Micro-PAT Principles, Strengths, and Weaknesses

By Susha Cheriyedath, M.Sc.Reviewed by Dr. Liji Thomas, MD

Photoacoustic tomography (PAT) is a sensitive and rapid imaging tool. PAT is a hybrid technique that involves acoustic detection of light absorption by endogenous chromophores like oxy- and deoxy-hemoglobin or exogenous contrast agents like nanoparticles and organic dyes. It is an emerging tool that finds widespread applications in preclinical research.

In the PAT technique, optical absorption contrast is measured and used to reveal the functional, anatomical, histological, and metabolic properties of a tissue. Micro-PAT imaging can be applied to visualize cells, organelles, organs, or tissues in small animals.

Principles of micro-PAT

In the micro-PAT technique, short laser pulses are applied to the sample tissue. Photoacoustic signals are generated when the sample absorbs light. The light is transformed into heat energy, which causes a rise in temperature and thermoelastic expansion, which is proportional to the wavelength of the optical source initially used. This results in the emission of ultrasound waves from the sample tissue. The ultrasound signals can be captured by a transducer. Hemoglobin, water, and melanin are typical optical absorbers present in endogenous tissue.

Strengths of micro-PAT

• Micro-PAT combines the high spatial resolution of acoustic imaging with the excellent sensitivity of optical imaging
• It is a fully non-invasive technique and hence ideal for brain tumor studies
• Micro-PAT penetrates deeper and offers better spatial resolution than purely optical techniques fluorescence tomography
• It offers better optical contrast with lesser artefacts than ultrasonic imaging tools
• Micro-PAT uses non-ionizing light source and is safer compared to tools such as positron emission tomography (PET) and X-ray computed tomography
• It is also more rapid and and is less expensive than MRI
• Being a hybrid technique, it can distinguish between different types of tissues, track contrast agents, and also identify hemodynamic responses

Applications of micro-PAT

Micro-PAT imaging technology has widespread applications in various fields of medicine such as oncology, vascular biology, neurology, dermatology, ophthalmology, and cardiology. Micro-PAT has enabled many clinical and life science research studies in small animals. This tool has evolved greatly over recent years and contributed to a lot of modern research work in chemistry, biology, and nanotechnology.

Some of the key biomedical applications of micro-PAT are as follows:

• High-resolution imaging of superficial organs such as the skin
• Cancer cells detection and imaging, which enables early detection of cancer
• Imaging small animals and measuring key biochemical information using optical absorption data
• Structural, functional, and molecular imaging of pathological tissues such as tumor cells

Weaknesses of micro-PAT technique

• The imaging depth of PAT is limited by the light attenuation by tissues. Currently, the maximum possible imaging depth using PAT is 8.4 cm. Newer illumination techniques may help overcome this limitation.
• The imaging speed of PAT is limited by the rate of pulse repetition of lasers. Scientists are counting on developments in laser technology to help improve PAT imaging speed.
• Quantitative analysis in PAT is relatively difficult because local fluence distribution measurement is complicated. Advanced algorithms may be a solution for this problem.
• Currently, not many commercially available PAT systems in the market are capable of 3D imaging.

References

1. http://rsfs.royalsocietypublishing.org/content/1/4/602
2. http://www.nature.com/nmeth/journal/v13/n8/abs/nmeth.3925.html
3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4311576/
4. https://wustl.edu/

Further Reading

• All Preclinical Imaging Content
• Micro-CT Principles, Strengths, and Weaknesses
• Micro-MRI Principles, Strengths, and Weaknesses
• Micro-PET Principles, Strengths, and Weaknesses
• Micro-PAT in Cancer Research