OPTICAL PHASE CONJUGATION FOR TURBIDITY SUPPRESSION IN BIOLOGICAL SAMPLES

Abstract

Elastic optical scattering, the dominant light interaction process in biological tissues, prevents tissues from being transparent. While scattering may appear stochastic, it is in fact deterministic in nature. We show that, despite experimental imperfections, optical phase conjugation (lambda = 532 nm) can force a transmitted light field to retrace its trajectory through a biological target and recover the original light field. For a 0.69 mm thick chicken breast tissue section, we can enhance point source light return by approximately 5x10(3) times and achieve a light transmission enhancement factor of 3.8 within a collection angle of 29 degrees . Additionally, we find that the reconstruction's quality, measured by the width of the reconstructed point source, is independent of tissue thickness (up to 0.69 mm thick). This phenomenon may be used to enhance light transmission through tissue, enable measurement of small tissue movements, and form the basis of new tissue imaging techniques.

Figure 1. Schematics of TSOPC setup and scattering medium

(a) Experimental setup to confirm the TSOPC phenomenon in biological tissues. Concentric (black) dot and circle represent vertical polarization whereas double arrow in the plane of paper symbolizes horizontal polarization. (b) and (c) show schematics for recording of tissue turbidity information and reconstruction of OPC light field, respectively. (d) Schematic of a scattering medium. a_i and b_i are the complex incident and scattered fields, respectively, at the i^th face of the scattering medium. Li; ith spherical lens; RL: relay lens; CP: compensation plate; Mi: ith mirror; WPi: ith half wave plate; Pi: ith polarization beam splitter; BS: 50/50 beam splitter; CCD: charge coupled device; S: scattering sample.

Figure 1. Schematics of TSOPC setup and scattering medium

(a) Experimental setup to confirm the TSOPC phenomenon in biological tissues. Concentric (black) dot and circle represent vertical polarization whereas double arrow in the plane of paper symbolizes horizontal polarization. (b) and (c) show schematics for recording of tissue turbidity information and reconstruction of OPC light field, respectively. (d) Schematic of a scattering medium. a_i and b_i are the complex incident and scattered fields, respectively, at the i^th face of the scattering medium. Li; ith spherical lens; RL: relay lens; CP: compensation plate; Mi: ith mirror; WPi: ith half wave plate; Pi: ith polarization beam splitter; BS: 50/50 beam splitter; CCD: charge coupled device; S: scattering sample.

Figure 1. Schematics of TSOPC setup and scattering medium

(a) Experimental setup to confirm the TSOPC phenomenon in biological tissues. Concentric (black) dot and circle represent vertical polarization whereas double arrow in the plane of paper symbolizes horizontal polarization. (b) and (c) show schematics for recording of tissue turbidity information and reconstruction of OPC light field, respectively. (d) Schematic of a scattering medium. a_i and b_i are the complex incident and scattered fields, respectively, at the i^th face of the scattering medium. Li; ith spherical lens; RL: relay lens; CP: compensation plate; Mi: ith mirror; WPi: ith half wave plate; Pi: ith polarization beam splitter; BS: 50/50 beam splitter; CCD: charge coupled device; S: scattering sample.

Figure 1. Schematics of TSOPC setup and scattering medium

(a) Experimental setup to confirm the TSOPC phenomenon in biological tissues. Concentric (black) dot and circle represent vertical polarization whereas double arrow in the plane of paper symbolizes horizontal polarization. (b) and (c) show schematics for recording of tissue turbidity information and reconstruction of OPC light field, respectively. (d) Schematic of a scattering medium. a_i and b_i are the complex incident and scattered fields, respectively, at the i^th face of the scattering medium. Li; ith spherical lens; RL: relay lens; CP: compensation plate; Mi: ith mirror; WPi: ith half wave plate; Pi: ith polarization beam splitter; BS: 50/50 beam splitter; CCD: charge coupled device; S: scattering sample.

Figure 2. Demonstration of the TSOPC phenomenon through 0.46 mm thick chicken breast tissue section

(a,b) show imaging of USAF target through 0.46 mm thick agarose and chicken breast tissue sections, respectively, using plane wave illumination. (c) Reconstruction of USAF target image through 0.46 mm thick chicken breast tissue using the OPC light field. The high quality of the reconstructed image attests that OPC light field can indeed suppress turbidity by retracing its initial trajectory through the tissue. RL: relay lens; L: imaging lens; CP: compensation plate; CCD: charged coupled device. Note that in all cases, the images are brought to the sharpest possible focus.

Figure 3. TSOPC using point source illumination

(a) Schematic of TSOPC experimental setup. (b)-(f) Average radial light intensity distributions of the reconstructed spots for 0.23 mm agarose and 0.23 mm, 0.46 mm, and 0.69 mm thick chicken breast tissue sections at 0μm, 2μm, 4μm, 6μm, and 8μm displacements, respectively. PZT: lead zirconate titanate actuator; S: Sample; D: displacement of the agarose and chicken breast tissue samples; BS: beam splitter; Li: ith imaging lens.

Figure 4. Strength of the reconstructed light field under OPC and non-OPC conditions

(a) Normalized peak intensity of the reconstructed spot versus (a) the displacement of chicken breast tissue sections of varied thicknesses and (b) the thickness of the chicken breast tissue sections for zero displacement. In Fig. 4(b), I is the normalized peak intensity of the reconstructed spot whereas α is the coefficient associated with the reconstruction efficiency drop. For comparison, the red line shows expected signal drop associated with coherence based detection methods. (c) Normalized return transmission reaching the CCD, within a collection angle of 29° from the PCM, versus the displacement of chicken breast tissue sections of different thicknesses. The error bar in each curve represents the standard error of the mean.

Figure 4. Strength of the reconstructed light field under OPC and non-OPC conditions

(a) Normalized peak intensity of the reconstructed spot versus (a) the displacement of chicken breast tissue sections of varied thicknesses and (b) the thickness of the chicken breast tissue sections for zero displacement. In Fig. 4(b), I is the normalized peak intensity of the reconstructed spot whereas α is the coefficient associated with the reconstruction efficiency drop. For comparison, the red line shows expected signal drop associated with coherence based detection methods. (c) Normalized return transmission reaching the CCD, within a collection angle of 29° from the PCM, versus the displacement of chicken breast tissue sections of different thicknesses. The error bar in each curve represents the standard error of the mean.

Figure 4. Strength of the reconstructed light field under OPC and non-OPC conditions

(a) Normalized peak intensity of the reconstructed spot versus (a) the displacement of chicken breast tissue sections of varied thicknesses and (b) the thickness of the chicken breast tissue sections for zero displacement. In Fig. 4(b), I is the normalized peak intensity of the reconstructed spot whereas α is the coefficient associated with the reconstruction efficiency drop. For comparison, the red line shows expected signal drop associated with coherence based detection methods. (c) Normalized return transmission reaching the CCD, within a collection angle of 29° from the PCM, versus the displacement of chicken breast tissue sections of different thicknesses. The error bar in each curve represents the standard error of the mean.

Figure 5. Quality of the reconstructed light field under OPC and non-OPC conditions

1/e² size of the reconstructed spot versus (a) the displacement of chicken tissue sections of different thicknesses and (b) the thickness of the chicken tissue sections for zero displacement. This illustrates that the reconstructed spot size for the ideal case (no sample displacement) is independent of the tissue thickness for tissues of up to 0.69 mm in thickness. The error bar in each curve represents the standard error of the mean.

Figure 5. Quality of the reconstructed light field under OPC and non-OPC conditions

1/e² size of the reconstructed spot versus (a) the displacement of chicken tissue sections of different thicknesses and (b) the thickness of the chicken tissue sections for zero displacement. This illustrates that the reconstructed spot size for the ideal case (no sample displacement) is independent of the tissue thickness for tissues of up to 0.69 mm in thickness. The error bar in each curve represents the standard error of the mean.