CMOS Time-Domain Imager for Functional Brain Imaging Using Gated Near-Infrared Spectroscopy
Authors: Alhemsi Hani , Deen M. Jamal 
Published: February 2020

Non-invasive imaging of brain activity enables novel studies in neuroscience and provides an alternative modality for clinical monitoring applications. Near-infrared spectroscopic imaging (NIRSI) is non-invasive, cheap, portable, and immune to electro-magnetic interference. NIRSI is also superior in terms of spatial or temporal resolution when compared to electro-encephalography (EEG) or magnetic resonance imaging (MRI), respectively. Utilizing the time-domain (TD) technique offers the richest information at the cost of being the most complex[1]. TD NIRS imagers utilize time-correlated single-photon-counting (TCSPC) measurements which require detectors with single-photon sensitivity like single-photon avalanche diodes (SPADs)[2] or Silicon photomultipliers (SiPMs) and very fast time-discriminators like time-discriminator circuits (TDCs). Implementing these circuits in well-established and mature CMOS technologies[3] is advantageous.

The main challenges in designing a TD NIRSI sensor deal with four issues: spectral responsivity, noise, fill-factor, and throughput. The spectral responsivity and noise are shaped by the technology process and structure (see figure 2) of the SPAD[4]. The fill-factor is determined by the ancillary circuitry needed to maintain the TD operation of the SPAD[4] (i.e., frontend, gating, and TDC). The throughput is dependent on the readout architecture of the array of pixels.

We are proposing a fully integrated scalable array of time-gated actively-quenched SPADs with shared time-gated ring-oscillator-based TDCs following H-tree-based architecture and a 3-transistor active-pixel sensor (3T APS) readout scheme with in-pixel storage capability to be implemented using standard deep sub-micron CMOS technology. For example, using 130 nm CMOS technology SPADs can be built with an area of 50 um2 which exhibit a dark count rate (DCR) of 18 KHz[5]. At a wavelength around 600 nm and an excess bias of 2 V, their photon detection probability (PDP) could reach 22%. These SPADs breakdown at 20 V and could resolve down to 90 ps for wavelengths around 654 nm. With the same technology a 0.04 mm2 TDC can be designed with a resolution or least significant bit (LSB) of 6 ps for a range of 11-bits[6]. Moreover, SPAD structure variations, novel gating schemes, smart resource sharing, and efficient array architecture a CMOS TD NIRSI sensor can be realized to meet requirements of functional human brain imaging applications.

References

[1] A. Torricelli, D. Contini, A. Pifferi, M. Caffini, R. Re, L. Zucchelli and L. Spinelli, "Time domain functional NIRS imaging for human brain mapping," J NeuroImage, 2013.

[2] E. Villella, O. Alonso, A. V. A. Montiel and A. Dieguez, "A low-noise time-gated single-photon detector in HV-CMOS technology for triggered imaging," Sensors and Actuators A: Physical, vol. 201, p. 342, 2013.

[3] C. Niclass, M. Soga, H. Matsubara, M. Ogawa and M. Kagami, "A 0.18-um CMOS SoC for a 100-nm-Range 10-Frame/s 200 x 96-Pixel Time-of-Flight Depth Sensor," IEEE Journal of Solid-State Circuits, vol. 49, no. 1, 2014.

[4] D. Palubiak, M. El-Desouki, O. Marinov, M. J. Deen and Q. Fang, "High-Speed, Single-Photon Avalanche-Photodiode Imager for Biomedical Applications," IEEE Sensors Journal, vol. 11, no. 10, p. 2401, 2011.

[5] E. Webster, L. Grant and R. Henderson, "A High-Performance Single-Photon Avalanche Diode in 130-nm CMOS Imaging Technology," IEEE Electron Device Letters, vol. 33, no. 11, p. 1589, 2012.

[6] M. Straayer and M. Perrott, "A multi-path gated ring oscillator TDC with first-order noise shaping," IEEE Journal of Solid-State Circuits, vol. 44, no. 4, p. 1089, 2009.

[7] M. Ferrari and V. Quaresima, "A brief review on the history of human functional near-infrared spectroscopy (fNIRS) development and fields of application," J NeuroImage, vol. 63, p. 921, 2012.

[8] L. Braga, L. Gasparini, L. Grant, R. Henderson, N. Massari, M. Perenzoni, D. Stoppa and R. Walker, "A Fully Digital 8 x 16 SiPM Array for PET Applications With Per-Pixel TDCs and Real-Time Energy Output," IEEE Journal of Solid-State Circuits, vol. 49, no. 1, 2014.