Abstract:
The method of the invention controls an ultrasound system to identify a boundary between a tissue region and a blood-filled region that lies within an ROI. The method initially administers a contrast agent to the region of interest and then transmits first and second ultrasound beams at a different power levels into the ROI. Signal returns from the first and second beams are processed to derive first and second digital values, respectively. It has been determined that, under certain circumstances, a phase change of echo returns occurs at the boundary between tissue and blood-containing contrast agent. Detection of the phase change enables precise identification of the boundary, based upon the time segment in which the phase change is detected. Accordingly, time segment values of the first and second stored digital values are then phase-compared to enable determination of a boundary location between the tissue region and the blood-filled region by detection of the phase change.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This is a divisional of application Ser. No. 09/502,383, filed Feb. 11, 2000 now U.S. Pat. No. 6,398,732. 

   FIELD OF THE INVENTION 
   This invention relates to ultrasound imaging systems and, more particularly, to a method for detecting the location of boundaries between tissue regions and blood-filled regions within an area of anatomy being imaged. 
   BACKGROUND OF THE INVENTION 
   In the presentation of ultrasound images of anatomical features, there is a continuing need to improve the ability to precisely identify tissue/blood boundaries. The availability of images that precisely identify such boundaries enable an examining physician to better discern physical abnormalities. Recently, to improve the contrast between blood-filled regions and tissue regions, ultrasound contrast agents have been used. Such contrast agents are substances which strongly interact with ultrasonic waves, returning echoes which may be clearly distinguished from those returned by both blood and tissue. 
   Certain contrast agents consist of microbubbles which present a significant acoustic impedance mismatch and a non-linear behavior in certain acoustic fields. Such non-linear behavior is readily detectable through special ultrasonic processing. One type of microbubble contrast agent comprises microbubbles of an inert gas that are coated with a thin biodegradable coating or shell. Such microbubbles are infused into the body and survive passage through the pulmonary system and circulate throughout the vascular system. 
   The non-linear response of microbubble contrast agents, as contrasted to a relatively linear response from tissue is illustrated in  FIG. 1 . There, the return from a contrast agent with increasing levels of acoustic transmit power is plotted versus echo magnitude. As can there be seen, the contrast agent echo response exhibits an exponential relationship whereas the response from tissue is approximately linear. Accordingly, at higher acoustic transmit power levels, the contrast agent echo response exhibits a larger difference from the tissue echo response than at lower acoustic transmit power levels. 
   U.S. Pat. No. 5,577,505 to Brock-Fisher et al., assigned to the same assignee as this Application, describes a method for enhancing the detection of echo returns from microbubbles in circulation relative to tissue. Brock-Fisher et al. achieve increased sensitivity to non-linear responses from the microbubbles by transmitting a first ultrasound signal at a first power level into a region of interest (ROI) to be imaged. The echo responses gathered from that ultrasound signal are stored, and a second ultrasound signal is applied, at a second power level and the ultrasound echo responses stored. 
   The ultrasound responses are gain compensated and further processed to subtract one from the other so as to remove most of the linear response values from the exponential echo response signal. What remains corresponds to the exponential response portion of the microbubble contrast agent backscatter. The determined difference values are color-coded and added back into the original image in the spatial areas from which the echo signals were generated, enabling better identification of the blood-filled regions carrying the contrast agent. 
   Notwithstanding the ability to better image the contrast agent in areas of blood circulation, there still is a need to better identify boundaries between tissue and contrast agent-containing regions of an ultrasound image. 
   SUMMARY OF THE INVENTION 
   The method of the invention controls an ultrasound system to identify a boundary between a tissue region and a blood-filled region that lies within an ROI. A first embodiment of the method initially administers a contrast agent to the region of interest and then transmits a first ultrasound beam at a first power level into the ROI. Signal returns from the first beam are processed and stored as first digital signal values. Thereafter, a second ultrasound beam is transmitted at a second power level into the ROI and the signal returns therefrom are processed to derive second digital signal values which may then be used to identify a boundary between a blood filled cavity and tissue. 
   It has been determined that, under certain circumstances, a phase change of echo returns occurs at the boundary between tissue and blood-containing contrast agent. Detection of the phase change enables precise identification of the boundary, based upon the time segment in which the phase change is detected. Accordingly, time segment values of the first and second stored digital values are phase-compared to enable determination, by detection of a phase change, of a boundary location between the tissue region and the blood-filled region. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plot of echo magnitude versus acoustic transmit power showing the response of both a contrast agent and tissue. 
       FIG. 2  comprises a series of schematics indicating the respective responses from tissue and blood-containing contrast agent and a combined response therefrom. 
       FIG. 3   a  illustrates the phase relationships of echo returns resulting from a single transmit event at a first low power level. 
       FIG. 3   b  illustrates the phase relationship of echo returns from a transmit event at a second higher power level. 
       FIG. 4  illustrates a block diagram of a system adapted to carry out the method of the invention. 
       FIG. 5  is a flow diagram illustrating the method of the invention, as implemented by the system of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 2 , the upper portion thereof illustrates an area of tissue  10  which borders a blood filled area  12 , both of which contain contrast agent  14 . Blood filled region  12  contains a higher concentration of contrast agent  14  than does tissue region  10 . The respective echo response signals from regions  10  and  12  are shown by signals  16  and  18 , respectively. The composite of the echo responses is shown by signal  20 . 
   While there may be an amplitude difference between signal responses  16  and  18 , the boundary between the respective signals is difficult to determine. It has been determined, however, that sum echo signal  20  exhibits a phase change at a position therein that is coincident with reflections from boundary  22  between tissue and blood-filled regions  10 ,  12 . The reason for this phase change relationship can be better understood by referring to  FIGS. 3   a  and  3   b.    
     FIG. 3   a  illustrates signal returns resulting from a first transmit event (i.e., line  1 ) that is delivered at a power P 1 . Time segment delta t is selected from the echo response signals. The tissue echo response signal is shown at  24  and the contrast agent echo response signal at  26 . Due to non-linearity of the response characteristic of signal echoes from contrast agent  14 , the ultrasound echoes from tissue  10  and blood  12  vary widely in phase and in a non-deterministic manner. 
   However, at a lower incident signal power level (e.g., P 1 ), echo returns from tissue predominate—due to the density of the tissue and the fact that the nonlinear response increase of contrast agent is not significant at that power level. By contrast, at a higher incident signal power level (e.g., P 2 ), the nonlinear increase in echo magnitude from contrast agent  14  causes it to dominate over the tissue echo response signals. Accordingly, if an echo signal  26  from contrast agent  14  is out-of-phase with an echo signal  24  from tissue  10  (which can randomly occur), at the higher power transmit level, the higher amplitude of echo signal  26  will cause the phase of echo signal  24  to be overridden and a phase reversal of the sum echo signal will be seen. This effect will not, however occur at the lower power level. Further, if power P 1  is at a relatively low level (e.g., see  FIG. 1 ), the amplitude of signal  26  is relatively low and sum signal  28 , which is the addition of signals  24  and  26 , is in-phase with tissue response signal  24  due to the predominance of signal  24 . This condition (at power P 1 ) will generally exist when time segment delta t captures a sum response signal that constitutes an echo signal from tissue region  10 . 
     FIG. 3   b  shows signal relationships when a second transmit event (line  2 ) is generated at a power level P 2 , where power level P 2  is greater than power level P 1 . In this instance, due to the non-linear relationship of the response characteristic from contrast agent  14  (see  FIG. 1 ), signal  26 ′ evidences a substantially higher amplitude than signal  24 ′ from tissue  10 . Accordingly, sum signal  28 ′ evidences a phase characteristic that is dominated by the contrast agent echo response signal  26 ′ and, evidences an out-of-phase relationship with tissue echo response signal  24 ′. 
   The change in amplitude of signal  26 ′ is due to the non-linear response characteristic of contrast agent  14  and is further enhanced when time segment delta t captures an echo response signal from the increased concentration of contrast agent  14  in blood  12 . By correlating the time segment delta t (wherein a phase change from in-phase to out-of-phase occurs, as determined from sum signals  28  and  28 ′ and tissue signals  24  and  24 ′), the location of boundary  22  between tissue  10  and blood  12  can be identified. 
   Note that since the effects discussed above occur on a random basis, if a change in phase occurs, it is saved as an indication of a picture element where a boundary point has been found. Over a plurality of transmit events, a number of phase change occurrences will probably be detected. Then, a “connect the dots” method can be used to fill in intervening picture elements where no phase change was detected. 
   Turning now to  FIG. 4 , ultrasound system  50  includes a transducer  52  that is powered via a transmit/receive switch  54  from a transmitter  55 . As will be hereafter understood, transmitter  55  is controlled by processor  64  to generate, sequentially, two ultrasound beams  60  and  62  at the same angle, using the same focal point, but at different power levels. Power level P 1  of ultrasound beam  60  is less than the power level P 2  of ultrasound beam  62 . 
   Echo signals are received via transducer  52  and passed via transmit/receive switch  54  to a beamformer  56  of conventional design. The output of beamformer  56  is fed to a variable gain amplifier  58  that is, in turn, controlled by an input from processor  64 . Prior to generation of beam  60 , variable gain amplifier  58  is set to evidence a gain of 1 and prior to generation of beam  62 , to a gain that is equal to P 1 /P 2  to scale the second echo response signal to the first echo response signal. The “line” of echo signals derived from beam  60  are passed by variable gain amplifier  58  with a unitary gain. The output of variable gain amplifier  58 , in that condition, includes both tissue response returns and contrast agent returns. The output of variable gain amplifier  58 , when higher power ultrasound beam  62  is generated and an echo response signal is received, is dominated by the non-linear portions of the return signal when the echo signals are those returned from the blood filled region  12  (see  FIG. 2 ). 
   Both the P 1  and P 2  echo return signals, in time sequence, are passed through an analog to digital converter  66  which transforms the respective outputs into a time series of digital values. The respective digital signal values are stored in memory  68  as line  1  and line  2 , respectively. Thereafter, processor  64  initiates a coherent phase comparator procedure  70  which selects time segments of the line  1  and line  2  digital signal values in a manner to assure time “coherence” therebetween. 
   Coherent phase comparator procedure  70  then subtracts from the line  2  values (P 2 ), the line  1  signal values (P 1 ). Subtraction of these two values leaves a difference signal that exhibits a phase relationship that changes at boundary  22  and is either out-of phase or in-phase with the tissue echo return signal (see  FIGS. 3   a ,  3   b ). If the signals are in-phase, it can be concluded that the signal is dominated by echo returns from either tissue or contrast agent and a boundary has not yet been reached. By contrast, if the difference signal evidences an out-of-phase relationship with the echo return signal, the boundary has been reached. The phase change occurs because the amplitude of the contrast agent echo return signal at the P 2  power level experiences a significant increase at the increased power level. Thus the phase of the contrast agent echo return signal dominates at the higher P 2  power level. Further, at boundary  22  the concentration of contrast agent changes markedly. 
   Accordingly, as coherent phase comparative procedure  70  steps along through the time segments of lines  1  and  2 , the first time segment to show the phase change relationship indicates the time point at which the boundary between tissue and blood is detected. The time segment of the phase change signal is then related to the remaining signals of the ultrasound image, allowing the position of the boundary in the image to be determined. The resultant detected boundary can then be enhanced by, for example, assignment of a color value to the picture elements that define the boundary region. 
   Turning now to  FIG. 5 , the method of the invention will be described in conjunction with the flow diagram shown therein. Initially, variable gain amplifier  58  is set at unity gain (step  100 ). Thereafter, ultrasound beam  60  is caused to be transmitted at an angle “a” with a focus “f” and a power equal of P 1  (step  102 ). The return signals are processed through variable gain amplifier  58 , at the unitary gain setting. The output signals therefrom are converted to digital values and stored as “line 1” in memory  68  (step  104 ). 
   Next, the gain of variable gain amplifier  58  is set to the ratio of P 1 /P 2  (step  106 ) and beam  62  is transmitted at the same angle and focus as beam  60 , but with an increased power P 2  (step  108 ). The return echoes are processed by variable gain amplifier  58 , using the gain setting of P 1 /P 2 . This gain setting scales the second response signal to the first response signal and the resulting scaled signal is converted to digital values and stored as “line 2” (step  110 ). 
   At this stage, coherent phase comparator procedure  70  selects succeeding time coherent segments of data from the line  1  and line  2  storage areas in memory  68  and then compares their respective phases (step  112 ). If no phase change is detected from a time segment signal in a previous time segment, no boundary has been reached. By contrast, if a phase change is detected, a boundary has been found. 
   As an alternative embodiment, the line  1  and line  2  signals may be subtracted to derive (line  2 −line  1 ). The difference value (line  2 −line  1 ) is then subjected to a threshold and if the difference value exceeds the threshold, the phases of the line  1  and (line  1 −line  2 ) time segment signals are then compared (step  112 ). Otherwise, the procedure repeats for a next time segment. 
   Thereafter, processor  64  (see  FIG. 4 ) outputs the resulting boundary data to image processing module  59  which superimposes a signal enhancement on the resulting ultrasound image to better identify the boundary region (step  114 ). The enhanced signals are then fed to display  61  (see  FIG. 4 ) for review by the user. 
   As indicated above, introduction of contrast agents can be used to improve the delineation of blood-filled structures, especially in patients that would otherwise have poor diagnostic quality images. In images obtained without contrast agents, the tissue structures normally are presented as brighter intensities, while blood-filled cavities, because of their relative lack of returned echo signals, appear darker than the tissues surrounding them. However, when contrast agents are introduced, because of the greater volumetric concentration of contrast agents in the cavities than in the tissues, the cavities subsequently appear with brighter intensities than the surrounding tissues. 
   In the prior art (see U.S. Pat. No. 5,195,521 to Melton et al.), a thresholding operation is applied to the image intensity data. The image samples that are of greater intensity than a threshold are classified as tissue and the image samples that are of lesser intensity than a threshold are classified as blood. A boundary is indicated where the areas of image samples classified as tissue meet areas of image samples classified as blood. Newer contrast detection techniques (see U.S. Pat. No. 5,577,505) serve to further increase the cavity-to-tissue intensity ratio when contrast agents are used. Further, the multi-power level procedure described herein provides an even better ratio of intensities as between blood pools containing contrast agent and tissue. 
   Accordingly, it has been determined that an improved method for defining tissue boundaries can be implemented when contrast agents are introduced into circulation. As the relative intensity relationship between tissue and the cavity areas is reversed, with blood pools being brighter than tissue (as described above), a threshold detection action can be used to classify a boundary between tissue and a blood filled cavity. More particularly, areas are classified as cavity where the image intensity is greater than a threshold, and areas are classified as tissue where the image intensity is less than a threshold. The regions where the classification changes are indicative of the position of a boundary. 
   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. For example, (i) one or more frequency components of one of the ultrasound beams may be modulated in phase with respect to the other ultrasound beam; (ii) the first and second ultrasound beams may be transmitted in phase coherence when considering a reference time; or (iii) the ultrasound beams may be transmitted in phase opposition when considering a reference time. In each case, the summed phase relationship of echoes that result from the beams will enable the determination of a boundary picture element. Further, while each of the procedures required to operate the invention have been described as loaded into memory, they may be stored on a memory device (e.g. a magnetic floppy disk) and loaded on an as-needed basis. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.