MHz SS-OCT – from biometry to live volumetric imaging

Abstract

Purpose : For various applications during ophthalmic surgery spectral-domain optical coherence tomography (SD-OCT) is limited through its low A-scan rate and imaging depth. We present a versatile swept-source OCT (SS-OCT) engine, which addresses a large collection of use cases, ranging from axial eye length measurements to live volumetric visualizations at MHz A-scan rates.

Methods : We developed a flexible SS-OCT engine and an add-on module to couple its sample arm to an ophthalmic surgical microscope. This engine includes a 1060nm tunable MEMS-VCSEL whose sweep repetition rate can be alternated between 100kHz, 400kHz or 1MHz. To increase the effective A-scan rate at the cost of axial resolution, we scanned at twice the speed and mathematically divided each sweep into two halves. The 100kHz mode was used for high resolution B-scan and axial eye length measurements with an axial resolution of 6.3mm and an imaging depth of 29mm in tissue. For 4D live imaging, effective A-scan rates of 800kHz and 2MHz allowed us to realize fields of view (FOVs) of 3.1-15.7mm with imaging depths of 4.3-10.5mm and an axial resolution of 12.6mm in tissue. We imaged anterior segment and retina mimicking phantom eyes, as well as ex vivo porcine eyes. All data was processed and rendered live.

Results : Using the same instrument, we acquired full eye scans, anterior and posterior segment B-scans, as well as 4D-OCT scans with volume rates of up to 17vol/s. Fig.1 shows an ocular biometry scan of a test eye captured at an A-scan rate of 100kHz. Sampling such enormous depths of 29mm in real-time allows for ocular distance measurements or solid state z-tracking. In Fig.2A-H an image sequence of a 17vol/s live rendered volume series can be seen. It visualizes an incision of a porcine cornea displayed at different viewing angles and zooms. An A-scan rate of 1MHz and axial resolution of 6.3μm resulted in a FOV of 3.1mm. To further enhance depth perception, depth is color-encoded from red (top) to blue (bottom) and the z-direction is indicated by an arrow.

Conclusions : We demonstrated SS-OCTs' potential to address multiple ophthalmic imaging applications with a single device. 4D OCT can be used for enhancing depth perception and visualizing sub-surface structures during surgery. Imaging at lower rates enables full eye OCT for high resolution B-scan imaging and biometry.

This is a 2021 Imaging in the Eye Conference abstract.

 

Ocular biometry scan of a model eye

 

4D volume series showing a corneal incision of a porcine eye