Optical tomography system

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical tomography system for obtaining an optical tomographic image by measurement of OCT (optical coherence tomography).

2. Description of the Related Art

As a system for obtaining a tomographic image of an object of measurement in a body cavity, there has been known an ultrasonic tomography system. In addition to such an ultrasonic tomography system, there has been proposed an optical tomography system where an optical tomographic image is obtained on the basis of an interference of light by low coherence light. See, for instance, Japanese Unexamined Patent Publication No. 2003-172690. In the system disclosed in Japanese Unexamined Patent Publication No. 2003-172690, an optical tomographic image is obtained by measuring TD-OCT (time domain OCT) and the measuring light is guided into the body cavity by inserting a probe into the body cavity from the forceps port of an endoscope by way of a forceps channel.

More specifically, low coherence light emitted from a light source is divided into measuring light and reference light and the measuring light is projected onto the object of measurement, while the reflected light from the object of measurement is led to a multiplexing means. The reference light is led to the multiplexing means after its optical path length is changed. By the multiplexing means, the reflected light and the reference light are superposed one on another, and interference light due to the superposition is detected by, for instance, heterodyne detection. In the TD-OCT measurement, a phenomenon that interference light is detected when the optical path of the measuring light conforms to the optical path of the reference light in length is used and the measuring position (the depth of measurement) in the object is changed by changing the optical path length of the reference light.

When measuring the OCT by inserting a probe into a body cavity, the probe is mounted on the system body to be demountable since disinfection, cleaning and the like of the probe after use are necessary. That is, a plurality of probes are prepared for one optical tomography system and the probes are changed by the measurement. However there is an individual difference in the length of the optical fiber due to the manufacturing errors and the like, and the optical path length of the measuring light can change each time the probe is changed. Accordingly, in Japanese Unexamined Patent Publication No. 2003-172690, on the basis of the reflected light from the inner surface of a tube (sheath) covering an optical fiber of the probe, the optical path length of the reference light is adjusted to conform to the optical path length of the measuring light.

Whereas, as a system for rapidly obtaining a tomographic image without changing the optical path length of the reference light such as disclosed in Japanese Unexamined Patent Publication No. 2003-172690, there have been proposed optical tomography systems of obtaining an optical tomographic image by spatially or time dividing the interference light (See, for instance, U.S. Pat. No. 5,565,986. Among those, there has been proposed an SS-OCT (swept source OCT) system where interference light is detected by spectrally dividing the interference light in time while the frequency of the light emitted from the light source is swept. In the SS-OCT system, an interferogram interference intensity signal is obtained without changing the optical path length by sweeping the frequency of the laser beam emitted from the light source to cause the reflected light and the reference light to interfere with each other by the use of a Michelson interferometer. Then a tomographic image is generated by carrying out a Fourier analysis on the interferogram signal in the region of an optical frequency.

SUMMARY OF THE INVENTION

In the SS-OCT measurement, it is not necessary to conform the optical path length of the measuring light to that of the reference light since information on the reflection in positions in the direction of depth can be obtained by carrying out frequency-analysis. However, the wavelength sweeping laser for SS-OCT is actually about 0.1 nm in the instant spectral width and about 10 mm in coherence length. When the optical path length difference between the measuring light and the reference light is equal to or larger than the coherence length, there is generated no interference. Accordingly, also in the SS-OCT measurement, it is still necessary to adjust the optical path length so that the optical path length of the measuring light conforms to that of the reference light and the measurement initiating position is adjusted to a position in which the object is included in the measurable range.

Further since the measurable range over which a tomographic image is obtainable by the SD-OCT measurement is limited in the direction of depth, it is necessary to adjust the optical path length of the reference light according to the distance between the probe and the object in order to adjust the measurement initiating position so that the object S is positioned in the measurable range. That is, in the SS-OCT measurement, it is necessary to adjust the measurement initiating position so that the object S is positioned in the measurable range in addition to that the optical path length must be adjusted in order to accommodate the individual difference of the probe such as shown in Japanese Unexamined Patent Publication No. 2003-172690.

In the TD-OCT measurement, since the measuring depth is changed by adjusting the optical path length of the reference light, the measurable range can be adjusted by adjusting the optical path length while observing the intensities or the waveforms of the signals obtained by a beat signal measurement or the interferogram measurement of the interference light. However, in the SS-OCT measurement, since the reflection information cannot be obtained unless frequency-analys is such as Fourier-transform is carried out on the detected interference light and even when the position of the object is confirmed to adjust the measurement initiating position, frequency-analysis is required, it takes a long time to adjust the measurement initiating position.

In view of the foregoing observations and description, the primary object of the present invention is to provide an optical tomography system in which the adjustment of the measurement initiating position can be carried out in a short time.

In accordance with the present invention, there is provided an optical tomography system for obtaining a tomographic image of an object to be measured comprising

a light source unit provided with a laser light source which emits laser light while sweeping the wavelength thereof and a low coherence light source which emits low coherence light,

a light dividing means which divides the laser light or the low coherence light emitted from the light source unit into measuring light and reference light,

an optical path length adjusting means which adjusts the optical path length of the measuring light or the reference light divided by the light dividing means,

a multiplexing means which multiplexes the reflected light from the object when the measuring light divided by the light dividing means is projected onto the object and the reference light,

an interference light detecting means which detects interference light of the reflected light and the reference light which have been multiplexed by the multiplexing means,

a tomographic image obtaining means which detects intensities of the interference light in positions in the direction of depth of the object by carrying out frequency-analysis on the interference light detected by the interference light detecting means and obtains a tomographic image of the object, and

a control means which switches between a measurement initiating position adjusting mode in which the position in the direction of depth of the object in which tomographic image signal is to be obtained is adjusted and a tomographic image obtaining mode in which a tomographic image of the object is to be obtained,

wherein the improvement comprises that

the control means controls the light source unit to emit the laser light and the tomographic image obtaining means to obtain the tomographic image from the interference light generated by the laser light in the image obtaining mode and controls the light source unit to emit the low coherence light and the tomographic image obtaining means to obtain the tomographic image from the interference light generated by the low coherence light in the measurement initiating position adjusting mode.

Further, the control means may have a function, in addition to the function of controlling the light source unit and the image obtaining means according to the mode, of automatically controlling the optical path length adjusting means so that the optical path length difference between the reference light and the measuring light is in an interference signal generating region. The “interference light generating region” means a region where the optical path length difference between the measuring light and the reference light is smaller than the coherence length and interference can occur.

The light source unit may be of any structure so long as it is provided with a light source emitting laser light while sweeping its wavelength. For example, the light source may be of various tunable lasers.

The low coherence light may be either visible light or invisible light. When the low coherence light is visible light, the control means may control the light source unit to emit both the laser light and the low coherence light in the image obtaining mode and to emit only the low coherence light in the measurement initiating position adjusting mode.

Further, the interference light detecting means may detect an interference light by the low coherence light as interferogram or a beat signal in the measurement initiating position adjusting mode. When the interference light detecting means detects the interference light as a beat signal, a phase modulation means which gives a frequency difference between the measuring light and the reference light is provided and the control means drives the phase modulation means in the image obtaining mode.

In accordance with the optical tomography system of the present invention, since the control means controls the light source unit and the tomographic image obtaining means so that a laser beam is emitted and the tomographic image signal is obtained by the tomographic image obtaining means on the basis of the interference light by the laser beam in the image obtaining mode, while controls the light source unit and the tomographic image obtaining means so that light is emitted and the tomographic image signal is obtained by the tomographic image obtaining means on the basis of the interference light by the low coherence light in the measurement initiating position adjusting mode, the time required for the signal processing to detect the measurement initiating position can be shortened and adjustment of the measurement initiating position can be carried out in a short time when the position in which a tomographic image is to be obtained is set in the measurement initiating position adjusting mode by obtaining the tomographic image and identifying the position of the object by a so-called TD-OCT measurement by the use of the interference light by the low coherence light not measuring the distance to the object by the use of the interference light by the laser light.

Further, when the control means controls the optical path length adjusting means so that the optical path length difference between the reference light and the measuring light is in an interference light generating region in the measurement initiating position adjusting mode, the optical path length adjustment can be automatically carried out, whereby the tomographic image signal can be efficiently obtained and the measurement initiating position can be surely adjusted.

Further, when the low coherence light is visible light, and the control means controls the light source unit to emit the laser light and the low coherence light in the image obtaining mode and to emit only the low coherence light in the measurement initiating position adjusting mode, since the low coherence light functions as the guiding light (aiming light) in the image obtaining mode, the measured part where a tomographic image is obtained can be easily checked on the basis of the low coherence light.

Further, when a phase modulation means which gives a frequency difference between the measuring light and the reference light is further provided and the control means drives the phase modulation means in the image obtaining mode, the interference light detecting means can detect the interference light as a beat signal that varies in intensity at the frequency difference, whereby the time required for adjustment of the measurement initiating position can be further shortened.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an optical tomography system in accordance with a preferred embodiment of the present invention,

FIG. 2 is a view showing fluctuation in wavelength of the laser beam output from the light source unit of FIG. 1,

FIG. 3 is a schematic diagram showing an optical tomography system in accordance with another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the optical tomography system of the present invention will be described in detail with reference to the drawings, hereinbelow. FIG. 1 is a schematic diagram that illustrates an optical tomography system in accordance with a preferred embodiment of the present invention. The optical tomography system 1 of this embodiment is for obtaining a tomographic image of an object of measurement such as a living tissue or a cell in a body cavity by measuring the SS-OCT. The optical tomography system 1 of this embodiment comprises: a light source unit 10 which emits a laser beam L or a low coherence light beam L10; a light dividing means 3 which divides the laser beam L or the low coherence light beam L10 emitted from the light source unit 10 into measuring light beam L1 and reference light beam L2; an optical path length adjusting means 20 which adjusts the optical path length of the reference light beam L2 divided by the light dividing means; a probe 30 which guides to the object S to be measured the measuring light beam L1 divided by the light dividing means 3; a multiplexing means 4 for multiplexing a reflected light beam L3 from the object S when the measuring light beam L1 is irradiated onto the object S from the probe 30, and the reference light beam L2; an interference light detecting means 40 for detecting interference light beam L4 of the reflected light beam L3 and the reference light beam L2 which have been multiplexed by the multiplexing means 4; and an image obtaining means 50 which detects intensities the measuring light L1 in positions in the direction of depth of the object by carrying out frequency-analysis on the interference light L4 detected by the interference light detecting means 40 and obtains a tomographic image of the object S.

The light source unit 10 comprises a laser 10A which emits a laser beam L while sweeping its wavelength and a low coherence light source 10B which emits a low coherence light beam L10. The laser 10A comprises a semiconductor optical amplifier (a semiconductor gain medium) 11 and an optical fiber FB10 connected to the semiconductor optical amplifier 11 at opposite ends thereof. The semiconductor optical amplifier 11 emits weak spontaneous light to one end of the optical fiber FB10 in response to injection of a drive current and amplifies light input from the other end of the optical fiber FB10. When a drive current is supplied to the semiconductor optical amplifier 11, a pulse-like laser beam L is emitted to the optical fiber FB1 from a resonator formed by the semiconductor optical amplifier 11 and the optical fiber FB10.

Further, an optical divider 12 is connected to the optical fiber F10 and a part of the light beam propagated through the optical fiber FB10 is emitted from the optical divider 12 toward the optical fiber FB11. Light emitted from the optical fiber FB11 travels through the collimator lens 13, the diffraction grating 14 and the optical system 15 and is reflected by the rotating polygon mirror 16. The reflected light is returned to the optical fiber FB11 by way of the optical system 15, the diffraction grating 14 and the collimator lens 13.

The rotating polygon mirror 16 rotates in the direction indicated by arrow R1, to vary the angle of each reflective surface thereof with respect to the optical axis of the optical system 15. Thereby, only a light beam having a specific frequency, from among the light spectrally split by the diffraction grating 14, is returned to the optical fiber FB11. The frequency of the light beam that reenters the optical fiber FB11 is determined by the angle formed by the optical axis of the optical system 15 and the reflective surface of the rotating polygon mirror 16. Light which comprises a specific frequency band and enters the optical fiber. FB1 enters the optical fiber FB10 from the optical divider 12, and as a result, a laser beam L comprising a specific frequency band is emitted to the optical fiber FB1. Accordingly, when the rotating polygon mirror 16 rotates in the direction indicated by arrow R1 at a constant speed, the wavelength of the light beam which reenters the optical fiber FB11 is swept at a period as shown in FIG. 2. As a result, a laser beam L which is swept in its wavelength at a period is emitted from the light source unit 10 toward the optical fiber FB1.

Whereas, the low coherence light source 10B emits low coherence light beam L10 such as, for instance, SLD (super luminescent diode) or ASE (amplified spontaneous emission). The low coherence light source 10B propagates the low coherence light beam L10 through the optical fiber FB10 by way of the fiber optic coupler 2.

The light dividing means 3 of FIG. 1 comprises, for instance, a 2×2 fiber optic coupler and divides the laser beam L or the ASE light beam L10 led thereto by way of the optical fiber FB1 from the light source unit 10 into the measuring light beam L1 and the reference light beam L2. The light dividing means 3 is optically connected to two optical fibers FB2 and FB3, and the measuring light beam L1 is propagated through the optical fiber FB2 while the reference light beam L2 is propagated through the optical fiber FB3. In FIG. 1, the light dividing means 3 also functions as the multiplexing means 4.

The probe 30 is optically connected to the optical fiber FB2 and the measuring light beam L1 is guided to the probe 30 from the optical fiber FB2. The probe 30 is inserted into a body cavity, for instance, through a forceps port by way of a forceps channel and is removably mounted on the optical fiber FB2 by an optical connector OC.

The optical path length adjusting means 20 is disposed on the side of the optical fiber FB3 radiating the reference light beam L2. The optical path length adjusting means 20 changes the optical path length of the reference light beam L2 in order to adjust the tomographic image obtaining area and comprises a collimator lens 21 and a reflecting mirror 22. The reference light beam L2 radiated from the optical fiber FB3 is reflected by the reflecting mirror 22 after passing through the collimator lens 21 and reenters the optical fiber FB3 through the collimator lens 21.

The reflecting mirror 22 is disposed on a movable stage 23 which is moved in the direction of arrow A by a mirror moving means 24. In response to movement of the movable stage 23 in the direction of arrow A, the optical path length of the reference light beam L2 is changed.

The multiplexing means 4 comprises a 2×2 fiber optic coupler, and multiplexes the reference light beam L2 which has been changed in its optical path length and shifted in its frequency by the optical path length adjusting means 20 and the reflected light beam L3 from the object S to emit the multiplexed light beam toward an interference light detecting means 40 by way of an optical fiber FB4.

The interference light detecting means 40 detects interference light beam L4 of the reflected light beam L3 and the reference light beam L2 which have been multiplexed by the multiplexing means 4 and comprises, for instance, a photodiode. The image obtaining means 50 obtains a tomographic image of the object S by carrying out frequency analysis on the interference light beam L4 detected by the interference light detecting means 40. Then the tomographic image is displayed in a display 60. In the embodiment shown in FIG. 1, an optical detector 40a which detects the intensity of the laser light beam L branched from an fiber optic coupler 2 of the optical fiber FB1 and an optical detector 40b which detects the intensity of interference light beam L4 are provided and the interference light detecting means 40 has a function of adjusting the balance of the intensity of the interference light beam L4 on the basis of the output of the optical detector 40a. This function suppresses unevenness in the light intensity by the frequency and permits to obtain a clearer image. The light detecting means 40 has such a spectral sensitivity that it can detect both the wavelength band of the laser beam L and the wavelength band of the. ASE light beam L10.

Here, detection of the interference light beam L4 in the interference light detecting means 40 and image generation in the image obtaining means 50 will be described briefly. Note that a detailed description of these two points can be found in M. Takeda, “Optical Frequency Scanning Interference Microscopes”, Optical Engineering Contact, Vol. 41, No. 7, pp. 426-432, 2003.

When the measuring light beam L1 is projected onto the object S, the reflected light L3 from each depth of the object S and the reference light L2 interfere with each other with various optical path length difference l. When the light intensity of the interference fringe at this time versus each optical path length difference is assumed to be S(l), the light intensity I(k) detected in the interference light detecting means 40 is expressed by the following formula.
$\begin{matrix} I (k) = \int_{0}^{\infty} S (l) [l + \cos (kl)] ⅆ l & (1) \end{matrix}$

wherein k represents the wave number and l represents the optical path length difference. Formula (1) may be considered to be given as an interferogram of a frequency range having a wave number of ω/c (k=ω/c) as a variable. Accordingly, a tomographic image is obtained by obtaining in the image obtaining means 50 information on the distance of the object S from the measurement initiating position and information on the intensity of reflection by carrying out frequency analysis by Fourier-transform on the spectral interference fringes detected by the interference light detecting means 40 and determining the intensity S(l) of the interference light beam L4. The tomographic image thus generated is displayed by a display 60.

Operation of the optical tomography system 1 having a structure described above will be described with reference to FIGS. 1 and 2, hereinbelow. When a tomographic image is to be obtained, the optical path length is first adjusted by moving the movable stage 23 in the direction of the arrow A so that the object S is positioned in the measurable area. The laser beam L sweeping the wavelength at a period is subsequently emitted from the light source unit 10 and the laser beam L is divided into the measuring light beam L1 and the reference light beam L2 by the dividing means 3. The measuring light beam L1 is led by the optical probe 30 into a body cavity and is projected onto the object S. Then the reflected light beam L3 from the object S and the reference light beam L2 reflected by the reflecting mirror 22 are multiplexed, and the interference light beam L4 of the reflected light beam L3 and the reference light beam L2 is detected by the interference light detecting means 40. A tomographic image is obtained by carrying out frequency analysis on a signal of the detected interference light beam L4 in the image obtaining means 50. In the optical tomography system 1 where a tomographic image is obtained by SS-OCT measurement, image information in the position in the direction of depth is obtained on the basis of the frequency and the intensity of the interference light beam L4, and the movement of the reflecting mirror 22 in the direction of arrow A is employed to adjust the measurement initiating position.

In the case where the measurement initiating position is adjusted by moving the reflecting mirror 22 in the arrow A, steps of first moving the reflecting mirror 22, carrying out detection of the reflected light beam L4 when the reflecting mirror 22 is in the position and signal processing such as frequency-analysis on the detected reflected light beam L4, and thereafter readjusting the position of the reflected mirror 22 is necessary. That is, what kind of interference light beam is detected in the new position of the reflecting mirror cannot be known until the signal processing is carried out, whereby adjustment of the measurement initiating position requires a long time.

Accordingly, in the optical tomography system of FIG. 1, there is provided a control means 70 which switches between a measurement initiating position adjusting mode where the position in which a tomographic image is to be obtained is adjusted in the direction of depth of the object S and an image obtaining mode where an image of the object S is obtained so that the system is switched to the image obtaining mode after the position in which a tomographic image is to be obtained is adjusted in the measurement initiating position adjusting mode and a tomographic image is obtained. The control means 70, in the measurement initiating position adjusting mode, controls the light source unit 10 to emit only the low coherence light beam L10 and controls the interference light detecting means 40 and the image obtaining means 50 to effect the TD-OCT measurement where the direction of depth of measurement changes in response to movement of the reflecting mirror 22.

Specifically, a phase modulating means 25 such as a piezoelectric element which shifts the frequency of the reference light beam L2 is provided in the optical fiber FB3. In the measurement initiating position adjusting mode, the control means 70 drives the phase modulating means 25 and controls so that the interference light detecting means 40 and the image obtaining means 50 detect the interference light beam L4 by the low coherence light beam L10 by heterodyne detection. Thereby the low coherence light beam L10 emitted from the light source unit 10 is divided into the measuring light beam L1 and the reference light beam L2 by the light dividing means 3, and the reflected light beam L3 from the object S is multiplexed with the reference light beam L2 by the multiplexing means 4 to generate the interference light beam L4. In the interference light detecting means 40, a beat signal which repeats strength and weakness at the frequency difference between the reflected light beam L3 and the reference light beam L2 is detected as a signal of the interference light beam L4 when the optical path lengths of the measuring light beam L1 and the reference light beam L2 are equal to each other. The image obtaining means 50 obtains a tomographic image signal from the interference light beam L4. As the optical path length is changed by the optical path length adjusting means 20, the optical path length difference between the measuring light beam and the reference light beam changes and when the optical path lengths of the measuring light beam and the reference beam light come to conform to each other, the beat signal is detected. Accordingly, the measurement initiating position is adjusted by adjusting the position of the reflecting mirror 22 in the optical path length adjusting means 20.

The optical path length adjusting means 20 may be arranged to cause the control means to automatically adjust the optical path length at this time. At this time, the optical path length adjusting means 20 is controlled so that the optical path length difference between the reference light beam L2 and the measuring light beam L1 is in an interference light generating region. The “interference light generating region” means a region where such an interference that the optical path length difference Al between the measuring light beam L1 and the reference light beam L2 is smaller than the coherence length takes place.

After the adjustment of the measurement initiating position, the control means 70 switches from the measurement initiating position adjusting mode to the image obtaining mode and a tomographic image is obtained. At this time, the control means 70 controls so that the wavelength fluctuating laser beam L is emitted from the light source unit 10 and controls the interference light detecting means 40 to detect the interference light beam L4 on which the reflection information in the positions in the direction of depth is superposed. The control means 70 stops the low coherence light source 10B and the phase modulating means 25. Then the image obtaining means 50 obtains a tomographic image on the basis of the interference light beam L4 detected by the interference light detecting means 40.

By the SS-OCT measurement, where it is not necessary to move the reflecting mirror 22 to obtain a tomographic image, a tomographic image can be obtained at a higher speed than by the TD-OCT measurement. However, the TDOCT measurement is wider than the SD-OCT measurement in the measurable range. On the other hand, the tomographic image need not be of a high resolution when the measurement initiating position is adjusted. Accordingly, by detecting the object to adjust the optical path length by the TD-OCT measurement in the measurement initiating position adjusting mode, the object S can be easily imaged in a tomographic image and the optical path length can be adjusted simply at high speed.

Though only the laser light beam L is emitted in the above embodiment in the image obtaining mode, the low coherence light beam L10 in the form of visible light may be emitted together with the laser light beam L and the interference light detecting means 40 may detect only the interference light L4 based on the laser light beam L. At this time, the low coherence light beam L10 functions as the guiding light. Accordingly, when the probe 30 is inserted into a body cavity, the position of the probe 30 can be known on the basis of the guiding light. When the interference light detecting means 40 has such a spectral sensitivity that it cannot detect the wavelength band of the low coherence light beam L10 which is visible light, the interference light detecting means 40 may be changed to that formed by the photodiodes or the like suitable for the wavelength band of the low coherence light beam L10 when the measurement initiating position adjusting mode and the image obtaining mode are switched.

Though, in the measurement initiating position adjusting mode in the above embodiment, the interference light beam L4 by the low coherence light beam L10 is detected as a beat signal, the interference light beam L4 may be detected as an interferogram by not providing the phase modulating means 25 in the optical path of the reference light beam L2 (e.g., the optical fiber FB3) as shown in FIG. 3.

Further, though the optical path length adjusting means 20 adjusts the optical path length of the reference light beam L2 in FIG. 1, the optical path length adjusting means 20 may adjust the optical path length of the measuring light beam L1. In this case, the above said optical path length adjusting means 20 is interposed, for instance, in the optical fiber FB2 for guiding the measuring light beam L1 and the mirror in the optical fiber FB3 is fixed.

In accordance with the above embodiments, since the optical tomography system 1 is provided with the control means 70 which switches between a measurement initiating position adjusting mode in which the position in the direction of depth of the object S in which tomographic image signal is to be obtained is adjusted and a tomographic image obtaining mode in which a tomographic image of the object S is to be obtained and controls the light source unit 10 so that a laser light beam L is emitted from the laser 10A in the image obtaining mode, and a low coherence light beam L10 is emitted from the low coherence light source 10B in the measurement initiating position adjusting mode, the measurement initiating position can be adjusted efficiently and simply from a tomographic image.

Optical tomography system

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