CABI

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Photoacoustics

Photoacoustic imaging is a promising new in vivo modality that uses laser generated ultrasound to image tissue on a mm-cm length scale. The technique relies upon the absorption of low energy nanosecond pulses of visible or near infrared laser light by specific tissue chromophores to excite broadband ultrasound waves. These waves are encoded with the optical properties of the tissue and, by recording them over the tissue surface using an array of ultrasound receivers, a 3D absorption based image can be reconstructed. The distinguishing feature of the technique is that it combines the spectroscopic based specificity of optical imaging with the high spatial resolution of ultrasound enabling functional as well as structural images to be obtained. The principal source of endogenous contrast is haemoglobin due to its strong optical absorption allowing high contrast 3D images of the vasculature to be acquired. Furthermore, by exciting at different laser wavelengths, spectroscopic maps of blood oxygen saturation can be obtained and, through the use of Doppler photoacoustic techniques, blood flow can be imaged. This makes for a powerful imaging tool for studying angiogenesis, inflammatory responses, ischaemia and other processes characterised by changes in the structure and function of the vasculature. There is the further prospect of revealing processes at a molecular level using targeted contrast agents or genetically expressed absorbers for studying gene transcription and other genetic processes. This ability to provide an anatomical, functional and molecular imaging capability all within a single modality offers tremendous opportunities for  pre-clinical studies of, for example, tumour pathophysiology, brain function and cardiovascular disease. Having recently demonstrated the feasibility of the technique, a novel multiwavelength photoacoustic scanner, the first of its type worldwide, is currently being installed in CABI. This will provide 3D non invasive in vivo images to sub-cm depths with a spatial resolution in the range 50µm to 200µm and will complement the current state-of-the-art MRI/PET/SPECT and other imaging modalities in CABI.


For information about Photoacoustics please have a look at the webpage of the UCL Photoacoustic Imaging Group

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Figure1................................................................................................................................................................
Photoacoustic image of the vasculature in the mouse brain obtained using an excitation wavelength of 590 nm. (left) Schematic of superficial cerebral vascular anatomy: A – Superior sagittal sinus, B – Transverse sinus, C – inferior cerebral vein. (top right) x–y maximum intensity projection (MIP) of 3D photoacoustic image and (bottom right) y–z MIP E. Zhang E, Laufer J, Raivich G, Beard PC, Applied Optics, Vol 48, (10), 2009

Fig2_hand

Figure 2..............................................................................................................................................................
In vivo photoacoustic image of the vasculature in the palm using an excitation wavelength of 670 nm. Left: photograph of the imaged region, middle: volume rendered image.

  • An animated representation of this image
    and right: lateral slices at different depths. The arrow ‘A’ indicates the deepest visible vessel, which is located 4 mm beneath the surface of the skin. Laufer J, Zhang E, Pedley RB, Beard PC, Physics in Medicine and Biology, 54, pp1035–1046, 2009

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