Non-contact thermoacoustic imaging based on laser and microwave vibrometry

Microwave-induced thermoacoustic imaging (TAI), which exploits the high resolution of ultrasound imaging and high contrast of microwave imaging, is an emerging modality in medicine. Traditional TAI employs a relatively narrow-band ultrasound transducer to detect TA signals, which requires acoustic coupling and physical contact between the transducer and the sample. In certain applications, physical contact is either undesirable or not feasible. In this paper, we investigate non-contact TAI, employing either a laser or millimeter-wave (W-band) vibrometer, to remotely detect thermoacoustic-induced surface vibrations. The sensitivity of each vibrometer was first evaluated using a 1 MHz ultrasound transducer embedded inside an Agarose™ gel. The detection thresholds for the laser and microwave vibrometers were 0.02 and 1.3 nm, respectively. The sensitivity and bandwidth of the laser vibrometer were sufficient to detect TA signals from a saline gel and produce an image of embedded Rexolite™ samples. The amplitude and frequency of the surface vibrations depended on the thickness of the gel and depth of the sample. Unlike the laser vibrometer, the W-band vibrometer did not require an optically reflective surface, performing well even with a rough surface. The two types of vibrometers, therefore, are complementary and could be especially useful for non-contact applications in medical imaging or characterization of materials in high-water content media.