Multi-exposure laser speckle contrast imaging using a high frame rate CMOS sensor with a field programmable gate array.

A system has been developed in which multi-exposure laser speckle contrast imaging (LSCI) is implemented using a high frame rate CMOS imaging sensor chip. Processing is performed using a field programmable gate array (FPGA). The system allows different exposure times to be simulated by accumulating a number of short exposures. This has the advantage that the image acquisition time is limited by the maximum exposure time and that regulation of the illuminating light level is not required. This high frame rate camera has also been deployed to implement laser Doppler blood flow processing, enabling a direct comparison of multi-exposure laser speckle imaging and laser Doppler imaging (LDI) to be carried out using the same experimental data. Results from a rotating diffuser indicate that both multi-exposure LSCI and LDI provide a linear response to changes in velocity. This cannot be obtained using single-exposure LSCI, unless an appropriate model is used for correcting the response.

A Single-Chip CMOS Pulse Oximeter with On-Chip Lock-In Detection.

Pulse oximetry is a noninvasive and continuous method for monitoring the blood oxygen saturation level. This paper presents the design and testing of a single-chip pulse oximeter fabricated in a 0.35 µm CMOS process. The chip includes photodiode, transimpedance amplifier, analogue band-pass filters, analogue-to-digital converters, digital signal processor and LED timing control. The experimentally measured AC and DC characteristics of individual circuits including the DC output voltage of the transimpedance amplifier, transimpedance gain of the transimpedance amplifier, and the central frequency and bandwidth of the analogue band-pass filters, show a good match (within 1%) with the circuit simulations. With modulated light source and integrated lock-in detection the sensor effectively suppresses the interference from ambient light and 1/f noise. In a breath hold and release experiment the single chip sensor demonstrates consistent and comparable performance to commercial pulse oximetry devices with a mean of 1.2% difference. The single-chip sensor enables a compact and robust design solution that offers a route towards wearable devices for health monitoring.

Design of a mid-field wireless power transmission system for deep-tissue implants.

BACKGROUND: Implantable medical devices are being valued as one of the developments of wireless biomedical technology. OBJECTIVE: This paper presents a mid-field wireless power transmission (WPT) system, which is designed for implantable applications and operates at the 2.40-2.48 GHz band of Industrial, Scientific, and Medical (ISM). METHODS: A new compact transmitter structure is proposed, and a small 4-C planar ring antenna is designed as the receiving element. A measurement setup is fulfilled on porcine tissues to verify the power transmission system. RESULTS: The experimental results show that the operating bandwidth is 2.2-2.62 GHz and the transmission coefficient can reach -26.32 dB at a distance of 50 mm. The effects of tissue differences, placement depth, and different transmission distances were also measured. The displacement and deflection tolerances between the transmitter and the implant receiver also have good performance. In the safety standard of specific absorption rate, for the 1 W output power from the mid-field transmitter, the receiving power of the implantable antenna at the mid-field distance can reach 79.6 mW. CONCLUSION: With measurements of different implantation and transmission distance on pork, the mid-field power transmission efficiency is proven and shows the high performance of the system.

Laser doppler blood flow imaging using a CMOS imaging sensor with on-chip signal processing.

The first fully integrated 2D CMOS imaging sensor with on-chip signal processing for applications in laser Doppler blood flow (LDBF) imaging has been designed and tested. To obtain a space efficient design over 64 × 64 pixels means that standard processing electronics used off-chip cannot be implemented. Therefore the analog signal processing at each pixel is a tailored design for LDBF signals with balanced optimization for signal-to-noise ratio and silicon area. This custom made sensor offers key advantages over conventional sensors, viz. the analog signal processing at the pixel level carries out signal normalization; the AC amplification in combination with an anti-aliasing filter allows analog-to-digital conversion with a low number of bits; low resource implementation of the digital processor enables on-chip processing and the data bottleneck that exists between the detector and processing electronics has been overcome. The sensor demonstrates good agreement with simulation at each design stage. The measured optical performance of the sensor is demonstrated using modulated light signals and in vivo blood flow experiments. Images showing blood flow changes with arterial occlusion and an inflammatory response to a histamine skin-prick demonstrate that the sensor array is capable of detecting blood flow signals from tissue.

Effect of object size and acoustic wavelength on pulsed ultrasound modulated fluorescence signals.

Detection of ultrasound (US)-modulated fluorescence in turbid media is a challenge because of the low level of fluorescent light and the weak modulation of incoherent light. A very limited number of theoretical and experimental investigations have been performed, and this is, to our knowledge, the first demonstration of pulsed US-modulated fluorescence tomography. Experimental results show that the detected signal depends on the acoustic frequency and the fluorescent target's size along the ultrasonic propagation axis. The modulation depth of the detected signal is greatest when the length of the object along the acoustic axis is an odd number of half wavelengths and is weakest when the object is an integer multiple of an acoustic wavelength. Images of a fluorescent tube embedded within a 22- by 13- by 30 mm scattering gel phantom (µ(s)∼15 cm(-1), g=0.93) with 1-, 1.5-, and 2 MHz frequency US are presented. The modulation depth of the detected signal changes by a factor of 5 depending on the relative size of the object and the frequency. The approach is also verified by some simple experiments in a nonscattering gel and using a theoretical model.

Application of a maximum likelihood algorithm to ultrasound modulated optical tomography.

In pulsed ultrasound modulated optical tomography (USMOT), an ultrasound (US) pulse performs as a scanning probe within the sample as it propagates, modulating the scattered light spatially distributed along its propagation axis. Detecting and processing the modulated signal can provide a 1-dimensional image along the US axis. A simple model is developed wherein the detected signal is modelled as a convolution of the US pulse and the properties (ultrasonic/optical) of the medium along the US axis. Based upon this model, a maximum likelihood (ML) method for image reconstruction is established. For the first time to our knowledge, the ML technique for an USMOT signal is investigated both theoretically and experimentally. The ML method inverts the data to retrieve the spatially varying properties of the sample along the US axis, and a signal proportional to the optical properties can be acquired. Simulated results show that the ML method can serve as a useful reconstruction tool for a pulsed USMOT signal even when the signal-to-noise ratio (SNR) is close to unity. Experimental data using 5 cm thick tissue phantoms (scattering coefficient µ(s) = 6.5 cm(-1), anisotropy factor g=0.93) demonstrate that the axial resolution is 160 µm and the lateral resolution is 600 µm using a 10 MHz transducer.

Low resource processing algorithms for laser Doppler blood flow imaging.

The emergence of full field laser Doppler blood flow imaging systems based on CMOS camera technology means that a large amount of data from each pixel in the image needs to be processed rapidly and system resources need to be used efficiently. Conventional processing algorithms that are utilized in single point or scanning systems are therefore not an ideal solution as they will consume too much system resource. Two processing algorithms that address this problem are described and efficiently implemented in a field programmable gate array. The algorithms are simple enough to use low system resource but effective enough to produce accurate flow measurements. This enables the processing unit to be integrated entirely in an embedded system, such as in an application-specific integrated circuit. The first algorithm uses a short Fourier transformation length (typically 8) but averages the output multiple times (typically 128). The second method utilizes an infinite impulse response filter with a low number of filter coefficients that operates in the time domain and has a frequency-weighted response. The algorithms compare favorably with the reference standard 1024 point fast Fourier transform in terms of both resource usage and accuracy. The number of data words per pixel that need to be stored for the algorithms is 1024 for the reference standard, 8 for the short length Fourier transform algorithm and 5 for the algorithm based on the infinite impulse response filter. Compared to the reference standard the error in the flow calculation is 1.3% for the short length Fourier transform algorithm and 0.7% for the algorithm based on the infinite impulse response filter.