Patent Application
20050089145
This invention relates generally to imaging systems and more particularly to systems and methods for reducing radiation dosage incident on a subject.
A third generation computed tomography (CT) scanner includes an x-ray source and a detector that are rotated together around a patient. An x-ray beam is passed through the patient and intensity of the x-ray beam is measured on the detector. In some CT imaging systems, an x-ray tube is used to create the x-rays. X-rays are produced when electrons are accelerated against a focal spot or an anode by a high voltage difference between the anode and a cathode of the x-ray tube. These x-rays typically diverge conically from the focal spot, and the diverging x-ray beam is typically passed through a pre-patient collimator to define an x-ray beam profile on the detector. Some CT imaging systems include detector cells arranged on an arc of constant radius from the source. If the collimator is linear, or rectangular, an x-ray beam profile on the detector will become curved along a fan of the detector as an aperture of the collimator is opened along a z-axis. The curvature can result in both unused x-ray dose and degradation in a CT image formed from the curved x-ray beam profile.
In one aspect, an imaging system is provided. The imaging system includes a radiation source configured to generate a beam, a collimator configured to collimate the beam to generate a collimated beam, and a detector configured to detect the collimated beam. The collimator is one of a first collimator with a curved contour proportional to a contour of the detector, a second collimator with blades, where slopes of two oppositely-facing surfaces of at least one of the blades are different from each other, and a third collimator having at least two sets of plates, where the plates in a set pivot with respect to each other.
In another aspect, a computed tomography imaging system is provided. The computed tomography imaging system includes an x-ray source configured to generate a beam, a collimator configured to collimate the x-ray beam to generate a collimated x-ray beam, and a detector configured to detect the collimated x-ray beam. The collimator is one of a first collimator with a curved contour proportional to a contour of the detector, a second collimator with blades, where slopes of two oppositely-facing surfaces of at least one of the blades are different from each other, and a third collimator having at least two sets of plates, where the plates in a set pivot with respect to each other.
In yet another aspect, a method for reducing dosage of radiation incident on a subject is provided. The method includes transmitting a beam of radiation toward the subject, collimating the beam of radiation before the beam reaches the subject, and detecting the collimated beam of radiation. The collimating is performed by one of a first collimator with a curved contour proportional to a contour of a detector that detects the collimated beam, a second collimator with blades, where slopes of two oppositely-facing surfaces of at least one of the blades are different from each other, and a third collimator having at least two sets of plates, where the plates in a set pivot with respect to each other.
In some known CT imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The x-ray beam passes through an object, such as a patient, being imaged. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of an x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In third generation CT imaging systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged such that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a “helical” scan may be performed. To perform a “helical” scan, the object is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Reconstruction algorithms for helical scanning typically use helical weighing algorithms that weight the collected data as a function of view angle and detector channel index. Specifically, prior to a filtered backprojection process, the data is weighted according to a helical weighing factor, which is a function of both the gantry angle and detector angle. The helical weighting algorithms also scale the data according to a scaling factor, which is a function of the distance between the x-ray source and the object. The weighted and scaled data is then processed to generate CT numbers and to construct an image that corresponds to a two dimensional slice taken through the object.
As used herein, an element or step recited in the singular and preceded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.
Also as used herein, the phrase “reconstructing an image” is not intended to exclude embodiments of the present invention in which data representing an image is generated but a viewable image is not. However, many embodiments generate (or are configured to generate) at least one viewable image.
Referring to
Rotation of gantry 12 and the operation of x-ray source 14 are governed by a control mechanism 26 of CT imaging system 10. Control mechanism 26 includes an x-ray controller 28 that provides power and timing signals to x-ray source 14 and a gantry motor controller 30 that controls the rotational speed and position of gantry 12. A data acquisition system (DAS) 32 in control mechanism 26 samples analog data from detector elements 20 and converts the data to digital signals for subsequent processing. An image reconstructor 34 receives sampled and digitized x-ray data from DAS 32 and performs high-speed image reconstruction. The reconstructed image is applied as an input to a computer 36 which stores the image in a mass storage device 38.
Computer 36 also receives commands and scanning parameters from an operator via console 40 that has a keyboard. An associated cathode ray tube display 42 allows the operator to observe the reconstructed image and other data from computer 36. The operator supplied commands and parameters are used by computer 36 to provide control signals and information to DAS 32, x-ray controller 28 and gantry motor controller 30. In addition, computer 36 operates a table motor controller 44 which controls a motorized table 46 to position object 22 in gantry 12. Particularly, table 46 moves portions of object 22 through gantry opening 48.
In one embodiment, computer 36 includes a device 50, for example, a floppy disk drive, CD-ROM drive, DVD drive, magnetic optical disk (MOD) device, or any other digital device including a network connecting device such as an Ethernet device for reading instructions and/or data from a computer-readable medium 52, such as a floppy disk, a CD-ROM, a DVD or an other digital source such as a network or the Internet, as well as yet to be developed digital means. In another embodiment, computer 36 executes instructions stored in firmware (not shown). Computer 36 is programmed to perform functions described herein, and as used herein, the term computer is not limited to just those integrated circuits referred to in the art as computers, but broadly refers to computers, processors, microcontrollers, microcomputers, programmable logic controllers, application specific integrated circuits, and other programmable circuits, and these terms are used interchangeably herein.
Furthermore, portions 76 and 78 may also generate artifacts on an image reconstructed from x-ray beam profile 74. A distance 80 between focal point 60 and collimator 62 corresponds to a point 82 on x-ray beam profile 74 and a distance 84 between focal point 60 and collimator 62 corresponds to a point 86 on x-ray beam profile 74. Distance 80 is shorter than distance 84 as a result of which the artifacts are created. Moreover, as width of aperture 70 of collimator 62 is further increased along the z-axis, an x-ray beam profile 88 is formed with shaded portions 90 and 92 that introduce a higher amount of artifacts than those introduced by x-ray beam profile 74.
When collimator 62 includes a tapered or a sloped aperture 94 with a slope, for instance, along the x-axis, the projection of collimated beam 64 forms an x-ray beam profile 96 on detector array 18. The taper of aperture 94 is set so that x-ray beam profile 96 is rectangular for a pre-determined size, such as a width along the z-axis, of aperture 94. Moreover, the taper of aperture 94 can be varied to optimize the taper for various sizes of aperture 94. However, it is difficult to manufacture aperture 94 having a variable taper because a level of smoothness of surfaces of aperture 94 cannot be achieved easily. If an x-ray beam profile 98 is generated by collimating x-ray beam 16 with collimator 62 not having the level of smoothness, x-ray beam profile 98 includes shaded portions 100, 102, 104, and 106. Portions 100, 102, 104, and 106 introduce artifacts in images generated from x-ray beam profile 98.
Moreover, as size of aperture 94 of collimator 62 is increased, an x-ray beam profile 108 with shaded portions 110 and 112 is generated. Portions 110 and 112 have a larger area than area of portions 100, 102, 104, and 106. Portions 110 and 112 introduce more artifacts in an image generated from x-ray beam profile 108 than artifacts introduced in an image generated from x-ray beam profile 98. The introduction of more artifacts with an increase in the size of aperture of collimator renders it difficult to provide an adequate range of sizes of aperture 94 of collimator 62. Furthermore, as size of apertures 70 and 94 is increased, the mass of collimator 62 used to absorb x-ray beam 16 becomes excessive.
X-ray source 14 transmits x-ray beam 16 towards collimator 122. Collimator 122 collimates or restricts x-ray beam 16 to generate a collimated beam 126. Collimated beam 126 falls on detector elements 20 and generates an x-ray beam profile 128. X-ray beam profile 128 is a projection of collimated beam 126. Curvature of x-ray beam profile 128 is minimal for all sizes, such as widths, of apertures formed by the cams of collimator 122.
A radius of curvature of collimator 122 is proportional to a radius of curvature of detector array 18. As an example, a radius of curvature of detector array 18 at a point 130 is x+y centimeters (cm), where x is a radius of curvature of collimator 122 at a distance 132 from focal point 60, and where x and y are real numbers greater than zero. In this example, a radius of curvature of detector array 18 at a point 134 is m+y cm, where m is a radius of curvature of collimator 122 at a distance 136 from focal point 60, and where m is a real number greater than zero. A radius of curvature of collimator 122 and detector array 18 is measured from focal point 60. Unlike distances 80 and 84, distance 132 is approximately equal to distance 136 because a contour of collimator 122 matches a contour of detector array 18.
Blades 152 and 154 are partially closed but do not overlap each other, as shown in an isometric view 164, to form an aperture with a large width between inner surfaces 160 and 162 of blades 152 and 154. An example of an aperture with a large width is an aperture whose x-ray beam profile has a width greater than 30 mm on detector array 18. When blades 152 and 154 are partially closed to obtain the aperture with the large width, distance between outer surfaces 156 and 158 is greater than distance between inner surfaces 160 and 162. Tapers of inner surfaces 160 and 162 can be optimized for apertures of large widths.
Alternatively, blades 152 and 154 are partially closed but do not overlap each other to form an aperture with a medium width between outer surfaces 156 and 158 of the blades. If blades 152 and 154 are in a position shown in isometric view 164, the blades are overlapped with each other and cross-over each other so that an aperture with a medium width is formed between outer surfaces 156 and 158 of the blades. An example of an aperture with a medium width is an aperture whose x-ray beam profile has a width from 1 mm to 30 mm on detector array 18. When blades 152 and 154 are partially closed to obtain the aperture with the medium width, distance between inner surfaces 160 and 162 is greater than distance between outer surfaces 156 and 158. Tapers of outer surfaces 156 and 158 can be optimized for apertures of medium widths.
In yet another alternative embodiment, blade 154 includes a slit 166 or an aperture having a small width through which x-ray beam 16 passes to form an x-ray beam profile on detector array 18. An example of an aperture with a small width is an aperture whose x-ray beam profile has a width of approximately 1 mm on detector array 18. Alternatively, cam 152 includes slit 166.
Each blade 152 and 154 is coupled to a respective shaft 168 and 170 that is coupled to a respective motor 172 and 174. Motors 172 and 174 provide rotational motion to blades 152 and 154 so that the blades can overlap and cross-over each other. Alternatively, a linear drive mechanism is used to operate blades 152 and 154. However, motors 172 and 174 have less susceptibility to wear and tear as compared to the linear drive mechanism.
Technical effects of the herein described systems and methods include reducing a curvature of an x-ray beam profile formed on detector array 18 while simultaneously supporting a wide range of apertures. For instance, collimator 150 provides apertures of large, medium, and small widths while simultaneously reducing curvature of x-ray beam profiles. It is noted that although CT imaging system 10 described herein is a “third generation” system in which both the x-ray source 14 and detector array 18 rotate with gantry 12, many other CT imaging systems including “fourth generation” systems where a detector is a full-ring stationary detector and an x-ray source rotates with the gantry, may be used. It is also noted that although a curved detector array is shown in
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
