以有限時域差分法分析在光學同調掃描術量測具有吸收效應的複數黑色素折射率

Abstract

黑色素是一種在生物體中廣泛分佈的天然色素，但現今黑色素精確的折射率仍不清楚。光學同調斷層掃描（OCT）是具有高速掃描的優點和更高的解析度，但重建圖像的穿透度低，因此它是一種非常合適應用到淺層結構的斷層攝影，像是表皮組織和眼睛的視網膜結構。 有限時域差分法（FDTD）是一種數值電磁模擬方法，可以很容易的模擬實驗中原始數據的軸向掃描（A-Scan）。我們假設光從樣本反射的機制是正向入射遵循菲涅爾公式且用一維的有限時域差分法來模擬實驗中原始數據的軸向掃描。 我們提出的方法是使用有吸收的反射訊號去重構組織單純不具吸收結構。得到不吸收的結構後，該結構可以是另一個無吸收的參考結構去模擬，以獲得另一個無吸收的反射訊號，藉此與之前的反射訊號做比較。 結論上來說，吸收係數（α）可以從訊號中第二反射的衰減來計算。結果顯示一些重建結構的缺陷可以歸因於細小的邊界由於在橫向方向上的厚度太薄。因此，我們比較與雙倍尺寸重建的結果，結果顯示較大的幾何形狀可減少由厚度小於解析度所引起的重建誤差。

Melanin is a widely distributed natural pigment in creature, but the exact refractive index of melanin is still not clear. Optical coherence tomography (OCT) is with the advantage of high-speed scan and much higher resolution, but with low image penetration. Therefore, it is very appropriate to apply the tomography to superficial in vivo sample like epidermis tissue and the structure of retina in the eye. The numerical electromagnetic simulation method: finite-difference time domain (FDTD) can easily simulate the axial scan (A-scan) from OCT experiment carrier raw data. We assumed that the mechanism of light reflected from sample is incident Fresnel’s equation and simulated the OCT carrier raw data by 1D-FDTD. The method we propose is to use the signal which are with absorption to reconstruct the sample without absorption structure first. After getting the structure without absorption, the structure can be a simulation scenario to get another reflection signal without absorption to compare with the first reflection. In conclusion, the absorption coefficient (α) can be calculated from the decay of second reflection and the result shows that the reconstructed sample with some defects can be attributed to the slight boundary that is with thin thickness in lateral direction. Therefore, we compare to the result with double sized structure, the difference shows that the larger geometry can reduce the reconstruct error caused by the thickness under the resolution.