Types/Applications of Photoacoustic Contrast Agents: A Review

Hyuncheol Kim,Jaehun Jung,Mingyun Kim,Yongho Jang

doi:10.3390/photonics8070287

Abstract

Ultrasound imaging, one of the common diagnosis techniques, is frequently used since it is safe, cost-efficient technique and real-time imaging can be conducted. However, various organs and tissues reflect ultrasonic waves, which leads to difficulty in imaging small biomolecules and to a low spatial resolution for deep-tissue images. As such, there have been significant advances in photonics and optical molecular probes in recent years, and photoacoustic (PA) tomography (PAT) has emerged as a promising modality that can overcome the limitations of ultrasound. PAT relies on the photoacoustic effect, which is the conversion of absorbed optical energy into acoustic energy. Since fewer biomolecules exhibit the photoacoustic effect compared to the scattering or reflection of ultrasound, PAT can be employed to generate high-resolution images. PAT also has a number of other advantages when compared to conventional biomedical imaging modalities such as optical tomography, ultrasound imaging, computed tomography, positron emission tomography and magnetic resonance imaging. This review provides a general overview of the contrast agents used for PAT, including organic, inorganic and hybrid contrast agents, and describes their application. This review also identifies limitations of current PAT contrast agents and suggests future research directions for their development.

Highlights

In the medical field, ultrasound imaging has become an important tool in the accurate diagnosis of disease
We reviewed the principles of the photoacoustic effect, techniques for photoacoustic imaging, various organic/inorganic contrast agents and applications of photoacoustic imaging
Imaging techniques using PA contrast agents lead to stable real-time imaging of deep tissue at a high resolution

Summary

Introduction

Ultrasound imaging has become an important tool in the accurate diagnosis of disease. Since fewer biomolecules exhibit a photoacoustic effect compared to the scattering or reflection of ultrasound waves, PAT can generate high-resolution images. In contrast with ultrasound imaging, PAT offers rich intrinsic and extrinsic optical contrast with various materials and is free of speckle artifacts. PAT offers rich endogenous and exogenous optical contrast based on signals from the optical absorption of externally injected molecules or molecules already present in the body. The molecular imaging of specific biomolecules can be achieved by injecting materials such as dyes, nanoparticles (NPs) and reporter genes, which affect the exogenous optical contrast of PAT. Materials used to increase the contrast-to-noise ratio (CNR) are referred to as PAT contrast agents. They exhibit a strong photoacoustic effect, producing a stronger ultrasonic signal under laser exposure. The final section highlights the limitations of current PAT contrast agents and suggests future research directions

Principles of the Photoacoustic Effect

Laser Irradiation in PAT

Photoacoustic Signal Generation

Inorganic PA Contrast Agents

Findings

Discussion and Conclusions