In Vivo Photoacoustic Microscopy of Subcutaneous Vasculature in the Human Wrist: Method Based on External Pressure Applied to the Investigated Region

Date(s) - 06/27/2013
8:00 am

Cristina Letizia- Masters Student

The imaging of the vasculature provides a powerful tool for studying the vasculature development as well as assessing structural and functional features of blood vessels. This kind of imaging becomes much more important if it is addressed to the study of tumor angiogenesis. Tumor angiogenesis is the proliferation of a network of blood vessels that penetrates into cancerous growths, supplying nutrients and oxygen and removing waste products. Adequate non-invasive imaging techniques can help physicians to monitor and control the tumor vasculature development as well as to assess functional and structural characteristics of the tumor vasculature.

A potential non-invasive imaging modality is photoascoustic (PA) imaging, a new biomedical imaging modality, emerged over the last decade. It consists in imaging of the internal distribution of optical energy deposition in biological tissues based on the detection of laser-induced ultrasonic waves, which reveals physiologically specific optical absorption contrast. Indeed a PA image is an ultrasound image in which the contrast depends not on the mechanical and elastic properties of the tissue, but by its optical properties, particularly optical absorption. Thus, this allows PA imaging to visualize anatomical features that contain an abundance of chromophores such as haemoglobin, deoxyhaemoglobin, melanin, lipids and water which absorb the light.

In this work, photoacoustic microscopy (PAM) has been used to image the structure of the vasculature and also to evaluate how this kind of acquisition is modified by the application of an external pressure on the target region. A pressure device has been designed and manufactured to apply an external pressure to the investigated region. Firstly, we imaged the structure of subcutaneous vasculature in the human wrist with PAM in absence of pressure. Then we performed PAM experiments on the same area when an external pressure was applied. In both cases, we chose 532 nm laser light, which is an isosbestic wavelength for haemoglobin and deoxyhaemoglobin. Finally, the effects generated by the external pressure have been assessed in terms of changes of flow rate and blood volume, by measuring changes in the amplitude of the PA signal.

The results of this work show that the response of the blood vessels to the pressure application can be divided into two groups. For certain vessels, the blood volume and blood flow progressively decrease when the pressure is exerted; the pressure application causes an increase in the blood flow and volume in other vessels. Other in-vivo experiments show that the blood volume increases for certain values of external pressure and decrease for others. The increase of the amplitude of the signal implies an enhancement of the final image, since the shape of the blood vessels is shown with a better definition and the signal from the vessels becomes stronger than that from the surrounding tissue. Although the concept of using an external pressure to improve the quality of the imaging has been demonstrated in this thesis, few other aspects concerning the organization of our experiments might be investigated in future studies. In future experiments, before applying a higher external pressure and start a new acquisition, enough time might be left to the circulation to return to its rest condition. A comparison of the current results and te new results might be interesting.