Abstract
Short optical pulses emitted from a tunable Q-switched laser (800 to 2000nm) generate laser ultrasound (LUS) signals at the surface of biological tissue. The LUS signal's acoustic frequency content, dependence on sample type, and optical wavelength are observed in the far field. The experiments yield a reference dataset for the design of noncontact LUS imaging systems. Measurements show that the majority of LUS signal energy in biological tissues is within the 0.5 and 3MHz frequency bands and the total acoustic energy generated increases with the optical absorption coefficient of water, which governs tissue optical absorption in the infrared range. The experimental results also link tissue surface roughness and acoustic attenuation with limited LUS signal bandwidth in biological tissue. Images constructed using 810-, 1064-, 1550-, and 2000-nm generation laser wavelengths and a contact piezoelectric receiver demonstrates the impact of the generation laser wavelength on image quality. A noncontact LUS-based medical imaging system has the potential to be an effective medical imaging device. Such a system may mitigate interoperator variability associated with current medical ultrasound imaging techniques and expand the scope of imaging applications for ultrasound.
Highlights
A noncontact medical laser ultrasound (LUS) imaging system could deliver repeatable, quantitative, and volumetric images without contacting or applying coupling material to the patient
The LUS source signals in the bovine sample, measured with experimental setup one, exhibit a similar form to LUS source signals observed in water11,12 [Fig. 2(a)]
The waveform and spectral shapes of LUS source signals observed in the tissues tested are broadly consistent with the theory for LUS source signals in pure water (Fig. 2) when acoustic attenuation, surface roughness, and the receiving transducer response are taken into account [Figs. 3(a) and 3(b)]
Summary
A noncontact medical laser ultrasound (LUS) imaging system could deliver repeatable, quantitative (sound, speed, and density), and volumetric images without contacting or applying coupling material to the patient. Noncontact LUS is identical to conventional ultrasound except there is no coupling agent and the piezoelectric source/receiver is replaced with a generation laser (for ultrasound wave generation) and a detection laser (for ultrasound wave detection). The use of optical (laser) systems in LUS enables ultrasonic imaging of objects without contact. The detection laser is a Doppler vibrometer (interferometer), which records direct or reflected LUS source waves (sensitive to sound speed and density distribution within the sample) at the surface of the sample.[13,14,15,16] All detection and generation of the acoustic waves occurs optically, without contact, at a distance, and without coupling agent or immersion of the sample.
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