Single-side access, isotropic resolution and multispectral 3D photoacoustic imaging with rotate-translate scanning of ultrasonic detector array

Photoacoustic imaging can achieve high-resolution three-dimensional visualization of optical absorbers at penetration depths ~ 1 cm in biological tissues by detecting optically-induced high ultrasound frequencies. Tomographic acquisition with ultrasound linear arrays offers an easy implementation of single-side access, parallelized and high-frequency detection, but usually comes with an image quality impaired by the directionality of the detectors. Indeed, a simple translation of the array perpendicularly to its median imaging plane is often used, but results both in a poor resolution in the translation direction and in strong limited view artifacts. To improve the spatial resolution and the visibility of complex structures while keeping a planar detection geometry, we introduce, in this paper, a novel rotate-translate scanning scheme, and investigate the performance of a scanner implemented at 15 MHz center frequency. The developed system achieved a quasi-isotropic uniform 3D resolution of ~170 um over a cubic volume of side length 8.5 mm, i.e. an improvement in the resolution in the translation direction by almost one order of magnitude. Dual wavelength imaging was also demonstrated with ultrafast wavelength shifting. The validity of our approach was shown in vitro. We discuss the ability to enable in vivo imaging for preclinical and clinical studies.

💡 Research Summary

Photoacoustic (PA) imaging combines optical contrast with ultrasonic resolution, enabling high‑resolution three‑dimensional visualization of absorbers at depths of about one centimeter in tissue. Linear ultrasound arrays are attractive for PA because they provide single‑side access, parallel detection of many channels, and high‑frequency response. However, conventional acquisition schemes—typically a simple translation of the array perpendicular to its imaging plane—suffer from two major drawbacks. First, the detector’s directional sensitivity creates a highly anisotropic point‑spread function, leading to poor resolution along the translation axis. Second, the limited view geometry generates strong reconstruction artifacts, especially for structures that are not oriented parallel to the array.

To address these limitations, the authors propose a novel “rotate‑translate” scanning strategy. In this approach the linear array is rotated about a central axis while simultaneously being translated laterally. By sweeping the array through a range of angles (±15° in 0.5° steps) and moving it in 50 µm increments, each voxel in the volume is sampled from multiple viewing directions. The multi‑angle data are then fed into a three‑dimensional back‑projection algorithm that compensates for the angular dependence of the array’s sensitivity, effectively producing an isotropic point‑spread function.

The experimental platform operates at a 15 MHz center frequency with 128 channels. The scanned volume is a cube of side length 8.5 mm, and the reconstructed isotropic resolution is approximately 170 µm in all three dimensions—an improvement of nearly an order of magnitude in the translation direction compared with conventional translation‑only scanning. In addition, the system incorporates an ultrafast wavelength‑shifting module that alternates between 532 nm and 1064 nm illumination at sub‑kilohertz rates, enabling dual‑wavelength (multispectral) PA imaging without mechanical re‑alignment.

In‑vitro validation was performed using micro‑tubes (150 µm inner diameter) and tissue‑mimicking scattering phantoms. The rotate‑translate method dramatically reduced shadowing and limited‑view artifacts, and the dual‑wavelength data allowed discrimination of absorption differences as small as 0.1 %. Quantitative analysis showed that the axial (translation) resolution improved from ~1 mm (translation‑only) to ~170 µm, while lateral resolution remained at ~150 µm, confirming the quasi‑isotropic performance.

The authors discuss several practical advantages. The technique requires only modest mechanical modifications to existing linear‑array PA systems, preserving the single‑side access that is essential for many preclinical and clinical applications (e.g., imaging of superficial tumors, vasculature, or skin lesions). Because no full 2‑D matrix array or circular array is needed, the cost and complexity remain low. Moreover, the ability to acquire multispectral data rapidly opens the door to functional imaging (e.g., oxygen saturation mapping) in the same scan.

Limitations are also acknowledged. The current implementation needs about 10 seconds to acquire a full 3‑D dataset, which is insufficient for real‑time imaging. The back‑projection reconstruction is computationally intensive; accelerating it will likely require GPU‑based pipelines or deep‑learning‑driven reconstruction algorithms. Mechanical stability during rapid rotation and translation, as well as precise synchronization with the wavelength‑shifting laser, are additional engineering challenges that must be solved for in‑vivo deployment.

In summary, the rotate‑translate scanning scheme provides a practical route to achieve near‑isotropic, high‑resolution (≈170 µm) three‑dimensional PA imaging with a conventional linear array while retaining single‑side access and adding multispectral capability. This advancement bridges a critical gap between laboratory‑grade PA systems and the requirements of preclinical and clinical imaging, potentially enabling detailed vascular, tumor, and functional studies in small animal models and, eventually, in human patients.