When configured in morphological imaging mode, OMAG imaged the intra-retinal and choroidal microstructures, with image quality similar to that of state of the art OCT systems. Figure 1 gives one such example that was obtained from the macular region of a healthy volunteer. Note that in this experiment, the B scan was set to cover on the retina. It is clear that OMAG is capable of visualizing the architectural morphology of the intraretinal and choroidal layers that correlate well with the intraretinal anatomy. For blood flow imaging, OMAG treats the microstructures as if they were noises to the system. This is achieved by the modified Hilbert transformation on the captured spatial-varying spectral interferograms to separate the blood flow signals from the static tissue signals. The detailed description of this method has been reported previously.4- 5 Here we introduce the directional imaging using OMAG. The B-scan interference signal captured by the OCT system can be written as:Display Formula
1where is the wavenumber; is the timing when a A-line was captured. is the light reflected from the reference mirror; is the spectral density of the light source used; is the refractive index of tissue; is the depth coordinate; is the amplitude of the back scattered light; is the velocity of moving blood cells in a blood vessel, which is located at depth . Because the light backscattered from the sample is quite weak compared to the light reflected from the reference mirror, here we do not consider the self cross-correlation between the light backscattered from different positions within the sample. We also do not consider the DC signals because they do not contribute to useful OMAG signals. The first term in Eq. 1 represents the spatial frequency components from the static tissue elements, which can be used to provide the static structural information of the sample, such as that shown in Fig. 1. The second term is the Doppler beating signal, determined by the moving blood cells. If we first apply high pass filtering with an appropriate selection of cut off frequency to exclude the structural information, and then apply the Hilbert transformation along B-scan direction, the result will be:Display Formula
2when (i.e., the blood cells moving towards the incident probe beam direction). Whereas when (i.e., the blood cells moving away from the incident probe beam direction), Eq. 2 becomes:Display Formula
3Mathematically, Eq. 3 is clearly the complex conjugate of Eq. 2. Therefore, according to Ref. 4, the directional blood flow is discriminated. The optical signals that represent the blood cells moving towards the probe beam will be situated at the positive output plane of OMAG, while those from the blood cells moving away from the probe beam direction will be located at the negative plane.