Abstract
The past two decades have seen tremendous advancements in photoacoustic tomography (PAT). 1 PAT is a hybrid technology that combines rich optical contrast with ultrasonic resolution, enabling high-resolution imaging deep into soft biological tissue. PAT's contrast mechanism comes from optical absorption of excitation light by either endogenous or exogenous chromophores, thereby generating heat and thermoelastic expansion. Subsequently, images in PAT are formed by detecting ultrasonic waves generated by thermoelastic expansion. Many flavors of PAT have been developed, spanning from subcellular-resolution imaging of the superficial tissue layer to submillimeter-resolution imaging several centimeters deep. In addition, PAT can be used to image numerous functional and structural tissue parameters, such as hemoglobin concentration and oxygen saturation, microvascular structure, temperature, blood flow, metabolic rate of oxygen, melanin, cell nuclei, lipids, water content, and various exogenous contrast agents used for molecular imaging. 2
The capabilities of PAT strongly motivate its implementation in preclinical brain studies. However, until recently, the adoption of PAT by neuroscience labs was relatively slow, in part due to technical complexity (absence of commercially available PAT systems) and cost. Although PAT neuroscience applications are rapidly increasing, PAT brain studies are still performed only on anesthetized rats and mice. In the meantime, other optical brain imaging technologies are already making significant strides toward awake-animal imaging, including both imaging under head-restrained conditions and imaging in ‘freely-moving’ behaving animals. 3 In particular, imaging in behaving animals removes the concerns about the anesthesia effects and enables animal interactions with the environment that are incompatible with head-restraint conditions.
In the current issue of JCBFM, Tang et al 4 present a significant advancement of PAT by developing a wearable PAT array system (wPAT), which enables PAT imaging of cerebral hemoglobin concentration and oxygen saturation over a large cortical area in awake, behaving rats. When designing their wPAT, Tang et al efficiently used some of the major strengths of PAT, such as the ability to image deep into brain tissue through an intact skull and scalp, to acquire images at multiple frames per second using ultrasonic transducer arrays, and to image hemoglobin concentration and oxygen saturation with high signal-to-noise ratio. The key component of the wPAT setup is a lightweight head probe, which includes a simplistic and efficient custom-built ultrasonic transducer array. The potential of the novel imaging system was demonstrated on imaging the hemoglobin concentration and oxygen saturation changes during generalized seizures induced by pentylenetetrazol injection. The observed difference in amplitude and timing of the hemodynamic response to seizures between awake behaving and anesthetized rats underlies an unresolved concern regarding the confounding effects of anesthesia on neurovascular coupling. The experiment also highlights the potential of wPAT to provide critically important information about brain physiology that is currently not accessible by other imaging modalities. With further improvements in spatial resolution and acquisition speed, and with inclusion of the arsenal of other functional and structural parameters accessible by PAT, wPAT shows considerable promise for future preclinical brain investigations.
DISCLOSURE/CONFLICT OF INTEREST
The author declares no conflict of interest.