Patent Application
20090310740
This disclosure relates generally to method and system of computed tomography that generates an image based on multiple sets of views.
Typically, in computed tomography (CT) systems, an x-ray source emits a fan-shaped x-ray beam or a cone-beam shaped x-ray beam toward a subject or object positioned on a support. The beam, after being attenuated by the subject, impinges upon a detector assembly. The intensity of the 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 x-ray beam intersects the object constantly changes. 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 processed to form an image of the object.
Known CT systems may be used to collect multiple datasets in order to show how a subject changes over a period of time. For example, the data may be used to show a portion of the subject during different phases of a cardiac cycle or to show how an injected contrast agent perfuses into the subject's tissue over time. One problem is that known CT systems may expose the subject to an x-ray dose that is higher than necessary for procedures intended to show change over a period of time.
The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification.
In an embodiment, a method of computed tomography includes acquiring a first plurality of views during a first helical pass and acquiring a second plurality of views during a second helical pass. The method also includes generating an image based on both the first plurality of views and the second plurality of views.
In an embodiment, a method of computed tomography includes rotating an x-ray source mounted to a gantry around an object. The method includes translating the object in a first direction with respect to the gantry while rotating the x-ray source. The method includes acquiring a first incomplete set of views of a volume of interest of the object while translating the object in the first direction. The method includes translating the object in a second direction with respect to the gantry that is generally opposite to the first direction while rotating the x-ray source. The method includes acquiring a second incomplete set of views of the volume of interest of the object while translating the object in the second direction and generating an image based on both the first incomplete set of views and the second incomplete set of views.
In another embodiment, a computed tomography system includes a gantry adapted to rotate around an object, an x-ray source mounted to the gantry, a table for supporting the object, wherein the table is adapted to translate the object with respect to the gantry, and a controller connected to the gantry, the x-ray source, and the table. The controller is configured to acquire a first incomplete set of views of the object during a first helical pass and a second incomplete set of views of the volume of interest of the object during a second helical pass. The controller is also configured to generate an image based on both the first incomplete set of views and the second incomplete set of views.
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.
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According to an embodiment, a pitch of the helical acquisition may remain constant during the first helical pass of step 102. For the purposes of this disclosure, the term pitch is defined to include the ratio of the relative translational movement between the gantry 14 and the object 26 in one gantry rotation to the collimator aperture width. It should be understood that the term helical pass also includes computed tomography procedures where the pitch varies. For example, according to an embodiment, the pitch of the helical pass is varied by adjusting a translational velocity of the moveable table portion 17 with respect to the gantry 14. According to other embodiments, the pitch of the helical pass may also be varied by adjusting a rotational speed of the gantry 14 or by adjusting both the translation velocity of the moveable table portion 17 with respect to the gantry 14 and the rotational speed of the gantry 14.
In order to reconstruct an image of the object 26, a complete set of views is desired. The complete set of views is a limit that varies based on the design of the CT system. The complete set of views may be mathematically calculated under an ideal and hypothetical set of operating conditions. For the purposes of this disclosure, the complete set of views is defined to include the minimum number of views per gantry rotation needed to accurately reconstruct an image of the scanned volume assuming that the transmission data in each view is exact. According to one exemplary embodiment, a complete set of views may include 1000 views per gantry rotation for a CT system with a 1:1 pitch. The exact number of views in the complete set of views may vary based on the design of the CT system and the scan parameters. For the purposes of this disclosure, an incomplete set of views includes a set of views that does not contain all of the views included in the complete set of views. According to an embodiment, the controller 22 controls the x-ray source 18 in order to collect the odd numbered views during the first helical pass at step 102.
At step 104, the method 100 determines if additional views are required. If additional views are required, the method 100 cycles back to step 102 where a second plurality of views are acquired during a second helical pass. According to an embodiment, the object 26 is translated in a second direction with respect to the gantry 14 during the second helical pass. The second direction is generally opposite to the first direction of translation of the first helical pass. According to an embodiment, the controller 22 controls the x-ray source 18 in order to collect the even numbered views during the second helical pass.
If no additional views are required at step 104, the method 100 advances to step 106 where an image is generated based on multiple sets of views. For example, according to an embodiment, an image is generated by combining the first plurality of views acquired during the first helical pass at step 102 and the second plurality of views acquired during the second helical pass at step 102. Since the first plurality of views comprises the odd numbered views and the second plurality of views comprises the even numbered views, by combining the first plurality of views and the second plurality of views, a complete set of views may be formed. According to an embodiment, the first image is generated by using a filtered back-projection algorithm as is well-known by those skilled in the art. It should be understood that other embodiments may use a different reconstruction algorithm, such as an iterative reconstruction algorithm. Also, embodiments may acquire views in a different pattern. For example, embodiments may use more than two helical passes in order to collect the complete set of views.
According to an embodiment, the method 100 may cycle between step 102 and step 104 ten times. The direction of the helical pass at step 102 alternates with each helical pass as the method 100 cycles between steps 102 and 104. The subject 26 may be advanced in a positive z-direction with respect to the gantry 14 during the even helical passes and the subject 26 may be advanced in a negative z-direction with respect to the gantry 14 during the odd helical passes. By “shuttling” the subject 26 back-and-forth, the method 100 allows for the efficient acquisition of views of a volume of interest of the subject 26. According to another embodiment, the method 100 may collect a complete set of views during the odd helical passes and a complete set of views during the even passes. Then at step 106, the method 100 is able to generate a first image based on the views collected during the even helical passes and a second image based on the views collected during the odd helical passes. This acquisition technique may be beneficial for circumstances where the acquisition of views during each helical pass is gated on a physiological function of the subject 26, such as a cardiac cycle or a respiratory cycle.
According to another embodiment, views may be acquired at two or more different x-ray energy levels during each helical pass at step 102. For example, an embodiment may alternate between acquiring one or more views at a higher x-ray energy level and one or more views at a lower x-ray energy level during each helical pass. According to an exemplary embodiment, switching between the higher x-ray energy level and the lower x-ray energy level results in a first subset of views at the higher x-ray energy level and a second subset of views at a lower x-ray energy level during each helical pass. After multiple helical passes, a complete set of views for both the higher x-ray energy level and the lower x-ray energy level will have been acquired. It should be understood that additional embodiments may acquire views at more than two x-ray energy levels during each helical pass.
Referring to
At step 206, a first incomplete set of views of a volume of interest (VOI) of the object 26 is acquired while translating the object 26 in the first direction. The first incomplete set of views is acquired by selectively activating the x-ray source 18 in order to acquire x-ray transmission data for only the desired views.
At step 208, the object 26 is translated in a second direction with respect to the gantry 14 while the x-ray source 18 is rotating around the object 26. According to an embodiment, the object 26 is translated in generally the opposite direction with respect to the gantry 14 as the object 26 was translated during step 204. At step 210, a second incomplete set of views of the volume of interest of the object 26 is acquired while translating the object in the second direction during step 208.
While acquiring the first incomplete set of views during step 206 and the second incomplete set of views during step 210, the intensity of the x-ray source 18 may be varied. The intensity of the x-ray source 18 may be varied to provide more uniform noise characteristics or to reduce the x-ray dose to which the object 26 is exposed. The intensity of the x-ray source 18 may be adjusted by altering an amount of current that is supplied to the x-ray source 18. The intensity of the x-ray source 18 may vary based on the translational velocity of the object 26 with respect to the gantry 14. According to an embodiment, the intensity of the x-ray source 18 may vary as a function of the translational velocity of the object 26 with respect to the gantry 14. According to another embodiment, the intensity of the x-ray source 18 may vary based on a gantry angle of the gantry 14 with respect to the gantry support 12. In cases where the subject 26 is being imaged, a lower intensity x-ray beam 24 may be used in the anterior-posterior direction compared to the lateral direction. Therefore, according to an embodiment, the x-ray source 18 may be configured to emit the x-ray beam 24 with a lower intensity for gantry angles where the x-ray beam 24 enters the subject 26 in generally the anterior-posterior direction compared to gantry angles where the x-ray beam 24 enters the subject 26 in generally the lateral direction. It should be appreciated that additional embodiments may utilize different methods of varying the intensity of the x-ray beam 24.
At step 212, an image is generated based on the first incomplete set of views and the second incomplete set of views. According to an embodiment, a filtered backprojection reconstruction algorithm is used to generate the image. However, additional embodiments may use other types of reconstruction algorithms. It should be appreciated that additional embodiments may collect more than two incomplete sets of views before generating an image.
According to another embodiment, the collimator aperture width may be adjusted during the method 200. For example, while the object 26 is being translated in a first direction during step 204, the x-ray beam 24 may expose a portion of the object 26 other than the volume of interest at both the start of the translation and at the end of the translation. Therefore, an embodiment may activate a shutter of the collimator (not shown) to minimize the amount of the x-ray beam 24 that extends beyond the volume of interest in the direction of the translation. In a similar manner, the shutter of the collimator may be activated during the second translation of step 208, or during any subsequent translation steps according to other embodiments.
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At step 308, the controller 22 determines if additional views are required. If additional views are required, the method 300 returns to step 304. At step 304, the object 26 is translated in the opposite direction from the previous helical pass. By switching the direction of translation of each successive helical pass it is possible to cover the same volume of interest (VOI) with each helical pass. At step 306, a second incomplete set of views of the volume of interest (VOI) of the object 26 is acquired. According to an exemplary embodiment, the second incomplete set of views includes views 3-4, 23-24, 43-44, . . . , 983-984. According to this embodiment, the computed tomography system 10 would take a total of 10 helical passes in order to collect enough information to generate an image. According to an embodiment, the third helical pass would collect views 5-6, 25-26, 45-46, . . . , 985-986. If the pattern of advancing the views collected on each helical pass continues in the same manner as described above, the method 300 would cycle through the loop between steps 304 and 308 ten times before a complete set of views was collected. It should be understood that according to an embodiment, the object 26 is translated in the opposite direction during each successive helical pass. For example, during helical passes 1, 3, 5, 7, and 9 the object 26 may be translated in generally the positive z-direction and during helical passes 2, 4, 6, 8, and 10 the object 26 may be translated in generally the negative z-direction.
After the method 300 has cycled through the loop between steps 304 and 308 ten times, the incomplete sets of views acquired during step 306 are combined during step 310. At step 312, an image is generated and displayed based on the combined sets of incomplete views.
At step 314, the x-ray source 18 is rotated around the object 26. According to an embodiment, the x-ray source may keep rotating from step 302 until step 314. At step 316, the object 26 is translated while x-ray source 18 is rotating. At step 318, the x-ray source 18 is activated during a portion of the time while the x-ray source 18 is rotating and the object 26 is being translated, and an updated incomplete set of views is acquired. Since a complete set of views had already been acquired, the updated incomplete set of views acquired at step 318 duplicates some of the views that were previously acquired. However, the views acquired at step 318 represent the object 26 at a later point in time. For example, according to one embodiment, the eleventh helical pass collects the same plurality of views as were collected during the first helical pass. In other words, the eleventh helical pass collects transmission data from views 1-2, 21-22, 41-42, . . . , 981-982. At step 320, the updated incomplete set of views are combined with the previously acquired incomplete sets of views in order to form an updated complete set of views. For example, according to an embodiment, the eleventh incomplete set of views are combined with the second through tenth incomplete sets of views acquired during step 306, and an updated image is generated and displayed at step 322. If additional updated images are required at step 324, the method 300 loops back to step 316 and another updated incomplete set of views is acquired. The updated incomplete set of views may then be combined with previously acquired incomplete sets of views from either step 306 or step 318 in order to form a complete set of views so that an updated image can be generated. According to an embodiment, the method 300 acquires the updated incomplete sets of views at step 320 in the same order as the incomplete sets of views were acquired-during steps 304-308. If no additional updated images are required, the method 300 ends. It should be appreciated by one skilled in the art that the updated incomplete sets of views may be acquired and/or combined in an order different from the one described above according to additional embodiments.
The method 300 may be used to show how an the object 26 changes with time. For example, the method 300 may be used to acquire and display perfusion or computed tomography angiography information. For example, the volume of interest may include a portion of the subject's 26 vasculature during an angiography study.
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.
