Photoacoustic imaging uses optical absorption as the image contrast, which gives information about the molecular composition of tissues. Photoacoustic imaging can be used to measure blood oxygenation or visualise the spread of medical dyes or therapeutic agents within tissue. We are interested in photoacoustic imaging combined with micro-ultrasound imaging to identify where tumours are for biopsy or resection, to understand how tissues change with disease progression, or to guide treatments such as phototherapies.
The photoacoustic effect is the result of absorption of light by molecules, resulting in small amount of heating and local elastic expansion, which is an acoustic wave. When a short, nano-second laser pulse is used to illuminate tissue, the resulting photoacoustic wave can be detected by ultrasound transducers and an image of the absorber can be formed. Different molecules have different absorption spectra, such that by illuminating tissues with multiple laser wavelengths, typically in the infrared range, the relative amounts of the molecules can be calculated. For example, the absorption spectra of de-oxygenated and oxygenated haemoglobin are different, so photoacoustic images acquired at each of their peak absorption wavelengths can be used to map the variation of oxygen saturation within tissue.
Both the receiving ultrasound system and the laser illumination need to be appropriate for the tissue being imaged. For example, there must be sufficient light fluence at the target tissues to produce a detectable photoacoustic signal, which means the illumination source (e.g. optical fiber) may need to be placed near or within the tissue. Similarly, the receiving transducer needs to be placed near the target tissues and acquire anatomical images with sufficient resolution, yielding a trade-off with image depth and photoacoustic sensitivity, which are ultrasound frequency dependent.
We are investigating combined micro-ultrasound and photoacoustic imaging systems for imaging prostate cancer, using an existing transrectal micro-ultrasound imaging system. We are developing a diffuse optical fiber for transurethral light delivery to the prostate to increase fluence in the prostate compared to transrectal illumination. We are testing photoacoustic adaptation of miniature micro-ultrasound arrays suitable for imaging in oral cavities and small spaces. The photoacoustic targets include endogenous contrast sources such as blood oxygenation and exogenous sources such as porphysome theranostic agents.
