Methods and apparatus for performing a computed tomography scan

A computed tomographic (CT) imaging system for performing a CT scan includes a detector array including a plurality of detector cells and a processor operationally coupled to the detector array. The processor is configured to receive first data regarding a first x-ray spectral range from a first detector cell, receive second data regarding a second x-ray spectral range different from the first x-ray spectral range from a second detector cell different from the first detector cell, and determine spectral information from the first data and the second data.

BACKGROUND OF INVENTION

This invention relates to computed tomographic (CT) imaging, and more particularly to methods and apparatus for measuring a spectral content and a total energy of an x-ray beam after attenuation by an object.

At least one known CT imaging system detector measures a current signal that is a representative of an energy of the x-ray beam received by the detector cells but does not measure both an energy of an x-ray beam and a spectral content of the x-ray beam after being attenuated by a patient.

SUMMARY OF INVENTION

In one aspect, a computed tomographic (CT) imaging system for performing a CT scan is provided. The CT system includes a detector array including a plurality of detector cells and a processor operationally coupled to the detector array. The processor is configured to receive first data regarding a first x-ray spectral range from a first detector cell, receive second data regarding a second x-ray spectral range different from the first x-ray spectral range from a second detector cell different from the first detector cell, and determine spectral information from the first data and the second data.

In another aspect, a method for scanning an object is provided. The method of scanning includes scanning an object by at least one of scanning the object while varying a peak kiloelectronvolt to an x-ray tube, scanning the object with a filter such that a plurality of x-ray spectra are received by a detector array, and scanning the object such that elements of a detector array discriminate between a plurality of x-ray spectra and generate signals based on the x-ray spectra.

In yet another aspect, a method for determining the presence of an analyte in an object with a computed tomographic (CT) imaging system is provided. The method includes receiving first data regarding a first x-ray spectral range from a first detector cell, receiving second data regarding a second x-ray spectral range different from the first x-ray spectral range from a second detector cell different from the first detector cell, and determining spectral information from the first data and the second data.

In still another aspect, a computed tomographic (CT) imaging system for performing a CT scan is provided. The CT system includes a detector array including a plurality of detector cells, an x-ray source positioned to emit x-rays toward the detector array, a plurality of x-ray energy filter elements separated by intervening air paths and oriented in a Z direction, and a processor operationally coupled to the detector array. The processor is configured to receive first data regarding a first x-ray spectral range from a first detector cell, receive second data regarding a second x-ray spectral range different from the first x-ray spectral range from a second detector cell different from the first detector cell, and determine spectral information from the first data and the second data.

DETAILED DESCRIPTION

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 being imaged, such as a patient. 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 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 patient 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.

Referring toFIGS. 1 and 2, a multi-slice scanning imaging system, for example, a computed tomography (CT) imaging system10, is shown as including a gantry12representative of a “third generation” CT imaging system. Gantry12has an x-ray source14that projects a beam of x-rays16toward a detector array18on the opposite side of gantry12. Detector array18is formed by a plurality of detector rows (not shown) including a plurality of detector elements20which together sense the projected x-rays that pass through an object, such as a medical patient22. Each detector element20produces an electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuation of the beam as it passes through object or patient22. During a scan to acquire x-ray projection data, gantry12and the components mounted thereon rotate about a center of rotation24.FIG. 2shows only a single row of detector elements20(i.e., a detector row). However, multislice detector array18includes a plurality of parallel detector rows of detector elements20such that projection data corresponding to a plurality of quasi-parallel or parallel slices can be acquired simultaneously during a scan.

Rotation of gantry12and the operation of x-ray source14are governed by a control mechanism26of CT system10. Control mechanism26includes an x-ray controller28that provides power and timing signals to x-ray source14and a gantry motor controller30that controls the rotational speed and position of gantry12. A data acquisition system (DAS)32in control mechanism26samples analog data from detector elements20and converts the data to digital signals for subsequent processing. An image reconstructor34receives sampled and digitized x-ray data from DAS32and performs high-speed image reconstruction. The reconstructed image is applied as an input to a computer36which stores the image in a mass storage device38.

Computer36also receives commands and scanning parameters from an operator via console40that has a keyboard. An associated cathode ray tube display42allows the operator to observe the reconstructed image and other data from computer36. The operator supplied commands and parameters are used by computer36to provide control signals and information to DAS32, x-ray controller28and gantry motor controller30. In addition, computer36operates a table motor controller44which controls a motorized table46to position patient22in gantry12. Particularly, table46moves portions of patient22through gantry opening48.

In one embodiment, computer36includes a device50, for example, a floppy disk drive or CD-ROM drive, for reading instructions and/or data from a computer-readable medium52, such as a floppy disk or CD-ROM. In another embodiment, computer36executes instructions stored in firmware (not shown). Computer36is 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.

FIG. 3illustrates a method60for performing a computed tomography (CT) scan. In one embodiment, method60includes receiving62first data regarding a first x-ray spectral range from a first detector cell20, receiving64second data regarding a second x-ray spectral range different from the first x-ray spectral range from a second detector cell20different from the first detector cell20, and determining66spectral information from the first data and the second data.

FIG. 4illustrates an x-ray tube14in which a kVp (peak kiloelectronvolt) is varied as the tube is moved in a Z direction. In one embodiment, method60includes using x-ray beam16including a spatially variant x-ray energy distribution in a Z direction. The spatially variant x-ray energy distribution in the Z direction is generated using an obtuse angle off an x-ray source target that includes a highly varying x-ray self absorption by the x-ray source target. In one embodiment, varying the x-ray source14in the Z direction produces images of object22in which each detector row corresponds to attenuation data from a different x-ray subspectrum and from which a plurality of spectral information about object22can be obtained. As used herein, subspectra and subspectrum can mean the full x-ray spectrum or any subpart thereof. Determining spectral information means taking a difference between two or more x-ray attenuation measurements collected with a plurality of x-ray subspectra. For example, the x-ray source is constructed such that when energized the x-ray source emits x-rays such that there is a spatially variant x-ray spectral distribution in the Z direction and a first detector row receives x-rays of a different spectrum than received at a second detector row. The data samples thus received from the first and second detector rows are then analyzed to determine the presence of a particular analyte. As used herein analyte refers to a specific material in an object such as a patient that one desires to identify such as bone, calcium, iodine or other contrast agent, and other types of agents or dyes that provide functional information and/or anatomical information.

FIG. 5illustrates an x-ray tube14which is switched between a plurality of different kVp's during a scan. Switching the x-ray tube kVp in turn alters the x-ray spectrum emitted by x-ray tube14. In this embodiment, the spatially variant x-ray energy distribution in the Z direction is generated by varying the x-ray source tube kVp for each rotation or half slice of data. For example, using an eight slice detector, eight different kvp's are cycled. Method60facilitates running continuous helical to obtain a continuous stream of slice data that includes elemental composition data in specific areas of interest of object22. For example, the x-ray source tube is energized at a first kVp when object22is at a first detector row to obtain a first data sample. The x-ray source is then energized at a second kVp when object22is at a different detector row to obtain a second data sample. The two data samples are then analyzed to determine the presence of a particular analyte. Accordingly, attenuation data obtained at two different subspectra of radiation are compared to determine a presence of the analyte.

In another embodiment, method60includes positioning a shaped x-ray filter in a Z direction between x-ray source14and a collimator90(shown inFIG. 14). In one embodiment, the filter includes, but is not limited to, a simple stepped filter, a sloped filter per Z location or per detector cell in the Z direction, a set of K edge filters per Z location, a set of paired K edge filters per Z location, or more elaborate and complicated designs. In another embodiment, the filter is positioned in collimator90. Gantry12, including x-ray source14, is then rotated about object22using an x-ray source pitch of one detector cell such that each detector cell20of detector18images the same slice of object22using a different x-ray spectrum. For example, using imaging system10including an eight slice detector, method60generates eight overlapping images each created with a different energy x-ray beam. The data from each of these eight slices is then analyzed to determine the spectral content associated with the anatomical CT slice data.

FIG. 6illustrates an x-ray beam16subjected to a variable filtration in a Z direction before reaching object22, with a plurality of alternating filters70and air paths72. In one embodiment, a plurality of filters70are alternated with air paths72such that individual detector rows receive filtered or unfiltered x-ray beam data. Attenuation data from filtered and unfiltered x-ray beams is subsequently analyzed to determine a spectral content associated with anatomical CT slice data. Filters70may all have the same characteristics, as shown inFIG. 6, or may have different characteristics, as depicted inFIG. 7. Also illustrated inFIG. 6are a plurality of filter-plus-air-path pairs74including a first pair76, a second pair78, a third pair80, a fourth pair82, a fifth pair84, a sixth pair86, and a seventh pair88. As patient22and filter-plus-air-path pairs74move relative to each other in the Z direction, dynamic information is obtainable. For example, an analyte is injected or otherwise provided in patient22at or near an area of interest, and first pair74passes over the area of interest at a first time T0. Detector18receives attenuated x-rays at time T0and a first spectral analysis determines the location of the analyte at T0. While the relative motion in Z continues, second pair78passes over the area of interest at a later time than time T0, say time T1, and a second spectral analysis determines the location of the analyte at T1. Additionally, third pair80through seventh pair88, pass over the area of interest at times T2though T6, and spectral analyses allow for the determination of the analyte's location at times T2through T6. By knowing the analyte's location at the various times, dynamic information regarding the analyte's movement thought patient's22body is provided. Although illustrated with seven filter-plus air-path pairs74, it is contemplated that the benefits of the invention accrue to embodiments with more than and less than seven pairs.

FIG. 7illustrates an x-ray beam16subjected to a variable filtration in a Z direction before reaching object22, with a plurality of alternating filters70and air paths72to analyze different materials. The filter-plus-air-path pairs in this embodiment are designed to detect such analytes including, but not limited to, bone, calcium, iodine or other contrast agent, and other types of agents or dyes that provide functional information and/or anatomical information.

FIG. 8illustrates an x-ray beam subjected to variable filtration in a Z direction before reaching object22. In one embodiment, at least one variable filter80rotates between a plurality of filter/air positions. Variable filter80comprises a plurality of discrete x-ray filters that rotate into position between x-ray source14and object22.FIG. 9illustrates two variable filter configurations as seen from the perspective of looking down toward object22. In another embodiment variable filter80comprises a stationary filter that has x-ray filtration properties which vary along the Z direction. In yet another embodiment, variable filter80is drum shaped including at least one filter section and at least one slit providing an air path. Alternatively, drum shaped variable filter80includes a plurality of filter sections with different filter characteristics.FIG. 10illustrates an x-ray beam16subjected to variable filtration in a Z direction between x-ray source14and object22, with variable filter80comprising a plurality of distinct parts each of which has different x-ray filtration properties.FIG. 11illustrates an x-ray beam16subjected to variable filtration in a Z direction between x-ray source14and object22, with variable filter80being a wedge-shaped block of a single substance wherein the x-ray filtration properties vary in the Z direction based solely on thickness.

FIG. 12illustrates an x-ray beam16subjected to variable filtration in a Z direction between object22and detector18, with a plurality of alternating filters70and air paths72to analyze different materials. In one embodiment, a plurality of filters70are alternated with air paths72such that individual detector rows receive x-ray data that is filtered or unfiltered after being attenuated by object22. Attenuation data from filtered and unfiltered x-ray beams is subsequently analyzed to determine a spectral content associated with anatomical CT slice data. The filter-plus-air-path pairs in this embodiment are designed to detect such analytes including, but not limited to, calcium, bone, iodine or other contrast agent, and other types of agents or dyes that provide functional information and/or anatomical information.

FIG. 13illustrates a detector18which simultaneously measures a plurality of different x-ray spectral ranges. In use, detector cells20can distinguish between x-ray spectra through means including, but not limited to, the use of a filter in front of each cell20or a cell variation which lends itself to detecting various x-ray energies so as to make detector cell20self-discriminating. In other words, detector cells20selectively respond to different x-ray subspectra and no filters are used. Detector cells20are made self-discriminating by methods including, but not limited to, manufacturing detector cells20from different materials that are sensitive to different portions of the x-ray spectrum and coating each detector cell20with a different scintillating material that is sensitive to a different portion of the x-ray spectrum.

In another embodiment, an effective x-ray source current (in milliamperes) is reduced for each slice to facilitate preventing an increased accumulated x-ray dose per CT slice. In use, the reduced x-ray source current can be used with a radio opaque contrast agent, such as, but not limited to, iodine, to gather dynamic information. Further, all eight slices of an eight slice detector can be used in this method. Alternatively, two sets of four slices of the eight slice detector can be used to generate multiple sets of data per detector18. Detectors with a higher number of cells20, i.e. sixteen or more, can have an even greater advantage in this operating mode. In another embodiment, detector18is designed to gather spectral information directly through the use of a filter in front of each cell20, or a cell variation which facilitates detecting various X-ray energies can be used. For example, method60as describe herein can be used on a plurality of combinations of systems and detectors, and can also be combined, such as utilizing both pre-patient and post patient spectral discrimination hardware etc.

Measuring both an energy of an x-ray beam and a spectral content of the x-ray beam after being attenuated by a patient facilitates allowing a two-dimensional (2D) determination of both an anatomical composition and elemental composition within a CT slice. Multiple samples taken over time also facilitate allowing a plurality of dynamic flow studies, such as, but not limited to, tracking an iodine contrast media perfusion or a blood flow study. Additionally, spectral content information obtained from a plurality of detector cells can be used to correct for beam hardening, thereby obtaining CT numbers with an increased accuracy.