## 1 Introduction  
Photoacoustic (PA) imaging is a fast-developing hybrid imaging modality that combines optical absorption and ultrasound detection techniques to visualize the hemodynamic changes in biological tissues [1; 2; 3; 4; 5; 6; 7]. In contrast to all-optical imaging methods [8; 9; 10], it uses light for illumination and ultrasound for detection. In PA imaging (PAI), a nanosecond pulsed laser deposits energy onto a tissue, causing a local temperature increase of light absorbing molecules and subsequent thermal expansion through the thermoacoustic effect [1; 6; 7; 11]. This thermal expansion causes a localized pressure rise, which propagates from the sample to be imaged by an ultrasound transducer (or an array of ultrasound transducers), similar to traditional ultrasound detection. Finally, depending on the configuration of the PAI system, a two-dimensional (2D) or a three-dimensional (3D) image is reconstructed from the absorption distribution across the tissue [12; 13; 14; 15; 16; 17]. Among different implementations of PAI, photoacoustic computed tomography (PACT) has characteristics that help its clinical translatability. Some of these characteristics are: (i) deep penetration, compared to all-optical imaging methods such as Functional Near-Infrared Spectroscopy (fNIRS), where in both illumination and detection paths, travelling photons are involved, and compared to the use of acoustic detection in PACT, which has orders of magnitude smaller signal attenuation; (ii) high frame rate, compared to magnetic resonance imaging (MRI), PACT can generate faster frame-rate volumetric images. It is worth noting, in the current study, the channel data was averaged many times and worsened the temporal resolution. Ideally, PACTs are developed using high-sensitivity transducers and fast repetition-rate (rep-rate) lasers; the frame rate is defined by the rep-rate of the laser or at least one-tenth of that if 10 iterations of acquired signals are averaged at each channel. The ability of PACT systems to render 3D volumetric images, i.e., 3D-PACT, relies on covering angles to which the acoustic waves are traveling to [18]. Such an imaging configuration will produce adequate channel data for image reconstruction because reconstruction algorithms are developed based on the assumption that the entire imaging target is surrounded by transducers [19; 20]. 3D-PACT has been studied for volumetric mammography, neuroimaging in small animal, and human neonates [7; 20; 21; 22; 23; 24; 25; 26; 27]. As for ultrasound detection implementation, one way to perform 3D imaging is to use a 2D ultrasound matrix array [28]. However, these arrays consist of many ultrasonic transducer elements, significantly increasing the system's complexity and cost [28]. A cost-effective alternative method for 3D imaging is to use a linear array and stack multiple cross-sectional images; this method causes motion artifacts and usually it is not time efficient [29; 30]. Apart from 2D arrays, expensive arc-shaped [31; 32] or circular ring arrays [33; 34; 35] along with some mechanical translations have also been used for 3D-PACT; these acoustic detection configurations have not been optimized for the best temporal resolution and/or spatial resolution. As for light illumination, an overhead illumination using a single large optical fiber or a full-field illumination using a fiber bundle, have been studied [15; 36; 37]. We have previously shown in [38; 39] that full-field illumination provides a more homogenous and deeper illumination than overhead illumination; using full-field illumination, we have access to a larger illumination area, and according to the literature [40], the larger the area of illumination, the more light we can couple to the tissue. This issue is of paramount importance especially in the areas of brain and breast imaging where there is a need for a large amount of fluence inside the tissue, which is to be maintained below the tissue damage threshold.