Imaging modalities capable of visualizing the human brain have led to major advances in neuroscience. Functional magnetic resonance imaging has enabled a better understanding of brain function and connectivity abnormalities in brain diseases and has become a workhorse in neuroimaging. Nuclear imaging technologies such as positron emission tomography and single-photon emission computed tomography assisted with radiolabelled tracers have further enabled molecular diagnosis and monitoring of brain diseases. Limitations of these established technologies, such as the high cost or the use of ionizing radiation as well as accessibility, have fostered the development of new imaging approaches that can complement or enhance their performance. Optoacoustic tomography (OAT) provides unique capabilities to study cerebral function by mapping changes in several hemodynamic parameters within the brain. It further provides molecular imaging capabilities that can facilitate disease diagnosis and treatment monitoring. However, OAT imaging of the human brain is severely hampered by acoustic attenuation and aberrations of ultrasound waves propagating through the skull. Herein, we performed transcranial OAT imaging through the temporal bone of an adult healthy volunteer based on a previously reported spherical ultrasound array. We validated the OAT results by using head-to-head time-of-flight (TOF) magnetic resonance angiography (MRA) and T1-weighted structural MRI. The superior middle cerebral vein in the temporal cortex was identified in the OAT images matching the observed location in TOF-MRA. This basic demonstration anticipates the development of new hardware and reconstruction algorithms, ultimately enabling accurate OAT imaging of the human brain cortex.
Abnormal microcirculatory changes in arterioles, venules and capillaries are key functional indicators of important diseases such as cancer, diabetes or cardiovascular disorders, as well as several neurodegenerative ailments. In vivo visualization of these structures has traditionally been restricted to the shallow depths <1-2 mm reachable with highresolution optical imaging methods. Optoacoustic (photoacoustic) tomography has enabled breaking through this light diffusion barrier to map functional hemodynamic parameters such as oxygen saturation in deep-seated vessels, but frequency-dependent attenuation of ultrasound waves limits the achievable resolution and prevents reaching the capillary level. Imaging of individual capillaries has been achieved via localization of microbubbles flowing in blood with pulseecho ultrasound. Optoacoustic localization of light-absorbing particles in mammalian tissues is however hampered by the strong background absorption of blood. Herein, we demonstrate the feasibility of in vivo localization optoacoustic tomography (LOT) with intravenously injected 5 m microdroplets having optical absorption 10000 higher than red blood cells for near-infrared wavelengths. Three-dimensional imaging of the mouse brain microvasculature through the intact scalp and skull was facilitated by the unidirectional propagation of ultrasound waves, yielding 20 μm resolution at 3 mm depth in turbid mouse brain tissues in vivo.
Fluorescence Molecular Tomography (FMT) constitutes a functional and molecular imaging tool complementing well-established anatomical imaging modalities such as magnetic resonance imaging (MRI) and X-ray computer tomography (CT). Preclinical applications of FMT in the fields of tumor imaging, breast imaging, proteomic research, and drug discovery have been reported. We have developed the Smart ToolkIt for Fluorescence Tomography (STIFT) software platform for simulating, reconstructing and optimizing FMT. However, accurate reconstruction of FMT data for inhomogeneous objects of complex shape, e.g. a live mouse, requires prior information about the anatomy. This work addresses this issue by incorporating a digital mouse atlas into STIFT. The Digimouse consists of high-resolution and well-registered multimodal volumetric images of a post-mortem 28g male C57Bl/6 mouse. Techniques including cropping, translation, and de-noising were performed on the Digimouse atlas (DA), the Digimouse CT volume (DCT), and our MR datasets. To decrease the discrepancy between the DCT and the DA, the skull regions of these two volumes were enhanced and the DA was deformed using the transformation matrix. Followed, was another automated registration, between an experimental MRI dataset and the newly coupled DCT-DA dataset by fixing the former. Finally, the registered DA was used for meshing and FMT simulations. The quality of registration was quantified and shown to have increased organ volume overlap using the Dice similarity coefficient as metric. The reconstructed fluorophore distribution matches well with the preset ground truth, demonstrating that the meshed atlas is compatible with STIFT.
Multimodal imaging combining optoacoustic tomography (OAT) with magnetic resonance imaging (MRI) enables spatiotemporal resolution complementarity, improves accurate quantification, and thus yields more insights into physiology and pathophysiology. However, only manual landmark based coregistration of OAT-MRI has been used so far. We developed a toolbox (RegOA), which frames an automated registration pipeline to align OAT with high-field MR images based on mutual information. We assessed the performance of the registration method using images acquired on one phantom with fiducial markers and in vivo/ex vivo data of mouse heads/brain. The accuracy and robustness of the registration are improved using a two-step registration method with preprocessing of OAT and MRI data. The major advantages of our approach are minimal user input and quantitative assessment of the registration error. The registration with MR and standard reference atlas enables regional information extraction, facilitating the accurate, objective, and rapid analysis of large groups of rodent OAT and MR images.
The International Photoacoustic Standardisation Consortium (IPASC) emerged from SPIE 2018, established to drive consensus on photoacoustic system testing. As photoacoustic imaging (PAI) matures from research laboratories into clinical trials, it is essential to establish best-practice guidelines for photoacoustic image acquisition, analysis and reporting, and a standardised approach for technical system validation. The primary goal of the IPASC is to create widely accepted phantoms for testing preclinical and clinical PAI systems. To achieve this, the IPASC has formed five working groups (WGs). The first and second WGs have defined optical and acoustic properties, suitable materials, and configurations of photoacoustic image quality phantoms. These phantoms consist of a bulk material embedded with targets to enable quantitative assessment of image quality characteristics including resolution and sensitivity across depth. The third WG has recorded details such as illumination and detection configurations of PAI instruments available within the consortium, leading to proposals for system-specific phantom geometries. This PAI system inventory was also used by WG4 in identifying approaches to data collection and sharing. Finally, WG5 investigated means for phantom fabrication, material characterisation and PAI of phantoms. Following a pilot multi-centre phantom imaging study within the consortium, the IPASC settled on an internationally agreed set of standardised recommendations and imaging procedures. This leads to advances in: (1) quantitative comparison of PAI data acquired with different data acquisition and analysis methods; (2) provision of a publicly available reference data set for testing new algorithms; and (3) technical validation of new and existing PAI devices across multiple centres.
Oxygen metabolism and matrix metalloproteinases (MMPs) play important roles in the pathophysiology of cerebral ischemia. Using multispectral optoacoustic tomography (MSOT) imaging, we visualized in vivo changes in cerebral tissue oxygenation during 1 h of transient middle cerebral artery occlusion (tMCAO) and at 48 h after reperfusion together with MMP activity using an MMP-activatable probe. The deoxyhemoglobin, oxyhemoglobin, and MMP signals were coregistered with structural magnetic resonance imaging data. The ipsi-/contralateral ratio of tissue oxygen saturation (SO2) was significantly reduced during 1 h of tMCAO and recovered after 48 h of reperfusion in tMCAO compared with sham-operated mice (n = 8 to 10 per group). A higher ipsi-/contralateral MMP signal ratio was detected at 48 h after reperfusion in the lesioned brain regions of tMCAO compared with the sham-operated animal (n = 4 to 6 per group). Ex vivo near-infrared fluorescence imaging of MMP signal in brain slices was used to validate in vivo MSOT measurements. In conclusion, noninvasive MSOT imaging can provide visualization of hemodynamic alterations and MMP activity in a mouse model of cerebral ischemia.
Beta-amyloid (Aβ) deposition and vascular dysfunction are important contributors to the pathogenesis in Alzheimer’s disease (AD). However, the spatio-temporal relationship between an altered oxygen metabolism and Aβ deposition in the brain remains elusive. Here we provide novel in-vivo estimates of brain Aβ load with Aβ-binding probe CRANAD-2 and measures of brain oxygen saturation by using multi-spectral optoacoustic imaging (MSOT) and perfusion imaging with magnetic resonance imaging (MRI) in arcAβ mouse models of AD. We demonstrated a decreased cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) in the cortical region of the arcAβ mice compared to wildtype littermates at 24 months. In addition, we showed proof-of-concept for the detection of cerebral Aβ deposits in brain from arcAβ mice compared to wild-type littermates.
KEYWORDS: Magnetic resonance imaging, Brain, Ischemia, In vivo imaging, Mouse models, Neuroimaging, Optoacoustics, Imaging systems, Animal model studies
Matrix metalloproteinases (MMPs) play important roles in the pathophysiology of cerebral ischemia. Here we visualized in vivo MMP activity in the transient middle cerebral artery occlusion (tMCAO) mouse model using multispectral optoacoustic imaging (MSOT) with a MMP-activatable probe. MSOT data was co-registered with structural magnetic resonance imaging (MRI) obtained at 7 T for localization of signal distribution. We demonstrated upregulated MMP signal within the focal ischemic lesion in the tMCAO mouse model using MSOT/MRI multimodal imaging. This convenient non-invasive method will allow repetitive measurement following the time course of MMP-lesion development in ischemic stroke animal model.
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