Communications – electrical: acoustic wave systems and devices – Echo systems – Speed determination
Patent
1991-11-08
1993-06-01
Pihulic, Daniel T.
Communications, electrical: acoustic wave systems and devices
Echo systems
Speed determination
G01S 1550
Patent
active
052166392
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a method for processing a Doppler signal, and more particularly to a method for displaying motion of a moving target or analyzing characteristics of the target by fetching a velocity of the moving target as Doppler frequency shift by using ultrasonics or microwaves.
In a sonar, for example, Doppler shift signals are used to detect and locate objects moving in the sea by using ultrasonics. These signals are also used in medical ultrasonic diagnostic equipment to measure the velocity of blood flowing in a patient's circulatory system. In a microwave radar, the Doppler shift signals are used to detect velocities of moving objects such as airplanes or ships.
DESCRIPTION OF THE RELATED ART
Heretofore Doppler shift techniques have been applied to detect motion of an object by using ultrasonics or microwaves without touching the object actually. Specifically, continuous or pulses waves having a predetermined frequency are sent toward the object. In return, an echo is reflected from the object. Since the echo is subject to Doppler frequency shift according to the motion of the object, it is possible to detect precisely the motion or velocity of the object based on the frequency shift of the received echo.
Such Doppler shift techniques have been recently applied to medical measurement using ultrasonics. Specifically, medical ultrasonic Doppler diagnostic equipments are in wide use to measure the velocity of blood flowing through a patient's circulatory system.
At an initial stage of such ultrasonic Doppler measurement, a continuous wave was transmitted to the object, from which an echo was reflected. A beat between the frequency of the echo and that of the transmitted signal was measured to detect Doppler frequency shift.
At recent, a pulsed Doppler system has been developed, in which pulse signals are transmitted intermittently to the object, making it possible to detect a velocity and location of the object simultaneously.
Also at an initial stage of such pulsed Doppler system, signal strength of an echo was too weak to ensure acceptable detection precision. Recent development of a high speed Fourier transformation technique has improved the detection precision to usable levels.
These Doppler techniques are applicable not only to ultrasonic devices but also to microwave radars, being applicable to a variety of fields.
Heretofore, data obtained from Doppler shift signals are mainly Doppler frequency shift resulting from the motion of the object. At recent, attempts have been made to apply the Doppler frequency shift to quantitative measurement through elaborate analysis of frequency components as well as detection of velocities of the objects. Before long, it will become possible to analyze a patient's somatologic organization with medical ultrasonic diagnostic equipment based on the Doppler frequency shift components.
Only a little real-time information is obtained based on the Doppler frequency shift using continuous or pulsed waves. Therefore it is difficult to fetch sufficient data for precise analyses of characteristics.
Further the Doppler shift frequencies are limited to certain ranges. Specifically, the higher the Doppler shift frequency, the more precise velocity detection is possible. Accordingly data sufficient for quantitative measurement can be fetched. However there occurs a phenomenon called "aliasing", wherein the velocity of the object exceeds a maximum velocity for a given ultrasonic transmitted frequency. Therefore, the fetched data would become too erroneous to assure precise measurement.
Aliasing would be indicated as mosaic patterns when the patient's blood flow is displayed in colors two-dimensionally on medical Doppler diagnostic equipment, which would result in poor image display on the diagnostic equipment.
As is well known, aliasing is concerned not only with the velocity of the object but also with the repetition frequency peculiar to the pulsed Doppler system in which pulses are transmitted intermittently. Aliasing can be minimized b
REFERENCES:
patent: 3798590 (1974-03-01), Jacobson et al.
patent: 4556067 (1985-12-01), Hokanson et al.
patent: 4680739 (1987-07-01), Lannuzel
Ohtsuki Shigeo
Tanaka Motonao
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