Photoacoustic temperature measurement based on dual-wavelength method

Photoacoustic temperature measurement is a novel technique in which photoacoustic effect is used to measure temperature. It has the advantages of non-invasiveness, high sensitivity and deep penetration depth, which is suitable for monitoring the temperature distribution for the safe deposition of heat energy and efficient destruction of tumor cells during thermotherapy or cryotherapy. However, the present reported methods usually use one single wavelength for photoacoustic temperature measuring and are vulnerable to systematic and environmental influence, including the instability of system caused by fluctuation of laser energy, position displacement of transducer, and tissue complexity, which could reduce the measuring accuracy and stability. To solve this problem, a new photoacoustic temperature measuring method by employing two laser wavelengths is proposed in this paper. Firstly a brief theoretical analysis of dual-wavelengths photoacoustic temperature method is performed based on the linear relationship between photoacoustic signal and tissue temperature under two different wavelengths. Then two different samples including phantom of graphite and ex vivo pig blood are experimented respectively. The experimental temperature is set to be in a range of 26 degrees C-48 degrees C, which is controlled by a precise hot plate. And for improving the detection accuracy, the dual-wavelengths are selected as 760 and 900 nm for graphite phantom, 820 nm and 860 nm for ex vivo pig blood according to their absorption spectrum repetitively. The obtained results reveal that the temperature measuring correlation coefficients by dual-wavelength method can reach to 0.98 in graphite phantom and 0.99 in ex vivo tissue, respectively. And the average measurement deviation decreases to 0.88 degrees C in dual-wavelength method from 1.31 degrees C for the traditional single wavelength method for graphite phantom. While in ex vivo tissue, the measurement deviation decreases to 0.90 degrees C in dual-wavelength method from the average value 1.45 degrees C for the single wavelength method. Furthermore, the standard deviations of error are respectively reduced by an average of 38% in graphite phantom and an average of 30% in ex vivo tissue, respectively. These results indicate that the dual-wavelength method of photoacoustic temperature measurement can improve both the measuring accuracy and stability, and has a potential to be applied to medical therapy and other biomedical fields.