Quadruple ultrasound, photoacoustic, optical coherence, and fluorescence fusion imaging with a transparent ultrasound transducer

Abstract

Ultrasound and optical imagers are used widely in a variety of biological and medical applications. In particular, multimodal implementations combining light and sound have been actively investigated to improve imaging quality. However, the integration of optical sensors with opaque ultrasound transducers suffers from low signal-to-noise ratios, high complexity, and bulky form factors, significantly limiting its applications. Here, we demonstrate a quadruple fusion imaging system using a spherically focused transparent ultrasound transducer that enables seamless integration of ultrasound imaging with photoacoustic imaging, optical coherence tomography, and fluorescence imaging. As a first application, we comprehensively monitored multiparametric responses to chemical and suture injuries in rats' eyes in vivo, such as corneal neovascularization, structural changes, cataracts, and inflammation. As a second application, we successfully performed multimodal imaging of tumors in vivo, visualizing melanomas without using labels and visualizing 4T1 mammary carcinomas using PEGylated gold nanorods. We strongly believe that the seamlessly integrated multimodal system can be used not only in ophthalmology and oncology but also in other healthcare applications with broad impact and interest.

Keywords: multimodal imaging; optical imaging; transparent ultrasound transducer; ultrasound imaging.

Fig. 1.

Fabrication process and assembly of a TUT. (A) The step-by-step fabrication process of a TUT. (B) Schematic illustration of the layer-by-layer TUT and enlarged view explaining each component. The center part of all components is transparent. (C) Photograph demonstrating the transparency of a TUT with an element size of 9 mm.

Fig. 2.

Acoustic and optical properties of a TUT. (A) Measured pulse echo response and frequency spectra, with a spatial pulse length of ∼0.1 μs and dual center frequencies (∼7.5 and ∼31.5 MHz) and bandwidths (∼13 MHz and 8 MHz). (B) Simulated pulse echo waveform and spectrum of the TUT in both the time and frequency domains. (C) Measured electrical impedance (53 Ω) and phase angle (−50°) spectra of the TUT, showing a good thickness-mode electromechanical coupling coefficient (0.68). (D) Light transmittance in the optical range from 200 nm to 900 nm. The yellow window represents a range of over 70% transparency of the TUT. (E) Comparison of the acoustic pressure fields of commercial UTs (20 and 30 MHz), a custom-made ring UT (20 MHz), and the TUT, measured by a hydrophone. All UTs are spherically focused. (F) Summary of SNR, peak-to-peak voltages, and lateral and axial FWHM.

Fig. 3.

Schematic diagrams of a seamlessly integrated quadruple fusion imaging system using a TUT: USI, OCT, and FLI. (A) Overall schematic. (B) Magnified schematic of the imaging head module. PC, personal computer; RF, radio frequency; M, mirror; FM, flipping mirror; ND, neutral density filter; C, collimator; CorL, correction lens.

Fig. 4.

In vivo quadruple fusion imaging of rats’ eyes before and after alkali burns. (A) PA MAP, depth-encoded PA MAP, B-scan, and enlarged B-scan images before (1–3) and after (4–6) alkali burns, and the quantified CNV area (7). (B) OCT MIP and B-scan images before (1 and 2) and after (3 and 4) alkali burns, respectively, and the quantified CCT (5). (C) US MIP and B-scan images before (1 and 2) and after (3 and 4) the alkali burns, respectively. (D) Overlaid FL and photographic images of the inflammation areas before (1) and after (2) the alkali burns, and the quantified inflammation area (3). The rat cornea was stained with fluorescein. (Scale bar = 1 mm.) The error bars in A, 7, B, 5, and D, 3 indicate the SD.

Fig. 5.

In vivo imaging of rat eyes before and after suture injuries, using the modalities of the quadruple fusion system. (A) Depth-encoded PA MAP and B-scan images acquired before (1 and 2) and on day 1 (3 and 4), day 4 (5 and 6), and day 7 (7 and 8) after suture injury, respectively, and the quantified CNV area (9). (B) OCT MIP and B-scan images before (1 and 2) and after (3 and 4) the suture injury, respectively, and the quantified CT (5). (C) US MIP and B-scan images before (1 and 2) and after (3 and 4) the suture injury, respectively. (Scale bar = 1 mm.) CT, corneal thickness.

Fig. 6.

In vivo multimodal imaging of subcutaneous melanomas in a mouse model. (A) PA MAP images before (1) and on day 3 after (2) melanoma injection. The vascular network and spectrally unmixed melanoma are overlaid in 2. PA B-scan image (3) on day 3 after the injection, acquired along the dashed line in A2. Overlaid PA sO₂ and melanoma image (4). Quantification of the melanoma thickness based on the PA B-scan image (5). (B) US MIP and B-scan images before (1 and 2) and after (3 and 4) the melanoma injection. Quantification of the melanoma thickness based on the US B-scan image (5). (C) OCT MIP and B-scan images before (1 and 2) and after (3 and 4) the melanoma injection. (Scale bar = 1 mm.) ED, epidermis; D, dermis; SC, subcutis; and M, muscle.

Fig. 7.

In vivo multimodal imaging of subcutaneous 4T1 breast carcinoma in a mouse model. (A) PA MAP and B-scan images before (1 to 3) and 2-h (4 to 6) and 4-h (7 to 9) after the PEG-GNRs labeling. Quantification of PA signal enhancements in tumor based on the PA MAP images (10). (B) US MIP and B-scan images before (1 and 2) and after (3 and 4) the PEG-GNRs labeling. (C) OCT MIP and B-scan images before (1 and 2) and after (3 and 4) the PEG-GNRs labeling. (Scale bar = 1 mm.) T, thickness.