A frequency‐domain non‐contact photoacoustic microscope based on an adaptive interferometer

Abstract

A frequency-domain, non-contact approach to photoacoustic microscopy (PAM) that employs amplitude-modulated (0.1-1 MHz) laser for excitation (638-nm pump) in conjunction with a 2-wave mixing interferometer (532-nm probe) for non-contact detection of photoacoustic waves at the specimen surface is presented. A lock-in amplifier is employed to detect the photoacoustic signal. Illustrative images of tissue-mimicking phantoms, red-blood cells and retinal vasculature are presented. Single-frequency modulation of the pump beam directly provides an image that is equivalent to the 2-dimensional projection of the image volume. Targets located superficially produce phase modulations in the surface-reflected probe beam due to surface vibrations as well as direct intensity modulation in the backscattered probe light due to local changes in pressure and/or temperature. In comparison, the observed modulations in the probe beam due to targets located deeper in the specimen, for example, beyond the ballistic photon regime, predominantly consist of phase modulation.

Review on practical photoacoustic microscopy

“…Third, most PAMs require a coupling medium such as water or US gel, which can limit many intraoperative applications [135]. Many researchers are already exploring ways to overcome these problems using arrayed US transducer [136], fast PRF laser [137], compressed sensing [99], ultra-broadband detector [128], and non-contact detection [106,138]. In spite of these challenges, PAM has tremendously impacted the life and medical science researches.…”

Towards non-contact photoacoustic imaging [review]

“…George et al [ 181 ] developed a non-contact photoacoustic microscope utilizing a photorefractive crystal-based interferometer for imaging of red blood cells and ex-vivo porcine retinal samples. The schematic diagram of the system and representative images recorded with the system are presented in Fig.…”

“…(C) (left) photoacoustic image of blood smear showing RBC; (right) corresponding bright field image. Reprinted with permission from [ 181 ]. …”

Full image reconstruction in frequency-domain photoacoustic microscopy by means of a low-cost I/Q demodulator

“…Therefore, in order to expand further the rich potential of PA microscopy in biomedical research, it is necessary to reduce drastically the cost of respective PA systems, improving also their multispectral imaging capabilities. Within this framework, emphasis has been lately given to the development of low-cost PA imaging devices integrating diode lasers, and implemented both in TD (through the generation of short light pulses) [3,4] and frequency-domain (FD), using a sinusoidal modulation of optical intensity [5,6]. Lock-in detection is quite common in FD imaging systems, offering high sensitivity in the recording of PA signals, however, such a solution is also rather expensive, especially when the required bandwidth is in the order of several MHz.…”