Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
2001-04-30
2003-04-01
Allen, Stephone (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
C356S432000
Reexamination Certificate
active
06542245
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for detecting a dynamic change in ultrasonic wave or the like propagating through a medium. Furthermore, the invention relates to an ultrasonic diagnostic apparatus having such a dynamic change detecting apparatus.
2. Description of a Related Art
In an ultrasonic diagnostic apparatus for so-called ultrasonic echo observation or the like, it is the general practice to use a piezo-electric material typically represented by PZT (Pb (lead) titanate zirconate) for an ultrasonic sensor section (probe).
FIGS. 11A and 11B
schematically illustrate the structure of a conventional probe:
FIG. 11A
is a whole perspective view of the probe, and
FIG. 11B
is an enlarged perspective view illustrating an array oscillator.
The probe
301
has a thin box shape as a whole and has a long and slender rectangular probing surface
302
. This probing surface
302
is brought into contact with a human body and an ultrasonic wave is radiated so as to receive an ultrasonic echo reflected from the depth of the body. In
FIG. 11A
, a cable
307
sending an ultrasonic receiving signal is connected to the upper side of the probe
301
.
A comb-shaped array oscillator
303
serving simultaneously as a transmitter and a receiver of ultrasonic wave is housed in the probing surface
302
. The array oscillator
303
is formed by providing a number of slits
306
(having a width of, for example, 0.1 mm) in a thin (having a thickness of, for example, 0.2 to 0.3 mm) strip-shaped PZT sheet and arranging many (for example, 256) comb-teeth-shaped individual oscillators
305
(having, for example, a width of 0.2 mm and a length of 20 mm).
Electrodes are formed in each individual oscillator
305
, and signal lines are connected thereto. An acoustic lens layer or an acoustic matching layer made of a resin material such as rubber is pasted to the surface side (lower side in
FIG. 11A
) of the array oscillator
303
, and a packing material is pasted onto the back side. The acoustic lens layer converges the transmitted ultrasonic waves effectively. The acoustic matching layer improves the transmission efficiency of ultrasonic waves. The packing material has a function of holding the oscillator and causes oscillation of the oscillator to be finished early.
These ultrasonic probes and ultrasonic diagnostic apparatuses are described in detail in the “Ultrasonic Observing Method and Diagnostic Method”, Toyo Publishing Co., and “Fundamental Ultrasonic Medicine”, Ishiyaku Publishing Co.
In the area of ultrasonic diagnosis, collection of three-dimensional data is demanded for obtaining more detailed information about the interior of an object's body. In order to comply with such a demand, it is required to make ultrasonic detecting elements (ultrasonic sensors) into a two-dimensional array. In the aforementioned PZT, however, refinement and integration of devices over the present status is difficult for the following reasons. That is, the processing technology of PZT materials (ceramics) is on a limit level, and further refinement leads to an extreme decrease in processing yield. This furthermore results in an increase in the number of wires, thus leading to an increase in electrical impedance of wiring. In addition, there is an increase in crosstalk between individual elements (individual oscillators). It is therefore considered difficult at the present level of art to achieve a two-dimensional array probe using a PZT.
A paper entitled “Progress in Two-Dimensional Arrays for Real-Time Volumetric Imaging” by E. D. Light et al., Duke University, appears in ULTRASONIC IMAGING 20, 1-15 (1998), disclosing a probe having a two-dimensional array of PZT ultrasonic sensors. The paper however states “In order to obtain an image of a similar quality, it is necessary to provide 128×128=16,384 elements of the two-dimensional array. However, it is complicated and expensive to make such many RF channels, and therefore, there is only a little chance of this solution in the future. It is furthermore very difficult to densely connect such many elements.” (page 2, lines 14-18).
On the other hand, a sensor using optical fibers is used as an ultrasonic sensor not using a piezo-electric material such as PZT. Such an optical-fiber ultrasonic sensor is suitable for measurement at a location largely affected by magnetic field or at a narrow site.
There is available a kind of optical fiber ultrasonic sensor which uses an optical fiber Bragg grating (hereinafter abbreviated as an “FBG”) (see TAKAHASHI, National Defense Academy, et al. “Underwater Acoustic Sensor with Fiber Bragg Grating” OPTICAL REVIEW, Vol. 4, No. 6 (1997) 691-694). An FBG is formed by alternately laminating two kinds of material layers (light propagating medium) having different values of refractive index in several thousand layers so that refractive index changes periodically at a pitch satisfying Bragg's reflection conditions. The pitch of the periodic structure is &Dgr;, a wavelength of incident light is &lgr;, and N is an arbitrary integer, then, Bragg's reflecting condition is expressed by the following formula:
2
N&Dgr;=&lgr;
Under the action of Bragg reflection, the FBG selectively reflects light having a particular wavelength satisfying the above formula, and causes light having the other wavelengths to pass through.
When an ultrasonic wave is caused to propagate to FBG, the wavelength &lgr; of the light selectively reflected changes because deformation of FBG leads to change in the pitch &Dgr; of the aforementioned periodic structure. In practice, there are slant zones in which the reflectance varies before and after a central wavelength showing the highest reflectance (lowest transmittance), and an ultrasonic wave is applied to the FBG while a detection light having a wavelength in these slant zones is made incident to the FGB. It is thus possible to observe change in intensity of the reflected light (or transmitting light) corresponding to intensity of the ultrasonic wave. The ultrasonic intensity can be determined by converting this change in light intensity into an electric signal.
There is available another kind of optical fiber ultrasonic sensor using a Fabry-Perot resonator (hereinafter abbreviated as an “FPR”) (see UNO et al., Tokyo Institute of Technology, “Fabrication and Performance of a Fiber optic Micro-Probe for Megahertz Ultrasonic Field Measurements”, T. IEE Japan, Vol. 118-E, No. 11, '98).
The sensor of UNO et al. is prepared by forming a half mirror through vapor deposition of gold at the leading end of a single-mode optical fiber (&lgr;=1.3 &mgr;m, core: 10 &mgr;m and clad: 125 &mgr;m), providing a cavity (length: 100 &mgr;m) by a member of polyester resin (n=1.55) at the leading end thereof, and forming a total reflection mirror by gold vapor deposition further at the leading end thereof.
Detection light having a wavelength &lgr; is made incident into this sensor from the half mirror side, and an ultrasonic wave is transmitted from the total reflection side. When the reflectance of the half mirror is r, the single pass gain is G, the length of the cavity is L, and the refractive index is n, then, the reflectance R of this sensor is determinable from the following formula:
R
=
(
r
-
G
)
2
+
4
r
G
sin
2
δ
(
1
-
r
G
)
2
+
4
r
G
sin
2
δ
where, &dgr; is calculated by means of the following formula:
&dgr;=2&pgr;
Ln/&lgr;
The formula expressing &dgr; suggests that change in an optical path length 2L of a reciprocation of the cavity caused by change in sound pressure of the ultrasonic wave, i.e., change in the optical path length L leads to change in the reflection property of the light from the sensor.
In practice, there are slant zones in which the reflectance varies before and after the central wavelength giving the lowest reflectance, and change in intensity of the reflected light corresponding to the intensi
Allen Stephone
Fuji Photo Film Co. , Ltd.
Sughrue & Mion, PLLC
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