Micro-photoacoustic tomography (PAT) is an emerging technique for full-body imaging of small animals in preclinical research. As small animals are being increasingly used for biomedical studies, in vivo whole body imaging of these plays a key role in these studies.
Micro-PAT is a hybrid imaging tool based on acoustic detection of absorption of light by endogenous chromophores or exogenous agents. Micro-PAT provides high-resolution images of small animal tissues with the help of near-infrared light. Endogenous hemoglobin contrast is used to image vascular and anatomical structures, while exogenous contrast agents are used in functional and molecular imaging.
Due to its excellent resolution, good penetration depth, real-time imaging potential, non-ionizing nature, user-friendliness, and easy availability of non-radioactive optical contrast agents, PAT is considered a promising technique in preclinical imaging. It can detect deep-seated tumors and their vasculature, circulating cancer cells, and micro-metastasis in lymph nodes and other organs. This greatly helps further understand the molecular pathways of various cancers. Micro-PAT can also help oncologists detect cancer at very early stages, which greatly improves prognoses and survival rates while also reducing costs associated with treatment of cancer.
PAT can also be combined with other molecular imaging modalities that are based on ultrasound to acquire images with better contrast and to obtain more molecular data. One of the most important benefits of PAT is that it is easy to attach to existing ultrasound machines in the clinical setting, thus extending the scope of traditional ultrasound imaging to cellular, as well as molecular, imaging.
Glucose metabolism imaging
Micro-PAT is widely used in the diagnosis of cancer due to the flexibility in light delivery and precise acoustic detection. The technique is helpful in preclinical cancer staging as well as treatment planning. Simultaneous imaging of anatomy and glucose metabolism plays a key role in planning cancer treatment. Earlier, this was achieved with the help of multiple imaging modalities such as PET-CT and PET-MRI. These techniques were expensive and offered low-resolution solutions. In addition, the use of ionizing radiation made longitudinal monitoring very difficult.
These limitations were resolved using a PAT-based technique by Chatni et al. The team used a single imaging modality - ring-shaped confocal PAT - to acquire high-resolution images of glucose metabolism and anatomy. The experiment was performed on a mouse with a skin tumor, and fluorescence imaging was used to confirm the images from RC-PAT. They also imaged 2 mice with kidney tumor cells in order to demonstrate RC-PAT’s depth imaging ability. In both animals, anatomical cross-sections showed that the cancerous kidneys were bigger than the healthy kidneys.
Micro-PAT in brain cancer studies
Brain cancer studies in the past have been hampered by the lack of a suitable imaging tool that helps study small animals in vivo. Conventional histological processes involved sacrificing animals at various stages to study the changes in brain tissue of small animals. Techniques such as micro-fMRI were very costly, offered low-resolution images, and required high wait times for image acquisition.
The micro-PAT technique offers significant improvement over existing neuro-imaging modalities. It is non-invasive, rapid, and offers a host of extra information. Micro-PAT offers excellent spatial resolution and quantifies functional parameters including SO2 and HbT. It also provides information that is extremely helpful in the quantification of a tumor.
Reviewed by Afsaneh Khetrapal BSc (Hons)