Wavefront-shaping for biomedical imaging

Optical scattering limits light penetration in tissue

Light traveling in biological tissue becomes scattered in seemingly random directions, resulting in a speckle pattern as shown in the image above. This effect limits the effective depth of optical imaging to mere millimeters beneath the skin. Beyond this depth, information carried by the light becomes scrambled, making it impossible to acquire a clear image.

Wavefront shaping corrects for scattering

Principles of wavefront shaping. (a) Light passing through a scattering object becomes distorted, resulting in low intensities within tissue. (b) By applying a phase pattern on the light, scattering can be corrected for and light is refocused within the tissue.

To overcome this issue, I have developed new methods to correct for optical scattering using wavefront shaping technology. In wavefront shaping, a liquid crystal screen called a spatial light modulator is used impart a phase shift on the optical field. This shift is applied in a two-dimensional pattern, which causes resulting light beam to change its shape.

To focus light into a scattering sample, I used photoacoustics or ultrasound to generate a signal that originates from within the sample. This signal is originally weak due to the low amount of light that arrives at the location. Then, by using a search algorithm, the pattern on the spatial light modulator is optimized to obtain the maximum signal. It turns out that this same pattern causes light to be focused within the scattering object.

Wavefront shaping improves photoacoustic signals by an order of magnitude

Experimental results showing the effect of wavefront shaping within a scattering object. (Left) The pattern on the SLM and (right) the resulting optical spot within the scattering object. When the optimized pattern is applied, a focal spot is generated.

I have applied this method to improve photoacoustic imaging. In this imaging technique, an intense pulse of light directed into tissue, where it is absorbed by an internal structure, such as a blood vessel. The absorbed energy is converted into hear, causing the structure to rapidly expand and contract, which in turn releases a pressure wave in the tissue. This wave can be measured on the outside using an ultrasonic sensor.

By using wavefront shaping, I showed that the photoacoustic signal could be increased up to several orders of magnitude. This finding has important findings for biomedical imaging, as it demonstrates that signals originating within thick tissue samples can be increased, thereby allowing deep structures to be imaged.

Future work

I am interested in continuing to develop this technology for biomedical imaging. In particular, I am putting together plans to apply this technology to study 3D biological structures at the single-cell level. Wavefront shaping could also be used to improve optical resolution in microscopy.