Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2002-06-11
2003-09-16
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06620102
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an ultrasonic diagnostic apparatus, or in particular to a technique effectively applicable to the correction of the delay time distribution of the received signals of a probe attributable to the fact that the interior of a subject is an ununiform medium.
BACKGROUND ART
In the conventional ultrasonic diagnostic apparatus, an ultrasonic wave is transmitted to a subject from an ultrasonic probe including an arrangement of a plurality of ultrasonic transducers (elements), the reflected wave thereof is received and given a delay time distribution as a received wave signal thereby to generate an ultrasonic beam having a directivity in a predetermined direction, thus producing a tomogram of the subject. The ultrasonic diagnostic apparatus, which is free of a radiation exposure unlike the other diagnostic apparatuses and can clearly plot the flesh easily without using a contrast medium as described above, is an indispensable diagnostic apparatus in wide fields of applications. As compared with the X-ray apparatus, however, the resolution of the ultrasonic diagnostic apparatus is still low and is expected to be improved further. In a technique for improving the resolution of the ultrasonic diagnostic apparatus, the amount of the phase shift caused by the ununiform medium in the subject is measured, and based on this phase shift amount, the delay time distribution applied to the received wave signals is corrected, and then the received wave signals corresponding to the respective ultrasonic transducers are added.
First,
FIG. 9
shows a diagram for explaining the delay time distribution for forming an ultrasonic beam of high resolution in a human body constituting an ununiform medium. The correction of the pulse wave front in the ununiform medium will be explained with reference to FIG.
9
. In the description that follows, only the operation at the time of receiving the ultrasonic wave will be explained for simplicity's sake.
In the case of a uniform medium with a known sound velocity, the wave front (the wave front of the received wave signal) of the pulse reflected from a reflector
906
reaches transducers
901
to
905
as an ideal wave front. In the process, due to the relative positions of the reflector
906
and the transducers
901
to
905
, the reflected pulse reaches the transducer
903
earliest and the transducers
901
,
905
latest. In order to assure the same arrival time of all the reflected pulses, the pulses received by the transducers
902
to
904
are given an appropriate delay. As a result, the same arrival time is secured for all the reflected pulses, and by the subsequent addition thereof, only the pulses received from the intended direction are amplified thereby to form a tomogram of a high resolution. In the case of a uniform medium of a known sound velocity, the delay to be given could be analytically determined as described below.
Assume that the distance between the transducers
901
to
905
and the reflector
906
is Li (1≦i≦5), the initial sound velocity of the ultrasonic diagnostic apparatus is c, the delay time given to the received wave signals of the transducers
901
to
905
is &tgr;i (1≦i≦5), and the maximum value of Li (1≦i≦5) is Lmax. Then, &tgr;i can be expressed by equation 1 below.
&tgr;
i=
(
Lmax−Li
)/
c
(1)
Actually, however, an ununiform medium
907
exists between the transducers
901
to
905
and the reflector
906
, and therefore the pulse wave front assumes a distorted wave front
908
. As a result, although &tgr;i is optimum as an initial delay time given to the received wave signal of each transducer, the initial delay time is required to be corrected by an amount taking the distorted wave front
908
into consideration in order to produce a tomogram of a high resolution.
A technique for measuring this correction amount is described in “IEEE Transactions on Ultrasonic, Ferroelectrics and Frequency Control, Vol. 39, No. 6, pp. 700-707, 1992 (hereinafter referred to as “reference 1”) or “IEEE 1991 Ultrasonics Symposium Proceeding pp. 1189-1193, 1991” (hereinafter referred to as “reference 2”). The technique described in these references is for correcting the effect that is had on the pulse wave front by an object having a different sound velocity, i.e. an ununiform medium which may exist between transducers for transmitting and receiving an ultrasonic wave and a reflector for reflecting the ultrasonic wave. According to this correcting technique, first, the amount of the phase shift of the reflected pulses resulting from the deviation of propagation time of the ultrasonic wave due to the ununiform medium is calculated by the correlating operation between all adjacent transducers. Then, based on the result of this calculation, the delay time of delay means is corrected thereby to correct the distortion of the pulse wave front due to the propagation of the ultrasonic wave through the ununiform medium for an improved resolution of the ultrasonic image.
The phase shift amount is determined using correlators for detecting adjacent phase shift of the outputs of the delay means for delaying the received wave signals of the transducers, for example. As a method of measuring the phase shift amount using the correlators, a technique is described in JP-A-1-135333 (hereinafter referred to as “reference 3”). According to the measuring technique described in reference 3, first, a delay time (initial delay time) is set as an initial value of delay means for giving a delay time distribution on the assumption that an human body is a uniform medium having a known sound velocity. Then, the delay process, i.e. the phasing of the received wave signals is carried out. After that, the phase shift amount between adjacent received wave signals after the delay process, i.e. the phase shift amount between the output signals of the delay means is calculated using correlators, and based on this arithmetic output, the phase shift amount for the initial delay time is corrected. In this way, the resolution of the ultrasonic wave is improved by correcting the phase shift caused by the ununiform medium in the human body.
DISCLOSURE OF THE INVENTION
As the result of studying the prior art described above, the present inventor has discovered the following problem points.
The conventional ultrasonic diagnostic apparatus poses the problem that as many correlators as the outputs of the delay means less 1, i.e. the number of the ultrasonic transducers less 1 are required for calculating the correlation between all the adjacent outputs of the delay means, resulting in a large circuit scale required for signal processing.
A technique for solving this problem is incorporated in an ultrasonic diagnostic apparatus described in JP-A-9-103429 (hereinafter referred to as “reference 4”) filed by the same applicant. The ultrasonic diagnostic apparatus described in reference 4 comprises delay means for giving a different delay time distribution for each of the received wave signals output from ultrasonic transducers, first adding means for reducing the number of signals by adding the received wave signals output from the delay means, i.e. the received wave signals after phasing, second adding means for generating a single ultrasonic beam by adding the received wave signals output from the first adding means, and correlating means for performing the correlation calculation of adjacent output signals (adjacent received wave signals) from the received wave signals output from the first adding means and measuring the phase shift amount between the adjacent output signals. This ultrasonic diagnostic apparatus is so configured as to comprise Na delay means corresponding to Na ultrasonic transducers, for example, and the first adding means adds each two or more of the Na adjacent inputs to reduce the number of the outputs to Nb. In the ultrasonic diagnostic apparatus described in reference 4, therefore, correlating means can be configured with Nb-1 correlators corresponding to the first adding
Baba Hirotaka
Miwa Yuichi
Shinomura Ryuichi
Tamano Satoshi
Hitachi Medical Corporation
Jaworski Francis J.
Mattingly Stanger & Malur, P.C.
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