Increased light penetration due to ultrasound-induced air bubbles in optical scattering media

Abstract

Light is an attractive tool for high spatial- and contrast-resolution imaging, highly sensitive molecular imaging, and target-selective therapy, and it does not exhibit the risks associated with ionizing radiation. The main limitation of using light in clinical applications is its superficial imaging and therapeutic depth caused by high optical scattering in biological media. Here, we demonstrate that the scattering and thus defocusing of the incident light can be alleviated when simultaneously delivered ultrasound generates air bubbles in the pathway of the incident light, thus increasing the light penetration. The bubbles are temporally induced by ultrasound with an intensity that is sufficiently low to avoid tissue damage and act as a Mie scattering medium in which light is scattered predominantly in the forward direction. The change in the optical scattering property caused by the ultrasound is undone after cessation of the insonification. From the results, it is expected that this proposed method will open a new route for overcoming the limitations of current optical imaging and therapeutic techniques.

Endoscopic Probe for Ultrasound-Assisted Photodynamic Therapy of Deep-Lying Tissue

“…For efficient US-assisted PDT, the size and position of the bubble cloud created by the endoscopic probe should be controllable. As shown in [20], the bubble cloud grows mainly in the axial direction as US intensity increases. Additionally, the cloud growth direction is towards an US transducer [26], which is also confirmed in Fig.…”

“…Implantable light sources can be used to reduce the distance that the light travels to the tumor site [19] if a module capable of elaborate insertions is available to avoid tissue perforation. As another approach, we proposed a way to mitigate the adverse effect of optical scattering itself on the penetration depth of light [20]. This method is based on the fact that ultrasound (US) can temporarily generate air bubbles in the light path and the bubbles act as a Mie scattering medium.…”

Ultrasound-assisted photothermal therapy and real-time treatment monitoring

“…Additionally, the increase rate of the PA signal strength outside the focal region was higher than that in the case of transmitting either US or laser only. This may result from scattering reduction due to the local temperature rise [ 33 ] and/or US-induced air bubbles [ 13 ] in the pathway of the incident laser (i.e., the US focal area), thus increasing the penetration depth of the laser. These causes of optical scattering reduction are other possible factors that lead to an increase in treatment depth in the proposed DTT because light is less attenuated and the light fluence contributing to the temperature rise of the target lesion increases.…”

“…To this end, various photothermal agents have been intensively developed although the safety of non-biological materials in clinical use still needs to be confirmed [ 8 , 10 , 11 ]. Another possible solution is to overcome the optical scattering effect itself by either the wavefront-shaping methods [ 12 ] or the temporal change of the optical scattering characteristics of a medium to Mie scattering by means of ultrasound-induced air bubbles [ 13 ]. However, more research is needed to apply those techniques to PTT in clinics although it seems promising.…”

Real-Time HIFU Treatment Monitoring Using Pulse Inversion Ultrasonic Imaging

“…At 5 s after the HIFU insonation, the echogenicity change indicated by the dashed white arrows and the tissue boundary were clearly observed (see Figure 5c). The hyperechoic spot appearing during the HIFU exposure (indicated by the dashed circle in Figure 5d) is mainly due to microbubbles induced by the HIFU [7,22] and partially acoustic impedance change of the region due to coagulation [23,24]. Since some microbubbles disappear without tissue damage after cessation of the HIFU exposure [22], the size of the hyperechoic spot may not match the actual size of the coagulation lesion measured in the tissue (see Figure 6).…”