The human brain accounts for only 2% of total body weight, yet consumes 20% of the body’s total oxygen. Neuronal function relies on a continuous supply of oxygen, and the brain remains in a state of high oxygen consumption even at rest. A few minutes of oxygen deprivation can cause irreversible neuronal damage. Therefore, imaging whole-brain vascular morphology and changes in blood oxygen saturation (sO 2 ) with high spatiotemporal resolution can reflect the physiological status and pathological alterations of the brain, which is of paramount importance for medical research and neuroscience. As a hybrid optical-acoustic imaging technology, photoacoustic computed tomography (PACT) combines high optical contrast with large penetration depth in biological tissues, offering unique advantages for whole-brain vascular structure and blood oxygen functional imaging. However, the performance of conventional PACT systems is still constrained by the intrinsic limitations of piezoelectric ultrasonic transducers, making it difficult to meet the challenge of high-resolution imaging in deep whole-brain tissues.
In a new paper published in Light: Science & Applications , a team of scientists, led by Professor Bai-Ou Guan from Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Institute of Photonics Technology, Jinan University, and co-workers have proposed a novel sheet-like focusing dual-frequency fiber ultrasound transducer (FUT). By harnessing the tight mechanical coupling between a quartz optical fiber and a polymer coating, this fiber transducer redistributes vibration energy into high- and low-frequency bands, enabling a single element to simultaneously capture dual-frequency photoacoustic signals. This design achieves deep penetration, high resolution, and precise functional imaging at the entire brain scale without requiring multiple transducers operating at different frequencies.
The core of this technology is the dual-frequency sheet-focused FUT. By exploiting the mechanical coupling between the bare silica fiber and the polymer coating to redistribute the ultrasonic oscillation energy, part of the energy from the intrinsic high-frequency transverse resonant modes of the fiber is transferred to the low-frequency range. Thus, a single transducer simultaneously detects low-frequency signals for deep tissue and high-frequency signals for fine vascular structures. “Remarkably, the energy distribution between the two bands can be freely tailored by adjusting the length ratio of the polymer coating to the bare fiber, allowing the dual-frequency response to be customized for different applications scenarios,” Prof. Guan supplemented.
Another key innovation is the lens-less sheet-like focusing design. By leveraging the high axial flexibility of optical fibers, both the silica fiber and the polymer coating are uniformly bent into an arc with a curvature radius of 4 cm, naturally creating a sheet-like ultrasound focus without any additional acoustic lenses. “This design brings three key advantages, an ultra-low detection limit of ~5.2 Pa at a 4 cm working distance for capturing weak deep signals, a thin focus layer with a slice thickness of ~400 µm that effectively suppresses out-of-plane interference, and perfect dual-frequency confocal alignment eliminating complex signal alignment and image co-registration,” Prof. Ma said.
Looking ahead, this dual-frequency FUT array-based PACT technology overcomes the fundamental limitations of conventional piezoelectric ultrasound transducers and opens new avenues for whole-brain vascular imaging and metabolism assessment. “We expect that further increasing the number of FUT array elements will eliminate mechanical scanning and enable real-time imaging,” the researchers forecast. Such progress could accelerate the clinical translation of PACT for brain disease diagnosis and neuroscience research.
Light: Science & Applications
Dual-frequency fiber-array photoacoustic computed tomography for high-resolution deep brain imaging