OCT Speckle Contrast

What is speckle?

Speckle is exhibited by a coherent imaging modality (e.g. ultrasound) and results from the coherent addition of multiple waves of different phases. It is what gives a rough surface illuminated by laser light its characteristic granular or mottled appearance. In OCT, speckle may arise from different processes as indicated in Fig. 1, but scattering of multiple waves from within the volume of sample probed by the system is nearly always present.

Figure 1. Scattering processes in OCT. Speckle results from the coherent superposition of multiple backscattered waves from particles within the sample volume.

Can speckle contain useful information?

Speckle is widely considered to be a corrupting influence in OCT images because it can mask the true structure of a sample, but can it provide useful information? We have shown that under appropriate conditions the speckle contrast ratio (CR) is correlated to scatterer density¹. The speckle CR is defined as the ratio of the OCT envelope signal’s standard deviation to its mean. Scatterer density is represented by the effective number of scatterers (ENS) in the coherence volume.

Figure 2. (a) Signal strength and CR vs. axial physical distance for 0.51-um microsphere water suspension; (b) CR peak vs. ENS for 0.51 and 1.01-um microsphere suspensions. Figures adapted from Refs. [1,2]

This permits us to obtain sample information that is below the resolution limit of the OCT system. A potential application of speckle CR measurement is detection of cancer, as cell disorder due to malignancy may affect scatterer density.

References

1. T. R. Hillman, S. G. Adie, V. Seemann, J. J. Armstrong, S. L. Jacques, and D. D. Sampson, Correlation of static speckle with sample properties in optical coherence tomography, Optics Letters, vol. 31, no. 2, pp. 190-192, 2006.
2. S. G. Adie, T. R. Hillman, D. D. Sampson, Investigation of speckle contrast ratio in optical coherence tomography in Proc. SPIE, vol. 6085, 608506, (BIOS 2006: Complex Dynamics and Fluctuations in Biomedical Photonics III, V. V. Tuchin, Ed., SPIE, Bellingham, WA), 2006.