Abstract:
Parametric ultrasonic measurements which characterize the structure of tissue, using information from an ultrasonic signal beyond amplitude information, are obtained by combining multiple ultrasonic signals acquired at different angles, thereby reducing the variance of the calculations. Such angular compounding may be applied to detecting scatterer size, spacing, density, and attenuation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application 60/464,678 filed Apr. 22, 2003 hereby incorporated by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   This invention was made with United States government support awarded by the following agencies: NIH CA39224 The United States has certain rights in this invention. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to ultrasonic imaging techniques, and in particular to “parametric” ultrasound imaging that characterizes parameters of the scanned tissue using information in the echo signal other than or in addition to echo amplitude. 
   Ultrasound imaging is widely regarded as a safe, cost-effective, and versatile medical imaging modality. In a typical echo-mode ultrasonic device, an ultrasonic signal is transmitted into the patient from a transducer and an echo signal is received from the patient and analyzed. In conventional B-mode imaging, only the amplitude of the echo signal is extracted and displayed. 
   In parametric ultrasound imaging, additional information is extracted from the echo signal beyond its amplitude. This information may include frequency and/or phase information of the echo signal and may be processed to characterize the “effective scatterers” of the tissue through a description of their shape, size, spacing, and density. 
   Such parametric measurements have a high degree of statistical fluctuation, which limits their practical use in medical diagnostics. 
   SUMMARY OF THE INVENTION 
   The present invention controls the statistical fluctuations of parametric imaging by using multiple angle acquisitions combined either before or after the relevant parameter is extracted. The inventors have determined that relatively small angular differences between the acquisitions provide the necessary statistical independence of these measurements. This “angular compounding” works with a variety of different parametric measurements including those measuring scatterer size, scatterer spacing, scatterer density and scatterer attenuation. 
   Specifically then, the present invention provides a parametric ultrasonic system using an ultrasonic transducer assembly adaptable to produce a series of echo signals at different angles of a plurality of voxels in a region of interest. The echo signals at different angles can be obtained by moving a single transducer, or by sweeping a phased array transducer with or without movement, or by other techniques known in the art. A processor receives the echo signals and extracts a parametric measurement for each of the voxels, the parametric measurement based on a combination of frequency spectra from the multiple echo signals at different angles. 
   Thus it is an object of the invention to improve the quantitative value of the measured parameter by using echo signals acquired at different angles. 
   The parameter may be scatterer size. In one embodiment, the processor may determine the spectrum of a portion of each echo signal and match the spectra to spectra of materials having known scatterer size to produce the parametric measurement of scatterer size. 
   It is thus another object of the invention to provide a versatile method of characterizing tissue. Matching spectra to a library of spectra of materials having known scatterer size provides a versatile method of identifying scatterer size. 
   The spectra of the echo signal and of the materials having known scatterer size may be corrected prior to matching for spectral coloring caused by the measurement environment, including the transducer and some aspects of the material through which the measurement is made. 
   Thus it is another object of the invention to improve the sensitivity of the parametric measurement to the tissue by removing other influences that may affect the echo spectra. 
   The parameter measured alternatively may be scatterer spacing. In one embodiment, scatterer spacing may be determined by analyzing the frequency content of the spectra. 
   Thus it is another object of the invention to extract additional information from the echo signal&#39;s spectra. 
   The parameter measured may alternatively be scatterer density. In one embodiment, this may be measured by matching a spectrum of a portion of the ultrasonic signal measurement to the spectra of materials having known scatterer size and then scaling the matched spectra to the ultrasonic signal measurement to determine scatterer density. 
   Thus it is another object of the invention to provide a more sophisticated analysis of echo signal strength than provided by conventional B-mode imaging. 
   Alternatively, the scatterer number density may be determined from the kurtosis of the echo signal, such as by taking the ratio of the signal kurtosis from a region to the kurtosis from the same region in a reference phantom having a known scatterer number density. 
   Thus it is another object of the invention to provide a means to calculate scatterer number density using the kurtosis of the signal from a region. 
   The parameter, alternatively, may be an ultrasonic attenuation (UA) value. In one embodiment, the processor may determine UA by taking a spectrum of each echo signal for adjacent voxels in the region of interest and determining a difference of these spectra whose slope is UA. 
   It is yet another object of the invention, therefore, to provide a highly resolved attenuation measurement of the tissue being imaged. 
   The system may provide a sensor attached to the ultrasonic transducer producing a position signal for each of the different angles of measurement and/or a position signal may be derived from beam steering commands given to a phased array transducer and known geometry of the transducer location and orientation, and the processor may receive the position signal to match corresponding portions of the echo signals for angular compounding. 
   Thus it is an object of the invention to provide a positive method of aligning the different echo signals for angular compounding. 
   Alternatively, the processor may provide a correlator correlating the echo measurements over each voxel to match corresponding portions of echo signals for the extractions of parameter measurements from each voxel. 
   Thus it is another object of the invention to provide angular compounding without fundamental modification to existing ultrasound machines. 
   These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a simplified schematic of an ultrasound machine suitable for use with the present invention using a hand-held transducer to obtain echo signals at different angles through the region of interest; 
       FIG. 2  is a schematic of a second embodiment of the machine in  FIG. 1  providing a mechanism for movement of the transducer; 
       FIG. 3  is a representation of echo signals re-binned into three measurement sets of parallel rays and different angles; 
       FIG. 4  is a flow chart showing the principal steps of the present invention such as may be implemented in software or hardware; 
       FIG. 5  is a graph of an echo signal received from the devices of  FIGS. 1 and 2  showing the ultrasonic signal and its amplitude used in conventional ultrasonic imaging; 
       FIG. 6  is a diagram showing the conversion of echo signal amplitude into a conventional B-mode image such as may also be output by the present invention; 
       FIG. 7  is a figure similar to that of  FIG. 6  showing conversion of spectra of portions of the underlying echo signals from three measurement sets of echo signals of different angles; 
       FIG. 8  is a diagram showing the collection of scans taken by the device of  FIG. 2  re-binned into the measurement sets of  FIG. 7  for parameter extraction and the combination of the extracted parameters to create an image; 
       FIG. 9  is a signal flow chart showing the extraction of a parameter from an echo signal to deduce scatterer size; 
       FIG. 10  is a figure similar to that of  FIG. 9  showing the extraction of scatterer spacing from an echo signal; 
       FIG. 11  is a figure similar to that of  FIGS. 9 and 10  showing steps added to  FIG. 9  to extract scatterer number density; 
       FIG. 12  is a figure similar to  FIGS. 9 and 10  showing subtraction of spectral data from two adjacent voxels to produce a local broad band attenuation measurement; 
       FIG. 13  is a fragmentary view of  FIG. 8  showing an alternative angular compounding technique where the measurement sets are combined prior to extraction of the parameter; 
       FIG. 14  is a perspective representation of the acquisition of scan data over a three-dimensional region of interest for parametric imaging of a volume rather than a single plane; and 
       FIG. 15  is a view of an alternative embodiment of the ultrasound transducer of  FIG. 1  showing scanning using a phased array linear or curvilinear multi-element transducer. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , an ultrasonic imaging system  10  suitable for use with the present invention may employ a standard ultrasonic imaging machine  11  alone or in combination with computer  30 . Generally, the ultrasonic imaging machine  11  provides the necessary hardware and a protocol to collect a series of ultrasonic echo signals that can be processed by a processor held within the ultrasonic imaging machine  11  or transmitted to the computer  30  for external processing. 
   An ultrasound transducer  12  associated with ultrasonic imaging machine  11  transmits ultrasonic beams  14  and  14 ′ at a number of different angles (only two being shown for clarity) toward a region of interest  18 . Each ultrasonic beam  14  provides a number of echo signals acquired along different measurement rays  16  extending within the ultrasonic beam  14  passing through volume elements (voxels)  26  within the region of interest  18 . 
   The echo signals are received by interface circuitry  22  of the ultrasonic imaging machine  11  which may provide amplification and digitization of the echo signals. These echo signals may then be transmitted to a memory  35  for storage and subsequent processing by a processor  33  within the ultrasonic imaging machine  11  or in the external computer  30  either executing a stored program as will be described below. 
   In both cases, an image will be generated that may be provided to a graphic display  32 . In both cases, input commands may be received via a keyboard  34  and/or a cursor control device  36  such as a mouse as is well understood in the art. 
   In one embodiment, the ultrasonic imaging machine  11  may be an Acuson 128XP10 scanner employing a V4 transducer with a center frequency of 3.5 MHz with a 6 dB bandwidth of 40 percent. Digitized echo signals from this ultrasonic imaging machine  11  may be captured by a Gage Applied Science 12100 A/D board and provided to the computer  30  for processing. More commonly, the ultrasound imaging machine will employ a linear or a curvilinear array transducer, and the echo signals will be processed directly by the machine. 
   Generally, as shown in  FIG. 1 , the ultrasound transducer  12  may be a single element transducer manually steered to transmit the different beams  14  and acquire echo signals along the different rays  16  or preferably as shown in  FIG. 15 , the ultrasound transducer  12  may be a multi-element ultrasonic transducer  12  producing a multiplicity of beams, each beam electronically steered by phased-array operation to transmit the different beams  14  and acquire echo signals along the different rays  16 . As will be understood in the art, the multi-element ultrasonic transducer  12  may also operate in a uniphased broadcast with phased array reception or phased array broadcast with uniphased reception or other variations known in the art. Significantly, the ultrasound transducer  12  must collect echo data from different angles through each voxel. A position sensor  17  optionally may be attached to the ultrasonic transducer  12  to obtain position data  41  indicating the position and orientation of the beams  14 ,  14 ′ from the ultrasonic transducer  12  whose use will be described below. Position data alternatively may be extracted from a correlation of the echo signal as will also be described. 
   Referring to  FIG. 2 , in an alternative embodiment, a mechanical scanning arm  40  may hold the multi-element ultrasonic transducer  12  to provide a linear scanning across the patient  15 . Alternatively, the scanning arm  40  may move in an arcuate or other pattern. The scanning arm  40  may provide a precise movement of the ultrasonic transducer  12  to produce a variety of different ultrasonic beams  14 ,  14 ′ and  14 ″, each acquiring echo signals along corresponding measurement rays  16 ,  16 ′ and  16 ″ at a variety of angles crossing a region of interest  18 . The scanning arm  40  may provide a position signal  41  or the position signal  41  may be deduced from commands to the scanning arm  40 . 
   The echo signal acquired with ultrasonic beams  14  of  FIGS. 1  or  2  may be collected into measurement sets  25 , either according to the particular ultrasonic beam  14  used to acquire the data, or as shown in  FIG. 3 , according to a re-binning so that the echo signals of each measurement set  25  is associated with a single angle of measurement rays  16 , and different measurement sets  25  have echo signals of different measurement rays  16 . 
   Referring now to  FIGS. 4 and 8 , in a first step of the present invention, as indicated by process block  50 , multiple ultrasonic beams  14  are used to collect echo signals at measurement rays  16  of different angles. The measurement rays  16  may differ by as little as 0.75 degrees and still provide sufficient independence of measurement to reduce the statistical deviation in the extracted parameter. Nevertheless, higher degrees of angular separation may also be used, and in a preferred embodiment, for example, forty-five different measurement sets  25  may be acquired, each with one degree of difference between them. Clearly higher angular differences produce even more independence in the measurement and angular separations of five degrees and greater may also be practical and angular ranges of less than 180 degrees, e.g., 90 degrees, are practical unlike tomographic systems. 
   At succeeding process block  52 , the data of the ultrasonic beams  14  may be re-binned optionally into measurement sets  25  having parallel rays. This is not necessary as a mathematical requirement, but can simplify later calculations. Alternatively, the measurement sets  25  may be formed of the echo signal associated with each particular ultrasonic beam  14 . 
   Referring now to  FIG. 5 , the echo signal  54  along each measurement ray  16  provides a time signal having both frequency and phase information. In conventional B-mode imaging, as will be described, an envelope signal  56  is extracted from the echo signal  54  and the amplitude of the envelope signal  56  alone is used. As indicated by process block  62 , this envelope signal  56  may be used to develop a B-mode image for each measurement set  25  acquired. 
   Referring to  FIG. 6 , the B-mode image maps the amplitude of the envelope signal  56  to pixels  59  of image  61 , such that samples  63  of the amplitude of the envelope signal  56  taken at different times in the echo signals  54  provide information for different pixels  59  in a column of pixels  59  of the image  61  and different echo signals  54  at corresponding times provide different pixels  59  for a given row of the image  61 . The magnitude of the envelope signal  56  for each pixel  59  is mapped to a color or gray scale. Each pixel  59  corresponds to a similarly located voxel  26  within a plane of the region of interest  18 . 
   Referring again to  FIGS. 4 and 8 , at process block  58 , the acquired measurement sets  25  may be further processed to extract parametric measurements as will be described in detail further below. Generally, each parametric value will be associated with a portion of an echo signal  54  related to an echo received from a voxel  26  within the patient  15 . 
   At succeeding process block  64 , the measurement sets  25  are aligned with each other as a prelude to combining the parameters extracted at process block  58 . This alignment process finds portions of different echo signals  54  that measure an echo from a common voxel  26  of the patient  15 . This in turn can be done by using the direction of acquisition of a steered beam, either alone or in combination with the known geometry of the scanning arm  40  and its position signal  41  or the position signal  41  from a position sensor attached to a freely movable ultrasonic transducer  12 , or a combination of tracking techniques. The time axis of the echo signal  54  is used to determine the depth of the echo from the patient  15  and position signal  41  provides the orientation of the measurement ray  16  of that echo signal so that the particular voxel  26  can be identified geometrically. 
   In an alternative embodiment of the invention, B-mode images  61  of each of the image sets  25  may be moved in translation and rotation to provide maximum correlation between their pixels  59 . This provides a matching of the different echo signals  54  of each of the image sets that may be used to match corresponding parametric measurements of a given voxel  26 . This may be accomplished by the use of a correlator implemented by the processor  33 . 
   Referring to  FIGS. 4 ,  7 , and  8 , at process block  72 , parameters associated with corresponding samples  63  of the echo signals  54  of three measurement sets  25   a ,  25   b , and  25   c  and thus with common voxels  26  measured by the three measurement sets  25   a ,  25   b , and  25   c , may be combined according to the alignment derived from process block  64  to produce a parametric pixel  76  of a parametric image  78 . 
   At process block  80 , this image  78  may be displayed along with quantitative information about the extracted parametric measurements, for example, an average value within a region of the image  78 . 
   At process block  80 , the B-mode images developed with respect to process block  62  may also be displayed for reference by the operator and may be combined in a tomographic type image as is well understood in the art. 
   Each of the above process blocks may be implemented in software or firmware on the ultrasonic imaging machine  11  or the computer  30 . 
   Referring still to  FIGS. 4 and 8 , the process of extraction of parametric values from the measurement sets  25  of process block  58  differs according to the parameter being extracted. Each of these processes is described below for a single pixel and will be repeated to generate parametric measurements for each of the pixels of an image. 
   For a determination of scatterer size, multiple samples  63  are taken of each echo signal  54  according to a window  82  corresponding roughly to the size of a voxel from which the parameter is being extracted as shown in  FIG. 9 . The tissue power spectrum  86  of this sample  63  is obtained by Fourier transform per block  84 , the tissue power spectrum  86  indicating the energy in the sample  63  at different frequencies as is understood in the art. 
   Referring to  FIGS. 4 ,  8 , and  9 , in the preferred embodiment, a second standard echo signal  54 ′ corresponding to echo signal  54  being analyzed, is obtained of a phantom simulating the generally attenuating characteristics of tissue of a standard patient as indicated by process block  60 . The window  82  is also applied to this echo signal  54  to obtain a sample  63 ′ which may also be transformed by a Fourier transform algorithm per block  84  to produce a machine power spectrum  86 ′, dependent principally on characteristics of the transducer  12 , the interface circuitry  22 , the amplification and depth dependent signal processing in the receiver, and the phantom. 
   This machine power spectrum  86 ′ may be subtracted from the tissue power spectrum  86  by subtractor  88  to produce a scatterer dependent power spectrum  90  having a distinctive curve  92 . 
   A library  94  of different curves  92 ′ representing scans performed of phantoms having known scatterer sizes, or representing power spectra modeled for different sized scatterers, are then compared to the curve  92  by a curve fitting process  96 . In the preferred embodiment, this curve fitting is insensitive to differences between curve  92 , and curves  92 ′ caused solely by a multiplicative constant, for example, as taught by Insana, et al. “Describing Small-Scale Structure In Random Media Using Pulse-Echo Ultrasound”, J. Acoust. Soc. Am. 1990; 87: 179-192.1990. 
   The particular one of the curves  92 ′ that matches is mapped to a gray or color scale value by a mapper  98  to produce an output pixel for that sample  63  that may be combined with other pixels per process block  64  and  72  described above. 
   Referring now to  FIG. 10 , alternatively, the parametric measurement may be scatterer spacing determined by again analyzing samples  63  selected by windows  82  from the echo signal  54 . As before, a tissue power spectrum  86  may be produced through the use of the Fourier transform per block  84 . A frequency analysis of the spectrum may be produced using the cepstrum operation indicated by process block  89  to identify a dominant frequency component  102 . Again, the frequency of this component  102  may be mapped by mapper  98  to a gray or color scale value to produce an output pixel for that sample  63  that may be combined with other pixels per process block  64  and  72  described above. 
   Referring now to  FIG. 11 , alternatively, the parametric measurement may be scatterer number density and the identified curve  92 ′ of  FIG. 9  may be scaled by a multiplicative constant by curve fitter  106  to fit to the actual curve  92  and this multiplicative constant may be provided to a mapper  98  to provide the pixel  76  indicating scatterer number density. Alternatively, instead of conducting a spectral analysis of the echo signal waveform, scatterer number density can be derived from statistical properties of the echo signal, the kurtosis as taught by Chen, et al., “A Method for Determination of Frequency Dependent Effective Scatterer Number Density”, J. Acoust. Soc. Am. 1994; 95: 77-85. Thus, the kurtosis of the signal from each of the overlapping measurement regions  26  is calculated as the ratio of the fourth moment to the square of the second moment of the echo signals. By comparing to the kurtosis derived from a reference phantom that has a known scatterer number density, the scatterer number densities of tissues mapped to measurement regions  26  are derived. 
   Referring now to  FIG. 12 , alternatively, the parametric measurement may be ultrasonic attenuation. In this case separate windows  82  and  82 ′ provide samples  63  and  63 ′ related to adjacent voxels of the same echo signal  54 . These samples  63  are processed by a Fourier transform per blocks  84  to produce separate spectra  86   a and  86   b . These spectrum  86   a  of the later sample  63  is subtracted from the spectrum of the earlier sample  86   b  to produce a spectral difference  112 , whose slope  114  provides the attenuation for the later voxel  26 , which may be mapped by mapper  98  to a value of pixel  76 . 
   Referring now to  FIG. 13 , it will be understood that the order of parameter extraction and parameter combination may be switched. Thus, for example, the measurement sets  25   a - 25   d  may be aligned and summed per summer  116  before the parameters are extracted from the combined measurement sets of process block  58  to produce the image  78 . 
   Referring now to  FIG. 14  for reasons of clarity, the invention has been described with respect to voxels  26  aligned in a single plane corresponding to a plane of the image  78 . However, it will be understood that the essential principle of summing together echo signals  54  taken at different angles to enhance parametric measurements may occur by moving the ultrasonic transducer  12  so as to collect multiple ultrasonic beams  14  that differ not only by their angle within a plane but also in angles over a three-dimensional curved or planar surface  118  so as to produce volumetric image data that may be displayed, one slice at a time, or rendered as a three-dimensional object. It will also be understood that this process can be done either by motion of the transducer or by beam steering with array region from different directions. 
   It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.