Abstract:
Methodology for determining the bounds of a patient&#39;s acoustic window is described. Medical application acoustic array designs with apertures accommodated by patient acoustic windows and merged acoustic windows are exemplified.

Description:
RELATED APPLICATIONS 
     This application is a continuation to concurrently filed and co-assigned U.S. patent application entitled “ Non - Invasive Turbulent Blood Flow Imaging System”  identified by Attorney Docket No. 9023-11 which corresponds to PCT/US97/20186 filed Nov. 10, 1997 (“the 20186 application”). This application is also related to co-pending and co-assigned patent application Ser. No. 09/136,933, entitled “ Thin Film Piezoelectric Polymer Sensor,”  and concurrently filed and co-assigned patent application identified by U.S. application Ser. No. 09/433,211, which claims the benefit of priority from Provisional Application Serial No. 60/107,616, filed Nov. 9, 1998 entitled “ Acoustic Window Identification.”  The contents of the above-identified applications are hereby incorporated by reference as if recited in full herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to arrays of acoustic sensors that facilitate the non-invasive detection of coronary artery disease (CAD). 
     BACKGROUND OF THE INVENTION 
     The 20186 application describes an invention for the non-invasive in vivo detection and localization of abnormal blood flow. Embodiments of that invention display the spatial distribution of phase coherence in the shear eave component of blood flow signals generated by an acoustic sensor array. An essentially uniform display indicates normal blood flow. A non-uniform display may indicate the presence of an occlusion and the presence or extent of abnormal, turbulent blood flow. Poor correlation of signals from the array sensors may adversely affect the display uniformity. 
     Acoustic sensor arrays are conventionally positioned above a measurement area defined as the hairless human chest skin located vertically between the sternum and a parallel line passing through the left nipple and horizontally 10 cm above and 6 cm below the left and right nipples. 
     A prior art acoustic sensor array comprising eight equally spaced sensors in two concentric circles having prime numbers of sensors in each circle and a ninth sensor at the common center of the concentric circle is illustrated by FIG. 6 of the 20186 application. 
     To reach sensors in a conventionally positioned prior art array as described in the 20186 application, sound waves must travel either directly through lung tissue or first to the body surface and then laterally with consequent attenuation of correlation. A study of the correlation by that array of patient data signals generated by the quiet interval revealed that only four or five of the nine sensors are well correlated. 
     It is known that a notch (“cardiac notch”) in the human left lung allows the heart to be in contact with the chest wall. Well correlated blood flow signals may be generated by acoustic sensors positioned on a human chest in a small area (“acoustic window”) located above the cardiac notch. The bounds of the acoustic window have been approximated by ultrasonic probe means as described in this application and by locating the portions of sensor corresponding to channels achieve the highest apparent signal to noise ratio (SNR) as described in the Stearns application. 
     DEFINITIONS 
     Acoustic Window—An area above the notch in the human left lung which allows the heart to be in contact with the chest wall. Well correlated acoustic blood flow signals of good quality may be generated by a sensor array positioned on a patient&#39;s chest within or substantially within the perimeter of an acoustic window. 
     Sensor or Accelerometer—Any current or voltage mode device which generates an electric signal from displacement or a derivative thereof upon detection of a sound wave. 
     Sensor Array—A pattern or spaced arrangement of a plurality of sensors on or to be placed on the body surface of a patient. 
     Sensor Array Aperture—The space or area within the perimeter of an array. 
     Sensor Array Geometry—The shape of the perimeter of a sensor array. 
     Channel—The path to a receiver followed by a signal from the sensor by which the signal is generated. 
     SUMMARY OF THE INVENTION 
     Pursuant to one embodiment of the invention, an acoustic window may be defined by ultrasonic probe means. The invention includes sensor arrays having an aperture locatable within or substantially within the bounds of an acoustic window when the array is positioned on the chest of a person. 
     An important aspect of the invention includes the identification of an acoustic window comprising the merged acoustic window sub-areas corresponding to two or more intercostal spaces (ICS&#39;s), and array designs to accommodate such acoustic windows. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 illustrates an ultrasonic probe acoustic window characterization method that provides a template for the positioning of sensors on a person&#39;s chest. All acoustic window data illustrated by FIGS. 2 through 8 and  10  through  12  was obtained by the FIG. 1 method. 
     FIG. 2 is a plot in polar format of the acoustic window size data obtained from 22 male and 7 female subjects (29 subjects). 
     FIG. 3 is a histogram of the window areas of the same 29 subjects from which the FIG. 2 data was obtained. 
     FIG. 4 shows acoustic window size in polar format. Maximum, minimum and average window size for all of the same 29 subjects is depicted. 
     FIG. 5 illustrates in Cartesian coordinates variations of the ICS ultrasonic probe data points from the left (□) and right (⋄) ICS. The statistical averages for ICS&#39;s 1 to 5 are shown in solid lines. A perimeter connecting the ends of the solid lines is a visualization of the average geometry of the six intercostal spaces. 
     FIG. 6 is a histogram that indicates ICS nearest to the centroids of the average window area (see FIG.  4 ). 
     FIG. 7 is a histogram indicating the distribution of the perpendicular distances from the centroid of the average window area (see FIG. 4) to the nearest ICS. 
     FIG. 8 is a histogram illustrating the distribution of the distance from the left side of the nearest ICS to the projection of the centroid of the average window area (see FIG.  4 ). 
     FIG. 9 depicts a prior art nine sensor array based on seismic accelerometers commercially available from Wilcoxon Research, 21 Firstfield Road, Gaithersburg, Md. 20878. The array comprises eight equally spaced sensors in two concentric circles having prime numbers of sensors in each circle and a ninth sensor in the common center of the concentric circles. 
     FIG. 10 depicts a 13 element array positioned over an acoustic window of average size (solid line, see FIG.  4 ). An acoustic window of maximum area is also shown (broken line). 
     FIG. 11 illustrates a 57 element small PVDF sensor array based on averaging of ICS data points. The array comprises five linear subarrays positioned above intercostal spaces 2 to 6. 
     FIG. 12 illustrates a 32 element array of large PVDF sensors based on averaging of ICS data points. The array comprises five linear subarrays positioned above intercostal spaces 2 to 6. 
     FIG. 13 illustrates a beam pattern in x for delay-and-sum (DS) (higher in value) and MVDR (lower in value) beamformers using prior art (HA) (dashed lines), bowling pin (BO) (dashed dotted lines), small PDVF (SP) (solid lines) and large PDVF (LP) (dotted lines) arrays. 
     FIG. 14 illustrates a beam pattern in y (dB) for delay-and-sum (DS) (broken lines) and MVDR (solid lines) beamformers using prior art (HA), bowling pin (BO), small PDVF (SP) and large PDVF (LP) arrays. 
     FIG. 15 illustrates a beam pattern in z (dB) for delay-and-sum (DS) (broken lines) and MVDR (solid lines) beamformers using Harris (HA), bowling pin (BO), small PDVF (SP) and large PDVF (LP) arrays. 
     FIG. 16 is a proposed array based on the medical Wilcoxon accelerometer. Only 12 elements are used due to the limitation of the data collection system. The broken line indicates the perimeter of an acoustic window. 
     FIG. 17 illustrates a 45-element PVDF sensor array comprising five nine element linear subarrays positioned above intercostal spaces 2 to 6. Large and small acoustic window perimeters with centroids near the fourth and fifth ICS&#39;s are shown. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention generally comprises the identification of an acoustic window and the design of arrays having geometry sized to fit within or substantially within and thus accommodate the perimeter of the window. The invention may include an average acoustic window and consolidated or merged window subareas and array geometry sized accordingly. 
     Ultrasonic Probe Determination of the Size and Location of the Acoustic Window—Design of Template for Sensor Positioning 
     The acoustic window may include one or a combination of the small areas (intercostal space window areas) of the patient&#39;s chest surface directly above the intercostal spaces one through six. Determination of the size of an acoustic window may be accomplished by steps (i) to (v). 
     (i) With the patient supine, i.e., lying on his back or side, draw a series of dots along the left sternal border at the beginning of each intercostal space (ICS) for spaces one through six. 
     (ii) Place an ultrasound probe at the left sternal border of the first intercostal space (ICS). Then move the probe along the intercostal space until the lung tissue is encountered. Place a dot on the chest to mark where the lung tissue begins. 
     (iii) Repeat step (ii) for intercostal spaces two through six. 
     (iv) Wipe the ultrasound gel off the chest, and draw a line following each intercostal space, connecting the two previously drawn dots. The lines should be similar to FIG.  1 . 
     (v) After the chest has been marked as above, place a sheet of tracing paper on the chest, and transfer the markings onto the paper to provide a template for positioning of sensors. 
     Average or “generic” templates may be prepared from average data determined in the same way from a plurality of persons. 
     A like procedure may be used to determine an acoustic window of a person lying slanted on a bed. 
     Table 1 lists the window areas in cm-squared for the three window sizes in two bed positions. For the maximum and average window cases, lying slanted on the bed produces a slightly larger (5% and 16%, respectively) window size than lying flat on the bed. In the case of the minimum window, lying flat does produce a significant 37% larger window area than lying slanted on the bed. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Differences in the Flat and Slanted Bed Positions 
               
             
          
           
               
                 Areas in cm 
                 Average Area 
                 Maximum Area 
                 Minimum Area  
               
               
                   
               
               
                 Slant Position 
                 58.08 
                 134.46 
                 6.73 
               
               
                 Flat Position 
                 48.82 
                 127.37 
                 9.22 
               
               
                 % Difference 
                 15.94 
                  5.26 
                 −36.87   
               
               
                   
               
             
          
         
       
     
     Based on these results, the flat position is more advantageous since it does not significantly reduce the acoustics window for subjects with large to medium window sizes and at the same time significantly opens up the smaller acoustic window sizes. 
     Another method for identifying an acoustic window entails examination of which sensor channels receive the highest signal to noise ratio (SNR) as measured by the optimal weights for summing channels. See copending, commonly assigned Stearns U.S. application identified by U.S. application Ser. No. 09/433,211, which claims the benefit of priority from Provisional Application Serial No. 60/107,616, filed Nov. 9, 1998. 
     Statistical Analysis of Acoustic Window Data 
     Acoustic window size data, collected pursuant to the described ultrasonic probe methodology, was obtained from 22 male and 7 female subjects. There are two types of data. 
     1. Measurements based on a Cartesian coordinate with X axis on the 6 th  intercostal space (ICS) and Y axis along the left end of the ICS. 
     2. Data estimate in polar coordinate centered at the centroid of the acoustic window mass. This data is derived from measuring the distance from the centroid to the edge of the window at 30 degree angle increments. There is a total of twelve data points per subject. 
     The Distribution of Polar Data 
     The distribution of the window size data in polar format was tested for normality using the normal probability plot from the MATLAB Statistics Toolbox. The purpose of a normal probability plot is to graphically assess whether the data could come from a normal distribution. If the data are normal, the plot will be linear. Other distribution types will introduce curvature in the plot. As shown in FIG. 2, the data points are virtually in a straight line, indicating that the polar coordinate data is Gaussian. 
     When the area for each of the 22 male windows was computed using AutoCad software, the ratio of the maximum to the minimum area was found to be 15. FIG. 3 illustrates the histogram of the window areas. 
     FIG. 4 shows the acoustics window size in polar format. In this figure, the outside perimeter is for maximum, the inside perimeter is for the minimum and the intermediate perimeter is for the average across all subjects. 
     Correlation Coefficients Between Acoustics Window Areas and Demographic Data 
     Computed correlation coefficients between the window area and the subject demographics data are shown in Table 2. This analysis was carried out based on data broken down by male (22 subjects), female (7 subjects) and a combination of both sexes. In general, there exists no strong correlation between the window area and demographics data, with the exception of strong negative correlation of −0.84 between the acoustics window area and the anterior/posterior (AP) diameter of the female subjects and a strong negative correlation between the acoustics window area and the sternum length in both male and female subjects. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Correlation w/ 
                 Correlation w/ 
                 Correlation w/ 
               
               
                   
                 Acoustics 
                 Acoustics 
                 Acoustics 
               
               
                 Demographic 
                 Window 
                 Window 
                 Window 
               
               
                 Feature 
                 Area (M&amp;F) 
                 Area (Male) 
                 Area (Female)  
               
               
                   
               
             
             
               
                 Age 
                 0.0043 
                 0.0152 
                 −0.0095 
               
               
                 Height 
                 −0.1846 
                 −0.4345 
                 0.2794 
               
               
                 Weight 
                 −0.3503 
                 −0.3579 
                 −0.6678 
               
               
                 Chest Circumference 
                 −0.3578 
                 −0.3800 
                 −0.3623 
               
               
                 AP Diameter 
                 −0.5904 
                 −0.5027 
                 −0.8690 
               
               
                 Aortic Valve Depth 
                 0.1868 
                 −0.0043 
                 0.5311 
               
               
                 ICS of Aortic Valve 
                 −0.0774 
                 −0.2081 
                 0.1810 
               
               
                 Sternum Length 
                 −0.6446 
                 −0.6959 
                 −0.6587 
               
               
                 Body Type 
                 −0.0874 
                 −0.1637 
                 0.0604  
               
               
                   
               
             
          
         
       
     
     Variations of the ICS End Points in Cartesian Coordinate 
     Examination of the data in Cartesian coordinate reveals the absence of common single reference point such as the centroid in the polar data case. The X-Y data was collected relative to the six intercostal spaces and was measured as left and right ICS. The only single common reference was made when the six ICS&#39;s were aligned on the x-axis so that the other spaces can be seen relative to this reference space. 
     As seen from FIG. 5, the XY coordinates of the left in (□) and right in (⋄) ICS are quite different across the subjects. The statistical averages of the left and right ICS are also shown. When connected, a visualization of the average geometry of the six intercostal spaces is provided. 
     The Distribution of Window Centroids Over ICS 
     The x-y coordinates of the acoustics window centroids were measured and correlated with the lines defined by the left and right ICS. The purpose of this correlation is to determine which of the six spaces the window centroid is near to and then to ascertain the best space(s) for location of the array. 
     A histogram of the ICS to which the centroids of the window areas are nearest is plotted in FIG.  6 . The result indicates that the fourth and fifth ICS are good candidates for positioning the array center, with the fourth ICS being more frequent than the fifth ICS. In practice, it is appropriate to consider these two ICS equally and pick one based on the best knowledge of which ICS has the best heartbeat sound. 
     The distribution of (a) the perpendicular distances from the centroid to the nearest ICS and (b) the distances from the left side of the nearest ICS to the projection of the centroid are histogrammed in FIGS. 7 and 8. These results provide guidelines as to the approximate location of the array center relative to the nearest intercostal space. 
     Array Design Based on Acoustics Window Data 
     Factors and Constraints in Array Geometry Design 
     Constraints imposed on array geometry include: 
     1. Limitation on the array aperture by the size of the acoustics window which varies from person to person. 
     2. Sensor size which limits the number of elements that can be put in the array aperture. For example, the medical Wilcoxon sensor diameter is about one cm. In the case of PVDF sensor, the prefabricated thin film strip size dictates how many sensors could be placed in the array aperture. 
     3. The anti-aliasing requirement of the array design at different operating frequencies. In principle, the inter-element spacing of the sensors is required to be less than half a wavelength at the highest operating frequency to avoid spatial aliasing in the plane wave case. This requirement is relaxed in the near field where source location is the objective. The use of irregularity in array geometry may also alleviate the aliasing problem when there is an inter-element spacing of more than half wavelength. 
     Array Geometries 
     Use of the acoustics window in array design based on the medical Wilcoxon accelerometer and the large and small PVDF sensors resulted in the three arrays depicted by FIGS. 10,  11  and  12 . 
     The 13-element array of FIG. 10 was based on the average window size described with reference to FIG.  4 . Using the actual dimensions of the medical Wilcoxon accelerometer on graph paper, each accelerometer was placed on straight lines starting from the center and populating the perimeter until space is occupied. A total of 13 elements that were fitted into this average window size. Clinical data indicates that 13 elements may not give optimum array gain especially when element signal-to-noise ratio of turbulent flow is low. 
     More elements per unit area are possible with PVDF technology. Because PVDF film is available in linear strips of 9 and 16 elements per unit, each strip can be put on the intercostal space to maximize signal reception. These factors motivate the array geometries illustrated in FIGS. 11 and 12. 
     In these two arrangements, five lines of PVDF film strip are placed along ICSs 2 to 6 at approximately the length of the average ICS as described with reference to FIG.  5 . The placement of these PVDF film strips as shown in FIGS. 11 and 12 are for illustration only and not necessarily the exact position and direction of the film strips. Also, because of the inherent variations in human anatomy, the actual placement of the PVDF sensor strips is expected to be different from person to person, in view of the effect of the ribs as a factor in signal reception. 
     At the end, 57 elements for the small PVDF and 32 elements for the large PVDF sensors were used in this array design. 
     Array Performance 
     The performance of the four sensor arrays depicted by FIGS. 9,  10 ,  11  and  12  is presented in terms of beam width and array gain by FIGS. 13,  14  and  15 . The beam pattern plots are for frequency at 250 Hz using Verberg propagation model and 10 dB element SNR. These figures show the beam patterns in x, y and z direction for a source 3 cm directly below the array center. The beam pattern for the conventional delay and sum (DS) beamformer is shown in dashed line, and the beam pattern for the MVDR beamformer is in solid line. 
     The figures show that for a conventional beamformer, the array gain is proportional to the number of elements. The effect of the number of elements on the array beamwidth is much more visible for the MVDR beamformer than for the DS beamformer. Also notable is the lack of array aperture in the z direction, as illustrated by the large beam width shown in FIG.  15 . 
     It is known that the MVDR beamformer provides an estimate of the signal power at the signal direction as can clearly be seen from FIGS. 13,  14  and  15 . At the source location, the output of the MVDR beamformer is 10 dB regardless of the number of element in the array. The effect of an increase in the number of elements is a narrower beam width, which is consistent with data showing that the beam width of an MVDR beamformer is inversely proportional to the number of elements (and the element SNR). 
     Array Geometries 
     Increasing the number of elements in constrained by acoustics window size and the physical dimensions of the individual sensor. 
     It became apparent from the performance of the array designs of FIGS. 10 to  14  that the use of the 4 th  and 5 th  intercostal spaces for centering purpose has merit in the array design process. For both designs, the acoustics window is the union of the two window areas for the 4 th  and 5 th  ICSs. These two windows are the average of the XY data obtained from the acoustics window study. The merging of the two windows increases the area available for the array aperture which is an advantage to array performance. 
     A Proposed Wilcoxon Accelerometer Array Based on the Dominant ICS Areas 
     A design for a Wilcoxon commercial accelerometer array is shown in FIG.  16 . In this design, there are a total of 16 elements that will fit the composite window area. The composite window area consists of the two averaged windows with controids near the 4 th  and 5 th  ICSs. In the current data collection system, only 12 elements are used. The missing elements are chosen such that the resulting array is as irregular as possible with at least one sensor pair very close to each other to prevent spatial aliasing. 
     A PVDF Sensor Array 
     For the same composite acoustics window, a 45-element PVDF array is shown in FIG.  17 . This array essentially consists of 5 rows of 9-element large PVDF linear array arranged in such a way that conforms to the human chest curvature and, if possible, lies within the lower ICS to adapt to the patient anatomy. One reason for a 5 by 9 linear PVDF array is in the manufacturing and logistics of the thin film technology. 
     It may not be possible to use all 45 elements for beamforming, since some of the array elements may fall out the acoustics window and thus will not be able to receive the heart sound. An acceptable data collection scheme includes estimation of the signal-to-noise ratio at each element, and weighting or eliminating the sensors that receive the noisiest signal. The use of this weighting technique enables the array to adapt to the differences in acoustic window size that are embodied in human anatomy.