Photoacoustic and Ultrasound Imaging with Nanosized Contrast Agents - Nanotechnology for Biomedical Imaging and Diagnostics - page 288

The produced PA waves easily travel in biological tissues, which are much more
transparent for US than light (by ~10 3 times). Furthermore, because the sound
velocity in tissues (~1480 m/s) is much slower than that of light (3 × 10 8 m/s), the
time-dependent PA signals can be easily detected by conventional US detectors. The
temporal information based on the arrival times of the PA waves is directly associ-
ated with the spatial distribution of the objects in the tissue. There are two ways to
localize the PA sources: hardware and software focusing. In hardware focusing, a
focusing acoustic lens is typically used, whereas an inverse algorithm is used in
software focusing.
The combination of these intrinsic properties brings several advantages: (i) The
maximum penetration depth can be extended beyond the one optical transport
mean free path and reach up to approximately 8 cm in tissues [36]. (ii) The spatial
resolution and penetration depth are scalable according to US parameters and
optical wavelengths. More importantly, the depth-to-resolution ratio of PAT remains
constant (~200) in all imaging depths [23, 27]. (iii) By tuning optical wavelengths,
PAT can probe different chemicals [26, 37]. (iv) Unlike optical coherence tomog-
raphy and US imaging, no speckle artifacts are visible in PA images [38]. (v) Existing
clinical US imaging systems can be adapted to provide dual-modality US and PA
imaging capabilities [36, 39-41]. (vi) PAT is completely free of harmful ionizing
radiation. Thus, the clinical translation potential of PAT is very high. Based on these
features, PAT can noninvasively supply structural (i.e., vascular structures [42], solid
tumors and angiogenesis [43-49], and internal organs [50, 51]), functional (i.e., total
hemoglobin concentration, oxygen saturation of hemoglobin [24, 25], blood flow in
microvasculature [52-54], oxygen partial pressure [55], pH [56-58], and metabolic
rate of oxygen consumption [59, 60]), and molecular (i.e., probing disease-specific
molecules at a molecular level using biomarkers and exogenous contrast agents)
information of tissues [26].

10.3 Pat modalities

PA imaging systems can be categorized into two types [23, 30]: (1) inverse algo-
rithm-based photoacoustic computed tomography (PACT) [25] and (2) point-by-
point detection-based photoacoustic microscopy (PAM) [50, 61, 62]. Both types can
be implemented in an endoscopic probe [63-68]. The first type, PACT, utilizes
array-based US detection systems for parallel detection and inverse algorithms to
reconstruct images in real time. As the first attempt, a circular scanning-based PACT
system using a single-element unfocused US transducer was developed for in vivo
PA imaging (Fig. 10.2a) [25, 69]. Although the system was successfully used for
providing anatomical [43], functional [25, 69], and molecular [37] information of
brains in small animals in vivo , it took 15 min to acquire one two-dimensional (2d)
cross-sectional image because of the mechanical scanning of the transducer. Later,
US array systems were adapted to enhance the image acquisition speed. To mimic
360° full circular scanning with a single-element transducer, a full-ring ultrasonic
transducer (512 elements) array was developed, and one 2d image acquisition speed