Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1999-05-28
2001-05-15
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06231512
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to ultrasound imaging and, more particularly, to harmonic imaging of the human anatomy for the purpose of medical diagnosis.
BACKGROUND OF THE INVENTION
Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which brightness of a pixel is based on intensity of the echo return. Conventional B-mode images are formed from a combination of fundamental and harmonic signal components, the former being direct echoes of the transmitted pulse and the latter being generated in a nonlinear medium, such as tissue, from finite-amplitude ultrasound propagation. In some instances, e.g., obese patients, ultrasound images can be improved by suppressing the fundamental and emphasizing the harmonic signal components.
Propagation of ultrasound beams in biological tissues is known to be nonlinear, giving rise to generation of harmonics. In harmonic imaging, energy is transmitted at a fundamental frequency ƒ
0
and an image is formed with energy at the second harmonic 2 ƒ
0
. Some of the characteristics of the nonlinearly generated second harmonic beams are: a narrower beam, lower sidelobes than the fundamental, and beam formation in a cumulative process, i.e., the second harmonic continually draws energy from the fundamental during propagation. These characteristics contribute to lateral resolution improvements, reduction of multiple reflections or other aberrations due to difficult windows, (i.e., body locations at which placement of a probe does not result in a good image) and clutter reduction due to inhomogeneities in the tissue and skin layers.
At least two methods for harmonic imaging in an ultrasound scanner are known. In one method, the transducer elements of a phased array are activated by waveforms that have a fundamental frequency and are time-delayed to produce an ultrasound beam which is focused at a transmit focal zone, transmission of a single focused beam being referred to as a “firing”. The echoes returned from the body being interrogated are transduced by the array elements into electrical signals and time-delayed to form a receive vector of acoustic data having both fundamental and harmonic signal components. A receive filter removes the fundamental signal component and isolates the harmonic signal component which is then detected, scan-converted and displayed.
In a second method, each transducer element is activated by a first waveform having one polarity during a first transmit firing and by a second waveform having the opposite polarity during a second transmit firing. Both waveforms are broadband pulses having a fundamental frequency. Activations of the transducer elements during each firing are time-delayed to produce an ultrasound beam which is focused at the same transmit focal zone. Each firing results in a respective receive vector of acoustic data, each vector having both fundamental and even harmonic signal components. When the receive vectors are vector summed, however, the fundamental signal components substantially cancel, thereby isolating an even harmonic signal component that is then detected, scan-converted and displayed.
Drawbacks to the first method include the following: (a) the received signal is narrowband and hence resolution is poor; (b) it is difficult to filter the large fundamental signal component completely, so there is some residual fundamental signal that degrades contrast improvement; and (c) if the transmit signal contains harmonic frequencies, it is not possible to filter out those harmonic frequencies.
The second method does not present the disadvantages of the first method. However, a major drawback of the second method is that it requires two firings to acquire harmonic data corresponding to a particular transmit focal zone and hence always decreases the frame rate by half. The second method is also susceptible to motion artifacts. For lower-frequency transducers, the second method is often not realizable.
SUMMARY OF THE INVENTION
A method and apparatus for performing parametric harmonic imaging enables different tissue types to be clearly differentiated in diagnostic ultrasound imaging. The nonlinear tissue response R(p) due to a given pressure pulse p(t) may be modeled as a power series:
R
(
p
(
t
)
)
=
∑
k
=
0
∞
α
k
p
k
(
t
)
(
1
)
where &agr;
k
are the harmonic response parameters. Since the harmonic response parameters may be significantly different between healthy and diseased tissue, the method of the invention images tissue using these harmonic response parameters.
In accordance with a preferred embodiment of the invention, a single transmit firing is used to image multiple harmonic response parameters. Parametric harmonic imaging consists of transmitting a pulse centered at frequency ƒ
0
and receiving the returned signal with a bandpass filter centered at a frequency less than ƒ
0
. The fundamental transmit pulse spectrum and the receive filter passband are chosen to have negligible overlap in order to bandpass substantially only harmonic signal components. This method detects the harmonic signals leaked into the passband from harmonic spectra centered at DC (zero frequency) or at ƒ
0
. The signal content leaked into the passband is a function of the entire set of harmonic response parameters.
REFERENCES:
patent: 5632277 (1997-05-01), Chapman et al.
patent: 5891038 (1999-04-01), Seyed-Bolorforosh et al.
patent: 5957852 (1999-09-01), Hossack et al.
patent: 5977911 (1999-11-01), Green et al.
patent: 6077226 (2000-06-01), Washburn et al.
patent: 6102858 (2000-08-01), Hatfield et al.
“A New Imaging Technique Based On The Nonlinear Properties Of Tissues,” Michalakis A. Averkiou, DN Roundhill, and JE Powers, IEEE Symposium 1997, 0-7803-4153-8/97/$10.00, pp. 1561-1566.
Chiao Richard Yung
Haider Bruno Hans
General Electric Company
Imam Ali M.
Jaworski Francis J.
Snyder Marvin
Stoner Douglas E.
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