Abstract
This work concerns the development and testing of a setup that uses laser-induced ultrasound sources to achieve synthetic transmit aperture ultrasound imaging. The sources are created by sequentially firing 32 contiguous multi-mode optical fibers to illuminate an optically absorbing film with nanosecond-pulsed laser light. Ultrasound is generated by the photoacoustic effect and insonifies the sample under investigation. Ultrasound that has interacted with the sample is detected in reflection mode using a conventional ultrasound transducer array. We present a custom-developed optical fiber multiplexing setup that enables sequential firing of the optical fiber array and characterize the acoustic fields produced by the laser-induced approach using hydrophone measurements. The integrated setup is used to make images of wire phantoms. Following this, images are taken of a breast-mimicking phantom as well as the wrist of one of the authors. Imaging results from the new approach and from conventional ultrasound imaging are compared. The lateral and axial point-spread function values show broad agreement between the two approaches, whereas the phantom and in vivo images exhibit some differences in contrast values. This work is, to our knowledge, the first instance of laser-induced ultrasound synthetic transmit aperture imaging using a clinical ultrasound array.
Highlights
Laser-induced ultrasound (LIUS), that is, the use of an external photoacoustic source for transmission and pulse-echo ultrasound imaging, is a relatively recent area of interest in the biomedical field.1 Such sources are readily made by the use of elastomeric composites doped with various types of optically absorbing particulate carbon.2–8 The efficiency of a material’s photoacoustic response is determined by the following equation:9p0(x, y, z) 1⁄4 ΓηthμaF(x, y)eÀμaz: (1)Here, ηth and μa are the heat conversion efficiency percentage and optical absorption coefficient, respectively, and F(x, y) represents the light fluence on the material surface
We present a custom-developed optical fiber multiplexing setup that enables sequential firing of the optical fiber array and characterize the acoustic fields produced by the laser-induced approach using hydrophone measurements
Several implementations of LIUS imaging in conjunction with photoacoustic tomography, generally for the purpose of speedof-sound mapping and compensation in photoacoustic tomographic reconstruction, have been shown in the previous decade
Summary
Laser-induced ultrasound (LIUS), that is, the use of an external photoacoustic source for transmission and pulse-echo ultrasound imaging, is a relatively recent area of interest in the biomedical field. Such sources are readily made by the use of elastomeric composites doped with various types of optically absorbing particulate carbon. The efficiency of a material’s photoacoustic response is determined by the following equation:9p0(x, y, z) 1⁄4 ΓηthμaF(x, y)eÀμaz: (1)Here, ηth and μa are the heat conversion efficiency percentage and optical absorption coefficient (mmÀ1), respectively, and F(x, y) represents the light fluence on the material surface. The Grüneisen parameter Γ depends on the thermal expansion coefficient β (KÀ1), the isothermal compressibility κ (PaÀ1), the density ρ (kg mÀ3), and the isochoric specific heat capacity CV (J kgÀ1 KÀ1) as follows: Γ 1⁄4 β κρCV : (2) This means that the elastomers are usually selected to have a high thermal expansion coefficient as well as good acoustic coupling with biological samples, materials like polydimethylsiloxane (PDMS) and epoxy are often used. Several implementations of LIUS imaging in conjunction with photoacoustic tomography, generally for the purpose of speedof-sound mapping and compensation in photoacoustic tomographic reconstruction, have been shown in the previous decade One such implementation describes 3D photoacoustic imaging of an in vivo mouse in a cylindrical geometry, including transmission speed-of-sound maps taken with LIUS.. Wurzinger et al demonstrated simultaneous photoacoustic and reflection mode ultrasound tomography in ex vivo transparent zebrafish.
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