Tuesday, January 24, 2012

PhotoAcoustic Imaging and its Application

Photoacoustic imaging is a method combines the advantages of optical imaging (high contrast) and ultrasonic imaging (high spatial resolution). This results in various new applications in the field of biology, medicine, industry and material science.

Principle
To generate photoacoustic signals short laser pulses (in the range of nanoseconds) are used. The electromagnetic energy (laser) is absorbed in the sample. Due to the thermoelastic expansion in the region of absorption – the so called photoacoustic effect – a broadband ultrasonic signal is launched. The photoacoustic signal is recorded at different positions at the surface of the sample by a special transducer. Taking such a set of data it is possible to calculate either slices of the objects or the whole volume using reconstruction algorithms such as Time Reversal, Back Projection, or other methods. In Fig. 1 a scheme of the principle of photoacoustic tomography can be seen. 

Fig 1. PhotoAcoustic Effect



Example Application: Material Inspection 
The remote contactless photoacoustic imaging with short excitation pulses on semitransparent solid polymer samples can be used for material inspection. Solid semitransparent samples are excited with pulses from a short pulse laser. The local absorption of the electromagnetic radiation leads to generation of broadband ultrasonic waves. Ultrasonic waves arriving on the sample surface are detected with a confocal Fabry-Perot interferometer. After data acquisition the absorbed energy density is reconstructed by utilizing an F-SAFT algorithm.
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