## 2 Materials and Methods
### Experimental Setup  
A Q-switched Nd: YAG laser (Changchun Industries Optoelectronics Tech. Co., Changchun, Jilin Province, China) with 10 Hz, 8.5 ns pulse width at 532 nm wavelength has been used for illumination purposes. For full-field illumination, 40, 5-mm diameter poly-methyl methacrylate (PMMA) (epef-10, Ever Heng Optical Co., Shenzhen, China) optical fibers were bundled into a custom-made aluminum fiber bundle holder and aligned to the laser beam using 1\({}^{\prime\prime}\) round top-hat 20\({}^{\circ}\) diffuser (EDI-C20-MD, Thorlabs, Newton, NJ, USA). The overhead illumination was done using a single fiber with a 12-mm diameter; the bundle was removed and a single optical fiber was coupled to the laser. The integral part of the system is the 3D-printed hemisphere cap to hold the distal end of the optical fibers and transducer-amplifier units. Based on the simulation results, we developed this hemispherical dome-shaped cap (Fusion 360, v2.0.5688-Year: 2019, San Rafael, CA, USA) with a total of 90 equidistant cylindrically hollow spaces (Figure 1b): 50 for the transducers and 40 for optical fibers. The experimental setup is shown in Figure 2a. The cap has a diameter of 12 cm and is made of polyactic acid (PLA). The cap has three holes at the base to hold it in upright position using three horizontal optical rods. These rods are connected to three vertical rods which are fixed to the optical table. Out of 90 hollow spaces, 40 equally spaced smaller holes around the hemisphere are dedicated to fit in the distal end of the 5 mm PMMA optical fibers (length: 1.75 m, NA: 0.70) for homogeneous illumination (see Figure 2a(ii)). These holes were left empty when the single fiber (length: 1 m, NA: 0.35) overhead illumination was used, where the fiber was held in place directly above the center of the hemisphere (see Figure 2a(iii)). The other 50 equidistantly located hollow spaces (diameter: 16 mm) throughout the cap are dedicated to hold 5 MHz single element ultrasound transducers (ISL-0504-GP, Technisonic Research Inc., Fairfield, CT, USA). These transducers are directional and would only provide decent imaging at the center due to the acoustic field overlap they create, as compared to that of focused transducers which is less homogenized but covers a larger field of view. The transducers were connected to low noise 24 dB amplifiers (ZFL-500 LN, Mini Circuits RF/Microwave Components, Brooklyn, NY, USA) to amplify the raw signals. SMA cables were used to establish a connection between transducer-amplifier units and the data acquisition (DAQ) system (NI PXIe-1078, National Instruments, Austin, TX, USA). The NI system contains seven eight-channel 14-bit data acquisition cards (NI PXIe-5170R). All 50 transducer-amplifier units were sampled simultaneously with the sampling frequency of 50 MHz. Two-hundred frames of data were collected and averaged for SNR improvement at each transducer location with a total acquisition time of 20s, considering that the laser repetition rate was 10 Hz. The trigger management for the laser and DAQ unit synchronization was done in Labview. System specification details are provided in Table 2.  
Figure 2: **(a)** Experimental setup, (i) measurement of saran wrap thickness using OCT imaging system, (ii) full-field illumination, and (iii) overhead illumination; (**b**) two imaging targets used in experimental configuration: photograph of (i) triangle phantom, (ii) four-leg phantom, (iii,iv) simulation results of triangle and four-leg phantoms, (v,vi) full-field illumination reconstruction results, and (vii,viii) overhead illumination reconstruction results; and (**c**) relative values of contrast to noise ratio of the reconstructed images of the triangle (blue) and four-leg (red) phantoms. To prepare the phantom, the hemisphere was held in place with the imaging target held by a 90\({}^{\circ}\) rod holder (RA90, ThorLabs, Newton, NJ, USA) inside the space of the hemisphere.