WO2013081902A1

WO2013081902A1 - System and method for improving image quality in vivo oct imaging

Info

Publication number: WO2013081902A1
Application number: PCT/US2012/065991
Authority: WO
Inventors: Jianhua Wang; Michael Renxun WANG
Original assignee: University Of Miami
Priority date: 2011-11-30
Filing date: 2012-11-20
Publication date: 2013-06-06

Abstract

A method and system for increasing scan depth in optical coherence tomography (OCT) imaging. The method and system effectively double OCT scanning depth as seen in dual-OCT systems, without the cost. The advantage is realized by either by including two or more reference mirrors to a single reference arm to increase the optical distance or by using a means for adjusting the optical distance of a reference arm having a single mirror (such as a translation stage, fiber stretching device, or fiber delay line). The system and method may be used in any SD-OCT or SS-OCT system without necessitating the addition of costly dual-OCT systems.

Description

SYSTEM AND METHOD FOR IMPROVING IMAGE QUALITY IN VIVO

OCT IMAGING

FIELD OF THE INVENTION

The present invention relates to a method and system for increasing scan depth of in vivo and in vitro optical coherence tomography imaging.

BACKGROUND OF THE INVENTION

Optical coherence tomography (OCT) is a non-contact and non-invasive imaging method that has been widely used for in vivo imaging in the field of ophthalmology. Two broad categories of OCT techniques are time-domain OCT and spectral domain OCT. Time-domain OCT requires the axial translation of the reference arm to obtain a reflectivity profile of the sample, with each pixel of image depth requiring movement of a reference arm of the OCT system. Spectral domain OCT (SD-OCT), on the other hand, allows the reference arm to remain in a single position, with the scan depth being calculated not by the distance of the reference arm, but by performing a Fourier-transform on the acquired data. The use of SD-OCT

(also referred to as Fourier-domain (FD) OCT) eliminates time-consuming reference arm motion.

The SD-OCT is based on a spectrometer containing a camera that collects the spectrum of light reflected from the eye and is much more sensitive than traditional time-domain OCT. SD-OCT has been used to image the anterior and posterior segments of the eye, including the tear film, cornea, crystalline lens, vitreous humour and membrane, and retina. The fast scanning speed enables a rapid scan of the eye in real time for structure and function, such as blood flow. Recently, the use of OCT has been demonstrated in imaging molecules in active and passive ways with or without the aid of scattering media in diagnosis and monitoring disease progress in humans and animals. Molecular imaging using OCT is showing promise in the diagnosis of ocular cancers.

Another type of spectral domain OCT called swept-source OCT (SS-OCT) uses a light source with changing wavelengths and a synchronized photodetector to perform spectral signal acquisition instead of a spectrometer used in SD-OCT. The imaging functionality of SS-OCT is similar to SD-OCT. Both SS-OCT and SD-OCT techniques offer a sensitivity advantage over the TD-OCT technique, thus producing clearer images with less noise.

However, all SD-OCT and SS-OCT systems have scan depth limitations. Present OCT systems include a sample arm (optical pathway between a light source and a tissue or item of interest, or "sample") and a reference arm (optical pathway between a light source and a reference mirror). In order to increase the scan depth of these systems, a second reference arm must be used, which necessitates the use of costly duplicate system components. The present invention allows for not only the acquisition of high-quality images typical for SD-OCT and SS-OCT systems, but also imaging at increased scan depths without using additional components (such as light sources, photodetectors, and/or spectrometers).

SUMMARY OF THE INVENTION

The present invention advantageously provides a method and system for increasing the scan depth in in vivo and in vitro SD-OCT and SS-OCT imaging. The method and system may address the limitation of decreased image quality with increased scan depth without the incorporation of an additional spectrometer, photodetector, light source, and optical scanning hardware on the sample arm. The advantage is achieved by mimicking the functionality of a second reference arm, by altering the reference arm length (or optical distance). This may be achieved either by including two or more reference mirrors to a single reference arm to increase optical distance or by using a means for adjusting the optical distance of a reference arm having a single mirror. Therefore, the system and method may be used in any SD- OCT and/or SS-OCT system without necessitating the addition of costly dual-OCT systems.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: FIG. 1A and IB show methods for increasing the optical coherence tomography (OCT) scan depth in accordance with the principles of the invention;

FIG. 2A shows a schematic representation of an SD-OCT system having the functionality of two reference arms through the use of two reference mirrors;

FIG. 2B shows a schematic representation of an SS-OCT system having the functionality of two reference arms through the use of two reference mirrors;

FIG. 3 shows a schematic representation of an OCT system having the functionality of two reference arms through the use of fast mirror repositioning;

FIG. 4 shows a schematic representation of an OCT system having the functionality of two reference arms through the use of fiber length stretching;

FIG. 5 shows a schematic representation of an OCT system having the functionality of two reference arms through the use of optical fiber delay line; and

FIG. 6 shows the image enhancement that is possible when doubling OCT scan depth using the system and methods described herein.

DETAILED DESCRIPTION OF THE INVENTION

The functionality of a dual-OCT system, including increased scan depth, may be achieved without the need for an additional light source, photodetector, and spectrometer. This functionality may be achieved by increasing the reference arm length, including through the use of multiple reference mirrors, fast mirror repositioning, fiber length stretching, and optical fiber delay line. The method used may depend on the desired speed, stability, and ease of use.

As used herein, the term "arm" of the OCT system refers to, in general, an optical path along which light from a light source travels. The "reference arm" is the optical path between the light source and one or more reflective elements (in the case of the reference arm, referred to as "reference mirrors"). Similarly, the "sample arm" is the optical path between the light source and tissue or other item to be visualized.

As used herein, the term "mirror" refers to any reflective element capable of reflecting light.

As used herein, the term "optical distance" or "optical path length" refers to the product of the geometric length of the path light follows (for example, a light transmission pathway) and the index of refraction of the medium through which it propagates (such as an optical fiber or air). For example, the optical distance of the reference arm is the length of the path light from the light source travels to a reference mirror. As described herein, the optical distance of the reference arm may be altered using various methods (using the same medium), such as the use of more than one reference mirror, or a single mirror positionable using a translation stage, fiber stretching device, or fiber delay line.

As used herein, the term "A-scan" (axial depth scan) refers to a reflectivity profile of tissue or other sample of interest, which contains information about the spatial dimensions and location of structures within the sample. A "B-scan" may be generated from a series of A-scans by laterally combining the series of A-scans.

Referring now to FIGS. 1A and IB, flow charts of two embodiments of a method for increasing OCT scan depth are shown. The general method involves lengthening the path of a reference arm light beam to generate different images. The OCT system may be programmed to record data (scan the sample) at a predetermined scan depth that does not change during the procedure (Step 1). To obtain images at depths different than the predetermined scan depth, the reference arm distance is increased using the means described herein. Referring now to FIG. 1A, during the first OCT scan, the reference arm light may be directed from the light source along the reference arm having a zero delay line and to a first reference mirror having a first optical pathway length from the light source to accomplish a first series of A-scans or record a first series of images (Step 2). If using a SD-OCT, images set by the Fourier transform computation include one image on top of the zero delay line and another below the zero delay line. One such image is used while the other is discarded. The first series of retained A-scans may then be combined to generate a first B-scan image.

During the second OCT scan (Step 3), a beam-switching element (such as a synchronized galvanometer, an optical chopper, a dial with a lens, an acousto-optic switch, an electro-optic switch, a magneto-optic switch, a thermo-optic switch, a MEMS (micro-electro-mechanical-system) based micro mirror switch, or a fast motorized translation stage) may be used to switch the reference arm light beam to be directed along the reference arm to a second reference mirror having a second optical pathway length from the light source to accomplish a second series of A-scan measurements. The second optical pathway length is greater than the first optical pathway length, and may be chosen to correspond to a sample depth that is approximately double that of the first optical pathway length. However, the second optical pathway length may be chosen to correspond to a sample depth that is greater than but less than double that of the first optical pathway length. The second series of A-scans may then be combined to generate a second B-scan image. The images (either A-scans or B-scans) of Steps 2 and 3 may be combined to generate a composite image that has an effective scan depth that is approximately the sum of the depth of each image (for example, up to approximately twice the predetermined scan depth) (Step 4). Additionally, any number of reference mirrors may be used to increase the reference arm length and achieve the desired scan depth. For example, it may be possible to image a patient's entire eye, from cornea to retina, using four reference mirrors. In this example, combined imaging depth using these four reference mirrors may be between approximately 30 mm and approximately 40 mm. In general, the more reference mirrors that are used to increase the reference arm length, the deeper the scan depth that may be achieved.

The method of FIG. IB uses the same principal as that of FIG. 1 A. That is, the method of FIG. IB may achieve a greater scan depth without adjusting the predetermined or programmed scan depth of the OCT system (Step 1) by increasing the reference arm length. In the method shown in the flow chart of FIG. IB, first and second scans (Steps 2 and 4) are accomplished not by redirecting the light from a first reference mirror to a second reference mirror, but by repositioning a single reference mirror from a first position to a second position (Step 3). For example, during the second OCT scan, an element such as a motorized translation stage may be used to physically reposition the reference mirror from a first optical distance (optical path length) to a second optical distance to accomplish a second series of A- scan measurements. Alternatively or additionally, the optical path length of the reference arm may be altered using a fiber stretching device (such as is shown in FIG. 4).

Alternatively or additionally, the light from the light source may be directed along a second optical path that includes a fiber delay line (such as is shown in FIG. 5). A first scan depth image may be generated when the reference mirror is at a first optical distance (or reference arm distance), and a second scan depth image may be generated when the reference mirror is at a second optical distance (or reference arm distance). The second optical distance is greater than the first optical distance. As discussed in FIG. 1A, the first and second series of A-scans may then be combined to generate a first and second B-scan image, respectively. The images (either A-scans or B-scans) of Steps 2 and 4 may be combined to generate a composite image that has an effective scan depth that is up to approximately twice the predetermined scan depth (Step 5).

Referring now to FIGS. 2A and 2B, schematic representations of an SD-OCT system 10 and SS-OCT system 10, respectively, having two reference mirrors 12, 14 is shown. The system 10 of FIG. 2A may include optical fiber 15, a light source 16 (for example, a laser or a low-coherence superluminescent diode (SLD)), a beam splitter 18 (for example, a 50:50 fiber coupler), a reference arm 20, and a sample arm 22. As a non-limiting example, the sample to be scanned may be an eye, as shown in FIGS. 2A and 2B. The OCT system 10 may further include an optical isolator 24, one or more polarization controllers 26, diffraction grating 28, one or more lenses 30, a line scan camera 32 (for example, a having a charge coupled device (CCD), a complementary metal-oxide-semiconductor (CMOS), or InGaAs camera), a digital/analog converter 33, a digital signal processing unit 34 (for example, being located within or integrated with a computer 36), and a video camera 37. The diffraction grating 28, line scan camera 32, and one or more lenses 30 may together make up a spectrometer 38 in the SD-OCT system 10. The system 10 may also include a power source, one or more user input devices, one or more video cameras, one or more monitors, cables, and one or more control units (not shown). The SS- OCT system 10 of FIG. 2B may include the same components as the SD-OCT system 10 of FIG. 2A, except that a photodetector 39 instead of a spectrometer 38 may be in communication with the signal processing unit 34 and/or computer 36.

Continuing to refer to FIGS. 2A and 2B, the reference arm 20 includes two reference mirrors 12, 14, although any number of reference arm mirrors may be used (as described in FIG. 1A). The reference arm 20 may also include one or more lenses 30, one or more filters 40 (such as a neutral density filter), a dispersion compensator 42, a switching mirror 44, and a synchronized galvanometer driver 46. However, different types of beam-switching elements other than a galvanometer driver 46 and switching mirror 44 may be used, such as an optical chopper, a dial with a lens, an acousto-optic switch, an electro-optic switch, a magneto-optic switch, a thermo-optic switch, a MEMS (micro-electro-mechanical-system) based micro mirror switch, or a fast motorized translation stage. The sample arm 22 may include one or more lenses 30 and an X-Y scanner 48.

Continuing to refer to FIG. 2, the reference mirrors 12, 14 may allow the system 10 to have the functionality of two reference arms. Unlike a dual-OCT configuration, which requires complex reference arm modulation and computation recovery of the OCT image, the present system may avoid the use of, for example, two light sources and two spectrometers. Likewise, scan depth may be increased without the need for phase unwrapping of the normally discarded image. As described in FIG. 1A, a beam-switching element may be used to selectively direct the reference arm 20 light along the optical reference arm to a first reference mirror 12 having a first optical pathway length to a second reference mirror 14 having a second optical pathway length to accomplish two series of scans.

Referring now to FIG. 3, an OCT system 10 having the functionality of two reference arms through the use of fast mirror repositioning is shown. In the embodiment shown in FIG. 3, the OCT system 10 includes only one reference mirror 50. Light travels from a light source 16 (not shown in FIG. 3) along the reference arm optical pathway 20 (such as through an optical fiber 15) and then through a lens 30, creating a collimated beam of light 52. The collimated beam of light 52 then travels to the reference mirror 50. Generally, the reference mirror 50 may be physically repositioned, such as by a translation stage 54. Repositioning the reference mirror 50 will adjust the optical path length (optical distance) of the reference arm, without changing the medium through which the light travels, thereby adding the functionality of a dual-OCT system. For example, the scan depth may be doubled using the method as described in FIG. IB above.

Referring now to FIG. 4, an OCT system 10 having the functionality of two reference arms through the use of fiber length stretching is shown. Fiber length stretching may be used to adjust the optical path length (optical distance) of the reference arm. In this system, an optical fiber 15 may be wound one or more times around a fiber stretching device 58, such as a piezoelectric modulator, which may expand and contract, thereby stretching the optical fiber 15. This stretching may induce a temporal delay on the light within, thereby affecting the optical distance. An increased scan depth may be achieved using the method as described in FIG. IB above.

Referring now to FIG. 5, an OCT system 10 having the functionality of two reference arms through the use of optical fiber delay line is shown. Similar to the system of FIG. 4, the induced delay allows for increased scan depth by increasing the optical path length (optical distance). In general, optical fiber delay as referred to herein may include one or more beam switching elements 60 (such as a MEMS (micro-electro-mechanical-system) based micro mirror switch) to adjust the optical distance by redirecting light from the light source along a second optical path (fiber delay line). For example, beam switching elements may be used to redirect light along either a straight (shorter) optical path 62 or a longer optical path 64, such as a path that includes one or more loops 66 of optical fiber 15 to extend the length of the optical path 64. However, other means for diverting light may also be used. Whether light is directed along the first optical path or the second optical path (fiber delay line), the light will still be directed to the reference mirror (as shown in FIG. 5). An increased scan depth may be achieved using the method as described in FIG. IB above.

Referring now to FIG. 6, the image enhancement that is possible when doubling OCT scan depth using the system and method described herein is shown. This non-limiting example is merely illustrative of a composite image that may be obtained using the methods and systems described herein. It will be understood that scans and composite images of different depths may be obtained. Using the embodiment shown in FIG. 2, for example, each scan 68A, 68B may be performed with a depth of 7.3 mm. The first scan 68A may be obtained using a first optical path length (for example, using a first reference mirror or a reference mirror in a first position). Likewise, the second scan 68B may be obtained using a second optical path length (for example, using a second reference mirror, a reference mirror in a second position, or an optical path length altered using any of the means described herein). The scan depth is effectively doubled to give a composite image 70 of 14.6 mm, provided that the length of the optical path to each of the two reference mirrors 12, 14 is such that the two depth pictures can be combined to form such a larger depth OCT image.

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.

Claims

What is claimed is:

1. A method of increasing scan depth of optical coherence tomography imaging, comprising:

providing an optical coherence tomography system, the system including a light source and an optical pathway along which light from the light source is directed, the optical pathway having an adjustable length;

performing a first scan at a first optical pathway length; and

adjusting the optical pathway length to a second optical pathway length and performing a second scan at the second optical pathway length.

2. The method of Claim 1, wherein the optical pathway includes a first reflective element, a second reflective element, and a beam- switching element.

3. The method of Claim 2, wherein the beam switching element includes a switching mirror and a synchronized galvanometer.

4. The method of Claim 2, wherein the beam switching element is selected from the group consisting of an optical chopper, a dial with a lens, an acousto-optic switch, and electro-optic switch, a magneto-optic switch, a thermo-optic switch, a micro- electro-mechanical mirror switch, and a translation stage.

5. The method of Claim 2, wherein the first optical pathway length is the distance the light travels from the light source to the first reflective element, and the second optical pathway length is the distance the light travels from the light source to the second reflective element.

6. The method of Claim 5, wherein the system further comprises an optical fiber positioned between the light source and first and second reflective elements.

7. The method of Claim 2, wherein the adjustment of the optical pathway between the first optical pathway length and the second optical pathway length is achieved by activating the beam switching element to selectively direct light from the light source to either the first reflective element or the second reflective element.

8. The method of Claim 1, wherein the second optical pathway length is greater than the first optical pathway length.

9. The method of Claim 1, wherein the light source is a low-coherence light source.

10. The method of Claim 9, wherein the light source is a superluminescent diode.

11. The method of Claim 1, further including combining the first scan and second scan to generate a composite image.

12. The method of Claim 11, wherein the system is programmed to perform scans at a predetermined depth.

13. The method of Claim 12, wherein the composite image shows a scan depth that is approximately twice the predetermined scan depth of the optical coherence tomography system.

14 The method of Claim 1, wherein the adjustment of the optical pathway between a first optical pathway length and second optical pathway length is achieved by adjusting the position of a reflective element using one of a translation stage, a fiber stretching device, and a fiber delay line.

15. The method of Claim 1, wherein the optical coherence tomography system is one of a swept-source optical coherence tomography system or a spectral-domain optical coherence tomography system.

16. A method of increasing scan depth of optical coherence tomography imaging comprising:

providing an optical coherence tomography system, the system including: a light source generating light;

a reference arm; and

a means for adjusting the optical length of the reference arm;

programming the optical coherence tomography system to perform scans at a predetermined depth;

generating a first series of images with the reference arm having a first optical length; increasing the optical length of the reference arm to a second optical length; generating a second series of images with the reference arm having a second optical length.

17. The method of Claim 16, wherein the position of the reference mirror is adjusted using one of a translation stage, a fiber stretching device, a fiber delay line, and use of two or more reference mirrors.

18. The method of Claim 17, wherein the position of the reference mirror is adjusted using a fiber delay line that includes one or more beam switching elements.

19. The method of Claim 18, wherein the one or more beam switching elements are selected from the group consisting of: optical chopper, dial with a lens, acousto- optic switch; electro-optic switch; magneto-optic switch; thermo-optic switch;

MEMS-based micro mirror switch; fast translation stage; and synchronized galvanometer scan mirror.

20. An optical coherence tomography system, the system comprising: a light source;

an optical pathway including a plurality of reflective elements, each reflective element having a different optical pathway length from the light source; and

a beam switching element for selectively directing the light to any of the plurality of reflective elements.