| dc.description.abstract | Near-infrared diffuse optical tomography (NIR DOT) is a new non-invasive and non-radiation biomedical imaging technique, and can be divided into two parts including opto-electrical measurement system and image reconstruction scheme. In this study, we improved frequency-domain (FD) opto-electrical measurement system to acquire both amplitude and phase information that are essential in image reconstruction computation. During the imaging procedure, NIR light through fibers illuminates a phantom, and neutral density (ND) filter, infrared band-pass interference (IR) filter are used to attenuate and filter out the unwanted light. Then the filtered light is detected and transformed into electrical signal by a photomultiplier tube (PMT). Eventually, the amplitude and phase information are computed through the demodulation of mixers and spectral analysis.
In order to effectively use the dynamic measurement range of a PMT with different control voltage, we improved the FD measurement system, previously using constant PMT control voltage, by auto-adjusting control voltage according to the received light power. Meanwhile, the correction of power amplitude and phase delay corresponding to variant PMT control voltage was performed to ensure the acquired data in an effective range.
Designated liquid phantoms that mimic the bio-tissue were used for justifying the proposed method. The design parameters of phantoms include size, optical absorption and scattering coefficients. Moreover, measurement was performed with using fixed or varying ND filter, and at different modulation frequency (20 and 60 MHz). Results show that the saturation of PMT occurs at source-detection angle below 67.5° when constant control voltage was used to measure Φ-80mm phantom, and limits to 22.5° while even using auto-adjusting control voltage.
The proposed method of auto-adjusting PMT control voltage in the study can fully utilizes the dynamic range of the PMT. According to the designated phantoms (different optical coefficients and size of the phantom), and the modulation frequency adopted, amplitude- and phase- correction functions can be established and employed for the improvement of NIR FD opto-electrical measurement system.
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