For example, it is common to add pulsers for digital beamforming and switches for rapid switching between the transmission and receiving events, and these electronics generally need to be one-to-one-mapped to the transducer elements to achieve a sufficiently high FR for fUS. Moreover, equipping a PAT system with fUS capability requires considerable hardware modification. Nevertheless, the relatively large element size and pitch of these USTAs hamper the implementation of fUS: the highly diffractive sidelobes prevent proper beam steering.

To suppress the limited-view problem, USTAs may adopt 2D ring-shaped or 3D bowl-shaped configurations tailored for maximum 2D and 3D detection coverages, respectively. In PAT, full- or wide-angular-view ultrasonic transducer arrays (USTAs) provide better imaging quality than linear or planar arrays. Also, images of acoustically smooth blood vessel boundaries suppress interior speckles. PAT's widely used panoramic detection scheme provides high sensitivity to blood vessels along the cortical surface over a large FOV. Ĭomplementing fUS with molecular specificity, photoacoustic (PA) tomography (PAT) uses thermo-elastically induced acoustic waves to measure the optical absorption of endogenous hemoglobin (Hb). For brain imaging, the standard protocol requires the ultrasonic transducer probe to be in direct contact with the head, so blood vessels distributed along the cortical surface are not well resolved. Despite its advantages, fUS inherently lacks molecular specificity and suffers decreased sensitivity to blood flowing perpendicularly to the acoustic axis. Ultrafast ultrasound imaging capitalizes on successive tilted plane wave transmissions, which are coherently compounded to form images at a few hundred to several thousand hertz, enabling functional ultrasound imaging (fUS). This scanning method limits the FOV and produces aliasing artifacts in measurements of high-velocity blood flow. Conventional Doppler ultrasound imaging scans the region of interest using a focused beam line by line at a frame rate (FR) typically below 60 Hz (corresponding to 5 cm in depth and 256 lines in width). Due to its low cost, flexibility, and ability to provide quantitative measurements at a greater depth, Doppler ultrasound has been extensively used for preclinical and clinical neuroimaging. Additionally, these optical methods have limited imaging depths.

Speckle-based methods, for example, laser speckle contrast imaging and multi-exposure speckle imaging, provide high spatiotemporal resolution with a large FOV, but they require mathematical modeling and calibration for quantitative flow measurements. The first two categories employ point scanning and offer quantitative measurements at the expense of the field of view (FOV). The optical methods fall into three main categories: Doppler-based laser flowmetry, red blood cell (RBC)-tracking-based laser scanning microscopy, and speckle-based imaging. Label-free optical and Doppler ultrasound methods have been widely employed to image CBF. The results demonstrate CRUST-PAT as a unique tool for studying cerebral hemodynamics.Ĭerebral blood flow (CBF) provides the key to unraveling neurovascular coupling in response to neuronal activities. Using CRUST-PAT, the CBF, oxygen saturation, and hemoglobin concentration changes of the mouse brain during sensory stimulation are measured, with a field of view of ≈7 mm in diameter, spatial resolution of ≈170 µm, and temporal resolution of 200 Hz. Superior to conventional fUS, whose sensitivity to blood flow is angle-dependent and low for perpendicular flow, the crossed transmission and panoramic detection fields of CRUST provide omnidirectional sensitivity to CBF. A full-ring-shaped ultrasonic transducer array whose imaging plane is orthogonal to the SFUST's acoustic axis receives scattered ultrasonic waves. CRUST employs a virtual point source from a spherically focused ultrasonic transducer (SFUST) to provide widefield excitation at a 4-kHz pulse repetition frequency. Here, cross-ray ultrasound tomography (CRUST) and its combination with PAT are presented.

Complementing each other, fUS and PAT, respectively, measure the cerebral blood flow (CBF) and hemoglobin concentrations, allowing synergistic characterization of cerebral hemodynamics. Recent advances in functional ultrasound imaging (fUS) and photoacoustic tomography (PAT) offer powerful tools for studying brain function.

Radia imaging