From Light to Sound: Photoacoustic and Ultrasound Imaging in Fundamental Research of Alzheimer's Disease

Abstract

Alzheimer's disease (AD) causes severe cognitive dysfunction and has long been studied for the underlining physiological and pathological mechanisms. Several biomedical imaging modalities have been applied, including MRI, PET, and high-resolution optical microscopy, for research purposes. However, there is still a strong need for imaging tools that can provide high spatiotemporal resolutions with relatively deep penetration to enhance our understanding of AD pathology and monitor treatment progress in fundamental research. Photoacoustic (PA) imaging and ultrasound (US) imaging can potentially address these unmet needs in AD research. PA imaging provides functional information with endogenous and/or exogenous contrast, while US imaging provides structural information. Recent studies have demonstrated the ability to monitor physiological parameters in small-animal brains with PA and US imaging as well as the feasibility of using US imaging as a therapeutic tool for AD. This concise review aims to introduce recent advances in AD research using PA and US imaging, provide the fundamentals, and discuss the potentials and challenges for future advances.

Keywords: Alzheimer’s disease; Photoacoustic imaging; brain imaging; ultrasound imaging.

Figure 1.. Schematics for various photoacoustic imaging systems.

(A) Reflection-mode OR-PAM system with an optical-acoustic combiner that transmits light and reflects sound. SOL, silicone oil layer; UT, ultrasonic transducer. (B) Dark-field AR-PAM system with excitation light only weakly focused. (C) PACT system with a full-ring transducer and widefield illumination from top. UTA, ultrasonic transducer array. (37)

Figure 2.. In vivo brain imaging of a Congo red-injected 10-month-old APP/PSI mouse through a cranial window.

(A) System schematic of the optical-resolution photoacoustic microscope for amyloid plaque imaging. (B) Conventional fluorescence microscopy imaging of exposed cortical region through the cranial window. The region of interest is labeled with a red-dashed box. (C) Multiphoton microscopy image of the region of interest; excitation at 800 nm. (D) OR-PAM image at 523 nm. (E) OR-PAM image at 570 nm. (F) Processed and labeled OR-PAM image (plaques colored green, blood vessels colored red). Arrows indicate plaques. (reproduced from [42], OSA publishing, all rights reserved)

Figure 3.. Perfusion MRI and PACT images of young and aged wild-type and arcAβ mice.

(A) Anatomical position of the coronal plane imaged by perfusion MRI from the sagittal view. Cortex is labeled red and thalamus is labeled blue. (B) Representative coronal cerebral blood flow (CBF) maps. (C) Reduced CBF in cortex is observed in aged arcAβ mice compared to aged wild-type mice. (D) Coronal view of SO₂ maps. The coronal plane was approximately at Bregma −1.5 ± 0.3 mm. White arrow indicates the middle cerebral artery (MCA) and superior sagittal sinus (SSS). (E, F) Quantification of brain oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO₂). OEF does not significantly differ among the four groups. However, CMRO₂ is lower in aged arcAβ mice compared to both aged wild-type and young arcAβ mice. All statistical results are mean ± standard deviation with p<0.05, one-way ANOVA with post-hoc Bonferroni correction for multiple comparison. (reproduced from [44], Photoacoustics, all rights reserved)

Figure 4.. PAI of CDnir7 localization in the mouse brain.

(A) Chemical structure of CDnir7. (B) PA signal of CDnir7 at various time points for both AD and healthy brain. (C) PA signal of CDnir7 in the cortex of both AD and healthy brain as a function of time. From 20 min post-injection onward, a stronger PA signal was observed in the cortex of AD brain. In contrast, the PA signal at SSS remained similar in both AD and healthy brain during the entire experiment. (D) PA signal of total hemoglobin in AD and healthy brain. No high signals in the cortex area in AD brain were observed during the course of the experiment. (reproduced from [48], Scientific Reports, all rights reserved)

Figure 5.. PAI of RPS1 probe in the mouse brain.

(A) Chemical structure of RPS1. (B) PA spectra of RPS1 and RPS1-Cu from 680 to 970 nm. The RPS1-Cu has an excitation peak at 710 nm. (C) PA images of AD and healthy mouse brain. The brain images of AD mice show strong PA signals from RPS1 in the cortex region. Weak PA signals observed in normal mice injected with RPS1 and AD mice injected with PBS were likely due to hemoglobin. (D) Within the region of interest, the PA signal from AD mice injected with RPS1 was approximately 9-fold stronger than those of the other two groups. (reproduced from [51], Angewandte Chemie, all rights reserved)

Figure 6.. Typical B-mode images and their corresponding high-frequency US elastography (HFUSE) images for 4-month-old and 11-month-old mice.

The hippocampus and cortex regions were identified in the B-mode image and are denoted by yellow and red lines, respectively. (reproduced from [59], IEEE, all rights reserved)

Figure 7.. Correlation matrices of the functional connectivity.

(A, C) Matrices at Bregma +0.84 mm and (B, D) matrices at Bregma −2.16 mm. (reproduced from [64], NPG group, all rights reserved)

Figure 8.. A comparison of vessel density between control WT mice and AD mice.

(A) B-mode structure of the brain. The image size is 3 mm × 7 mm. (B) The corresponding power Doppler image. (C) Directional flow images. (reproduced from [65], IEEE, all rights reserved)

Figure 9.. Establishing repeated scanning ultrasound (SUS) in an AD mouse model.

(A) Simplified system schematics. (B) Evans blue dye was injected as an indicator. In response to US treatment, the single-entry point reveals a focal opening of the BBB. (C) Odyssey fluorescence LI-COR scan of brain slices, showing widespread opening of the BBB 1 hour after SUS. (D) Simplified schematic for treatment and analysis. (E) The sequence of arm entries in the Y-maze was used to obtain a measure of alternation, reflecting spatial working memory. US-treated mice exhibited improved performance for three memory tasks: the Y-maze, the novel object recognition test, and the active place avoidance task. (F) Total number of arm entries did not differ between groups. (reproduced from [67], AAAS, all rights reserved)