Acoustic Estimation of Thermal Distribution in the Vicinity of Femtosecond Laser-Induced Optical Breakdown

• Published in: IEEE transactions on biomedical engineering Vol. 53; no. 11; pp. 2347 - 2355
• Main Authors: Zohdy, M.J., Tse, C., Ye, J.Y., O'Donnell, M.
• Format: Journal Article
• Language: English
• Published: United States IEEE 01.11.2006; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Acoustic signal detection; Acoustic thermal estimation; Acoustics - instrumentation; bioeffects; Biomedical acoustics; Biomedical optical imaging; bubbles; Computer Simulation; Electric breakdown; Equipment Design; Equipment Failure Analysis; femtosecond lasers; high-frequency ultrasound; Laser modes; Laser Therapy - instrumentation; Laser Therapy - methods; laser-induced breakdown; Models, Biological; Optical pulses; Temperature; Thermography - instrumentation; Thermography - methods; Ultrafast optics; Ultrasonic imaging; Ultrasonic technology; Ultrasonic transducers
• ISSN: 0018-9294; 1558-2531
• DOI: 10.1109/TBME.2006.877111

Laser-induced optical breakdown (LIOB), or photodisruption, can generate individual microbubbles in tissues for biomedical applications. We have previously developed a co-localized high-frequency ultrasound system to detect and characterize these laser-induced microbubbles. Because ultrasound speed varies with temperature, this system can also be used to directly estimate thermal effects in the vicinity of photodisruption. In this study, individual bubbles (sizes 60-100 mum) were created at the bottom of a water tank using a 793-nm, 100-fs Ti:Sapphire laser pulsed at 250 kHz. During and after breakdown, pulse-echoes from the tank bottom in the region surrounding a bubble were recorded with a single-element 85-MHz ultrasonic transducer, and temperature-dependent pulse-echo displacements were calculated using phase-sensitive correlation tracking. These displacements were then fit to a finite-element heat transfer model to estimate the effective thermal distribution. Estimates were calculated for laser exposure times ranging from 6.25 to 312.5 ms (1600 to 78 000 laser pulses), at 1.5 and 4 J/cm 2 fluences. Results suggest a minimal temperature increase (<1deg C) within 100 mum of a bubble created with <1600 laser pulses at 1.5 J/cm 2 fluence. This implies that LIOB can be controlled to be thermally noninvasive in the bubble vicinity