We present a 3D photoacoustic and ultrasound tomographic system intended for imaging of breast phantoms with the capacity to detect millimetric objects. The speciality of the ultrasound imaging part is that transmitters based on laser-induce ultrasound (LIUS) are used for acoustic generation. We describe the design and development of transmitters including an absorbing layer for photoacoustic generation.
We present laser-induced ultrasound (LIUS) imaging, using a conventional linear ultrasound probe as a receiver. The LIUS source consists of a 40 μm thick film of Carbon Black-doped PDMS. Illumination of this LIUS transmitter with a 10 ns pulsed Nd:YAG laser with a 10 Hz repetition rate leads to the generation of a short, unipolar ultrasound pulse as a consequence of the photoacoustic effect. Two synthetically focused imaging techniques will be presented: coherently compounded multi-angled plane wave imaging (PWI) and synthetic transmit aperture imaging (SAI) . In the PWI case a planar LIUS transmitter, matched in size to the conventional probe aperture, is used. In the SAI case, the same film is illuminated sequentially at different locations along the aperture by an array of multimode optical fibres. For both PWI and SAI a comparison between conventionally acquired and LIUS images is made, as well as a cross-comparison between PWI and SAI. Images of wire phantoms, speckle analysis and finally images of tissue-mimicking phantoms demonstrate the image quality and advantages offered by LIUS sources. Aside from generating shorter pulses for enhanced resolution, the continuous nature of the absorber and the illumination spot provides a cleaner, more homogeneous plane wave field. The outlook for these unconventional US sources and their relative advantages and disadvantages are discussed.