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Virtual Journal for Biomedical Optics

Virtual Journal for Biomedical Optics

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  • Editor: Gregory W. Faris
  • Vol. 5, Iss. 3 — Feb. 10, 2010
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Second harmonic generation imaging via nonlinear endomicroscopy

Hongchun Bao, Alex Boussioutas, Reynolds Jeremy, Sarah Russell, and Min Gu  »View Author Affiliations


Optics Express, Vol. 18, Issue 2, pp. 1255-1260 (2010)
http://dx.doi.org/10.1364/OE.18.001255


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Abstract

Abstract: A compact endomicroscope is the only solution for transferring second harmonic generation (SHG) imaging into in vivo imaging and real time monitoring the content and structure of collagen. This is important for early diagnoses of different diseases associated with collagen change. A compact nonlinear endomicroscope using a double clad fiber (DCF) is newly employed in SHG imaging. The experiment shows the core of the DCF can maintain the linear polarization of the excitation laser beam in particular directions, and the degree of polarization of the excitation laser beam directly affects signal to noise ratio of SHG imaging. The nonlinear endomicroscope can display clear three dimensional (3D) SHG images of mouse tail tendon without the aid of contrast agents, which reveals the collagen fiber structure at different depths. The high resolution of SHG imaging from the endomicroscope shows that SHG imaging can reveal additional information about the orientation and degree of organisation of proteins and collagen fibers than two-photon-excited fluorescence imaging. Therefore SHG imaging offers endomicroscopy with additional channel of imaging for understanding more about biological phenomena.

© 2010 OSA

1. Introduction

Second harmonic generation (SHG) is a process where two photons simultaneously interact with non-centrosymmetrical structures without absorption, producing radiation at exactly half of the excitation wavelength [1

1. W. R. Zipfel, R. M. Williams, and W. W. Webb, “Nonlinear magic: multiphoton microscopy in the biosciences,” Nat. Biotechnol. 21(11), 1369–1377 (2003). [CrossRef] [PubMed]

3

3. E. Brown, T. McKee, E. diTomaso, A. Pluen, B. Seed, Y. Boucher, and R. K. Jain, “Dynamic imaging of collagen and its modulation in tumors in vivo using second-harmonic generation,” Nat. Med. 9(6), 796–801 (2003). [CrossRef] [PubMed]

]. SHG allows for the unique ability to observe fine structural details of connective tissues such as collagen, tendon and muscle in animals and humans without the aid of contrast agents [4

4. M. Stroh, J. P. Zimmer, D. G. Duda, T. S. Levchenko, K. S. Cohen, E. B. Brown, D. T. Scadden, V. P. Torchilin, M. G. Bawendi, D. Fukumura, and R. K. Jain, “Quantum dots spectrally distinguish multiple species within the tumor milieu in vivo,” Nat. Med. 11(6), 678–682 (2005). [CrossRef] [PubMed]

, 5

5. P. J. Campagnola and L. M. Loew, “Second-harmonic imaging microscopy for visualizing biomolecular arrays in cells, tissues and organisms,” Nat. Biotechnol. 21(11), 1356–1360 (2003). [CrossRef] [PubMed]

].

The properties of SHG offer several advantages for live cell or tissue imaging. Unlike fluorescence imaging, SHG does not involve excitation of endogenous florescent molecules. Thus, it does not suffer from phototoxicity effects or photo bleaching, which limits the usefulness of fluorescence imaging [6

6. D. A. Dombeck, K. A. Kasischke, H. D. Vishwasrao, M. Ingelsson, B. T. Hyman, and W. W. Webb, “Uniform polarity microtubule assemblies imaged in native brain tissue by second-harmonic generation microscopy,” Proc. Natl. Acad. Sci. U.S.A. 100(12), 7081–7086 (2003). [CrossRef] [PubMed]

, 7

7. N. Uldbjerg, A. Malmström, G. Ekman, and U. Ulmsten, “The integrity of cervical collagen during pregnancy and labor,” Gynecol. Obstet. Invest. 20(2), 68–73 (1985). [CrossRef] [PubMed]

]. The excitation of SHG imaging uses near-infrared wavelengths, which allows for a large penetration depth, making it a prime candidate for analysis of thick tissue specimens with the enhanced axial depth discrimination. SHG signals have well defined polarization, so SHG polarization anisotropy can be used to determine the absolute orientation and the degree of organization of proteins in tissues [7

7. N. Uldbjerg, A. Malmström, G. Ekman, and U. Ulmsten, “The integrity of cervical collagen during pregnancy and labor,” Gynecol. Obstet. Invest. 20(2), 68–73 (1985). [CrossRef] [PubMed]

9

9. L. Fu and M. Gu, “Polarization anisotropy in fiber-optic second harmonic generation microscopy,” Opt. Express 16(7), 5000–5006 (2008). [CrossRef] [PubMed]

]. The SHG excitation is a nonlinear process and the signal intensity is directly proportional to the square of the excitation intensity. This provides SHG imaging with intrinsic confocality and deep sectioning in complex tissues.

SHG microscopy is presently being introduced as a tool for basic biomedical research to noninvasively visualize and obtain high-resolution images of unstained collagen structures [1

1. W. R. Zipfel, R. M. Williams, and W. W. Webb, “Nonlinear magic: multiphoton microscopy in the biosciences,” Nat. Biotechnol. 21(11), 1369–1377 (2003). [CrossRef] [PubMed]

5

5. P. J. Campagnola and L. M. Loew, “Second-harmonic imaging microscopy for visualizing biomolecular arrays in cells, tissues and organisms,” Nat. Biotechnol. 21(11), 1356–1360 (2003). [CrossRef] [PubMed]

, 10

10. M. E. Llewellyn, R. P. J. Barretto, S. L. Delp, and M. J. Schnitzer, “Minimally invasive high-speed imaging of sarcomere contractile dynamics in mice and humans,” Nature 454(7205), 784–788 (2008). [PubMed]

]. However, to date almost all research in SHG imaging has been carried out using a conventional bench top microscope system. Such a system does not provide the adjustment flexibility needed for in vivo imaging, where specimens need to be mounted on the microscopy stage, limits the system’s use. A biopsy is required for SHG imaging which will introduce intrusion to animal or human and variation of collagen structures because of excision from a live body. An endomicroscope system having a long flexible fiber and a miniaturized probe is necessary for translating SHG imaging into in vivo imaging [11

11. H. Bao and M. Gu, “Reduction of self-phase modulation in double-clad photonic crystal fiber for nonlinear optical endoscopy,” Opt. Lett. 34(2), 148–150 (2009). [CrossRef] [PubMed]

15

15. L. Fu, X. Gan, D. Bird, and M. Gu, “Polarization characteristics of a 1×2 fibre coupler under femtosecond pulsed and continuous wave illumination,” Opt. Laser Technol. 37(6), 494–497 (2005). [CrossRef]

].

In this letter, we demonstrate SHG imaging using our recently developed compact handheld nonlinear endomicroscope which is capable of in vivo 3D imaging [16

16. H. Bao, J. Allen, R. Pattie, R. Vance, and M. Gu, “A fast handheld two-photon fluorescence microendoscope with a 475µmx475µm field of view for in vivo imaging,” Opt. Lett. 33(12), 1333–1335 (2008). [CrossRef] [PubMed]

]. Experimental results show that the core of a double clad fiber (DCF) which the endomicroscope uses has a “polarization maintaining” feature in particular directions. Using this “polarization marinating” feature, the nonlinear endomicroscope can observe clear 3D collagen structure of a mouse tail tendon within a depth of 88 µm. High resolution SHG imaging from the endomicroscope has shown more micro-structure information of biological tissue than two-photon fluorescence imaging.

2. The nonlinear endomicroscopic system for SHG imaging

The SHG endomicroscope is schematically depicted in Fig. 1
Fig. 1 Schematic representation of experimental setup for SHG imaging using the designed nonlinear endomicroscope.
, where a passive mode-locked Ti:Sapphire laser generates an 80 MHz 100 fs optical pulse train with wavelength tuneable between 730 nm and 870 nm. 800 nm pulses can introduce high SHG signal from mouse tail tendon and are used for SHG imaging. The pulses propagate through a pair of gratings which provide a negative frequency chirp. The pulses pass through a half wave plate (λ/2 plate) and a Glen Thompson polarizer being used to control the polarization direction of the optical pulses by rotating the direction of the λ/2 plate and the Glen Thompson polarizer accordingly. The optical pulses are then coupled into the core of a 2.5 m long DCF by an objective lens (Olympus MA20X/0.4), where the chromatic dispersion of the DCF is compensated by the pre-chirp in the pulses. The pulse width at the output of the endomicroscope is 1.5 ps.

3. Degree of polarization of the excitation laser beam delivered by the nonlinear endomicroscope

The DCF that the nonlinear endomicroscope uses has a circular core and it should not be polarization maintaining. As a linearly polarized excitation laser beam is coupled into the core of the DCF, we found that the degree of polarization, defined as γ = (Imax - Imin)/(Imax + Imin) by measuring the maximum (Imax) and minimum (Imin) intensity of the output laser beam, changes by varying the polarization direction of the incident linearly polarized laser beam. Figure 2
Fig. 2 Degree of polarization of the excitation laser beam and signal to noise ratio of SHG imaging as a function of the polarization direction of the input linearly polarized laser beam
shows the measured degree of polarization of the output laser beam at wavelength 800 nm from the fiber probe as a function of the incident polarization direction. The degree of polarization γ of the excitation laser beam from the output of the probe is changed between 0.35 and 0.96 for different incident polarization directions. The perfect circular structure of the core of the DCF should not have such features. However, minor deviations in the symmetry of the DCF during fabrication causes the 2.5 m DCF “polarization maintaining” along two directions: 0° and 90°. Since the DCF is only 2.5 m long and free running, polarization maintaining of the DCF is stable during imaging.

The degree of polarization of the excitation laser beam from the nonlinear endomicroscope directly affects the signal to noise ratio of SHG imaging. The SHG intensity and the background noise of SHG imaging of a mouse tail tendon fiber by the nonlinear endomicroscope are measured. The signal to noise ratio (SNR), defined as the ratio of the SHG intensity to the background noise, is also displayed in Fig. 2. The SNR of SHG imaging has a similar trend as the degree of polarization γ of the excitation laser from the endomicroscope. SHG imaging has a high SNR as the output laser beam is close to be linearly polarized, and the SNR of SHG imaging reduces while γ of the output laser beam decreases.

4. Three-dimensional SHG imaging by nonlinear endomicroscopy

The nonlinear endomicroscope permits 3D anatomical and functional imaging of organs with a large field of view and high resolution [9

9. L. Fu and M. Gu, “Polarization anisotropy in fiber-optic second harmonic generation microscopy,” Opt. Express 16(7), 5000–5006 (2008). [CrossRef] [PubMed]

]. Figure 3
Fig. 3 SHG images of a mouse tail tendon with an axial step of 8 µm. The image size is 475 µm × 475 µm. The excitation wavelength is 800 nm. The excitation power is 30 mW on the sample. Arrows are the polarization directions of the excitation laser beam
is 3D SHG images of a mouse tail tendon obtained by the nonlinear endomicroscope with axial distance of 8 µm, where polarization direction of incident laser beam is 0°, aligning with the “polarization maintaining” direction of the DCF. The size of the images is 475 µm × 475 µm. Figure 3 shows clear structural details of collagen in the mouse tail tendon at different depth. This nonlinear endomicroscope can observe the collagen structure of a mouse tail tendon 88 µm under the surface.

5. SHG imaging and two-photon excited fluorescence imaging by nonlinear endomicroscopy

The nonlinear endomicroscope can observe two-channel of images – a SHG image and a two-photon excited fluorescence (TPF) image, by injecting a mouse with fluorescein and switching between the filter for SHG imaging and the filter for TPF imaging, which is a 3 mm thick BG18 glass filter (Schott Pty Ltd). Figure 4
Fig. 4 Two channel images of a mouse tail tendon by the nonlinear endomicroscope. (a), (d) SHG images. (b), (e) TPF images. (c), (f) Combination of two-channel images. Red: TPF image. Green: SHG image. The size of the images: 150 µm × 150 µm. Arrows are the polarization directions of the excitation laser beam
reveals high resolution two-channel images of a mouse tail tendon.

The structure of the SHG images and the TPF images is different. The TPF images show the distribution of fluorescein in the mouse tail tendon absorbed from the bloodstream. On the other hand, SHG images display non-centrosymmetrical orientation and structures of proteins in the mouse tail tendon. Therefore, SHG offers the nonlinear endomicroscope with another channel of image which helps to understand orientation and degree of organisation of proteins and collagen fiber in the tissues.

5. Conclusions

The nonlinear endomicroscope can be used for in vivo SHG imaging. The experimental results show that the degree of polarization of the excitation laser beam from the output of the endomicroscopy probe changes with different incident polarization directions. The circular DCF that the nonlinear endomicroscope uses shows a “polarization maintaining” feature at particular directions due to minor deviations in the symmetry of the DCF during fabrication. As a result, the SNR of the SHG images obtained by the endomicroscope changes with different incident polarization directions. With the polarization direction of the incident laser beam aligning with the “polarization maintaining” direction of the DCF, the nonlinear endomicroscope shows clear 3D collagen structures of a mouse tail tendon. Comparing with two-photon excited fluorescence imaging by the nonlinear endomicroscope, the high resolution SHG imaging offers further information about the orientation and degree of organisation of proteins and collagen fiber in biological objects.

Acknowledgements

The authors thank the Australian Research Council for its support.

References and links

1.

W. R. Zipfel, R. M. Williams, and W. W. Webb, “Nonlinear magic: multiphoton microscopy in the biosciences,” Nat. Biotechnol. 21(11), 1369–1377 (2003). [CrossRef] [PubMed]

2.

L. Moreaux, O. Sandre, M. Blanchard-Desce, and J. Mertz, “Membrane imaging by simultaneous second-harmonic generation and two-photon microscopy,” Opt. Lett. 25(5), 320–322 (2000). [CrossRef]

3.

E. Brown, T. McKee, E. diTomaso, A. Pluen, B. Seed, Y. Boucher, and R. K. Jain, “Dynamic imaging of collagen and its modulation in tumors in vivo using second-harmonic generation,” Nat. Med. 9(6), 796–801 (2003). [CrossRef] [PubMed]

4.

M. Stroh, J. P. Zimmer, D. G. Duda, T. S. Levchenko, K. S. Cohen, E. B. Brown, D. T. Scadden, V. P. Torchilin, M. G. Bawendi, D. Fukumura, and R. K. Jain, “Quantum dots spectrally distinguish multiple species within the tumor milieu in vivo,” Nat. Med. 11(6), 678–682 (2005). [CrossRef] [PubMed]

5.

P. J. Campagnola and L. M. Loew, “Second-harmonic imaging microscopy for visualizing biomolecular arrays in cells, tissues and organisms,” Nat. Biotechnol. 21(11), 1356–1360 (2003). [CrossRef] [PubMed]

6.

D. A. Dombeck, K. A. Kasischke, H. D. Vishwasrao, M. Ingelsson, B. T. Hyman, and W. W. Webb, “Uniform polarity microtubule assemblies imaged in native brain tissue by second-harmonic generation microscopy,” Proc. Natl. Acad. Sci. U.S.A. 100(12), 7081–7086 (2003). [CrossRef] [PubMed]

7.

N. Uldbjerg, A. Malmström, G. Ekman, and U. Ulmsten, “The integrity of cervical collagen during pregnancy and labor,” Gynecol. Obstet. Invest. 20(2), 68–73 (1985). [CrossRef] [PubMed]

8.

R. M. Williams, W. R. Zipfel, and W. W. Webb, “Interpreting second-harmonic generation images of collagen I fibrils,” Biophys. J. 88(2), 1377–1386 (2005). [CrossRef]

9.

L. Fu and M. Gu, “Polarization anisotropy in fiber-optic second harmonic generation microscopy,” Opt. Express 16(7), 5000–5006 (2008). [CrossRef] [PubMed]

10.

M. E. Llewellyn, R. P. J. Barretto, S. L. Delp, and M. J. Schnitzer, “Minimally invasive high-speed imaging of sarcomere contractile dynamics in mice and humans,” Nature 454(7205), 784–788 (2008). [PubMed]

11.

H. Bao and M. Gu, “Reduction of self-phase modulation in double-clad photonic crystal fiber for nonlinear optical endoscopy,” Opt. Lett. 34(2), 148–150 (2009). [CrossRef] [PubMed]

12.

D. Bird and M. Gu, “Compact two-photon fluorescence microscope based on a single-mode fiber coupler,” Opt. Lett. 27(12), 1031–1033 (2002). [CrossRef]

13.

L. Fu, X. Gan, and M. Gu, “Use of a single-mode fiber coupler for second-harmonic-generation microscopy,” Opt. Lett. 30(4), 385–387 (2005). [CrossRef] [PubMed]

14.

L. Fu and M. Gu, “Double-clad photonic crystal fiber coupler for compact nonlinear optical microscopy imaging,” Opt. Lett. 31(10), 1471–1473 (2006). [CrossRef] [PubMed]

15.

L. Fu, X. Gan, D. Bird, and M. Gu, “Polarization characteristics of a 1×2 fibre coupler under femtosecond pulsed and continuous wave illumination,” Opt. Laser Technol. 37(6), 494–497 (2005). [CrossRef]

16.

H. Bao, J. Allen, R. Pattie, R. Vance, and M. Gu, “A fast handheld two-photon fluorescence microendoscope with a 475µmx475µm field of view for in vivo imaging,” Opt. Lett. 33(12), 1333–1335 (2008). [CrossRef] [PubMed]

17.

M. Goetz, C. Fottner, E. Schirrmacher, P. Delaney, S. Gregor, C. Schneider, D. Strand, S. Kanzler, B. Memadathil, E. Weyand, M. Holtmann, R. Schirrmacher, M. M. Weber, M. Anlauf, G. Klöppel, M. Vieth, P. R. Galle, P. Bartenstein, M. F. Neurath, and R. Kiesslich, “In-vivo confocal real-time mini-microscopy in animal models of human inflammatory and neoplastic diseases,” Endoscopy 39(4), 350–356 (2007). [CrossRef] [PubMed]

18.

R. Kiesslich, P. R. Galle, and M. F. Neurath, Atlas of endomicroscopy. Springer Medizin Verlag Heidelberg (2008).

OCIS Codes
(110.2350) Imaging systems : Fiber optics imaging
(110.3080) Imaging systems : Infrared imaging
(110.6880) Imaging systems : Three-dimensional image acquisition
(170.2150) Medical optics and biotechnology : Endoscopic imaging
(180.4315) Microscopy : Nonlinear microscopy

ToC Category:
Medical Optics and Biotechnology

History
Original Manuscript: November 12, 2009
Revised Manuscript: December 28, 2009
Manuscript Accepted: December 30, 2009
Published: January 11, 2010

Virtual Issues
Vol. 5, Iss. 3 Virtual Journal for Biomedical Optics

Citation
Hongchun Bao, Alex Boussioutas, Reynolds Jeremy, Sarah Russell, and Min Gu, "Second harmonic generation imaging via nonlinear endomicroscopy," Opt. Express 18, 1255-1260 (2010)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-18-2-1255


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References

  1. W. R. Zipfel, R. M. Williams, and W. W. Webb, “Nonlinear magic: multiphoton microscopy in the biosciences,” Nat. Biotechnol. 21(11), 1369–1377 (2003). [CrossRef] [PubMed]
  2. L. Moreaux, O. Sandre, M. Blanchard-Desce, and J. Mertz, “Membrane imaging by simultaneous second-harmonic generation and two-photon microscopy,” Opt. Lett. 25(5), 320–322 (2000). [CrossRef]
  3. E. Brown, T. McKee, E. diTomaso, A. Pluen, B. Seed, Y. Boucher, and R. K. Jain, “Dynamic imaging of collagen and its modulation in tumors in vivo using second-harmonic generation,” Nat. Med. 9(6), 796–801 (2003). [CrossRef] [PubMed]
  4. M. Stroh, J. P. Zimmer, D. G. Duda, T. S. Levchenko, K. S. Cohen, E. B. Brown, D. T. Scadden, V. P. Torchilin, M. G. Bawendi, D. Fukumura, and R. K. Jain, “Quantum dots spectrally distinguish multiple species within the tumor milieu in vivo,” Nat. Med. 11(6), 678–682 (2005). [CrossRef] [PubMed]
  5. P. J. Campagnola and L. M. Loew, “Second-harmonic imaging microscopy for visualizing biomolecular arrays in cells, tissues and organisms,” Nat. Biotechnol. 21(11), 1356–1360 (2003). [CrossRef] [PubMed]
  6. D. A. Dombeck, K. A. Kasischke, H. D. Vishwasrao, M. Ingelsson, B. T. Hyman, and W. W. Webb, “Uniform polarity microtubule assemblies imaged in native brain tissue by second-harmonic generation microscopy,” Proc. Natl. Acad. Sci. U.S.A. 100(12), 7081–7086 (2003). [CrossRef] [PubMed]
  7. N. Uldbjerg, A. Malmström, G. Ekman, and U. Ulmsten, “The integrity of cervical collagen during pregnancy and labor,” Gynecol. Obstet. Invest. 20(2), 68–73 (1985). [CrossRef] [PubMed]
  8. R. M. Williams, W. R. Zipfel, and W. W. Webb, “Interpreting second-harmonic generation images of collagen I fibrils,” Biophys. J. 88(2), 1377–1386 (2005). [CrossRef]
  9. L. Fu and M. Gu, “Polarization anisotropy in fiber-optic second harmonic generation microscopy,” Opt. Express 16(7), 5000–5006 (2008). [CrossRef] [PubMed]
  10. M. E. Llewellyn, R. P. J. Barretto, S. L. Delp, and M. J. Schnitzer, “Minimally invasive high-speed imaging of sarcomere contractile dynamics in mice and humans,” Nature 454(7205), 784–788 (2008). [PubMed]
  11. H. Bao and M. Gu, “Reduction of self-phase modulation in double-clad photonic crystal fiber for nonlinear optical endoscopy,” Opt. Lett. 34(2), 148–150 (2009). [CrossRef] [PubMed]
  12. D. Bird and M. Gu, “Compact two-photon fluorescence microscope based on a single-mode fiber coupler,” Opt. Lett. 27(12), 1031–1033 (2002). [CrossRef]
  13. L. Fu, X. Gan, and M. Gu, “Use of a single-mode fiber coupler for second-harmonic-generation microscopy,” Opt. Lett. 30(4), 385–387 (2005). [CrossRef] [PubMed]
  14. L. Fu and M. Gu, “Double-clad photonic crystal fiber coupler for compact nonlinear optical microscopy imaging,” Opt. Lett. 31(10), 1471–1473 (2006). [CrossRef] [PubMed]
  15. L. Fu, X. Gan, D. Bird, and M. Gu, “Polarization characteristics of a 1×2 fibre coupler under femtosecond pulsed and continuous wave illumination,” Opt. Laser Technol. 37(6), 494–497 (2005). [CrossRef]
  16. H. Bao, J. Allen, R. Pattie, R. Vance, and M. Gu, “A fast handheld two-photon fluorescence microendoscope with a 475µmx475µm field of view for in vivo imaging,” Opt. Lett. 33(12), 1333–1335 (2008). [CrossRef] [PubMed]
  17. M. Goetz, C. Fottner, E. Schirrmacher, P. Delaney, S. Gregor, C. Schneider, D. Strand, S. Kanzler, B. Memadathil, E. Weyand, M. Holtmann, R. Schirrmacher, M. M. Weber, M. Anlauf, G. Klöppel, M. Vieth, P. R. Galle, P. Bartenstein, M. F. Neurath, and R. Kiesslich, “In-vivo confocal real-time mini-microscopy in animal models of human inflammatory and neoplastic diseases,” Endoscopy 39(4), 350–356 (2007). [CrossRef] [PubMed]
  18. R. Kiesslich, P. R. Galle, and M. F. Neurath, Atlas of endomicroscopy. Springer Medizin Verlag Heidelberg (2008).

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