Assessment of Brain Functional Activity Using a Miniaturized Head-Mounted Scanning Photoacoustic Imaging System in Awake and Freely Moving Rats

Abstract

Understanding the relationship between brain function and natural behavior remains a significant challenge in neuroscience because there are very few convincing imaging/recording tools available for the evaluation of awake and freely moving animals. Here, we employed a miniaturized head-mounted scanning photoacoustic imaging (hmPAI) system to image real-time cortical dynamics. A compact photoacoustic (PA) probe based on four in-house optical fiber pads and a single custom-made 48-MHz focused ultrasound transducer was designed to enable focused dark-field PA imaging, and miniature linear motors were included to enable two-dimensional (2D) scanning. The total dimensions and weight of the proposed hmPAI system are only approximately 50 × 64 × 48 mm and 58.7 g (excluding cables). Our ex vivo phantom experimental tests revealed that a spatial resolution of approximately 0.225 mm could be achieved at a depth of 9 mm. Our in vivo results further revealed that the diameters of cortical vessels draining into the superior sagittal sinus (SSS) could be clearly imaged and continuously observed in both anesthetized rats and awake, freely moving rats. Statistical analysis showed that the full width at half maximum (FWHM) of the PA A-line signals (relative to the blood vessel diameter) was significantly increased in the selected SSS-drained cortical vessels of awake rats (0.58 ± 0.17 mm) compared with those of anesthetized rats (0.31 ± 0.09 mm) (p < 0.01, paired t-test). In addition, the number of pixels in PA B-scan images (relative to the cerebral blood volume (CBV)) was also significantly increased in the selected SSS-drained blood vessels of awake rats (107.66 ± 23.02 pixels) compared with those of anesthetized rats (81.99 ± 21.52 pixels) (p < 0.01, paired t-test). This outcome may result from a more active brain in awake rats than in anesthetized rats, which caused cerebral blood vessels to transport more blood to meet the increased nutrient demand of the tissue, resulting in an obvious increase in blood vessel volume. This hmPAI system was further validated for utility in the brains of awake and freely moving rats, showing that their natural behavior was unimpaired during vascular imaging, thereby providing novel opportunities for studies of behavior, cognition, and preclinical models of brain diseases.

Revisiting the cerebral hemodynamics of awake, freely moving rats with repeated ketamine self-administration using a miniature photoacoustic imaging system

“…Despite the miniaturized size, the proposed hmPAI system provided good spatial resolution (i.e., an axial resolution of 0.225 mm 22 ) and successfully captured rapid cerebral hemodynamic changes (2 lines/s for PA A-line signals) in awake and freely moving rats. Previously, we conducted ex vivo phantom tests and in vivo experiments to demonstrate the capability of the hmPAI system for small animal PA imaging 15 .…”

“…According to optical absorption spectra information found in the literature, the wavelength of 800 nm is sensitive to changes in the CBV 23 , 24 . Furthermore, PA imaging at 800 nm contains various signals that are generated by different types of tissues, such as changes in the CBV 22 , 25 . Therefore, the PA images acquired at 800 nm were used to detect changes in the CBV.…”

“…59 Our previous experimental study reported that our hmPAI system can be successfully used to detect changes in the CBV and blood vessel diameter in rats in anesthetized and awake rat models in both B-scan and C-scan PA imaging modes. 22 In the present study, we compared cerebral hemodynamics in response to saline or ketamine injection in freely moving normal rats and . This finding indicates that ketamine has no obvious effect on the dynamics of cerebral blood vessels in addicted rats, which may be because the addicted rats have adapted to ketamine; thus, the cerebral blood vessels do not undergo any pronounced changes.…”

“…23,24 Furthermore, PA imaging at 800 nm contains various signals that are generated by different types of tissues, such as changes in the CBV. 22,25 Therefore, the PA images acquired at 800 nm were used to detect changes in the CBV. Thus, independent measurements of changes in the CBV can be achieved by the developed wearable scanning PA imaging system with PA imaging at 800 nm being used to monitor changes in the CBV.…”

A photoacoustic patch for three-dimensional imaging of hemoglobin and core temperature

“…Insets show the photoacoustic images of the vein at three different moments. The image pixels with a normalized value >0.5 were counted into the vein area 62 , as labeled by the white boundaries. No pressure was applied in the first 1 min, thus no significant area change was observed.…”