Multi-spectral and Speckle imaging of hemodynamic parameters for functional activation and stroke research

Two methods of optical imaging; Speckle contrast and multi-spectral reflectance imaging (MSRI), use shared optics to give simultaneous and co-localized data of both changes in blood flow (CBF) and tissue perfusion. The data can then be combined to calculate changes in cerebral metabolic rate of oxygen consumption (CMRO₂).

Coherent light (from a laser) reflecting from a surface generates a random pattern of fringes known as a speckle pattern. If the blood cells from which the light is reflected are moving, the pattern becomes blurred. The change in speckle contrast (K), is inversely proportional to the change in blood velocity (v).

 

 

 

 

 

low speckle contrast

 

high speckle contrast

 

Raw speckle image

Speckle contrast

Speckle Contrast

Multi-spectral imaging

Oxygenated hemoglobin (HbO), and deoxygenated (reduced) hemoglobin (HbR) have different absorption spectra, which is why deoxygenated blood is more blue in colour. In the diffuse reflection regime, absorption coefficient is inversely proportional to the amount of light reflected.

Therefore, it is possible to detect changes in HbO and HbR by monitoring changes in reflectance in two or more

wavelengths. The Beer-Lambert approximation shown above creates a set of simultaneous equations which can be solved so long as the number of chromaphores (HbO,HbR) is lower than the number of wavelengths used. We use 6 wavelengths changing sequentially at a frequency of 18 Hz.

 

 

 

 

 

R₀ & R= reflectance before and after a change

ΔC = Change in concentration of HbO or HbR

εi = Absorption coefficient at a particular wavelength