Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/29—Measurement performed on radiation beams, e.g. position or section of the beam; Measurement of spatial distribution of radiation
  • G01T1/2914—Measurement of spatial distribution of radiation
  • G01T1/2985—In depth localisation, e.g. using positron emitters; Tomographic imaging (longitudinal and transverse section imaging; apparatus for radiation diagnosis sequentially in different planes, steroscopic radiation diagnosis)
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
  • A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
  • A61B6/027—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis characterised by the use of a particular data acquisition trajectory, e.g. helical or spiral

Definitions

• the invention relates to computed tomography (CT) and. more particularly, to an apparatus and a method for use in a volumetric CT scanning system that enable the resolution, scanning speed, axial coverage (the extent of the patient that is being imaged) and other parameters associated with an area detector to be varied in order to optimize the scanning procedure for a particular application.
• CT computed tomography
• Computed tomography is a technique that generally involves subjecting a patient to X-rays, acquiring digital X-ray projection data of a portion of the patient ' s body, and processing and back-projecting the digital X-ray projection data to produce an image that is then displayed on a display monitor of the CT system.
• CT systems typically comprise a gantry, a table, an X-ray tube, an X-ray detector array, a computer and a display monitor.
• the computer sends commands to controllers of the gantry to cause the controllers to rotate the X-ray tube and/or the detector array at a particular rotational speed.
• third generation CT systems relative rotational motion is produced between the detector array and the X-ray tube about the patient ' s body.
• the computer controls the data acquisition process performed by the X-ray tube and the detector array to acquire digital X-ray radiographs.
• the computer then processes and back-projects the digital X-ray radiograph data by performing a reconstruction algorithm and displays the reconstructed CT image on the display monitor.
• CT systems in use today utilize a single row of detectors in the gantry, which is normally referred to as a linear array of detector elements.
• Advanced CT systems use two to four linear arravs of detectors to construct a multi-row detector.
• the multi-row detector facilitates patient scanning since a specified axial coverage of the patient can be scanned in less time by increasing the helical pitch of the CT system.
• Helical pitch is typically defined as the ratio of the displacement of the table supporting the patient during one rotation of the gantry to the detector pitch.
• a helical pitch of one refers to translating the patient table an amount equal to the detector pitch during one revolution of the CT gantry of the CT system.
• the use of the multi-row detector is revolutionizing scanning protocols by enabling whole-organ scanning to be accomplished in one breath hold of the patient (i.e.. the period of time that the patient is able to hold his/her breath during scanning to thereby minimize motion of the portion of the patient being imaged).
• An area detector is a rectangular grid of individual detector elements (pixels) that have dimensions on the order of hundreds of microns.
• the rectangular detector grid may have thousands of pixels per side.
• the CT system is typically referred to as a volumetric CT system, or a VCT system.
• This area detector technology is replacing X-ray film in the clinical environment in state- of-the-art facilities for planar radiography, thereby making a single two-dimensional image of the patient at a certain orientation of the X-ray tube and detector, which corresponds to the attenuation of the X-ray beam through the patient.
• these facilities are migrating to "filmless " radiology departments.
• area detector technology enables the scanning time to be reduced. In one revolution of the gantry, axial coverage of a complete organ can be obtained. In contrast, many revolutions of a gantry that utilizes a multi-row CT system are needed to attain the same axial coverage. It is generally known in the art that, with area detector technology, it is necessary to acquire additional data to generate a mathematically complete data set. However, this additional data can be acquired in a single linear scan of the patient, typically referred to as the scout scan. In general, a scout scan of the patient is typically acquired before axial scanning commences and ensures that the patient is positioned properly in the VCT scanner. This, in turn, ensures that the appropriate axial coverage of the patient is scanned. Therefore, in general, it is possible to reduce the scanning time of the patient by one to two orders of magnitude using area detector technology.
• In-plane resolution is affected by the detector resolution and the physical geometry of the CT imaging system.
• the in-plane resolution of CT reconstructions of data collected using multi-row detector technology is on the order of 0.5 millimeters.
• Axial resolution can be as high as 1.25 mm. assuming that no overscanning techniques are used to effectively improve the resolution.
• Individual detector elements in area detector arrays have a resolution that can be an order of magnitude less than its multi-row counterparts.
• the in-plane resolution of CT data reconstructed from data collected with area detector technology can also be an order of magnitude less than reconstructions computed from data collected with multi-row detector technology.
• the reconstructed volume has isotropic voxel resolution, i.e.. the in-plane resolution of the CT reconstructions matches the axial resolution of the reconstructions (i.e.. the effective slice thickness of the CT reconstruction is directly related to the finite dimensions of the detector).
• CT images in the reconstructed volume are equal. This characteristic of reconstructed data allows data to be reformatted to generate sagittal and coronal views of the data that was reconstructed on several axial planes without a loss of resolution in the reformatted data.
• linear or multi-row detectors are not square. Normally the slice thickness or axial resolution of the reconstructed image is larger than the in-plane resolution. If one observes an axial image of the patient, which is generally the preferred orientation, the resolution in both dimensions is identical. However, by reformatting the data in a sagittal or coronal cross section, an observer will notice that the horizontal and vertical resolution in the image is not equal.
• isotropic voxel resolution in data reconstructed from radiographs obtained using area detector technology has yet to be realized because the clinical community has not previously recognized the capabilities and flexibilities of high-resolution imaging.
• Scanning time is the time interval during which the patient is being subjected to X-rays.
• Axial coverage is the extent of coverage projected by the X-ray source onto the patient who is being imaged during the CT examination.
• Spatial resolution is the dimension of a pixel element in the reconstructed image and is used herein to represent in-plane and/or axial resolution. Spatial resolution is affected by the dimensions of the detector elements and by the geometric configuration of the CT gantry.
• the present invention provides a method and apparatus for use in a volumetric computed tomography (VCT) system.
• the VCT system comprises an X-ray source. an area detector, a gantry that generates relative rotational motion between the object and the X-ray source and area detector during a particular scanning time.
• the VCT system also comprises a data acquisition component and read-out electronics.
• the X- ray source projects X-rays on the object as the gantry rotates in such a way that a particular axial coverage of X-rays on the object occurs.
• the detector elements generate electrical signals in response to X-rays impinging thereon.
• a switching device in communication with the detector elements is selectively controlled to select which electrical signals generated by certain detector elements of the detector are output from the detector elements at any particular time and sent to certain analog-to- digital converters (ADCs). which convert them into digital signals.
• ADCs analog-to- digital converters
• the spatial resolution of the reconstructed image can be selectively controlled by enabling the number of detector element signals that are ganged together and sent to the ADCs to be varied.
• Other scanning parameters may be varied as well, such as. for example, the axial coverage, the scanning time, and the number of views acquired by the area detector as the gantry rotates. By making trade-offs between certain of these scanning parameters, the scanning of the patient can be optimized for a given imaging application.
• Fig. 1 is a block diagram illustrating the volumetric CT scanning system of the present invention in accordance with the preferred embodiment.
• Fig. 2 is a flow chart illustrating the method of the present invention in accordance with one scenario.
• Fig. 3 is a flow chart illustrating the method of the present invention in accordance with another scenario.
• Fig. 4 is a flow chart illustrating the method of the present invention in accordance with another scenario.
• VCT volumetric CT
• trade-offs may be selected or determined between the scanning time of the patient, the in-plane resolution, the axial resolution, the azimuthal sampling, and coverage achievable by the VCT scanner, for example. No such tradeoffs have, as of yet. been implemented. Therefore, the full benefits of area detector technology have not yet been discovered and implemented, as will be understood by those skilled in the an.
• Area detector technology provides a high-resolution two-dimensional (2D) rectilinear grid of detector elements that, in general, allows for isotropic image reconstruction since the detector dimensions usually are symmetric. Although it is desirable for the detector elements to be symmetric, this requirement is not essential for the present invention.
• 3D reconstruction with isotropic voxel dimensions have several limitations, the most significant being that reformats of the reconstructed data along coronal and sagittal planes in the patient have varying resolution in the 2D reformatted data.
• Surface and volume rendering techniques that utilize the reconstructed 3D data set also are limited by the nature of the data.
• an area detector in a VCT system, it is possible to acquire X-ray projection data over a large region of the patient in a significantly smaller time interval when compared with existing single-slice or multi-slice scanners available today.
• the spatial resolution of the area detector technology can be an order of magnitude better than that obtained by the linear or multi-row detector technology.
• area detectors have many more rows of detector elements than multi-slice detectors; in most cases, two to three orders of magnitude more rows. Higher resolution and more rows of detector elements in the area detector has many advantages, but also results in significant difficulties in digitizing the signal in each detector cell in an area detector at each view angle of the gantry for a given scanning time due to the large number of detector elements, as discussed below in detail.
• Area array detector technology generally employs storage diode technology so that each detector element can be read out sequentially.
• Each detector element integrates a signal that is related to the X-ray energy impinging on that particular element and stores this information.
• the signals in the detector elements are then sequentially digitized using multiplexers and analog-to-digital converters (ADC s).
• ADC s analog-to-digital converters
• the readout time of the panel decreases for each view of projection data acquired by the data acquisition system 6 (variable scanning time), which may be necessary for an imaging application where, for example, patient or organ motion is an issue.
• variable scanning time a section of the detector array will be illuminated by X-rays at each view angle of" the gantry (i.e., variable coverage). It is possible to vary the resolution, scanning time, and/or axial coverage of the scanner to meet the objectives of the particular scanning protocol.
• a general discussion of the VCT system of the present invention will be provided with reference to Fig. 1.
• Fig. 1 is a block diagram of a volumetric CT scanning system that is suitable for implementing the method and apparatus of the present invention.
• the Volumetric CT scanning system will be discussed with respect to its use in reconstructing an image of an anatomical feature of a patient, although it will be understood that the present invention is not limited to imaging any particular object.
• the present invention may also be used for industrial processes, as will be understood by those skilled in the art.
• the present invention is not limited to medical CT equipment, but includes industrial systems where the X-ray source and detector geometry are held fixed while the object is rotated during the scanning time.
• a volumetric CT scanning system the gantry is rotated about an object, such as a human patient, and projection data are acquired.
• a computer 1 controls the operations of the volumetric CT scanning system.
• that phrase is intended to denote rotation of the X-ray tube 2 and/or rotation of the detector 3. which preferably is a high resolution area detector.
• the X-ray tube 2 and the area detector 3 are comprised by the gantry.
• the controllers 4A and 4B are controlled by the Volumetric CT scanning system computer 1 and are coupled to the X-ray tube 2 and to the detector 3. respectively.
• the controllers 4A and 4B cause the appropriate relative rotational motion to be imparted to the X-ray tube 2 and/or to the detector 3. Individual controllers are not necessary.
• a single controller component may be used to rotate the gantry.
• the computer 1 controls the variations in image scanning time, image resolution and/or axial coverage in order to implement the methods of the present invention.
• the computer 1 controls the data acquisition process by instructing the data acquisition system 6 as to when to sample the detector 3 and by controlling the speed of the gantry. Additionally, the computer 1 instructs the data acquisition system 6 to configure the resolution of the radiographs obtained by the area detector 3. thereby allowing the resolution of the system to be varied.
• the data acquisition system 6 comprises the read out electronics, as shown, which can be controlled in such a way that the resolution of the system can be varied, as discussed below in detail.
• the area detector 3 is comprised of an array of detector elements (not shown).
• Each detector element measures an intensity value associated therewith that is related to the amount of X-ray energy that impinges on the detector element.
• a new volumetric CT scanning system is created. Therefore, the present invention also provides a new volumetric CT scanning system.
• the present invention is not limited to any particular computer for performing the data acquisition and processing tasks of the present invention.
• the term "computer " as that term is used herein, is intended to denote any machine capable of performing the calculations, or computations. necessary to perform the tasks of the present invention. Therefore, the computer utilized to perform the control algorithm 10 of the present invention may be any machine that is capable of performing the necessary tasks.
• ADC analog-to-digital converter
• a plurality of ADCs will be used for this purpose, so that the time required to digitize the signal from each detector element will not be prohibitively long.
• the scanning time i.e., the time required to scan the targeted region of the patient, since fewer signals need to be digitized.
• the scanning time of the patient By multiplexing several detector elements into the ADCs. the scanning time of the patient further decreases, assuming the same axial coverage.
• the resolution of the projection data acquired also decreases in this case since the outputs of the several detector elements are multiplexed together and output to the ADCs.
• the scanning time may be held constant because the effective number of channels to be digitized remains unchanged. In essence, as many signals as necessary are multiplexed together so that the total number of signals to be digitized remains constant.
• This scenario is represented by the flow chart shown in Fig. 2.
• the projection data acquired by the VCT system is reconstructed, as indicated by block 14.
• the manner in which this type of projection data can be reconstructed is known in the art.
• a variety of algorithms are known that are suitable for performing the reconstruction, as will be understood by those skilled in the art.
• the Feldkamp algorithm is suitable for this purpose and is well known in the art.
• axial coverage may be sacrificed. or. in other words, decreased, in favor of increasing spatial resolution.
• spatial resolution may be sacrificed, or. in other words, decreased, in favor of increasing axial coverage.
• scanning time may be held constant while increasing spatial resolution and/or axial coverage, but doing so will require that the number of views acquired as the gantry rotates about the patient be decreased.
• scanning time may be held constant while also increasing the number of views acquired as the gantry rotates about the patient, but doing so will require that the spatial resolution and/or the axial coverage be decreased.
• a constant axial coverage may be maintained while scanning time and spatial resolution are increased. Conversely, a constant axial coverage may be maintained if scanning time and spatial resolution are decreased.
• axial coverage may be held constant while increasing spatial resolution and/or decreasing scanning time, but doing so will require that the number of views acquired as the gantry rotates about the patient be decreased. Furthermore, axial coverage may be held constant while also increasing the number of views acquired as the gantry rotates about the patient if the spatial resolution is decreased and/or the scanning time is increased.
• a constant spatial resolution may be maintained if scanning time is increased.
• the axial coverage may also be increased without causing the spatial resolution to be varied. Conversely, if scanning time is decreased, for a given spatial resolution, axial coverage may decrease so that the detector signals digitized by the
• ADC are held constant.
• spatial resolution may be held constant while increasing axial coverage and/or decreasing scanning time, but doing so will require that the number of views acquired as the gantry rotates about the patient be decreased.
• spatial resolution may be held constant while also increasing the number of views acquired as the gantry rotates about the patient if the axial coverage is decreased and/or the scanning time is increased.
• the number of views acquired as the gantry rotates is held constant.
• the number of view angles acquired as the gantry rotates is another parameter that may be taken into account in making the aforementioned trade-off decisions.
• Scenario (1) generally indicates that, if the scanning time is selected and is not variable, as indicated by block 12. spatial resolution can be increased, but the axial coverage will have to be decreased in order to acquire the data in the same time interval. Alternatively, if the scanning time is held constant, or is not variable for some reason (e.g., scanning an area where there is motion, such as the lungs), axial coverage can be increased, but doing so will decrease the spatial resolution. Both situations will be useful under certain circumstances.
• This scenario is generally represented by Fig. 2.
• Fig. 2 indicates that, for a selected scanning time and a constant number of projection views, the axial coverage and spatial resolution must be determined, as indicated by block 13. For a constant scanning time, the spatial resolution and axial coverage are inversely related to each other.
• Scenario (2) generally indicates that if it is desirable to utilize a particular axial coverage, this can be achieved by increasing the scanning time, which can result in an increase in spatial resolution since the read out electronics have more time to read out the detector element signals. Conversely, a particular axial resolution can be obtained by decreasing the scanning time, which results in more detector element signals being multiplexed to the same ADCs. This, in turn, results in a lower resolution image being obtained.
• This scenario is represented generally by Fig. 3. As shown in Fig. 3. if a particular axial coverage is selected, as indicated by block 18. the resolution and scanning time can be selected or determined, as indicated by block 19 either by a computer or by the system operator. Trade-offs can be made between the resolution and scanning time to achieve the desired scanning protocol for the particular application.
• Scenario (3) generally indicates that if a selected spatial resolution is not varied, or is held constant for some reason, scanning time and axial coverage may be increased without causing the spatial resolution to be varied. However, if the scanning time is increased by a particular amount, care must be taken in selecting the proper increase in axial coverage. In essence, if scanning time is increased, the amount to which axial coverage can be increased is limited by the amount to which scanning time was increased. In contrast, if axial coverage is increased, the scanning time must be increased by a proper amount in order to maintain the selected spatial resolution.
• both the scanning time and the axial coverage may be decreased without causing the spatial resolution to vary.
• a particular resolution can be maintained by decreasing both the axial coverage and the scanning time. This means that a smaller area is being scanned for a shorter period of time, which can result in obtaining the same resolution as in the former situation.
• a decrease in scanning time will limit the amount by which the axial coverage is decreased in order to maintain a constant spatial resolution.
• a decrease in axial coverage will also limit the amount by which the scanning time is decreased in order to maintain a constant spatial resolution.
• the spatial resolution is first selected, as indicated by block 23.
• the scanning time and the axial coverage are then determined or selected based on the desired spatial resolution, as indicated by block 24. Therefore, trade-offs can be made between scanning time and the axial coverage in accordance with the desired scanning protocol and the particular application in order to achieve the desired resolution.
• the image is reconstructed, as indicated by block 25.
• the manner in which the X-ray beam can be collimated to obtain a certain axial coverage is known. If it is desired to obtain a greater axial coverage (e.g., to obtain an image of the entire thorax in the same amount of time), then a greater slice thickness must be obtained if the scanning time is held constant. Since more data is being obtained, the read out electronics of the data acquisition system 6 must blend more detector element signals together when sending them to the ADCs since more data is being acquired during the same period of time. This also corresponds to a lower resolution since more detector element signals are combined together and sent to the ADCs. Therefore, the collimator (not shown) is opened up to obtain this higher axial coverage, but spatial resolution is sacrificed.
• the collimator may be narrowed to obtain a higher resolution image of the region of interest and to reduce the overall dose to the patient. Since less data may be acquired, the read out electronics do not have to gang as many signals of the detector elements together when they are multiplexed to the ADCs. Therefore, the multiplexing of the signals together is controlled to obtain the desired resolution, as is the axial coverage. The scanning time may not need to be increased since the axial coverage has decreased, which is consistent with scenario ( 1).
• a high-resolution scan may be performed for a thin cross-section (reduced axial coverage) of the thorax.
• the scanning time may increase so that less detector element outputs are multiplexed together to particular ADCs. i.e.. the read out electronics have more time to read out the outputs of the individual detector elements.
• the scanning time may even decrease in this case. In other words, it may be possible to reduce the scanning time and increase resolution at the same time, provided that the decrease in the digitization time resulting from the reduced axial coverage more that offsets the increase in time required to digitize the high-resolution data.
• a helical scan can be performed, which may maintain a certain scanning time and decrease axial coverage. This, in turn, may allow an increase in resolution since the read-out electronics have more time to read out the detector element signals (i.e.. a lesser amount of detector element signals need to be multiplexed to the same ADCs).
• the present invention enables scanning speed to be traded off for resolution and/or axial coverage.
• resolution can be traded off for scanning speed and/or axial coverage.
• axial coverage can be traded off for scanning speed and/or resolution.
• the present invention has been discussed with respect to certain embodiments. However, the present invention is not limited to these embodiments.
• the three scenarios discussed are not meant to be all inclusive of the manner in which tradeoffs of the aforementioned parameters can be utilized to obtain the proper mode of operation of the VCT system. These scenarios are discussed in order to illustrate the concepts of the present invention and the manner in which these fundamental parameters can be traded off in order to achieve the proper scanning protocol.
• the trade-offs are not limited to a scanning protocol, i.e. they apply to both axial scanning (the patient table is not moved during the scanning period) and helical scanning protocols. Those skilled in the art will understand the manner in which these concepts can be utilized and extrapolated to achieve other area detector scanning protocols that are useful for particular applications.

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• 2000
  • 2000-04-14 DE DE10081368T patent/DE10081368T1/de not_active Withdrawn
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