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.

Keywords: IVUS; OPUS; imaging; intravascular ultrasound; optical ultrasound; optoacoustics.

Figure 1

(A) Generation and reception of OpUS. The schematic (top) includes both a transmitter and a receiver. Lower-left inset (dashed green box): scanning electron microscopy image of the transmitter fiber-tip coated with nanocomposite (scale bar: 50 μm) (15). Lower-right inset (dashed red box): optical microscopy of the fiber-optic receiver (19). (B) 2D longitudinal M-mode OpUS imaging of ex-vivo human coronary artery tissue with a lipid pool (LP), calcification (Ca), and corresponding histology (haematoxylin and eosin staining) (scale bars: 2 mm).

Figure 2

(A) Schematic of side-viewing optical ultrasound imaging probe with pulsed excitation light transmitted along an optical fiber (yellow), reflected at a mirror (M) and absorbed at the nanocomposite (NC) coating surface of an epoxy-based (E) optical transducer housing. Ultrasound pulses reflect from tissue and the echoes are received through interferometric interrogation of the Fabry-Pérot (FP) cavity at the distal end of the receiving optical fiber (blue). The probe is rotated about its axis to achieve circumferential imaging. (B) Rotational optical ultrasound images obtained within an ex-vivo swine carotid artery (20). (C) Co-registered optical ultrasound (gray-scale) and photoacoustic images of human aortic tissue (HA) using a gold nanoparticle/PMDS composite transmitter. Areas of lipid are displayed as a color signal. The tissue was secured to a cork ring (CR) and metal base (MB) for imaging. Corresponding histology of the green boxed area highlights a lipid pool (L), tunica media (T) and adventitia (A) (9).