Abstract:
An auscultation system aids a clinician&#39;s diagnosis of the heart sounds by visually displaying at least an S 1  heart sound and an S 2  heart sound, and ascertaining an onset of at least one of the heart sounds. A corresponding audio representation of the heart sounds can be provided to the clinician. The auscultation system includes a sensor for sensing heart sounds from at least one chest location of the patient and for transducing the heart sounds into electrical signals. The auscultation system also includes a signal processor for selectively filtering the electrical signals thereby highlighting frequency differences of the heart sounds, and further includes a video display for selectively displaying the selectively filtered electrical heart signals. In some embodiments, the auscultation system also displays calipers corresponding to the time domain and the frequency domain of the heart sounds, permitting the clinician to zoom in and out portions of the heart sounds of particular interest and also to take more accurate measurements of these portions of the heart sounds.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to medical electronic devices for analysis of auscultatory cardiac sounds. More particularly, this invention relates to a method for recording, analyzing and audiovisual representation of heart sounds at the point of care, in humans, to enable differential diagnosis. 
         [0002]    Auscultatory sounds have long been the primary inputs to aid in the detection of various physiological conditions. For instance the stethoscope is the primary tool used by a clinician to monitor heart sounds to detect and diagnose the condition of a subject&#39;s heart. Auscultation itself is extremely limited by a number of factors. It is extremely subjective and largely depends on the clinician&#39;s expertise in listening to the heart sounds and is compounded by the fact that certain components of the heart sounds are beyond the gamut of the human ear. In addition, auscultation relies on correctly determining which of the primary heart sounds correspond to the systole diastolic phase of the heart. This is made more difficult when ectopic beats occur. 
         [0003]    A number of improvements have been developed to circumvent such bottlenecks, ranging from relatively noise-free electronic auscultation, to complex computer algorithms that can analyze the cardiac sounds, calculate various numerical values like heart rate, ascertain the heart sound phases etc. For example, algorithms are available that allow heart sounds in electronic format to be visualized on a personal computer screen and analyzed. 
         [0004]    Accordingly, personal computer (PC) based auscultatory devices like the Acoustic Cardioscan from Zargis Medical Corporation of Stamford, Conn., and software packages like the Veteran Phonocardiograph monitor from BioSignetics Corporation of Exeter, N.H., are capable of a wide range of operations and manipulations of heart sounds offline. However, the above described PC based platforms suffer from the following shortcomings and bottlenecks. These PC based systems call for a separate data gathering device to record heart sounds in the format that can be processed by the PC based algorithm. In addition, there is a critical time delay between the time the clinician auscultates the subject and the time the clinician applies the PC based analysis to the recorded heart sounds. There are also portability issues associated with the PC based system setup. 
         [0005]    Currently, handheld auscultatory devices have been developed in an attempt to circumvent some of the above described problems with PC based computer systems. These handheld devices do incorporate the data gathering mechanism in the device itself, obviating the need for separate data gathering. Handheld devices sold under the brand names Cadiscope (from Caditec AG Medical Instruments of Switzerland) and the Visual Stethoscope (from MC21 Meditech Group) are instances of such handheld auscultatory devices. However handheld devices have their own shortcomings. For example, some handheld devices are designed such that the chest piece is housed in the device itself thereby rendering sterilization processes difficult, or at least call for involved and expensive methods of cleaning. Further, the mere display of the heart sounds or ECG signals, in addition to the audio of the heart sounds is insufficient for the user to ascertain the condition of the heart. 
         [0006]    It is therefore apparent that an urgent need exists for an improved auscultatory device that is easy to use, accurate, portable, cost-effective and easy to sterilize and maintain. 
       SUMMARY OF THE INVENTION 
       [0007]    To achieve the foregoing and in accordance with the present invention, a method and system of analyzing and displaying heart sounds is provided. Such an auscultation system is useful for a clinician to efficiently and cost-effectively auscultate patients. 
         [0008]    In one embodiment, the auscultation system includes a sensor for sensing heart sounds from at least one chest location of the patient and for transducing the heart sounds into electrical signals. The auscultation system also includes a signal processor for selectively filtering the electrical signals thereby highlighting frequency differences of the heart sounds, and further includes a video display for selectively displaying the selectively filtered electrical heart signals. 
         [0009]    The auscultation system aids the clinician&#39;s diagnosis of the heart sounds by visually displaying at least an S 1  heart sound and an S 2  heart sound, and ascertaining an onset of at least one of the heart sounds. A corresponding audio representation of the heart sounds can be provided to the clinician. 
         [0010]    In some embodiments, in addition to displaying the heart sounds, the auscultation system also displays calipers corresponding to the time domain and the frequency domain of the heart sounds, permitting the clinician to zoom in and out portions of the heart sounds of particular interest and also to take more accurate measurements of these portions of the heart sounds. 
         [0011]    These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]    In order that the present invention may be more clearly ascertained, one embodiment will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0013]      FIG. 1  is a block diagram showing one embodiment of an auscultation device for analyzing and displaying heart sounds in accordance with the present invention; 
           [0014]      FIG. 2  is a block diagram illustrating a heart sound signal acquirer for the auscultation device of  FIG. 1 ; 
           [0015]      FIG. 3  is a block diagram illustrating a heart sound signal conditioner for the auscultation device of  FIG. 1 ; 
           [0016]      FIG. 4  is a flow diagram illustrating heart sound signal decomposition for the auscultation device of  FIG. 1 ; 
           [0017]      FIG. 5  is a flow diagram illustrating heart sound signal playback for the auscultation device of  FIG. 1 ; 
           [0018]      FIG. 6  is a flow diagram illustrating display zooming for the auscultation device of  FIG. 1 ; 
           [0019]      FIG. 7  is a flow diagram illustrating display calipers for the auscultation device of  FIG. 1 ; 
           [0020]      FIG. 8  is a flow diagram illustrating heart sound signal storage for the auscultation device of  FIG. 1 ; 
           [0021]      FIG. 9  is a flow diagram illustrating various functions of the auscultation device of  FIG. 1 ; and 
           [0022]      FIGS. 10A-10G  show screenshots illustrating the various functions of the auscultation device of  FIG. 1 . 
           [0023]      FIGS. 11A &amp; 11B  are isometric and top views, respectively, of another embodiment of a heart sound signal acquirer for the auscultation device of  FIG. 1 . 
           [0024]      FIGS. 12A &amp; 12B  show an isometric and two side views of another embodiment of the auscultation device of  FIG. 1 . 
           [0025]      FIGS. 13 and 14  illustrate two additional embodiments of the auscultation device of  FIG. 1 . 
           [0026]      FIGS. 15 and 16  show embodiments of the auscultation device of  FIG. 1  wherein the heart sound signal acquirer is attached directly to the main body of the auscultation device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    The present invention will now be described in detail with reference to several embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention. The features and advantages of the present invention may be better understood with reference to the drawings and discussions that follow. 
         [0028]    To facilitate discussion,  FIG. 1  is a block diagram showing one embodiment of an auscultation device  100  for analyzing and displaying heart sounds in accordance with the present invention. Device  100  includes heart sound signal acquirer  110 , signal conditioner  120 , signal processor  130 , memory  140 , user interface  150 , video display  160  and audio input/output device  170 . 
         [0029]    Memory  140  can be fixed or removal memory, and combinations thereof. Examples of suitable technologies for memory  140  include solid-state memory such as flash memory, or a hard disk drive. 
         [0030]    User interface  150  can be a keypad, a keyboard, a thumbwheel, a joystick, and combinations thereof. Video display  160  can be an LCD screen, or can be an LED display or a miniature plasma screen. It is also possible to combine video display  160  with user interface  150  by use of technologies such as a touch screen. Contrast and brightness control capability can also be added to display  160 . 
         [0031]    Audio input/output (I/O) device  170  includes a microphone, and speakers, earphones or headphones, any of which can be internal or external with respect to device  100 . It is also possible to use wireless audio I/O devices such as a Bluetooth-based headset. Volume control of device  170  can also be provided. 
         [0032]      FIG. 2  is a block diagram illustrating heart sound signal acquirer  110  in greater detail. Acquirer  110  includes an acoustic sensor  210  and a preamplifier  215  which are coupled to signal conditioner  120 . In this embodiment, sensor  210  is a unidirectional microphone housed in a chest piece assembly. Preamplifier  215  is solid-state and provides pre-amplified heart sounds to signal conditioner  120 . 
         [0033]      FIG. 3  is a detailed block diagram illustrating heart sound signal conditioner  120  which includes an input buffer  310 , one or more band pass filter(s)  320 , a variable gain amplifier  330 , a gain controller  340  and an output buffer  350 . Output buffer  350  is coupled to signal processor  130  which in turn is coupled to gain controller  340 . 
         [0034]    In this embodiment, filter  320  is a 4 th  order Butterworth pass band of 5 Hz to 2 kHz which limits the analysis of the heart sound signal to frequencies less than 2 kHz, thereby ensuring that all frequencies of the heart sounds are faithfully captured and at the same time eliminating noise sources that typically exist beyond the pass band of filter  320 . Variable gain amplifier  330  of signal conditioner  120  serves to vary the signal gain based on a user-selectable input parameter, and also serves to ensure enhanced signal quality and improved signal to noise ratio. The conditioned heart sound signal after filtering and amplification is then provided to signal processor  350  via output buffer  350 . 
         [0035]      FIG. 4  is a flow diagram illustrating an exemplary “Decomposition” of heart sound signals by one embodiment of signal processor  350 . In step  405 , heart sound signals are retrieved from input buffer  310 . Using the appropriate key sequence on user interface  150 , the user can select Visual Display mode and/or Audio Playback mode as shown in step  410 . 
         [0036]    Two sets of filters of different frequencies pass bands pertain to two modes of operation, namely, a “Bell” mode and a “Diaphragm” mode. These two operational modes emulate the respective functions of a combined Bell/Diaphragm head found in traditional acoustic (non-electronic) stethoscopes that many experienced clinicians are accustomed to using. These two sets of filters pertain to audio filtering as shown in steps  460  and  465 , as well as video filtering for subsequent visual display on display  160 . Depending on the user selection between audio playback mode and visual display mode, the pertinent set of audio or video filters is enabled. 
         [0037]    As shown in step  415 , the user&#39;s visual analysis of the decomposed the heart sounds is based on the Bell or Diaphragm mode selected through user interface  150 . Referring also to  FIG. 10F  which shows an exemplary “Bell” Visual Display mode, the composite heart sound signal which is between 1-500 Hertz is decomposed into two component frequency ranges; a low frequency component between 0-150 Hertz and a high frequency component between 150-500 Hertz (steps  420 ,  425 ). The low frequency components highlights S 1 , S 2 , S 3 , S 4 , and low frequency murmurs, while the high frequency components highlights S 1 , S 2 , low frequency murmurs (prominent) and medium frequency murmurs (suppressed). 
         [0038]    In  FIG. 10D  which illustrates a “Diaphragm” Visual Display mode, the composite heart sound signal which is between 60-600 Hertz is also decomposed into two component frequency ranges, a low frequency component between 60-250 Hertz and a high frequency component between 250-600 Hertz (steps  430 ,  435 ). The low frequency components highlighting the S 1 , S 2 , medium frequency murmurs (prominent) and low frequency murmurs (suppressed). The high frequency components highlighting the medium frequency murmurs and high frequency murmurs. 
         [0039]    In this embodiment, the frequencies captured in “Bell” mode include the complete range of Bell frequencies. Similarly the frequencies captured in “Diaphragm” mode include the complete range of the Diaphragm frequencies. Other customized decomposition modes with user definable component frequency ranges are also possible. As discussed above, display  160 , e.g., an LCD display, provides the visual representation of the heart sounds to the user by storing the waveforms in output buffer  350  prior to visual display (steps  440 ,  445 ). Meanwhile audio output device  170 , e.g., a set of headphones, provides an auditory representation of the same heart sounds to the user by a digital-to-analog conversion (DAC) prior to audio playback (steps  470 ,  475 ). Preferably, both visual and auditory representations of the heart sounds as experienced by the user are synchronized. 
         [0040]    In another embodiment, the sensor head has two opposing sensors (not shown), i.e., a Bell-side sensor and a Diaphragm-side sensor, like a traditional acoustic stethoscope. Accordingly, instead of the user manually selecting the Decomposition mode, device  100  automatically selects the appropriate decomposition mode by sensing whether the Bell side sensor or the Diaphragm side sensor of the sensor head is touching the chest wall of the patient and hence is generating a stronger heart sound signal. The heart sounds are then analyzed by the corresponding Bell or Diaphragm filters which are also automatically selected by processor  130 . 
         [0041]    In yet another embodiment illustrated by the isometric and top views of  FIGS. 11A and 11B , respectively, heart signal acquirer  1110  also includes a selector switch  1115  which the user can use to select from two or more pre-determined modes, e.g., a “Bell” mode or a “Diaphragm” mode, using a finger of the same hand that is hold signal acquirer  1110  against the chest wall of the patient. While an exemplary two-position slider switch assembly  1115 ,  1116  is shown, it is understood that other selector switches are also possible, including push button switches, rocker switches and rotary switches. Switch  1115  can be located on the top of or on the side of signal acquirer  1110 . 
         [0042]    Referring now to  FIG. 5 , which is a flow diagram illustrating heart sound signal audio playback as facilitated by the user inputting the “Playback” command using user interface  150 , such as a keypad (step  510 ). In the Playback function, the user can select “Normal” playback or “Stereophonic” playback (steps  520 ,  530 ). The user can also select “Bell” mode or “Diaphragm” mode (steps  550 ,  570 ). In step  560 , when Bell mode has been selected, display  160  such as an LCD screen, shows the composite Bell waveform as well as the respective component low frequency and the high frequency waveforms. Similarly, in step  580 , when Diaphragm mode has been selected, display  160  such as an LCD screen, shows the composite Diaphragm waveform as well as the respective component low frequency and the high frequency waveforms. 
         [0043]    As shown in  FIG. 10G , a vertical “Play” cursor scrolls across the three waveforms on display  160  synchronously with the audio playback described above, thereby ensuring that the user can visually see on display  160  what he/she is hearing via audio output device  170 . 
         [0044]      FIG. 6  is a flow diagram illustrating the “Zoom” command for video display  160  of auscultation device  100 . By selecting the Zoom command using user interface  150 , the user is able to Enlarge or Reduce the heart sound waveforms along the horizontal-axis, i.e., along the time domain, displayed on video display  160  (step  610 ). When enlarging, the Enlarge key press will have no effect once the maximum Enlargement has been reached (steps  620 ,  630 ,  640 ). Similarly, when reducing, the Reduce key press will have no effect once the maximum Reduction has been reached (steps  650 ,  670 ,  680 ). 
         [0045]      FIG. 7  is a flow diagram illustrating soft “Calipers” for time measures of heart sound waveforms. The user activates the Caliper function on video display  160  by pressing keys on user interface  150 , causing soft calipers to appear on display  160  as a pair of lines (step  705 ). Along with both X and Y calipers, the time widths enclosed by the respective calipers also appear on display  160 . 
         [0046]    Referring also to  FIG. 10E , if “X” calipers are selected, a pair of vertical calipers appears. X calipers allow the user to make accurate measurements of the time periods of the different heart sound phases. Conversely, as shown in  FIG. 10F , if “Y” calipers are selected, a pair of horizontal calipers appears. Y calipers allow the user to ascertain the murmur grades, whenever murmurs are detected in the heart sounds. 
         [0047]    The user is able to ascertain pathologic heart conditions using device  100  because of most conditions can be associated with their respective characteristic frequencies and amplitude durations. For example under the right conditions, mitral value regurgitation can be diagnosed with approximately 60% certainty. 
         [0048]    By pressing appropriate key on user interface  150 , the calipers can be repositioned by moving left or right relative to its current position. For example, as shown in steps  710 ,  714 , calipers can be repositioned to the left until the calipers are at the end of the page, thereby causing the “previous page” of the heart sound waveforms to appear on display  160  (step  718 ). Alternatively, the calipers can be repositioned to the right until the calipers are at the end of the page (steps  740 ,  744 ), thereby causing the “next page” of the heart sound waveforms to appear on display  160  (step  748 ). Other display positioning modes are possible. For example, it is also possible to move the display window by partial page increments or portions thereof. 
         [0049]    In addition the calipers on display  160  can be resized by expanding or reducing the size of the calipers. In steps  720 ,  724 , the calipers can be enlarged until a maximum size is reached, and further key presses will no longer have any effect (step  728 ). Similarly, the calipers can be reduced until a minimum size is reached, and further key presses will no longer have any effect (steps  730 ,  734 ,  738 ). 
         [0050]      FIG. 8  is a flow diagram illustrating the storage of heart sound signals acquired by auscultation device  100 . In step  810 , the user selects the “Save” function by pressing keys on user interface  150 , causing device  100  to download the heart sound signal, and associated patient identification and any annotation into a removable or an external memory device (step  820 ). In this embodiment, the patient ID and annotations can be added using voice recordings thereby minimizing the need for additional keystrokes. The local memory of device  100  can now be freed up for recording new heart sound signals (steps  830 ,  840 ). 
         [0051]    In some embodiments, speech recognition technology known to one skilled in the art can be incorporated into device  100 , enabling a textual record of the patent identification and annotations to be included instead or in addition to an audio recording. Speech recognition capability can also be used to activate the various functions of device  100 , thereby resulting in a user-friendly and relatively hands-free auscultation device. Accuracy and/or efficiency of speech recognition can be increased by limiting the vocabulary and/or training the synthesizer to recognize the user&#39;s vocal characteristics. 
         [0052]    It is also possible to incorporate speech synthesis capability into device  100  so as to enhance the ease of use with prompts, instructions and/or feedback. For example, device  100  can ask a user whether device  100  should be sensing in “Bell” or “Diaphragm” mode, or to inform the user that an invalid command/mode has been selected. 
         [0053]    Having described several of the functions of auscultation device  100  in detail, the flowchart of  FIG. 9  and the screenshots of  FIGS. 10A-10G  are now used to illustrate a typical sequence of the various functions that a user may activate while using auscultation device  100  to diagnose the heart sounds of a patient. 
         [0054]    In one embodiment as shown in  FIG. 10A , a heart sound signal acquirer  110 , e.g., a microphone embedded in a chestpiece, and audio input device  170 , e.g., earphones, are electrically coupled to signal processor  130  of device  100 . The user turns device  100  on by pressing the “Function Select” key  1054 .  FIG. 10B  shows device  100  during the “Power On” cycle, while  FIG. 10A  shows battery level  1011  upon completion of the “Power On” which enables the user to keep track of the power needs of device  100 . 
         [0055]    To conserve power, device  100  goes into a sleep mode if there are no key presses after a timeout period, e.g., after two minutes. While in this sleep mode, any key press causes device  100  to return to the last state of operation. 
         [0056]    The user pre-selects a suitable duration of heart diagnosis, e.g., X seconds, of heart sound signals to be acquired (step  910 ). As illustrated by  FIG. 10C , device  100  displays function “Acquire”  1013  and enables the user to make a voice recording of the associated patient information including patient ID (step  920 ). The user places chestpiece  110  on the patient&#39;s chest (step  930 ), which causes device  100  to output the heart sound  1064  on video display  160  as shown in  FIG. 10C  (device response  935 ). 
         [0057]    Together with the user&#39;s training and experience, the “Original” heart sound  1064  enables the user to interpret the graphical representation of the complete heart waveforms, thereby providing the user with a general idea of the condition of the patient&#39;s heart. Note that device  100  initially displays the default audio volume level as an adjustable “Speaker” icon  1012 , the default signal gain level as a “Dial” icon  1016 , and the default zoom as a “Percentage” icon  1015  on video display  160 . 
         [0058]    In step  400 , the user selects “Bell” or “Diaphragm” mode by pressing “Mode Select” key  1053 , thereby causing device  100  to indicate the appropriate mode, in this example, “Diaphragm”  1014 , on video display  160  (device response  942 ). Referring now to  FIG. 10D , when the user presses “Function Select” key  1052  to activate the “Decompose” function (step  940 ), which is followed by a lapse of X seconds, original heart sound  1067 , and decomposed low frequency heart sound  1066  and high frequency heart sound  1065  are displayed by device  100  (device responses  944 ,  946 ). The decomposed heart sounds  1065 ,  1066  enable the user to identify the various heart sound phases and also to detect the presence of heart murmurs. 
         [0059]    By manipulating the “Play/Pause” key  1059  as shown in step  500 , the user causes device  100  to playback and/or record the heart sound signal, and also enables the user to select between “Normal” and “Stereophonic” playback modes (device response  955 ). 
         [0060]    Referring to  FIG. 10E , by manipulating Function Select key  1054  and “Directional” keys  1055 ,  1057  (step  600 ), the user is able to “Zoom In” and “Zoom Out” on the heart sounds in the time domain, i.e., along the X-axis of display  160  (device response  965 ), thereby allowing the user to observe a closer expanded view of the heart sounds. In step  700 , by activating “X-Calipers”  1072   a ,  1072   b , device  100  enables the user to make more accurate time measurements of the heart sound phases (device response  975 ). This ability to zoom in/out and to measure the heart sounds in the time domain is particularly important when one of more of the heart sound phases exceed a particular “normal” time limit, and is indicative of a pathological condition. 
         [0061]    Conversely, as shown in  FIG. 10F , by manipulating Function Select key  1054  and Directional keys  1056 ,  1058 , the user is able to activate and position “Y-Calipers”  1071   a,    1071   b  to provide measurements of the heart sounds in the frequency domain, i.e., along the Y-axis of display  160 , as illustrated by step  700  and device response  965 . The ability to accurately measure the amplitude of the heart sounds facilitates the user to ascertain the grade of the murmurs, based on the width enclosed by the Y-Calipers. 
         [0062]      FIG. 10G  depicts device  100  during playback of the heart sounds, as indicated by “Playback” mode  1017  on display  160 . Note vertical line cursor  1073  scrolls across display  160  during playback, synchronizing the video display with the audio playback of the heart sounds, and enabling the user to visually observing on display  160  what he or she is hearing on audio output device  170 . 
         [0063]    After playback, the user has the option of saving the heart sounds in memory  140  for future analysis before initiating a new recording by pressing “Home” key  1051  (step  800  and device response  985 ). The user can now initiate a new heart sound recording by pressing Function Select key  1054  as shown in step  900 . 
         [0064]      FIG. 12A  illustrates another embodiment  1200  in which heart sound acquirer  1210  and display  1260  are both coupled to audio input device  1270 .  FIG. 12B  show side views of the “open” and “close” positions, respectively, of device  1200 . The “power-on” function of device  1200  can be activated by sliding open device  1200  which simultaneously exposes user interface  1250 . Conversely, sliding close device  1200  conceals user interface  1250  and powers-down device  1200 . 
         [0065]      FIG. 13  is an isometric view of an additional embodiment of device  100 . Device  1300  includes a display  1360  which can be a touch-screen large enough to incorporate all or a portion of the user interface for device  1300 . It is possible for device  1300  to be worn like a watch on the wrist of the user by adding a wrist strap. 
         [0066]      FIG. 14  is an isometric view of yet another embodiment of device  100 . Device  1400  is a compact version with display  1460  supported by audio input device  1470 . In addition, display  1460  of device  1400  can be conveniently flipped open resulting in a hands-free display capability. In this embodiment, heart sound acquirer  1410  is attached to display  1460 . 
         [0067]    Other modifications to device  100  are also possible. As shown in  FIGS. 15 ,  16 , it is also possible to incorporate sensors  1510 ,  1610  with devices  1500 ,  1600 , respectively, resulting in very compact auscultation system designs. In addition to displaying the heart sounds, it is also possible for device  100  to generating gating signals S 1 , S 2  for heart imaging systems. When tuned to appropriate frequencies, device  100  can also be used to sense and record lung sounds, including the higher frequency ranges associated with pulmonary problems such as wheezing. 
         [0068]    In sum, device  100  provides many advantages over the existing auscultatory devices, including ease of use, accuracy, portability, cost-effectiveness and ease of sterilization and maintenance. 
         [0069]    While the present invention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrative and that the invention scope is not so limited. In addition, the various features of the present invention can be practiced alone or in combination. Alternative embodiments of the present invention will also become apparent to those having ordinary skill in the art to which the present invention pertains. Such alternate embodiments are considered to be encompassed within the spirit and scope of the present invention. Accordingly, the scope of the present invention is described by the appended claims and is supported by the foregoing description.