Versatile digital lock-in detection technique: application to spectrofluorometry and other fields

Enhancement of signal-to-noise ratio for fluorescence endoscope image based on fast digital lock-in algorithm

In this paper, the signal-to-noise ratios (SNR) of two image channels were enhanced with the fast digital lock-in algorithm. In order to simultaneously improve the quality of white and fluorescence images obtained by fluorescence endoscope, and improve the SNR to achieve a better image processing effect, two sources of white light and near-infrared light of a fluorescence endoscope were modulated, then the acquired images were demodulated into white and fluorescence images. A fluorescent endoscope experimental platform was setup to acquire endoscopic images of a target dyed by indocyanine green. The experimental results showed that the SNR of white and fluorescent images without the lock-in algorithm were 36.56 dB and 33.47 dB, respectively. However, with the lock-in algorithm, the SNR of white and fluorescent images were 39.54 dB and 35.70 dB, respectively. The SNR of white and fluorescent images was increased by 8.2% and 6.7%, respectively, by appling the digital lock-in algorithm. Therefore, this novel fluorescence endoscope based on the fast digital lock-in algorithm can rapidly and simultaneously obtain two-channel images of white light and fluorescence, effectively enhance the SNR of white and fluorescent images, and improve the imaging quality.

A Two-Dimensional Lock-In Algorithm for Signal Analysis in Patterned Images

GOAL

We extend a classic signal processing algorithm, the lock-in amplifier, to two dimensions to extract the signal in patterned images.

METHODS

The algorithm was evaluated using simulated image data and experimental microscopy images to determine the fluorescence signal of fluorescently labeled proteins adsorbed on surfaces patterned with chemical vapor deposition.

RESULTS

The algorithm was capable of retrieving the signal with a signal-to-noise ratio as low as -20 dB. Optimal parameters were determined for the pattern design.

CONCLUSION

The lock-in algorithm is a powerful technique for 1/f noise removal.

SIGNIFICANCE

The methodology holds promise not only for the measurement of adsorption events on patterned surfaces but in all situations where a signal has to be extracted from a noisy background in two or more dimensions.

Optimization of a digital lock-in algorithm with a square-wave reference for frequency-divided multi-channel sensor signal detection

A digital lock-in detection technique is commonly used to measure the amplitude and phase of a selected frequency signal. A technique that uses a square wave as the reference signal has an advantage over the one using a sinusoidal wave due to its easier implementation and higher computational efficiency. However, demodulating multiple-frequency composite signals using square wave reference may result in interference between channels. To avoid interference between channels and reduce the computational complexity, we modify the calculations and determine the optimal parameter settings of the low-pass filter and carrier frequency, as detailed in this paper. The results of our analysis show that when the length of the average filter and carrier frequencies are properly set, the interference between the channels is removed. This optimization produces the digital lock-in detection suitable for measuring multi-channel sensor signals.

Note: Differential configurations for the mitigation of slow fluctuations limiting the resolution of digital lock-in amplifiers

The resolution of digital lock-in amplifiers working with a narrow bandwidth (<100 Hz) is limited by slow fluctuations, which can be two orders of magnitude larger (μV range) than the noise of the input amplifier (tens of nV). In order to tackle this issue, affecting state-of-the-art laboratory instrumentation and here systematically quantified, three differential sensing configurations are presented. They adapt to different setup conditions and are based on manual and automatic tuning of dummy references, allowing a 25-fold resolution improvement for enhanced long-term tracking of impedance sensors.

Note: High precision measurements using high frequency gigahertz signals

Generalized lock-in amplifiers use digital cavities with Q-factors as high as 5 × 10(8) to measure signals with very high precision. In this Note, we show that generalized lock-in amplifiers can be used to analyze microwave (giga-hertz) signals with a precision of few tens of hertz. We propose that the physical changes in the medium of propagation can be measured precisely by the ultra-high precision measurement of the signal. We provide evidence to our proposition by verifying the Newton's law of cooling by measuring the effect of change in temperature on the phase and amplitude of the signals propagating through two calibrated cables. The technique could be used to precisely measure different physical properties of the propagation medium, for example, the change in length, resistance, etc. Real time implementation of the technique can open up new methodologies of in situ virtual metrology in material design.

Synchronous digitization for high dynamic range lock-in amplification in beam-scanning microscopy

Digital lock-in amplification (LIA) with synchronous digitization (SD) is shown to provide significant signal to noise (S/N) and linear dynamic range advantages in beam-scanning microscopy measurements using pulsed laser sources. Direct comparisons between SD-LIA and conventional LIA in homodyne second harmonic generation measurements resulted in S/N enhancements consistent with theoretical models. SD-LIA provided notably larger S/N enhancements in the limit of low light intensities, through the smooth transition between photon counting and signal averaging developed in previous work. Rapid beam scanning instrumentation with up to video rate acquisition speeds minimized photo-induced sample damage. The corresponding increased allowance for higher laser power without sample damage is advantageous for increasing the observed signal content.

Generalized lock-in amplifier for precision measurement of high frequency signals

We herein formulate the concept of a generalized lock-in amplifier for the precision measurement of high frequency signals based on digital cavities. Accurate measurement of signals higher than 200 MHz using the generalized lock-in is demonstrated. The technique is compared with a traditional lock-in and its advantages and limitations are discussed. We also briefly point out how the generalized lock-in can be used for precision measurement of giga-hertz signals by using parallel processing of the digitized signals.

A rapid and sensitive assay of intercellular coupling by voltage imaging of gap junction networks

Background

A variety of mechanisms that govern connexin channel gating and permeability regulate coupling in gap junction networks. Mutations in connexin genes have been linked to several pathologies, including cardiovascular anomalies, peripheral neuropathy, skin disorders, cataracts and deafness. Gap junction coupling and its patho–physiological alterations are commonly assayed by microinjection experiments with fluorescent tracers, which typically require several minutes to allow dye transfer to a limited number of cells. Comparable or longer time intervals are required by fluorescence recovery after photobleaching experiments. Paired electrophysiological recordings have excellent time resolution but provide extremely limited spatial information regarding network connectivity.

Results

Here, we developed a rapid and sensitive method to assay gap junction communication using a combination of single cell electrophysiology, large–scale optical recordings and a digital phase–sensitive detector to extract signals with a known frequency from Vf2.1.Cl, a novel fluorescent sensor of plasma membrane potential. Tests performed in HeLa cell cultures confirmed that suitably encoded Vf2.1.Cl signals remained confined within the network of cells visibly interconnected by fluorescently tagged gap junction channels. We used this method to visualize instantly intercellular connectivity over the whole field of view (hundreds of cells) in cochlear organotypic cultures from postnatal mice. A simple resistive network model reproduced accurately the spatial dependence of the electrical signals throughout the cellular network. Our data suggest that each pair of cochlear non−sensory cells of the lesser epithelial ridge is coupled by ~1500 gap junction channels, on average. Junctional conductance was reduced by 14% in cochlear cultures harboring the T5M mutation of connexin30, which induces a moderate hearing loss in connexin30^T5M/T5M knock–in mice, and by 91% in cultures from connexin30^−/− mice, which are profoundly deaf.

Conclusions

Our methodology allows greater sensitivity (defined as the minimum magnitude of input signal required to produce a specified output signal having a specified signal−to−noise ratio) and better time resolution compared to classical tracer–based techniques. It permitted us to dynamically visualize intercellular connectivity down to the 10th order in non−sensory cell networks of the developing cochlea. We believe that our approach is of general interest and can be seamlessly extended to a variety of biological systems, as well as to other connexin−related disease conditions.

Digital optical tomography system for dynamic breast imaging

Diffuse optical tomography has shown promising results as a tool for breast cancer screening and monitoring response to chemotherapy. Dynamic imaging of the transient response of the breast to an external stimulus, such as pressure or a respiratory maneuver, can provide additional information that can be used to detect tumors. We present a new digital continuous-wave optical tomography system designed to simultaneously image both breasts at fast frame rates and with a large number of sources and detectors. The system uses a master-slave digital signal processor-based detection architecture to achieve a dynamic range of 160 dB and a frame rate of 1.7 Hz with 32 sources, 64 detectors, and 4 wavelengths per breast. Included is a preliminary study of one healthy patient and two breast cancer patients showing the ability to identify an invasive carcinoma based on the hemodynamic response to a breath hold.

Quick-EXAFS implementation on the general purpose EXAFS beamline at ESRF

The new implementation of QEXAFS acquisition on the general purpose EXAFS beamline BM29 at the European Synchrotron Radiation Facility is presented. By adopting a continuous-scan mode, a high signal-to-noise ratio can be maintained, together with a fixed exit beam and full compatibility with step-by-step operation, while reducing acquisition times to a few seconds. The new equipment implemented on the beamline is described and the potential of this application is demonstrated with a few examples.

Studying RNA using site-directed spin-labeling and continuous-wave electron paramagnetic resonance spectroscopy

In site-directed spin-labeling (SDSL), a stable nitroxide radical is attached to a specific location within a macromolecule and electron paramagnetic resonance (EPR) spectroscopy is used to interrogate the local environment surrounding the nitroxide. The SDSL strategy enables probing site-specific structural and dynamic features of RNA in solution without being limited by the size of the molecule, thus serving as a unique tool in biophysical studies of RNA. This chapter describes the use of continuous-wave (cw)-EPR to study dynamic features of RNAs as well as to monitor interactions between them. Various approaches for attaching nitroxide spin labels to nucleic acids are described, followed by detailed descriptions of cw-EPR spectral acquisition and processing procedures. Specific examples are subsequently used to illustrate analysis of EPR spectra, showing how information regarding the parent RNA can be extracted.

Ultrasensitive fluorescence detection of single protein molecules manipulated electrically on Au nanowire

Proteins assembled on an Au nanowire are manipulated by an electrical potential applied on the nanowire, which leads to the modulation of molecular fluorescence. The molecular modality can be unequivocally correlated with the modulated fluorescence, which enables the specific fluorescence from a single target protein to be unambiguously distinguished from background noise and nonspecific fluorescence. As demonstrated through a thrombin assay, this simple method can significantly improve the sensitivity and specificity of the protein detection down to the single molecule level.

Digital-signal-processor-based dynamic imaging system for optical tomography

In this article, we introduce a dynamic optical tomography system that is, unlike currently available analog instrumentation, based on digital data acquisition and filtering techniques. At the core of this continuous wave instrument is a digital signal processor (DSP) that collects, collates, processes, and filters the digitized data set. The processor is also responsible for managing system timing and the imaging routines which can acquire real-time data at rates as high as 150 Hz. Many of the synchronously timed processes are controlled by a complex programmable logic device that is also used in conjunction with the DSP to orchestrate data flow. The operation of the system is implemented through a comprehensive graphical user interface designed with LABVIEW software which integrates automated calibration, data acquisition, data organization, and signal postprocessing. Performance analysis demonstrates very low system noise (approximately 1 pW rms noise equivalent power), excellent signal precision (<0.04%-0.2%) and long term system stability (<1% over 40 min). A large dynamic range (approximately 190 dB) accommodates a wide scope of measurement geometries and tissue types. First experiments on tissue phantoms show that dynamic behavior is accurately captured and spatial location can be correctly tracked using this system.

Digital lock-in algorithm for biomedical spectroscopy and imaging instruments with multiple modulated sources

Digital lock-in detection provides spectroscopic and imaging instruments a means of measuring physical quantities with improved signal to noise ratios compared to analogue detection schemes. We introduce a digital lock-in detection algorithm for measuring the amplitude and phase of multiple amplitude modulated signals simultaneously by using particular modulation and sampling constraints and averaging filters. The technique exhibits exceptional reduction in both noise and inter-source distortion. It is shown that the digital lock-in technique can be performed as a simple matrix multiplication in order to reduce computation time. The digital lock-in algorithm is described and analyzed under certain sampling and modulation conditions. Results are shown for experimental data.