Patent Publication Number: US-2013231558-A1

Title: System and Method for Reducing Angular Geometric Distortion in an Imaging Device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a system and method for reducing angular geometric distortion in an imaging device. 
     2. Background of the Related Art 
     A number of diagnostic tools are used in the field of Interventional Cardiology to diagnose and treat heart disease. Among other things, Intravascular Ultrasonic (IVUS) imaging systems are used to estimate an extent of coronary artery plaque, allowing an interventionalist to detect heart disease, estimate the severity of the disease and its effect on blood flow, and choose appropriate therapies. One such system is a fluoroscopy system, which injects a radio-opaque contrast dye into the bloodstream, and then uses a real time x-ray imaging technique to watch as the contrast dye passes through the vascular tree. However, fluoroscopy systems do not image the vessel tissue, but rather the blood pool inside the vessel lumen. 
     In an attempt to obtain an image of the vessel wall, and not just the blood pool, ultrasonic imaging transducers are mounted at the distal end of catheters and positioned in the coronary arteries. This approach allows the visualization of the tissues of the artery walls, and more particularly, visualization of the atherosclerotic plaque that forms in these vessels. However, these images are compromised in that the assumed angular orientation of the ultrasonic transducer at the distal tip of the catheter is not, in fact, the actual orientation of the transducer. 
     More particularly, in many IVUS imaging systems, the proximal end of a torque cable is spun at a uniform velocity by an electric motor. For example, a motor rotated at approximately 1800 RPM generates a complete circular cross section at a rate of approximately 30 frames, or images, each second. However, a catheter has a number of bends and twists in its length necessary to get from the entry point into the human body to a point at which an image is acquired. One of the most common distortions generated by these types of IVUS systems is Non-Uniform Rotational Distortion (NURD). 
     NURD can be caused by a number of different sources, including, for example, friction between the spinning torque cable and the stationary sheath that encloses the cable, or the torque cable and transducer assembly being not perfectly cylindrically symmetrical, causing the cable to resist bending more at some angles than at other angles. When rotated, these asymmetries will cause the cable to store more energy in some angular orientations and then to release that energy as the cable is rotated past that angle. In either case, the angular velocity of the transducer varies with each cycle even if the motor maintains a constant speed at the proximal end of the catheter. In many situations, an absolute angular orientation of a particular lesion or section of a lesion may be critical in performing an accurate, timely diagnosis and prescribing appropriate treatment. Thus, it is important to substantially eliminate, or at least significantly reduce, NURD so that these measures of angular extent are accurate and reliable. 
     To this end, new IVUS systems have been proposed in which the catheter is manually rotated in either direction so as to produce an image of a sector of an artery, rather than spinning the catheter in a single direction. However, this change in rotational direction leads to another form of NURD caused by the “windup” and resulting “backlash” of the transducer when the rotational direction is changed. Electronically steered imaging systems produce images without rotating the transducers on the distal end of the catheter, and thus do not produce any rotational distortion. However, this comes at the expense of increased transducer, catheter, and imaging system complexity and cost. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter. 
     Embodiments of the invention are directed to a system and method for reducing angular geometric distortion in an imaging device. The system and method according to the invention may be utilized with any type of device used to image and/or inspect an interior portion of a cavity, such as a substantially tubular cavity, in which it is advantageous to significantly reduce or substantially eliminate angular geometric distortion. 
     Further, embodiments of the invention are directed to a system and method capable of significantly reducing or substantially eliminating NURD in both continuously rotated IVUS systems, in which the direction of rotation remains substantially constant, and in manually rotated IVUS systems, in which the direction of rotation is changed and windup and backlash NURD are also present. 
     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein: 
         FIG. 1A  is an exemplary IVUS imaging system by means of which embodiments of the invention may be implemented; 
         FIG. 1B  is a block diagram of a processor according to an embodiment of the invention; 
         FIG. 2  is an exemplary data table generated and stored by the processor shown in  FIG. 1B ; 
         FIGS. 3A-3G  are flow charts which show a number of different combinations and sequences of processing steps or methods which may be implemented in the exemplary IVUS imaging system shown in  FIG. 1A  and/or implemented by the processor shown in  FIG. 1B ; 
         FIG. 4  is a flow chart of a process to collect raw angle and echo line data such as that shown in  FIG. 2 , in accordance with an embodiment of the invention; 
         FIG. 5  is a flow chart of a backlash and/or windup NURD removal process, in accordance with an embodiment of the invention; 
         FIG. 6  is a flow chart of an angular correlation process, in accordance with an embodiment of the invention; 
         FIG. 7  is a flow chart of a line-to-line correlation process, in accordance with an embodiment of the invention; 
         FIG. 8A  is a flow chart of a data table rewriting process, in accordance with an embodiment of the invention; and 
         FIG. 8B  is a flow chart of a data table rewriting process, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following detailed description of embodiments of the invention is discussed in the context of NURD reduction for an IVUS system. However, the invention may be utilized with any type of device used to image and/or inspect an interior portion of a cavity, such as a substantially tubular cavity, in which it is advantageous to significantly reduce or substantially eliminate angular geometric distortion. 
     NURD reduction for an IVUS system can be viewed as a process that more accurately assigns angles that are associated with each acoustic line in an IVUS frame compared to angles that are assumed to be present at a proximal end of a catheter as imaging data is collected. The systems and methods according to certain embodiments of the invention apply algorithms which may be implemented in either hardware or software, and may operate in real time on data as it is collected from a transducer to produce images without any objectionable distortions caused by NURD. 
     It should be noted that, as discussed above, medical devices, and more particularly, IVUS devices, are referred to herein merely for ease of discussion, and that the systems and methods according to embodiments of the invention may be applied to any device used to image and/or inspect an interior portion of a cavity, such as a substantially tubular cavity, to significantly reduce or substantially eliminate angular geometric distortion. Further, although the term “IVUS system” refers to ultrasonic imaging systems, the systems and methods apply as well to other forms of intravascular imaging systems, such as, for example, those that use other forms of energy to make images. These systems may include, for example, optical imaging systems, such as, for example, Optical Coherence Tomography. 
       FIG. 1A  illustrates an exemplary IVUS imaging system by means of which embodiments of the invention may be implemented. The system  1  of  FIG. 1A  includes a processor  20  which controls an electronics module  18 . The electronics module  18  transmits an ultrasonic pulse to a transducer assembly  14  positioned at a distal tip  28  of a catheter  10  via data a path  12 , and an angle encoder  16  records the relative angle of the catheter and transmits it back to the electronics module  18 . Incremental angle information sensed by the angle encoder  16  is transmitted to the electronic module  18  via a data path, or cable  22 , and a data path, or cable  24  provides the power and ground for the angle encoder  16 . Echo data is returned to the electronics module  18  via the data the path  22 , and the processor  20  accepts the echo digitized data from the electronics module  18  and stores it in a data table, such as the exemplary data table shown in  FIG. 2 . Software running in the processor  20 , or alternatively, hardware or firmware running in the electronics module  18 , manipulates the information in the data table to remove the NURD artifact and an intravascular image is then created by scan conversion of the information in the data table and displayed on a display module  21 . In some embodiments of the invention, the processor  20  may be a separate unit, as shown in  FIGS. 1A-1B , while in other embodiments of the invention, the capabilities of the processor  20  may be included in the electronics module  18 . 
       FIG. 1B  is a block diagram of a processor in accordance with an embodiment of the invention. The processor  520  may be implemented, for example, by software provided in processor  20  of  FIG. 1A  or by hardware or firmware provided in electronics module  18  of  FIG. 1A . The processor  520  of  FIG. 1B  includes a data collection processor unit  530  and one or more of a backlash and/or windup NURD removal processing unit  540 , an angular correlation processor unit  550 , and a line-to-line correlation processor unit  560 . The processor units  540 ,  550 , and  560  may all be provided and used as needed to significantly reduce and/or substantially eliminate NURD based on the application and/or the desired correction. Alternatively, one or more of the processor units  540 ,  550 , and  560  may be eliminated, for example, to reduce costs or processing time based on the application and/or the desired correction. 
     The data collection processor unit  530  functions to collect raw data, including angle and echo line data. The raw data may be stored in a data table, such as the exemplary table shown in  FIG. 2  and further discussed below. For example, the data collection processor unit  530  may implement the methodology discussed below with respect to FIG.  4  of the present application. However, other data collection methodology may also be utilized. 
     The backlash and/or windup NURD removal processor unit  540  functions to remove NURD caused by backlash in the catheter when the direction of rotation is changed and a fixed amount of windup NURD is assumed to be present in a given imaging situation. For example, the backlash and/or windup NURD removal processor unit  540  may implement the methodology discussed below with respect to  FIG. 5  of the present application. However, other methodologies capable of removing NURD caused by backlash in a catheter when the direction of the rotation is changed and a fixed amount of windup NURD is assumed to be present in a given imaging situation may be utilized. 
     The angular correlation processor unit  550  functions to remove backlash NURD by performing an angular correlation on the collected data. For example, the angular correlation processor unit  550  may implement the methodology discussed below with respect to  FIG. 6  of the present application. However, other methodologies capable of removing backlash NURD by performing an angular correlation on the data collected may be utilized. 
     The line-to-line correlation processor unit  560  functions to calculate a cross-correlation between adjacent data lines and then discard redundant lines. For example, the line-to-line correlation processor unit  560  may implement the methodology discussed below with respect to  FIG. 7  of the present application. However, other methodologies capable of calculating a cross-correlation between adjacent data lines and then discarding redundant lines of data may be utilized. 
     As discussed above, embodiments of the invention may be implemented using a plurality of processing steps in various combinations. Such plurality of processing steps or methods include collecting raw angle and echo line data, which may be stored in a data table, such as that shown in  FIG. 2 , removing backlash and/or windup NURD, performing angular correlation on the collected data and discarding redundant data, and performing line-to-line cross-correlation and discarding redundant lines of data. Each of these processing steps or processes will be discussed in more detail below. The processing steps may be used in various combinations to significantly reduce and/or substantially eliminate NURD, based on the application, for example, the particular imaging system utilized, and/or the desired correction.  FIGS. 3A-3G  show various exemplary combinations of the process steps, as discussed below. 
       FIG. 3A  is a flowchart of an exemplary arrangement of processing methods or steps in accordance with an embodiment of the invention. First, image data is collected in step S 100  and then NURD is reduced through the removal of redundant data using known backlash data (S 200 ), performing angular cross-correlation (S 300 ), and performing line-to-line cross correlation (S 400 ). Each of the NURD reducing processing steps can reduce NURD depending on the source of the NURD and the particular imaging system used. Further, each of the NURD reducing processing steps can also be used as standalone processing steps as necessary to, for example, reduce processing time and/or system cost. The steps which remove backlash/windup NURD (S 200 , S 300 ) may be utilized with IVUS systems which change rotational direction, while the line-to-line correlation processing step (S 400 ) may be applied to both unidirectional and bi-directional systems. Although the processing steps are all shown, sequentially, in  FIG. 3A , any number of combinations and/or sequences, such as, for example, those illustrated in the flow charts shown in  FIGS. 3B-3E , may be applied to significantly reduce and/or substantially eliminate NURD in an IVUS system, depending on the application or system used, and the desired correction or results. 
     The processing steps may be considered successive manipulations of information contained in a data table such as, for example, the data table shown in  FIG. 2 . In some IVUS systems, a line rate, or data collection rate, is fixed, while the number of lines in an image is variable, and the number of lines then depends on a somewhat variable catheter rotation rate. Consequently, the number of lines in a data table for such a system is also variable. In contrast, in other IVUS systems, the number of acoustic lines is fixed, and the line rate varies based on an angular velocity of the catheter. The processing steps can be applied to either type of IVUS system in a number of different ways, including, for example, a software algorithm, firmware controlling a Field Programmable Gate Array (FPGA), an Application Specific Integrated Circuit (ASIC), a pipeline processor implemented totally in hardware, or a combination of one or more of these. 
     Each of the processing steps will now be discussed in more detail. 
       FIG. 4  is a flow chart of the data collection processing step or method (S 100 ) shown in  FIGS. 3A-3G . The data collection processing step simply collects angle and echo line data. The data may be stored in a data table, such the exemplary data table  50  shown in  FIG. 2 . The length of the data table may be established based on the amount of memory available. Further, the data table may contain all of the acoustic echo lines from a few revolutions of a catheter in a motor driven, unidirectional IVUS system, or 10 to 15 direction reversals in a manually rotated, bi-directional IVUS system. The data collection processing step builds the initial raw data table of angles and associated image lines. It is well understood that the line data that represents the returned echoes from a constantly increasing radius or depth is, in fact, an array. However, for ease of discussion, each line of information will be treated as if it is a simple scalar rather than an array. 
     Referring again to  FIG. 1A , ultrasound scanning is initiated by electronics module  18  with the transmission of a pulse of sound to the transducer assembly  14 . Echoes are returned to the transducer assembly  14  from the acoustic impedance discontinuities encountered by the pulse in the tissue being interrogated, and are processed into a time series of discrete samples. This can be accomplished in a number of ways, including, for example, by digitizing the returning radio frequency signal directly, or by mixing it down to an intermediate or baseband frequency, or by detecting the information and digitizing the detected signal. In any case, the sampling rate is preferably consistent with the Nyquist criteria of at least twice the bandwidth of the signal to be sampled. It is preferable to digitize the detected data to minimize the number of sampled points in an echo line. 
     For example, for a ˜40 MHz center frequency transducer with a ˜60% fractional bandwidth, the information bandwidth of the amplitude modulated signal is ˜24 MHz. According to the Nyquist criteria, this signal must be sampled at least ˜48 million times each second. If, for example, a ˜50 MHz sampling rate is used, and it is desired to collect an echo line of ˜10 mm in length, then the number of samples in the echo line is: 
     
       
         
           
             
               
                 
                   
                     
                       
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     The number 2 in the denominator of Equation 1 reflects the fact that the sound must travel to the point of interest, and the echo must return from that point to the transducer, effectively progressing at half the speed of sound in the tissue. Along with the echo data, the proximal angle for each scan line at the time of pulse transmission is recorded by the electronics module  18 . 
     Each time the data collection processing step is invoked, incoming pairs of proximal angle and echo data are used to construct a new version of the data table. As previously discussed, the data table may contain a list of angles and the echo data that is captured at each particular angle. For ease of discussion, the exemplary data table shown in  FIG. 2  is represented as an array with three columns and “N” rows. The first column serves as an array index “n” which is an ordered integer from 1 to “N” where “N” is the number of rows (i.e. proximal angle/scan line pairs) obtained. The second column is the Angle(n) and the third column is the EchoData(n,t). Each entry in the EchoData column is a time series or array of discrete samples. The time values start from the moment the transmit pulse occurs until the last echo of interest is returned from the maximum depth to be displayed in the image. These time values correspond linearly with distance or depth from the face of the transducer. 
     The number of rows “N” in the data table may be a function of a number of design considerations. In general, a shorter data table results in a shorter overall latency period through all of the processing steps. Input data sets as short as one proximal angle/echo data pair are allowed. This yields a data table size of N=2 due to the optional buffering of the last line from one iteration of the data collection processing step to the next; this configuration supports pipelined operation in which data flows continuously through all processing steps. The determination of the end-of-input-data condition includes, but is not limited to, the following conditions: 
     1. A predetermined number of scan lines have been received (1 . . . N). 
     2. A predetermined number of complete 360° image frames have been received (1 . . . F). 
     3. A predetermined time interval has elapsed. 
     4. A predetermined number of direction reversals have occurred (1 . . . R). 
     5. A predetermined time of no input activity has occurred. 
     As shown in  FIG. 4 , a line counter is initially set to 1, in step S 110 , and the counter is increased by 1, in step S 115 , as the data is collected and stored. The method determines if the end of input data has been reached, in step S 120 , based on the criteria set forth above. If the end of input data has been reached, the last line in the data table  150  is saved to the last line buffer  117 , in step S 155 , and the data table  150  is sent to the next processing step, or for scan conversion, as appropriate, in step S 160 . If the end of input data has not been reached, the proximal angle(n) is obtained, in step S 125 , and written to the data table  150 , in step S 130 , and scan line echo data(n) is obtained, in step S 135 , and written to the data table  150 , in step S 145 . The counter is then increased by 1, in step S 145 , and the process repeats until the end of input data is reached, in step S 120 . 
       FIG. 5  is a flow chart of the backlash and/or windup NURD processing step or method shown in  FIGS. 3A-3C  and  3 F. This processing step senses when the catheter has reversed its rotational direction, and removes the lines corresponding to a predetermined minimum backlash angle from the data table. The remaining lines are then rotated by the predetermined backlash angle to remove the angular distortion. An estimate of the minimum amount of backlash that will always occur upon a change of rotational direction for any particular imaging situation is predetermined and loaded into an a priori backlash data table. Many factors influence these backlash angles, including, for example, the type of guide catheter used, the access site to the patient, the guidewire employed, a patient body type, and other such factors which have had their minimum backlash angles experimentally measured for each of the catheters that may be compatible with a particular imaging system being used. This processing step can be used with a catheter that can image while being rotated in both directions, i.e., a bi-directional IVUS system. 
     When the rotational direction is reversed, the distal tip of the transducer is assumed to be stationary until the minimum backlash angle has passed at the proximal end of the catheter. The backlash angle is, for purposes of discussion, assumed to be the total angle necessary to unwind any twist in the catheter added to the angle necessary to rewind the catheter in the opposite direction so as to start the distal tip moving again in the opposite direction. After the proximal end of the catheter has exceeded the backlash angle, the method begins writing lines according to the incremental angles measured at the proximal end. Conceptually, the best case backlash scenario occurs when the catheter is held as straight as possible with no bends other than those created by the guide catheter. All other scenarios will typically have more backlash since, in general, more friction will be present due to the catheter being forced to bend by contact with the vessel wall. Since guide catheters of various shapes are typically used with IVUS imaging systems, putting the IVUS catheter inside the guide catheter and measuring the backlash when there is no load on the part of the catheter that extends distal to the tip of the guide catheter, or the part that is proximal to the proximal end of the guide catheter, will be a reasonable best case minimum backlash measure. 
     As discussed above, the backlash/windup NURD removal processing step or method acts to remove NURD caused by backlash in the catheter when the direction of rotation is changed. This fixed amount of windup NURD is assumed to be present in a given imaging situation. The a priori backlash data table may provide measured, estimated, or calculated backlash data for a variety of combinations of guide catheter-IVUS catheter combinations, and allows this processing step to significantly or completely remove the minimum amount of NURD that will always occur for any one of the combinations available. 
     In the method of  FIG. 5 , the a priori backlash data table  212  and the data table  250  are made accessible as the method is initialized, in step S 210 , and a processing loop is entered where the lines in the data table  250  are examined, in steps S 215 , S 220 . A rotational direction change is identified, in step S 225 , when the catheter angles in the data table  250  go from monotonically increasing to monotonically decreasing, or vice-versa. If no direction change has occurred, no change to the data table  250  is made, and the next angle in the data table  250  is examined, in step S 215 , until the end of the data table  250  is reached, in step S 230 . 
     However, when a direction change has occurred, in step S 225 , the data table  250  is rewritten to remove all of the angles and echo data lines that are contained within a preset backlash angle, in step S 227 . All of the data table  250  entries in the intervening backlash angle are removed, and the angles for all subsequent rows in the data table  250  are adjusted, in step S 227 , by subtracting or adding the preset backlash angle from the a priori backlash data table  212 , plus the current adjustment angle  229  from the original angle depending on whether the direction change was from clockwise to counterclockwise, or from counterclockwise to clockwise. 
     Referring again to the system shown in  FIG. 1A  as an example, this assumes that the transducer assembly  14  at the distal end  28  of the catheter  10  will be stationary after the change in direction, but before the complete backlash angle has been subtended, so echo data for all of the lines in the intervening backlash angle will be nearly identical. The backlash/windup removal processing step is computationally very simple and its implementation is inexpensive either from a parts cost or a computer processing time perspective. As such, it could eliminate the backlash lines without the expense of the additional computer cycles required for the angular or line-to-line correlation steps. If the line-to-line correlation processing step or the angular correlation processing step is employed in addition to the backlash/windup removal processing step, then it is preferable that the backlash/windup removal processing step precede either or both of the other two steps. More particularly, the line-to-line correlation step and the angular correlation step will remove the redundant lines in the backlash angle because they will be highly correlated. The backlash/windup removal processing step would then simply sense the change in direction and remove an additional set of lines that would in fact be valid data that is not redundant. It can, however, be desirable to have the backlash removal processing step employed in conjunction with the other processing steps because it is a more efficient way to remove that backlash artifact, thereby saving computer cycles for other things, such as, for example, scan converting the acoustic lines into images, monitoring and responding to the human interface, and any signal processing tasks that the system employs. 
     After all of the rows in the incoming data table have been processed, in step S 230 , the modified data table can be sent to the next processing step for further NURD reduction, or sent to be scan converted and displayed, in step S 235 . The current adjustment angle  229  is updated by adding or subtracting the backlash angle and retained for the next invocation of the backlash/windup NURD removal processing step. 
       FIG. 6  is a flow chart of the angular correlation processing step or method shown in  FIGS. 3A ,  3 C- 3 D, and  3 G. Like the backlash/windup NURD removal processing step, this processing step senses a reversal in the direction of rotation. The process then calculates an angular cross-correlation between sectors that occur before and after the direction change. This may be, for example, complete 360-degree images, or sectors of images, and the same processing step may be applied when changing from clockwise to counterclockwise rotation as when changing from counterclockwise to clockwise rotation. The angle that produces the maximum of the angular cross-correlation is used to rotate the image sector so as to properly align the two images sectors without the intervening backlash. In this case, angle values in the data table are shifted by the angle that corresponds to the maximum cross-correlation angle, while the echo data remains unchanged. 
     The angular correlation processing step may follow the backlash/windup NURD removal processing step, or may be used immediately following the data collection processing step. When used subsequent to the backlash/windup NURD removal processing step, the angular correlation processing step removes any remaining backlash that occurs in addition to the minimum that was stored in the a priori backlash data table. This additional backlash angle is a consequence of the less than ideal circumstances that were measured, estimated, or calculated in the a priori backlash data table. For example, higher catheter rotation velocity, more total bending of the catheter as it accesses the desired vasculature, and contact with the vessel walls after exiting the guide catheter all cause more frictional drag and, consequently, more backlash. The angular correlation processing step may be carried out on the raw data in the data table, or the acoustic lines can be moved to the nearest display line and the intervening pixels left uncalculated 
     As set forth above, the angular correlation processing step aligns the sectors of an image just prior to and subsequent to a change in catheter rotation direction. As shown in  FIG. 6 , a loop index “k” is initially set to zero, in step S 310 , the loop index “k” is incremented for each change in catheter rotation direction, in step S 315 , and a rotational direction change is detected, in step S 320 , as discussed in more detail with respect to  FIG. 8A . Once the direction changes are located in the data table  350 , Sector k , which represents the set of rows that represent the image information a set amount before the change in direction and Sector k+1  which represents the image information a set amount after the change in direction can be determined, in steps S 320 , S 325 , and S 330 . For ease of discussion, forty degrees is chosen as the set amount, as a 40 degree sector angle would provide sufficient image area to provide a robust angular cross-correlation but is small enough so that an operator who may be manually sweeping out the image would sweep through the entire sector angle in normal practice; however, other sector angles may also be appropriate. This corresponds to the numerical integration from −20 degrees to +20 degrees in Eqn. 2. This angle can be a predetermined number stored in a table much like the a priori backlash data table discussed above, and may represent an optimum angle depending on any particular imaging situation that can be known in advance. 
     The 40 degree sector just prior to a catheter rotational direction change is compared to the 40 degree sector subsequent to the direction change, and the rotation angle necessary to align these two sectors is calculated, in step S 335 , based on an angular cross-correlation formula shown below in Equation 2. Preferably, the angular cross correlation is performed on the “pre-scan-converted” lines in the data table with the angular position of the acoustic lines rounded to the nearest display line. Once the angular cross-correlation has been calculated, in step S 335 , the actual angles are used for scan conversion purposes. 
     
       
         
           
             
               
                 
                   
                     
                       
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             Where: AC k  (θ) is the Angular-Correlation as a function of the offset angle θ, the angle θ is allowed to vary over + and −20 degrees, and α is the angular index over which the correlation is integrated.
 
It will be obvious to one skilled in the art that this calculation is in fact a summation over as much as 80 degrees of image data.
 
           
         
       
    
     The angular cross-correlation function in Equation 2 will have a maximum value θ MaxAC(k)  for the angular rotation (θ) that best aligns the two sectors of the IVUS image. Once the angular cross-correlation is calculated, in step S 335 , the maximum of the function is determined, in step S 340 , Sector k+1  is rotated by that angle, and the data table  350  is rewritten to remove redundant lines and adjust the remaining angles, in step S 345 . The rotation of the sector involves only the addition of the θ maxAC(k)  value, plus the current adjustment angle  355 , to the Angle(n) column for the EchoData(n) line that marks the beginning of Sector k+1  through the end of the data table  350 . 
     Once the angles in the data table  350  have been rewritten, in step S 345 , if the end of the data table  350  has not been reached, in step S 360 , the loop index “k” is incremented, in step S 315 , the next change in direction is found, in step S 320 , and the process repeats until the last direction change has been processed. When the last direction change has been processed, the data table  350  is ready to be sent to the next processing step or scan-converted and displayed, in step S 365 . The current adjustment angle  355  is updated by adding in the net sum of all the θ MaxAC  for all the sectors in the data table and retained for the next invocation of this processing step. 
       FIG. 7  is a flow chart of the line-to-line correlation processing step or method shown in FIGS.  3 A and  3 E- 3 G. This processing step is applicable whether the catheter is unidirectional or bi-directional, and is used to calculate a cross-correlation between adjacent lines. If the cross-correlation is above a preset threshold, it is assumed that the distal end of the transducer assembly is substantially stationary, that the lines are taken from the same angular location, and therefore one of them is redundant and should be discarded. This is considered to be a NURD sector of the IVUS image, and would likely show up as a “streaked” sector where the lines have the same echo pattern for many different apparent angles. Thus, all but the first or last line are discarded until the correlation coefficient drops below the preset threshold. A reduction in the cross-correlation indicates that the transducer is again rotating and presumably tracking the proximal angle encoder values, and the image lines are written accordingly. With a frame buffer and sufficient line density, the changing lines at either edge of the sector which would have been “streaked” (i.e. the NURD sector) can be repositioned across the (to be discarded) highly correlated sector to reduce the geometric distortion. 
     This processing step can assume that the transducer is moving at a constant velocity when the line-to-line correlation coefficients are no longer approximately one. Further, it may be assumed that the transducer will accelerate before reaching a certain velocity that it will maintain until either the drive on the proximal end is changed or the line-to-line correlation again approaches one. Similarly, for the lines just preceding the NURD sector where the lines are highly correlated, it may be assumed that the transducer decelerates to zero angular velocity at the edge of the leading edge of the NURD sector. Either of these corrections allows a more accurate geometric presentation of the received data than the constant velocity assumption that is implicit in conventional mechanically rotated IVUS systems. 
     The line-to-line correlation processing step may accept the raw data table from the data collection processing step, and/or may accept the data table as processed in either the backlash/windup NURD reduction processing step and/or the angular correlation processing step. The method is initialized, in step S 410 , and the line number in the data table  450  is incremented, in step S 415 . A correlation between adjacent lines is calculated, in step S 420 . The LineCorr(n) is compared to a predetermined threshold (Th o ), in step S 430 , and if the LineCorr(n) is greater than the predetermined threshold (Th o ), these highly correlated lines are discarded and the remaining lines are reoriented, in step S 435 . The process is repeated until the end of the data table  450  is reached, in step S 440 , and the data table  450  is ready for scan conversion and/or display, in step S 460 . The formula for the line-to-line correlation calculation, in step S 420 , is shown in Equation 3. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           LineCorr 
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                             EchoData 
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     This correlation value would typically be between 0.90 and 1.00 if the two lines were received while the transducer, at the distal end of the catheter, was in substantially the same position. However, it is not exactly 1.00, because there is always some level of thermal noise in the receiver and the transducer is never perfectly stationary in a living system due to respiration, heart beat, patient motion, and other such external influences. If the line-to-line correlation coefficient is low, then the adjacent acoustic lines represent substantially independent acoustic lines. Equation 3 can be implemented on the EchoData signals after the DC component has been removed (AC coupled) or with the DC component intact with similar results. 
     The correlation coefficient, LineCorr(n), for the n th  acoustic line and the n+1 st  acoustic line is then compared to a preset threshold value (Th o ) of for example, 0.95, in step S 430 . If the correlation between the lines exceeds the preset threshold (Th o ), then it is assumed that the transducer is not moving during the time the two lines were acquired, one of the lines is discarded, and the data table  450  is rewritten accordingly, in step S 435 . The process of re-writing the data table  450  is discussed in more detail with respect to  FIGS. 8A-8B . If the correlation between the lines is less than the preset threshold (Th o ), in step S 430 , then the lines are assumed to be valid lines, substantially without NURD, and the next pair of lines is examined. When all of the lines in data table  450  have been processed, in step S 440 , the data table  450  may be sent to another processing step, or scan-converted and displayed, in step S 460 . The current adjustment angle  445  is updated, by adding in the net angle of image sector data removed and angle of the image sector data retained for the next invocation of this processing step. 
       FIGS. 8A-8B  are flow charts illustrating data table rewriting processes. The basic difference in these two processes is in the assumptions that are made for the correct replacement of remaining lines after aberrant lines have been removed. In the event of NURD caused by backlash during a direction reversal, it is assumed that the distal tip of the catheter was stationary during the backlash angle. Once the backlash angle is exceeded, the distal tip moves at the same angular velocity as the proximal end of the catheter, thus causing the sector of image that occurs after the change in rotational direction to be discarded as the catheter traverses the backlash angle, and the remaining angles in the sector to be adjusted to rotate the image accordingly. 
     In contrast, when there is no change in rotational direction, it is assumed that the distal tip of the catheter decelerates just prior to the occurrence of NURD, remains substantially stopped during the NURD sector, and accelerates just subsequent to the NURD sector. Although, for ease of discussion, it is assumed that the deceleration and acceleration are approximately of the same magnitude and duration, this is not a necessary condition, as the acceleration and deceleration profiles may be stored in a table, and may reflect any profile that the actual catheter is likely to experience. In this situation, the remaining lines may be repositioned prior and subsequent to the NURD sector to fill in the gap of discarded lines, and subsequent data tables will have their initial angle adjusted to reflect the cumulative angles and lines removed from previous versions of the data table. This cumulative angle adjustment from sector to sector, or from data table to data table, is referred to as current adjustment angle. 
       FIG. 8A  is a flow chart illustrating a data table rewriting process invoked by either of the backlash/windup NURD removal processing step or the angular correlation processing step. The direction change counter “k” is initialized, in step S 805 , and angles for pairs of adjacent lines are examined, in step S 810 , to determine if the rotational direction has changed, in step S 820 . If not, the search for a change in direction continues as step S 810  is repeated. If a direction change is found, in step S 820 , the index “k” is incremented, in step S 830 , and the NURD lines are deleted according to the algorithm in either the backlash/windup NURD removal processing step or the angular correlation processing step, in step S 840 . The sector is then rotated again by an angle specified in the appropriate algorithm, in step S 850 . This process continues until the entire data table has been processed, in step S 860 , and the data table is returned to the appropriate processing step, in step S 870 . 
       FIG. 8B  is a flow chart illustrating a data table rewriting process invoked by in the line-to-line correlation processing step when there is no change in rotational direction. This process requires a pre-calculated table which dictates how valid lines are to be repositioned to cover a gap left behind by redundant lines, referred to as the acceleration/deceleration profile  900 . If the NURD sector is relatively wide, then the acceleration and deceleration may be spread over a wider sector of good data. However, if the NURD occurs in only a few lines, then the acceleration and deceleration of the catheter most likely occurs over only a few adjacent lines before a constant angular velocity is restored. The algorithm may include an accumulator to count the total number of adjacent lines with high correlation coefficients and then choose an appropriate acceleration/deceleration profile from a two-dimensional data set that varies each as a function of the number of lines to be discarded. However, for ease of discussion, a simple one-dimensional table is shown in  FIG. 8B . 
     The method of  FIG. 8B  is initiated when the line-to-line correlation processing step detects adjacent lines with a high correlation coefficient. This implies that the transducer is likely stationary, and the proximal end of the catheter is turning at a nearly constant velocity. First, one of the two adjacent lines is discarded, in step S 910 , and an index “i” that starts the deceleration profile is initialized, in step S 920 . The deceleration is assumed to occur over a given number of lines, for example, 10 lines, as shown in the exemplary acceleration/deceleration profile  900 . A small adjustment may be made to the angle of the line that is 10 lines prior to the discarded line to account for the assumed deceleration that occurs prior to the discarded line, in step S 930 . The index “i” is incremented, in step S 940 , and a larger angle is subtracted from the next line until the assumed deceleration brings the transducer angular velocity to substantially zero at the location of the discarded line. At this point, a large increment followed by decreasing increments are subtracted from the angles of the respective lines until the index is 10 lines away from the discarded line, in step S 950 , as the transducer accelerates. Once the full acceleration/deceleration profile  900 , in this example, 10 deceleration adjustments and 10 acceleration adjustments have been used to adjust the data table, in step S 950 , and the data table is returned to the line-to-line correlation processing step, in step S 960 , to search for the next redundant line. 
     Proper implementation of the acceleration/deceleration profile  900  may also help ensure precise orientation of a rotating device inserted in a cavity as rotation of the device is interrupted to, for example, more closely examine and/or remove tissue or plaque from a particular portion, such as may be done when, for example, using an ablation device in combination with an imaging system, when precise placement is critical. More particularly, as the imaging device rotates within the cavity and a corresponding image is displayed to an interventionalist for examination, the interventionalist may choose to interrupt rotation of the device for more close examination and/or to remove or sample tissue at a particular location. However, as discussed above, when rotation at the proximal end of the device is stopped, the distal end of the device may experience some delay before its corresponding rotation is stopped, and thus may continue to rotate for a short period after rotation at the proximal end is stopped. Without correction using the appropriate acceleration/deceleration profile  900 , this delay at the proximal end may result in imprecise, inaccurate positioning of the imaging and/or ablation device. 
     The invention as embodied and broadly described herein allows NURD to be significantly reduced, and/or substantially eliminated without significant increases in system complexity and cost. The resulting significant reduction or elimination of NURD provides a more accurate, reliable measure of angular. orientation and image of anomalies observed during inspection of the vasculature, thus providing more efficient diagnosis and more appropriate treatment options. 
     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the invention. The present teaching can be readily applied to other types of apparatuses. The description of the invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.