# A Novel Dictionary-Based Image Reconstruction for Photoacoustic Computed Tomography
## 3 Results  
To evaluate the performance of the reconstruction algorithms, we made a gel phantom with imaging target inside. The phantom was made by 3% Agar powder in water. Eight straight graphite pencil leads with a 0.5 mm diameter were placed co-centered in the same plane embedded in the gel. The size of the phantom was 30 \(\times\) 30 mm (Figure 2b). The phantom was imaged using a single-transducer PACT system (Figure 2a). A Quanta-Ray PRO-Series Nd:YAG laser (Spectra-Physics Inc., Santa Clara, CA, USA) pumped a VersaScan V1.7 optical parametric oscillator (OPO) (Spectra-Physics Inc., USA), generating wavelengths in the range of 398-708 nm, with a pulse width of 7 ns and a repetition rate of 30 Hz. In this experiment, the illumination wavelength was 532 nm. A large graded index plastic optical fiber with a diameter of 10 mm and a numerical aperture of 0.55 was used on the top of the ring, 5 cm away from the sample, forming an uniform illumination. A cylindrically focused ultrasound transducer, V326-SU (Olympus Inc., Center Valley, PA, USA) with the element size of 0.375 inch, central frequency of 5 MHz and the focal length of 0.625 inch was positioned on a cylindrical construct. The position of the transducer was in the same plane as the pencil leads were. The diameter of the construct was 75 mm. The cylinder was rotated using a stepper motor (Applied Motion Products Inc., Watsonville, CA, USA) with a driver controlled by LabVIEW. The transducer was smoothly rotated around the phantom with the speed of 0.0125 round/s to collect the PA signal in 360 degrees. The data acquisition is performed using our FPGA based National Instrument (NI) system. Specification of the experiment is presented in Table 1. A photograph of the PAI system is shown in Figure 3. The system was placed on a 12 ft \(\times\) 4 ft optical table (New Port, Inc., Irvine, CA, USA). Figure 2: (**a**) Schematic of our PACT system; (**b**) gel phantom with eight 0.5 mm pencil leads. For image reconstruction, we used a laptop with i7 core, 2.3 GHz CPU, and 8 GB Memory. Figure 4 demonstrates a set of results of different reconstruction algorithms on the phantom data acquired by our PACT system. They are C-scan images with their dimensions annotated in the right image. The reconstructed images with BP and CS with different sparsifying methods for different numbers of views (30, 60, 90, and 120) have been presented. As seen in the first column, BP generates images with a high level of artifacts and imaging noise. Even though increasing the number of angles improves the image quality with lower imaging noise and reconstruction artifacts, the images are still affected. The CS-based WT removes a considerable amount of artifacts presented in the BP image (initial image). However, it blurs the edges of the reconstructed image. Even for a low number of angles, the real structure of the imaging target is compromised. It can be seen that adding the TV sparsifying method to the WT sharpens the edges, especially for 90 and 120 angles. While improvement is obtained, it removes the artifacts in the image along with useful information, lowering the accuracy of the reconstructed image. Finally, the proposed method removes the artifacts effectively and more accurately by sharpening the edges while retaining the significant information in the image. It should be noted that, in all the reconstruction implementations, investigated in this study, the media is assumed \begin{table}
\begin{tabular}{c c} \hline Transducer & 5 MHz \\ Laser energy & 20 mJ/cm\({}^{2}\) \\ Laser pulse width & 7 ns \\ Laser rep-rate & 30 Hz \\ Wavelength & 532 nm \\ Amplifier & ZFL500 LN \\ DAQ & National instrument \\ \hline \end{tabular}
\end{table}