Classifications

• • 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
  • A61B8/481—Diagnostic techniques involving the use of contrast agents, e.g. microbubbles introduced into the bloodstream
• • 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
  • G01S15/8959—Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques using coded signals for correlation purposes
• • 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
  • G01S15/8959—Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques using coded signals for correlation purposes
  • G01S15/8961—Short-range imaging systems; Acoustic microscope systems using pulse-echo techniques using coded signals for correlation purposes using pulse compression
• • 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
  • G01S7/52038—Details of receivers using analysis of echo signal for target characterisation involving non-linear properties of the propagation medium or of the reflective target

Definitions

• This invention relates to ultrasound imaging systems and more, particularly, to methods for harmonic ultrasound imaging using coded excitation.
• Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements which transmit an ultrasound beam and then receive the reflected beam from the object being studied. Such operation comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display.
• transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line.
• the receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
• the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages.
• the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam.
• the beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam.
• the beam with its focal point can be moved in a plane to scan the object.
• a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
• the transducer probe is employed to receive the reflected sound in a receive mode.
• the voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object.
• this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element.
• An ultrasound image is composed of multiple image scan lines.
• a single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time.
• the focused transmit energy is referred to as a transmit beam.
• one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or time delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time.
• the resulting focused sensitivity pattern is referred to as a receive beam.
• Resolution of a scan line is a result of the directivity of the associated transmit and receive beam pair.
• the output signal of the beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
• Conventional ultrasound transducers transmit a broadband signal centered at a fundamental frequency f 0 , which is applied separately by a respective pulser to each transducer element making up the transmit aperture.
• the pulsers are activated with time delays that produce the desired focusing of the transmit beam at a particular transmit focal position.
• the transducer array is used to convert these ultrasound echoes into electrical signals, which are processed to produce an image of the tissue.
• These ultrasound images are formed from a combination of fundamental (linear) and harmonic (nonlinear) signal components, the latter of which are generated in nonlinear media such as tissue or a blood stream containing contrast agents.
• the received signal With scattering of linear signals, the received signal is a time-shifted, amplitude-scaled version of the transmitted signal. This is not true for acoustic media which scatter nonlinear ultrasound waves.
• the echoes from a high-level signal transmission contain both linear and nonlinear signal components.
• ultrasound images may be improved by suppressing the fundamental signal and emphasizing the harmonic (nonlinear) signal components. If the transmitted center frequency is at f 0 , then tissue/contrast nonlinearities will generate harmonics at kf 0 and subharmonics at f 0 1 k , where k is an integer greater than or equal to 2. (The term "(sub)harmonic" refers to harmonic and/or subharmonic signal components.) Imaging of harmonic signals has been performed by transmitting a narrow-band signal at second harmonic frequency / 0 and receiving at a band centered at frequency 2f Q followed by receive signal processing.
• Tissue-generated harmonic imaging is capable of greatly improving B- mode image quality in difficult-to-image patients.
• One fundamental problem faced by tissue-generated harmonic imaging is low harmonic-to-noise ratio (HNR) since the harmonic signals are at least an order of magnitude lower in amplitude than the fundamental signal.
• HNR high harmonic-to-noise ratio
• a secondary problem is insufficient isolation of the harmonic signal from the fundamental as measured by a low harmonic-to-fundamental ratio (HFR).
• Coded Excitation is the transmission of long encoded pulse sequences and decoding of the received signals in order to improve image SNR and/or resolution.
• the energy contained in a long transmit pulse sequence is compressed into a short time interval on receive by virtue of the code.
• Coded excitation is a well-known technique in medical ultrasound imaging. For example, the use of Golay codes is disclosed in U.S. Patent No. 5,984,869 issued on November 16, 1999 and assigned to the instant assignee..
• tissue harmonic imaging and harmonic imaging using contrast agents are known. Harmonic imaging images the nonlinear signal components produced inside the body that are used to both reduce clutter when imaging tissue and to enhance contrast agent signal when imaging blood flow.
• the technique of tissue harmonic imaging is presented in Averkiou et al., "A New Imaging Technique Based on the Nonlinear Properties of Tissues," Proc. 1997 IEEE Ultrasonics Symp., pp. 1561-1566 (1998), while harmonic imaging using contrast agents is presented in de Jong et al., “Characteristics of Contrast Agents and 2D Imaging," Proc. 1996 IEEE Int'l Ultrasonics Symp., pp. 1449-1458 (1997).
• Tissue harmonics can greatly improve B-mode image quality in difficult-to-image patients, while contrast harmonics can greatly improve vascular studies.
• Harmonic imaging using harmonic Golay-coded excitation encodes the fundamental and second harmonic signals, and subsequently performs decoding on reception to suppress the fundamental signal and to compress the second harmonic signal. Using this technique, the SNR and/or resolution of the second harmonic image can be improved.
• four encoded sequences are transmitted to generate two fundamental and two second harmonic Golay-encoded pulse sequences on reception. Upon decoding of these received sequences, the fundamental signal is suppressed and the second harmonic signal is compressed.
• the amplitude of the transmitted pulse sequences is set sufficiently high to generate harmonic signals from the nonlinearity of the tissue.
• four separately encoded sequences are transmitted, received, filtered, and combined to form the decoded (compressed) second harmonic signal with fundamental component suppressed. This process is then repeated for subsequent focal zones to form an entire image frame.
• the transmit sequences are encoded using QPSK implemented as quarter-cycle circular rotations or shifts of the base pulse. This is implemented by time shifting the chips of the transmit sequence encoded with a “j " or “-j " code symbol by 1/4 fractional cycle at center frequency relative to the chips encoded with a "1" or "-1” code symbol.
• the QPSK transmit code symbols are "1", “-1", “j “ and “ —j ".)
• the phases of the chips of the encoded transmit sequence are 90 ° apart, which is implemented by circularly shifting the one chip by a quarter cycle in a transmit sequence memory.
• circularly shifting as used herein means that the time samples which are dropped at the front end of a shifted chip are added at the back end of the shifted chip.
• a different QPSK transmit code is used for each of four transmits A, B, C and D.
• the transmit codes are selected such that (A-B) and (C-D) are encoded by Y and - X, respectively, while (A - B ) and (C - D ) are encoded by X and Y, respectively, where X and Y form a Golay code pair.
• the coding and decoding technique disclosed herein achieves fundamental suppression and second harmonic compression for a given Golay code pair X and Y.
• the received signal from the second transmit is subtracted from that of the first transmit, and the received signal from the fourth transmit is subtracted from that of the third transmit to form the Golay-encoded fundamental and Golay-encoded second harmonic signals.
• Formation of the encoded harmonic signal from the difference of two received signals is necessary to equalize the spectra of the second harmonic signals representing the "plus” and "minus” code symbols, since the second harmonic signals generated from different QPSK transmit pulses may not be exactly inverted in phase (e.g., the second harmonic response of the "j " pulse may not be equal to the negative of the second harmonic response of the "1" pulse).
• the Golay- encoded harmonic signals are then compressed by matched filtering and summing the filter outputs, thereby canceling the fundamental signal to leave the second harmonic signal for image formation.
• FIG. 1 is a block diagram of a conventional ultrasound imaging system.
• FIG. 2 is a block diagram showing of an ultrasound imaging system in accordance with one preferred embodiment of the invention.
• FIGS. 3-6 are pulse diagrams showing a base sequence (FIG. 3), an oversampled code sequence (FIG. 4), an encoded transmit sequence for fundamental imaging (FIG. 5), and an encoded transmit sequence for harmonic imaging (FIG. 6).
• FIG. 7 is a block diagram illustrating a coding and decoding scheme for a Golay code pair (X, Y) in accordance with one preferred embodiment of the invention.
• FIG. 8 is a block diagram showing an implementation of a coding and decoding scheme for a Golay code pair (X,Y) in accordance with another preferred embodiment of the invention.
• FIG. 1 An ultrasonic imaging system in which the present invention can be incorporated is depicted in FIG. 1.
• the system comprises a transducer array 10 made up of a plurality of separately driven transducer elements 12, each of which produces a burst of ultrasonic energy when energized by a pulsed waveform produced by a transmitter 14.
• the ultrasonic energy reflected back to transducer array 10 from the object under study is converted to an electrical signal by each receiving transducer element 12 and applied separately to a receiver 16 through a set of transmit receive (T/R) switches 18.
• the T/R switches 18 are typically diodes which protect the receive electronics from the high voltages generated by the transmit electronics.
• the transmit signal causes the diodes to shut off or limit the signal to the receiver.
• Transmitter 14 and receiver 16 are operated under control of a master controller (e.g., a host computer) 20 responsive to commands supplied by a human operator via an operator interface (not shown).
• a master controller e.g., a host computer
• a complete scan is performed by acquiring a series of echoes in which transmitter 14 is gated ON momentarily to energize each transducer element 12, and the subsequent echo signals produced by each transducer element 12 are applied to receiver
• a channel may begin reception while another channel is still transmitting.
• Receiver 16 combines the separate echo signals from each transducer element to produce a single echo signal which is used to produce a line in an image on a display monitor 22.
• transmitter 14 drives transducer array 10 such that the ultrasonic energy is transmitted as a directed focused beam.
• respective time delays are imparted to a multiplicity of pulsers 24 by a transmit beamformer 26.
• Master controller 20 determines the conditions under which the acoustic pulses will be transmitted. With this information, transmit beamformer 26 determines the timing and amplitudes of each of the transmit pulses to be generated by pulsers 24.
• each transmit pulse is generated by an apodization generation circuit (not shown).
• Pulsers 24 in turn send the transmit pulses to each of elements 12 of the transducer array 10 via the T/R switches 18, which protect time-gain control (TGC) amplifiers 28 from the high voltages which may exist at the transducer array.
• TGC time-gain control
• the echo signals produced by each burst of ultrasonic energy reflect from objects located at successive ranges along each transmit beam.
• the echo signals are sensed separately by each transducer element 12 and a sample of the magnitude of the echo signal at a particular point in time represents the amount of reflection occurring at a specific range. Due to differences in the propagation paths between a reflecting point and each transducer element 12, the echo signals are not detected simultaneously and their amplitudes are not equal.
• Receiver 16 amplifies the separate echo signals via a respective TGC amplifier 28 in each receive channel. TGC is carried out by increasing or decreasing gain as a function of depth.
• the amount of amplification provided by the TGC amplifiers is controlled through a control line (not shown) that is driven by a TGC circuit (not shown), the latter being set by the host computer and hand operation of potentiometers.
• the analog echo signals are then sent to receive beamformer 30.
• receive beamformer 30 tracks the direction of the transmitted beam, sampling the echo signals at a succession of ranges along each beam.
• Receive beamformer 30 imparts the proper time delays and receive apodization weightings to each amplified echo signal and sums them to provide an echo signal that accurately indicates the total ultrasonic energy reflected from a point located at a particular range along one ultrasonic beam.
• the receive focus time delays are computed in real-time using specialized hardware or are read from a lookup table.
• the receive channels also have circuitry for filtering the received pulses. The time- delayed receive signals are then summed.
• the beamformer output frequency is shifted to baseband by a demodulator 31.
• a demodulator 31 One way of achieving this is to multiply the input signal by a complex sinusoidal e' ⁇ ' , where f d is the frequency shift required to bring the signal spectrum to baseband.
• the demodulated signals are then supplied to a signal processor 32.
• Signal processor 32 converts the demodulated signals to display data.
• the display data would be the envelope of the signal with some additional processing, such as edge enhancement and logarithmic compression.
• the RF signals are summed, equalized and envelope detected without intervening demodulation to baseband.
• the display data are converted by scan converter 34 into X — Y format for video display.
• the scan-converted frames are passed to a video processor (not shown) incorporated in display subsystem 22.
• the video processor maps the video data for display and sends the mapped image frames to the display subsystem.
• the images displayed by the video monitor (not shown) of display subsystem 22 are produced from an image frame of data in which each datum indicates the intensity or brightness of a respective pixel in the display.
• An image frame may, e.g., comprise a 400 x 500 data array in which each intensity datum is an 8-bit binary number that indicates pixel brightness.
• the brightness of each pixel on the display monitor is continuously refreshed by reading the value of its corresponding element in the data array in a well-known manner.
• Each pixel has an intensity value which is a function of the backscatter cross section of a respective sample volume in response to interrogating ultrasonic pulses.
• FIG. 2 shows a preferred embodiment of the invention employing coded excitation for the display of a harmonic image.
• each transducer element in the transmit aperture is pulsed using an encoded tiansmit sequence, each pulse in the sequence being commonly referred to as a chip.
• the encoded transmit sequence is formed by convolving a base sequence (comprising a sequence of +1 and -1 elements) with an oversampled code sequence (comprising an n -digit code, each digit being either of two code symbols, +1 and -1).
• the base sequence is phase encoded, using an n -digit code sequence, to create an n -chip encoded transmit sequence.
• four different n -chip encoded transmit sequences are stored in a transmit sequence memory 36.
• FIGS. 3-6 show an exemplary encoded transmit sequence for use in harmonic imaging in accordance with a preferred embodiment of the invention.
• FIG. 5 shows an encoded transmit sequence wherein the second chip is phase-shifted by 180° relative to the first chip.
• FIG. 6 shows an encoded transmit sequence wherein the second chip is phase-shifted by 90° relative to the first chip.
• the start of the chip encoded with the second code symbol of the code sequence is designated by the letter "A" in FIGS. 5 and 6.
• the encoded transmit sequence can be transmitted much more efficiently since its spectrum is better matched to the tiansducer passband with proper selection of the base sequence.
• T l/(2Nf 0 ) sec
• f 0 the fundamental (i.e., transmit) center frequency in MHz.
• 90° phase shift is then circularly shifted in time by 3 time samples, the first of the 3 shifted time samples being shaded in FIG. 6. This is implemented by circularly shifting the second chip by a quarter cycle in the transmit sequence memory.
• each encoded transmit sequence read out of transmit sequence memory 36 controls activation of a multiplicity of bipolar pulsers 24' during a respective transmit firing.
• the encoded transmit sequence for a given focal position is transmitted with sufficient amplitude such that harmonic signals are generated from nonlinear propagation in tissue.
• Pulsers 24' drive the tiansducer elements
• transmit focus time delays stored in a lookup table 38 are imparted to the respective pulsed waveforms produced by the pulsers.
• the ultrasonic beams can be focused at a multiplicity of transmit focal positions to effect a scan in an image plane.
• bipolar pulsers 24' can be excited by an encoded transmit sequence provided from transmit sequence memory 36 or from specialized hardware.
• the echo signals from transducer elements from transducer elements 12 are fed to respective receive channels 40 of the receive beamformer.
• Each receive channel has both a TGC amplifier and an analog-to-digital converter (not shown in FIG. 2).
• the receive beamformer tracks the direction of the transmitted beam.
• Receive beamformer memory 42 imparts the proper receive focus time delays to the received echo signals and sums the signals to obtain an echo signal that accurately represents the total ultrasonic energy reflected from a particular tiansmit focal position.
• the time-delayed receive signals are summed in receive beamsummer 44 for each transmit firing.
• the beamsummed receive signals acquired following each tiansmit firing are supplied to a matched filter 46 that convolves each beamsummed receive signal with a respective receive code.
• matched filter 46 comprises a finite impulse response (FIR) filter.
• FIR finite impulse response
• Suitable filter coefficients are stored in memory 48 and are supplied to matched filter 46 at the appropriate times.
• the summed receive signals from first through fourth successive firings are supplied to matched filter 46, which convolves the first summed receive signal with a first receive code for the first transmit firing, the second summed receive signal with a second receive code for the second transmit firing, and so forth.
• the match-filtered signals derived from the first through fourth transmit firings focused at the same transmit focal position are summed in a vector summer 50. Matched filter 46 and vector summer 50 together perform pulse compression of the harmonic signal and suppression of the fundamental signal.
• the filtered and summed receive signal is demodulated by demodulator 31 and supplied to signal processor 32 (FIG. 1).
• signal processing includes envelope detection, edge enhancement and logarithmic compression. After signal processing and scan conversion, a scan line is displayed on the display monitor.
• the transmit sequences are generated using QPSK implemented as quarter-cycle circular rotations or shifts of the base pulse.
• the QPSK tiansmit code symbols are "1", "-1", “_/ " and "-j ".
• the QPSK is implemented by time-shifting the chips of the transmit sequence encoded with a "J " or "-j " code symbol by 1/4 fractional cycle at center frequency relative to the chips encoded with a "1" or "-1” code symbol, as demonstrated in FIG. 6.
• the base pulse [1, 1, - 1, - 1] can be transmitted, implying that the phase- inverted base pulse [-1, -1, 1, 1] would correspond to the "-1" symbol.
• the right quarter-cycle rotation of the base pulse [-1, 1, 1, - 1] can be further assigned to be the
• QPSK tiansmit code is used for each of four transmits A, B, C and D.
• the transmit codes are selected such that (A-B) and (C-D) are encoded by Y and ⁇ X, respectively, while (A - B ) and (C - D ) are encoded by X and Y, respectively, where X and Y form a Golay code pair,
• Y and Y
• FIG. 7 shows the coding and decoding method for to achieving fundamental suppression and second harmonic compression for a given Golay code pair X and Y.
• the received signal 54 resulting from the second transmit B is subtracted from the received signal 52 resulting from the first transmit A in an adder/subtracter 60, and the received signal 58 resulting from the fourth transmit D is subtracted from the received signal 56 of the third transmit C in an adder/subtracter 62 to form the Golay-encoded fundamental 64 and Golay-encoded second harmonic 66 signals.
• the second harmonic signals generated from different QPSK transmit pulses may not be exactly inverted in phase (e.g., the second harmonic response of the " j " pulse may not be equal to the negative of the second harmonic response of the "1" pulse).
• the Golay-encoded harmonic signals are then compressed by matched filtering 68 and 70,) and summing the filter output signals 72 and 74 in a summer 76). With the particular selection of Golay code on the fundamental signal given above, this same filtering and summation cancels the fundamental signal to leave the second harmonic signal for image formation.
• received signal 52 from the first transmit A is match filtered with X (filter 68)
• received signal 54 from the second transmit B is match filtered with -X (filter 68')
• received signal 56 from the third transmit C is match filtered with Y (filter 70)
• received signal 58 from the fourth transmit D is match filtered with -Y (filter 70').
• all four filtered signals are summed in a vector summer 80 to produce the compressed second harmonic signal with suppressed fundamental signal.
• the disclosed algorithm may be demonstrated with an example.
• the notation used in this example is as follows: the subscripts "1" and “2” denote the resulting fundamental and second harmonic signals, respectively, while “u” and “v” denote the second harmonic signal resulting from the "1" and “j” pulses, respectively.
• the second harmonic signal is related to the square of the fundamental, and it is assumed that a difference exists between the second harmonic signals generated by the " 1 " and " j " fundamental transmit pulses.
• a 2 [u,u,u,v]
• the second harmonic component is match filtered with X, -X, Y, and -Y, and then summed to produce a fully compressed second harmonic signal:
• the imaging system incorporating the structure shown in FIG. 8 can also operate by demodulating the RF echo signals to baseband and downsampling before or after beamsummation. In this situation, the oversampled sequences would also be demodulated to baseband and downsampled.
• the matched filter can be implemented in software or hardware at the beamformer output, as shown in FIG. 2, or at the demodulator output (not shown), hi the latter situation, the filter coefficients must be matched to the demodulated signals.
• the I and Q matched filters receive different sets of filter coefficients and thus form a complex filter.
• the filter coefficients are matched to the respective demodulated signal component.

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• 2001
  • 2001-01-11 US US09/757,762 patent/US6491631B2/en not_active Expired - Fee Related
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