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

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

Spatio-temporal analysis of blood perfusion by imaging photoplethysmography

Imaging photoplethysmography (iPPG) has attracted much attention over the last years. The vast majority of works focuses on methods to reliably extract the heart rate from videos. Only a few works addressed iPPGs ability to exploit spatio-temporal perfusion pattern to derive further diagnostic statements. This work directs at the spatio-temporal analysis of blood perfusion from videos. We present a novel algorithm that bases on the two-dimensional representation of the blood pulsation (perfusion map). The basic idea behind the proposed algorithm consists of a pairwise estimation of time delays between photoplethysmographic signals of spatially separated regions. The probabilistic approach yields a parameter denoted as perfusion speed. We compare the perfusion speed versus two parameters, which assess the strength of blood pulsation (perfusion strength and signal to noise ratio). Preliminary results using video data with different physiological stimuli (cold pressure test, cold face test) show that all measures are in fluenced by those stimuli (some of them with statistical certainty). The perfusion speed turned out to be more sensitive than the other measures in some cases. However, our results also show that the intraindividual stability and interindividual comparability of all used measures remain critical points. This work proves the general feasibility of employing the perfusion speed as novel iPPG quantity. Future studies will address open points like the handling of ballistocardiographic effects and will try to deepen the understanding of the predominant physiological mechanisms and their relation to the algorithmic performance

Laser Speckle Imaging: A Quantitative Tool for Flow Analysis

Laser speckle imaging, often referred to as laser speckle contrast analysis (LASCA), has been sought after as a quasi-real-time, full-field, flow visualization method. It has been proven to be a valid and reliable qualitative method, but there has yet to be any definitive consensus on its ability to be used as a quantitative tool. The biggest impediment to the process of quantifying speckle measurements is the introduction of additional non dynamic speckle patterns from the surroundings. The dynamic speckle pattern under investigation is often obscured by noise caused by background static speckle patterns. One proposed solution to this problem is known as dynamic laser speckle imaging (dLSI). dLSI attempts to isolate the dynamic speckle signal from the previously mentioned background and provide a consistent dynamic measurement. This paper will investigate the use of this method over a range of experimental and simulated conditions. While it is believable that dLSI could be used quantitatively, there were inconsistencies that arose during analysis. Simulated data showed that if the mixed dynamic and static speckle patterns were modeled as the sum of two independent speckle patterns, increasing static contributions led to decreasing dynamic contrast contributions, something not expected by theory. Experimentation also showed that there were scenarios where scattering from the dynamic media obscured scattering from the static medium, resulting in poor estimates of the velocities causing the dynamic scattering. In light of these observations, steps were proposed and outlined to further investigate into this method. With more research it should be possible to create a set of conditions where dLSI is known be accurate and quantitative

Endoscopic Laser Speckle Contrast Analysis for Tissue Perfusion

Laser speckle contrast analysis (LASCA), as a method of measuring blood flow speed and tissue perfusion, is a full field imaging technique requiring simple configurations and data processing, which is important for the application in real time in vivo. But LASCA is sensitive to changes in environmental factors. The application in vivo is also limited to superficial detection due to the limitation of the light penetration depth. Therefore this thesis aims to develop an endoscopic LASCA system to extend the access to internal detection and explore the relationship between the contrast and experimental parameters. Firstly the relationship between the contrast and speckle size, flow mode, quantity of stationary scatterers and the signal intensity were investigated. Theoretical models for the relationship between the contrast and the mean intensity of the speckle pattern were deduced and the correction methods were introduced to correct the contrast bias due to the intensity difference. Then a flexible single wavelength endoscopic laser speckle contrast analysis system (ELASCA) was developed using a leached fibre image guide (LFIG). A Butterworth filter and defocus were used to remove the fibre pattern to retrieve the contrast images. This system and the data processing methods were used on a customized phantom demonstrating that this ELASCA system can detect the flow speed changes in an imaging domain. Afterwards a dual-wavelength ELASCA was developed for functional imaging of the blood circulation. The test on a human fingertip and rabbit uterine blood vessels show that this system can monitor the change of blood flow speed and the oxygen saturation introduced by occlusion, in addition to the cardiac pulse and respiration rate. Then a novel application of LASCA to visualize the ultrasound pressure field and the propagation of the shear wave is presented for the application of locating area of interest (AOI) and detecting tissue variation

Exercise intensity as a mediator of central and peripheral vascular integrity

Vascular homeostasis is a vital element of health. Exercise plays a role in maintaining the integrity of the vascular endothelium via transient increases in endothelial shear stress that accompany exercise-induced hyperaemia. This hyperaemia and stimulus it presents to the vasculature is governed by exercise intensity. Exercise is the primary therapy in cardiac rehabilitation contexts, a population with typically elevated cardiovascular risk factors and compromised vascular integrity. In the UK, cardiac rehabilitation may be ineffective for improving long-term health outcomes. This thesis demonstrates that the exercise intensities achieved by patients in UK cardiac rehabilitation were variable and generally low. The exercise performed had no impact upon indices of peripheral vascular structure or function and little effect upon short-term health outcomes. A service-level intervention was implemented to increase the dose of exercise achieved by patients via an increase in intensity and examine its effects upon the vasculature. In a subsequent cohort, the intervention was unsuccessful at modifying the exercise intensities that were achieved or indices of peripheral vascular structure or function. However, a positive relationship between the intensity achieved at the end of the programme and changes in vascular function was found. Although peripheral vascular integrity is easily studied it is perhaps less relevant to health compared to central vascular integrity. Assessments of the central circulation during exercise have previously relied upon high-risk, invasive techniques. Therefore, to understand the stimulus presented to the central vasculature by exercise at intensities akin to those achieved in cardiac rehabilitation, a novel technique was applied: using cardiac magnetic resonance imaging during cycling in a healthy cohort. This technique was unable to capture the changes in perfusion of the central circulation that should have occurred with exercise at different intensities but successfully demonstrated reproducible assessments of cardiac dynamics during exercise of different intensities

Ultrasound Contrast Agents for Imaging and Therapy

The aim of this thesis is to unravel the relation between shell properties and the acoustic response of single microbubbles. Next, the most stable and acoustically best performing UCAs are also investigated in vitro for therapeutic applications by means of sonoporation, and for in vivo diagnostic imaging applications