High-frequency annular array with coaxial illumination for dual-modality ultrasonic and photoacoustic imaging

Abstract

This paper presents a combined ultrasound and photoacoustic (PA) imaging (PAI) system used to obtain high-quality, co-registered images of mouse-embryo anatomy and vasculature. High-frequency ultrasound (HFU, >20 MHz) is utilized to obtain high-resolution anatomical images of small animals while PAI provides high-contrast images of the vascular network. The imaging system is based on a 40 MHz, 5-element, 6 mm aperture annular-array transducer with a 800 μm diameter hole through its central element. The transducer was integrated in a cage-plate assembly allowing for a collimated laser beam to pass through the hole so that the optical and acoustic beams were collinear. The assembly was mounted on a two-axis, motorized stage to enable the simultaneous acquisition of co-registered HFU and PA volumetric data. Data were collected from all five elements in receive and a synthetic-focusing algorithm was applied in post-processing to beamform the data and increase the spatial resolution and depth-of-field (DOF) of the HFU and PA images. Phantom measurements showed that the system could achieve high-resolution images (down to 90 μm for HFU and 150 μm for PAI) and a large DOF of >8 mm. Volume renderings of a mouse embryo showed that the scanner allowed for visualizing morphologically precise anatomy of the entire embryo along with corresponding co-registered vasculature. Major head vessels, such as the superior sagittal sinus or rostral vein, were clearly identified as well as limb bud vasculature. © 2013 AIP Publishing LLC.

Coherence-Weighted Synthetic Focusing Applied to Photoacoustic Imaging Using a High-Frequency Annular-Array Transducer

“…A pulse energy of 0.4 mJ was found to be sufficient to obtain an SNR above 30 dB at the geometric focus. The PA-signal amplitude peaked at 10 mm and then decreased with increasing depth (data not shown here, but previously presented in Filoux et al 9 ). Depth-dependent reduction in transducer sensitivity and loss due to acoustic attenuation contribute to this depth-dependent signal decay.…”

“…A 800-μm diameter hole was precisely drilled along the central axis of the array to accommodate coaxial laser illumination. 9 The central hole reduced the surface area of the central element by 10%, which reduced the sensitivity by 2 dB, and did not alter the transducer-beam characteristics appreciably. 9 The array had a central frequency of 38 MHz and a −6 dB pulse-echo bandwidth of 40%, which were characterized using pulse-echo measurements from a rigid quartz reflector while using a broadband pulser (Panametrics 5900, Panametrics, Waltham, MA) for excitation.…”

“…9 The periscopic assembly consisted of two portions: one that moved with the annular array in 2D (shown in dashed box) and one that moved along one of the scan axes (shown in solid box). This arrangement facilitated delivery of the laser beam through the center hole during mechanical scanning of the annular array.…”

Transversally travelling ultrasound for light guiding deep into scattering media

“…The direct translation of the method requires an axially extended uniform positive instantaneous pressure region and provides a difficult challenge. However, utilizing annular arrays with coaxial opening 48 and US Bessel beams 49 , an on-axis series of two-dimensional gradient refractive index medium lenses might be induced, that are potentially capable of light waveguiding or even relay imaging 27 . We suggest to investigate such arrangements further for possible in vivo applications with single-sided access.…”

Fiber-optic annular detector array for large depth of field photoacoustic macroscopy

“…To suppress these aberrations, arrays of ring-shaped transducers with varying diameters can be used. Annular detector arrays for photoacoustic microscopy were, e.g., presented in [14] , [15] , [16] , [17] . A piezoelectric annular detector array for large DOF photoacoustic imaging was demonstrated by Passler et al [18] .…”