Instrumentation and calibration methods for the multichannel measurement of phase and amplitude in optical tomography

In this article, we describe the multichannel implementation of an intensity modulated optical tomography system developed at Helsinki University of Technology. The system has two time-multiplexed wavelengths, 16 time-multiplexed source fibers and 16 parallel detection channels. The gain of the photomultiplier tubes (PMTs) is individually adjusted during the measurement sequence to increase the dynamic range of the system by ${10}^4$ . The PMT used has a high quantum efficiency in the near infrared (8% at $800\mathrm{nm}$ ), a fast settling time, and low hysteresis. The gain of the PMT is set so that the dc anode current is below $80\mathrm{nA}$ , which allows the measurement of phase independently of the intensity. The system allows measurements of amplitude at detected intensities down to $1\mathrm{fW}$ , which is sufficient for transmittance measurements of the female breast, the forearm, and the brain of early pre-term infants. The mean repeatability of phase and the logarithm of amplitude $\left(\ln \mathrm{A}\right)$ at $100\mathrm{MHz}$ were found to be 0.08° and 0.004, respectively, in a measurement of a $7\mathrm{cm}$ phantom with an imaging time of $5\mathrm{s}$ per source and source optical power of $8\mathrm{mW}$ . We describe a three-step method of calibrating the phase and amplitude measurements so that the absolute absorption and scatter in tissue may be measured. A phantom with two small cylindrical targets and a second phantom with three rods are measured and reconstructions made from the calibrated data are shown and compared with reconstructions from simulated data.