The capture of tomographic images is one of the most essential measurement techniques in biophotonic systems, especially in biomedical applications, such as in the field of ophthalmology or when used in combination with endoscopy, for example for cardiovascular medicine. Other medical applications also include dental or skin tissue examinations or other areas of medicine. Optical coherence tomography (OCT) is a non-contact and non-invasive imaging technique to obtain fine resolution and three-dimensional cross-sectional images of tissue structure on the micron scale (μm), such as the retina, cornea, anterior chamber of eyes, cell imaging, tissue characterization, live blood flow imaging, etc. It avoids the physical cutting of samples, thereby rendering non-invasive in vivo imaging possible (optical biopsy).
Conventional optical coherence tomography (OCT) has recently been accepted in both industry and the laboratory, due to its fine resolution and non-invasive nature. It provides depth information (optical biopsy) and avoids the physical cutting of samples, thereby rendering non-invasive in vivo imaging possible. OCT is able to obtain ˜10 μm resolutions and 2-3 mm imaging depths in highly scattering biological tissues based on low-coherence interferometery and fiber optic technology.
In its most basic form, time-domain OCT (TD-OCT) consists of a Michelson-type interferometer with a focused sample arm beam and lateral-scanning mechanism.
OCT uses this concept by replacing the mirror in the sample arm with the sample 15 to be imaged, which sample has several reflecting structures. The reference arm is then scanned in a controlled manner and the light intensity is recorded on the detector.
The transverse or x-y localization of the sample structure is simpler. The broadband light source beam that is used in OCT is focused to a small spot (on order of few microns) and scanned over the sample.
Fourier-domain OCT provides an efficient way to implement the low-coherence interferometry. Instead of recording the intensity at different locations of the reference mirror, the intensity is recorded as a function of wavelengths or frequencies of the light. The intensity modulations when measured as function of frequency are the spectral interference. The rate of variation of intensity over different frequencies is indicative of the location of the different reflecting layers in the samples. It can be shown that a Fourier transform of spectral interference data provides information equivalent to the one obtained by moving the reference mirror.
There are two common methods of obtaining spectral interference in OCT. One involves using a spectrometer as the detectors and is called Spectral-domain OCT (“SD-OCT”) (
Fourier-domain allows for much faster imaging as all the back reflections from the sample are being measured simultaneously. This speed increment introduced by Fourier-domain OCT opened a whole new arena of applications. Video-rate OCT imaging can be easily obtained using commercial systems.
To afford better diagnostic ability, rapid acquisition rates are necessary to reduce artifacts due to patient motion, to capture fast dynamics and to generate 3D volumetric images within reasonable time constraints. Increases in OCT imaging speed have been achieved with the SS-OCT detection technique.
For a typical SS-OCT system, different reflecting depth would result in different interference frequencies after the signal detector, i.e., the interferometer. Then it is required to perform the Fourier transformation on the interference fringes in order to obtain the tomographic images.
The resolution of the OCT is fully determined by the spectral bandwidth. Thus, to achieve better resolution, larger spectral bandwidth is required. This process is similar to the resolution of the spatial microscope, which is limited by the numerical aperture (NA) of the objective lens. The similarity of these two schemes is connected by the space-time duality, since the Fourier transformation process can be achieved at the focal plane (Fourier plane) of the imaging modality. Fortunately, there are some ingenious imaging modalities in achieving super-resolution in microscopic applications, without increasing the NA, e.g. the stimulated emission depletion (STED) microscopy in the fluorescence imaging. It achieves super-resolution by generating a doughnut-shape de-excitation spot to subtract from the original diffracted-limited spot, such that the remaining area will become much smaller in the spot size.
Inspired by STED microscopy in the spatial domain, as well as the space-time duality, the present inventors discovered that the OCT spatial process can be transform into the temporal domain, and this turns out to be particularly suitable for an OCT system. As a result, of this insight, the present inventors have developed a new method to capture tomography images which they call Phase-inverted sidelobe-annihilated optical coherence tomography (PISA-OCT) in which super-resolution is achieved by suppressing the sidelobe of the original pulse profile. This results in captured images with higher resolution than those achieved with conventional swept-source OCT (SS-OCT).
Phase-inverted sidelobe-annihilated optical coherence tomography (PISA-OCT) is an entirely new scheme, which allows the capture of tomography images (layers) with a higher resolution than the diffraction limit, based on one of the fastest and most promising optical tomography modalities, i.e., swept-source OCT (SS-OCT). For a typical SS-OCT system, since the illuminating light is a swept-source, different reflecting depth would result in different interference frequencies after the interferometer. Then it is required to perform the Fourier transformation on the interference fringes in order to obtain the tomographic images, and its “line width” (or resolution) will be limited by the bandwidth of the laser source deployed. By optically engineering the point spread function (PSF) of one frame into a two-peak (or doughnut) shape, while the other frame is kept with the original Gaussian shape, a super-resolution image can be obtained by the subtraction of these two frames, because the doughnut shape creates a negative value around the real layer. Benefitting from the subtraction, the DC component and the noise level will be suppressed, thus better signal-to-noise ratio (SNR) and detection sensitivity are obtained. In addition to narrowing the resolution of the tomographic layers, the PISA-OCT system also eliminates those ghost fringes introduced by the interference between different sample layers. Unlike the advanced super-resolution technologies in the microscope system (through the spatial domain), this invention achieves super-resolution in a tomography system (through the temporal domain) by PISA-OCT, which will perform way better than the conventional OCT systems available in the market.
By optically engineering the point spread function (PSF) of one frame into a two-peak (or doughnut) shape in the frequency domain, while the other frame is kept as the original Gaussian shape, a super-resolution image is obtained by the subtraction of these two frames, because the doughnut shape create negative value around the real layer. In the PISA-OCT scheme, only a temporal phase modulation (a stepped π-phase shift on the reference signal) is required in the reference arm, which is a simple and low-cost solution.
The PISA-OCT system makes possible a first generation super-resolution tomography product; or alternatively, it can also provide an upgrade option to the conventional SS-OCT on the reference arm. Furthermore, the current manifestation in the optical domain can be further extended to other electromagnetic wave devices such as those in the terahertz (THz) and microwave frequencies.
The advantages of PISA-OCT include: 1) minimal adjustment on the existing swept-source OCT setup (i.e., by simply introducing a phase modulator in the reference arm); 2) achieving sharper resolution without increasing the required bandwidth of the swept-source; 3) removing ghost fringes introduced by the self-interference between sample layers, similar to the balanced detection technology; and 4) enhancing the sensitivity by suppressing the noise floor. Therefore, the SA-OCT system provides a very simple solution in achieving better tomographic imaging quality, based on the conventional swept-source OCT.
The foregoing and other objects and advantages of the present invention will become more readily apparent when considered in connection with the following detailed description and appended drawings in which like designations denote like elements in the various views, and wherein:
a)-1(d) are diagrams showing the arrangement and different modes of operation of conventional optical coherence tomography;
a)-4(g) show the operational sequence of the PISA-OCT system
a)-5(g) show characteristic waveforms of the PISA-OCT;
a)-6(c) illustrate the ghost imaging feature of the OCT system; and
a)-7(h) compare images from a conventional swept source OCT versus the PISA-OCT for fish eye and a human finger; and
a)-8(h) compare images from a conventional swept source OCT versus the PISA-OCT for orange slices and an onion slices.
Phase-inverted sidelobe-annihilated optical coherence tomography (PISA-OCT) leverages a π-step phase modulation to introduce a two-peak shape in the frequency domain. This two-peak shape causes the system to achieve a sharper resolution than the resolution that is diffraction-limited by the spectral bandwidth. The essential part of PISA-OCT is introducing a phase modulator in the reference arm of a conventional swept-source OCT.
A conventional swept-source OCT, and its working principle is shown in
where c is the light velocity, d is the reflective depth in the sample arm, and Δω is the frequency (or spectral) bandwidth. We can obtain the resolution from Eq. (1), i.e. ROCT=2 ln 2λ2/πΔλ. If the swept bandwidth Δλ=80 nm, and is centered at 1550 nm, its temporal aperture is T0=10 μs, and the single scattered layer sample is delayed by 200 μm. The obtained resolution is 10.6 μm, which matches with the theoretical calculation (12.4 μm). As shown in the bottom of
where D+(x)=exp(−x2)∫0xexp(t2)dt is the Dawson function, and D+(0)=0, its absolute value is shown as a two-peak shape. Similarly, there are three peaks observed in
The experimental setup of the PISA-OCT versus the conventional swept-source OCT is shown in
In order to realize the benefits of the present invention, an electro-optical reference arm is substituted for the pure optical reference arm of the prior art. Thus, the optical signal from coupler 32, instead of using path 1, uses path 2 where it first encounters an optical delay line 36 which helps to balance the timing of the signal with that of the sample arm. The optical signal then engages the reflective pulse modulator 35, which reflects the optical signal and introduces a 180-degree phase inversion in the interference pattern during alternate sweeps of the beam scanner according to its electrical input, which is shown in
The circulator 38 and balanced detector 14 are designed for balanced detection in the OCT system, which helps to improve the detection sensitivity by 6 dB, and to remove the interlayer interferences. Since there is π-phase shift between the two arms of the 50/50 coupler, the two ports of the balanced detector also receive the interference fringes with π-phase shift, thus the subtraction between these two arms will enhance the fringe intensity by 3 dB, and will remove some intensity noise and DC components.
As shown in
f) shows that after Fourier transformation, the single frequency peak with the phase inversion has a two-peak shape for the phase modulated period, while the single frequency peak without the phase modulation or inversion remains a single peak with a Gaussian shape.
This PISA-OCT is first characterized by a single reflective mirror in the reference arm, as shown in
Some bio-samples are measured by the PISA-OCT, and these images are compared with the conventional swept-source OCT, as shown in
Similar to
The following table is a comparison of the PISA-OCT with a commercially available OCT systems, i.e., the Vivolight OCT, the Thorlabs OSC1310V1 and the Thorlabs OCS1300SS.
The comparisons were conducted under the following conditions indicated by the notes in the table:
(a) Control of the scanning range to 3 mm;
(b) 512 A-scans per frame;
(c) Measurements at 10 dB line width;
(d) Since the single line is too narrow (around 1 μm), the resolution is defined as equal to the depth's point separation;
(e) Assume that the refractive index is 1.5;
(f) Sensitivity depends on the background fringes, which is estimated to be around 90 dB;
(g) The 12-dB sensitivity improvement is calculated from the SNR of the roll-off trace, where the noise ground was decreased by around 6 dB.
The Vivolight OCT, a product of Shenzhen Vivolight Medical Device & Technology Co., Ltd of Shenzhen, P.R.C., and the Thorlabs systems are products of Thorlabs Inc. of Newton, N.J., USA. The Vivolight is the base system of the current invention, i.e., the current invention can be used as an add-on module to the system such that the depth resolution and sensitivity of the system are improved by 50% and 13%, respectively.
Advantages of the present invention include:
While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.