High-Resolution Photoacoustic Imaging of Ocular Tissues

Abstract

Optical coherence tomography (OCT) and ultrasound (US) are methods widely used for diagnostic imaging of the eye. These techniques detect discontinuities in optical refractive index and acoustic impedance respectively. Because these both relate to variations in tissue density or composition, OCT and US images share a qualitatively similar appearance. In photoacoustic imaging (PAI), short light pulses are directed at tissues, pressure is generated due to a rapid energy deposition in the tissue volume, and thermoelastic expansion results in generation of broadband US. PAI thus depicts optical absorption, which is independent of the tissue characteristics imaged by OCT or US. Our aim was to demonstrate the application of PAI in ocular tissues and to do so with lateral resolution comparable to OCT. We developed two PAI assemblies, both of which used single-element US transducers and lasers sharing a common focus. The first assembly had optical and 35-MHz US axes offset by a 30° angle. The second assembly consisted of a 20-MHz ring transducer with a coaxial optics. The laser emitted 5-ns pulses at either 532-nm or 1064-nm, with spot sizes at the focus of 35-μm for the angled probe and 20-μm for the coaxial probe. We compared lateral resolution by scanning 12.5-μm diameter wire targets with pulse/echo US and PAI at each wavelength. We then imaged the anterior segment in whole ex vivo pig eyes and the choroid and ciliary body region in sectioned eyes. PAI data obtained at 1064 nm in the near infrared had higher penetration but reduced signal amplitude compared to that obtained using the 532-nm green wavelength. Images were obtained of the iris, choroid and ciliary processes. The zonules and anterior cornea and lens surfaces were seen at 532 nm. Because the laser spot size was significantly smaller than the US beamwidth at the focus, PAI images had superior resolution than those obtained using conventional US.

Assessing the mechanical properties of tissue-mimicking phantoms at different depths as an approach to measure biomechanical gradient of crystalline lens

“…In our feasibility study, the ultrasound transducer and the OCT imaging lens are separated. An integrated probe with OCT beam located inside the ultrasound transducer has been proposed and utilized in photoacoustic imaging techniques [35]. This configuration is an ideal fit for the proposed method and will be applied for our future in situ experiments on the crystalline lens.…”

Novel Retinal Imaging Technologies

“…We acquired 3D images of the posterior pole (retina, choroid, optic nerve) of the rabbit eye by ultrasound and photoacoustic imaging [18][19][20]. Silverman et al, [21] imaged fresh ex vivo pig eyes by PAI and US. They demonstrated choroidal/scleral images, and the PAI image shows sharp depiction of the choroidal surface but with limited penetration due to the strong absorption of the green laser by melanin.…”

New Frontiers in Retinal Imaging

“…PA can be a potential tool for noninvasive and sensitive imaging of retinal and choroidal structures [105] . Apart from the photoacoustic tomography (PAT) used to visualize the entire eye [106,107] , photoacoustic ophthalmoscopy (PAOM) is emerging as a promising tool. With the guidance of OCT, PAOM has been reported to be capable of image the retinal vessels [108,109,110] , RPE cells [109,110] , and choroidal vessels [108,110] in rodent animals with a axial resolution of around 20 μm [111,112] and a higher resolution that can reach 4 μm [112,113] .…”