Development of a time‐resolved optical tomography system for neonatal brain imaging

Abstract

This Ph.D. project involved the development and evaluation of a prototype medical optical tomography system. The time‐resolved 32‐channel instrument is presented, and various instrumental aspects and performance issues are discussed. It has been designed primarily as a continuous bedside monitor for obtaining functional images of premature infants' brains that are at an increased risk of injury due to dysfunction in cerebral oxygenation or haemodynamics. The fully automated device employs 32 source fibers that sequentially deliver near infrared pulsed laser radiation of picosecond duration to the tissue. Transit time measurements of very high temporal resolution (∼100 ps FWHM) and stability (∼5 ps/h drift, and jitter of similar magnitude) are made between these sources and 32 detector optodes also located on the skin surface. Photons transmitted diffusely through the tissue and collected by the optodes are transmitted to four ultra‐fast eight‐anode Microchannel Plate Photomultiplier Tube detectors. Thirty‐two fully simplex time‐correlated single photon counting channels simultaneously record histograms of the photon flight times at rates of up to about 300,000 counts per second per channel. These so‐called temporal point spread functions represent the raw data for the image reconstruction. Separate maps of the internal absorption and scattering properties can be reconstructed from purely temporal data without recourse to reference or baseline measurements. The effectiveness of this instrument has been demonstrated by successfully imaging various tissue‐equivalent phantoms and an adult forearm. A preliminary post‐mortem study suggested that a sufficient number of photons can be transmitted across the head of the neonate at safe exposure levels and durations to obtain an adequate signal.