A review of microwave-induced thermoacoustic imaging: Excitation source, data acquisition system and biomedical applications

Abstract

Microwave-induced thermoacoustic imaging (TAI) is a noninvasive modality based on the differences in microwave absorption of various biological tissues. TAI has been extensively researched in recent years, and several studies have revealed that TAI possesses advantages such as high resolution, high contrast, high imaging depth and fast imaging speed. In this paper, we reviewed the development of the TAI technique, its excitation source, data acquisition system and biomedical applications. It is believed that TAI has great potential applications in biomedical research and clinical study.

Sound Out the Deep Colors: Photoacoustic Molecular Imaging at New Depths

“…[76][77][78][79] For example, breast cancer tissue has up to 10 times higher microwave absorption than normal tissues, providing high imaging contrast for TAT. 73 Conventional TAT systems usually use high-power pulsemodulated carrier-frequency generators, and the resultant transmitted microwaves operate at frequencies ranging from 434 MHz to 3 GHz with a pulse width of a few microseconds. [80][81][82] The excitation pulse is prolonged to deposit more energy at the expense of spatial resolution.…”

“…Meanwhile, it is challenging to direct microwave propagation direction. 73 Internal light delivery. In many clinical applications, the deeply-seated organs of interest are relatively close to some body cavities.…”

“…Long wavelengths in the microwave spectrum have been employed as radiation sources in PAT to achieve greater penetration depth, a technique also referred to as thermoacoustic tomography (TAT). 73 Unlike PAT, which relies on the optical absorption contrast, the contrast in TAT stems from tissue’s dielectric properties. 74 , 75 In the frequency range from 0.1 to 10 GHz, the relative dielectric constant in soft tissues ranges from 5 to 70, and the conductivity ranges from 0.02 to 3 S/m, leading to a penetration depth of TAT up to 15 cm, 73 , 74 which is sufficient for imaging whole-body small animals and many deep organs in humans.…”

Ultraefficient thermoacoustic conversion through a split ring resonator

“…24,25 By leveraging the strong local field confinement enabled by a split ring resonator (SRR)-a building block of microwave metamaterials, 26,27 we demonstrate conversion of microwave energy to US waves at an unprecedented conversion efficiency that is three orders of magnitude higher than that of reported TA contrast agents. 15,16 With as low as 100-W peak power, which is three orders lower than that used for TA imaging, 28 our SRR generates a strong US at close to 40 dB signal-to-noise ratio (SNR) without averaging. More importantly, by leveraging the pulse wave modulation (PWM) on the input microwave pulses, the SRR presents itself as a single versatile acoustic emitter with the precisely controlled frequency of ultrasonic emission.…”

Thermoacoustic elastography: recovery of bulk elastic modulus of heterogeneous media using tomographically measured thermoacoustic measurements

“…Here, we add a new method to the family of elastography based on the contrast mechanism of thermoacoustic tomography (TAT). TAT (19)(20)(21)(22)(23)(24)(25) is an emerging imaging technique based on the thermoacoustic (TA) effect, and provides high microwave contrast, high spatial resolution, and large penetration depth. Thermoacoustic elastography (TAE) has a wide range of potential medical applications, e.g., in the detection of cirrhosis, atherosclerosis, and cancers (25)(26)(27).…”