WO2012100816A1

WO2012100816A1 - Optical coherence tomography apparatus and method with speckle suppression

Info

Publication number: WO2012100816A1
Application number: PCT/EP2011/050944
Authority: WO
Inventors: Maciej Szkulmowski; Iwona GORCZYNSKA; Daniel SZLAG; Maciej Wojtkowski; Andrzej Kowalczyk
Original assignee: Optopol Technology S.A.
Priority date: 2011-01-25
Filing date: 2011-01-25
Publication date: 2012-08-02

Abstract

The invention relates to an optical coherence tomography apparatus and method. It improves speckle suppression by modifying the scanning trajectory (18) of the probing light beam so that groups of A scans close to the imaged segments (22) can be recorded and averaged. The invention facilitates effective speckle suppression combined with high resolution and little dependence of sample motions during the measurement.

Description

Optical coherence tomography apparatus and method with speckle suppression

The invention relates to apparatus according to the pream^¬ ble of claim 1 and a method according to the preamble of claim 10.

Optical coherence tomography (OCT) is a technique for ex^¬ amination of two or three dimensional structures of samples consisting of partially transparent matter. According to this method, partially coherent light is divided into two portions. One portion is used for illuminating the sample under investigation. The second portion is led through a reference path to be recombined with the light returning from the sample. The recombined light carries an interfer- ence signal containing information about the internal structure of the sample. This information can be retrieved in various ways known to the person skilled in the art. For instance, in an OCT modality known as "Time domain OCT" (TdOCT) , a scannable optical path delay is introduced in the reference path, which delay causes interference fringes to occur when the optical distance of the first and second light portion approximates minimum values. This enables de^¬ termination of the relative distances of scattering struc^¬ tures within the sample (see Huang et al . , Science, Vol. 254, 1991, p. 1178 - 1181) . In another OCT modality, known as "Fourier domain OCT" (FdOCT) , the structural information is retrieved by spectral analysis of the recombined light, e. g. by use of a spectrometer ("Spectral OCT" = SOCT, see Szkulmowska et al . , Journal of Physics D: Applied Physics, Vol. 38, 2005, 2606 - 2611) or by use of a tuned light source ("Swept Source OCT" = SS-OCT, see R. Huber et al . , Optics Express, Vol. 13, 2005, 3513 - 3528).

In order to obtain two dimensional tomograms or three di^¬ mensional images, hereinafter jointly referred to as the OCT image, from the sample, the first portion of light is shaped to a light beam directed on the sample and scanned across the sample along a predetermined trajectory. The ex^¬ traction of depth-resolved information about elements within said sample from the light beam at one scanning po- sition is called A scan. Shifting the position of said light beam across the sample along the scanning trajectory is called B scan, whereby several A scans are recorded along a line. The trajectory may be a single straight or curved line so that a two dimensional tomogram is obtained. A three dimensional image can be obtained by consecutively changing the direction of the light beam so that the beam crosses the sample several times or by interrupting the measurement and resuming it at another position so that several B scans are recorded.

A major problem associated with OCT is the occurrence of speckle patterns caused by the partial coherence of the light source on which the technique is based. As this im^¬ plies a considerable limitation of the resolution of OCT, numerous attempts have been made to reduce the effects of speckle patterns. These attempts can be grouped in software and hardware based speckle reduction techniques. Software solutions are applicable without any modification of the OCT setup. The OCT image is acquired in a conventional way and processed with numerical algorithms, e. g. by axial and/or lateral local averaging, averaging with rotation kernels, filtering with adaptive, median, Lee or other fil- ters, wavelet transformations etc. A major drawback of all these approaches is the requirement of high computational power .

Hardware based speckle reduction techniques are based on recording several OCT images, between which a decorrelation of speckle patterns is introduced by means of controlled variations of the interference signal in time, space, fre^¬ quency, or polarization. The data of these OCT images are averaged to smooth out the speckle patterns. The main chal- lenge of these techniques is to acquire the OCT images in such a way that the speckle patterns differ but the un^¬ avoidable change of the imaged structure is as small as possible. The variations have to be chosen very carefully, because either averaging will not have any effect or it will cause resolution loss.

The hardware based speckle reduction techniques may be sub^¬ divided in serial and parallel techniques. The serial tech^¬ niques utilize subsequent recording of the OCT image data. The most straightforward realization is to acquire several complete OCT images one after another. If the shift of the scanning trajectory between image acquisitions is less than the lateral resolution, speckle patterns will be partially decorrelated but the imaged structure will remain the same within an error the order of magnitude of which is within the resolution. However, the consecutive image acquisitions extend the time needed for one measurement. As a conse- quence, serial techniques are highly susceptible to motions of the sample during the measurement.

In parallel hardware based speckle reduction techniques, several OCT images used for averaging are recorded at the same time. To this end, setups using two incoherent inter^¬ ference signals with light from two light sources with dif^¬ ferent central wavelengths or polarizations have been pro^¬ posed. All these techniques require, however, substantial modifications to the optical setup which add significantly to the complexity and overall cost of such systems.

The object of the invention is to provide an improved speckle reduction technique for OCT which requires little hardware modifications of known OCT setups and has a mini^¬ mum susceptibility to sample motions during the measure^¬ ment. This is achieved by the subject-matter as defined by claims 1 and 10. The dependent claims define further im^¬ provements of the invention.

The use of the terms "optical" and "light" within the con^¬ text of the invention means that the apparatus and method according to the invention operate with electromagnetic ra^¬ diation with wavelengths within the range of 300 nm to 1,700 nm. It follows that infrared radiation outside the visible range is included.

An image according to the invention consists of a two- or three-dimensional set of segments, whereas the first dimen- sion is defined by the direction of a probing light beam through the sample and the other one or two dimensions are defined by an image trajectory across the sample, along which the segment positions are scanned. The direction pointing towards the sample in the first dimension may be called z direction, and the directions perpendicular thereto and across the sample may be called x and y direc^¬ tions. If the image is three-dimensional, it will thus be segmented into segments at (x, y, z) positions. If the im^¬ age is a two-dimensional tomogram taken along a line f (x, y) in the xy plane, it will be segmented into segments at (f(x, y) , z) positions. In this case f (x, y) may be a straight line, e. g. with f (x, y) = x. Segments with iden- tical (x, y) may be regarded as segment sets p(z) . The im^¬ age trajectory determines the order of segment sets p(z) in the (x, y) plane.

As the invention is applicable for any OCT modality, the recording means can be of any kind. In particular, it may be a recording means for SOCT, i. e. it comprises a spec^¬ trometer. As SOCT is a comparatively fast OCT modality and thus little susceptible to sample motions, the invention is particularly advantageous when combined with SOCT.

The image scanning means usually comprises one or two scan^¬ ning mirrors. It may be a single mirror combined with a galvanometric drive so that the mirror pivots about one or two pivoting axes. Alternatively, it may be a combination of two mirrors, each of which pivoting about a different axis. In this case, one or both mirrors may be driven by a galvanometric drive. In case only one of the mirrors is driven by such a drive, the other may be driven by a reso^¬ nant scanner.

The segment scanning means may also comprise a scanning mirror. However, it is desirable that the segment scanning means operates at a higher speed than the image scanning means. So the segment scanning means preferably comprises a scanning device selected from the group of resonant scan^¬ ners, micro-electro-mechanical scanners and acousto-optic scanners. The segment scanning means may be combined with the image scanning means so that the same device carries out image scanning and segment scanning. In such a case it is preferred that this device is selected from the group of resonant scanners, micro-electro-meachanical scanners or acousto-optic scanners. Another mirror of the image scan- ning means may be a galvanometrically driven mirror.

The segment scanning means preferably generates periodic segment trajectories with a frequency of above 3 kHz, more preferably above 3.5 kHz. This reduces the susceptibility to sample motions during the measurement. If this frequency is chosen to be 4 kHz, e. g. with a resonant scanner, a cy^¬ cle will last 0.25 ms. If the sample moves with a speed of 5 mm/s in z direction, as is typical in in vivo eye meas^¬ urements, it will travel only 1.25 ym during this cycle which is less than the typical OCT resolution in the z direction of around 4 ym. If a group of A scans is recorded during a part of such a cycle, said sample motion has al^¬ most no effect. As for sample motions in the x and z directions, the inven^¬ tion' s susceptibility to such disturbances may be reduced by selecting the speed at which the probing light beam travels along the segment trajectory above 1 m/s, prefera^¬ bly above 1.5 m/s, more preferably above 4 m/s . To this end, the segment scanning means is preferably configured for allowing the probing light beam to travel along the segment trajectory at the surface of the imaged region of the sample a speed above 1 m/s, more preferably above 1.5 m/s and most preferably above 4 m/s . The surface of the im^¬ aged region of the sample is the layer with the lowest z values, i. e. the imaged layer which is closest to the ap^¬ paratus. The skilled person will select parameters like frequency of the segment scanning means, location and elon^¬ gation accordingly. Lateral sample motions at tens of mm per second, as typical for in vivo retinal measurements, will hardly have any influence. In practical applications, the sampling rate at which A scans are recorded should be above 40 kHz, e. g. 50 kHz. In most recent experiments by the inventors, sampling rates at 100 kHz and 200 kHz, i. e. above 90 kHz have been used. Further, sinusoidal segment trajectories turned out to be particularly advantageous, especially if all A scans of one group used for averaging are taken along the quasilinear part of the sine curve, i. e. within the -n/4 to n/4 or 3n/4 to 5n/4 phases of the sine curve. The number of A scans in a group was preferably at least 8, e. g. even 16. With these parameters, the goal of effective speckle suppression with high resolution and fast imaging was excellently met.

Moreover, good results were obtained if the distance be^¬ tween adjacent A scans was chosen to be about half the beam diameter, i. e. between 40 % and 70 % of the diameter of the probing light beam in the imaged region of the sample, especially if combined with the above parameters of sinu^¬ soidal segment trajectories, recording within the quasilin^¬ ear parts and/or using at least 8 A scans for a group. Per^¬ fect speckle decorrelation can be obtained for distances of 100 % or higher, but only in trade for resolution loss. In order to maintain a good resolution, the distances should not be higher than 100 %. As the recording means is arranged for recording a group of at least two A scans at different positions along a segment trajectory, the information of these A scans may be used for the decorrelation of speckle patterns. To this end, each segment of the image is computed with an averaging op^¬ eration on the information extracted from a group of A scans recorded along the segment trajectory of that segment position . Advantageously, the segment scanning means and the re^¬ cording means are arranged to perform A scans offside the image trajectory. In this case, multiple A scans in the close neighbourhood of a position corresponding to a seg^¬ ment position may be recorded, which allows to obtain ef- fective speckle suppression and a high resolution along the scanning trajectory at the same time. This renders the in^¬ vention to work very effectively. In this case, it is fur^¬ ther preferred that the segment scanning means and the re^¬ cording means are arranged to record the A scans in such a way that within said groups of A scans, i. e. for the A scans used for one averaging operation, the density of A scans, i. e. the number of A scans per unit length, along the image trajectory is high than perpendicularly thereto. Preferably, the segment scanning means is arranged for mov^¬ ing the probing light beam faster in a direction perpendicular to the image trajectory than it moves in the direc^¬ tion of the image trajectory. This allows for effective speckle decorrelation and high resolution along the image trajectory at the same time. Where value of the speed of the probing light beam is not constant, its average values in the direction perpendicular and parallel to the image trajectory apply in this respect, while changes of direc^¬ tion may be neglected in this regard.

It is further advantageous if the segment scanning means is arranged to superimpose segment trajectories in form of pe^¬ riodic cycles on the probing light beam motion at consecu^¬ tive segment positions. This allows easy control of the beam motion. In this case, it is further advantageous if the recording means is arranged to record A scans in con- secutive cycles at identical relative positions within their cycles. In this case the averaging operation can be performed identically on the A scans of the consecutive cy^¬ cles . Advantageously, the segment scanning means is located up^¬ stream of the image scanning means. The term "upstream" means that it is more distant from the sample than the scanning means. In such a configuration, it is easier to superimpose identical segment trajectories on the motion of the probing light beam along the entire scanning trajec^¬ tory .

It is further preferred that the segment scanning means comprises a scanning device selected from the group of resonant scanners, micro-electro-mechanical scanners and acousto-optic scanners.

Moreover, it is advantageous if the segment scanning means is configured to trigger the A scan recording. This allows for rapid operation. The method according to the invention may be performed with an apparatus according to the invention. Therefore, the aforementioned aspects apply to the method as well. The scanning trajectory is the trajectory along which the probing light beam actually travels. Advantageously, the scanning trajectory comprises points offside the image tra^¬ jectory at which points A scans are recorded. As with the apparatus according to the invention, this renders the method to be very effective. In this case, it is further preferred that within said groups of A scans, i. e. for the A scans used for one averaging operation, the density of A scans, i. e. the number of A scans per unit length, along the image trajectory is high than perpendicularly thereto.

Preferably, the segment trajectories are periodic cycles, e. g. triangular, sinusoidal, or Lissajous curves. This al^¬ lows for easy control of the scanning motion. It is in this case further preferred that A scans along consecutive cy- cles are recorded at identical relative positions within their cycles. In this case the averaging operation can be performed identically on the A scans of the consecutive cy^¬ cles. It is further advantageous if the scanning trajectory between the first and the last A scan belonging to one group describes a monotonous curve with respect to the im^¬ age trajectory. This facilitates a rapid operation.

A group of A scans consists preferably of at least two, more preferably of at least five and most preferably of at least ten A scans taken at different positions.

The scope of invention is not left if the probing light beam is interrupted between A scan operations. Also, the invention may be used for a segment of an image, while other parts of the image are generated in a conventional way. Such a segment would thus represent the "image" as de^¬ fined in the claims.

As an example, an embodiment of the invention will be de^¬ scribed in greater detail hereinbelow with reference to drawings, wherein:

Fig. 1 is a schematic view of an apparatus according

the invention;

Fig. 2 is a diagram illustrating a beam deviation pattern according to the invention;

Fig. 3A shows OCT images recorded without utilizing the in^¬ vention and

Fig. 3B shows OCT images recorded with utilizing the invention.

Partially coherent light is emitted from a light source 1, e. g. a superluminescent diode (SLD) . The light is fed into an optical fibre system 2, along which an optical insulator 3 is located in front of the light source 1 to prevent light from returning into the source. From here, the light proceeds to an optical 2 x 2 coupler 5 which divides the light into two portions. One portion is led though on port into a sample arm 6 and the other portion through another port into a reference arm 7.

In the sample arm 6, the light exits the optical fibre sys^¬ tem 2 to be shaped to a light beam 4 by a collimating lens 8 and reach a resonant scanner 9 acting as segment scanning means. From here, the light is further directed to an xy scanner 10 acting as image scanning means. The xy scanner 10 may consist of a combination of two mirrors, each of them pivoting about an axis with a galvanometric drive. The xy scanner 10 may cause the light beam 4 to scan along an image trajectory across the sample 11, which is the retina of an eye in this example. The z direction for the image to be generated from sample 11 is marked with an arrow, the x and y directions are perpendicular thereto. The light beam 4 does not follow exactly this image trajectory as a motion induced by the resonant scanner 9 is superimposed. It can be seen that the resonant scanner 9 is located upstream the xy scanner 10. The light beam 4 leaving the xy scanner 10 is guided and shaped by an arrangement of lenses 10.1 and 10.2 so that it is pivoted about a point roughly at the ocular lens in or^¬ der to obtain a precise scanning motion of the beam 4 along the retina 11. Likewise, the light beam 4 leaving the reso- nant scanner 9 is guided and shaped by an arrangement of lenses 9.1 and 9.2 so that it reaches the xy scanner 10 al^¬ ways at the same position, but at different angles caused by the motion of the resonant scanner 9. From the retina 11, the light beam 4 travels back the same way along the sample path 6 to enter the coupler 5 and pro^¬ ceed into detection arm 12, where it exits the optical fi^¬ bre system 2 to be collimated by a collimating lens 13 and spectrally decomposed into spectral components by a grating 14. A further lens 15 focuses the spectral components on the sensors of a line array 16, the signals of which are relayed to a computation unit 17. The computation unit 17 is connected to the drives of the resonant scanner 9, the xy scanner 10 and to the light source 1, so that it may control the entire measurement operation. Likewise, the resonant scanner 9 may trigger the A scan recording through the computation unit 17.

The reference arm 7 in this example comprises components known from conventional OCT such as polarisation controllers 7.1, lenses 7.2 and 7.5 for guiding the reference light, a neutral density filter 7.3 for adjusting the in- tensity of the reference light, a dispersion controller 7.4 and a reference mirror 7.6.

Fig. 2 shows the scanning trajectory 18 which an uninterrupted probing light beam follows across the sample (not shown; it is actually the background of the drawing) . Solid points 18.1.1, 18.1.2 etc. mark the locations where A scans are performed. The xy scanner 10 causes the light beam to move along direction 20, while the resonant scanner 9 su^¬ perimposes a perpendicular back and forth motion so the probing light beam actually progresses along a triangular scanning trajectory in the general direction 20. So the segment scanning means is arranged to superimpose periodic, in this example triangular, segment trajectories on the probing light beam motion at consecutive segment positions. Further, the periodic arrangements of solid points 18.1.1 etc. shows that the recording means is arranged to record A scans at corresponding positions along the segment trajec^¬ tories at consecutive segment positions. The image trajectory is marked with reference numeral 21.3. The points 18.1.3, 18.2.3 etc. falling on this line repre^¬ sent actual segments of the image, while the other points falling in the areas 21.1., 21.2, 21.4 and 21.5 lie offside the image trajectory. However, according to the invention they are averaged with neighbouring points to suppress speckle effects. In this example, the box 22 marked with a dashed line represents a segment comprising five positions 18.1.1, 18.1.2, 18.1.3, 18.1.4 and 18.1.5 where A scans are recorded and averaged to produce the image data for posi^¬ tion 18.1.3. The same procedure is carried out for the next five positions 18.2.1, 18.2.2 etc. forming the next segment etc. So the segment trajectories are periodic cycles, the first segment trajectory starting at point 18.1.1 and end^¬ ing at point 18.2.1 etc. A scans along consecutive cycles are recorded at identical relative positions within their cycles, namely, at five equidistant points 18.1.1 to 18.1.5 etc. along the monotonously rising curves of the triangular scanning trajectory.

As shown, the density of A scans along the image trajectory 21.3 is higher than perpendicularly thereto in direction 19. Therefore, speckle decorrelation can be achieve effec- tively by the distances between the A scans in the direc^¬ tion 19, while a high resolution along the image trajectory is obtained by close relative distances of A scans in di^¬ rection 20. The probing light beam may travel along the scanning tra^¬ jectory 18 at constant speed. In this case probing light beam is moved faster in a direction perpendicular to the image trajectory 21.3 than it moves in the direction of the image trajectory 21.3.

Fig. 3A and 3B contain two OCT images of the same section of a human retina, one of which was generated with conven^¬ tional OCT (Fig. 3A) , where a disturbing speckle pattern is apparent. The other image was generated with an apparatus according to the invention with a method according to the invention (Fig. 3B) . As can be seen, the speckles are effectively smoothed out.

Claims

Apparatus for generating an image of a sample (11) with optical coherence tomography (OCT) which image consists of a two- or three-dimensional set of segments, whereas the first dimension is defined by the direction of a probing light beam through the sample (11) and the other one or two dimensions are defined by an image trajectory across the sample (11), along which the seg^¬ ment positions are scanned, whereas the apparatus com^¬ prises recording means (12) arranged for extracting segment information along the first dimension from the probing light beam returning from the sample (11) (A scan), image scanning means (10) arranged for scanning said probing light beam across the sample (11) along said image trajectory, computing means (17) arranged for computing the set of segments from segment informa^¬ tion of a plurality of A scans performed at different scan positions, characterized in that the apparatus further comprises segment scanning means (9) arranged for modifying the scanning motion of the probing light beam for each segment position along the image trajectory so that segment trajectories are superimposed on the scanning motion of the probing light beam caused by the image scanning means (10), whereas the recording means (12) is arranged for recording a group of at least two A scans at different positions along a seg^¬ ment trajectory, and whereas the computing means (17) is arranged for computing the segments for each segment position along the image trajectory with an averaging operation on the information extracted from the group of A scans recorded along the segment trajectory of that segment position.

Apparatus according to claim 1, characterized in that the segment scanning means (9) and the recording means (12) are arranged to perform A scans offside the image traj ectory .

Apparatus according to claim 2, characterized in that said segment scanning means (9) is arranged for moving the probing light beam faster in a direction perpendicular to the image trajectory than it moves in the direction of the image trajectory.

Apparatus according to any of the preceding claims, characterized in that said segment scanning means (9) is arranged to superimpose segment trajectories in form of periodic cycles on the probing light beam motion at consecutive segment positions.

Apparatus according to claim 4, characterized in that the recording means (12) is arranged to record A scans in consecutive cycles at identical relative positions within their cycles.

Apparatus according to any of the preceding claims, characterized in that the segment scanning means (9) is located upstream of the image scanning means.

Apparatus according to any of the preceding claims, characterized in that the segment scanning means (9) comprises a scanning device selected from the group of resonant scanners, micro-electro-mechanical scanners and acousto-optic scanners.

Apparatus according to any of the preceding claims, characterized in that the segment scanning means (9) is configured for allowing the probing light beam to travel along the segment trajectory at the surface of the imaged region of the sample (11) a speed above 1 m/s, more preferably above 1.5 m/s and most preferably above 4 m/s.

Apparatus according to any of the preceding claims, characterized in that the segment scanning means (9) is configured to trigger the A scan recording.

Method for generating an image from a sample (11) with optical coherence tomography (OCT) , which image consists of a two- or three-dimensional set of segments, whereas the first dimension is defined by the direction of a probing light beam through the sample (11) and the other one or two dimensions are defined by an image trajectory across the sample (11), along which the seg^¬ ment positions are scanned, whereas segment information along the first dimension is extracted from probing light beam returning from the sample (11) (A scan), said probing light beam is scanned across the sample (11) along a scanning trajectory, the set of segments is computed from segment information of a plurality of A scans taken at different scan positions, characterized in that said scanning trajectory is determined by said image trajectory and superimposed segment trajec^¬ tories for each segment position along the image tra^¬ jectory, whereas groups of at least two A scans are re- corded at different positions along each segment tra^¬ jectory, and whereas the computation of segments for each position along the image trajectory involves an averaging operation on the information extracted from the group of A scans recorded along the segment trajec^¬ tory of that segment position.

Method according to claim 10, characterized in that the scanning trajectory comprises points offside the image trajectory at which points A scans are recorded.

Method according to claim 11, characterized in that within said groups of A scans, the density of A scans along the image trajectory is higher than perpendicularly thereto.

13. Method according to any of preceding claims 10 to 12, characterized in that the segment trajectories are pe^¬ riodic cycles.

14. Method according to claim 13, characterized in that a scans along consecutive cycles are recorded at identi^¬ cal relative positions within their cycles.

15. Method according to claim 11 to 14, characterized in that the scanning trajectory between the first and the last A scan belonging to one group describes a monoto^¬ nous curve with respect to the image trajectory.