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

Abstract

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 ͑PMTs͒ 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 nm͒, a fast settling time, and low hysteresis. The gain of the PMT is set so that the dc anode current is below 80 nA, which allows the measurement of phase independently of the intensity. The system allows measurements of amplitude at detected intensities down to 1 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 ͑ln A͒ at 100 MHz were found to be 0.08°and 0.004, respectively, in a measurement of a 7 cm phantom with an imaging time of 5 s per source and source optical power of 8 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.

Hybrid photomultiplier tube and photodiode parallel detection array for wideband optical spectroscopy of the breast guided by magnetic resonance imaging

“…Most DOT systems operate at two to four wavelengths 11 and, due to the limited sensitivity of PMT detectors, use light between 650 and 850 nm. [12][13][14] Later systems have evolved to include additional wavelengths. However, the larger lipid and water absorption peaks both lie at 930 and 975 nm, respectively.…”

Neuroimaging with functional near infrared spectroscopy: From formation to interpretation

“…The computational efficiency of the regularized reconstruction is also critical for real-time imaging [112]. To account for source and detector optode losses, calibration strategies can also be used [113,114,115].…”

Compensation of optode sensitivity and position errors in diffuse optical tomography using the approximation error approach

“…If the coupling coefficients ζ 0 are not estimated by the pre-calibration techniques [5,6,9,10], they are typically fixed to the ideal "no losses" values, i.e., ε 1 (ζ 0 ) ≡ 0. In addition, if the random measurement errors have zero mean (ē = 0), the MAP estimate (18) has the usual form…”

“…A calibration method using a homogeneous diffusive phantom with source placed at the center and optodes placed equidistant from the center to calibrate relative optode sensitivities was used in [6]. Furthermore, a calibration method based on rotational symmetry of source and detector positions in a measurement setup was utilized in [5,9,10]. Difference imaging and calibration approaches assume that the source and detector coupling coefficients do not change after the calibration.…”