# Demonstration of Subretinal Injection Using Common-Path Swept Source OCT Guided Microinjector
## 2 Materials and Methods
### Micrinjector System Configuration  
The SMART injection system consists of three main modules: (1) fiberoptic OCT probe for distal sensing, (2) microinjector and pump for fluid delivery, and (3) workstation for computing and control. The OCT probe consists of a swept source OEM engine (AXSUN, Billerica MA USA, central wavelength \(\uplambda\)0: 1060 nm, sweeping rate: 100 kHz, 3 dB axial resolution: 8 \(\upmu\)m, scan range: 3.7 mm in air), a balanced photodetector and a digitizer with a sampling rate of up to 500 MSPSwith 12-bit resolution, a Camera Link DAQ Board, and a Camera Link frame grabber (PCIe-1433, National Instruments, Austin, TX USA). In contrast to typical 2D imaging OCT setup, the OCT setup for distal sensing does not require the scanning module, which has to be synchronized with OCT module including swept source engine and digitizer. Thus, the distal-sensing setup is simpler than the imaging setup. Additionally, the common-path fiberoptic OCT probe is advantageous for the system to be compact in that the system does not require a reference arm since it uses the reflection beam from the interface between fiber and outside medium as a reference beam. Additionally, both reference and sample reflected light share the same beam path; as a result, CP-OCT is naturally immune to dispersion and polarization noises [15; 16]. The design of the microinjector, as shown in Figure 2b, involves a 34 G needle (Hamilton, Reno NV USA, OD: 0.1842 mm, ID: 0.0826 mm) inside a 26 G guide needle (Hamilton, OD: 0.4636 mm, ID: 0.260 mm), a needle-to-tube connector (RN Coupler-p/n 55752-01, Hamilton), linear piezoelectric motors (LEGS-LL1011A, PiezoMotor, Uppsala Sweden), a fiberoptic probe, and a handpiece of the injector. A prototype was 3D printed using acrylonitrile butadiene styrene (ABS) thermoplastic. The pump module is composed of a 100 \(\upmu\)L capacity syringe (Hamilton) with a 1.46 mm inner barrel diameter and 0.60 mm forward push, which corresponds to 1 \(\upmu\)L volume injection. A 1 \(\upmu\)m-precision linear actuator (850 G Actuator, Newport, Irvine CA USA) was used to accurately drive the syringe. To connect the injection needle remotely from the syringe, we used a 1 m length of 1/16-inch PEEK tube (Tub PEEK Blu, IDEX, Lake Forest, IL USA). The sampled OCT spectral data for the distal sensing is sequentially processed in the workstation following the three main steps as shown in Figure 3. The detail of the signal processing is precisely described in our previous work [24]. The first step of the processing is to transform the spectral data to A-scan data. As the sweeping rate is 100 kHz, the OCT engine returns 128 buffered spectral data every 1.28 ms. These oversampled data are useful to increase the signal-to-noise ratio because the temporally and spatially averaged data within a restricted window based on target size and speed effectively remove speckle noise, which is the one of the main noise factors in A-scan. However, the increased processing time due to the oversampled data has serious adverse effects on intraoperative guidance capabilities. Parallel processing using a GPU significantly reduces the processing time. In detail, 128 consecutive spectra consisting of 1376 data points transmitted from the frame grabber to the GPU in the workstation are passed to a high-pass filter in order to remove the high DC component in the spectrum data. Then, low-pass filtering is consecutively applied to remove the high-frequency noise components. Next, zero padding, FFT, averaging, and background subtraction are conducted on the A-scan data. This produces an A-scan with 2048 data points that covers a 3686.4 \(\upmu\)m axial distance from the end of the fiber probe. Figure 1: Free hand insertion test performed by a retinal surgeon: (**a**) Needle tip position tracking data during the insertion trial, (**b**) a magnified area in red ellipse of graph (**a**). The two red dotted lines mark the range of insertion depth variation during the microinjection.