Abstract:
A method and apparatus for reducing trigger jitter from a CT system position encoder including the steps, for each trigger pair in the encoder signal, identifying the integer portion of average period corresponding to N preceding trigger pairs, identifying a modulus-N residual corresponding to the N preceding trigger pairs as a lag value, adding the lag value to a lag count, determining when the lag count exceeds N and, where the lag count exceeds N, incrementing the integer portion by one, identifying a modulus-N residual corresponding to the lag count, setting the lag count equal to the residual corresponding to the lag count and generating a final binary trigger signal corresponding to the integer portion.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to computerized tomography and more particularly to a method and apparatus for minimizing the effects of gantry jitter on image quality. 
     In computerized tomography (CT) a patient is positioned on a support table with a portion of a patient to be imaged (hereinafter “a region of interest”) disposed within an imaging area, an X-ray photon source and a detector array are mounted to an annular gantry on opposite sides of the imaging area and the gantry, including the detector and the source, are rotated about the imaging area so that photon rays from the source are directed through the region of interest toward a detector opposite the source. In addition to the rotational motion, the support table may be translated through the imaging area so that the rays sweep a helical path through the region of interest. Attenuated rays are detected by the detector, the amount of attenuation indicative of the make up (e.g. bone, flesh, air pocket, etc.) of the region of interest through which the rays traverse. 
     The attenuation data is processed and grouped into separate “views” about the patient where each view corresponds to a specific gantry orientation and hence a specific source position or angle with respect to the imaging area. Thereafter, the views are back-projected according to a reconstruction algorithm to generate an image of the region of interest. Generally, the “back projection” is performed in software but, as the name implies, is akin to physically projecting views from many different angles within an image plane through the image plane, the view rays passing through the same image voxels being combined in some manner to have a combined effect on the voxel in the resulting image. 
     In order to group the attenuation data into separate views, ideally, the 360 degrees of gantry rotation are equally divided into view angle ranges (hereinafter “view ranges”) corresponding to the required number of views and then the data collected within each separate view range is binned together to form a corresponding view. For instance, where 984 separate views are required to construct an image, the 360 degree range is divided into 984 separate view ranges of approximately 0.3659 degrees each. Thereafter, data collected within each separate view range during a single source rotation is stored as a separate view. In operation, during data acquisition, the system tracks source orientation and, when the source transitions from a first position within a first view range to a second position within a second view range, the system generates a trigger signal causing the system to begin binning the data in a new view corresponding to the second view range. 
     One useful method for identifying source orientation and determining if the source angle is within a specific angle range has been to provide a ring encoder linked to the gantry that senses gantry position and generates source location trigger signals. For instance, an exemplary encoder may be capable of differentiating 106,496 separate and equispaced source orientations (i.e., the encoder has a 106,496 position resolution). Where 984 separate views are required, the 106,496 encoder positions are divided into 984 separate position ranges corresponding to the 984 views. Hereinafter the ratio of encoder resolution to required views is referred to as the encoder-view ratio. 
     In the example above, where the gantry rotates at one rotation per second, the encoder generates a signal having a frequency of precisely 106,496 Hz and data corresponding to 984 views is collected for every gantry rotation. Where gantry rotation frequency is increased, the encoder frequency and view frequency increase proportionally. For instance, where rotation frequency is 2 Hz, the encoder and view frequencies are doubled to 212,992 Hz and 1968 Hz, respectively. Hereinafter the encoder and view acquisition frequencies will be referred to generally as encoder frequency and view frequency, respectively. 
     Encoder type systems like the system described above have two important shortcomings. First, often the encoder frequency does not divide evenly by the view frequency. For instance, in the example above, the encoder-view frequency ratio is 108.23 (i.e., 106,496 Hz/984 Hz) and therefore the encoder positions cannot be precisely and directly converted into view ranges. 
     Second, as well known in the CT industry, while attempts have been made to manufacture robust and precise encoders, even high quality encoders tend to jitter (i.e., vibrate) during gantry rotation so that the instantaneous encoder frequency may vary appreciably. For instance, in one CT system the rotating portion of the gantry includes a ring having externally extending teeth and the encoder includes a relatively smaller gear having teeth that mate with the gantry teeth so that the encoder gear spins as the gantry rotates. In an exemplary case the encoder gear rotates at 13 times the gantry rotational frequency and includes 300 teeth. In this case, where the gantry rotation frequency is 1 Hz, the frequency of the encoder signal will often include 13 and 300 Hz noise components. 
     While not discussed here in detail, it should be noted that the CT imaging environment is often very noisy and therefore there are many other noise sources that pollute the encoder trigger signal so that the trigger signal sequence that is generated often does not precisely reflect the gantry and source position. As with most mechanical systems, the encoder accuracy problems are exacerbated as the encoder components wear over time. 
     While jitter and encoder-view ratio related frequency inaccuracies may be acceptable in certain applications, in many applications such variations cause image artifacts that appreciably reduce the diagnostic value of resulting images. For instance, where long CINE scans of several minutes are performed, in order to align views from consecutive gantry rotations, the scans require that the first trigger in every rotation occur within 10% of the first trigger in the first scan rotation. This 10% registration requirement requires that the system have minimal trigger signal drift. For instance, if a system were to lose 0.99 triggers every rotation, after only two rotations the drift would be too great for the 10% registration requirement. 
     To account for fractional encoder-view ratios, many systems feed the encoder signals to a phase locked loop (PLL) circuit. PLLs are well known in the art and therefore will not be explained here in detail. It should suffice to say, in this regard, that a PLL circuit typically receives the encoder signal and generates an output trigger signal every N encoder signals where N is the encoder-view ratio. For instance, in the present example, where the encoder-view ratio is 108.23 (i.e., 106,496/984), the PLL trigger signal is generated approximately every 108.23 encoder signals. 
     Unfortunately, while the PLL reduces the affects of system noise somewhat, the encoder noise is at least in part reflected in the PLL output trigger signal. For example, where the encoder signal has an instantaneous frequency range of between plus and minus 10% of the ideal encoder frequency (i.e., the encoder frequency that would precisely correspond to gantry position), the PLL trigger signal frequency may have a range of plus or minus 5% of the ideal view frequency. Five percent variance in the frequency spectrum is too great for many applications. 
     SUMMARY OF THE INVENTION 
     It has been recognized that the mass of a gantry and components attached thereto is typically large and therefore, during gantry rotation, the gantry and attached components typically maintain their rotational frequency over small rotational ranges despite system noise and instantaneous encoder frequency changes. For this reason recent gantry rotational frequency history can be used to relatively precisely identify trigger times corresponding to different view ranges. 
     To this end, the present invention includes a filter apparatus that receives the PLL trigger signals and identifies a moving average of the periods corresponding to separate view ranges during data acquisition. Thereafter, the filter apparatus generates a filtered trigger signal that occurs at the average of the most recent view range periods. The filter, in effect, substantially eliminates the effects of jitter from the trigger signals so that the resulting trigger signal more closely mirrors the gantry and source position. For instance, in a typical case, the filtered trigger signal frequency is plus or minus 1% of the actual instantaneous gantry frequency and therefore the trigger signals are more precisely aligned with the gantry position. 
     In addition, the inventive apparatus retains a running filter error and uses the running error to compensate for drift that the filter could introduce into the trigger signals. To this end, in at least some embodiments, where the moving average period is determined over N trigger cycles, the modulus-N values for consecutive cycles are summed until the modulus-N sum, referred to herein as a lag count, exceeds N. When the lag count exceeds N for any cycle, the integer portion of the moving average is incremented by one to eliminate the effects of drift. In addition, when the lag count exceeds N for any cycle, a modulus-N value for the lag count is determined and the lag count is reset to the modulus-N value. 
     Consistent with the above, the present invention includes a method for use with a CT system including a gantry mounted position encoder that provides a digital encoder position signal including signal pulses that indicate gantry positions, the system including a phase locked loop (PLL) that receives the position signal and generates an intermediate trigger signal every X/Y position signals, each two consecutive intermediate trigger signals comprising a trigger pair, the method comprising the steps of beginning with the first trigger pair and working toward the last trigger pair in the intermediate signal, for each trigger pair: identifying the average period corresponding to N preceding trigger pairs; and generating a final trigger signal as a function of the average period. 
     In at least some embodiments the step of identifying includes the steps of identifying the integer portion of the average period, identifying drift in the integer portion, determining when the drift exceeds a threshold value and modifying the integer portion to compensate for the drift when the drift exceeds the threshold value. In addition, the step of identify drift may include the steps of identifying a modulus-N residual corresponding to the N preceding trigger pairs as a lag value and adding the lag value to a lag count. 
     The step of determining when drift exceeds a threshold value may include the steps of determining when the lag count exceeds N. Moreover, the step of modifying the integer portion may include, when the lag count exceeds N, incrementing the integer portion by one. Here, the method may further include the step of, when the lag count exceeds N, identifying a modulus-N residual corresponding to the lag count and setting the lag count equal to the residual corresponding to the lag count. N may include the N immediately preceding pulse pairs. N, in many embodiments, corresponds to a fraction of a complete gantry rotation. Typically N corresponds to 3 to 50 percent of a complete gantry rotation. 
     Where the trigger signal is a binary signal including a high time followed by a low time, in at least some embodiments the step of generating the final trigger signal further includes dividing the integer portion by two to generate a half period, rounding the half period up and down to generate ceiling and floor periods, respectively, setting a one of the low and high times equal to one of the floor and ceiling periods&#39; and setting the other of the low and high times equal to the other of the floor and ceiling periods. Here, the steps of setting the low and high times further may include setting the high time equal to the floor period and setting the low time equal to the ceiling period. 
     The invention also includes a method for use with a CT system including a gantry mounted position encoder that provides a digital encoder position signal including signal pulses that indicate gantry positions, the system including a phase locked loop (PLL) that receives the position signal and generates an intermediate trigger signal every X/Y position signals, each two consecutive intermediate trigger signals comprising a trigger pair, the method comprising the steps of, beginning with the first trigger pair and working toward the last trigger pair in the intermediate signal, for each trigger pair: identifying the integer portion of an average period corresponding to N preceding trigger pairs, identifying a modulus-N residual corresponding to the N preceding trigger pairs as a lag value, adding the lag value to a lag count, determining when the lag count exceeds N and, where the lag count exceeds N: (i) incrementing the integer portion by one, (ii) identifying a modulus-N residual corresponding to the lag count, (iii) setting the lag count equal to the residual corresponding to the lag count and generating a final binary trigger signal including a high time followed by a low time, the step of generating including, dividing the integer portion by two to generate a half period, rounding the half period up and down to generate ceiling and floor periods, respectively, setting one of the low and high times equal to one of the floor and ceiling periods and setting the other of the low and high times equal to the other of the floor and ceiling periods. 
     Moreover, the invention includes an apparatus for use with a CT system including a gantry mounted position encoder that provides a digital encoder position signal including signal pulses that indicate gantry positions, the system including a phase locked loop (PLL) that receives the position signal and generates an intermediate trigger signal every X/Y position signals, each two consecutive intermediate trigger signals comprising a trigger pair, the apparatus comprising a program running a pulse sequencing program to perform the steps of, beginning with the first trigger pair and working toward the last trigger pair in the intermediate signal, for each trigger pair: identifying an average period corresponding to N preceding trigger pairs and generating a final trigger signal as a function of the average period. 
     In some embodiments the program causes the processor to perform the step of generating by performing the steps of, identifying the integer portion of the average period, identifying a modulus-N residual corresponding to the N preceding trigger pairs as a lag value, adding the lag value to a lag count, determining when the lag count exceeds N and, where the lag count exceeds N: (i) incrementing the integer portion by one, (ii) identifying a modulus-N residual corresponding to the lag count, (iii) setting the lag count equal to the residual corresponding to the lag count and generating a final binary trigger signal corresponding to the integer portion. 
     More specifically, where the final trigger signal includes a high time followed by a low time, the program may cause the processor to perform the step of generating by dividing the integer portion by two to generate a half period, rounding the half period up and down to generate ceiling and floor periods, respectively, setting one of the low and high times equal to one of the floor and ceiling periods and setting the other of the low and high times equal to the other of the floor and ceiling periods. 
     The program may also cause the processor to perform the steps of setting the low and high times by setting the high time equal to the floor period and setting the low time equal to the ceiling period. 
    
    
     These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a CT apparatus used to practice the present invention which includes a detector array having rows and columns of detector elements and fan beam source; 
     FIG. 2 is a block diagram of CT control system which may be used to control the CT apparatus of FIG.  1  and which is useful for the purposes of practicing the present invention; 
     FIG. 3 is a schematic diagram illustrating processor components and exemplary system signals according to the present invention; and 
     FIG. 4 is a flow chart illustrating an exemplary method according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A. Hardware 
     Referring now to FIG. 1, a CT scanner for use with the present invention includes a gantry  20  having an opening (i.e., defining an imaging area) supporting an x-ray source  10  oriented to project a fan beam  40  of x-rays along the beam axis  41  through a patient  42  to a supported and opposed detector array  44 . The gantry  20  rotates to swing the beam axis within a gantry plane  38  defining the x-y plane of a Cartesian coordinate system. Rotation of the gantry  20  is measured by beam angle B from an arbitrary reference position within the gantry plane  38 . 
     A patient  42  resets on a table  46  which may be moved along a translation axis  48  aligned with the Z-axis of the Cartesian coordinate system. Table  46  crosses gantry plane  38  and is radio-translucent so as not to interfere with the imaging process. 
     The x-rays of the fan beam  40  diverge from the beam axis  41  within the gantry plane  38  across a transverse axis  50  generally orthogonal to both the beam axis  41  and the translation axis  48  at a fan beam angle γ. The x-rays of beam  40  also diverge slightly from the beam axis  41  and the gantry plane  38  across the translation axis  48 . Referring also to FIG. 3, a maximum beam angle γ is identified by symbol Γ. 
     After passing through patient  42 , the x-rays of the fan beam  40  are received by detector array  44  which has multiple columns of detector elements  18 ′. The detector elements  18 ′ in exemplary array  44  are arranged in eight rows (i.e., array  44  is an eight slice detector) extending along the traverse axis  50  that subdivide array  44  along the Z-axis and a plurality of columns extending along Z or translation axis  48 . The width of detector array  44  is measured along Z-axis  48 . The surface of detector array  44  may be planar or may follow a section of a sphere or cylinder having a center at focal spot  26  or alternatively at the system isocenter. 
     The detector elements  18 ′ each receive x-rays and provide intensity measurements along separate rays of the fan beam  40 . Each intensity measurement describes the attenuation via a line integral of one fan beam ray passing through a portion of volume  43  of patient  42 . The dimension of volume  43  along Z-axis  48  is greater than the Z-axis width of eight slice array  44 . 
     Referring to FIGS. 1 and.  2 , an exemplary control system for controlling the CT imaging system of FIG. 1 includes gantry associated control modules collectively identified by numeral  52 , a table motor control  58 , slip rings  64 , a central processing computer  60 , an operator&#39;s console  65 , a mass storage device  66  and an encoder  69 . Modules  52  include an x-ray control  54 , a gantry motor control  56 , a data acquisition system  62  and an image reconstructor  68 . X-ray control  54  provides power and timing signals to the x-ray source  10  to turn it on and off as required under the control of a computer  60 . Gantry motor control  56  controls the rotational speed and position of the gantry  20  and provides information to computer  60  regarding gantry position. Data acquisition system  62  samples and digitizes intensity signals from the detector elements  18 ′ of detector array  44  provides the digitized signals in the form of helical data row views to computer  60  for storage in mass storage device  66 . Reconstructor  68  is linked to computer  60  for receiving slice image data there from and back projects the received data to, as its label implies, construct a slice image for viewing or that can be manipulated in some other manner. 
     Each of the above modules is connected to associated gantry mounted components via slip rings  64  and is also linked to computer  60  for control purposes Slip rings  64  permit gantry  20  to rotate continuously through angles greater than 360° to acquire projection data. 
     The speed and position of table  46  along translation axis  48  is controlled by computer  60  by means of table motor control  58 . In addition, computer  60  runs a pulse sequencing program to perform the inventive data processing method as described in more detail below. Computer  60  receives commands and scanning parameters via operator console  65  that generally includes some type of visual interface device (e.g., a CRT display) and one or more input devices (e.g., a keyboard, a mouse controlled display cursor, etc.). Console  65  allows an operator to enter parameters for controlling a data acquiring scan and to display constructed image and other information from computer  60 . 
     Mass storage device or memory  66  provides a means for storing operating programs for the CT imaging system, as well as image data for future reference by the operator. Both computer  60  and the image reconstructor  68  have associated electronic memory (not shown) for storing data and pulse sequencing programs. 
     Encoder  69  is mounted to the gantry  20  for measuring gantry and source position during data acquisition and, to that end, provides an encoder output signal that is provided to computer  60 . For the purposes of this explanation it will be assumed that encoder  69  generates 106,496 separate position signals during each rotation of the gantry to identify a like number of gantry positions. In addition, unless indicated otherwise, it will be assumed that the gantry  20  is rotating at 1 Hz and therefore the encoder, over the course of each gantry rotation, averages an encoder output signal frequency of 106,496 Hz. An exemplary encoder output signal  78  is illustrated in FIG.  3  and includes a digital signal having either a high value (i.e., akin to a 1) or a low value (i.e., akin to a 0). 
     As discussed above, encoder signal  78 , while having a frequency of 106,496 over the course of each gantry rotation, typically has an instantaneous frequency during the course of each rotation that appreciably varies about 106,496 Hz. In FIG. 4 an exemplary encoder signal frequency spectrum  88  is shown as 106,496+/−10%. 
     In operation, gantry motor control  56  brings gantry  20  up to a rotational speed and table motor control  58  begins translation of table  46  along translation axis  48 . The x-ray control  54  turns on x-ray source  10  and projection data is acquired on a continuous basis. The table  46  translation speed relative to the gantry rotation rate is referred to as the operating “pitch”. At each gantry angle, the projection data acquired comprises intensity signals corresponding to each detector element  18 ′ at each particular column and row of array  44 . The collected data is stored in storage device  66  as helical data including row views correlated by gantry angle into separate views. 
     Computer  60  uses the encoder signals to identify divisions between the separate views. In the present example it will be assumed that 984 separate views are acquired during each gantry rotation. Thus, because the encoder generates 106,496 separate gantry position signals during each gantry rotation, the computer must identify a separate trigger signal every 108.23 encoder signal pulses to precisely identify the temporal boundaries between data acquired for consecutive views. 
     Referring still to FIG.  2  and also to FIG. 3, according to the present invention, in addition to other components, computer  60  includes a PLL  80  and an averager-drift compensator circuit  82 . The PLL  80  is a conventional PLL which should be well understood by one of ordinary skill in the art and therefore is not explained here in detail. Suffice it to say that PLL  80  receives a high frequency encoder signal  78  that, in the present example, is approximately 106,496 Hz and converts that signal into a relatively low 984 Hz frequency intermediate trigger signal  84 . The PLL conversion has some filtering effect on the encoder signal and therefore the PLL output trigger signal  84  will typically have less relative variance in its frequency spectrum. Thus, the exemplary intermediate trigger signal frequency spectrum in FIG. 3 is illustrated as being 984 Hz+/−5%. 
     The averager-drift compensating circuit  82  receives the intermediate trigger signal and performs a moving average and drift compensation process on the intermediate signal to reduce the frequency spectrum variation appreciably. To this end, it has been recognized that, while some instantaneous frequency variation occurs during each gantry rotation, because the gantry has a relatively large mass and, during acquisition, is typically characterized by a relatively large inertia, the frequency variation that occurs is typically within a relatively small frequency range and therefore the view trigger signal frequency spectrum also should remain in a small frequency range. This small frequency range limitation is particularly true over partial gantry rotations such as 5 to 10 percent of a complete rotation. 
     Thus, circuit  82  is programmed to, for each intermediate trigger signal cycle, identify a preceding moving average cycle period and generate a corresponding final trigger signal that essentially maintains the moving average cycle period. More specifically, circuit  82  takes the integer portion of the moving average and generates a corresponding trigger signal that follows the preceding trigger signal by a period equal to the integer portion. This process of using the integer portion to generate the trigger signal is referred to hereinafter as integer rounding. 
     In at least one embodiment, to make sure that the moving average reflects recent gantry rotation, the average is taken over a period corresponding to only a fraction of a complete gantry rotation. For instance, in one embodiment, the moving average may be taken over 64 intermediate trigger cycles. In this case, because 984 intermediate trigger cycles occur during each gantry rotation, the moving average corresponds to approximately 6.4% or 23 degrees of gantry rotation. 
     In addition, as its label implies, averager-drift compensating circuit  82  tracks drift that the integer rounding process causes and compensates for that drift. One exemplary drift compensating process is described in more detail below. 
     Referring now to FIG. 4, an exemplary method  98  according to the present invention is illustrated. Referring also to FIGS. 1 and 2, it should be appreciated that, when referring to FIG. 4, it is assumed that gantry  20  has already achieved a data acquiring rotational speed, in the present case, a rotational speed of 1 Hz. In addition, it is assumed that encoder  69  has already generated at least enough encoder signal pulses for a processor to perform a moving average cycle period calculation. To this end, the letter N will be used to refer to the number of cycles used to perform the moving average. For instance, where N is 64, the moving average process would include averaging signal cycles over 64 consecutive signal cycles. Herein the first intermediate signal cycle period for which the inventive process is performed will be referred to as P 1  meaning that the encoder  69  has provided at least N−1 signal cycles prior to period P 1.    
     Referring still to FIGS. 1,  2  and  4  and also to FIG. 3, initially the processor sets a counter i value equal to 1 at block  99 . At block  100 , the processor measures period P i . At block  102 , the processor calculates the moving average period corresponding to the N intermediate signal cycle periods including period P i  and the N−1 periods that precede period P i . The averaging equation can be expressed as follows:                P   ave     =       (       P   i     +     P     i   -   1       +     P     i   -   2       +   …   +     P     i   -   N         )     N             Eq   .              1                                
     At block  104 , the processor identifies the integer portion P int  of the moving average period P ave . At block  106 , the processor identifies the modulus-N of the sum period corresponding to the N periods ending with period P i  as a residual or lag period P lag . The calculation solved to identify the lag period P lag can be expressed as follows: 
     
       
           P   lag =( P   i   +P   i−1   P   i−2   + . . . +P   i−N )mod N   Eq. 2 
       
     
     Continuing, at block  108 , the processor adds the lag period identified in Equation 2 to a running lag count P lagct  thereby increasing the value of the running lag count P lagct . This summation process can be expressed as follows: 
     
       
           P   lagct   =P   lagct   +P   lag   Eq. 3 
       
     
     Referring still to FIGS. 1-4, at block  110  the processor determines whether or not the lag count is equal to or greater than value N. Where the lag count is less than value N, control passes to block  118  where final trigger pulse high and low times are set according to the following two equations: 
     
       
           T   high =floor( P   int /2)  Eq. 4 
       
     
     
       
           T   low =ceiling( P   int /2)  Eq. 5 
       
     
     The “floor” operator corresponds to a function that rounds a corresponding value down to the nearest integer value. Similarly, the operator begins “ceiling” operator corresponds to a function that rounds a corresponding value up to the nearest integer. For examples, referring to Equation 4, if the integer portion P int  is 9, Equation 4 would yield a T high  value of 4 and Equation 5 would yield a T low  value of 5. After block  118 , control passes to block  116  where counter i is incremented by 1 prior to control passing back up to block  100  again where the process is repeated. 
     Referring again to block  110 , where the lag count P lagct  is greater than or equal the threshold value N, the cumulative drift caused by circuit  82  is at a sufficiently high value that the drift must be compensated. To this end, control passes to block  112  where the integer portion P int  is incremented by 1. Next, at block  114  the lag count P lagct  is set equal to the modulus-N value of the P lagct . This calculation can be represented according to the following equation: 
     
       
           P   lagct =( P   lagct )modn  Eq. 6 
       
     
     After block  114  process control passes to block  118  where the high and low trigger times are set according to Equations 4 and 5 above. Thereafter, control again passes to block  116  where counter i is incremented by 1. Next, as above, control passes back up to block  100  where the process is repeated for the next signal cycle. 
     The  T   high  and  T   low  values are used to generate the corrected trigger signal  86  in FIG. 3 that reflects gantry and source position more precisely and that, in the present example, would have a view frequency range of approximately 984 Hz±1%. The corrected trigger signal  86  is then used to divide acquired data into separate views as known in the art. 
     It should be understood that the methods and apparatuses described above are only exemplary and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall under the scope of the invention. For example, the invention may be used either during data acquisition or post acquisition to divide the data into corresponding views. In addition, other averaging algorithms may be employed and, in certain cases, the N value may be larger or smaller depending upon system configuration.