In vivo photoacoustic imaging dynamically monitors the structural and functional changes of ischemic stroke at a very early stage

Abstract

Ischemic stroke (IS) is one of the leading causes of death and accounts for 85% of stroke cases. Since the symptoms are not obvious, diagnosis of IS, particularly at an early stage, is a great challenge. Photoacoustic imaging combines high sensitivity of optical imaging and fine resolution of ultrasonography to non-invasively provide structural and functional information of IS. Methods: We adopted three rapid photoacoustic imaging systems with varying characteristics, including a portable handheld photoacoustic system, high-sensitivity bowl-shaped array photoacoustic computed tomography (PACT), and high-resolution photoacoustic microscopy (PAM) to assess the stereoscopic and comprehensive pathophysiological status of IS at an early stage. Two representative models of IS, referring to photothrombosis and middle cerebral artery occlusion (MCAO) models, were established to verify the feasibility of photoacoustic imaging detection. Results: Non-invasive, rapid PACT of the IS model in mouse provided structural information of the brain lesion, achieving early disease identification (5 min after the onset of disease). Moreover, it was able to dynamically reflect disease progression. Quantitative high-resolution PAM allowed observation of pathological changes in the microvascular system of mouse brain. In terms of functional imaging, significant differences in oxygen saturation (sO₂) levels between infarcted and normal areas could be observed by PACT, permitting effective functional parameters for the diagnosis of IS. Conclusions: We used PACT to perform full-view structural imaging and functional imaging of sO₂ in IS at the macroscopic level, and then observed the microvascular changes in the infarcted area at the microscopic level by using PAM. This work may provide new tools for the early diagnosis of IS and its subsequent complications as well as assessment of disease progression.

Keywords: early diagnosis.; ischemic stroke; photoacoustic imaging.

Scheme 1

Schematic illustration of structural and functional photoacoustic imaging on photothrombosis/MCAO model mice at an early stage in vivo. Scale bar, 1 mm.

Figure 1

PACT and MRI of mouse brain in a photothrombosis model at an early stage in vivo. (A-D), (I-L) Representative PA images of a photothrombosis mouse model right followed by EB dye injection at varied time points upon injection at 680 nm. (E-H), (M-P) Representative MR images of a photothrombosis mouse model at varied time points. (Q) Normalized PAI and MRI signals of mice brains (n = 5; the error bars show the standard deviation) in infarcted areas at varied time points upon EB dye injection. (*) P < 0.05. (R) Triphenyl tetrazolium chloride staining in the brain of model mice. Scale bar, 1 mm.

Figure 2

PACT and MRI of mouse brain in MCAO model at an early stage in vivo. (A-F) Representative PA images of an MCAO model mouse at varied time points without EB dye at 680 nm. (G-L) Representative PA images of an MCAO model mouse right followed by EB dye injection at varied time points upon injection at 680 nm. (M-S) Representative MR images of an MCAO model mouse at varied time points. (T) Normalized PA signals of mice brains (n = 5; the error bars show standard deviation) in the region of interest (regions I, II, III, IV) at varied time points (*) P<0.05. (U) MR image in the brain of a MCAO model mouse at 24 h. Scale bar, 1 mm.

Figure 3

PACT of mouse brain in a hemorrhagic transformation model at an early stage in vivo. (A-F) Representative PA images of a hemorrhage transformation model mouse at varied time points at 800 nm. (G) Normalized PA signals of mice brains (n = 5; error bars show the standard deviation) in a hemorrhagic region at varied time points. (H, I) PA images from sagittal and transverse planes, respectively, of a model mouse brain. Scale bar, 1 mm.

Figure 4

PAM of mouse brain in photothrombosis and MCAO model in vivo. (A, B) Representative PAM images of a mouse brain before and 30 min after photothrombosis modeling at 532 nm. (C, D) Representative PAM images of another mouse brain before and 2 h after photothrombosis modeling with EB dye injection, respectively. (E, F) Representative PAM images of a mouse brain before and after MCAO modeling after 6 h at 532 nm. (G, H) Representative PAM images of a mouse brain after MCAO modeling at 532 nm following EB injection after 1 h and 3 h, respectively.

Figure 5

Oxygen saturation mapping of mouse brain of IS using PACT in vivo. (A-C) Ultrasonograms and 2D and 3D PA functional images of brain sO₂ of photothrombosis model mice at different time points, respectively. Organ labeling part a (1) scalp, (2) skull, and (3) cerebral cortex. (D-F) Ultrasonograms and 2D and 3D PA functional images of brain sO₂ of MCAO model mice at different time points, respectively. (G, H) Statistical results of sO₂ mapping in apoplectic and normal areas at different time points of photothrombotic mice and MCAO mice (n = 5; the error bars show standard deviation), respectively. (*) P < 0.05.