Twente Optical Perfusion Camera: system overview and performance for video rate laser Doppler perfusion imaging

We present the Twente Optical Perfusion Camera (TOPCam), a novel laser Doppler Perfusion Imager based on CMOS technology. The tissue under investigation is illuminated and the resulting dynamic speckle pattern is recorded with a high speed CMOS camera. Based on an overall analysis of the signal-to-noise ratio of CMOS cameras, we have selected the camera which best fits our requirements. We applied a pixel-by-pixel noise correction to minimize the influence of noise in the perfusion images. We can achieve a frame rate of 0.2 fps for a perfusion image of 128×128 pixels (imaged tissue area of 7×7 cm2) if the data is analyzed online. If the analysis of the data is performed offline, we can achieve a frame rate of 26 fps for a duration of 3.9 seconds. By reducing the imaging size to 128×16 pixels, this frame rate can be achieved for up to half a minute. We show the fast imaging capabilities of the system in order of increasing perfusion frame rate. First the increase of skin perfusion after application of capsicum cream, and the perfusion during an occlusion-reperfusion procedure at the fastest frame rate allowed with online analysis is shown. With the highest frame rate allowed with offline analysis, the skin perfusion revealing the heart beat and the perfusion during an occlusion-reperfusion procedure is presented. Hence we have achieved video rate laser Doppler perfusion imaging

Pulsatile microvascular blood flow imaging by short-time Fourier transform analysis of ultrafast laser holographic interferometry

We report on wide-field imaging of pulsatile microvascular blood flow in the exposed cerebral cortex of a mouse by holographic interferometry. We recorded interferograms of laser light backscattered by the tissue, beating against an off-axis reference beam with a 50 kHz framerate camera. Videos of local Doppler contrasts were rendered numerically by Fresnel transformation and short-time Fourier transform analysis. This approach enabled instantaneous imaging of pulsatile blood flow contrasts in superficial blood vessels over 256 x 256 pixels with a spatial resolution of 10 microns and a temporal resolution of 20 ms.Comment: 4 page

Real-time full field laser Doppler imaging

We present a full field laser Doppler imaging instrument, which enables real-time in vivo assessment of blood flow in dermal tissue and skin. This instrument monitors the blood perfusion in an area of about 50 cm2 with 480 × 480 pixels per frame at a rate of 12–14 frames per second. Smaller frames can be monitored at much higher frame rates. We recorded the microcirculation in healthy skin before, during and after arterial occlusion. In initial clinical case studies, we imaged the microcirculation in burned skin and monitored the recovery of blood flow in a skin flap during reconstructive surgery indicating the high potential of LDI for clinical applications. Small animal imaging in mouse ears clearly revealed the network of blood vessels and the corresponding blood perfusion

Review of methodological developments in laser Doppler flowmetry

Laser Doppler flowmetry is a non-invasive method of measuring microcirculatory blood flow in tissue. In this review the technique is discussed in detail. The theoretical and experimental developments to improve the technique are reviewed. The limitations of the method are elaborated upon, and the research done so far to overcome these limitations is critically assessed

Linear array CMOS detectors for laser doppler blood flow imaging

Laser Doppler blood flow imaging is well established as a tool for clinical research. The technique has considerable potential as an aid to diagnosis and as a treatment aid in a number of situations. However, to make widespread clinical use of a blood flow imager feasible a number of refinements are required to make the device easy to use, accurate and safe. Existing LDBF systems consist of 2D imaging systems, and single point scanning systems. 2D imaging systems can offer fast image acquisition time, and hence high frame rate. However, these require high laser power to illuminate the entire target area with sufficient power. Single point scanning systems allow lower laser power to be used, but building up an image of flow in skin requires mechanical scanning of the laser, which results in a high image acquisition time, making the system awkward to use. A new approach developed here involves scanning a line along a target, and imaging the line with a 1D sensor array. This means that only one axis of mechanical scanning is required, reducing the scanning speed, and the laser power is vastly reduced from that required for a 2D system. This approach lends itself well to the use of integrated CMOS detectors, as the smaller pixel number means that a linear sensor array can be implemented on an IC which has integrated processing while keeping overall IC size, and hence cost, lower than equivalent 2D imaging systems. A number of front-end and processing circuits are investigated in terms of their suitability for this application. This is done by simulating a range of possible designs, including several logarithmic pixels, active pixel sensors and opamp-based linear front-ends. Where possible previously fabricated ICs using similar sensors were tested in a laser Doppler flowmetry system to verify simulation results. A first prototype IC (known as BVIPS1) implements a 64x1 array of buffered logarithmic pixels, chosen for their combination of sufficient gain and bandwidth and compact size. The IC makes use of the space available to include two front-end circuits per pixel, allowing other circuits to be prototyped. This allows a linear front-end based on opamps to be tested. It is found that both designs can detect changes in blood flow despite significant discrepancies between simulated and measured IC performance. However, the signal-noise ratio for flux readings is high, and the logarithmic pixel array suffers from high fixed pattern noise, and noise and distortion that makes vein location impossible. A second prototype IC (BVIPS2) consists of dual 64x1 arrays, and integrated processing. The sensor arrays are a logarithmic array, which addresses the problems of the first IC and uses alternative, individually selectable front-ends for each pixel to reduce fixed-pattern noise, and an array of opamp-based linear detectors. Simulation and initial testing is performed to show that this design operates as intended, and partially overcomes the problems found on the previous IC - the IC shows reduced fixed pattern noise and better spatial detection of blood flow changes, although there is still significant noise

Modern Applications in Optics and Photonics: From Sensing and Analytics to Communication

Optics and photonics are among the key technologies of the 21st century, and offer potential for novel applications in areas such as sensing and spectroscopy, analytics, monitoring, biomedical imaging/diagnostics, and optical communication technology. The high degree of control over light fields, together with the capabilities of modern processing and integration technology, enables new optical measurement systems with enhanced functionality and sensitivity. They are attractive for a range of applications that were previously inaccessible. This Special Issue aims to provide an overview of some of the most advanced application areas in optics and photonics and indicate the broad potential for the future

Low bandwidth laser doppler blood flowmetry

Laser Doppler Blood Flowmetry (LDBF) has for several decades been applied to measure the flow of red blood cells in living tissue. Laser Doppler Perfusion Imaging (LDPI), a recent advancement which enables full-field blood flow visualisation, is gaining clinical acceptance in fields such as burn diagnostics. However, video-rate full-field imagers with appropriate sensor and processing capability require large financial and physical resources and this has prompted the development of under-specified systems. These systems may reduce the bandwidth and processing complexity but the question of how they perform compared to their fully specified counterparts remains. The advantages of these cheaper and often highly reconfigurable systems are recognised and so it is beneficial to ask whether any novel processing schemes can reduce the resultant error. Here a reduced bandwidth LDBF signal processing system has been modelled. Bayesian Inference has been used to show that the Pareto distribution is a likely model for the LDBF power spectrum, despite often being cited as exponential. Methods of evaluating microvascular blood flow have been described and compared. Additionally, one fast algorithm's effectiveness has been explained, and a novel and accurate method using the Hilbert transform has been presented. By understanding how aliasing modifies the frequency distribution, Bayesian Inference has been used to correct the blood flow output towards gold-standard values. The technique has been shown to correct the output of a low bandwidth CMOS camera imaging a rotating diffuser. Low bandwidth LDPI systems may be suitable for certain clinical applications where sensitivity to high flow is not required. However, where sensitivity to higher flow than baseline is required, e.g. in burn diagnostics, low bandwidth systems may underestimate the true blood flow leading to misdiagnosis. Nevertheless, low bandwidth systems could be used in this scenario if reliable post-processing is employed, such as that suggested by this thesis