beam trajectory substantially parallel to the cylinder axis. X-rays are detected after said x-rays pass through the examination region.
One advantage resides in providing a larger cone angle that is not limited by the heel effect.
Another advantage resides in providing improved image resolution uniformity.
Yet another advantage resides in improved thermal characteristics of an x-ray tube by distributing heating across a cylindrical anode.
Still yet another advantage resides in obtaining an axial scanning component by sweeping the x-ray beam or beams rather than by moving a subject support couch.
Numerous additional advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments.
The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for the purpose of illustrating preferred embodiments and are not to be construed as limiting the invention.
With reference to
A subject couch or other subject support 20 is linearly movable in a z-direction as indicated in
With continuing reference to
An electron source 54, such as a heated filament or film, disposed in a cathode cup 56 generates electrons. The cathode cup 56 is shaped and electrically biased relative to the cylindrical anode 30 to accelerate and focus the generated electrons toward a selected focal spot 58 on a target outer surface region of the cylindrical anode 30. The accelerated electrons are focused to define an electron beam 60. An electron deflector including biased electrodes 64, 68 electrostatically or electromagnetically deflect the electron beam 60 to sweep the selected spot 58 along a longitudinal sweep trajectory indicated by thick arrow 70. At the end of the sweep 70 a fast retrace indicated by thin arrow 72 returns the selected spot 58 to the initial point of the sweep trajectory 70 to complete a sweep cycle. Alternatively, the selected spot 58 can be swept back in the opposite direction in a reciprocating fashion.
With continuing reference to
With continuing reference to
The fan angle α of the conebeam 76 is related to an angle β of the electron beam 60 relative to a direction toward the imaging subject (where at β=0° the electron beam 60 strikes the anode perpendicularly to the direction of the source distance S) in that β>α. Thus, a larger electron beam angle β supports a larger fan angle α. This is countered, however, by a decrease in instantaneous power as the electron beam angle β increases. The electron beam angle β is preferably selected as being about 3°-4° greater than the fan angle α to maximize power output. For an exemplary source distance S=57.5 cm and a field of view Rfov=25 cm, the fan angle α should be about 26°, and the electron beam angle β is preferably about 29° to 30°. If Rfov is reduced to 12.5 cm, the fan angle α should be about 130, and the electron beam angle β is preferably about 16° to 17°.
The reduced spot length L providing improved resolution uniformity comes at a cost in x-ray power. The maximum instantaneous x-ray power output P is related to parameters of the x-ray tube 12 according to:
By distributing the heating along the beam sweep trajectory 70, the cylinder radius Ra can be reduced compared with the diameter of a conventional disk anode. Reducing the cylinder radius Ra can increase the target velocity. Although the target velocity of Equation (1) given by Ra·ω is proportional to anode radius Ra, reducing the radius Ra also reduces the moment of inertia of the cylindrical anode, which is proportional to mRa 2 where m is the anode mass. For a solid cylindrical anode, the mass m is proportional to the anode volume which is given by πRa 2h where h is the cylinder height. Hence, the moment of inertia of the cylindrical anode 30 is proportional to Ra 4, and so the reduction in target velocity due to the reduced radius Ra can be overcome by a larger increase in rotational velocity ω due to the substantial decrease in moment of inertia with decreasing radius Ra.
With continuing reference to
To accommodate the sweep 70, the rotating helical-slot collimator 90 has a helical slot 92 of pitch Pcoll as indicated in
With continuing reference to
In another embodiment, the spacer elements 104 can be replaced by a single continuous helical spacer element having a pitch equal to that of the slot 92. With such a helical spacer element, the cylindrical shells 100, 102 can be threaded or screwed onto the helical spacer element. In yet another embodiment, the spacer elements 104 are replaced by a solid cylindrical shell of thickness dcoll made of a material that is substantially transparent to the x-rays. For example, a rigid foam cylindrical support shell can be used. Moreover, the two cylindrical shells 100, 102 can be replaced by a single-piece hollow cylindrical shell with the helical slot formed therein that has a substantial annular thickness that provides the selected collimating depth dcoll in the direction of emission of the conebeam 76.
Advantageously, the rotating cylindrical helical-slot collimator 90 can support simultaneous collimation of a plurality of axially spaced x-ray beams. As shown in
Similarly, three or more conebeams spaced by the collimator slot pitch Pcoll can be supported by including suitable corresponding filament/cathode cup generating elements. The number of x-ray beams that can be so supported is limited by geometrical constraints such as the axial extent of the anode 30, the pitch Pcoll and the axial spread of the beams at the x-ray detector 16. The latter constraint is geometrically related to the cone angle γ of the beams and the source-to-detector distance. Axial sweeping of the plurality of beams can be accomplished using the electrodes 64, 68 of the electrostatic beam deflector (see
The outer and inner cylindrical shells 100, 102 provide the collimation depth dcoll that together with the selected collimator radius ensures that x-rays from the multiple x-ray generating spots are each collimated by a single turn of the helical slot 92, so that for example the portion of the helical slot 92 that collimates the conebeam 76 does not also allow x-rays generated by the x-ray generating spot corresponding to the conebeam 76′ to pass. Moreover, a fixed axially-limiting collimator 106 is preferably provided to provide a relatively sharp and fixed cutoff (indicated by dashed lines in
With returning reference to
An anode sweep correction processor 112 receives a sweep position signal from a beam sweep controller 114 and makes suitable longitudinal position and axial angular orientations adjustments of the acquired projections to account for the axial position of the x-ray beam or beams. The corrected projection data is input to a reconstruction processor 120 that performs image reconstruction by applying three-dimensional filtered backprojection or another suitable reconstruction algorithm. The resulting reconstructed image is stored in an image memory 122, processed by a video processor 124, and displayed to a radiologist or other operator on a video monitor of a user interface 126, for example, as a slice, plurality of slices, a three-dimensional rendering, or other suitable human-viewable representation. The reconstructed image can also be printed by a graphical printer, stored in a database, transmitted over a local area network or the Internet, or otherwise processed.
Preferably, the user interface 126 also enables the radiologist or other operator to communicate with a computed tomography controller 130 to control the computed tomography scanner 10. It will be appreciated that axial movement of the conebeam relative to an imaging subject disposed on the subject support 20 can be accomplished in three ways: (i) the beam sweep 70 can be used to sweep the conebeam while the subject support 20 (and hence the imaging subject) remains stationary; (ii) a combination of the beam sweep 70 and linear movement of the imaging subject via the subject support 20 can be employed; or (iii) the subject support 20 can move the subject linearly in the axial direction while the beam sweep 70 across the cylindrical anode is turned off.
In approach (i), longitudinal scanning is accomplished solely through sweeping of the selected spot 58. Hence, the beam sweep trajectory 70 should span the imaging volume. In cardiac imaging, for example, a longitudinal scan of about 12 cm is typically desired, and so the beam sweep trajectory 70 should span at least 12 cm. Moreover, the longitudinal beam sweep should be coordinated with angular rotation of the gantry 22 to ensure sufficient angular coverage for each voxel in the imaged volume. Preferably, a synchronization circuit 132 receives the rotating gantry angular position as an input, and outputs a synchronization signal to the beam sweep controller 114 to coordinate the revolving of the x-ray source 12 and the longitudinal scanning in the data acquisition.
In approach (ii), longitudinal scanning is accomplished by a combination of longitudinal beam sweep and longitudinal motion of the subject support 20. In this case, the beam sweep trajectory 70 can be less than the longitudinal length of the imaging volume. In one contemplated variation, the sweep trajectory 70 is relatively small, the beam sweep speed vspot is substantially higher than the longitudinal motion of the subject support 20, and the beam sweep is not relied upon to provide longitudinal data acquisition coverage, but rather only to provide heat distribution across the cylindrical anode 30. In this variation, the synchronization circuit 132 does not synchronize the beam sweep with gantry rotation. In other contemplated variations, the beam sweep trajectory 70 contributes to the longitudinal component of the scanning and is coordinated by the synchronization circuit 132 with the gantry rotation to provide selected angular coverage characteristics in the data acquisition. U.S. patent application Ser. No. 10/280,734 by Heuscher, filed Oct. 25, 2002, describes suitable longitudinal sweep/gantry rotation synchronization schemes for obtaining optimized temporal and/or spatial resolution using approaches (i) and (ii). In another variation, the beam sweep and the support movement are the same such that the beam remains stationary relative to the subject while still distributing heat across the anode.
The third approach (iii) in which the beam sweep 70 is turned off operates as a conventional computed tomography system having a fixed axial position of the x-ray source relative to the x-ray detector. In approach (iii), the anode sweep correction processor 112 is obviated. A substantial thermal disadvantage arises with approach (iii), however, because without sweeping the x-ray generating spot 58 follows a rapidly repeating circular path around the cylindrical anode 30. Most of the surface of the cylindrical anode 30 is unused, and so heat is not distributed across the cylindrical anode.
With reference to
A filament 254 and a cathode cup 256 (which are similar in operation to the filament 54 and cup 56 of the x-ray tube 12) generate an electron beam 260 that strikes the cylindrical anode 230 to define an x-ray generating spot. Unlike the x-ray tube 12, there is no beam deflector for sweeping the electron beam longitudinally across the anode 230. Rather, to provide heat distribution across the anode without shifting the focal spot, the anode 230 reciprocates relative to the electron beam 260. Reciprocation of the anode 230 is suitably effected using one or more solenoid windings 270 disposed outside the evacuated frame 236 that produce magnetic fields interacting with the motor rotor windings 250 and/or the drive shaft 240 to effect longitudinal motion of the drive shaft 240 and the attached cylindrical anode 230 and end shaft 242. It will be noted that both the motor stator windings 252 and the solenoid windings 270 extend over a longitudinal distance substantially corresponding to the distance of anode reciprocation. In other words, the motor rotor windings 250 remain inside the motor stator windings throughout the reciprocating. The x-ray tube 212 does not have a sweeping focal spot, and so there is no cylindrical collimator. Rather, an x-ray emission window 278 is formed into the evacuated frame 236 and provides collimation in both the fan and cone angle directions. Alternatively or in addition, an external pinhole, slat, or other collimator can be arranged outside the evacuated frame 236 to provide collimation.
Back-and-forth reciprocation of the cylindrical anode 230 provides heat distribution across the anode surface. This motion is independent of any longitudinal motion of helical scanning, and is preferably optimized to provide uniform heat distribution
With reference to
Although only a single filament 254 and cathode cup 256 are shown in
To mechanically balance the axial reciprocating motion of the anode 230 on the rotating gantry 22, one or more reciprocating counterweights (not shown) are optionally provided. In one contemplated balancing configuration, two reciprocating counterweights are arranged on opposite sides of the x-ray tube 212, with each counterweight having one-half of the combined mass of the reciprocating anode 230 and shafts 240, 242 and having axial reciprocating motion in opposition to the reciprocating motion of the anode 230.
The cylindrical anodes 30, 230 are shown as solid cylinders which are preferably made of a high x-ray yield material, that is, a material that produces a high x-ray yield responsive to irradiation by electrons of the electron beam 60, 260. Moreover, the material should be a refractory metal or other material capable of withstanding the instantaneous peak temperature Tpeak at the point where the x-rays are being generated as well as the elevated base background temperature Tbase of the cylindrical anode during operation. The drive shaft 40, 240 and end shaft 42, 242 are secured to the anode 30, 230. Alternatively, the drive shaft 40, 240 and end shaft 42, 242 form a single unitary shaft, and the anode 30, 230 has a hollow center through which the single unitary shaft passes. A disadvantage of the solid anode 30, 230 is that the high x-ray yield material may be non-optimal in terms of density or thermal conductivity. For example, tungsten is relatively dense, which increases the moment of inertia of the cylindrical anode 30, 230, making high speed anode rotation more difficult.
With reference to
With reference to
The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.