Diffuse Correlation Spectroscopy (DCS) allows the optical and label-free investigation of microvascular dynamics. Commonly, DCS is implemented with highly sensitive and ultra fast single-photon avalanche diodes (SPAD) for blood flow measurements from around 1-1.5cm deep inside tissue (source detector separation of 2.5-3 cm). In parallelized DCS (pDCS), we use arrays of multiple SPADs to boost the signal-to-noise ratio by averaging many independent DCS measurements. In this study, we explored the capabilities of an innovative, massively parallelized SPAD array with 500x500 single pixels for DCS for up to 250,000 parallel DCS measurements. We can show that this massively parallelized array enables viable blood flow measurements at 2cm depth (4cm source detector separation) in human subjects. Furthermore, we applied a dual detection strategy, where a secondary SPAD array probes the superficial blood flow simultaneously as a build-in reference measurement. In addition to our main results, we test and discuss methods to correct the deep flow measurement, by including simultaneously measured flow dynamics deep and superficial tissue layers via our novel dual-SPAD array measurement setup.
Diffuse correlation spectroscopy (DCS) is an optical technique that allows for non-invasive measurements of tissue perfusion, and is often used for neuromonitoring applications. However, a major challenge of DCS is low SNR for deep tissue measurements. Recent works have demonstrated the potential for SPAD arrays to provide significant SNR increases by averaging autocorrelation signals from individual speckles. Such methods may still be suboptimal for efficient signal extraction, as the individual signals may each be low fidelity. In this work, we explore alternative methods of integrating parallelized DCS signals in low photon regimes for accurate blood flow estimation.
The SwissSPAD2/3 camera family is based on quarter megapixel single-photon avalanche diode (SPAD) time gated imagers. The 16.38-µm low-noise pixels feature a single-bit memory and built-in all-solid-state nanosecond time gating without the need for external image intensifiers. Microlenses have also been made available to increase the overall system sensitivity, including for high NA applications. SwissSPAD2/3 are coupled to FPGA platforms enabling a virtually noiseless streaming at up to 100 kpfs. A 1-bit accumulation of frames to reconfigurable number of bits was programmed on the FPGA for applications such as fluorescence lifetime imaging microscopy (FLIM). In other applications, a burst-mode read-out of 130,000 binary frames to a DDD3 memory of one sensor half was programmed on one FPGA for applications requiring full bitplanes. These initial configurations were extended to dual-FPGA systems capable of streaming data at near 100 kfps in continuous mode for long acquisition times. In such configuration one FPGA streams data from one sensor half to the other FPGA, which then sends the combined data stream to a host PC over PCIe at up to 3 GB/s. The eight PCIe lanes require careful design with differential routing and controlled impedance and the whole development presented significant hardware and firmware challenges. We also achieved full synchronization of two SwissSPAD2 camera systems over PCIe and characterized the pixel-to-pixel exposure timing alignment error to better than 150 ps with a time gate of 10 ns. The resulting platforms are unique enablers for quantum imaging applications, such as plenoptic maging, quantum LIDAR or quanta burst photography.
The Piccolo gated sensor features a 32x32 SPAD array of single-photon avalanche diodes (SPADs) operating in time-correlated single-photon counting (TCSPC). The chip enables event-driven readout and a maximum count rate of 220 Mcps. The sensor is based on the original Piccolo architecture, whereas the pixel was redesigned to accommodate a sub-nanosecond time gating circuitry. As a result, the pitch was increased by 3 μm to 31 μm with a slightly lower fill factor of 23.7%. The time-gating circuitry comprises active recharge to activate the gate and a fast switch to de-activate the SPAD. The sensor is equipped with 128 dynamically allocated, 50 ps time-to-digital converters (TDCs) at the bottom of the array. Four TDCs are shared among 32 SPADs in each column, where a mechanism of reallocation is used to optimize the use of TDCs and to minimize photon loss. Time gating can reduce both uncorrelated and correlated noise by reducing overall active time and by increasing relaxation time after detection, respectively. Upon acquisition of TCSPC data, the FPGA reorganizes it in histograms, which may be dynamically allocated and reduced in the number of bins to optimize memory use and data transfer from the FPGA to an external Mac/PC. The TDCs may also be calibrated to suppress differential and integral nonlinearities on-FPGA. Timestamps are stored in DDR3 and streamed out of the FPGA through PCIe with a data rate of 5.12 Gbps. Thanks to these techniques, the maximum count rate of the sensor was increased by about 3×. The time gating feature was implemented to extend dynamic range, and therefore depth, of near-infrared optical tomography (NIROT) and g(2) multi-depth time-domain diffuse correlation spectroscopy (TD-mDCS). Time gating is especially useful in NIROT and mDCS, as it helps suppress large numbers of early photons reflected back from the sample’s surface, e.g. the skull or skin. Thus, the Piccolo-gated architecture could show its suitability in these imaging modality.
SwissSPAD3 is the latest of a family of widefield time-gated SPAD imagers developed for fluorescence lifetime imaging (FLI) applications. Its distinctive features are (i) the ability to define shorter gates than its predecessors (width W < 1 ns), (ii) support for laser repetition rates up to at least 80 MHz and (iii) a dual-gate architecture providing an effective duty cycle of 100%. We present widefield macroscopic FLI measurements of short lifetime NIR dyes, analyzed using the phasor approach. The results are compared with those previously obtained with SwissSPAD2 and to theoretical predictions.
We introduce a new generation of 3D imaging devices based on quantum plenoptic imaging. Position-momentum entanglement and photon number correlations are exploited to provide a scan-free 3D image after post-processing of the collected light intensity signal. We explore the steps toward designing and implementing quantum plenop- tic cameras with dramatically improved performances, unattainable in standard plenoptic cameras, such as diffraction-limited resolution, large depth of focus, and ultra-low noise. However, to make these new types of devices attractive to end-users, two main challenges need to be tackled: the reduction of the acquisition times, that for the commercially available high-resolution cameras would be from tens of seconds to a few minutes, and a speed-up in processing the large amount of data that are acquired, in order to retrieve 3D reconstructions or refocused 2D images. To address these challenges, we are employing high-resolution SPAD (single photon avalanche diode) arrays and high-performance low-level programming of ultra-fast electronics, combined with compressive sensing and quantum tomography algorithms, with the aim of reducing both the acquisition and the elaboration time by one or possibly two orders of magnitude. Moreover, in order to achieve the quantum limit and further increase the volumetric resolution beyond the Rayleigh diffraction limit, we explored dedicated pro- tocols based on quantum Fisher information. Finally, we discuss how this new generation of quantum plenoptic devices could be exploited in different fields of research, such as 3D microscopy and space imaging.
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