Research On Improving The Sampling Depth Of Laser Speckle Blood Flow Imaging

Laser speckle imaging (LSI) is a full-field blood flow imaging technique with highspatio-temporal resolution and easily controlled imaging area. It has attracted extensiveattention in biomedical applications，such as intraoperative imaging, mechanism ofneurovascular coupling and evaluating drug efficiency. However, this method suffers fromthe problem of shallow sampling depth of blood flow, so it mainly reflects the superficialblood flow information of biological tissue. Though combining the laser speckle imagingof blood flow with the cross-polarization method to reduce the influence of specularreflection can improve the sampling depth of blood flow, the improvement is notsignificant. Here，we present a line beam scanning method which only analyzes theinformation of the indirectly illuminated area to improve the sampling depth of blood flow:This lateral laser speckle blood flow analysis combined with line beam scanningillumination is simple to control. Only the light source section of the laser speckle imagingsystem should be revised without changing the acquisition maner of the raw spckle images.The main contents of this thesis include:1）Monte Carlo simulation is performed to study the mean depths that the detectedphotons traveled inside the medium with full-field illumination and line beam scanningillumination, respectively. According to the comparison results, we present a laser speckleimaging system with line beam scanning illumination combined with a lateral laserspeckle blood flow analysis method to improve the sampling depth of blood flow imaging.2）It is demonstrated by both the animal and the phantom experiments that theimprovement of sampling depth using our technology is larger than that using thecross-polarization method combined with full-field illumination. Furthermore, thistechnology can improve the sensitivity to the change of blood flow velocity. The depthsensitivity was also be evaluated.3）The influences of the spacing between line-scans (also called move step in this thesis), the line beam width and the removed width on sampling depth are studied. Through theMonte Carlo simulation, when the spacing of line-scans and the removed width areconstant values, the mean depth of the detected photons will decrease with the increase ofline beam width; when the spacing of line-scans and the line beam width are constantvalues, the mean depth of the detected photons will increase with the increase of removedwidth; while the mean depth of the detected photons does not depend on the spacingbetween line-scans. Moreover, the mean imaging depth across the pixels in the imagedarea does not vary in a semi-infinite homogeneous turbid medium with an adequate largeratio of illuminating/scanning area to imaging area. However, in practice the meanimaging depth across the pixels in the imaged area varies due to the limited ratio ofilluminated area to imaged area. Specifically, when the illuminated area equals to theimaged area, the mean imaging depths of the pixels in the central areas will be higher thanthat in the edges of the image for both full-field method and our scanning illuminationmethod.4）The influences of optical properties of biological tissues on sampling depth arestudied. Through the Monte Carlo simulation, the improvement of sampling depth willdecrease with the increase of scattering coefficient and the increase of absorptioncoefficient, while it will increase with the increase of the anisotropy factor.