Instrumentation for medical optical tomography with applications

Near-infrared spectroscopy (NIRS) and imaging techniques based on it are a field which has been the subject of intensive research during the past 15 years. In NIRS, the tissue is illuminated with visible red or near-infrared (NIR) light of several wavelengths, and the light which is reflected back from the tissue or transmitted through it is measured using sensitive photodetectors. As the distance between the source and the detector is increased, the sensitivity of the measured intensity of light to changes in the internal parts of the tissue increases. Two main optical parameters can be defined which characterize the tissue: the scattering and absorption coefficients. The absorption coefficient is sensitive to the concentrations of oxygenated and deoxygenated hemoglobins in the tissue. The partial blood volume and oxygen saturation in the tissue studied can be calculated if the concentrations are known. The optical measurements are non-invasive and patient-friendly, and they can be performed in the presence of electromagnetic interference. The most interesting applications of optical imaging include the imaging of hemodynamic changes related to neural activity in adults and infants, the imaging of hematoma and circulatory defects in premature infants, mammography, and imaging of muscle tissue during exercise. The goal of this thesis was to develop a multichannel intensity-modulated optical imaging system, which can be used for the imaging of activation-related hemodynamic changes in the adult and neonatal cortex as well as optical tomography. The system can be calibrated to give absolute measurements of amplitude and phase, which reflect the absolute attenuation and temporal delay of the light as it passes through the tissue from source to detector. Both difference imaging (imaging of the differences between two functional states of a tissue, or between two phantoms) and absolute imaging are successfully demonstrated using phantoms which have realistic background values close to the optical properties of premature neonates' heads, and small targets with contrasts that occur between background tissue and, e.g., breast cancer, bone, or hematoma. Many activation measurements were also performed successfully using the system, including the measurement of hemodynamic changes in the right motor cortex during transcranial magnetic stimulation of the left motor cortex in adults, as well as bilateral auditory activation measurements on healthy full-term neonates.reviewe