Photoacoustics

Photoacoustics in Bio-photonic Research

Photoacoustics (or optoacoustics) is based on photoacoustic (PA) effect which relates acoustic wave to be generated by modulated or pulsed light to excite medium. The absorbed energy of light pulses in the medium is changed to heat, resulting fast thermal expansion and restoration to generate acoustic (usually ultrasonic) wave. The various techniques based on the PAeffect have been widely used in industrial measurements and material science. Nowadays, PA techniques are important method in biomedical and biological research, especially in two aspects: PA imaging and bio-PA spectrum. The key advantage of PA imaging for tissues is it combining the merits of both pure ultrasound imaging and pure optical imaging, i.e., high ultrasonic resolution and high optical contrast. Bio-photoacoustic spectrum simultaneously senses biologic tissues’ light absorption and thermal properties which are closely related to tissues’ physical characteristics, metabolism and pathological status, etc.

Photoacoustics was used to study human glucose and its effect in liquid and tissues in our early research. We found that PA signal amplitudes to be excited at wavelength of 905 nm ware increased by adding glucose in water, milk suspensions, bio-tissue samples, and human blood. The signal increase is due to the contribution of glucose to thermal elastic parameters (such as thermal expansion coefficient and specific of heat), matching of index of refraction, and reducing the optical scattering in turbid material. Therefore, PA measurement has higher detection sensitivity for glucose comparing with pure optical and ultrasonic transmission methods.  Some results are shown in below graphs.

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Fig 1. Relationship between PA amplitude change (at 905 nm wavelength) and glucose concentration in water and 3% milk (a), in tissue sample (b), and in the fresh human blood sample (glucose was added in the blood with initial glucose level of 88.2 mg/dl, measured at room temperature).

Beside glucose, mitochondrion and yeast suspensions were sill studied by photoacoustics, as PA method has higher detection sensitivity, pure optical absorption measurement and much less interferoence by background scattering in comparison with traditional absorption spectroscopy. When the mitochondria sample was fully oxidized and then fully reduced, the photoacoustic signal at 355 nm increased 26%, much larger than the case of spectrophotometry (1.24%). Although fluorometry has a higher sensitivity, it is less suitable for rapid and easy determination of NADH (nicotinamide adenine dinucleotide + hydrogen) content of intact mitochondria because of calibration problems.

Fig 2. The PA signal amplitude plotted against time when different amounts of mitochondria and inhibitors or substrates of their metabolism were added in a 2-mm cuvette. (The orange arrows indicate the times when mitochondria were added, the red arrows point the times when inhibitors or substrates were added, and the thick green line highlights the periods of stable signal amplitude).

Laser-induced micro-bubbles in nanoparticle suspensions and the micro-bubble explosion may produce shock wave which is much stronger than the thermal elastic PA wave. This effect has significant application in biomedicine such as greatly improving the diagnostic sensitivity of early tumors and cancer cells, the photo-thermal therapy, and cosmetic or drug delivery in skin. We investigated threshold energy fluence of shock wave generation in gold, titanium dioxide and zinc oxide nanoparticle suspensions, respectively. The experiment results demonstrated that gold nanoparticles (NPs) generate shock wave with lower threshold value of laser fluence and higher efficiency than oxide NPs do. Smaller gold particles have higher threshold energy fluence of micro-bubble generation in suspensions than larger gold particles do, where the situation is contrary in the case of TiO2 particles.

 

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Fig 3. Signal amplitude-rank distributions from 40-nm (a) and 100-nm (b) gold sphere suspensions as a function of particle number in the optical path length, where the energy fluence of laser pulse was 90 mJ/cm2 at both 520-nm (a) and 570-nm (b) wavelengths. The data was collected in 1000 laser pulses for every recording.

Depth imaging of blood vessel, tumor and tissue inhomogeneity in human body is important for medical diagnosing and evaluating arterial diseases, breast cancers and other tissue abnormality. The traditional X-ray radiography and ultrasonic imaging has low contrast, magnetic resonance imaging is limited by its expensive cost, whereas optical tomography usually has a low resolution because of high optical scattering in tissue. Laser photoacoustic (PA) imaging combines the advantages of high optical contrast and ultrasonic high resolution, therefore, it is suitable for deeply imaging thin blood vessels, as well as small tumors which is very important for early cancer diagnose and treatment. The most common exciting source in PA tissue imaging is a high cost and high energy Q-switched Nd:YAG laser, but we attempted to choose a pulsed laser diode (with pulse energy ~20 µJ @ 905 nm) for developing low cost and portable instrument. The experimental results showed that blood vessel with diameter as small as 0.2 mm and tumor with size of 2.5 mm can be monitored in the depth up to 12 mm at least.

Fig 4. PA transmission-mode signals of two blood vessels with diameters of 0.2 mm and 0.4 mm (marked by red lines in left graph) and a 2.5 mm-tumor phantom (red spot in right graph) in tissue phantom. For convenient showing, the received PA signals (blue lines) by the piezoelectric transducers are overlapped with phantoms.

PA excitation was combined with skeletal quantitative ultrasound (QUS) for multi-mode ultrasonic assessment of human long bones. This approach permits tailoring the ultrasonic excitation and detection to efficiently receive the fundamental flexural guided wave (FFGW) through a top soft-tissue of bone. The FFGW is a clinically relevant indicator of cortical thickness. The approach probably uses for replacing the commonly used dual-energy x-ray absorptiometry (DEXA or DXA) and peripheral quantitative computed tomography (pQCT) for osteoporosis evaluation which both modalities lack assessment of elastic properties of bone, and the devices are expensive and have x-ray radiation. Based on theoretical analysis and experiment result, we optimized optical wavelength of 1250 nm for effectively exciting low frequencies of FFGW in porcine leg bones with a top soft coating. By applying a specific signal processing, the phase velocities of FFGW in coated bone samples can be extracted from scanning distance-time graphs, and cortical thickness of the bones can be deduced fitting to theoretical models. The work cooperates together with University of Jyväskylä and University of Helsinki.

Fig 5. Scanning distance-time graph from a coated bone, in which 1 marks first arriving wave (FAS) (Slow guide wave (SGW) is not seen in the graph, because of the top soft coating.).

Fig 6. Phase velocity results for coated bone samples at a fixed frequency of 50 kHz. Dashed line represents the A0 model and solid line the BL1 model.

References

Z. Zhao and R. Myllylä, “Photoacoustic determination of glucose concentration in whole blood by a near-infrared laser diode”, Biomedical Optoacoustics II, Proc. SPIE 4256, 77-83 (2001). [PDF]

Z. Zhao and R. Myllylä, “The scattering effect of glucose on near-infrared photoacoustic detection sensitivity in tissue measurement” Asian J. Phys. 10(4), 487-492 (2001).

Z. Zhao, R. Myllylä and M. Kinnunen, “Photoacoustic depth monitoring of human blood in tissue phantom by diode laser”, Proceeding Northern Optics, Helsinki, P061 (2003).

M. Kinnunen, Z. Zhao and R. Myllylä, “Effect of Glucose on Optical Properties of Intralipid –Measurements with Photoacoustic and Optical Techniques”, Proceedings of OSAV´2004, the International Topical Meeting on Optical Sensing and Artificial Vision, Saint Petersburg, Russia, 248-255 (2004).

Z. Zhao and R. Myllylä, “Measuring the optical parameters of weakly absorbing, highly turbid suspensions by a new technique: photoacoustic detection of scattering light” Appl. Opt. 44, 7845-7852 (2005). [PDF]

Z. Zhao and R. Myllylä, “Scattering photoacoustic study of weakly-absorbing substances in aqueous suspensions” J. de Phys. IV 137, 385-390 (2006). [PDF]

Z. Zhao, J. Hast, R. Myllylä, M. Känsäkoski, "Pulsed photoacoustic measurements of suspensions: in case study of mitochnodrial NADH and its phantom", Proc. SPIE 7142, 714213 (2008). [PDF]

Z. Zhao, R. Myllylä, “Laser-induced microbubbles in gold and oxide nanoparticle suspensions: photoacoustic detection”, Proc. SPIE 7376, 737606 (2010). [PDF]

Z. Zhao, P. Moilanen, R. Myllylä, et al, “Photo-Acoustic Excitation and Detection of Guided Ultrasonic waves in bone samples covered by a soft coating layer”, 2012 Photonics Asia, Proc. SPIE 8553, 85531E (2012).[PDF]

Z. Zhao, R Myllylä “Multiparameter measurement of absorbing liquid by time-resolved photoacoustics”, Applied Optics 51, 1061-1066 (2012). [PDF]

Last updated: 9.9.2016