Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2002-06-11
2004-05-11
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06733455
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to filtering in general and to filtering ultrasound data in particular.
2. Prior Art
Ultrasonic imaging is a frequently used method of analysis for examining a wide range of media and objects. Ultrasonic imaging is especially common in medicine because of its relatively non-invasive nature, low cost, and fast response times. For example, ultrasonic imaging is commonly used to detect and monitor the growth and health of fetuses, or to detect and assist in the diagnosis of liver and kidney pathology. Typically, ultrasonic imaging is accomplished by generating and directing ultrasonic sound waves (an ultrasonic beam or signal) into a medium under investigation using a set of ultrasound generating transducers and then observing reflections generated at the boundaries of dissimilar materials, such as tissues within a patient, also using a set of ultrasound receiving transducers. The generating and receiving transducers may be arranged in arrays and a single transducer may be used for both generating and receiving ultrasonic signals. The reflections are converted to electrical signals by the receiving transducers and then processed, using techniques known in the art, to determine the locations of echo sources. The resulting data is displayed using a display device, such as a monitor.
Typically, the ultrasonic signal transmitted into the medium under investigation is generated by applying continuous or pulsed electronic signals to an ultrasound generating transducer. In diagnostic imaging, the transmitted ultrasonic signal is generally in the radio frequency A) range of 1 MHz to 15 MHz, which corresponds to ultrasonic wavelengths in the range of 0.1 mm to 1.5 mm. The ultrasonic signal propagates through the medium under investigation and reflects off interfaces, such as boundaries, between adjacent tissue layers. Scattering of the ultrasonic signal refers to the deflection of the ultrasonic signal in many directions by interfaces that are much smaller than the ultrasonic wavelength. Attenuation of the ultrasonic signal is the loss of ultrasonic signal as the signal travels. Reflection of the ultrasonic signal is the bouncing off of the ultrasonic signal from an object (e.g., a vessel wall) that is similar in size or larger than the ultrasonic wavelength. Transmission of the ultrasonic signal is the passing of the ultrasonic signal through a medium. As it travels, the ultrasonic signal is scattered, attenuated, reflected, and/or transmitted. The portions of the reflected and/or scattered ultrasonic signals that return to the transducers are detected as echoes.
In the ultrasound art, steering refers to changing the direction of an ultrasonic beam. Aperture refers to the size of the transducer or group of transducer elements being used to transmit or receive an ultrasonic signal. The transmit aperture is the size of the transducer or group of transducers used to transmit an ultrasound signal, and receive aperture is the size of the transducer or group of transducers used to receive an ultrasound signal. Apodization refers to applying a weighting profile to the signals across the transducer aperture to produce ultrasound beams with reduced sidelobe spreading. Electronic focusing refers to applying relative time and/or phase shifts to signals across the transmit or receive transducer array elements to account for time-of-flight differences.
A conventional process of producing, receiving, and analyzing an ultrasonic signal (or beam) is called beam forming. The production of ultrasonic signals optionally includes apodization, steering, focusing, and aperture control. In conventional beamforming, RF echo data is acquired across a transducer array and processed to generate a one-dimensional set of echolocation data In a typical implementation, a plurality of ultrasonic beams are used to scan a multi-dimensional volume.
In electronic focusing, the transmit aperture of the transducer is apodized and electronically focused to form a transmit beam, and a large number (typically over 100) of transmit beams are generated and steered (as for a sector scan) along different scan lines to cover the entire scan plane.
To create two-dimensional (2D) B-mode images of tissue and 2D color flow images of moving blood, the echoes are detected and converted into electronic signals by the receive transducer aperture elements. Through parallel electronic channels the signals in different frequency bands are subject to amplification, digitization, frequency downshifting, apodization, focusing, steering and other filtering operations in order to generate echolocation data along the scan direction. Depending on the front-end architecture design, the order in which the above processing are performed may vary. Any processing such as amplification, which occurs before digitization would be implemented using analog electronic circuits.
In most ultrasound receivers, the echo signals are shifted down in frequency by means of frequency mixers and filters, to generate the in-phase (I) and quadrature (Q) signals which are centered at a much reduced RF frequency, but contain the same information bandwidth as the RF signals. For color flow processing, the RF spectrum is shifted down to baseband and the resultant I/Q components are also referred to as baseband components. The advantage of using I/Q echo components is that they can be digitized and processed at much lower sampling rates due to their reduced Nyquist bandwidths.
The I/Q echo data is furnished to the B-mode and color flow image processors for amplitude and motion detection respectively. For B-mode, the echo amplitude can be computed simply by taking the square root of I
2
+Q
2
. The detected data from different transmit events are compiled into 2D acoustic data sets, which are then converted by the scan-converter into X-Y format of, for example, 480×640 pixels (picture elements), for video display.
In B-mode imaging, the brightness of a pixel is based on the detected echo amplitude, whereas in color flow imaging, the color of a pixel is based on mean velocity and/or power of detected echoes from moving parts of the medium under investigation. In color flow imaging, the color flow image is formed within a region of interest (ROI), which is over-written onto the B-mode image by a video image processor such that, for example, the composite image depicts blood flow within the ROI according to a color scale, while surrounding stationary tissues are displayed in a gray-scale.
In color flow imaging, for each scan line within the user-specified ROI, a set of transmit beams are fired repeatedly at some pulse repetition frequency (PRF), in order to detect moving blood. Fundamentally, any motion of the medium under investigation relative to the ultrasound transducer produces the well-known Doppler effect in which the frequency of the reflected echo is shifted from that of the transmit frequency f
o
by an amount f
d
that is proportional to the target speed in the direction of the ultrasonic beam. That is, the frequency of the reflected signal is f
o
+f
d
. A medium under investigation that is moving towards the transducer will compress the incident ultrasonic wave thereby producing a positive Doppler frequency shift in the reflected echo. Conversely, a target that is moving away from the transducer will produce a negative Doppler frequency.
Mathematically, the Doppler frequency shift f
d
can be derived as follows. Suppose the target (e.g. red blood cells) is moving at velocity v, which makes an angle &phgr; with respect to the sound beam. This means that the target velocity component in the direction of the sound waves is u=v cos(&phgr;). Over a short time interval &Dgr;t, the change in round-trip distance between the target and the ultrasonic source (transducer) is &Dgr;d=2u&Dgr;t. Assuming u<<c (speed of sound), &Dgr;d translates into to a phase shift &Dgr;&thgr;=2&pgr;&Dgr;d /&lgr;, where &lgr;=c/f
o
is the ultrasound wavelength. Hence, the Doppler frequency s
Chou Ching-Hua
Ji Ting-Lan
McLaughlin Glen W.
Mo Larry Y. L.
Carr & Ferrell LLP
Jaworski Francis J.
Zonare Medical Systems, Inc.
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