Time-resolved Optical Tomography instrumentation for fast 3D functional imaging.

Optical Tomography is a medical imaging method for creating three-dimensional images using near infrared light. An instrument, named MONSTIR (Multi-channel Opto-electronic Near-infrared System for Time-resolved Image Reconstruction), has been designed and built to measure the flight times of photons between two positions on the surface of the tissue being imaged. The measurements are used to generate functional images of the newborn infant brain, and to detect and classify breast disease. The aim of my PhD project has been to design a new imaging system, improving on the original design of MONSTIR in order that the images produced from data collected in clinical measurements will lead to improved diagnosis of infant brain trauma and female breast disease, and new functional studies of the neonatal brain. MONSTIR has been used for conducting fundamental investigations into optical tomography, but has been found to be awkward to use in a clinical environment, inefficient, and has components coming to the end of their useful lifetime. New ultrafast timing electronics have been purchased and tested in conjunction with other new instrumentation intended to reduce measurement errors. The new system's capabilities have been quantified, and improved performance tested in a clinical environment. The data accuracy of MONSTIR whilst in the presence of a large magnetic field has also been quantified, as simultaneous acquisition with magnetic resonance imaging (MRI) could provide useful a priori information for reconstruction. The project has focussed on three major instrumentation changes to MONSTIR: new timing electronics, a new laser calibration method, and updated optical attenuators. First, the original timing electronics required a 12 s delay between switching source positions to upload data to the control PC. This dead-time is undesirable during clinical studies as the patient must remain still for 12 minutes, whilst collecting only 6 minutes of data. Second, temporal variations have been found in the laser's power output. A new instrument has been designed and tested, resulting in a reduction in intensity errors between repeated measurements over one hour from 40 % to 1 %. Finally, variable optical attenuators are used to compress the dynamic range of the detected signal and protect the detectors from excessive illumination. The original design, using eight apertures of various sizes, does not provide repeatable attenuation, causing a 3 % variation in intensity measurements. A new design has been implemented using x-ray film that has been variably exposed in ten discrete sections, reducing intensity variations to below 1 %.and increasing the maximum attenuation from 2.2 to 3.7 OD. Implementation of the new instrumentation has improved the system for clinical use. The system is half its original size, takes less than 3 hours to reach thermal stability, and acquires data at 5 s per source containing intensity and meantime uncertainties of 1.9 % and 5.2 ps respectively. The new MONSTIR system requires a delay of 1.5 s between sources, with the result that full 32-channel datasets can be acquired in 3.5 minutes. The increase in acquisition speed allows datasets to be acquired in rapid succession, further reducing data errors in functional imaging. Three dimensional images of breast disease have been produced, and validated using tissue-equivalent phantom studies