Mr. Rajas Poorna Profile

Rajas Poorna

at Indian Institute of Science

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Proceedings Article | 5 March 2021 Poster + Presentation + Paper

Square-waves for frequency multiplexing for fully parallel 3D diffuse optical tomography measurement

Rajas Poorna, Rajan Kanhirodan, Manob Jyoti Saikia

Proceedings Volume 11639, 116391S (2021) https://doi.org/10.1117/12.2578500

KEYWORDS: 3D metrology, Optical testing, Diffuse optical tomography, Multiplexing, Light emitting diodes, Photodiodes, Embedded systems, Tissue optics, Structural imaging

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Diffuse Optical Tomography (DOT) is an emerging non-invasive medical imaging method that can measure structural and functional properties of tissue, enabling it to detect abnormalities and tumors that other methods would miss. It may be described, in layman’s terms, as “3D ultrasound – but in color, and low resolution”. We present a fully parallel source and detector frequency-multiplexing method that enabled us to implement a fast 3D continuous-wave DOT (CW-DOT) system using low-cost embedded systems. Six simultaneously driven NIR LEDs and 24 photodiodes were fiber-optically coupled to a cylindrical phantom. A current driver board was developed to drive each LED with either a precise pre-set current or none. This was connected to an Arduino Due, which triggered each LED digitally with a precise frequency (0.5 – 2.5 kHz). The Due’s hardware timers were used to ensure frequency precision. This “division of labor” for precision in frequency and intensity allowed by square-wave modulation enabled significant reductions in cost and complexity and removed the need for multiple DACs for generating precise sinewaves. Signals from the 24 photodiodes were sampled simultaneously and processed using Fast Fourier Transform by a single-board computer (Raspberry Pi 3). We also describe a simple algorithm to mathematically remove the frequency crosstalk observed in the source LEDs, provided that all source intensities are also measured throughout. Experimental studies demonstrate a 2400x speed-up in the CW-DOT scan (1 s/scan) compared to the pre-existing serial system, which took 40 min/scan.

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