Abstract:
The present invention provides a method to detect tube spits and to reduce spit related artifacts in a computed tomography imaging system. A way of detecting tube spit is to monitor the generator kilovolt or milliamp waveforms. Changes in kV waveform follow closely with those in offset-corrected projection data. If a tube-spit event is not detected, processing proceeds without tube spit correction. If a tube-spit event is detected, a tube spit correction is performed. The objective of tube-spit correction is to remove image artifacts due to the occurrence of a tube-spit event.

Description:
BACKGROUND OF INVENTION  
       [0001]     This invention relates generally to methods and apparatus for CT imaging and other radiation imaging systems, and more particularly to detecting and correcting data for tube-spit events.  
         [0002]     In at least some computed tomography (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 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 detectors are acquired separately to produce a transmission profile.  
         [0003]     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. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent the scintillator.  
         [0004]     The term “tube-spit” refers to temporary electrical short-circuit that sometimes occurs inside an x-ray tube. Typically, upon the occurrence of tube-spit, the supply of power to the x-ray tube is temporarily interrupted to prevent arcing. Power is restored to the tube after a time interval of, for example, about one millisecond. During tube-spit recovery, no x-ray photon is emitted from the x-ray tube. As a result, detector measurements taken during the recovery are invalid.  
         [0005]     Generally, tube-spit recovery time should be shorter than the data acquisition sampling interval to avoid image artifacts. As data sampling rates increase, a recovery time of about one millisecond is too long to avoid artifact generation.  
       SUMMARY OF INVENTION  
       [0006]     What is needed is a method for tube-spit detection and correction that can be performed utilizing software processing rather than increasing demands on hardware by reducing the time interval between switching power to the x-ray tube off and then on again.  
         [0007]     The algorithm described in the present method is used to detect tube spits and to reduce spit related artifacts. Detection of tube spit is the first and most important step of the algorithm. When tube spits, both voltage and current from the generator drop significantly, if not to zero. Therefore, one way of detecting tube spit is to monitor the generator kilovolt or milliamp waveforms. When plotted, kV waveform and related projection view data collected from a CT system with a spitting tube show that changes in kV waveform at a spit location follow closely with those in offset-corrected projection data. Therefore, generator kV waveforms can be used to detect views corrupted by tube spits.  
         [0008]     If a tube-spit event is not detected, processing proceeds without tube spit correction. If a tube-spit event is detected, a tube spit correction is performed. In addition, if a tube-spit event is detected, power may still be interrupted to the x-ray tube. The tube-spit correction described herein, however, eliminates a need to increase demands on hardware by reducing the time interval between interrupting power to the x-ray tube as compared to the time interval described above. That is, even with increased sampling rates, the time interval for interrupting power to the x-ray tube need not be reduced. Generally, the objective of tube-spit correction is to remove image artifacts due to the occurrence of a tube-spit event. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]      FIG. 1  is a pictorial view of a CT imaging system.  
         [0010]      FIG. 2  is a block schematic diagram of the system illustrated in  FIG. 1 .  
         [0011]      FIG. 3  is a set of kV waveform and related offset-corrected projection data collected from a CT system experiencing a tube spit.  
         [0012]      FIG. 4  is a flow chart illustrating tube-spit detection and correction processing. 
     
    
     DETAILED DESCRIPTION  
       [0013]     Referring now to the drawings in detail, wherein like numbered elements refer to like elements throughout,  FIGS. 1 and 2  refer to a computed tomography (CT) imaging system  10 . The 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 an object, such as a medical patient  22 . 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 an object or a 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 . In one embodiment, and as shown in  FIG. 2 , detector elements  20  are arranged in one row so that projection data corresponding to a single image slice is acquired during a scan. In another embodiment, detector elements  20  are arranged in a plurality of parallel rows, so that projection data corresponding to a plurality of parallel slices can be acquired simultaneously during a scan.  
         [0014]     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 .  
         [0015]     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 .  
         [0016]     X-ray source  14  includes an x-ray generator  15  for providing power to x-ray tubes. The x-ray generator  15  converts electrical power from a commercial AC power source to high-voltage DC at a selected voltage and for a suggested duration as instructed by the x-ray controller  28 . The x-ray generator  15  preferably also regulates the power supply to the x-ray tubes. The x-ray controller  28  may be separate or integrated with a universal control system.  
         [0017]      FIG. 4  is a flow chart depicting a method for tube spit detection and correction processing  100  in accordance with one embodiment of the present invention. Detection and correction of the tube spits is performed by a processor in DAS  32 , image reconstructor  24 , or computer. Rather than increasing the demands on hardware by reducing the timer interval between interrupting power to the x-ray tube and then re-energizing the tube during a recovery period, the detection and correction processing in  FIG. 4  is performed utilizing software control processing. Therefore, the difficulties associated with hardware and speed limitations as described above are substantially, if not entirely, avoided and not dependent on the sampling rate of DAS  32 .  
         [0018]     Reference is now made specifically to  FIG. 4 , which provides in detail the method for tube spit monitoring  100  disclosed by the present invention. In brief, the projection data is continuously monitored  102 . Each kilovolt value kV k  is then compared to a threshold value (kV spit threshold )  104  multiplied by a prescribed kV (kV prescribed ). If the value is above the threshold, the computer automatically processes the next value  106 . If, however, the measured value is below the threshold, the view may be corrupted and the computer moves on to a preliminary processing stage  108 . The only activity at this stage is to determine how many of the immediately preceding views have been corrupted. If the number of corrupted views is less than the maximum allowable number of corrupted views, N view , a view interpolation is performed  110 . While there are many methods for tube spit detection, one such method is described below in much more detail.  
         [0019]     The general objective of tube spit detection  104  is to determine whether the x-ray source experiences a drop in power.  FIG. 3A  and  FIG. 3B , show a graph of the generator output in kilovolts and the related projection data for an x-ray experiencing a tube spit event. In particular, where the generator output drops, as indicated by reference numeral  201 ,  202  and  203 , the projection data, as shown in  FIG. 3A  is corrupted. The corrupted data  301 ,  302  and  303  correspond to the generator output drops. Therefore, unlike earlier methods, the present method monitors the generator waveform directly. If the generator output drops to a threshold value, a tube spit event is declared. For example, the operator could decide that for a particular scan, it is important that very little data is corrupted, therefore, the kV threshold may be set at 125 kV. After tube spit detection  104 , tube spit correction is performed  110 , the image is processed further, and the image is reconstructed.  
         [0020]     First, two parameters have to be set in order for the program to perform properly. The first value is the threshold value, kV threshold , for the generator voltage kV prescribed . The present method calls for entry of that voltage as a fraction of the nominal voltage. Whenever the generator voltage dips below the specified fraction of the nominal voltage, the method for tube spit correction is initiated. Obviously, this value should be optimized to maximize the spit correction rate. The second value required is the maximum allowable number of corrupted views N view . Obviously, as the maximum allowable number of corrupted images rises, the quality of the image decreases. (However, in general, these two parameters are set by the manufacturer of the CT scanner.)The monitoring block  102  simply receives information as to the value of the generator voltage kV k . The monitoring block  102  then reports the value to the detection block  104 . The detection block compares kV k  to kV threshold *kV prescribed . kV prescribed  is a value set by the user during scan protocol prescription. kV threshold  is also a fractional value determined previously. Following detection of the tube spit criteria, a counter is initiated  106 . If the counter exceeds the maximum number of allowable corrupted images N view , a warning message is posted to the operator, and service personnel would be contacted for a possible tube change.  
         [0021]     If, however, a momentary tube spit occurred, and the number of unacceptable n view  views is under the threshold  108 , new views will be generated in place of the corrupted views using linear or high-order interpolation between the adjacent non-corrupted views  110 . Other methods for interpolation, which are well known in the art could also be used. The preferred interpolation method is shown below. 
 
 P   ij ( k+n )=(( n   view   −n )/( n   view +1)) P   ij ( k− 1)+(( n+ 1)/( n   view −1)) P   ij ( k+n   view ) 
 
 Note that P ij (k+n) is the projection at channel i, detector row j, view number k+n. k is the view number right before the tube spit. 
 
         [0023]     There are many possible methods for monitoring tube spit. For example, the most basic method would be to provide a voltmeter or ammeter to measure the voltage output to the x-ray source. The voltmeter would then be electronically connected to the computer which would initiate the tube spit correction method in the event a tube spit occurred.  
         [0024]     Another possible method would be to use the x-ray controller  28  to monitor the voltage directly. The x-ray controller  28  is electronically connected to the computer  36  such that, in the event that a voltage decrease is detected by the x-ray controller, a tube spit event is declared by the computer  36  and the method for tube spit correction begins.  
         [0025]     Yet another example would provide a voltage or current measuring device connected in an electronic manner to the DAS  32  or computer  36 , which would initiate the method for tube spit correction. There are many devices for measuring electrical current and/or voltage and many devices capable of communicating the voltage and/or current measurement to a processor or computer and several have been described above. The inventors do not intend for the devices used to measure the voltage and/or current or the devices that communicate the voltage and/or current to be limitations of the present invention.  
         [0026]     Yet another example is to store the history of the tube spit occurrence. If the frequency or the magnitude of the tube spit exceed certain threshold, operator and service engineers will be notified for tube replacement. Note that frequent tube spit is an indication of “end-of-life” for the tube. This feature allows the replacement of the tube before its complete failure to reduce the down time of the system.  
         [0027]     In summary, the method of the present invention provides for a method comprising the steps of: providing an x-ray controller for monitoring the output of a CT system generator; providing a computer to monitor the generator output from a CT system generator; setting a voltage threshold that, if the voltage to the x-ray controller falls below, a tube-spit event is declared; determining the number of corrupted views; warning the operator if the maximum number of corrupted views has been exceeded; and if a tube spit occurred, performing tube spit correction. The method also provides a warning to the operator that the maximum allowable number of corrupted views has been exceeded. The method of the present invention further provides for using some form of view interpolation, whether it is linear, high-order or follows the form: 
 
 P   ij ( k+n )=(( n   view   −n )/( n   view +1)) P   ij ( k− 1)+(( n+ 1)/( n   view +1)) P   ij ( k+n   view ) 
 
 wherein P ij (k+n) is the projection at channel i, detector row j, view number k+n. The method of the present invention further comprises the step of storing the history and magnitude of tube spit occurrences and notifying the operator and/or service personnel of the need to change the x-ray tube. 
 
         [0029]     The method of the present invention uses kV generator waveforms to detect tube spit. Compared with prior methods this new method is easier to implement and faster because it doesn&#39;t require projection data processing to determine if tube spit occurred. Prior devices used projection data to detect tube spit, which is much more computation intensive and difficult to implement.  
         [0030]     Although the inventors have very specifically described the preferred embodiments of the invention herein, it is to be understood that changes can be made to the improvements disclosed without departing from the scope of the invention. Therefore, it is to be understood that the scope of the invention is not to be overly limited by the specification and the drawings, but is to be determined by the broadest possible interpretation of the claims.