Abstract:
Elastographic imaging of heart tissue may be used to provide strain images by mapping strain magnitude to brightness and strain sign to hue and thus provide improved clinical indication of compression and distension of heart muscle. An areal cursor may be used to obtain quantitative measurements of strain at predetermined periods in the heart cycle. Multiple area measurements of strain may be combined to provide a quantitative index of cardiac health.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with United States government support awarded by the following agencies: NIH CA 39224. The United States has certain rights in this invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a device for medical imaging and diagnosis, and in particular, to the use of elastography for the evaluation of cardiac health. 
     Elastography is a new imaging modality that reveals the stiffness properties of tissues, for example, axial strain, lateral strain, Poisson&#39;s ratio, Young&#39;s modulus, or other common strain and strain related measurements. The strain measurements may be collected over an area and compiled as a two-dimensional array of data, which may then be mapped to a gray scale to form a strain “image”. 
     In “quasi static” elastography, two conventional images of the tissue are obtained using ultrasound, computed tomography (CT), or magnetic resonance imaging (MRI). The first image provides a base line of the tissue at a given state of compression or distention and the second image is obtained with the tissue under a different compression or distention. The tissue may be compressed by an external agency such as a probe or the like or may be compressed by its own muscular action, for example, in the case of the heart, or by movement of adjacent organs. Displacement of the tissue between the two images is used to deduce the stiffness of the tissue. Quasi-static elastography is thus analogous to a physician&#39;s palpation of tissue in which the physician determines stiffness by pressing the tissue and detecting the amount that the tissue yields under this pressure. 
     In “dynamic” elastography, a low frequency vibration is applied to the tissue and the tissue vibrations accompanying the resulting elastic wave are measured, for example, using ultrasonic Doppler detection. 
     Elastography has recently been investigated as a method of detecting cardiac dysfunction. Normal, periodic myocardial thickening, associated with proper heart function, may be revealed in the strains shown in an elastographic image. Tissue ischemia or infarction may thus be detected as a reduction of myocardial thickening. 
     Despite the promise of elastography for cardiac evaluation, effective methods for displaying myocardial strain and of relating elastographic measurements to cardiac disease have not yet been developed. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides an improved method and apparatus for producing elastographic images of the heart to detect cardiac disease. 
     The invention includes in its several embodiments: a visually improved mapping of the two dimensions of strain (direction and sign) to a color scale, an area cursor quantifying strain measurements within predefined regions, and a quantitative metric of cardiac function comparing different predefined heart regions to reduce operator variability in the assessment of cardiac disease. 
     Specifically, the present invention provides an elastography apparatus including a medical imaging system, operating on in vivo tissue, to provide at least a two-dimensional array of strain values related to points in the tissue. Each strain value has a magnitude and sign indicating an amount of strain at a point and whether the strain is compression or distension, respectively. The apparatus further includes an image generator mapping the array of strain values to colors at pixels in an image such that brightness of the colors varies monotonically with absolute value (magnitude) strain value and hue of the colors is related to strain value sign. 
     Thus, it is one object of the invention to provide a visually intuitive color mapping for strain by independently mapping two dimensions of strain to brightness and hue. 
     Zero absolute value strain may map to black. 
     It is another object of the invention to visually de-emphasize regions of low strain. 
     The compressive tissue strain may map to warm hues and distensive tissue strain may map to cool hues. 
     It is thus another object of the invention to provide a clear visual distinction between compressive and distensive strains. 
     In one embodiment, the signal processing circuitry may provide a second image of the heart tissue indicating relatively time invariant tissue quantities. 
     Another object of the invention can be to provide a separate image to serve as a point of reference for the strain image. 
     The two images may be side-by-side on a single display device and a first and second movable cursor may be superimposed on corresponding regions of the images. 
     The two images may also be superimposed on a single display device with a cursor used to navigate about the strain image, with the wall location identified by the gray-scale ultrasound image. 
     Thus, it is another object of the invention to simplify navigating about the strain image. One of the cursors can be located on a region identified in the conventional image to locate the corresponding region in the strain image. 
     The cursor may define a region of interest and the signal processing circuitry may provide a quantitative display of strain of tissue within the region of interest. 
     Thus, it is another object of the invention to provide a quantitative and less observer dependent measurement of tissue strain. 
     The apparatus may include a means for identifying a phase of the beating heart and the quantitative display may be related to the phase of the beating heart. For example, the quantitative display may provide an indication of strain of the tissue within the region of interest at the end of the systolic phase or the end of the diastolic phase of the beating heart. 
     It can thus be another object of the invention to provide a robust repeatable measurement of strain that may be useful for generating a standardized index for cardiac function. 
     The apparatus may provide strain measurements at several predefined regions in the heart tissue. The quantitative display may then be a comparison of strains in these regions. 
     Thus, it is another object of the invention to provide a standardized index for cardiac function that makes use of a multi-point quantitative assessment, difficult for an unassisted observer. 
     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 DRAWINGS 
     FIG. 1 is a simplified block diagram of an ultrasound scanner suitable for use with the present invention in scanning heart tissue; 
     FIG. 2 is a graphical representation of an ultrasonic signal received by the ultrasound scanner of FIG. 1 showing the analysis of one waveform of the signal taken at two successive times with different strain of the heart tissue showing a shifting of the signals corresponding to such strain; 
     FIG. 3 is a block diagram of the processing of the scan data of FIG. 2 by the ultrasound scanner of FIG. 1 to deduce stiffness using a time-domain analysis technique; 
     FIG. 4 is a figure similar to that of FIG. 3 using a frequency domain analysis technique; 
     FIG. 5 is a representation of the screen of the display of the apparatus of FIG. 1 showing a juxtaposed conventional, and strain tissue images and showing tracking cursors for navigation and quantitative display of the strain measurement in numerical and graphical form; 
     FIG. 6 is a table indicating a mapping of strain data to color of the strain image of FIG. 5; and 
     FIGS. 7 and 8 are detailed presentations of the graphical forms of quantitative display of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, an ultrasonic imaging system  10  suitable for use with the present invention may include a standard ultrasound machine  11  alone or in combination with a stand-alone computer  30 . Generally, the ultrasonic imaging system  10  provides a graphic display  32 , a keyboard  34  for data entry and a cursor control device  36 , such as a mouse, as is well understood in the art for providing user input. 
     The ultrasound machine  11  forming part of the ultrasonic imaging system  10  may be a GE Vingmed Vivid FiVe ultrasound system (commercially available from GE Vingmed of Forton, Norway) communicating with a 2.5 Megahertz phased array transducer  12  transmitting and receiving a beam  14  of ultrasonic energy along a number of rays  16 . For cardiac imaging, the transducer  12  is placed against a patient  15  and directed in to provide an apical or parasternal view of the heart  18 . In the latter, parasternal or long axis view, a measurement of the anterior septal (AS) wall, the posterior medial papillary muscle (PM), and the posterior wall (PW) may be made. 
     As is understood in the art, during each data acquisition, the transducer  12  transmits an ultrasound beam  14  into the heart  18  and receives echo data at each of numerous transducer elements. This data is transmitted via cable  20  to the ultrasonic imaging system  10  where it is received and processed by interface circuitry  22 . Alternatively, echo data are formed into signals representing echoes from along each of the rays  16  and then transmitted to imaging system  10 . In the preferred embodiment, the data may be sampled at twenty megahertz or higher, and repeated acquisitions are taken at a frame rate of at least 50 frames per second. 
     The patient  15  may also have ECG electrodes  24  attached to the patient&#39;s skin for the acquisition of electrocardiogram data received by acquisition circuit  26 . Such ECG data will be keyed to the acquired ultrasound data so that it is referenced to a phase of the heartbeat. 
     The processed ultrasound data will be assembled into conventional B-mode images  38  providing a real-time representation of a plane through the heart  18  according to well-known techniques. Further processing, according to the present invention (as will be described below), may be performed by a processor  33  executing a stored program contained in memory  35  residing either in the standard ultrasound machine  11  or the stand-alone computer  30 . 
     Referring now also to FIG. 2, each image  38  is composed of a series of time-domain signals  56  corresponding approximately with the rays  16 , and having a varying amplitude mapped to brightness of pixels  54  forming the columns of the image  38 . As such, the time axis of each signal  56  generally reflects distance from the ultrasound transducer  12  to the tissue of the heart  18 . 
     The strain within the tissue of the heart  18  may be determined by comparing corresponding time-domain signals  56   a  and  56   b  from two sequential ultrasound echo images  38  measuring the heart tissue at different degrees of compression during its normal beating phase. As shown, the second time-domain image signal  56   b  exhibits an expansion in time reflecting an expansion or distention of the heart tissues away from the ultrasound transducer  12 . More generally, the later time-domain image signal  56   b  might represent either relative distention or relative compression with respect to earlier time-domain image signal  56   a.    
     A general translation of the tissue of the heart  18  (rather than local compression or distension) would cause an equal offset between all points in time-domain image signal  56   a  and  56   b . However, the elasticity of the tissue causes local tissue compression or distension, which in turn produces a gradient in the phase offset of the time-domain image signals  56   a  and  56   b  as a function of time and distance from the ultrasound transducer  12 . 
     For the example shown, the phase offset  58  between the time-domain image signals  56   a  and  56   b  at early times and hence near the ultrasound transducer  12  will be smaller than the phase offset  60  at later times and for tissue further away from the ultrasound transducer  12 . The rate of change of these displacements at points over the region of the heart  18  provides a series of strain values having magnitude and sign that may be used to produce an elastographic image of the tissue of the heart  18 . 
     Referring to FIG. 3, more specifically, ultrasonic scan data  64  is collected being at least two images  38  containing successive time-domain image signals  56   a  and  56   b , the latter linked to ECG data  61 . At process block  65 , these signals are processed to determine tissue displacement along an axis from the ultrasound transducer  12  through the heart  18 . In principle, short segments of the time-domain image signals  56   a  and  56   b  are analyzed by moving one segment with respect to the other until a best match is obtained and the amount of movement needed for the best match determines tissue displacement. The matching process may be implemented by means of mathematical correlation of the segments. 
     The displacement of signal  66  output by process block  65  is further processed by the process block  68 , which determines strain as a gradient of the displacement signal. The strain values  71  may be mapped to an elastic graphic image  72  also linked to the ECG signal  61  and thus having a defined phase with respect to the heartbeat. 
     As each successive frame is obtained by the system of FIG. 1, a new elastic graphic image may be obtained by comparing that frame to the predecessor frame to determine displacement as has been described, and thus the strain is relative to the last image  38 . Alternatively, a base image approximating the heart at rest may be used to produce strain relative to that image or a peak or root-mean-square value or other similar measure can be adopted. 
     Referring momentarily to FIG. 4, alternative algorithms may be used to create the elastographic images  72 . In one such algorithm, the time-domain image signals  56   a  and  56   b  may be received by process block  81  to extract a spectra of the time-domain image signals  56   a  and  56   b  using, for example, the well-known fast Fourier transform algorithm. The spectra of the time-domain image signals  56   a  and  56   b  will be shifted according to the Fourier transformation property that causes dilation in a time-domain signal to produce a down-frequency shift in its frequency-domain spectrum. The amount of shift may be determined at process block  83  using correlation techniques similar to those used in process block  65  but executed on the frequency-domain signals. 
     The shift between the spectra taken of different segments of the time-domain signals  56   a  and  56   b  centered at increasing time delays, provides a gradient signal to produce elastographic images  72 . While the results are similar to the technique of FIG. 3, this approach may have some advantages in terms of robustness against noise and the like. 
     Each of these process blocks may be implemented through a combination of hardware and software in the ultrasonic imaging system  10  and/or the stand-alone computer  30  as is well understood to those of ordinary skill in the art. 
     Referring now to FIGS. 3 and 6, the strain values  71  for each pixel  74  of the images  72  will have a magnitude and sign. The magnitude indicates the amount of the distension or compression of the tissue and the sign indicates whether it is a compression or distention with positive signs normally denoting compression and negative signs by convention noting distension of the tissue. FIG. 6 provides a mapping table  89  used in at least one embodiment of the present invention accepting as arguments compressive strains positive one through three and distensive strains negative one through three. The mapping table  89  maps the absolute value of the strains (magnitude) to brightness of the corresponding pixels  74  in the elastographic image  72  and maps the sign of the strains to particular hues for the corresponding pixels  74 . In a preferred embodiment strains with positive signs (indicating compression) map to warm hues such as yellow, orange, and red, and strains with negative signs (indicating distension) map to cool hues such as violet, blue, and indigo. 
     The brightness is the perceived brightness of the pixel  74  and this may be affected in part by the hues, as the eye is more sensitive to some hues than it is to others. For this reason, the ordering of the hues may be selected to augment the intended brightness. Generally, it is desired that the brightness be monotonic meaning that it only increases or only decreases for each of the positive and negative ranges. 
     This system can be contrasted to a color mapping scheme in which a full range of hues are mapped to the full range of strain, for example, by applying the full spectrum red, orange, yellow, green, blue, indigo, and violet, to the full range of strains from negative three to positive three. The advantage of the present system is that the peak strains both positive and negative are emphasized. Regions of positive and negative strain tend to separated by black or dark moats of color. 
     Referring now to FIGS. 1 and 5, the processor  33  executing the stored program in memory  35  may juxtapose the conventional B-mode image  38  (typically in a gray scale) next to a elastographic image  72  and also provide for a series of cursors  80  and  82  that may be positioned over the images  38  and  72 , respectively, through the use of the cursor control device  36  and keyboard  34 . The images  38  and  72  may be updated in real time and sized and oriented to show the same region of heart tissue. Image  38  shows relatively time invariant qualities of the heart tissue, such as tissue interfaces, and further provides a higher resolution image of the heart in which anatomical features may be more readily distinguished. Cursor  80  and  82 , in any case, are positioned to track each other so as to constantly contain a region of interest  84  centered on the same structure in both the images  38  and  72 . In this manner, the image  38  may be used to identify particular anatomy of the heart  18  and the strain may be investigated by reviewing the region within the cursor  82 . 
     A quantitative readout  86  may be provided on the graphics display  32  providing statistics related to the strain of the tissue contained in the region of interest of the cursor  82 . In the simplest embodiment, a current strain relative to the last image  38  may be displayed or alternatively a peak strain, absolute strain, or average strain magnitude may be displayed. 
     Alternatively and in the preferred embodiment, a strain value at a particular phase of the beating of the heart  18  may be displayed at quantitative readout  86  through the use of the keyed electrocardiograph data  61  linked to the images  72 . Preferably, the strain measured at the end of the systolic or end of the diastolic heartbeat phases may be used. Selection of these times provides large strain values providing an improved signal to noise ratio and a consistent and repeatable point at which strain may be measured quantitatively. 
     Multiple cursors  80  and  82  may be used as part of an index to provide a standard measurement of cardiac function. In this embodiment, one cursor  80  is placed in the anterior septal wall of the heart. A second cursor is  80 ′ is placed on the posterior medial papillary muscle and a third cursor  80 ′ is placed on the posterior wall of the heart  18  as guided by image  38 . Corresponding cursors  82 ,  82 ′, and  82 ″ appear in the image  72 . 
     Measurements of strain in each of these cursor locations is then obtained at the end of the systole and end of the diastole and this data is presented in graphs  90  also shown on graphics display  32 . 
     Referring now to FIG. 7, the plot  91  of strain values at the end of systole for a patient having coronary artery disease may be readily distinguished from the plot  92  derived from a group of normal patients having no cardiac dysfunction. 
     Likewise, referring to FIG. 8, the plot  91  of strain values at the end of diastole for a patient having coronary artery disease may be readily distinguished from the plot  92  derived from a group of normal patients having no cardiac dysfunction 
     The data of these graphs may be distilled to a single quantitative number that may be empirically related to cardiac dysfunction and displayed as well. 
     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. For example, the present invention though preferably used with ultrasonic elastography, has application for Doppler and other kinds of elastography and may be used with both transmission and reflection ultrasound.