Abstract:
The present invention, in one form, is a system for correcting thermal drift in an imaging system. More specifically, a correction algorithm determines an adjusted focal spot position based upon a first temperature focal spot position and a second temperature focal spot position. The adjusted focal spot position is utilized in the reconstruction process to correct for focal spot movement resulting from thermal drift.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to imaging systems and more particularly, to correcting x-ray source focal spot thermal drift in an imaging system. 
     In at least one known computed tomography (CT) imaging system configuration, 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 the “imaging plane”. The x-ray beam passes through the 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 beam radiation received at the detector array is dependent upon the attenuation of the 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 known 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 so 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. 
     The x-ray source typically includes an evacuated glass x-ray envelope containing an anode and a cathode. X-rays are produced by applying a high voltage across the anode and cathode and accelerating electrons from the cathode against a focal spot on the anode. The x-rays produced by the x-ray tube diverge from the focal spot in a generally conical pattern. 
     To produce a quality image from an axial scan in CT scanners such as, for example, a third-generation CT scanner, it is desirable for the focal spot to be properly aligned in the x-axis. At least one known CT imaging system aligns the x-ray source at a single temperature, for example, an ambient temperature. However, as various elements of the x-ray tube heat during use, thermal expansion causes small mechanical displacements of critical x-ray source structures and a corresponding shift in focal spot position. As a result of this thermal drift, the single temperature alignment does not accurately reflect the position of the x-ray source focal spot during a scan of an object. Such thermal drift of the x-ray source focal spot results in aliasing and reduced image quality. 
     It would be desirable to provide a correction algorithm which is effective in correcting for focal spot position thermal drift. It would also be desirable to provide such an algorithm which improves image quality without increasing the costs of the system. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other objects may be attained by a correction algorithm which, in one embodiment, determines an adjusted focal spot position based on a first temperature focal spot position and a second temperature focal spot position. The adjusted focal spot position is then used during the reconstruction process to correct the projection data for the x-ray source thermal drift. Particularly and in accordance with one embodiment, a first temperature focal spot position is determined. After operating the imaging system for a determined period of time, a second temperature focal spot position is determined. Using the first temperature focal spot position and the second temperature focal spot position, the adjusted focal spot position is determined. More particularly and in one embodiment, the adjusted focal spot position is determined to be a midpoint of the first temperature focal spot position and the second temperature focal spot position. 
     After determining the adjusted focal spot. position, projection data collected from a detector array is corrected for x-ray source focal spot thermal drift. Specifically, after collecting the projection data, the adjusted focal spot position is used in the reconstruction process to correct for the thermal drift of the focal spot. 
     The above described algorithm corrects for focal spot position thermal drift so that aliasing is minimized and image quality is improved. Such algorithm also does not significantly increase the costs of the imaging system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial view of a CT imaging system. 
     FIG. 2 is a block schematic diagram of the system illustrated in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a computed tomograph (CT) imaging system  10  is shown as including a gantry  12  representative of a “third generation” CT scanner. Gantry  12  has an x-ray source  14  that projects a beam of x-rays  16  toward a detector array  18  on the opposite side of gantry  12 . Detector array  18  is formed by detector elements  20  which together sense the projected x-rays that pass through a medical patient  22 . Detector array  18  may be fabricated in a single slice or multi-slice configuration. Each detector element  20  produces an electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuation of the beam as it passes through patient  22 . During a scan to acquire x-ray projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 . 
     Rotation of gantry  12  and the operation of x-ray source  14  are governed by a control mechanism  26  of CT 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 patient  22  in gantry  12 . Particularly, table  46  moves portions of patient  22  through gantry opening  48 . Additional details regarding the above described imaging system are set forth in co-pending U.S. patent application Ser. No. (15-CT-4641), entitled Scalable Multislice Imaging System, which is assigned the present assignee and hereby incorporated herein, in its entirety, by reference. 
     The correction algorithm described below may be implemented in computer  36  and practiced using data collected by DAS  32 . It will be apparent to those skilled in the art, of course, that such algorithm could be practiced in other components. For example, the algorithm may be practiced directly in image reconstructor  34  so that corrected data is supplied to computer  36 . 
     In accordance with one embodiment of the present invention, the position of an x-ray source x-axis focal spot is determined for at least two separate temperatures of system  10 , specifically, x-ray source  14 . Utilizing the determined focal spot positions, an adjusted focal spot position is determined. 
     Specifically, a focal spot position is determined and stored when system  10 , specifically source  14 , is at a cold, or ambient temperature, for example 70 degrees F. In one embodiment, the focal spot position is determined by scanning a metal pin (not shown) positioned near an iso-center (not shown) of system  10 . More specifically, the metal pin is positioned approximately about 1.4″ above and approximately about 1.4″ right of the iso-center of system  10 . Particularly, a ⅛″ diameter round metal pin is properly positioned by attaching the pin to the end of gantry table  46  nearest gantry  12 , or on a phantom holder (not shown). The pin is then scanned and the x-ray focal spot is determined. Specifically, utilizing attenuation data collected from detector array  18 , the x-ray focal spot is determined. In alternative embodiments, other methods may be utilized to determine the position of the x-ray focal spot. 
     A number of scans are then performed by system  10  until source  14  achieves a second temperature, for example, approximately about a maximum operating temperature of source  14 . A second temperature focal spot position is then determined and stored in a similar manner as described above. The second temperature focal spot position may be different than the first temperature focal spot position as a result of thermal drift of the focal spot. 
     Utilizing the first temperature focal spot position and the second temperature focal spot position, an adjusted focal spot position is determined. Specifically and in one embodiment, the adjusted focal spot position exists in a range between the first temperature focal spot position and second temperature focal spot position. For example, a position difference is determined between the first temperature focal spot position and the second temperature focal spot position. The adjusted focal spot position is then determined by adding a percentage of the position difference to the first temperature focal spot position so that the adjusted focal spot position is between the first temperature and second temperature focal spot positions. 
     For example, where a focal spot position at a first temperature, T1, is determined to be at location 373 and a focal spot position at a second temperature, T2, is determined to be at location 374, the position difference is (374−373)=1. As a result of thermal drift of the focal spot, the adjusted focal spot position may be determined to be within a range of 373 and 374. The adjusted focal spot position within the range is then determined. Specifically, where T1 is a room ambient temperature, T2 is a maximum operating temperature and as a result of source  14  typically operating at a temperature closer to the maximum operating temperature than an ambient temperature, the adjusted focal spot position may be selected to be closer to the T2 focal spot position. More specifically, the typical operating temperature of x-ray source  14  may be 70% of the maximum operating temperature. As a result, the adjusted focal spot position may include 70% of the position difference. Particularly, the adjusted focal spot position may be ((70%×(374−373))+373)=373.7. 
     Of course, other factors may be considered in determining the adjusted focal spot position in system  10 . For example, the adjusted focal spot position may be determined to be at a midpoint between the first temperature focal spot position and the second temperature focal spot position. Additionally, the adjusted focal spot position within the range may be altered as a result of the type of test being conducted. For example, in some known clinical environments, x-ray source  14  is operated at 50% to 75% of the maximum operating temperature. By properly determining the adjusted focal spot position within the position difference, the actual focal spot position and the adjusted focal spot position used in the reconstruction process will be very close. As a result, image quality will be optimized without significantly increasing the system costs and without requiring additional processing time to determine focal spot position during a scan. 
     The adjusted focal spot position is then used during the reconstruction process to generate the image data. Specifically, the adjusted focal spot position may be used in known reconstruction algorithms to correct the data collected from detector array  18 . More specifically and in one embodiment, to minimize aliasing of high contrast high resolution objects, the adjusted focal spot position may be used to correct a backprojector isocenter in the reconstruction process. The isocenter is corrected for thermal drift of focal spot  60 . In other embodiments, the adjusted focal spot position may be used in other portions of the reconstruction process so that the data gathered during the scan is corrected for thermal drift of the focal spot. 
     The above described algorithm facilitates reducing aliasing of objects as a result of focal spot thermal drift. As a result, reconstructed image quality is improved. In addition, such improved quality is without significantly increasing the system cost and without additional processing time to determine focal spot position during a scan. 
     From the preceding description of various embodiments of the present invention, it is evident that the objects of the invention are attained. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. For example, the imaging system described herein is a CT system. Many other imaging systems may be used, for example, x-ray imaging. In addition, the CT system described herein is a “third generation” system in which both the x-ray source and detector rotate with the gantry. Many other CT systems including “fourth generation”. systems wherein the detector is a full-ring stationary detector and only the x-ray source rotates with the gantry, may be used if individual detector elements are corrected to provide substantially uniform responses to a given x-ray beam. Moreover, the system described herein performs an axial scan, however, the invention may be used with a helical scan although more than 360° of data are required. Accordingly, the spirit and scope of the invention are to be limited only by the terms of the appended claims.