Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-01-11
2002-12-10
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S458000
Reexamination Certificate
active
06491631
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to ultrasound imaging systems and more particularly, to methods for harmonic ultrasound imaging using coded excitation.
BACKGROUND OF THE INVENTION
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 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. 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 f
0
, then tissue/contrast nonlinearities will generate harmonics at kf
0
and subharmonics at f
0
/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 f
0
and receiving at a band centered at frequency 2f
0
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., “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 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., “1” and “j”, where j
2
=−1) has been disclosed by Takeuchi in “Coded Excitation for Harmonic Imaging,” 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., s
Chiao Richard Yung
Rhyne Theodore Lauer
Breedlove Jill M.
Cabou Christian G.
General Electric Company
Imam Ali M.
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