JP2009268899A - High velocity switching method in double energy computer tomography (ct) system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a projection noise optimization method to make switching between two different kVps easier without remodeling an X-ray tube in a double energy computer tomography (CT) system. <P>SOLUTION: The method to make switching between two different peak kilovoltage (kVp) projections easier in a double energy computer tomography (CT) system (100) is disclosed. The method includes a step to set one current value in an X-ray tube (104) which emits X-rays at two or more different kVp to a subject. By substantially keeping the current setting constant, relative view time (s) is adjusted between two kVps to modulate current view time (mAs) product. Further, a duration ratio (R) is calculated based on the mAs product. The duration ratio (R) defines the duration until kVp is switched. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates generally to computed tomography (CT) systems, and more specifically to a method that facilitates fast switching between x-ray tube voltages in a dual energy CT system.

  CT systems typically include an x-ray tube that projects an x-ray beam through an object, eg, a patient. The X-ray beam projected toward the object is attenuated after passing through the object. A detector array detects the attenuated x-ray beam from various angular positions with respect to the object and generates one or more signals. The signal is then processed to obtain processed data called projection data. Furthermore, the projection data is combined to reconstruct a target image representing the attenuation of the X-ray beam by the target. Typically, the x-ray tube and detector array are mounted on a gantry. Projection data from a detector array at one gantry angle is called a “view”. The scan of the object includes a set of views formed at various gantry angles or projection angles during one revolution of the x-ray source and detector array.

  In a dual energy X-ray CT system, the projection data for a given path is measured twice in two separate peak-kilovoltage (kVp) projections to obtain the exact X-ray attenuation coefficient and material composition of the object being scanned. Provides additional information to be scanned. In some systems, these two separate kVp projections (at high and low kVp) are generated by a single x-ray tube or a dual x-ray tube. In a single x-ray tube system, switching between two separate kVp should be fast enough to effectively avoid misalignment between the two sets of projections due to object motion. is there. It is widely understood that the sensitivity of dual kVp systems for material differentiation relies primarily on the energy separation of the two x-ray spectra generated at the two kVp settings. In addition, in order to reduce the image noise in the specific material image, the optimum current (mA) ratio is applied at the high kVp setting and the low kVp setting when the sampling times at the high projection view and the low projection view are equal. Should. For example, to avoid excessive noise in low kVp projections, it is necessary to increase mA at low kVp durations. In conventional X-ray tubes, the mA change is achieved by changing the tube filament current. However, there is a time delay due to the thermal constant of the filament before tube current changes are observed. This time delay is usually greater than the time to switch kVp. Thus, with conventional constant view time trigger generation, optimization of projection noise (ie, mA) at high and low kVp is extremely difficult to achieve due to the undesirable characteristics of this x-ray tube.

  Several dual energy systems have been proposed that overcome the above mentioned problems. (A) A system including a photon counting detector. This system provides excellent performance even when operating with very low x-ray flux. However, this system is unable to operate at high x-ray flux due to the fact that photon counting detectors are not fast enough. (B) Double kVp system with two scans each time at a different kVp. In this system, the projection between the high kVp scan and the low kVp scan is not acquired sufficiently fast in time, resulting in misalignment due to object motion. (C) Two tube / two detector system with tubes / detectors mounted approximately 90 ° apart. In this system, the kVp of the two tubes can be set to different values. While this system provides a relatively good alignment, there is still residual misalignment due to angular misalignment between the two tubes. (D) Single pipe system. In this system, kVp is switched quickly between a high setting and a low setting. Typically, switching occurs every view or every few views. This system provides a good balance between image quality, projection / image alignment, and system simplicity. This system can provide competitive image quality if the relative projection noise between high and low kVp can be dynamically adjusted according to kVp switching.

Therefore, there is a need for a projection noise optimization method that facilitates switching between two separate kVp without modifying the x-ray tube.

  In a first aspect of the invention, a method is provided that facilitates switching between at least two separate peak kilovoltage (kVp) projections in an imaging system. The method includes adjusting the relative view time (s) between at least two kVp projections (high and low kVp projections) while maintaining a relatively constant current (mA). The current-view time product (mAs) is modulated for these at least two separate kVp projections by adjusting the relative view time between the at least two separate kVp projections. This view time adjustment has the same effect as the current (mA) adjustment in a constant view time triggered system for high / low kVp projection noise optimization. The method also includes calculating a duration ratio (R) for at least two separate kVp projections based on the calculated optimal mAs ratio at high and low kVp. The view duration is modulated according to kVp switching to compensate for the lack of fast control for mA.

  In a second aspect of the invention, a computed tomography (CT) system is provided. The CT system is configured to emit a gantry that is configured to accommodate a scan target and at least two separate peak kilovoltage (kVp) projections disposed within the gantry. X-ray tube. The CT system also includes a detector array that includes a plurality of detector elements. The detector array measures projection data with at least two separate kVp projections. The CT system further includes a processor configured to adjust the relative view time (s) between at least two separate kVp projections to facilitate modulation of the current view time (mAs) product. Contains. The processor is further configured to calculate a duration ratio (R) for at least two separate kVp projections based on the optimal mAs ratio between the different kVp.

  These and other features, aspects and advantages of the present invention will be more fully understood when the following detailed description is read in conjunction with the accompanying drawings. In the drawings, like reference characters designate like parts throughout the drawings.

1 is a block diagram illustrating a CT system according to an example embodiment of the present invention. 2 is a flow diagram illustrating an example of a method that facilitates switching between two separate peak-kilovoltage projections in the CT system of FIG. It is a graph which shows the view time change by the method of FIG.

  Various embodiments of the present invention provide a method that facilitates switching between separate peak-kilovoltage projections in a computed tomography (CT) system, particularly a dual energy CT system.

  FIG. 1 is a block diagram illustrating a CT system 100 according to an example embodiment of the present invention. The CT system 100 includes a gantry 102, an x-ray tube 104, and a detector array 108. The gantry 102 controls the position of the scanning object 110. The X-ray tube 104 projects an X-ray beam toward the detector array 108. The detector array 108 includes a plurality of detector elements 112 that detect attenuated X-rays after passing through the object 110. In the illustrated embodiment, the CT system 100 includes a single row of a plurality of detector elements 112. In various embodiments of the present invention, the detector array 108 may include a plurality of parallel rows of detector elements 112. The plurality of detector elements 112 generate signals based on the attenuation of the X-ray beam 106. Further, these signals are processed to obtain projection data.

  In one embodiment, the x-ray beam may correspond to at least two distinct peak kilovoltage (kVp) projections. The x-ray tube 104 and detector array 108 rotate with the gantry 102 so that the angle at which the x-ray beam intersects the object 110 changes constantly. A single scan of object 110 obtains x-ray attenuation measurements at one gantry angle called the projection angle.

  The gantry 102 and components mounted on the gantry 102 rotate around a center of rotation 114 when performing a scan that acquires projection data from the object 110. The rotation of the components provided in the gantry 102 and the operation of the X-ray tube 104 are controlled by the control mechanism 116 of the CT system 100. The control mechanism 116 includes an X-ray controller 118, a gantry motor controller 120, and a data acquisition system (DAS) 122. The X-ray controller 118 provides a power signal and a timing signal to the X-ray tube 104. The gantry motor controller 120 controls the rotational speed and position of the components of the gantry 102. The DAS 122 of the control mechanism 116 samples the analog data from the detector element 112 and converts the analog data into a digital signal for further processing.

  In one embodiment, the analog data may be projection data corresponding to at least two distinct peak kilovoltage (kVp) projections. Further, processor 124 receives sampled and digitized projection data from DAS 122. The processor 124 may view the relative view between the two separate kVp projections to facilitate modulation of the current-view time product (mAs) for the two separate kVp projections by maintaining a constant view speed. It is configured to adjust the time (s). Furthermore, a relatively constant current setting is maintained in high kVp projection / low kVp projection. The processor 124 calculates the optimum mAs ratio R of the high kVp projection and the low kVp projection by using the kVp setting and the average projection value. Thereby, the processor 124 calculates the view time in the high kVp projection and the view time in the low kVp projection using the mAs ratio R. In one embodiment, the constant view speed may include keeping the temporal center of view constant. The processor is also configured to calculate a duration ratio (R) for two separate kVp projections based on the optimal mAs ratio. In one embodiment, the relative view time (s) is defined as the time to switch kVp. The relative view time (s) is adjusted based on at least one of the average object 110 attenuation profile and the projection data.

  The processor 124 reconstructs the image using projection data acquired by two separate kVp projections. The reconstructed image is applied as an input to the computer 126 and stored in the memory 128 of the computer 126. Examples of the memory 128 include, but are not limited to, a read only memory (ROM). The computer 126 can receive commands and scanning parameters from an operator via the operation console 130. In one embodiment, the operation console 130 may include a keyboard. A display 132 allows the operator to view the reconstructed image and other data from the computer 126. Examples of the display 132 include, but are not limited to, a cathode ray tube (CRT) display, a liquid crystal display (LCD), and a plasma display. The instructions supplied by the operator are used by computer 126 to provide control signals and information to control mechanism 116. In addition, the computer 126 can operate the table motor controller 134. The table / motor controller 134 controls the movement of the table to place the object 110 in the gantry 102. Specifically, the table moves the object 110 through the gantry opening.

  In one embodiment, computer 126 includes a device that reads instructions and data from a computer-readable medium. Examples of this device include, but are not limited to, flexible disk drives, CD-ROM drives, DVD drives, magneto-optical disk (MOD) devices, or any other network connection device including Ethernet devices (Ethernet is a trademark). There are digital devices. Examples of computer readable media include flexible disks, CD-ROMs, DVDs, or digital sources such as networks or the Internet, and developing digital means. In other embodiments, computer 126 executes instructions stored in firmware (not shown). Examples of firmware include, but are not limited to, a basic input / output system (BIOS) that is built into the computer 126. The computer 126 is programmed to perform the operations described in this document, and the term “computer” used in this document is not limited to an integrated circuit referred to as a computer in the art, but includes a computer, a processor, and a microcontroller. Broadly refers to microcomputers, programmable logic controllers, application specific integrated circuits, and other programmable circuits.

  Although the method described herein is described for a CT system, it is believed that the advantages of the present invention can be obtained in X-ray imaging modalities other than CT such as PET-CT, MRI-CT and SPECT. Also, although the methods and apparatus described herein are described in a medical environment, such as systems typically used in industrial or transportation environments, such as, but not limited to, baggage scanning systems at airports or other transportation locations. Such non-medical imaging systems will also benefit from the advantages of the present invention.

  FIG. 2 is a flow diagram illustrating an example of a method that facilitates switching between at least two separate peak-kilovoltage projections in CT system 100. In one embodiment, each step shown in the flowchart may be performed automatically by computer 126 under appropriate program control. However, these steps do not have to be executed as an automatic process. In one embodiment, the program control is a firmware program, such as a system basic input / output system (BIOS) built into the computer 126. In another embodiment, the program control is a device firmware program, such as a device driver stored in the memory 128 of the CT system 100.

  Optimization of view time values at high and low kVp is achieved as shown in FIG. In step 202, an optimal mAs ratio R of high kVp scan to low kVp scan is calculated using equation (1). First, a scout scan is performed. From the scout scan, the region of interest (ROI) of interest is identified. The projection over the ROI is averaged to obtain an effective attenuation over the ROI region. Thus, R is calculated using the average projection for a given target image format (basis material decomposition image, density image, effective Z image, etc.).

R = f (kVp1, kVp2, p _ave ) (1)
In equation (1), kVp1 and kVp2 are the two kVp settings for the scan, and p _ave is the average projection value over the ROI. The function f () depends on the imaging system and can be obtained through imaging system calibration. Thereafter, in step 204, using the equations (2) and (3), the view time at the high kVp and the view time at the low kVp are calculated based on the optimum mAs ratio obtained in step 202.

V _H = mA _L * R / (mA _L * R + mA _H ) * T _t (2)
V _L = mA _H / (mA _L * R + mA _H ) * T _t (3)
Where V _H and V _L are view time values at high kVp and low kVp, respectively. T _t is the sum of view times at high kVp and low kVp. During kVp switching, the tube filament current of the x-ray tube of the imaging system is kept constant. However, the tube filament current varies from mA _H and mA _L as a result of the change in electron extraction field strength induced by kVp modulation. In step 206, kVp is switched based on the view time. Variable view time sampling may also be used to optimally switch kVp scanning.

FIG. 3 is a graph showing changes in view time according to the method of FIG. Two separate voltage projections kVp _L and kVp _H , and associated tube anode currents mA _H and mA _L (not shown in FIG. 3) at a constant filament current setting, are reduced by the X-ray tube 104 at low and high voltages. It is generated corresponding to each voltage. The x-ray tube scans the object 110 switching between two separate voltage projections (kVp _L and kVp _H ). The high voltage projection kVp _H is kept high for the duration T _H and the low voltage projection kVp _L is kept low for the duration T _L. Furthermore, for constant view sampling, T _H = T _L = T _t / 2. The relative view time (s) is adjusted between kVp _L and kVp _H to modulate mAs at the corresponding tube currents (mA _H and mA _L ). In other words, the center of the peak-kilovoltage projection kVp _{H and} the center of the peak-kilovoltage kVp _L are kept constant when the mAs product is modulated. In one embodiment, the relative view time (s) is adjusted based on the average target attenuation profile and projection data. In one embodiment, the duration ratio is calculated based on the attenuation profile of the object 110 at a predefined projection angle. Note that the center of each view and the sum of the view times at high and low kVp remain the same as in the conventional constant view sampling configuration, regardless of the individual view sampling time changes at high and low kVp. Become.

  Various embodiments of the present invention provide a method that facilitates switching between two separate peak-kilovoltage (kVp) projections by using a variable view time data acquisition method. This method modulates mAs by changing the view time (s) when the current (mA) is relatively constant. Furthermore, the variable view time scan can operate at maximum tube power while maintaining an optimal mAs ratio for dual kVp scans. In addition, this system also provides better time resolution and better sensitivity to basis material degradation. Therefore, an image having a better image quality can be reconstructed.

  While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. Therefore, it is to be understood that the claims are intended to cover all modifications and variations as fall within the spirit of the invention.

DESCRIPTION OF SYMBOLS 100 CT system 102 Gantry 104 X-ray tube 106 X-ray beam 108 Detector array 110 Inspection object 112 Detector element 114 Center of rotation of gantry 116 Control mechanism 118 X-ray controller 120 Gantry motor controller 122 Data acquisition system 124 Processor 126 Computer 128 Memory 130 Operator console 132 Display 134 Table motor controller 202 Calculate optimal mAs ratio R for high kVp scan to low kVp scan 204 View time at high kVp and low kVp based on optimal mAs ratio Calculate view time 206 Switch kVp based on view time

Claims (9)

  A method for facilitating switching between at least two separate peak-kilovoltage (kVp) projections in an imaging system (100), the at least two types based on a target attenuation profile and projection data Adjusting the view time for separate peak kVp projections, and operating the imaging system (100) at a substantially constant current setting while providing an optimal current-time (mAs) ratio of two or more kVp projections. How to make it possible.   The optimal ratio of the mAs values is set based on one or more of the at least two distinct kVp settings, target dimensions, and target attenuation profiles at one or more projection angles. The method described.   The method of claim 1, wherein adjusting the relative view time (s) comprises maintaining a constant view speed and current (mA) setting for the at least two kVp projection views.   The method of claim 1, wherein adjusting the relative view time (s) is based on at least one of an average object attenuation profile and projection data.   The method of claim 1, further comprising calculating a duration ratio of the at least two distinct peak kVp projections.   The method of claim 1, wherein the imaging system is a computed tomography (CT) system (100).   The method of claim 1, wherein the imaging system (100) is an x-ray imaging system. An x-ray tube (104) disposed within the imaging system (100) and configured to emit at least two distinct peak kilovoltage (kVp) projections;
A detector array (108) comprising a plurality of detector elements and measuring projection data by said at least two separate kVp projections;
An imaging system (100) comprising a processor (124), the processor (124) comprising:
Is configured to adjust a view time (s) between the at least two separate kVp projections based on the attenuation profile of the object and projection data, the adjustment making the imaging system substantially constant An imaging system (100) that allows operation at a current (mA) setting.   The imaging system of claim 9, wherein the processor (124) is further configured to maintain a constant view speed.

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