Studies increasingly support the idea that fluorescence lifetime imaging (FLI) offers superior tumor detection contrast, but conventional methods are limited by low frame rates and high computational load. We present a compact, real-time FLI system based on a QVGA SPAD sensor with in-pixel switched-capacitor circuits that compute the average photon time-of-arrival in the analog domain. This single-shot, center-of-mass approach avoids photon binning and gated integration, enabling high frame rate FLI up to 30 FPS. Experimental validation with visible (fluorescein and acridine orange) and near-infrared fluorophores (ICG and IRDye800CW), confirms accurate performance. Future work will evaluate the system in preclinical in vivo settings for fluorescence-guided surgical applications.

Applying Current-Assistance (CA) for time-resolved imaging in high-resistivity epilayers reduces its power consumption, but introduces strong nonlinearities in the generated drift fields. This work models these drift fields by demonstrating that the injected majority currents are Space-Charge-Limited (SCL) rather than purely Ohmic. We derive an analytical model for the electric field, validated by TCAD simulations, and introduce the concept of the Virtual Cathode (VC) for CA. The SCL nature of the majority current is verified through I - V measurements. These insights establish a physical understanding for the design of future current-assisted photodetectors in high-resistivity epilayers.

A video-rate fluorescence lifetime (FLT) imaging system is presented featuring in-pixel lifetime calculation based on the center of mass of the times of arrival (ToAs) of photons received by the single-photon avalanche diode of the pixel. The ToAs of photons received during a gating window placed in the decay of the returned fluorescent emission are averaged by a switched-capacitor circuit, implementing the center-of-mass method (CMM) in a compact pixel circuit. Binning of ToAs or complex time-to-digital circuits are avoided, and instead, optimal use of all received photons is achieved on a small pixel area with low communication and computational overhead. The system is validated through analytical modeling, Monte Carlo simulations, and real-world experiments using fluorophores with known lifetimes, demonstrating agreement with the analytical model and achieving accurate FLT estimations across a large range of photon counts. A dedicated quarter video graphics array (QVGA) (320 x 240 pixel) image sensor that includes all necessary circuitry for the CMM currently reaches 10 frames per second and demonstrates the real-time FLT imaging capability of the system.

Fluorescence imaging has been widely used in fields like (pre)clinical imaging and other domains. With advancements in imaging technology and new fluorescent labels, fluorescence lifetime imaging is gradually gaining recognition. Our research department is developing the tauCAMTM, based on the Current-Assisted Photonic Sampler, to achieve real-time fluorescence lifetime imaging in the NIR (700–900 nm) region. Incorporating fluorescence lifetime into endoscopy could further improve the differentiation of malignant and benign cells based on their distinct lifetimes. In this work, the capabilities of an endoscopic lifetime imaging system are demonstrated using a rigid endoscope involving various phantoms and an IRF-free deep learning-based method with only 6-time points. The results show that this application’s fluorescence lifetime image has better lifetime uniformity and precision with 6-time points than the conventional methods.

A 128 x 128-pixel current-assisted photonic sampler (CAPS) image sensor is designed for macroscale fluorescence lifetime imaging (MFLI). Current assistance is used for swift detection and time gating of the photogenerated carriers within the whole substrate. The sensor is fabricated in a commercial 350-nm CMOS process on a 15-mu m epilayer, with 19-mu m pixels and a 62% FF. The pixel features gating widths below 1 ns subject to a 42-ps standard deviation on the gate width uniformity and a 27-ps standard deviation on its temporal position. The pixel has a quantum efficiency (QE) above 40% for near-infrared (NIR) wavelengths, whereas the peak QE is 60% at 675 nm. The gate's intrinsic decay is observed to be 280 ps, which allows for the MFLI of subnanosecond lifetime dyes. A multiexposure method has been implemented, where the pixel is nondestructively read-out multiple times-with different exposure times-after each reset. This allows for imaging with an extended dynamic range (DR) and is validated on commercial fluorescence phantoms. The MFLI capabilities are further demonstrated in a preclinical experiment.

Correlation-Assisted Direct Time-of-Flight (CA-dToF) is demonstrated for the first time on a large 320 × 240-pixel SPAD array sensor that includes on-chip high-speed timing support circuitry. SPAD events are processed in-pixel, avoiding data communication over the array and/or storage bottlenecks. This is accomplished by sampling two orthogonal triangle waves that are synchronized with short light pulses illuminating the scene. Using small switched-capacitor circuits, exponential moving averaging (EMA) is applied to the sampled voltages, delivering two analog voltages (VQ2, VI2). These contain the phase delay, or the time of flight between the light pulse and photon’s time of arrival (ToA). Uncorrelated ambient photons and dark counts are averaged out, leaving only their associated shot noise impacting the phase precision. The QVGA camera allows for capturing depth-sense images with sub-cm precision over a 6 m range of detection, even with a small PDE of 0.7% at an 850 nm wavelength.

In vivo fluorescence lifetime (FLT) imaging is an emerging and promising modality with the potential to provide additional biological information compared to fluorescence intensity (FI) imaging. Until now, nearly all studies evaluating the benefits of FLT imaging in vivo have been conducted using cyanine dyes. While these fluorophores are highly successful for conventional in vivo fluorescence imaging due to their unmatched brightness in the NIR-I region (700-900 nm), they might be suboptimal for FLT imaging. Indeed, NIR-I cyanines are characterized by short singlet excited-state lifetime (generally below 1 ns) making it challenging to differentiate them from tissue autofluorescence. Therefore, there is a pressing need for biocompatible fluorophores with longer FLTs. Herein, we report a novel bioconjugatable water-soluble NIR-emissive aza-BODIPY dye with extended FLT values in physiological conditions, compared to cyanine dyes (increase of 100% compared to IRDye® 800CW). The suitability of aza-BODIPY for FLT imaging when conjugated to an antibody was assessed to enhance this technology. Beyond rational design and synthesis of a long-wavelength fluorophore with attractive features for in vivo FLT imaging, this study also highlights for the first time the impact of the conjugation method on the FLT characteristics of the resulting fluorescent antibody conjugates.

This letter proposes a novel single photon avalanche diode (SPAD)-based pixel, designed for direct Time-of-Flight (ToF) imaging with in-pixel averaging, which provides a promising advancement in low-power and potentially high image resolution for outdoor applications. By utilizing a laser pulse and two orthogonal sinusoidal signals, the pixel averages out the detected ambient light while accumulating the laser pulse round-trip time. A prototype pixel array was fabricated using a 180 nm CMOS process, featuring a commercial SPAD cell. By characterizing one pixel and employing a 100 klux solar emulator as an ambient light source with a fixed 40 ambient-to-signal ratio over a 360(degrees) phase shift, equivalent to 6 m detection range, the maximum detected accuracy error was 3.3%, with a 5 cm precision.

Recent studies demonstrated the added-value of fluorescence lifetime (FLT) imaging in tumor identification to intensity-based imaging. The tauCAM is a novel macroscale FLT imaging system that detects the FLT of a fluorophore by measuring fluorescence in the time-domain. FLT images are made simultaneously to fluorescence intensity- and grayscale reflectance images, under ambient illumination. In vivo imaging in subcutaneous tumor bearing-mice revealed that FLT imaging with the tauCAM provides additional information on environmental parameters between tumor and healthy tissue, for tracers sensitive to their physiological environment. This research emphasizes the importance of fluorophore selection for future (pre-) clinical FLT imaging trials.

Fluorescence imaging has been widely used in fields like (pre)clinical imaging, as well as in other domains. With advancements in imaging technology and the development of new fluorescent labels, fluorescence lifetime imaging is gradually gaining recognition. Our research department is developing the CAPS camera, based on the Current-Assisted Photonic Sampler, to achieve real-time fluorescence lifetime imaging in the NIR (700-900nm) region. Incorporating fluorescence lifetime into endoscopy would further differentiate malignant and benign cells based on their distinct lifetimes. Therefore, the capabilities of the system are demonstrated using a rigid endoscope involving various phantoms and various lifetime processing approaches to get the uniform lifetime image because of the independence of fluorescence lifetime to the intensity when the intensity image is not.

Macroscale fluorescence lifetime (FLT) imaging is emerging as a promising tool to improve tumor margin delineation and enhance contrast between tumor and healthy tissue during fluorescence-guided surgery. We have been developing the tauCAM, a custom-built time-gated CMOS camera with a high quantum efficiency (QE) in NIR (46%) and a resolution of 128x128 pixels, specifically for this application. The images obtained from the tauCAM are overlaid on high-resolution color images acquired with a dedicated color camera. We demonstrate the capabilities of our camera by performing in vivo studies on subcutaneous mice tumor models administered with nanobody- and antibody-based NIR tumor-targeted fluorescent contrast agents. The results indicate that FLT imaging enhances the contrast between tumor and healthy tissue.

A 64 x 64 pixel current-assisted photonic sampler (CAPS) image sensor is designed for real-time fluorescence lifetime (FLT) imaging. Current assistance is used for swift detection and time-gating of the photogenerated carriers within the whole substrate. An additional bias node is used to decrease the dynamic power consumption, and a photogate is applied to the sensor to reduce the carrier transfer time. The sensor is fabricated in a commercial 350-nm CMOS process on a 15- mu m epi-layer, with 30- mu m pixels and a 61% fill factor. The pixel features gating widths below 1 ns subject to a 17-ps standard deviation on the gate width uniformity and a 31-ps standard deviation on its temporal position. The pixel has a quantum efficiency (QE) above 40% for near-infrared (NIR) wavelengths, whereas the peak QE is 61% at 675 nm. The gate's intrinsic decay is observed to be 320 ps, which allows for the FLT imaging of sub-nanosecond lifetime dyes. The real-time FLT imaging capabilities are demonstrated on commercial fluorescence phantoms and in a preclinical experiment.

Fluorescence imaging is widely used in many different domains, one of which is fluorescence-guided surgery. The temporal behavior called fluorescence lifetime gives deeper insights, making it an additional tool. Current fluorescence-guided surgery imaging typically utilizes two different cameras for natural color and NIR fluorescence imaging, making the system complex and expensive. We propose an alternative approach of sequential RGB+NIR fluorescence lifetime imaging and overlaying the images on each other. Sequential RGB+NIR fluorescence imaging is achieved with a high-speed time-gated camera combined with an RGB LED ring illumination for natural color imaging and a picosecond NIR pulsed laser source for fluorescence lifetime imaging.

Time-gated image sensors with (sub-) nanosecond gating times enable applications in multiple domains. Commercial Intensified CCD cameras or the more recent time-gated SPAD image sensors suffer from low Quantum Efficiency. We present an imaging system based on the tauCAM, a compact time-gated camera with our new 128x128-pixel CAPS sensor. The higher resolution is achieved with a new pixel architecture, yielding an improved fill factor. The capabilities of the new imaging system are demonstrated in a pre-clinical experimental setup. In vivo imaging in subcutaneous mice tumor models reveal that FLT imaging with the tauCAM provides additional contrast between tumor and healthy tissue, in particular for tracers sensitive to their physiological environment.

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Time-resolved fluorescence spectroscopy is a promising and versatile spectroscopy technique. While conventional fluorescence spectroscopy is concerned with the spectral features of fluorescence emission, time-resolved fluorescence spectroscopy (TRFS) also captures the temporal behavior of the excited molecules. In addition to applications in rapid medical diagnoses, several applications were reported in forensics, chemical analysis and biology. The existing TRFS systems are based on complex optical setups which results in bulky and large systems. Therefore, using them in practice is still not feasible. In this work, we present a miniaturized system that combines a compact Czerny-Turner based spectrometer layout with our high-speed time-gated image sensor of 32×32 pixels based on the Current-Assisted Photonic Sampler (CAPS) sensor. The system operates in the visible wavelength range with a high spectral resolution. The novel image sensor is capable of measuring down to sub-nanosecond lifetimes. The proof-of-concept module was assembled and characterized for three fluorophores.

The performance of the current-assisted single-photon avalanche diode (CA-SPAD) can be demonstrated to its full extent when used in conjunction with an active quenching circuit (AQC) located on the same chip. Counting rates, photon detection probability and single-photon timing resolution improve substantially as is evidenced by measurements in this paper. It is also demonstrated that a PDP of 20 % at 940 nm can be reached, making the CA-SPAD one of the best performing front-side illuminated (FSI) CMOS SPADs for near-infrared (NIR) operation.

In this paper, we present the tauCAM, a compact sub-nanosecond time-gated video framerate imaging camera, with our new 64×64-pixel Current-Assisted Photonic Sampler (CAPS) sensor. It features gating widths of less than 1 ns with a jitter of less than 60 ps rms on the gating width and less than 50 ps rms on the position. It has a quantum efficiency (QE) of more than 40% for near-infrared wavelengths. It's peak QE is 61% at 675 nm. The sensor is fabricated in a conventional 350 nm CMOS process. We give an overview of the design of the new camera hardware, with characterization of the timing circuitry and gating signals. Furthermore, the new sensor is discussed in detail. The revised pixel design yields improvements in QE, with high QE even for very short gating windows. Additionally, the Instrument Response Function of the pixel is characterized. The capabilities of the tauCAM are demonstrated in an experimental setup where we image a scene that is sequentially illuminated with red, green and blue light, on a nanosecond time scale. Due to its fast-gating mechanism, our CAPS sensor can distinguish and image these colors separately, while the human eye merely sees white illumination.

Fluorescence imaging has been used for quite some time in microscopy, preclinical and medical imaging, as well as in other domains. There has been interest in using the time-behaviour of fluorophores to gain additional information. This time-behaviour, called the fluorescence lifetime, is a property of the fluorophore and can also reveal information about its environment through modulation of this lifetime. Fluorescence-lifetime imaging could improve imaging of existing fluorescent contrast agents or could enable novel applications. The tauCAM, based on the Current-assisted photonic sampler (CAPS), is being developed at our research department with the purpose of achieving real-time fluorescence lifetime imaging. Previous versions have already shown some results in doing lifetime imaging through experiments and demonstrations. Elaborate characterisation of its imaging capabilities in this domain has yet to be performed due to the lack of standardisation and phantoms available for lifetime imaging. In this publication we will discuss the (real-time) lifetime and intensity imaging capabilities of the tauCAM, the latest iteration of our CAPS camera, housing a new 64×64-pixel sensor. The groundwork will be laid for the development of phantoms for lifetime imaging based on phantoms made for fluorescence intensity imaging. These will in turn be measured using reference equipment and used to characterize the accuracy and precision of the results from the tauCAM.