Optically Generated Ultrasound for Intracoronary Imaging

Abstract

Conventional intravascular ultrasound (IVUS) devices use piezoelectric transducers to electrically generate and receive US. With this paradigm, there are numerous challenges that restrict improvements in image quality. First, with miniaturization of the transducers to reduce device size, it can be challenging to achieve the sensitivities and bandwidths required for large tissue penetration depths and high spatial resolution. Second, complexities associated with manufacturing miniaturized electronic transducers can have significant cost implications. Third, with increasing interest in molecular characterization of tissue in-vivo , it has been challenging to incorporate optical elements for multimodality imaging with photoacoustics (PA) or near-infrared spectroscopy (NIRS) whilst maintaining the lateral dimensions suitable for intracoronary imaging. Optical Ultrasound (OpUS) is a new paradigm for intracoronary imaging. US is generated at the surface of a fiber optic transducer via the photoacoustic effect. Pulsed or modulated light is absorbed in an engineered coating on the fiber surface and converted to thermal energy. The subsequent temperature rise leads to a pressure rise within the coating, which results in a propagating ultrasound wave. US reflections from imaged structures are received with optical interferometry. With OpUS, high bandwidths (31.5 MHz) and pressures (21.5 MPa) have enabled imaging with axial resolutions better than 50 μm and at depths >20 mm. These values challenge those of conventional 40 MHz IVUS technology and show great potential for future clinical application. Recently developed nanocomposite coating materials, that are highly transmissive at light wavelengths used for PA and NIRS light, can facilitate multimodality imaging, thereby enabling molecular characterization.

A Comprehensive Review on Photoacoustic-Based Devices for Biomedical Applications

“…At its core, the photoacoustic effect lies in the formation of a US wave from light absorption in a material with specific characteristics [ 73 ]. Thereupon, a source of modulated or pulsed light, a material/composite with both high optical absorption and high thermal expansion coefficient, and a US detector are indispensable to efficiently convert the light pulse into the US and subsequently detect it [ 34 , 50 , 54 , 63 , 68 , 74 , 75 ]. The supplied energy should be absorbed as much as possible by the target material and converted into heat, thus increasing its temperature.…”

“…Furthermore, since the signal generation and detection are based on light, which has an extremely small wavelength and may be transported by very thin fibers, this methodology also entails a great potential for system miniaturization. Many studies obtained photoacoustic-based transmitters with sizes between 0.84 and 2.5 mm with frequencies from 1 to 150 MHz, peak-to-peak US pressures between 2 kPa and 70 MPa, intensities of 1000–10 000 W/cm 2 , axial resolutions ranging from 40 to 380 μm, lateral resolutions from 88 to 480 μm, and penetration depth between 1 mm and 5 cm [ 1 , 59 , 60 , 63 , 64 , 75 , 76 , 79 , 80 , 81 , 82 , 83 , 84 ]. Although the developed transmitters had broad bandwidths [ 61 , 80 , 81 ], a few systems exhibited poor durability and stability as their coatings degraded with time [ 76 ].…”

“…An efficient generation of acoustic waves urges target materials with certain properties, and the choice of a suitable photoacoustic material is a determinant to successfully generating US waves [ 92 ]. Firstly, the light energy must be greatly absorbed and converted into heat, wherefore a material with a large optical absorption coefficient is convenient [ 60 , 62 , 63 , 75 ]. A target material with a high thermal expansion coefficient is also mandatory to generate powerful acoustic pressures [ 60 , 62 , 63 , 75 , 82 , 83 , 93 ].…”

“…Firstly, the light energy must be greatly absorbed and converted into heat, wherefore a material with a large optical absorption coefficient is convenient [ 60 , 62 , 63 , 75 ]. A target material with a high thermal expansion coefficient is also mandatory to generate powerful acoustic pressures [ 60 , 62 , 63 , 75 , 82 , 83 , 93 ]. Additionally, the thickness and thermal conductivity of the material with photoacoustic properties, laser pulse width, and absorption depth are other factors that also influence photoacoustic efficiency [ 92 ].…”

Toward Fluoro-Free Interventions: Using Radial Intracardiac Ultrasound for Vascular Navigation

“…Catheter-based US technologies such as IVUS and ICE are already indispensable components of cardiac catheterization labs, assisting in the assessment of the disease and device placement. The recent introduction of optical US (opUS) technology also indicates the great potential for the use of catheter-based US for cardiovascular interventions (Little et al 2020). US offers a radiation-free alternative for real-time image guidance.…”