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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 18 — Aug. 29, 2011
  • pp: 17143–17150
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In vivo near-realtime volumetric optical-resolution photoacoustic microscopy using a high-repetition-rate nanosecond fiber-laser

Wei Shi, Parsin Hajireza, Peng Shao, Alexander Forbrich, and Roger J. Zemp  »View Author Affiliations


Optics Express, Vol. 19, Issue 18, pp. 17143-17150 (2011)
http://dx.doi.org/10.1364/OE.19.017143


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Abstract

Optical-resolution photoacoustic microscopy (OR-PAM) is capable of achieving optical-absorption-contrast images with micron-scale spatial resolution. Previous OR-PAM systems have been frame-rate limited by mechanical scanning speeds and laser pulse repetition rate (PRR). We demonstrate OR-PAM imaging using a diode-pumped nanosecond-pulsed Ytterbium-doped 532-nm fiber laser with PRR up to 600 kHz. Combined with fast-scanning mirrors, our proposed system provides C-scan and 3D images with acquisition frame rate of 4 frames per second (fps) or higher, two orders of magnitude faster than previously published systems. High-contrast images of capillary-scale microvasculature in a live Swiss Webster mouse ear with ~6-µm optical lateral spatial resolution are demonstrated.

© 2011 OSA

1. Introduction

Photoacoustic imaging is an emerging technology that involves firing short laser pulses into tissue and recording acoustic signals due to light absorption-induced thermoelastic expansion. Image contrast is principally due to optical absorption. High-resolution photoacoustic imaging has captured considerable attention in the imaging community as its utility is extending to areas of interest to biologists and clinicians. One embodiment, termed Dark-Field Photoacoustic Microscopy (PAM) [1

1. K. Maslov, G. Stoica, and L. V. Wang, “In vivo dark-field reflection-mode photoacoustic microscopy,” Opt. Lett. 30(6), 625–627 (2005). [CrossRef] [PubMed]

] achieves high resolution by using a mechanically-scanned high frequency, high numerical aperture single-element ultrasonic transducers to receive photoacoustic signals. In 2006, Zhang et al. [2

2. H. F. Zhang, K. Maslov, G. Stoica, and L. V. Wang, “Functional photoacoustic microscopy for high-resolution and noninvasive in vivo imaging,” Nat. Biotechnol. 24(7), 848–851 (2006). [CrossRef] [PubMed]

] reported a lateral resolution of 45 μm at a maximum depth of 3 mm by using a 50 MHz focused ultrasonic transducer to demonstrate functional imaging of hemoglobin oxygen saturation. To realize micron-scale acoustic resolution, the transducer frequency would need to be in the hundreds of MHz to GHz range, where penetration depth is limited to less than ~100 μm due to high ultrasonic attenuation in tissue at high frequencies. An alternative technology to achieve high resolution is referred to as Optical-Resolution Photoacoustic Microscopy (OR-PAM). OR-PAM is capable of realizing high optical resolution images because its lateral spatial resolution is determined by the focused optical spot size (limited by the diffraction-limited focal spot size) rather than the width of the ultrasound focal zone. OR-PAM systems are depth-limited to approximately one transport mean-free path (~1 mm in tissue). Because of multiple scattering, diffraction-limited focusing is difficult to achieve past this depth. Beginning with the work of Maslov et al. [3

3. K. Maslov, H. F. Zhang, S. Hu, and L. V. Wang, “Optical-resolution photoacoustic microscopy for in vivo imaging of single capillaries,” Opt. Lett. 33(9), 929–931 (2008). [CrossRef] [PubMed]

], OR-PAM has been used for structural [4

4. S. Hu, K. Maslov, and L. V. Wang, “Noninvasive label-free imaging of microhemodynamics by optical-resolution photoacoustic microscopy,” Opt. Express 17(9), 7688–7693 (2009). [CrossRef] [PubMed]

] and functional [4

4. S. Hu, K. Maslov, and L. V. Wang, “Noninvasive label-free imaging of microhemodynamics by optical-resolution photoacoustic microscopy,” Opt. Express 17(9), 7688–7693 (2009). [CrossRef] [PubMed]

8

8. J. Yao, K. I. Maslov, Y. Shi, L. A. Taber, and L. V. Wang, “In vivo photoacoustic imaging of transverse blood flow by using Doppler broadening of bandwidth,” Opt. Lett. 35(9), 1419–1421 (2010). [CrossRef] [PubMed]

] imaging in mice. Oxygen saturation imaging, and imaging of blood velocity has been demonstrated [6

6. S. Hu, K. Maslov, and L. V. Wang, “In vivo functional chronic imaging of a small animal model using optical-resolution photoacoustic microscopy,” Med. Phys. 36(6), 2320–2323 (2009). [CrossRef] [PubMed]

] [7

7. L. Song, K. Maslov, and L. V. Wang, “Multifocal optical-resolution photoacoustic microscopy in vivo,” Opt. Lett. 36(7), 1236–1238 (2011). [CrossRef] [PubMed]

], and stunning trans-cranial images of whole-brain murine cortical capillary networks have shown the potential power of the technique for neuro-functional imaging [8

8. J. Yao, K. I. Maslov, Y. Shi, L. A. Taber, and L. V. Wang, “In vivo photoacoustic imaging of transverse blood flow by using Doppler broadening of bandwidth,” Opt. Lett. 35(9), 1419–1421 (2010). [CrossRef] [PubMed]

]. Other applications have included in vivo imaging of amyloid plaques in a transgenic murine model of Alzheimer’s disease [9

9. S. Hu, P. Yan, K. Maslov, J. M. Lee, and L. H. Wang, “Optical-resolution photoacoustic microscopy of amyloid-β deposits in vivo,” Proc. SPIE 7564, 75643D, 75643D-4 (2010). [CrossRef]

], high-resolution functional imaging and chronic monitoring of angiogenesis in a transgenic mouse model [10

10. S. Hu, J. Yao, K. Maslov, and L. H. Wang, “Optical-resolution photoacoustic microscopy of angiogenesis in a transgenic mouse model,” Proc. SPIE 7564, 756406, 756406-5 (2010). [CrossRef]

], ocular microvasculature [11

11. S. Hu, B. Rao, K. Maslov, and L. V. Wang, “Label-free Photoacoustic Ophthalmic Angiography,” Opt. Lett. 35(1), 1–3 (2010). [CrossRef] [PubMed]

], and others. E. Zhang et al [12

12. E. Zhang and P. Beard, “Ultra high sensitivity, wideband Fabry Perot ultrasound sensors as an alternative to piezoelectric PVDF transducers for biomedical photoacoustic detection,” Proc. SPIE 5320, 222–229 (2004). [CrossRef]

] demonstrated a unique Fabry-Perot etalon-based approach for OR-PAM. Xie et al. [13

13. Z. X. Xie, S. L. Jiao, H. F. Zhang, and C. A. Puliafito, “Laser-scanning optical-resolution photoacoustic microscopy,” Opt. Lett. 34(12), 1771–1773 (2009). [CrossRef] [PubMed]

], developed a laser-scanning OR-PAM (LS-OR-PAM) system based on the pioneering work of L.V. Wang’s group. In Xie’s studies, a lateral resolution of 7.8 μm in a 6 mm-diameter circular field-of-view (FOV) was achieved.

2. Methods

An eight channel PCI data acquisition card (CS8289, Gage Cobra, Gage Applied Systems, Inc.) with 12-bit dynamic range and up to 125 MSamples/s sample rate was used to acquire the photodiode signal (to trigger data acquisition), the feedback positions of the scanning mirrors (to determine laser spot position on the image plane), and the photoacoustic signals which were detected by an ultrasonic transducer, and amplified by an ultrasound pulser-receiver (5900PR, Olympus NDT Inc). Further data processing and analysis were conducted by using MATLAB programs (Mathworks, Inc) in the data acquisition PC.

3. Results

Figure 2(a)
Fig. 2 (a) Image of a human hair; (b) Image of a 7.5-μm carbon fiber target positioned 1.5 mm below the probe membrane; (c) Photoacoustic signal (circles) of a slice of data selected perpendicular to the carbon fiber on 2D image with Gaussian fit (line) in clear water, FWHM = 9 μm.
shows a maximum-amplitude-projection (MAP) image of a human hair target positioned 1.5mm below the probe membrane in a clear medium. By setting the 2D scanning galvanometer mirror system with scanning frequencies of 2 Hz and 400 Hz in the Y direction (slow scanning axis) and X direction (fast scanning axis), respectively at optical angles within ± 1.6 degrees, the OR-PAM system realized raster scanning at 4 fps. The image in Fig. 2(a) shows the target diameter of around 100 μm which is roughly the size of a hair. For resolution studies, an MAP image of a ~7.5-μm carbon fiber target positioned ~1.5 mm below the probe membrane was obtained as shown in Fig. 2(b). To analyze the photoacoustic signal resolution, we extracted out a slice of data on the image perpendicular to the carbon fiber. The circles in Fig. 2(c) indicate the photoacoustic signals along the slice direction, and the line curve is its Gaussian fitting which shows the photoacoustic signal full width at half maximum (FWHM) as ~9 μm. Since the measured FWHM shown in Fig. 2(c) are partly due to the width of the carbon fiber itself, we computed the convolution of a 2D Gaussian beam with a carbon fiber (simulated as a 2D rectangular absorption region) [15

15. W. Shi, S. Kerr, I. Utkin, J. C. Ranasinghesagara, L. Pan, Y. Godwal, R. J. Zemp, and R. Fedosejevs, “Optical resolution photoacoustic microscopy using novel high-repetition-rate passively Q-switched microchip and fiber lasers,” J. Biomed. Opt. 15(5), 056017 (2010). [CrossRef] [PubMed]

], which shows the corresponding lateral resolution as ~7μm.

For in vivo studies, we used pulse energies of ~0.15 μJ (measured after the scanning mirror system). While 4 fps were achieved in phantom studies, we chose to image a larger field of view (1 mm × 1 mm) for in-vivo studies. We set the 2D scanning galvanometer mirror system with scanning frequency in Y direction as 1 Hz, and in X direction as 400 Hz at optical angles of ± 1.6 degrees, which enabled raster scanning at 2 fps. Frame-rates for this FOV are limited by the fast-scanning mirror system but higher frame-rates are possible for smaller field of view. All experimental animal procedures were conducted in conformity with the laboratory animal protocol approved by the University of Alberta Animal Use and Care Committee. The Swiss Webster mouse was anesthetized using a breathing anesthesia system (E-Z Anesthesia, Euthanex Corp.) during image acquisition. Figure 3
Fig. 3 (a) in vivo image of microvasculature in a Swiss Webster mouse ear acquired at 2 fps, which shows an image of a pair of parallel arteries or veins surrounded by capillaries; Media 1 shows the volumetric visualization of the Swiss Webster mouse ear microvasculatures by OR-PAM; (b) Another in vivo image of microvasculature in a Swiss Webster mouse ear; Media 2 shows corresponding volumetric visualization of the Swiss Webster mouse ear microvasculature.
shows snapshots from 2 volumetric images of the microvasculature in a Swiss Webster mouse ear in vivo. The volumetric images were processed by using 3D Vesselness filtering [19

19. A. F. Frangi, W. J. Niessen, P. J. Nederkoorn, J. Bakker, W. P. Th. M. Mali, and M. A. Viergever, “Quantitative analysis of vascular morphology from 3D MR angiograms: In vitro and in vivo results,” Magn. Reson. Med. 45(2), 311–322 (2001). [CrossRef] [PubMed]

] and displayed using Volview software (Kitware, Inc., Volview 3.2). Figure 3(a) clearly depicts a pair of parallel arteries or veins surrounded by many capillaries, while a corresponding volumetric rending is shown in Media 1. Figure 3(b) shows another snapshot with photoacoustic signal FWHM of ~6 μm obtained from resolution studies, while a corresponding volumetric rending is shown in Media 2.

4. Discussion

In our studies on photoacoustic imaging systems with fiber laser, we have demonstrated an OR-PAM system with lateral resolution of ~6μm, which is close to the objective lens diffraction-limited focal spot size of ~4.3 μm at 532 nm. A smaller optical focal spot size can be achieved by using an objective lens with higher numerical aperture; however, this would require adjustment of the light delivery system.

The pixel separations along the Y-axis were determined by the ratio of the fast axis scanning speed over the slow axis scanning speed. The maximum scanning speed of around 500 Hz within optical angles of ± 1.6 degrees limits the maximum frame rate to 2.5 fps for an image area of 1 mm × 1 mm at average pixel pitch of around 2.5μm. We chose not to push these scanning-speed limits to prevent heat buildup in the galvanometer system. If active cooling higher speed 2D galvanometer mirror systems are used, with our fiber laser operating at its maximum PRR of 600 kHz, this system is capable of achieving a maximum frame rate of ~15 fps at an average pixel pitch of 2.5 μm for an image size of 500 μm × 500 μm, which provides near-realtime volumetric imaging. Realtime or near-realtime frame-rates will be possible in the near future, which will permit clinical applications.

For our 2 fps in vivo images, each 3D frame takes nearly 96 Mbytes. This is calculated as follows: we used 100 samples per A-scan, and 160,000 laser-shots per 3D image (with laser rep-rate of 320 kHz at 2 fps, with a higher number of laser shots needed for 600 kHz PRR). Note that we also acquire position feedback signals and laser-diode signals for a total of 4 channels. Thus each frame requires (100 × 160,000 × 4) samples/image × 12 bits/sample / (8 bits/byte) = 96 Mbyte/image. Since we have only 128 Mbytes on-board storage capacity we are limited in the number of frames we can acquire before transferring data to the PC RAM. To sustain realtime imaging rates, very high data transfer rates between the data acquisition card and the PC RAM are required and these transfer rates are beyond the capabilities of our current hardware. Future work will aim to store only peak-to-peak values, reduce the number of data channels required, increase the onboard RAM, and use higher data-throughput data acquisition hardware for sustained realtime acquisition and display.

Future work will involve demonstrating imaging of dynamic processes, minimizing the channel count and system complexity by eliminating mirror position feedback, increasing the imaging FOV, developing multi-wavelength high-repetition-rate sources for functional imaging by using nonlinear photonic crystal fibers [23

23. Y. N. Billeh, M. Liu, and T. Buma, “Spectroscopic photoacoustic microscopy using a photonic crystal fiber supercontinuum source,” Opt. Express 18(18), 18519–18524 (2010). [CrossRef] [PubMed]

] or Raman shift crystals, exploring nonlinear photoacoustic phenomena for single-wavelength functional imaging, minimizing the system footprint [24

24. P. Hajireza, W. Shi, P. Shao, S. Kerr, and R. J. Zemp, “Optical-resolution photoacoustic micro-endoscopy using image-guide fibers and fiber laser technology,” Proc. SPIE •••, 78990P, 78990P-6 (2011). [CrossRef]

] and expanding clinical and biological applications.

5. Conclusion

In our research on OR-PAM imaging system with a diode-pumped pulsed Ytterbium fiber laser, we have demonstrated: a) a photoacoustic imaging system presenting optically-defined lateral resolution of around 6 μm; b) an OR-PAM system capable of volumetric imaging at 4 fps by using a high repetition rate 532nm fiber laser and high speed 2D scanning galvanometer mirror system; c) our system has adequate laser parameters in terms of PRR, pulse energy, and spatial resolution to be used for near real-time optical-resolution photoacoustic microscopy.

Acknowledgements

We gratefully acknowledge funding from NSERC (355544-2008, 375340-2009, STPGP 396444), Terry- Fox Foundation and the Canadian Cancer Society (TFF 019237, TFF 019240, CCS 2011-700718), the Alberta Cancer Research Institute (ACB 23728), the Canada Foundation for Innovation, Leaders Opportunity Fund (18472), Alberta Advanced Education & Technology, Small Equipment Grants Program (URSI09007SEG), Microsystems Technology Research Initiative (MSTRI RES0003166), University of Alberta Startup Funds, and Alberta Ingenuity / Alberta Innovates scholarships for graduate and undergraduate students.

References and links

1.

K. Maslov, G. Stoica, and L. V. Wang, “In vivo dark-field reflection-mode photoacoustic microscopy,” Opt. Lett. 30(6), 625–627 (2005). [CrossRef] [PubMed]

2.

H. F. Zhang, K. Maslov, G. Stoica, and L. V. Wang, “Functional photoacoustic microscopy for high-resolution and noninvasive in vivo imaging,” Nat. Biotechnol. 24(7), 848–851 (2006). [CrossRef] [PubMed]

3.

K. Maslov, H. F. Zhang, S. Hu, and L. V. Wang, “Optical-resolution photoacoustic microscopy for in vivo imaging of single capillaries,” Opt. Lett. 33(9), 929–931 (2008). [CrossRef] [PubMed]

4.

S. Hu, K. Maslov, and L. V. Wang, “Noninvasive label-free imaging of microhemodynamics by optical-resolution photoacoustic microscopy,” Opt. Express 17(9), 7688–7693 (2009). [CrossRef] [PubMed]

5.

S. Hu, K. Maslov, V. Tsytsarev, and L. V. Wang, “Functional transcranial brain imaging by optical-resolution photoacoustic microscopy,” J. Biomed. Opt. 14(4), 040503 (2009). [CrossRef] [PubMed]

6.

S. Hu, K. Maslov, and L. V. Wang, “In vivo functional chronic imaging of a small animal model using optical-resolution photoacoustic microscopy,” Med. Phys. 36(6), 2320–2323 (2009). [CrossRef] [PubMed]

7.

L. Song, K. Maslov, and L. V. Wang, “Multifocal optical-resolution photoacoustic microscopy in vivo,” Opt. Lett. 36(7), 1236–1238 (2011). [CrossRef] [PubMed]

8.

J. Yao, K. I. Maslov, Y. Shi, L. A. Taber, and L. V. Wang, “In vivo photoacoustic imaging of transverse blood flow by using Doppler broadening of bandwidth,” Opt. Lett. 35(9), 1419–1421 (2010). [CrossRef] [PubMed]

9.

S. Hu, P. Yan, K. Maslov, J. M. Lee, and L. H. Wang, “Optical-resolution photoacoustic microscopy of amyloid-β deposits in vivo,” Proc. SPIE 7564, 75643D, 75643D-4 (2010). [CrossRef]

10.

S. Hu, J. Yao, K. Maslov, and L. H. Wang, “Optical-resolution photoacoustic microscopy of angiogenesis in a transgenic mouse model,” Proc. SPIE 7564, 756406, 756406-5 (2010). [CrossRef]

11.

S. Hu, B. Rao, K. Maslov, and L. V. Wang, “Label-free Photoacoustic Ophthalmic Angiography,” Opt. Lett. 35(1), 1–3 (2010). [CrossRef] [PubMed]

12.

E. Zhang and P. Beard, “Ultra high sensitivity, wideband Fabry Perot ultrasound sensors as an alternative to piezoelectric PVDF transducers for biomedical photoacoustic detection,” Proc. SPIE 5320, 222–229 (2004). [CrossRef]

13.

Z. X. Xie, S. L. Jiao, H. F. Zhang, and C. A. Puliafito, “Laser-scanning optical-resolution photoacoustic microscopy,” Opt. Lett. 34(12), 1771–1773 (2009). [CrossRef] [PubMed]

14.

L. Wang, K. Maslov, J. Yao, B. Rao, and L. V. Wang, “Fast voice-coil scanning optical-resolution photoacoustic microscopy,” Opt. Lett. 36(2), 139–141 (2011). [CrossRef] [PubMed]

15.

W. Shi, S. Kerr, I. Utkin, J. C. Ranasinghesagara, L. Pan, Y. Godwal, R. J. Zemp, and R. Fedosejevs, “Optical resolution photoacoustic microscopy using novel high-repetition-rate passively Q-switched microchip and fiber lasers,” J. Biomed. Opt. 15(5), 056017 (2010). [CrossRef] [PubMed]

16.

Y. Wang, K. Maslov, Y. Zhang, S. Hu, L. Yang, Y. Xia, J. Liu, and L. V. Wang, “Fiber-laser-based photoacoustic microscopy and melanoma cell detection,” J. Biomed. Opt. 16(1), 011014 (2011). [CrossRef] [PubMed]

17.

B. Rao, L. Li, K. Maslov, and L. H. Wang, “Hybrid-scanning optical-resolution photoacousticmicroscopy for in vivo vasculature imaging,” Opt. Lett. 35(10), 1521–1523 (2010). [CrossRef] [PubMed]

18.

J. C. Ranasinghesagara, Y. Jian, X. H. Chen, K. Mathewson, and R. J. Zemp, “Photoacoustic technique for assessing optical scattering properties of turbid media,” J. Biomed. Opt. 14(4), 040504 (2009). [CrossRef] [PubMed]

19.

A. F. Frangi, W. J. Niessen, P. J. Nederkoorn, J. Bakker, W. P. Th. M. Mali, and M. A. Viergever, “Quantitative analysis of vascular morphology from 3D MR angiograms: In vitro and in vivo results,” Magn. Reson. Med. 45(2), 311–322 (2001). [CrossRef] [PubMed]

20.

S. Hu, K. Maslov, and L. V. Wang, “Second-generation optical-resolution photoacoustic microscopy with improved sensitivity and speed,” Opt. Lett. 36(7), 1134–1136 (2011). [CrossRef] [PubMed]

21.

Laser Institute of America, American National Standard for Safe Use of Lasers ANSI Z136.1–2007 (American National Standards Institute, Inc., 2007).

22.

J. B. Pawley, Handbook of Biological Confocal Microscopy, 3rd ed. Springer Science + Business Media, LLC, New York (2006).

23.

Y. N. Billeh, M. Liu, and T. Buma, “Spectroscopic photoacoustic microscopy using a photonic crystal fiber supercontinuum source,” Opt. Express 18(18), 18519–18524 (2010). [CrossRef] [PubMed]

24.

P. Hajireza, W. Shi, P. Shao, S. Kerr, and R. J. Zemp, “Optical-resolution photoacoustic micro-endoscopy using image-guide fibers and fiber laser technology,” Proc. SPIE •••, 78990P, 78990P-6 (2011). [CrossRef]

OCIS Codes
(140.3510) Lasers and laser optics : Lasers, fiber
(170.5120) Medical optics and biotechnology : Photoacoustic imaging
(170.5810) Medical optics and biotechnology : Scanning microscopy

ToC Category:
Medical Optics and Biotechnology

History
Original Manuscript: June 13, 2011
Revised Manuscript: July 12, 2011
Manuscript Accepted: July 16, 2011
Published: August 17, 2011

Virtual Issues
Vol. 6, Iss. 9 Virtual Journal for Biomedical Optics

Citation
Wei Shi, Parsin Hajireza, Peng Shao, Alexander Forbrich, and Roger J. Zemp, "In vivo near-realtime volumetric optical-resolution photoacoustic microscopy using a high-repetition-rate nanosecond fiber-laser," Opt. Express 19, 17143-17150 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-18-17143


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References

  1. K. Maslov, G. Stoica, and L. V. Wang, “In vivo dark-field reflection-mode photoacoustic microscopy,” Opt. Lett. 30(6), 625–627 (2005). [CrossRef] [PubMed]
  2. H. F. Zhang, K. Maslov, G. Stoica, and L. V. Wang, “Functional photoacoustic microscopy for high-resolution and noninvasive in vivo imaging,” Nat. Biotechnol. 24(7), 848–851 (2006). [CrossRef] [PubMed]
  3. K. Maslov, H. F. Zhang, S. Hu, and L. V. Wang, “Optical-resolution photoacoustic microscopy for in vivo imaging of single capillaries,” Opt. Lett. 33(9), 929–931 (2008). [CrossRef] [PubMed]
  4. S. Hu, K. Maslov, and L. V. Wang, “Noninvasive label-free imaging of microhemodynamics by optical-resolution photoacoustic microscopy,” Opt. Express 17(9), 7688–7693 (2009). [CrossRef] [PubMed]
  5. S. Hu, K. Maslov, V. Tsytsarev, and L. V. Wang, “Functional transcranial brain imaging by optical-resolution photoacoustic microscopy,” J. Biomed. Opt. 14(4), 040503 (2009). [CrossRef] [PubMed]
  6. S. Hu, K. Maslov, and L. V. Wang, “In vivo functional chronic imaging of a small animal model using optical-resolution photoacoustic microscopy,” Med. Phys. 36(6), 2320–2323 (2009). [CrossRef] [PubMed]
  7. L. Song, K. Maslov, and L. V. Wang, “Multifocal optical-resolution photoacoustic microscopy in vivo,” Opt. Lett. 36(7), 1236–1238 (2011). [CrossRef] [PubMed]
  8. J. Yao, K. I. Maslov, Y. Shi, L. A. Taber, and L. V. Wang, “In vivo photoacoustic imaging of transverse blood flow by using Doppler broadening of bandwidth,” Opt. Lett. 35(9), 1419–1421 (2010). [CrossRef] [PubMed]
  9. S. Hu, P. Yan, K. Maslov, J. M. Lee, and L. H. Wang, “Optical-resolution photoacoustic microscopy of amyloid-β deposits in vivo,” Proc. SPIE 7564, 75643D, 75643D-4 (2010). [CrossRef]
  10. S. Hu, J. Yao, K. Maslov, and L. H. Wang, “Optical-resolution photoacoustic microscopy of angiogenesis in a transgenic mouse model,” Proc. SPIE 7564, 756406, 756406-5 (2010). [CrossRef]
  11. S. Hu, B. Rao, K. Maslov, and L. V. Wang, “Label-free Photoacoustic Ophthalmic Angiography,” Opt. Lett. 35(1), 1–3 (2010). [CrossRef] [PubMed]
  12. E. Zhang and P. Beard, “Ultra high sensitivity, wideband Fabry Perot ultrasound sensors as an alternative to piezoelectric PVDF transducers for biomedical photoacoustic detection,” Proc. SPIE 5320, 222–229 (2004). [CrossRef]
  13. Z. X. Xie, S. L. Jiao, H. F. Zhang, and C. A. Puliafito, “Laser-scanning optical-resolution photoacoustic microscopy,” Opt. Lett. 34(12), 1771–1773 (2009). [CrossRef] [PubMed]
  14. L. Wang, K. Maslov, J. Yao, B. Rao, and L. V. Wang, “Fast voice-coil scanning optical-resolution photoacoustic microscopy,” Opt. Lett. 36(2), 139–141 (2011). [CrossRef] [PubMed]
  15. W. Shi, S. Kerr, I. Utkin, J. C. Ranasinghesagara, L. Pan, Y. Godwal, R. J. Zemp, and R. Fedosejevs, “Optical resolution photoacoustic microscopy using novel high-repetition-rate passively Q-switched microchip and fiber lasers,” J. Biomed. Opt. 15(5), 056017 (2010). [CrossRef] [PubMed]
  16. Y. Wang, K. Maslov, Y. Zhang, S. Hu, L. Yang, Y. Xia, J. Liu, and L. V. Wang, “Fiber-laser-based photoacoustic microscopy and melanoma cell detection,” J. Biomed. Opt. 16(1), 011014 (2011). [CrossRef] [PubMed]
  17. B. Rao, L. Li, K. Maslov, and L. H. Wang, “Hybrid-scanning optical-resolution photoacousticmicroscopy for in vivo vasculature imaging,” Opt. Lett. 35(10), 1521–1523 (2010). [CrossRef] [PubMed]
  18. J. C. Ranasinghesagara, Y. Jian, X. H. Chen, K. Mathewson, and R. J. Zemp, “Photoacoustic technique for assessing optical scattering properties of turbid media,” J. Biomed. Opt. 14(4), 040504 (2009). [CrossRef] [PubMed]
  19. A. F. Frangi, W. J. Niessen, P. J. Nederkoorn, J. Bakker, W. P. Th. M. Mali, and M. A. Viergever, “Quantitative analysis of vascular morphology from 3D MR angiograms: In vitro and in vivo results,” Magn. Reson. Med. 45(2), 311–322 (2001). [CrossRef] [PubMed]
  20. S. Hu, K. Maslov, and L. V. Wang, “Second-generation optical-resolution photoacoustic microscopy with improved sensitivity and speed,” Opt. Lett. 36(7), 1134–1136 (2011). [CrossRef] [PubMed]
  21. Laser Institute of America, American National Standard for Safe Use of Lasers ANSI Z136.1–2007 (American National Standards Institute, Inc., 2007).
  22. J. B. Pawley, Handbook of Biological Confocal Microscopy, 3rd ed. Springer Science + Business Media, LLC, New York (2006).
  23. Y. N. Billeh, M. Liu, and T. Buma, “Spectroscopic photoacoustic microscopy using a photonic crystal fiber supercontinuum source,” Opt. Express 18(18), 18519–18524 (2010). [CrossRef] [PubMed]
  24. P. Hajireza, W. Shi, P. Shao, S. Kerr, and R. J. Zemp, “Optical-resolution photoacoustic micro-endoscopy using image-guide fibers and fiber laser technology,” Proc. SPIE •••, 78990P, 78990P-6 (2011). [CrossRef]

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