Abstract:
The invention provides a method for controlling, in an ultrasound system, the loci of maximum power regions (e.g., focal points) of plural transmit lines of an ultrasound scan. Initially, a region being imaged is displayed and the user superimposes on the display a desired path to be traced by the loci of focal points of a scanned ultrasound beam. Data defining the path is then used to control an ultrasound transducer to produce a sequence of ultrasound beams that scan the region and to adjust the foci of the individual ultrasound beams so that they track the desired path. It is preferred that a graphical input device be used to trace the path of the loci on the display. The invention further enables data derived from the selected loci of maximum power regions to enable adjustment of both power and focal depth of the respective beams.

Description:
FIELD OF THE INVENTION 
     This invention relates to ultrasound imaging systems and, more particularly, to a method and apparatus for enabling the user to modify and configure the locus of ultrasound beam focal points during a scan action. 
     BACKGROUND OF THE INVENTION 
     The use of linear and two-dimensional phased arrays of ultrasound transducers to achieve focus ultrasound beams is well known in the prior art. Such transducers employ a plurality of ultrasound-generating elements which are controlled to provide the ultrasound beam with a determined focal points. Such is achieved by control of the time of firing of the respective transducer elements. The scan of the ultrasound beam is accomplished by pulsing the ultrasound elements at different relative times to produce each beam (or “line”). Accordingly, as the relative timing of the energizations of the respective transducer elements changes, the azimuth taken by the beam moves in the direction of the scan. 
     Since the focusing of the ultrasound beam occurs through the control of the time of firing of the respective elements, the focus is predetermined at the time of firing of the elements. It is a characteristic of an ultrasound beam, at its focus, that a maximum level of ultrasound pressure is present. This feature is important when a contrast agent is utilized during the ultrasound imaging action. 
     The most widely used contrast agent comprises microbubbles that produce a strong nonlinear response when illuminated by an ultrasound beam. If the ultrasound beam exhibits, at its focus, an acoustic pressure which exceeds a threshold value, the contrast agent microbubbles can be burst. Use of this phenomenon during flow imaging enables a user to determine the time required to re-perfuse a flow region after the contrast agent therein has been destroyed by a high pressure ultrasound beam. 
     In current ultrasound systems, only one transmit focus is typically present per acoustic line or beam. To achieve the benefits of multiple transmit foci along a single scan direction, the prior art has used a “splice mode” wherein multiple acoustic lines are generated along the same azimuth, but with different focal depths as a result of altered timing of energizing pulses to the transducer elements. Data from the multiple co-linear acoustic lines are then combined to form a single composite image line with multiple foci. Other than the splice mode, a sector scan format used by current ultrasound systems produces focus loci which are semicircles that are centered at the apex of the scan. 
     The concentration of acoustic power at the focal point of an ultrasound beam provides a number of benefits. The echo signal-to-noise ratio in a region of interest is generally greater in echo signals received from the beam&#39;s focal point. Greater destruction of a contrast agent is achievable at the beam&#39;s focal point. This is especially useful in harmonic imaging methods that include subtraction of pre and post destruction image data. 
     As stated above, current ultrasound systems exhibit loci of focal points that are positioned along fixed, regular curves, e.g., in a sector scan, the locus of all beam foci are along a semicircle. There are, however, many anatomical features that present irregular outlines which are not aligned with the regular loci of focal points found in current ultrasound systems. For instance, when imaging cardiac walls, it would be desirable to control the loci of the ultrasound beam focal points to track the cardiac walls so as to achieve improved imaging thereof, or when contrast agent is being used, to enable improved its imaging or destruction. 
     Accordingly, due to the variability of anatomical structures, it would be useful to enable the user to define a path over which beam foci would track so as to enable an improved imaging of irregular anatomical structures. 
     SUMMARY OF THE INVENTION 
     The invention provides a method for controlling, in an ultrasound system, the loci of maximum power regions (e.g., focal points) of plural transmit lines of an ultrasound scan. Initially, a region being imaged is displayed and the user superimposes on the display a desired path to be traced by the loci of focal points of a scanned ultrasound beam. Data defining the path is then used to control an ultrasound transducer to produce a sequence of ultrasound beams that scan the region and to adjust the foci of the individual ultrasound beams so that they track the desired path. It is preferred that a graphical input device be used to trace the path of the loci on the display. The invention further enables data derived from the selected loci of maximum power regions to enable adjustment of both power and focal depth of the respective beams. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a system block diagram of an ultrasound system that is adapted to implement the invention. 
     FIG. 2 illustrates a sector scan of an apical view of cardiac walls, illustrating a power locus profile superimposed thereon. 
     FIG. 3 illustrates a frame table that includes the angle and quantized focal depth entries for each imaging line, according to a mapped power locus path. 
     FIG. 4 illustrates a table of transmit delays of pulses applied to an element of a phased array transducer. The exemplary transducer is assumed to have 128 ultrasound elements. 
     FIG. 5 is a logic flow diagram which illustrates the method for creation and use of a power locus profile. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, ultrasound system  10  comprises four main units, display monitor  12 , display subsystem  14 , scanner subsystem  16 , and acoustic transducer  18 . Display subsystem  14  includes a microprocessor  20  and a program read-only memory (ROM)  22  which, together with image processing hardware  24 , controls the operation of display monitor  12 . Video and data random access memories  26  and  28  store, respectively, video data that is to be presented on display monitor  12  and data which is used by microprocessor  20  and image processing hardware  24  during the processing of the video data. 
     A trackball  30  serves as a graphical input device for display subsystem  14  and enables graphical images to be formed on the face of display monitor  12 . The position of such a graphical image on display monitor  12  is determined and operated upon by display subsystem  14 . A keyboard  32  provides an alternative user input for display subsystem  14 . 
     Scanner subsystem  16  is controlled by a microprocessor  34  in conjunction with data stored in program ROM  36 . A frame table  38  includes control data that is used to control the operations of transmit beam former  40  and receive beam former  42 . A coefficient generator  44  is coupled to both transmit beam former  40  and receive beam former  42  and outputs coefficients that control the timing of actuation of the acoustic elements that comprise transducer  18 . 
     Briefly stated, transmit beam former  40 , based on the input data from frame table  38  and coefficient generator  44 , outputs transmit pulses via a probe interface  46  to each of the acoustic elements that comprise transducer  18 . In accordance with the timing of pulses applied to the acoustic elements, the beam emanating from transducer  18  will be caused to translate in a manner which forms a scan, such as by displacing each successive beam by a given distance or angle in space. Further, the timing of such pulses (and their magnitudes) control the focal points and beam powers that are generated by acoustic transducer  18 . Received echo signals are fed to receive beamformer  42  which, in turn, feeds acoustic line samples to image detector  47 . The outputs from image detector  47  comprise detected, sequenced line data that are transmitted to display subsystem for further processing and display. 
     Insofar as the overall operation of ultrasound system  10  is concerned, it is generally conventional and in conformance with the prior art. However, the operation of ultrasound system  10  departs from the prior art in that scanner subsystem  16 , in response to inputs from trackball  30  (or some other graphical input device) is enabled to alter the energization of the acoustic elements in ultrasound transducer  18  in such a manner as to cause the foci of the plural output beams to track a path that is input by the user, rather than to traverse a fixed, invariable path. In such manner, the image presentation appearing on display monitor  12  is improved. The improvement results from the locus of the foci of the beams following an anatomical structure, rather than taking a fixed scan path. 
     In the preferred embodiment of the invention, a user draws an ideal, desired power locus profile (hereafter “ideal PLP”) on display monitor  12  over an existing ultrasound anatomical image, such as the sector scan image shown on display monitor  12  and its expanded version shown in FIG.  2 . The image may be live (from continuous scanning) or frozen (such as with triggered scanning, or as a result of activating the “freeze” control). Image  50  shown in FIG. 2 is an apical view of the cardiac walls. Image  50  is generated by a combination of software and hardware and through use of standard ultrasound imaging procedures. The software portion resides in program ROM  22  in display subsystem  14 , and executes on microprocessor  20 . The input image data to display subsystem  14  is detected, sequenced, line data from the Scanner subsystem  16 . The standard ultrasound image processing methods for sector scan formats are well understood and documented elsewhere. 
     The user draws a line on display monitor  12  by touching a key on keyboard  32  to enter a “PLP Draw” mode. In response, a cursor shown on the display changes to indicate the draw mode is active. The user then moves the system trackball to move the cursor, which draws a line  52  on the display as a pixel trail. Line  52  preferably tracks the region of the displayed anatomical image (i.e., cardiac wall  50  whose presentation is to be enhanced. A standard line erasing/editing facility allows the user to redraw or correct the ideal PLP. 
     The ideal PLP is stored, pixel-by-pixel, as a graphics overlay to the ultrasound image in video RAM  26  of display subsystem  14 . When the user has finished drawing the ideal PLP, the user touches another key on keyboard  32  to confirm the entry, and the system responds by calculating and displaying a quantized PLP  54 . Alternatively, display subsystem  14  can calculate and update a portion of quantized PLP  54  as the user is drawing, allowing the user to back up and make quick corrections more readily. 
     Quantized PLP  54  is a piecewise linear approximation to ideal PLP drawn by the user and manifests system constraints on the PLP. The primary constraint is a quantization of focus for every individual scan line. The PLP can only occupy a distinct set of transmit foci available in the system. Although transmit coefficients for any focus depth can be generated algorithmically, only a practical subset of foci are characterized for transmit power, subject to FDA limits. Therefore, the choice of focus for any scan line must be taken from an available set of pre-determined foci F j  for the particular probe and scan format, where j is the index of available foci. 
     FIG. 2 shows, as an example,  6  discrete foci. They range from 2 to 12 cm. In the example shown in FIG. 2, actual PLP  52  must intersect only one focus for any scan line. (Representative scan lines  56  are shown to the left of FIG.  2 ). The curved portions of actual PLP  52  represent groups of scan lines that have the same focal depth F. 
     In forming the graphical representation of actual PLP  52  as visual feedback to the user, the straight radial segments of quantized PLP  54  each coincide with a single scan line  56 , and represent the boundaries between groups of lines that share the same discrete focus. The radial segments of actual PLP  52  are thus construction elements of the locus only which allow the user to better see the path of the locus. 
     Quantized locus  54  is found by taking user-drawn ideal PLP  52  from the graphics overlay in video RAM  26  and converting it from a set of X-Y points (i.e., Cartesian coordinates) to a radial format (i.e., a set of points in polar coordinates r, q). Ideal PLP  52  in r, q coordinates is mapped to the nearest focus F j  for each scan line angle q i , where i is the index of scan lines  56 . For each point r, q in ideal PLP  52 , the nearest q i  is found first by searching, and then the nearest F j . The mapping can be done either in display subsystem  14  or in scanner subsystem  16 , under control of one of the subsystem programs stored in the subsystem&#39;s program ROM and executed on the subsystem&#39;s microprocessor. 
     The result of the mapping is a Frame Table, an example of which is shown in FIG.  3 . The Frame Table is indexed by scan line number i, and includes the angle and quantized focal depth entries for each line according to mapped ideal PLP  52 . The Frame Table also typically contains many other data fields in each line entry for control of other aspects of the scan generation, such as image processing parameters (e.g. B-Mode, Color Flow, PW Doppler). Frame Table entries may also contain other coefficients that affect power locus (as described below). The Frame Table is stored in RAM in the scanner subsystem  16  and is used in the scanning process to control acquisition of acoustic data. 
     Once the focal depth for each scan line is known, transmit coefficients are generated in scanner subsystem  16  under control of microprocessor  34  and program ROM  36 . Coefficient generators  44  for both transmit and receive coefficients supply data to transmit and receive beamformers  40  and  42 , respectively. For both transmit and receive, a primary function of the coefficients is to control focus, steering, amplification and apodization of the acoustic beams. For transmit beamformer  40 , the coefficients may also determine the following parameters of the transmit waveforms, element-by-element: 
     pulse burst length (number of cycles), 
     pulse burst amplitude envelope shape, 
     pulse amplitude (peak-to-peak transmit voltage in a burst), 
     center frequency, 
     spectral bandwidth, and 
     aperture (which elements, in what pattern are active). 
     For the purposes of this example, only the path of the transmit focus across the scan lines is used as the determinant of power locus. For each scan line i, a set of transmit delays is calculated by Coefficient Generators  44  for the elements of probe  18 &#39;s phased array. The transmit burst from each probe element is timed by this set of delays in order to create the desired steering angle q i  and focus F i  for the line i. 
     FIG. 4 shows a table of transmit delays for 128 system channels. (Probe  18  may have greater or fewer elements than  128 ), Each channel is connected through probe interface  46  to an element of phased array probe  18 . The distance between elements on the probe face is accounted for in calculating the transmit coefficients for the scan. 
     During scanning, microprocessor  34  in scanner subsystem  16  steps through each line number i in sequence. For each scan line, control information is sent from the Frame Table to the beamformers, causing the pre-calculazed transmit and receive coefficients to be activated and used in beamforming. In transmit beamformer  40 , the coefficients will have been modified to use quantized PLP  52 . Accordingly, the coefficients for each scan line enable the acoustic elements to adjust the focus of the ultrasound beam so that succeeding beam foci follow the route of quantized PLP  54 . 
     A logic flow diagram for the process of creating and using the PLP is summarized in FIG.  5 . Note that the user is enabled to alter ideal PLP  52  until it is set to follow the path of the anatomical image along which enhanced images are desired. 
     The control of transmit parameters for locating the path of maximum acoustic power in the medium has been described above. 
     “Multi-burst” techniques employ more than one transmit event per scan line and have been shown to enhance the detection of non-linear acoustic media, especially in the presence of contrast agents. An example is cited in U.S. Pat. No. 5,577,505, wherein transmit power modulation from line to line allows suppression of linear signal response and amplification of non-linear response. Use of a power locus profile in a region of interest will complement and augment power modulation and similar multi-burst techniques. 
     Splicing techniques are well known in ultrasound imaging and enable two or more transmit events to be emitted in the same direction but with different focal depths. The received data from both events are combined by splicing the sections of the receive lines that are close to the corresponding transmit line&#39;s focus. The result is a single, composite receive line that benefits from narrower average focus over the whole line depth. Using a splice technique, the PLP can be extended such that its path may cross radial scan lines at more than one point, because for a given scan line, multiple foci from multiple transmit events may be spliced together. 
     The embodiment discussed above uses a radial scan line pattern as its basis. Scan line patterns other than sector scans are in common use, including rectangular and trapezoidal. Those patterns require only changes to the coefficient generation formulae for determining element by element delay, for a given focal depth. Thus a PLP, as described above, can be used to set the pattern of focal depths in such scan patterns. In the focus generation step, the mapping of ideal to actual PLP must use the coordinates of the actual scan lines in a coordinate system most suited to the scan format. For instance, Cartesian coordinates are best suited to rectangular shaped scans. 
     The invention is not only useful for contrast imaging, but is useful in any ultrasound scanning of non-linear media, including body tissues. Because the non-linear effect is most pronounced where acoustic power is most concentrated, it is desirable to allow control over the locus of maximum power. For instance, in “tissue harmonic” imaging, PLP will aid in image enhancement because harmonic echoes (from non-linearity in tissues) are exploited to enhance the image contrast and to reduce clutter. 
     The embodiment described above relates to 2-dimensional ultrasound scanning, where the acoustic beams formed by the scan lines are all coplanar. (The acoustic lines each have an elevation focus profile, but the scanner does not vary it.) That limitation is unnecessary in that the PLP can occupy 3-dimensional space. Standard 3-D rendering equipment can allow the user to construct a 3-D ideal PLP. The locus of transmit foci can be varied in 3 dimensions, using a 2-dimensional phased array probe. In a 3-D scanning system, each scan line interrogates the surrounding volume, which can optimally be a frustum if the scan lines have a common apex. In that scanning configuration, the focus of each scan frustum is determined by a mapped 3-D PLP. Again, transmit parameters other than just focus can be applied to each frustum. 
     It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.