Optical‐resolution photoacoustic microscopy with ultrafast dual‐wavelength excitation

Abstract

Fast functional and molecular photoacoustic microscopy requires pulsed laser excitations at multiple wavelengths with enough pulse energy and short wavelength-switching time. Recent development of stimulated Raman scattering in optical fiber offers a low-cost laser source for multiwavelength photoacoustic imaging. In this approach, long fibers temporally separate different wavelengths via optical delay. The time delay between adjacent wavelengths may eventually limits the highest A-line rate. In addition, a long-time delay in fiber may limit the highest pulse energy, leading to poor image quality. In order to achieve high pulse energy and ultrafast dual-wavelength excitation, we present opticalresolution photoacoustic microscopy with ultrafast dual-wavelength excitation and a signal separation method. The signal separation method is validated in numerical simulation and phantom experiments. We show that when two photoacoustic signals are partially overlapped with a 50-ns delay, they can be recovered with 98% accuracy. We apply this ultrafast dual-wavelength excitation technique to in vivo OR-PAM. Results demonstrate that A-lines at two wavelengths can be successfully separated, and sO 2 values can be reliably computed from the separated data. The ultrafast dual-wavelength excitation enables fast functional photoacoustic microscopy with negligible misalignment among different wavelengths and high pulse energy, which is important for in vivo imaging of microvascular dynamics. K E Y W O R D Sfast dual-wavelength excitation, functional photoacoustic imaging, multiwavelength, signal separation

Five-wavelength optical-resolution photoacoustic microscopy of blood and lymphatic vessels

“…For example, a deconvolution method has been developed to separate two overlapping signals generated from ultrafast dual-wavelength excitation. 47 In the OR-PAM probe, the laser beam from the 2-m fiber is focused by a pair of achromatic doublets (AC064-013-A, Thorlabs Inc.). The focused optical beam is reflected on an optical/acoustic beam combiner, is transmitted through a planoconcave lens (45-697, Edmund Optics), then illuminates the sample.…”

Quickly Alternating Green and Red Laser Source for Real-time Multispectral Photoacoustic Microscopy

“…Conventional multi-wavelength switchable SRS laser sources with a wavelength of 532, 545, and 558 (green to yellow colors) have been used to spectrally limited applications such as measuring oxygen saturation in hemoglobin, the concentration of hemoglobin, and cerebral blood flow [ [24] , [25] , [26] , 30 ]. In addition, these conventional laser sources have also been used in broader spectral applications with Prussian blue and gold nanoparticles that exhibit the highest absorption peaks beyond 600 nm.…”

“…Recently, new methods have been described to switch the excitation wavelengths of the SRS laser source by using a delay line [ 24 ], polarization modulation [ 6 , 25 ], and frequency-domain approaches [ 26 ]. In the delay-line method, light with multiple wavelengths of 532, 545, and 558 nm at PRR of 1 MHz was obtained by passively switching the wavelengths using an optical delay line composed of long optical fibers [ 24 ].…”

“…The polarization modulation method uses an EOM for actively switching the unconverted 532 nm beam and the (2 nd Stokes order) 558 nm Raman beam at a PRR of 300 kHz [ 25 ]. The frequency-domain separation method was used to recover the partially overlapped photoacoustic signals when the multi-wavelength excitation time was shorter than the A-line duration [ 26 ]. These multi-wavelength switchable SRS laser studies demonstrated limited wavelength range from 532 to 560 nm (green to yellow colors), which is suitable for vascular morphology, detection of oxygen saturation, blood flow, and oxygen metabolism because oxyhemoglobin and deoxyhemoglobin have strong absorbance in this limited spectral region.…”

Optical coherence tomography angiography for mapping cerebral microvasculature based on normalized differentiation analysis

“…Incorporating the analysis of time series OCT signals, optical coherence tomography angiography (OCTA) has been widely used for microvascular imaging and quantification of blood flow [1,2], including cerebral imaging [3][4][5][6], retinal imaging [7][8][9][10], skin imaging [11][12][13][14], and so forth. In vivo vascular imaging usually needs high imaging speed and short processing time to obtain dynamic physiological information [15]. In cerebral imaging, the OCTA can detect the dynamics of blood flow during localized ischemia [3,6] and measure the changes of blood flow before and after drug administration, light and electric stimulations [16,17].…”