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.

Figures

Figure 1

Schematic of the annular-array transducer with (a) side laser illumination using light guides and (b) coaxial, collimated illumination through a hole drilled along the central axis of the array. In both cases, light diffuses due to scatterers in the medium. When using a coaxial, collimated laser beam, the illuminated area is smaller and off-axis PA signals are reduced.

Figure 2

(a) Picture of the PA system allowing for a coaxial arrangement of the laser beam with the acoustic beam of the annular array. Four mirrors (M1, M2, M3, and M4) were used so that the alignment was maintained during scanning. The whole periscopic assembly was mounted on a 2-axis, linear motorized stage to acquire volumetric HFU and PA data. (b) Detail of the drilled annular-array transducer mounted in a manual, 2-axis stage for precise alignment with the laser beam.

Figure 3

(a) Composite B-mode HFU image of a wire phantom (13 μm diameter) at different depths in water. (b) Composite PA image of a hair phantom (80 μm diameter) at the same depths in water. HFU and PA data were processed using a SF algorithm to focus the beam at all depths. Each figure is displayed with 35 dB of dynamic range.

Figure 4

Lateral resolution versus depth for the imaging system in HFU and PA modes, measured from SF images of the wire and hair phantoms in water, respectively.

Figure 5

SNR versus depth measured from SF images of the HFU wire and PA hair phantoms in water.

Figure 6

Normalized, log-compressed envelope of pulse-echo HFU and PA signals acquired from the wire and hair phantoms in water, respectively. The phantoms were aligned with the central axis of the transducer and were positioned at the geometric focus (12 mm).

Figure 7

(a) HFU image of the mid-sagittal plane of the embryonic mouse head with a dynamic range of 45 dB. (b) Co-registered PA image displayed with a dynamic range of 20 dB. (c) Overlaid HFU and PA images that illustrate co-registration. (3V and 4V = third and fourth cerebral ventricles; CP = choroidal plexus; BA = basilar artery; VA = vertebral artery; SGS = superior sagittal sinus; AQ = aqueduct.)

Figure 8

(a) Volumetric PA (color) image of E12.5 embryo overlaid on the HFU (gray scale) image. HFU depicted anatomical features such as the head, eye, tail, and limb. (b) Rotated transparency-adjusted PA image visualized major blood vessels and the vascular plexus. (SGS = superior sagittal sinus; RV = rostral vein; LBV = limb blood vessel.)