Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/46—Ultrasonic, sonic or infrasonic diagnostic devices with special arrangements for interfacing with the operator or the patient
  • A61B8/461—Displaying means of special interest
  • A61B8/463—Displaying means of special interest characterised by displaying multiple images or images and diagnostic data on one display
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/06—Measuring blood flow
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/08—Clinical applications
  • A61B8/0808—Clinical applications for diagnosis of the brain
  • A61B8/0816—Clinical applications for diagnosis of the brain using echo-encephalography
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
  • A61B8/48—Diagnostic techniques
  • A61B8/488—Diagnostic techniques involving Doppler signals
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
  • G01S15/88—Sonar systems specially adapted for specific applications
  • G01S15/89—Sonar systems specially adapted for specific applications for mapping or imaging
  • G01S15/8906—Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques
  • G01S15/8979—Combined Doppler and pulse-echo imaging systems
  • G01S15/8984—Measuring the velocity vector
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
  • G01S15/88—Sonar systems specially adapted for specific applications
  • G01S15/89—Sonar systems specially adapted for specific applications for mapping or imaging
  • G01S15/8906—Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques
  • G01S15/8979—Combined Doppler and pulse-echo imaging systems
  • G01S15/8988—Colour Doppler imaging
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
  • G01S7/52017—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging
  • G01S7/52023—Details of receivers
  • G01S7/52036—Details of receivers using analysis of echo signal for target characterisation
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
  • G01S7/52017—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging
  • G01S7/52046—Techniques for image enhancement involving transmitter or receiver
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
  • G01S7/52017—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging
  • G01S7/52053—Display arrangements
  • G01S7/52057—Cathode ray tube displays
  • G01S7/52071—Multicolour displays; using colour coding; Optimising colour or information content in displays, e.g. parametric imaging
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
  • G01S7/52017—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging
  • G01S7/52053—Display arrangements
  • G01S7/52057—Cathode ray tube displays
  • G01S7/52073—Production of cursor lines, markers or indicia by electronic means

Definitions

• the invention relates generally to medical monitoring and diagnostic procedures and devices, and more particularly to a Doppler ultrasound method and apparatus for monitoring blood flow.
• Doppler ultrasound has been used to measure blood flow velocity for many years.
• the well-known Doppler shift phenomenon provides that ultrasonic signals reflected from moving targets will have a shift in frequency directly proportional to the target velocity component parallel to the direction of the ultrasound beam.
• the frequency shift is the same for any object moving at a given velocity, whereas the amplitude of the detected signal is a function of the acoustic reflectivity of die moving object reflecting the ultrasound.
• Pulse Doppler ultrasound systems commonly produce a spectrogram of the detected return signal frequency (i.e., velocity) as a function of time in a particular sample volume, with the spectrogram being used by a physician to determine blood flow characteristics of a patient.
• Doppler ultrasound systems also have the capability to detect and characterize emboli flowing in the bloodstream.
• An example Doppler ultrasound system with embolus detection capability is described in U.S. Patent No. 5,348,015, entitled “Method And Apparatus For Ultrasonically Detecting, Counting, and/or Characterizing Emboli," issued September 20, 1994, to Moehring et al., the disclosure of which is incorporated herein by reference.
• Such ultrasound systems are advantageously used both for diagnostic exams (to determine the presence and significance of vascular disease or dysfunction) and during surgical interventions (to indicate surgical manipulations that produce emboli or alter/interrupt blood flow).
• a user of ultrasound equipment finds it rather difficult to properly orient and position an ultrasound transducer or probe on the patient, as well as to select a depth along the ultrasound beam corresponding to the desired location where blood flow is to be monitored. This is particularly true in ultrasound applications such as transcranial Doppler imaging (TCD).
• TCD transcranial Doppler imaging
• the blood vessels most commonly observed with TCD are the middle, anterior, and posterior cerebral arteries, and the vertebral and basilar arteries.
• the Doppler transducer must be positioned so the ultrasound beam passes through the skull via the temporal windows for the cerebral arteries, and via the foramen magnum for the vertebral and basilar arteries. The user ofthe ultrasound equipment may find it difficult to locate these particular windows or to properly orient the ultrasound probe once the particular window is found.
• a complicating factor in locating the ultrasound window is determination of the proper depth at which the desired blood flow is located.
• the user does not know if he is looking in the correct direction at die wrong depth, the wrong direction at the right depth, or whether the ultrasound window is too poor for appreciating blood flow at all.
• Proper location and orientation of the Doppler ultrasound probe, and the proper setting of depth parameters is typically by trial and error. Not only does this make the use of Doppler ultrasound equipment quite inconvenient and difficult, it also creates a risk that the desired sample volume may not be properly located, with the corresponding diagnosis then being untenable or potentially improper.
• an information display in connection with Doppler ultrasound monitoring of blood flow.
• the information display includes two simultaneously displayed graphical displays.
• One graphical display is a blood locator display that indicates locations along the axis of the ultrasound beam at which blood flow is detected.
• the blood locator display includes a location indicator, such as a pointer directed to a selected one of the locations.
• the other graphical display is a spectrogram indicating velocities of monitored blood flow at the selected location.
• the blood locator display may include a color region corresponding with the locations at which blood flow is detected. The intensity of the color may vary as a function of detected ultrasound signal amplitude or as a function of detected blood flow velocities.
• the blood locator display allows a user to quickly locate blood flow along the ultrasound beam axis.
• the location of blood flow of particular interest can be further refined by the user adjusting the aim of the ultrasound probe to produce a greater displayed intensity or spatial extent at the particular location of interest.
• the user may then select the position ofthe pointer to view the corresponding spectrogram.
• the user may also use the two simultaneously displayed graphical displays to locate a particular blood vessel by detecting temporal or other variations in the displays that are consistent with the blood vessel.
• a method of detecting and characterizing an embolus is also provided. Locations in which blood does and does not flow are determined, as well as the direction of blood flow.
• a first ultrasound signal that may be an embolus is evaluated to determine if it corresponds with the locations where blood does and does not flow, as well as determining if it corresponds with the direction and rate of blood flow. If the first ultrasound signal does not correspond with blood flow direction or rate, then it is identified as non-embolic. If the first ultrasound signal does correspond with blood flow direction, and if it corresponds solely with locations where blood flows, then the first ultrasound signal is identified as an embolic signal of a first type. If the first ultrasound signal does correspond with blood flow direction, and if it corresponds both with locations where blood does and does not flow, then the first ultrasound signal is identified as an embolic signal of a second type.
• Figure 1 is a graphical diagram depicting a first Doppler ultrasound system display mode in accordance with an embodiment ofthe invention.
• Figure 2 is a graphical diagram depicting velocity and signal power parameters used in preparation ofthe display mode of Figure 1.
• Figure 3 is a graphical diagram depicting velocity and signal power parameters used in preparation of an alternative embodiment ofthe display mode of Figure 1.
• Figure 4 shows the alternative embodiment of the display mode of Figure 1 in color.
• Figure 5 is a graphical diagram depicting the display mode of Figure 4 and its use to identify the pulmonary artery.
• Figure 6 is a graphical diagram depicting a second Doppler ultrasound system display mode in accordance with an embodiment of the invention.
• Figure 7 shows two views ofthe display mode of Figure 6 in color.
• Figure 8 is the graphical diagram of the display mode shown in Figure 1, further depicting and distinguishing embolic signals from artifact signals.
• Figure 9 is a functional block diagram depicting a Doppler ultrasound system in accordance with an embodiment ofthe invention.
• FIGS 10 and 11 are functional block diagrams depicting particular details of pulse Doppler signal processing circuitry included in the Doppler ultrasound system of Figure 9.
• Figures 12-16 are process flow charts depicting particular operations performed by the pulse Doppler signal processing circuitry of Figures 10 and 11. DETAILED DESCRIPTION OF THE INVENTION
• FIG. 1 is a graphical diagram depicting a first display mode of Doppler ultrasound information in accordance with an embodiment of the invention.
• this first display mode referred to as an Aiming mode 100
• two distinct ultrasound displays are provided to the user.
• a depth-mode display 102 depicts, with color, blood flow away from and towards the ultrasound probe at various depths along the ultrasound beam axis (vertical axis) as a function of time (horizontal axis).
• the depth-mode display 102 includes colored regions 104 and 106.
• Region 104 is generally colored red and depicts blood flow having a velocity component directed towards the probe and in a specific depth range.
• Region 106 is generally colored blue and depicts blood flow having a velocity component away from the probe and in a specific depth range.
• the red and blue regions are not of uniform color, with the intensity of red varying as a function of the detected intensity of the return Doppler ultrasound signal.
• Those skilled in the art will understand that such a display is similar to the conventional color M- mode display, in which variation in red and blue coloration is associated with variation in detected blood flow velocities.
• the Aiming mode 100 also includes a displayed spectrogram 108, with Figure 1 depicting a velocity envelope showing the characteristic systolic- diastolic pattern.
• the spectrogram 108 includes data points (not shown) within the velocity envelope that are colored in varying intensity as a function of the detected intensity of the return ultrasound signal.
• the particular sample volume for which the spectrogram 108 applies is at a depth indicated in the depth-mode display 102 by a depth indicator or pointer 109.
• the depth-mode display 102 readily and conveniently provides the information concerning the range of appropriate depths at which a meaningful spectrogram may be obtained.
• the color intensity of regions 104 and 106 preferably vary as a function of the detected intensity of the return ultrasound signal.
• a graphical diagram depicts how such color intensity is determined.
• signals that may be intense but low velocity are ignored and not displayed in the depth-mode display 102 of Figure 1.
• This is referred to as clutter filtering and is depicted in Figure 2 as the threshold magnitude clutter cutoff limits for positive and negative velocities.
• low power signals associated with noise are also ignored and not displayed in the depth-mode display 102 of Figure 1.
• the user can determine the upper power limit for the color intensity mapping by selecting a power range value.
• color intensity mapping as a function of signal intensity, and further colored red or blue according to flow directions towards or away from the probe
• color intensity as a function of detected velocity may be employed instead.
• color intensity varies from the clutter cutoff magnitude to a maximum velocity magnitude, corresponding with one-half the pulse repetition frequency (PRF).
• PRF pulse repetition frequency
• Detected signals having a power below the noise threshold or above the selected upper power limit are ignored.
• Figure 4 is a color figure that shows the Aiming mode display 100 in which the color intensity of the regions 104 and 106 vary as a function of detected velocity.
• Both the depth-mode display 102 and the spectrogram 108 are displayed relative to the same time axis, and the depth-mode display shows variation both in spatial extent and in color intensity with the same periodicity as the heart beat.
• the depth-mode display shows variation both in spatial extent and in color intensity with the same periodicity as the heart beat.
• Those skilled in the art will also appreciate that instead of varying color intensity solely as a function of signal amplitude or solely as a function of velocity, one could advantageously vary color intensity as a function of both signal amplitude and velocity.
• Figure 1 shows a simplified display of a single, well-defined red region 104 and a single, well-defined blue region 106.
• Those skilled in the art will appreciate that the number and characteristics of colored regions will vary depending on ultrasound probe placement and orientation. Indeed, a catalogue of characteristic depth-mode displays can be provided to assist the user in determining whether a particularly desired blood vessel has, in fact, been located. Once the user finds the characteristic depth-mode display for the desired blood vessel, the user can then conveniently determine the depth at which to measure the spectrogram 108.
• the Aiming mode 100 enables the user to quickly position the ultrasound probe, such as adjacent to an ultrasound window through the skull so that intracranial blood flow can be detected.
• Use of colorized representation of signal amplitude is particularly advantageous for this purpose, since a strong signal is indicative of good probe location and orientation.
• the use of colorized representation of flow velocity may not be as advantageous, except where blood flow velocities vary significantly over blood vessel cross-section. However, when attempting to monitor blood flow near appreciably moving tissue (e.g., cardiac motion above clutter cutoff velocity), colorized representation of flow velocities may be preferred.
• use of the Aiming mode 100 is shown in connection with identifying a particular blood vessel, such as the pulmonary artery or femoral vein.
• a colorized representation of flow velocity is advantageously used in the depth-mode display 102, because of the high variation in blood flow velocities in these particular blood vessels.
• a user can identify optimal location of he pulmonary artery as follows: (1) the depth-mode display of the pulmonary artery will be blue with the same periodicity as the heart beat; (2)the blue region will typically reside between 4 and 9 cm depth; (3) along the time axis, the blue signal will be relatively intense in the middle of systole, corresponding to peak velocity; and (4) the signal will have the largest vertical extent in the depth-mode display, indicating that the user has positioned the probe such that the longest section of the pulmonary artery is aligned coincident with the ultrasound beam during systole.
• the user can then adjust other parameters, such as gate depth for the displayed spectrogram 108 and clutter filter parameters.
• the Aiming mode 100 also indicates to the user where to set the depth of the pulse Doppler sample gate so that the spectrogram 108 will process Doppler shifts from desired blood flow signals. It is the spectrogram 108 that is of primary clinical interest, allowing the user to observe and measure parameters associated with a particular blood flow and providing information that might suggest hemodynamically significant deviations in that blood flow.
• the information displayed to a user also typically includes well-known numerical parameters associated with the spectrogram, such as mean peak systolic velocity, mean end diastolic velocity, pulsatility index, and the relative change in mean peak systolic velocity over time.
• mean peak systolic velocity mean end diastolic velocity
• pulsatility index pulsatility index
• the Aiming mode display 100 of Figure 1 is particularly useful in positioning and orienting the Doppler ultrasound probe, and in first selecting a depth at which to measure the spectrogram 108. Following probe location and orientation and range gate selection, the user will typically prefer to have an information display emphasizing the clinically valuable spectrogram 108.
• a second display mode is shown that is referred to as a Spectral mode 110. In this mode, the spectrogram 108 occupies a larger display area. Instead of the full depth-mode display 102, a compressed depth-mode display 112 is provided.
• This compressed depth-mode display 112 on a shortened time scale, provides information concerning the depth of the sample volume at which the spectrogram 108 is taken, and the status ofthe blood flow in that sample volume, towards or away from the probe Thus, the user is continually informed concerning the desired sample volume depth and associated blood flow. This allows for quick understanding and compensation for any changes in the location of the desired sample volume relative to the blood flow, such as due to probe motion. This also allows a user of the ultrasound system to fine tune the sample volume depth even while focusing primary attention on the clinically important spectrogram 108.
• Figure 7 shows two different views of the Spectral mode 110 in color.
• the selected depth indicated by the pointer 109 in the compressed depth-mode display 112 is not a location at which blood flows, and consequently no there are no blood flow signals in the displayed spectrogram 108.
• the selected depth indicated by the pointer 109 does coincide with blood flow, and a corresponding spectrogram 108 is displayed.
• the color intensity of the region 104 varies as a function of detected velocity, and shows a characteristic color variation that may be associated with variation in blood velocity across blood vessel cross-section, a variation with depth in the alignment of the detected blood flow relative to the ultrasound beam axis, or both.
• the simultaneous presentation of the depth- mode display 102 and spectrogram 108 can also provide important information for detecting embolic signals and differentiating such signals from non-embolic artifacts.
• Figure 8 depicts three events: A, B, and C.
• event A the depth- mode display 102 shows a particularly high intensity signal having a non-vertical slope — i.e., a high-intensity signal that occurs at different depths at different times.
• event A the signal exists only within the boundary of one of the colored blood flow regions 104 and 106.
• a particularly high intensity signal is seen to have different velocities, bounded by the maximum flow velocity, within a short temporal region within the heartbeat cycle.
• Event A is strong evidence of an embolus passing through a blood flow region near the selected sample volume.
• Event B is another likely candidate for an embolus.
• the high-intensity signal seen in the depth-mode display 102 is non-vertical, but does not appear exclusively within a range of depths where blood is flowing. While this signal is strong enough and/or has a long enough back scatter to appear outside the blood flow margin in the depth-mode display 102, the spectrogram display 108 still shows the characteristic high intensity transient signal associated with an embolus.
• Event B is also evidence of an embolus, but likely an embolus different in nature from that associated with event A. Although the particular signal characteristics of various emboli have not yet been fully explored in the depth-mode display, the distinction between events A and B is likely that of different embolus types.
• event A may be associated with a particulate embolus
• event B may be associated with a gaseous embolus, with the different acoustic properties of a gas bubble causing the particularly long back scatter signal and the appearance of occurrence outside the demonstrated blood flow margins.
• Event C is an artifact, whether associated with probe motion or some other non-embolic event.
• Event C appears as a vertical line in the depth- mode display 102, meaning that a high-intensity signal was detected at all depth locations at precisely the same time — a characteristic associated with probe motion or other artifact.
• the high-intensity signal displayed in the spectrogram display 108 is a vertical line indicating a high-intensity signal detected for a wide range of velocities (including both positive and negative velocities and velocities in excess of the maximum blood flow velocities) at precisely the same time.
• Event C then is readily characterized as an artifact signal, and not embolic in nature.
• the simultaneous display of the depth-mode display 102 and the spectrogram 108 provides not only convenient means for locating the desired sample volume, but also provides a particularly useful technique for distinguishing embolic signals from artifact signals, and perhaps even for characterizing different embolic signals. Such embolic detection and characterization is easily observed by the operator, but can also be automatically performed and recorded by the ultrasound apparatus.
• Automatic embolus detection is provided by observing activity in two or more sample gates within the blood flow at the same time.
• the system (Mscriminates between two different detection hypotheses: (1) If the signal is embolic, men it will present itself in multiple sample gates over a succession of different times.
• FIG 9 is a functional block diagram that depicts an ultrasound system 150 in accordance with an embodiment of the invention.
• the ultrasound system 150 produces the various display modes described above in connection with Figures 1-8 on an integrated flat panel display 152 or other desired display format via a display interface connector 154.
• the signal processing core of the Doppler ultrasound system 150 is a master pulse Doppler circuit 156 and a slave pulse Doppler circuit 158.
• the Doppler probes 160 are coupled with other system components by a probe switching circuit 162.
• the probe switching circuit 162 provides both presence-detect functionality and the ability to distinguish between various probes, such as by detecting encoding resistors used in probe cables or by other conventional probe-type detection.
• two separate ultrasound probes 160 may be employed, thereby providing unilateral or bilateral ultrasound sensing capability (such as bilateral transcranial measurement of blood velocity in the basal arteries of the brain).
• the master and slave pulse Doppler circuits 156 and 158 receive the ultrasound signals detected by the respective probes 160 and perform signal and data processing operations, as will be described in detail below. Data is then transmitted to a general purpose host computer 164 that provides data storage and display.
• a suitable host computer 164 is a 200 MHz Pentium processor-based system having display, keyboard, internal hard disk, and external storage controllers, although any of a variety of suitably adapted computer systems may be employed.
• the ultrasound system 150 also provides Doppler audio output signals via audio speakers 166, as well as via audio lines 168 for storage or for output via an alternative medium.
• the ultrasound system 150 also includes a microphone 170 for receipt of audible information input by the user. This information can then be output for external storage or playback via a voice line 172.
• the user interfaces with the ultrasound system 150 primarily via a keyboard or other remote input control unit 174 coupled with the host computer 164.
• Figures 10 and 11 depict particular details of the master and slave pulse Doppler circuits 156 and 158. To the extent Figures 10 and 11 depict similar circuit structures and interconnections, these will be described once with identical reference numbers used in both Figures. Figure 10 also depicts details concerning the input and output of audio information to and from the ultrasound system 150 via the microphone 170, the speakers 166, and the audio output lines 168 & 172, the operations of which are controlled by the master pulse Doppler circuit 156.
• each of the pulse Doppler circuits 156 and 158 includes a transmit/receive switch circuit 175 operating under control of a timing and control circuit 176 (with the particular timing of operations being controlled by the timing and control circuit 176 of the master pulse Doppler circuit 156).
• the timing and control circuit 176 also controls operation of a transmit circuit 178 that provides the output drive signal causing the Doppler probes 160 (see Figure 9) to emit ultrasound.
• the timing and control circuit 176 also controls an analog-to-digital converter circuit 180 coupled to the transmit/receive switch 175 by a receiver circuit 182.
• the function and operation of circuits 175-182 are well known to those skilled in the art and need not be described further.
• the primary signal processing functions of the pulse Doppler circuits 156 and 158 are performed by four digital signal processors P1-P4.
• PI is at the front end and receives digitized transducer data from the receiver 182 via the analog-to-digital converter circuit 180 and a data buffer circuit or FIFO 186.
• P4 is at the back end and performs higher level tasks such as final display preparation.
• a suitable digital signal processor for PI is a Texas Instruments TMS320LC549 integer processor
• suitable digital signal processors for P2- P4 are Texas Instruments TMS320C31 floating point processors, although other digital signal processing circuits may be employed to perform substantially the same functions in accordance with the invention.
• Received ultrasound signals are first processed by the digital signal processor PI and then passed through the signal processing pipeline ofthe digital signal processors P2, P3, and P4.
• the digital signal processor PI constructs quadrature vectors from the received digital data, performs filtering operations, and outputs Doppler shift signals associated with 64 different range gate positions.
• the digital signal processor P2 performs clutter cancellation at all gate depths.
• the digital signal processor P3 performs a variety of calculations, including autocorrelation, phase, and power calculations. P3 also provides preparation of the quadrature data for stereo audio output.
• the digital signal processor P4 performs most of the calculations associated with the spectrogram display, including computation of the spectrogram envelope, systole detection, and also prepares final calculations associated with preparation of the Aiming display.
• Each ofthe digital signal processors P1-P4 is coupled with the host computer 164 (see Figure 9) via a host bus 187 and control data buffer circuitry, such as corresponding FIFOs 188(1) - 188(4).
• This buffer circuitry allows initialization and program loading of the digital signal processors P1-P4, as well as other operational communications between the digital signal processors P1-P4 and the host computer.
• Each of the digital signal processors P2-P4 is coupled with an associated high-speed memory or SRAM 190(2) - 1 0(4), which function as program and data memories for the associated signal processors.
• the digital signal processor PI has sufficient internal memory, and no external program and data memory need be provided.
• the ultrasound data processed by the digital signal processor P4 is provided to the host computer 164 via data buffer circuitry such as a dual port SRAM 194.
• the digital signal processor P4 ofthe master pulse Doppler circuit 156 also processes audio input via the microphone 170, as well as controlling provision of the audio output signals to the speakers 166 and audio output lines 168, 172.
• P4 controls the audio output signals by controlling operations of an audio control circuit 196, which receives audio signals from both the master and the slave pulse Doppler circuits 156 and 158.
• digital signal processor PI the operations of digital signal processor PI are as follows:
• PRF Doppler pulse repetition frequency
• Br and Bi are the digitally demodulated quadrature Doppler values for a series of N/4 different gate depths.
• the subtractions here remove DC bias from the data.
• LOW-PASS FILTER COEFFICIENTS Br and Bi contain frequencies up to carrier/4, and need to be further filtered to remove noise outside the bandwidth of the Doppler transmit burst.
• the coefficients for accomplishing this low-pass filtering are determined by a creating, with standard digital filter design software such as MATLAB, an order 21 low- pass FIR filter.
• the normalized cutoff of this filter is 2/(T*fs), where T is the time duration ofthe transmit burst, and fs is the sample rate ofthe data in Br and Bi (2MHz). Call this filter C(l:21).
• the coefficients of this filter will vary as the transmit burst length is changed by the user, and a bank of several different sets of filter coefficients is accordingly stored to memory.
• indices into the Br and Bi arrays are used that correspond to values falling closest to multiples of 1mm, as a means to decimating Br and Bi to 1mm sampling increments. This is done by having a prestored array of indices, D 1(1:64), corresponding to depths 29:92mm for 8kHz PRF, and indices D2(l:64) and D3(l:64) with corresponding or deeper depth ranges for 6.25kHz and 5kHz PRFs. 5.
• LOW-PASS FILTER AND DECIMATION OF QUADRATURE DATA The Br and Bi arrays are low-pass filtered and decimated to 64 gates by the following rules (note ⁇ a,b> is the 32 bit accumulated integer dot product of vectors a and b): 8kHz PRF:
• Er and Ei, 128 values altogether comprise the Doppler shift data for 1 pulse repetition period, over a set of 64 different sample gates spaced approximately 1mm apart. These arrays are passed to P2 with each new transmit burst.
• Gr, Gi, Hr and Hi are passed to P3 for further processing.
• embolus characterization e.g., distinguishing a particle from a bubble
• the method routes to a subroutine described below in connection with Figure 16.
• COMPLEX BANDPASS FILTER FOR USE IN AUDIO OUTPUT PREPARATION The min and max frequencies resulting from user specified spectral unwrapping of the spectrogram are used to determine a complex bandpass filter for making the audio output sound congruent with what is shown on the spectrogram display. For example, if the unwrapping occurs at [-l,7]kHz, then the audio complex bandpass filter has edges at -1kHz and +7kHz.
• a bank of several sets of complex bandpass filter coefficients, corresponding to different unwrap ranges, is generated offline and placed in memory. Each coefficient set corresponds to one of the unwrapping selections the user can make. Let the operative set of filter coefficients be called UWa(l:0) and UWb(l:0), where O is the filter order plus one.
• AUDIO OUTPUT PREPARATION RESAMPLE.
• the Doppler shift signals are to be played out the audio speakers. Before doing so, some prepping of the audio signals is important to match the user-selected spectral unwrapping.
• AUDIO OUTPUT PREPARATION COMPLEX BANDPASS. Apply a complex bandpass filter to Qr+jQi in order to remove the extra images introduced by multiplexing the data with zeros:
• R(n) UWb(l)*Q(n)+UWb(2)*Q(n-l)+...+UWb(0)*Q(n-0+l)
• AUDIO OUTPUT PREPARATION HILBERT TRANSFORM.
• the audio data in the sequence R(n) is in quadrature format and needs to be converted into stereo left and right for playing to the operator. This is done with a Hubert transform, and a 95 point transform, H(l:95), is used in this work — the coefficients can be obtained with formulas in the literature or standard signal processing software such as MATLAB.
• the application of the Hilbert transform to a data sequence is done as an FIR filter.
• digital signal processor P4 the operations of digital signal processor P4 are as follows:
• ENVELOPE Compute the maximum frequency follower or "envelope" function, E ), which indicates the upper edge of the flow signals in the spectrogram. This is an integer between 0 and 63, and is indexed by FFT calculation — i.e., for every spectral line calculation there is one value of E. Those skilled in the art will know of a variety of algorithms for making this calculation.
• POWERd4 and d multiply each of the power values by 1, 0 or -1, depending respectively on whether the associated ANGLE value is greater than the "filter cutoff parameter", less in absolute value than the filter cutoff parameter, or less than the negative of the filter cutoff parameter. This results in 64 values (one per gate depth) in the range of [-64,+63].
• This modified aiming array, POWERd5 is ready to display after sending to the host computer.
• AUDIO OUTPUT Send the arrays RR and RL, the right and left speaker audio outputs, to the speakers via port writes.
• INPUT MICROPHONE Sample M values into vector MIC from the input microphone port (M is # of transmit pulse repetitions within an 8ms period).
• EMBOLUS DETECTION BACKGROUND POWER IN POWER TRACES. For each of the four power traces, POWER_TRACEl..POWER_TRACE4, corresponding to the four preset gate depths, compute a background power level. Recall that POWER_TRACEn contains M values, where M is # of transmit pulse repetitions within an 8ms period). The background power value is obtained by a delta-follower for each trace, and is denoted by ⁇ l, 52, 53, and d 54.
• This update in the background values is done once every M power values, or every 8ms.
• EMBOLUS DETECTION PARABOLIC FIT. Apply a parabolic fit algorithm to the power trace each gate and determine if an event is occurring during the 8ms period. This fit must be applied to successive data windows spaced apart by at most 1ms. If the parabolic fit is concave down, and has a peak that exceeds the background power for the gate depth by 6 dB (an arbitrary threshold), then an event is detected.
• EMBOLUS DETECTION TIME DETERMINATION. For any single- gate events, compute the exact time of the event by analyzing the power trace between the -6dB points on either side of the peak power of the event. Record event results and times so that current events may be compared to past ones.
• Tl transmit burst length in microseconds.
• T2 pulse repetition period, in microseconds.
• the length of Hk(il:i2) is z2-zl. IF z2-zl ⁇ Rl, then no embolus is present. If Rl ⁇ z2-zl ⁇ R2, then a particulate is present. If z2-zl>R2, then a bubble is present.
• each ofthe circuits whose functions and interconnections are described in connection with Figures 9-11 is of a type known in the art. Therefore, one skilled in the art will be readily able to adapt such circuits in the described combination to practice the invention. Particular details of these circuits are not critical to the invention, and a detailed description of the internal circuit operation need not be provided. Similarly, each one of the process steps described in connection with Figures 12- 16 will be understood by those skilled in the art, and may itself be a sequence of operations that need not be described in detail in order for one skilled in the art to practice the invention.
• a user interface in accordance with the present invention may be provided by means other than a video display, such as a printer or other visual display device.
• a printer or other visual display device such as a printer or other visual display device.

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