Abstract:
Apparatus and methods for use in a diagnostic ultrasound system are disclosed. A display device has a first window for displaying live ultrasound images and a second window for displaying a reference cine image. The reference cine image is an image captured prior to the currently displayed live ultrasound images. The images of the first and second windows are synchronized by ECG gating. The apparatus and methods may be employed to image the heart, providing operators with a way to monitor the heart and detect subtle changes in heart condition from the time that a reference image was captured.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/316,445, filed Aug. 31, 2001. 
    
    
     BACKGROUND OF INVENTION 
     Certain embodiments of the present invention are directed generally to the field of diagnostic ultrasound imaging. More particularly, certain embodiments of the present invention are directed to the field of ultrasonic cardiac monitoring systems and methods. 
     When a patient is given anesthesia, there is a risk of myocardial infarction when the patient is in the induction phase or is in the phase of being awakened. One of the first signs of myocardial infarction is a change in left ventricular wall motion. Cardiologists, anesthesiologists, and other medical personnel have a need for early detection of signs of changes in left ventricular wall motion. 
     Conventional methods of monitoring patients&#39; cardiac activity include watching the contraction of the left ventricle on an ultrasound diagnostic imaging system that obtains ultrasound data using a TEE probe lowered into the patient&#39;s esophagus. Employment of a TEE probe lowered into the patient&#39;s esophagus to monitor cardiac activity during surgery is potentially inaccurate because a physician has nothing to compare to the observed data and, therefore, the physician must memorize the previous functioning of the observed heart. Thus, if the difference between the current cardiac activity and previous cardiac activity is subtle, then the physician will not notice the change. 
     Another conventional method for monitoring cardiac activity involves using an ultrasound diagnostic imaging system to express general heart function using a single number representing the ejection fraction. The ejection fraction is the fractional change in the observed cross-sectional area of the left ventricle. Greater change in the cross-sectional area of the left ventricle over the cycle of a heartbeat indicates, better health of the heart. Physicians would like to observe changes in the single number representing ejection fraction because changes in that number may represent a significant change in the health of the heart. To obtain the number that represents ejection fraction, an algorithm is used to automatically trace the boundary of the endocardium by distinguishing between blood and heart tissue. Tracing is based on edge detection. Edge detection is performed by detecting sudden high video contrast between the heart muscle and the endocardium cavity, the former being displayed in shades of gray and the latter being displayed in black. Because tracing is based on edge detection, the operator may position a marker, using a track ball or other controller, in the left ventricle prior to tracing. Once the marker is positioned, the endocardium is automatically traced, identifying the inner border of the left ventricle. Tracing, however, may be lost from time to time if the image of the left ventricle is not of high quality because tracing will stop if the algorithm fails to detect the boundary between the endocardium cavity and heart tissue. Also, in some circumstances, tracing is not robust enough for obtaining an accurate number. When tracing is not very robust, tracing has too little contrast to obtain an accurate number to represent the ejection fraction. 
     Rather than using a TEE probe or edge detection to detect left ventricle motion, a Doppler method has also been employed to calculate cardiac output. Cardiac output is the volume of the blood that is ejected per unit time from the left ventricle when the left ventricle contracts. Cardiac output can be considered the volume of blood passing through the aorta per unit time, typically expressed in terms of liters per minute. Cardiac output is related to ejection fraction and heart rate. The Doppler method of detecting left ventricle motion requires probe manipulation by an operator, which increases the potential for human error. Measurements made by Doppler are proportional to the cosine of the angle formed between the directional velocity of the flow of blood and the beam direction. Because the angle formed between the directional velocity of the flow of blood and the beam direction changes over time, there is inaccuracy in the Doppler measurements. Also, the cross-sectional area of the vessel being analyzed must be measured, because the volume of blood passing through the vessel equals the cross-sectional area of the vessel multiplied by the velocity. But the cross-section of the vessel changes over time, because the vessel is elastic. Also, the angle from which the vessel is measured changes, changing the measured cross-section of the vessel. For example, the vessel must be measured from an angle perpendicular to the longitudinal axis of the vessel if the measurement is to accurately represent the actual vessel cross-section. If the vessel is not measured from an angle perpendicular to the longitudinal axis of the vessel, then the measurement will be the cross-sectional area of an ellipse, which is greater than the actual cross-sectional area of the vessel. Thus, the use of ultrasound measurements to determine cardiac output is an ineffective method of detecting the subtle changes in left ventricular wall motion that may precede a myocardial infarction. 
     SUMMARY OF INVENTION 
     One embodiment of the present invention is an apparatus for displaying a plurality of images in a diagnostic ultrasound system. The apparatus may comprise a display device having a first window for displaying a live ultrasound image and a second window for displaying a reference cine image. The images displayed in the first and second windows are synchronized by ECG gating. The images may be cardiac images. The reference image makes it easy for an operator to compare the current condition of the heart or other imaged region with the condition of that region at the time the reference image was captured. 
     The reference image may be captured and re-captured automatically at predetermined intervals. Alternatively, the reference image may be captured when the operator elects to capture the image, in which case the reference cine image is repeated until the operator elects to capture a new reference image. The reference cine image may be continuously updated so that the reference cine image lags behind the live ultrasound image by a pre-determined amount of time. 
     Alternative embodiments include a plurality of windows for displaying reference images. Embodiments having a plurality of windows for displaying reference images permit an operator to have a plurality of reference views of an area of interest that is displayed as a live ultrasound image in the first window. 
     A method in accordance with one embodiment of the present invention comprises the steps of displaying a live ultrasound image, displaying a reference cine image, and synchronizing, by ECG gating, the live ultrasound image and the reference cine image. The live ultrasound image and the reference cine image may be images of the heart. In a further embodiment, the method comprises the additional step of displaying a second reference cine image, wherein a view of the heart displayed in the first reference cine image is different from a view of the heart displayed in the second reference cine image. The reference cine image or images may be captured manually or may be captured and re-captured at a pre-determined interval. Alternatively, the reference cine image or images may be continuously updated so that the reference cine image or images lag behind the live ultrasound image by a pre-determined amount of time. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing summary, as well as the following detailed description of the preferred embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the preferred embodiments of the present invention, there is shown in the drawings, embodiments which are presently preferred. It should be understood, however, that the present invention is not limited to the arrangements and instrumentality shown in the attached drawings. 
     FIG. 1 is a block diagram of an ultrasound system formed in accordance with an embodiment of the present invention. 
     FIG. 2 is a schematic diagram of a display system formed in accordance with one embodiment of the present invention. 
     FIG. 3 is a schematic diagram of a display system formed in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a schematic block diagram of a medical diagnostic ultrasound system  5 . A probe  10  connected to the ultrasound system  5  is used to transmit ultrasound waves  7  into a subject S by converting electrical signals  15  into ultrasound energy. The probe  10  converts backscattered ultrasonic energy  8  to analog electrical signals  16 . A front-end subsystem  20  comprising a receiver, transmitter, and beamformer is used to create transmitted waveforms, beam patterns and receiver filtering techniques that are used for various imaging modes. The front-end subsystem  20  interfaces at an analog interface to the probe  10  and interfaces at a digital interface over a digital bus  70  with a non-Doppler processor  30  and a Doppler processor  40  and interfaces over a digital bus  69  with a host processor  50 . The digital buses  69  and  70  may comprise several digital sub-buses, each sub-bus having a unique configuration and providing digital data interfaces to various parts of the ultrasound system  5 . 
     The non-Doppler processor  30  performs an amplitude detection function and data compression functions used for imaging modes such as B-mode, M-mode and harmonic imaging. The non-Doppler processor  30  and the Doppler processor  40  accept received signal digital data  71  from the front-end subsystem  20 , process the signal digital data  71  into sets of signal values, and pass the signal values to the host processor  50  and/or a display  75  over the digital bus  69 . 
     The display  75  includes a display processor  80  that performs a scan-conversion function, color mapping function, and tissue/flow arbitration functions based on the digital signals  41  and  73  from the non-Doppler processor  30  and the Doppler processor  40 . Digital data  72 , representing a location of a pattern of indicia, is accepted from host processor  50 . The display processor  80  processes, maps, and formats the digital data  72  accepted from the host processor  50  for display, converts the digital display data to analog display signals  89 , and passes these analog display signals  89  to a monitor  90 . The monitor  90  accepts the analog display signals  89  from the display processor  80  and displays a resultant image  87  to the operator on the monitor  90 . 
     A user interface  60  allows user commands to be input by the operator to the ultrasound system  5 . The user interface  60  comprises a keyboard, mouse, switches, knobs, buttons, track ball, and on screen menus. The host processor  50  is the main, central processor of the ultrasound system  5 . The ultrasound system  5  has been described as an example of an ultrasound system compatible with certain embodiments of the present invention. Embodiments of the present invention may be used in connection with ultrasound systems other than the ultrasound system S. 
     In FIG. 2 is shown an embodiment of the present invention comprising a display device, such as the monitor  90 , having two windows  100 . The display processor  80  (FIG. 1) has the capability of providing the monitor  90  with a plurality of windows  100 . For example, in order to provide a plurality of windows  100 , the display processor  80  may provide to the monitor  90  compound dynamic images produced by computer image manipulation. A first window  105  displays live ultrasound images  107 . A second window  110  displays a reference cine image  114 . The images  107 ,  114  may be images of the heart. The two windows  105 ,  110  are ECG gated to the ECG of the live ultrasound images  107  for synchronization. 
     The reference cine image  114  is an image captured earlier than the live images  107  currently being displayed. The reference cine image  114  may be captured at one time by the operator and then repeatedly displayed until the operator captures a reference image to replace it. Capture performed by an operator is called manual capture. With manual capture, the same cine image of a heart cycle, for example, keeps playing over and over while the live images  107  in the first window  105  are displayed until the operator captures a replacement reference cine image. The operator may perform capture, for example, by pressing a button on the user interface  60  (FIG.  1 ). 
     Alternatively, the second window  110  displays a reference cine image  114  that is automatically captured. An automatically captured image is a cine image that is updated or re-captured by the ultrasound system after a pre-determined amount of time passes. The operator may enter a number to indicate how much time must pass between updates of the reference image in the second window  110 . 
     As seen in FIG. 2, an operator may annotate the windows with information  116 . Annotations may include patient information. The window  100  that displays live images  107  may have a timer indicating how long that image  107  has been running. For example, the live image  107  of FIG. 2 has been running for five minutes and  22  seconds. The window  100  displaying a reference image  114  may have a number indicating the time at which the reference cine image  114  was captured, the time being the time that had been displayed with the live image  107  of the first window  105  when the reference image  114  was captured. For example, the reference image  114  in FIG. 2 was captured when the live image  107  had been running for one minute and five seconds. The reference image  114  of FIG. 2 has been running for four minutes and  17  seconds. If the reference image  114  is an automatic reference image, then the time of capture shown in that window  100  will be updated each time that the reference cine image  114  is updated (i.e., automatically captured). Additionally or alternatively, the time displayed in connection with either the manual or the automatic reference images  114  may be the time elapsed since the most recent capture (as noted above, an example of elapsed time of four minutes and  17  seconds is shown for the reference image  114  of FIG.  2 ). Although not shown in the Figures, for the automatic reference images  114 , the pre-determined time between automatic captures may also be displayed. 
     In other embodiments the display screen may have more than two windows  100 . For example, FIG. 3 shows an example of a display with four windows  100 . A first window  120  displays live ultrasound images  107 . Second and third windows  123 ,  127 , respectively, may be manually captured reference images  114  of a cardiac cycle showing different views such as the long axis and short axis. A fourth window  130  may be an automatically captured reference cine image  114  of a cardiac cycle. All four windows  120 ,  123 ,  127 , and  130  are ECG gated to the ECG of the live images  107  for synchronization. The windows shown in FIG. 3 may display annotations, capture times, elapsed times, and other times, as described in connection with the embodiment of FIG.  2 . An annotation  131  describes the view of the displayed image in the window  100  in which the annotation  131  is located. For example, the annotation  131  may read “Long Axis” in a window  100  displaying a long axis view, and a different annotation  131  may read “Short Axis” in a window  100  displaying a short axis view. Other information may be included, additionally or alternatively, in the annotation  131 , such as whether the image in the window  100  is a view of the heart during inhalation or whether the image is a view of the heart during exhalation. In FIG. 3, the annotation “Time” is not used to indicate a single particular type of time display but rather indicates that one or more of a variety of time display formats may be displayed. For example, an operator may select the desired time display, whether it be elapsed time, time of capture, or some other time display. 
     The reference images  114  allow an operator to readily compare the live image  107  of the heart with past images of the heart. Reference images  114  may be particularly helpful soon after a patient has been given a fluid or other treatment that affects heart function. An annotation  132  may be used to display the amount of time that has elapsed since administration of anesthesia was initiated. With the reference image or images  114 , operators can determine whether the wall of the heart has moved after the treatment or whether other subtle changes may have occurred. Reference images  114  and live images  107  may be employed to monitor areas of the human body other than the heart, however, ECG triggering would still be employed to synchronize such images  114  and  107 . 
     The positions of the live images  107  and the one or more reference images  114  may be determined by the operator. Thus, although shown on the left in FIG. 2, and in the upper left in FIG. 3, the live ultrasound images  107  may be shown in a different window  100 , and a reference cine image  114  may be displayed in the left window (or upper left window in the embodiment of FIG.  3 ). 
     Any window  100  that is not displaying the live ultrasound image  107  may display either a manual reference image  114  or an automatic reference image  114 . Thus, although FIG. 3 displays two manually captured reference windows  114  and one automatically captured reference window  114 , embodiments of the present invention may have any combination of manually captured and automatically captured reference windows  114 . Thus, an embodiment with a total of four windows  100  may comprise three manually captured reference windows  114 , or three automatically captured reference windows  114 , or other combinations of reference windows  114 . The display device may also display data  135  such as a blood pressure graph, an ECG graph, and a breathing monitor. 
     All images (live images  107  and reference cine images  114 ) may be stored to an archiving system for review postoperatively. With a large enough amount of memory, the automatic reference images  114  could be updated continuously so that the automatic reference images  114  remain behind (i.e., lag behind) the live image  107  by a pre-determined amount of time. As time would pass, the memory or buffer storing the cine image would be updated so that the displayed automatic reference image  114  would be continuously changed to correspond to what the live image  107  had been at the pre-determined time earlier. If an image buffer can hold cine images of a duration equal to the pre-determined amount of time that the automatic reference image  114  is supposed to lag behind the live image  107 , then the image buffer would be able to update the automatic reference images  114  continuously. If there is not enough memory to make such adjustments (or updates) to the automatic reference cine images  114 , then the automatic reference cine images  114  will simply repeat until the pre-selected time has elapsed and a new reference cine image  114  is automatically captured. The pre-selected time may be determined by the operator. 
     Memory for storing the live and reference images  107 ,  114 , respectively, may be provided by a computer. Images may be compressed to save memory, if desired. To conserve memory usage, images may be gated to only the contractility portion of the heart cycle. The windows  100  would then display only the contractility portion of the heart cycle which, for many applications, is the portion of the heart cycle of most interest to operators. The image  107  and reference image  114  data may be transmitted to remote locations over the Internet. Data compression techniques such as those mentioned above can facilitate transmission over the Internet. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.