Ultrasonic diagnosis apparatus

Details Ultrasonic diagnosis apparatus Ultrasonic diagnosis apparatus

2001-02-16

2002-10-01

Lateef, Marvin M. (Department: 3737)

Surgery

Diagnostic testing

Detecting nuclear, electromagnetic, or ultrasonic radiation

C600S458000

Reexamination Certificate

active

06458084

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ultrasonic diagnosis apparatus, and more particularly to an apparatus for performing diagnosis based on secondary harmonic information generated by a living body.
2. Description of the Related Art
In recent years, a physical quantity known as the “non-linear parameter” has gained attention in the field of observation and diagnosis of the interior of a living body using ultrasound. This non-linear parameter represents the degree of non-linear interaction of an acoustic wave with a medium, such as body tissue or an ultrasonic contrast agent comprising microbubbles. It is presumed that, based on the non-linear parameter, information such as the water content of the body tissue can be obtained, and the contrasting effect of the ultrasonic contrast agent can be achieved.
Based on the phenomenon that sound velocity increases as sound pressure becomes higher, distortion generated in an ultrasound propagating through a body is induced by the acoustic non-linearity of the body tissue, thereby accumulatively producing secondary harmonic component. In addition, secondary harmonic echo is generated based on the non-linear vibration characteristic of the ultrasonic contrast agent. It is known that, when distortion of an ultrasound is not large, the amplitude of the produced secondary harmonic is proportional to the intensity (square of the amplitude) of the fundamental. Further, the amplitude of the secondary harmonic depends on the non-linear parameter of the medium.
In view of the above, transmission of the fundamental of the center frequency f
0
to the medium, and then defining the non-linear parameter based on the secondary harmonic component of the frequency 2f
0
included in the received echo, has been conventionally proposed.
When the intensity of the transmitted ultrasound is P
0
(f
0
), the distance from the probe is z, and the frequency-dependent attenuation coefficient &agr;(f,z) is a function of the frequency f and the distance z, the amplitude A
2
(z) of the secondary harmonic of the echo signal received from scatterers having a backscattering characteristic &ggr;(f,z) can be represented by the following equation.
A
2
(
z
)=
P
0
(
f
0
)·exp (−2∫&agr;(
f
0
,z
)
dz
)·exp (−∫&agr;(2
f
0
,z
)
dz
)·&ggr;(2
f
0
,z
)·∫
h
(
z
)
dz
  (1)
The factor “P
0
(f
0
)·exp(−2∫ &agr;(f
0
,z)dz)” on the right side of the equation (1) represents the intensity of the transmitted fundamental which has been attenuated by the distance z. The factor “2” in the exponent is derived from the fact that the distortion is proportional to the square of the amplitude (intensity) of the fundamental. The next factor “exp(−∫ &agr;(2f
0
,z)dz)” represents the attenuation to which the secondary harmonic scattered wave was subjected in the distance z until reaching the probe. The factor “h(z)” in the final integrating factor is a term reflecting the non-linear parameter (B/A) of the medium in the distance z. This term can be represented by the following equation including the sound velocity C
0
and the density &rgr;
0
of the medium during equilibrium. The value of B/A is known to be about 5 to 11 in a body tissue, while much greater in a microbubble medium.
h=
(
B/A
+2)·2&pgr;
f
0
/(4&rgr;
0
C
0
3
)  (2)
In the body tissue, the final factor is obtained by integrating h(z) with respect to the distance z. This factor is provided corresponding to the accumulation of the secondary harmonic information generated along the propagation of the transmitted ultrasound. It is assumed, however, that secondary harmonic component generated by the non-linear vibration of the ultrasonic contrast agent does not accumulate, and that h(z) is not integrated with respect to the distance.
As is understood from the equation (1), secondary harmonic component included in an echo signal includes a factor dependent on the frequency-dependent attenuation characteristic &agr; and the backscattering characteristic &ggr;. Such secondary harmonic information, can therefore not be directly used as an evaluation value of the non-linear parameter.
Under the above circumstances, Akiyama et al. proposed, in Japanese Journal of Applied Physics, vol.30, supplement 30-1 (1991), re-transmitting the fundamental toward the same location in the body with the center frequency of the transmitted fundamental set to 2f
0
, and removing the influence of the attenuating characteristic and the scattering characteristic by making use of the phenomenon that the echo signal is subjected to the same attenuating and scattering characteristics as those of the secondary harmonic A
2
(z).
Specifically, in the system proposed by Akiyama, the influence of the attenuating and scattering characteristics is removed by the following processing. Assuming that the amplitude of the transmitted fundamental having the center frequency of 2f
0
is A
0
(2f
0
), the fundamental amplitude A
II
(z) of the received echo signal is represented by the following equation.
A
II
(
z
)=
A
0
(2
f
0
)·exp (−2∫&agr;(2
f
0
,z
)
dz
)·&ggr;(2
f
0
,z
)  (3)
The constant “2” in the exponent of the attenuating factor in the equation (3) is provided corresponding to the roundtrip propagation.
The frequency-dependent attenuation characteristic &agr; of the body tissue is generally linear, and therefore satisfies the following equation.
&agr;(2
f
0
,z
)=2·&agr;(
f
0
,z
)  (4)
By dividing the equation (1) by the equation (3), and using the equation (4), the following equation in which the influence of the attenuating characteristic &agr; and the scattering characteristic &ggr; is eliminated can be obtained.
A
2
(
z
)/
A
II
(
z
)=(
P
0
(
f
0
)/
A
0
(2
f
0
))·∫
h
(
z
)
dz
  (5)
Upon differentiating the above equation by the distance z, h(z) can be given by the following equation.
h
(
z
)=
d{A
2
(
z
)/
A
II
(
z
)}/
dz·{A
0
(2
f
0
)/
P
0
(
f
0
)}  (6)
Because P
0
(f
0
) and A
0
(2f
0
) are the intensity and the amplitude of the transmission and are known, h(z) reflecting the non-linear parameter (B/A) can be estimated from the equation (6) using A
2
(z) and A
II
(z).
In one method for obtaining the amplitude A
2
(z) of the secondary harmonic from the echo signal when the fundamental is transmitted, the band of the fundamental may be removed from the echo signal by using a band pass filter (BPF). However, using this method, the secondary harmonic component, being weak, cannot be accurately detected when the band of the fundamental and the band of the secondary harmonic overlap one another.
As a technique for solving the above problem, Kamakura et al. proposed a method in The Journal of the Acoustical Society of Japan, vol.46, No.10 (1990). In this method, two pulses both having the center frequency f
0
which differ from one another only in the signs are transmitted, thereby allowing separation of the fundamental and the secondary harmonic in the time domain to be performed by simple addition or subtraction of the echo signals of the two transmissions. According to this method, by adding the echo signals, the two fundamental components opposite in polarity cancel out one another, and only the secondary harmonic component can be extracted. On the other hand, by subtracting the echo signals of the two transmissions, only the fundamental component can be extracted.
As described above, according to the conventional techniques, in order to extract the secondary harmonic component at high precision so as to calculate the evaluation value of the non-linear parameter from which the influence of the attenuating and scattering characteristics is removed, it is necessary to twice transmit fundamentals having the frequency f
0
and differing polarity, and additionally transmit the fundamental having the frequency 2f
0
. In other words, the number of transmissions and receptions must undesirably be increased. When the number transmissions and receptions

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