Photoacoustic Tomography

Photoacoustic tomography uses high-intensity laser pulses to illuminate the body and obtain three-dimensional images.  Blood vessels and certain tissues below the skin's surface absorb the laser light and emit acoustic waves in response.  These acoustic waves result from the photoacoustic effect, a phenomenon originally discovered by Alexander Graham Bell.  Today, we recognize that these photoacoustic waves provide rich information about the blood vessels and tissues below the skin.  Photoacoustic tomography is the process of generating, then measuring the ultrasonic emissions, and finally reconstructing the measured photoacoustic waves into three-dimensional images representative of tissues below the surface.  Superior image resolution at depth is the main advantage photoacoustic tomography has over other optical imaging methods.  Furthermore, photoacoustic tomography has exquisite spectral sensitivity to molecular constituents such as haemoglobin, deoxyhaemoglobin, and melanin.  Because of these unique properties, photoacoustic tomography is thought to be a promising technology for early cancer detection.

Recently, our group developed photoacoustic imaging for intraoperative margin assessment.  The technique generates 3D images of the optical characteristics of tissues from sound waves generated in response to laser pulses. In breast tissue, fat has distinct optical absorption peaks in the near infrared region of the electromagnetic spectrum, which we hypothesized could be used to distinguish between tumour and healthy tissue using photoacoustic imaging.  Studies have shown that healthy breast tissue has significantly higher fat levels compared to cancerous tissue. Using a custom photoacoustic tomography system, we tested the hypothesis on human breast specimens collected from over 100 lumpectomy procedures.  We found that photoacoustic tomography had a sensitivity of 80% and a specificity of 89% for predicting cancerous tumour margins. It was estimated that photoacoustic tomography could potentially reduce repeat surgeries by 75% if the results were used by the surgeon to intervene during surgery.