Handheld optoacoustic probe facilitating nearfield investigations through a transparent detector

Abstract

Modern medicine relies strongly on measurement devices, enabling the physician to investigate the human body in ever greater detail. In addition to established techniques of optical microscopy, ultrasound, x-ray and magnetic resonance imaging, optoacoustic (OA) imaging is on its path to enter the clinics. The research field of optoacoustics already produced a variety of remarkable setups, from high resolution microscopy to deep penetrating tomography. Through the broad range of wavelengths available for this technique, it is capable of detecting the concentration of endogenous as well as exogenous contrast agents, even blood oxygenation levels can be determined in real time. Depending on the application, different OA setups can be created, customized to best address the specific task.

This thesis is concerned with the development of a handheld optoacoustic setup to determine the thickness of melanoma. Penetration depth is the most important factor in staging of skin tumors. To facilitate near field measurements the detector is designed to be transparent, which allows illumination through the detector. Indium tin oxide electrodes are sputtered on a piezoelectric polymer film to create a circular detector area. Transparency was confirmed using spectrophotometric measurements in the visible and near infrared light spectrum. To characterize the capabilities of the transparent detector, far field measurements on hydrogel samples with layers containing different concentrations of melanin were performed. An OA measurement series on a mole under laboratory conditions showcased the possibility using wavelengths in the range from 432-652 nm with this detector. For logistical reasons, only 532 nm were used in the other measurements.

Near field measurements on a coated glass plate are compared with simulation, confirming the validity of the data processing algorithm to remove the pyroelectric signal and deconvolve the instrument response function from the OA signal. In a small clinical study, suspicious nevi were investigated using the setup developed here. The obtained OA signals are discussed in relation with the histology of the respective nevus. Even though their thicknesses could not yet be determined reliably, the results are promising that further improvements with regards to noise reduction will allow real time measurements of the absorption depth profile.