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[ EN ]


This invention relates to medical diagnostic ultrasound systems and methods and, in particular, to echocardiographic ' systems and methods for analyzing left ventricular function.
Evaluation of left ventricular function is of interest for both diagnostic and therapeutic
applications. During normal cardiac function the cardiac chambers observe consistent time-dependent relationships during the systolic (contractile) phase and the diastolic (relaxation) phase of the cardiac cycle. During cardiac dysfunction associated with pathological conditions or following cardiac-related surgical procedures, these time-dependent mechanical relationships are often altered. This alteration, when combined with the effects of weakened cardiac muscles, reduces the ability of the ventricle to generate contractile strength resulting in
hemodynamic insufficiency.
Ventricular dyssynchrony following coronary artery bypass graft (CABG) surgery is a problem encountered relatively often, requiring post-operative temporary pacing. Atrio-biventricular pacing has been found to improve post-operative hemodynamics following such procedures. See Weisse et al., Thorac. Cardiovasc. Surg. 2002; 41:131-135. A widely accepted, standardized method for selecting pacing sites that provide the greatest hemodynamic benefit to the patient during the critical recovery phase, however, has not been available.
An imaging modality in current use for
evaluating ventricular dyssynchrony is
echocardiography. Tissue Doppler ultrasound is used in echocardiology to measure the motion and timing of the myocardium. Tissue Doppler ultrasound is an adaptation of the ultrasound techniques used for analyzing blood velocity: color flow mapping, and spectral and audio pulsed-wave (PW) Doppler. In these blood flow techniques, a clutter filter rejects the strong, slow tissue echo so that the very weak echoes from faster moving blood flow can be seen. Tissue Doppler typically does not use a clutter filter in this way, and the slow tissue echo that is analyzed is the dominant signal, far above that of blood, noise, and reverberation.
An important use for tissue Doppler is to compare the timing of regions of the lateral wall and septum of the left ventricle to each other and to the ECG, for diagnosis and pacemaker lead placement.
Currently this is done by deriving velocity vs. time graphs from a stored loop of color tissue Doppler imaging using a commercially available analysis software package. The timing analysis of these graphs can be manually or automatically derived, but is not done during live imaging. The graphs that are produced from the stored color tissue Doppler loops generally need a frame rate of about 100 Hz for several heart cycles. These loops of hundreds of frames represent a very large amount of data to transfer and store, for a relatively meager amount of clinical information that is finally derived. Also, the quality of the acquired loop data for timing analysis is typically not known until the time of this post-processing analysis, possibly after the patient is gone and not readily available for the acquisition of better data. Thus, there is a need for a system design that makes the procedure for tissue motion timing analysis more efficient, both in time and data size.
In accordance with the principles of the present invention, a diagnostic ultrasound system and method are described which enable live, real time
acquisition of tissue Doppler images and production of clinically diagnostic timing graphs. In an example described below, the clinician denotes locations of interest (LOIs) on the myocardium of a tissue Doppler image. Velocity vs. time graphs corresponding to these locations appear and update in real time on the screen while live imaging occurs . To enable the clinician to quickly explore timing relationships at different locations of the heart wall, the LOIs can be moved and repositioned, with graphs produced in real time at each set of
locations . Once the clinician is observing graphs of cardiac function which provide the basis for a diagnosis, a small amount of data is acquired and stored, enabling reliable, relevant clinical
information to be conveyed in a relatively small amount of data.
In the drawings:
FIGURE 1 illustrates in block diagram form an ultrasonic diagnostic imaging system constructed in accordance with the principles of the present invention.
FIGURE 2 illustrates is an example of the display screen of an ultrasound system constructed to operate in accordance with the principles of the present invention.
FIGURE 3 illustrates a second example of the display screen of an ultrasound system constructed to operate in accordance with the principles of the present invention.
Referring first to FIGURE 1, an ultrasound system constructed in accordance with the principles of the present invention is shown in block diagram form. A probe 10 including an array transducer 14 transmits ultrasonic beams and receives echoes in response from an image region 14 of a subject which contains organs and blood vessels 16. In this illustration the image field 14 is shown as sector-shaped as would be scanned by a phased array
transducer. In this particular example the
ultrasound system is to be used to analyze the motion of the myocardium of the left ventricle. The ultrasonic energy is transmitting and received by elements of the transducer array 14. If a two dimensional image plane is to be scanned the array will comprise a one-dimensional array of transducer elements, and if elevation focusing is used or a three dimensional volume is to be scanned in real time, the array will comprise a two-dimensional array of elements . The received echoes are coupled to a beamformer 20 which produces coherent echo signals. These echo signals are demodulated by a quadrature demodulator 22.
In the system shown in this example the echo signals are processed further in three different ways. Amplitude detector 32 performs an amplitude detection of the received echo signals and the detected signals are compressed by a log compression 34. The resultant B mode echo values are coupled to a scan converter 50then mapped to display values by a grayscale map 36. This processing will produce a B mode image of the moving tissue of interest, in this example, the myocardium. The quadrature demodulated signals are filtered by a wall filter 42 to remove signals from stationary tissue when performing blood flow imaging, and ensembles of the filtered signals undergo Doppler processing by a color Doppler processor 46. The Doppler processor 46 can produce different selectable representations of motion such as velocity, acceleration, variance or Doppler power. The resultant phase shift or intensity estimates are mapped to corresponding colors or hues by a color map 48coupled to the scan converter 50. This processing produces a two or three dimensional color overlay of the tissue motion which can be aligned with and overlay the structural B mode image. As mentioned above, for a tissue Doppler signal where the signals of interest are strong and of low velocity, a wall filter may not always be necessary and may be bypassed, set to pass all signals, or set with a low pass function. The B mode image signals and the color motion image signals are converted to the desired display format by theoupled to a scan converter 50 where they are combined in the desired spatial format for display as a two or three
dimensional colorflow or color tissue Doppler image. The echo values are then mapped to color or grayscale display values by a grayscale and color mapping processor 36. The display values are coupled to a video processor 80 for display.
In accordance with the principles of the present invention the quadrature demodulated signals from a number of LOIs are coupled to an autocorrelator 44. The autocorrelator may be of an adjustable lag order. In this example the autocorrelator is set to be a lag-1 autocorrelator and operates to multiply an echo sample from an LOI by the complex conjugate of the previous sample, an operation which may be expressed as Sn+1*conj [Sn] . The autocorrelator operates on the echo samples in windowed groups. For instance, a sixty-four sample window can be used. The samples in the window are generally weighted with the higher weighting functions used in the center of the window. The window will generally overlap for the desired time resolution. For instance the first window may include samples 1-64, the second window samples 16-80, the third window samples 32-96, and so on. When a higher PRF is employed lag-2 autocorrelation may be preferable, which would operate on every other sample in the sequence. The lag-1 multiplication will yield a relatively imprecise phase shift angle estimate at a very precise time, the time interval of the two consecutive samples. The autocorrelator 44 increases the angle estimate precision by summing the products of the window and taking the angle of the result, which is expressed as a complex number having a real and an imaginary part. This angle estimate is applied to arc tan. calculator 68 which looks up or calculates the phase shift angle value to be used for a velocity vs. time graph, as the tissue velocity is proportional to the phase shift angle determined by the arc tangent of the autocorrelation result.
In this manner a sequence of windowed
autocorrelation velocity estimates are produced sequentially in time. These velocity estimates are to be plotted as a velocity vs. time graph by a graphics processor 72 for display on the display screen 90. The velocity estimates are applied to an interpolator 66 which forms a smooth curve of the sequence of velocity data points. The resultant curve is coupled to the graphics processor 72 which plots in real time a plurality of velocity vs. time curves for display. Multiple curves are produced from the multiple LOIs at the same time, much in the same way spectral Doppler may be produced from dynamically tracked multiple sample volumes at the same time as shown in US Pat. 5,365,929 (Peterson). In this example the graphical velocity vs. time curves are produced by a graphics processor 72 and coupled to the video processor 80scan converter 50 for display separately or alongside the color tissue Doppler image. The images are displayedproduced by the scan converter are coupled to a video processor 80 for display on an image display 90.
Locations on the left ventricular wall in the tissue Doppler image where motion is to be shown as velocity vs. time graphs can be selected
automatically or by user manipulation of a cursor on the display screen with a mouse or trackball of a control panel 70. The user can select the number of LOIs to be graphed and this information is retained by an LOI selector 62. As the user manipulates the LOIs to their desired positions on the heart wall, the LOI selector updates the position of each LOI with respect to the tissue Doppler image. This LOI location information is coupled to the graphics processor 72 so that the graphics processor can display icons representing the LOIs over the tissue Doppler image in their current locations. The LOI icons can be placed in static locations on the screen with the tissue Doppler image of the heart expanding and contracting in real time beneath them.
Alternatively the LOI icons, once placed, can be displayed to move with their locations on the heart wall. Preferably the LOI icons are positioned while performing live imaging, in a similar manner to which clinicians manipulate cursors on a moving heart image. However, it is also possible to freeze the live images and manipulated the LOI icons on a frozen image and, once placed, are translated the LOI icon positions to live images at the same phase of the heart cycle as that of the frozen image . In either case, tThe placement locations of the LOIs are tracked by an LOI tracker 64. The LOI tracker can employ border detection tracking, speckle tracking, or any other method which is able to follow a particular point in the anatomy through a real time image sequence. When the LOI tracker employs speckle tracking, the LOI tracker tracks the initial
placement of the LOI positions through successive images by tracking the speckle pattern produced by the local tissue at each LOI. The LOI tracker identifies regions of pixels around the LOI points in the adjacent myocardium. The speckle patterns of these pixels is saved and compared with speckle patterns in the same regions of the successive images and the speckle patterns matched by block matching. Further details on speckle tracking may be found in US patent application serial number 60/734,662, filed November 8, 2005, the contents of which are
incorporated herein by reference. The LOI tracker may employ other anatomy tracking techniques such as tracking image characteristics which are greater than a wavelength in size. For instance, the movement of specific anatomical features may be tracked. As another example, tissue texture may be tracked. It will also be appreciated that the targeted
characteristics may be tracked in either pre-scan converted or post-scan converted image data. Yet another technique which the LOI tracker may employ is border detection. A border is automatically traced around the endocardial wall of the myocardium and the relationship of the position of each LOI with respect to each updated tracing of the endocardial border is maintained through each cardiac cycle as described in US Pat. 6,491,636 (Chenal et al . ) Another example is to maintain the LOI icons in static positions over the live, moving tissue Doppler image, but use the LOI tracker to dynamically track the initial positions of the LOIs as the myocardium moves. This provides an image of the LOI icons which is not distractingly movingstationary while
maintaining the precision of the locations for which the velocity vs. time graphs are produced.
One example of a display produced by an
ultrasound system of the present invention is shown in FIGURE 2. A real time tissue Doppler image 120 of the heart is shown above a plurality of velocity vs . time graphs 122. In the tissue Doppler image the left ventricle 100 is plainly visible with the septal wall 102 on the left and the lateral wall 104 at the right. The apex of the heart is at the top of the image in this subcostal apical 4-chamber view. In this example the user has chosen to use eight LOIs around the wall of the left ventricle. The LOI icons in this display are small hexagons numbered 1-6.
When the user first chooses the number of LOIs the LOI icons are evenly distributed around the
endocardial wall of the left ventricle as in the manner of control points of a border tracing as described in the aforementioned US Pat. 6,491,636.

The user can then manipulate the screen cursor 106 to click on an LOI icon and drag and drop it to its desired location on the myocardium. As previously mentioned this is preferably done on a frozen image to allow placement on a stationary image of the myocardium. After the LOIs are in their desired locations on the septal and lateral walls as shown in the drawing, the LOI tracker will track the locations of the underlying tissue as described above, from which data is acquired to produce velocity vs. time curves of the LOI locations in real timereal time display is resumed from the same phase of the heart cycle as the frozen image, which is retained by the ECG timing reference 108 at the bottom of the image. The six LOI icons will then either move with the live tissue Doppler image or stay stationary over the moving heart image as determined by the user.
As the live image contracts and expands during each heart cycle, the processors 44, 68, 66
continually receive the quadrature data from each LOI location as directed by the coupling of the LOI tracker 64, which tracks these locations, to the autocorrelator 44. The LOI tracker 64 is also coupled to the beamformer 20 to provide the LOI locations to the controller of the beamformer in case special transmit interleaving or beam density is desired for the LOI locations. The processors 44, 68, 66 thus continually calculate and update velocity curves over the time of each heart cycle. One such set of curves 110 is shown in FIGURE 2. Six of the curves 110 correspond to the six LOI points on the wall of the left ventricle in the ultrasound image 120. Typically the graphs 110 for multiple spatial locations of the LOIs are be overlaid as shown in the drawing, using different colors corresponding to different colors of the LOI icons on the ultrasound image 120. When an LOI icon is repositioned on the ultrasound image its corresponding graph is
preferably highlighted, such as by intensity or line width. The graphs 110 are updated as a Doppler spectrum or M-mode trace is, with an erase bar or a scrolling trace moving at a predetermined rate. For example, after a set of graphs for one heartbeat are drawn as in this example, the updated graph lines are drawn over the old ones, replacing them in real time.

The graphs in this example are scaled to show exactly one heartbeat, but can also be scaled to show multiple heartbeats. The ECG graph 108 is updated and scaled in the same way so that relative timing can be easily discerned.
FIGURE 3 shows another example of a display produced by an ultrasound system of the present invention. In this example the velocity graphs 122 are scrolling across the bottom of the display screen. In addition to the six velocity lines representing the six LOIs of the ultrasound image, a seventh line 132 is produced, which is an average of the lines of the six sampled locations. This graphical display readily shows graphs with timing that is not in synchronism with the average, which may alternatively be a mean or median calculation. The ECG graph 108 moves along the bottom of the graphical display in synchronism with the scrolling of the velocity graphs, continually showing the time reference of the graphs. In this example three heartbeats are visible at one time in the scrolling display.
Other variations of examples of the present invention will readily occur to those skilled in the art. The velocity data for the graphs can be derived from traditional color tissue Doppler data or from PW tissue Doppler data. Sampling of the multiple LOIs can be interleaved in time because the PRF needed for tissue is relatively low. The live imaging that is displayed for location reference can be a normal grayscale image or a color tissue Doppler image as described above. The image does not necessarily need to exhibit very high frame rate. The graphs can also show strain rate or strain vs. time, using a small region around each location of interest for the strain measurement.
A system of the present invention can, if desired, also analyze the velocity vs. time graphs from several locations to derive concise numerical results that are directly relevant to diagnosis and treatment. For example, time delay from ECG R-wave to peak velocity, or difference in time delays for two LOIs can be compared. The derived numerical results can if desired be displayed superimposed on the live ultrasound image in a variety of ways, such as with color, brightness, or simply numerically. Audio output can be produced from the velocity vs. time graph data, where pitch corresponds to velocity as with spectral/audio Doppler. A suitable Doppler feature is described in my co-pending US patent application serial number 60/749,214, filed December 8, 2006 which is hereby incorporated by reference. The audio can correspond to one selected location of interest, or could be produced from several locations simultaneously.