Ultra high-speed swept source Fourier domain mode locking (FDML) OCT at 1.68, 3.3 and 6.7 MHz - Image quality of retinal cross sectional scans

Abstract

 

Purpose:

 

To evaluate the image quality of cross sectional scans obtained with swept source Fourier domain mode locking (FDML) OCT at 1.68 MHz, 3.3 MHz and 6.7 MHz.

 

Methods:

 

A 1050nm swept source FDML OCT system was constructed running at 1.68 MHz, 3.3 MHz. Effective imaging speed was additionally doubled to 6.7 MHz two spots in parallel. Scanning time for a wide-field coverage consisting of two million axial scans was 1.32 sec (1.68 MHz), 0.66 sec (3.3 MHz) and 0.33 sec (6.7 MHz), respectively. Total incident power was 1.6 mW in accordance with Laser Safety Standards (ANSI, USA). Even in the case of two-spot imaging, the combined total power did not exceed 1.6mW. For each scanning speed, averaged OCT cross sectional scans of a healthy subject were compared for image quality.

 

Results:

 

Measured sensitivity at 1.68 MHz, was 91.5 dB, corresponding to shot noise limited detection for the employed interferometer configuration. Shot noise limited detection at 3.3 MHz and 6.7 MHz was 88.5 dB and 85.5 dB respectively. Evaluation and comparison of averaged FDML OCT images showed reasonable contrast and resolution of individual retinal layers at 1.68 MHz and 3.3 MHz with inferior image quality at 6.7 MHz.

 

Conclusions:

 

Ultra high-speed FDML OCT is feasible at 1.68 MHz, 3.3 MHz, and, for the first time at 6.7 MHz, see Figure 1. Reasonable image quality can be achieved with 1.68 MHz, followed by 3.3 MHz. Based on our observations we suggest a sensitivity threshold of a minimum of 90 dB for retinal imaging, which currently limits imaging speed to around 1.68 MHz axial scan rate.However, we would like to point out that for MHz and especially multi-MHz imaging, the fast scanning speed and short illumination duration of down to 0.3 sec may allow for higher incident power levels in accordance with safety standards, especially in the case of multi-spot imaging.  

 

Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • image processing • imaging/image analysis: clinical