Noninvasive quantification of metabolic rate of oxygen (MRO_2) by photoacoustic microscopy

Many diseases, normal decay and physiological functions are closely related to alterations in the metabolic rate of oxygen (MRO_2). In this study, we demonstrate that all the parameters for MRO_2 quantification can be simultaneously obtained by optical-resolution photoacoustic microscopy (OR-PAM). MRO_2 of the mouse ear under normothermia (31 °C skin temperature) and controlled systematic hyperthermia (42 °C skin temperature) was studied. As a result of hyperthermia, the MRO_2 increased by 34.1%. The tumor hypermetabolism was also demonstrated by longitudinally monitoring a melanoma growing on a mouse ear. The results show that OR-PAM, as a single noninvasive imaging modality, is well suited for quantitative MRO_2 measurement in microenvironments

In vivo noninvasive monitoring of microhemodynamics using optical-resolution photoacoustic microscopy

Microvascular autoregulation is an intrinsic ability of vascular beds to compensate for the fluctuation in blood flow and tissue oxygen delivery. This function is crucial to maintaining the local metabolic activity. Here, using optical-resolution photoacoustic microscopy (OR-PAM), we clearly observed vasomotion and vasodilation in the intact mouse microcirculation in vivo in response to the changes in physiological state. Our results show that a significant lowfrequency vasomotion can be seen under hyperoxia but not hypoxia. Moreover, significant vasodilation is observed when the animal status is switched from hyperoxia to hypoxia. Our data show that arterioles have more pronounced vasodilation than venules

Reflection-mode multifocal optical-resolution photoacoustic microscopy

Compared with single-focus optical-resolution photoacoustic microscopy (OR-PAM), multifocal OR-PAM utilizes both multifocal optical illumination and an ultrasonic array transducer, significantly increasing the imaging speed. A reflection-mode multifocal OR-PAM system based on a microlens array that provides multiple foci as well as an ultrasonic array transducer that receives the excited photoacoustic waves from all foci simultaneously is presented. Using a customized microprism to reflect the incident laser beam to the microlens array, the multiple optical foci are aligned confocally with the focal zone of the ultrasonic array transducer. Experiments show the reflection-mode multifocal OR-PAM is capable of imaging microvessels in vivo, and it can image a 6×5×2.5  mm^3 volume at 16 μm lateral resolution in ∼2.5  min, which was limited by the signal multiplexing ratio and laser pulse repetition rate

In vivo Photoacoustic Tomography of Total Blood Flow and Potential Imaging of Cancer Angiogenesis and Hypermetabolism

Blood flow is a key parameter in studying cancer angiogenesis and hypermetabolism. Current photoacoustic blood flow estimation methods focus on either the axial or transverse component of the flow vector. However, the Doppler angle (beam-to-flow angle) is needed to calculate the total flow speed, and it cannot always be estimated accurately in practice, especially when the system's axial and lateral resolutions are different. To overcome this problem, we propose a method to compute the total flow speed and Doppler angle by combining the axial and transverse flow measurements. The method has been verified by flowing bovine blood in a plastic tube at various speeds and Doppler angles. The error was experimentally determined to be less than 0.3 mm/s for total flow speed, and less than 15° for the Doppler angle. In addition, the method was tested in vivo on a mouse ear. We believe that the proposed method has the potential to be used for cancer angiogenesis and hypermetabolism imaging

In vivo photoacoustic tomography of total blood flow and Doppler angle

As two hallmarks of cancer, angiogenesis and hypermetabolism are closely related to increased blood flow. Volumetric blood flow measurement is important to understanding the tumor microenvironment and developing new means to treat cancer. Current photoacoustic blood flow estimation methods focus on either the axial or transverse component of the flow vector. Here, we propose a method to compute the total flow speed and Doppler angle by combining the axial and transverse flow measurements. Both the components are measured in M-mode. Collating the A-lines side by side yields a 2D matrix. The columns are Hilbert transformed to compare the phases for the computation of the axial flow. The rows are Fourier transformed to quantify the bandwidth for the computation of the transverse flow. From the axial and transverse flow components, the total flow speed and Doppler angle can be derived. The method has been verified by flowing bovine blood in a plastic tube at various speeds from 0 to 7.5 mm/s and at Doppler angles from 30 to 330°. The measurement error for total flow speed was experimentally determined to be less than 0.3 mm/s; for the Doppler angle, it was less than 15°. In addition, the method was tested in vivo on a mouse ear. The advantage of this method is simplicity: No system modification or additional data acquisition is required to use our existing system. We believe that the proposed method has the potential to be used for cancer angiogenesis and hypermetabolism imaging

Photoacoustic Doppler axial flow measurement of homogenous media using structured illumination

We propose time and frequency domain methods for homogenous flow measurement based on the photoacoustic Doppler effect. Excited by spatially modulated laser pulses, the flowing medium induces a Doppler frequency shift in the received photoacoustic signals. The frequency shift is proportional to the component of the flow speed projected onto the acoustic beam axis. These methods do not rely on particle heterogeneity in the medium. A red-ink phantom flowing in a tube immersed in water was used to validate the methods in both frequency and time domains

Cross-correlation-based transverse flow measurements using optical resolution photoacoustic microscopy with a digital micromirror device

A cross-correlation-based method is proposed to quantitatively measure transverse flow velocity using optical resolution photoacoustic (PA) microscopy enhanced with a digital micromirror device (DMD). The DMD is used to alternately deliver two spatially separated laser beams to the target. Through cross-correlation between the slow-time PA profiles measured from the two beams, the speed and direction of transverse flow are simultaneously derived from the magnitude and sign of the time shift, respectively. Transverse flows in the range of 0.50 to 6.84  mm/s 6.84  mm/s are accurately measured using an aqueous suspension of 10-μm-diameter microspheres, and the root-mean-squared measurement accuracy is quantified to be 0.22  mm/s 0.22  mm/s. The flow measurements are independent of the particle size for flows in the velocity range of 0.55 to 6.49  mm/s 6.49  mm/s, which was demonstrated experimentally using three different sizes of microspheres (diameters: 3, 6, and 10 μm). The measured flow velocity follows an expected parabolic distribution along the depth direction perpendicular to the flow. Both maximum and minimum measurable velocities are investigated for varied distances between the two beams and varied total time for one measurement. This technique shows an accuracy of 0.35  mm/s 0.35  mm/s at 0.3-mm depth in scattering chicken breast, making it promising for measuring flow in biological tissue