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. Typically, 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.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied 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. For a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. For a linear array, 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 same principles apply when 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. As with the transmission mode, 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. During the time after transmit, 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 f0, 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. As the transmit beam propagates through tissue, echoes are created when the ultrasound wave is scattered or reflected off of the boundaries between regions of different density. 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. 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. In certain instances ultrasound images may be improved by suppressing the fundamental signal and emphasizing the harmonic (nonlinear) signal components. If the transmitted center frequency is at f0, then tissue/contrast nonlinearities will generate harmonics at kf0 and subharmonics at f0/k, where k is an integer greater than or equal to 2. (The term xe2x80x9c(sub)harmonicxe2x80x9d 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 f0 and receiving at a band centered at frequency 2f0 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. 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. Pat. No. 5,984,869 issued on Nov. 16, 1999 and assigned to the instant assignee.
Likewise the techniques of 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., xe2x80x9cA New Imaging Technique Based on the Nonlinear Properties of Tissues,xe2x80x9d Proc. 1997 IEEE Ultrasonics Symp., pp. 1561-1566 (1998), while harmonic imaging using contrast agents is presented in de Jong et al., xe2x80x9cCharacteristics of Contrast Agents and 2D Imaging,xe2x80x9d 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 that uses two transmits with 180-degree phase shifts has been disclosed. Pulse inversion between the two transmits suppresses the fundamental signal and leaves the harmonic signal to form the image. Harmonic coded excitation that uses pulse sequences with 0 and 90-degree phase symbols (e.g., xe2x80x9c1xe2x80x9d and xe2x80x9cjxe2x80x9d, where j2=xe2x88x921) has been disclosed by Takeuchi in xe2x80x9cCoded Excitation for Harmonic Imaging,xe2x80x9d Proc. 1996 IEEE Int""l Ultrasonics Symp., pp. 1433-1436 (1997) and by Chiao et al. in U.S. patent application Ser. No. 09/494,465 filed on Jan. 31, 2000. However, a method to suppress the fundamental signal on reception was not specified in those disclosures. Harmonic coded excitation using Quadrature Phase Shift Keying (QPSK) (i.e., symbols 1, xe2x88x921, j and xe2x88x92j) with suppression of the fundamental signal on reception was disclosed in U.S. Pat. No. 6,050,947 issued on Apr. 18, 2000 and assigned to the instant assignee.
There is need for a harmonic imaging technique which does not use transmits that are 180 degrees apart and in which complete pulse compression is not prevented due to spectral mismatch between xe2x80x9cplusxe2x80x9d and xe2x80x9cminusxe2x80x9d harmonic pulses (particularly for broadband pulses).
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. In accordance with the preferred embodiments of the invention, 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.
More specifically, the amplitude of the transmitted pulse sequences is set sufficiently high to generate harmonic signals from the nonlinearity of the tissue. For each transmit focal zone, 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.
In accordance with the preferred embodiments, 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 xe2x80x9cjxe2x80x9d or xe2x80x9cxe2x88x92jxe2x80x9d code symbol by xc2xc fractional cycle at center frequency relative to the chips encoded with a xe2x80x9c1xe2x80x9d or xe2x80x9cxe2x88x921xe2x80x9d code symbol. (The QPSK transmit code symbols are xe2x80x9c1xe2x80x9d, xe2x80x9cxe2x88x921xe2x80x9d, xe2x80x9cjxe2x80x9d and xe2x80x9cxe2x88x92jxe2x80x9d.) For the second harmonic signal, the phases of the chips of the encoded transmit sequence are 90xc2x0 apart, which is implemented by circularly shifting the one chip by a quarter cycle in a transmit sequence memory. (The term xe2x80x9ccircularly shiftingxe2x80x9d 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 xe2x88x92X, respectively, while (A2-B2) and (C2-D2) are encoded by X and Y, respectively, where X and Y form a Golay code pair. X is a sequence such that X=x(n), n=0, 1, 2, . . . , (Nxe2x88x921), and X denotes the reversal of X given by X=x(Nxe2x88x921xe2x88x92n) for n=0, 1, 2, . . . , (Nxe2x88x921). The same is true for Y and Y. A Golay code pair X and Y satisfies the complementarity property X*X+Y*Y=xcex4(n), where the xe2x80x9c*xe2x80x9d symbol denotes convolution and xcex4(n) is a Kronecker delta function.
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 xe2x80x9cplusxe2x80x9d and xe2x80x9cminusxe2x80x9d 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 xe2x80x9cjxe2x80x9d pulse may not be equal to the negative of the second harmonic response of the xe2x80x9c1xe2x80x9d 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.
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