Two-dimensional spatiotemporal monitoring of temperature in photothermal therapy using hybrid photoacoustic–ultrasound transmission tomography

Abstract

Abstract. Recently, we presented an add-on to a photoacoustic (PA) computed tomography imager that permits the simultaneous imaging of ultrasound (US) transmission parameters such as the speed of sound (SOS), without additional measurements or instruments. This method uses strong absorbers positioned outside the object in the path of light for producing laser-induced US to interrogate the object in a conventional PA imager. Here, we investigate the feasibility of using this approach, first with PA to pin-point the location of photothermal therapeutic agents and then with serial SOS tomograms to image and monitor the resulting local temperature changes when the agents are excited with continuous wave (CW) light. As the object we used an agar-based tissue-mimicking cylinder carrying beads embedded with different concentrations of gold nanospheres. PA and SOS tomograms were simultaneously acquired as the gold nanospheres were photothermally heated using a 532-nm CW laser. In a first approximation, using the relation between SOS of water and temperature, the SOS tomograms were converted into temperature maps. The experimental results were verified using simulations: Monte Carlo modeling of light propagation through a turbid medium and using the obtained absorbed energy densities in heat diffusion modeling for spatial temperature distribution.

Passive ultrasound aided acoustic resolution photoacoustic microscopy imaging for layered heterogeneous media

“…15,16 Recently, the passive element optical excitation technique is introduced for hybrid ultrasonic and photoacoustic tomography imaging. [17][18][19] By fixing an additional strong absorber in front of laser lights, the ultrasound wave is excited by the laser shooting on the absorber. After performing a similar USCT type of measurement, a fan-beam reconstruction allows the SOS tomograms to be generated 20,21 from this laser-induced ultrasound.…”

Thermometry and ablation monitoring with ultrasound

“…Lihong Wang and colleagues reported similar success applying photoacoustic thermometry in phantoms (188) and chicken tissue (189). In a dual modality experiment with ultrasound transmission tomography, a group at the Univeristy of Twente (Netherlands) combined a photoacoutic tomography (PAT) image with speed-of-sound tomography to measure temperature increase of up to 9° C (190). …”