Dual-Wavelength Photothermal OCT for Depth-Resolved Measurement of Microvascular Hemoglobin Oxygen Saturation (SaO2)

Abstract

 

Purpose:

 

Objective of this study was to estimate depth resolution of Dual-Wavelength Photothermal Optical Coherence Tomography (DWP-OCT) for measuring hemoglobin oxygenation saturation (SaO2) in a murine brain model.

 

Methods:

 

DWP-OCT was used to measure photothermal-induced optical pathlength (op) variations versus depth in murine brain in vivo. The DWP-OCT probe was directed at a 30 µm diameter arteriole under guidance of a surgical microscope. Blood in the arteriole in vivo was excited with intensity-modulated light at 770 nm and 800 nm. Ratio of op changes at the back of the arteriole wall normalized by fluence of photothermal excitation light can be used to compute in-vivo SaO2 levels. We investigated depth resolution provided by DWP-OCT for measuring in vivo SaO2 levels.

 

Results:

 

DWP-OCT depth resolution can be estimated from depth-resolved op measurements in tissue containing a single arteriole. Amplitude of op variation per mW of photothermal excitation light (800 nm) with depth (orange) and its approximation (blue) are shown in Fig. 1. We approximate depth dependence of op by assuming spatial variation of temperature increase is Guassian so that op varies as an error function: op ∝ Erf(a[z-z0]), where z0 = 568 µm is the center of absorption (arteriole) and a = 0.031 µm-1 is inversely proportional to the effective longitudinal size of the temperature increase. The longitudinal full width at half maximum (FWHM) of the temperature increase z*= 2(ln2)^0.5/a = 45 µm gives DWP-OCT depth resolution to measure SaO2 levels.

 

Conclusions:

 

This study provides a numerical estimate of depth resolution of DWP-OCT to measure SaO2 levels. DWP-OCT may be useful in layer-specific quantification of abnormal tissue oxygenation in several retinal disorders such as diabetic retinopathy.  

 

Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • diabetic retinopathy • oxygen