Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2002-03-05
2003-05-13
Manuel, George (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06564089
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical imaging device which irradiates the subject with low coherence light, to produce a tomogram of the subject, from data on light scattered by the subject.
2. Description of Related Art
Recent U.S. Pat. No. 5,321,501, for example, discloses a device which can obtain optical information from the inside of the tissue, in order to diagnose the biological tissue, and which performs optical coherence tomography (OCT) of interference type, using low coherence light, to produce a tomogram of the subject. U.S. Pat. No. 5,321,501 also discloses an optical probe which has a flexible insertion unit to be introduced into the patient's body cavity, and which enables the doctor to introduce the insertion unit into a blood vessel, using an optical probe that includes a single mode fiber for sending low coherence light to the inside so that the doctor can observe the inside of the patient's body cavity, using a catheter or endoscope. When introduced into the patient's body cavity, the optical probe is necessarily bent according to the tortuosity of the body cavity. Because of the bending, stress-induced birefringence is induced in the optical fiber which varies according to the extent of the bending. In OCT, the doctor uses an interference signal of a reference beam with reflection from the subject, to observe the subject. It is necessary, generally, to orient polarization of the reflection from the subject, toward that of the reference beam so that interference with the reference beam may be maximized. The stress-induced birefringence varies depending on the bending in the patient's body cavity; polarization, therefore, varies depending on the bending. As introduced into the patient's body cavity, the insertion unit varies interference contrast. The doctor may turn an integrated irradiation optical system that includes the single mode fiber. Every rotation of the bent system, particularly in such cases, causes a great variation in the stress-induced birefringence of the fiber. Interference contrast varies, so that detection sensitivity greatly varies depending on the direction of rotary scanning. Such a great variation is a problem.
“Polarization-insensitive fiber optic Michelson interferometer” (Electr. Lett. Vol. 27, pp. 518-519, 1991 ) discloses a method of inserting an element that rotates polarization by 45 degrees in a non-reciprocal manner such as a Faraday rotator, in order to compensate for a variation in interference contrast due to the fluctuations in stress-induced birefringence of such a fiber. A general Faraday rotator, however, requires a garnet crystal, and magnetic material that provides a magnetic field for the garnet crystal. It is impossible to provide such substances at the narrow probe tip to be introduced into the patient's body cavity.
“In vivo video rate optical coherence tomography” (A. M. Rollins et al., Internet, 1998 Optical Society of America) discloses a method of high-speed scanning of an interference location in OCT, with the reference arm group delay mechanism using a galvanometer mirror. To scan the location at a high speed by rotating the mirror, the inertia of the mirror will dictate that the rotational position of the mirror, if plotted with respect to time, would approach a sine wave oscillation. The depth scan of the interference location is proportional to the mirror rotational position. Because the mirror rotates first in one direction and then in the other, it is difficult to reproduce a two-dimensional image from the obtained interference signals. If the device uses an interference signal obtained by scanning in only one direction, it will neglect half the actually obtained signal data. In such cases, if the doctor continues rotary scanning of the optical system in the optical probe, he or she will provide only half the resolution and dynamic range for a two-dimensional image. Such a decrease in resolution and dynamic range is a problem.
In the above-mentioned high-speed scanning, the use of a resonant scanner, for example, ensures that the time vs. scanning angle curve is a sine wave, not linear. A two-dimensional position and intensity graph, however, may be used to represent an interference signal. Otherwise a shaded image may be used to express intensity as shades, based on the two-dimensional interference locations and detection positions. In such cases, to display interference signals accurately, it is impossible to use the interference signals obtained in time series, because the interference signals are linear with respect to time, and because the interference location is nonlinear with respect to time. A tomogram of the biological tissue inside, for example, may be produced. In such cases, if the device fails to indicate accurately the interference location and the detection position, the image produced will be warped, and it is impossible to accurately measure length with a scale.
As described above, the device may use a nonlinear scanning means such as a resonant scanner. Depending on the scanning angle, in such cases, the Doppler frequency (which characterizes the interference signal produced) varies in proportion to the scanning speed of the interference location. Optical heterodyne detection in OCT uses this Doppler frequency for detection; therefore, it ensures high signal-to-noise ratio. If the scanning is nonlinear, the Doppler frequency may vary. In such cases, if the frequency characteristics of the demodulator circuit are set so that the demodulator can detect all possible interference frequencies in a wide range, then the detection will include excess noise, and result in a decrease in the signal-to-noise ratio. The inferior signal-to-noise ratio is a problem.
If the subject is scanned in a two-dimensional manner, methods of enlarging a part of the displayed image include: a method of cutting unnecessary parts away from that part; and a method of changing the scanning range. Changing the scanning range may require adjusting the speed of scanning the interference location, to the change in scanning range. To improve the signal-to-noise ratio, scanning the interference location may be slowed. In these cases, the Doppler frequency may also vary; therefore, setting the frequency characteristics of the demodulator will pose a similar problem.
As disclosed in U.S. Pat. No. 5,321,501, an optical probe to be introduced into the patient's body cavity, generally, needs to be detachable from the observation device body for the purpose of cleaning and sterilizing. If detachable, a broken optical probe can be easily replaced with a new one. During assembly, single mode fibers undergo various stresses. Consequently, when provided in each optical probe, each single mode fiber may produce a distinct intrinsic birefringence. Whenever replacing probes, the doctor has to use a polarization plane adjustment means to orient, toward polarization of the reference beam, polarization of reflection that can be obtained by the optical probe, from the subject. It is necessary to maximize contrast of interference with the reference beam. The work is troublesome.
As disclosed in “In vivo video rate optical coherence tomography” (A. M. Rollins et al.), a galvanometer mirror or resonant scanner mirror may be used for high-speed scanning of the interference location in OCT. Because of the inferior temperature characteristics, the galvanometer mirror and the resonance scan mirror vary the scanning range and scanning speed, depending on the variation in temperature.
As disclosed in the above cited Rollins et al. Publication “In vivo video rate optical coherence tomography”, a galvanometer mirror or resonant scanner mirror may be used for high-speed scanning of the interference location in OCT. In such cases, in order to maximize drive speed, the mirror will be driven so that the drive curve may approach a sine wave. As described above, the nonlinear drive results in a nonlinear relationship between the interference location and inter
Hirata Tadashi
Horii Akihiro
Iizuka Shuhei
Izatt Joseph A.
Rollins Andrew
Manuel George
Pilsbury Winthrop LLP
University Hospital of Cleveland
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