Fractional anisotropy from diffusion tensor imaging correlates with acute astrocyte and myelin swelling in neonatal swine models of excitotoxic and hypoxic‐ischemic brain injury

Abstract

The specific cytopathology that causes abnormal fractional anisotropy (FA) and mean diffusivity (MD) from diffusion tensor imaging (DTI) after neonatal hypoxia‐ischemia (HI) is not completely understood. The panoply of cell types in the brain might contribute differentially to changes in DTI metrics. Because glia are the predominant cell type in brain, we hypothesized that changes in FA and MD would signify perturbations in glial microstructure. Using a 3‐Tesla clinical scanner, we conducted in vivo DTI MRI in nine neonatal piglets at 20–96 h after excitotoxic brain injury from striatal quinolinic acid injection or global HI. FA and MD from putamen, caudate, and internal capsule in toto were correlated with astrocyte swelling, neuronal excitotoxicity, and white matter injury. Low FA correlated with more swollen astrocytes immunophenotyped by aquaporin‐4 (AQP4), glial fibrillary acidic protein (GFAP), and glutamate transporter‐1 (GLT‐1). Low FA was also related to the loss of neurons with perineuronal GLT‐1+ astrocyte decorations, large myelin swellings, lower myelin density, and oligodendrocyte cell death identified by 2′,3′‐cyclic nucleotide 3′‐phosphodiesterase, bridging integrator‐1, and nuclear morphology. MD correlated with degenerating oligodendrocytes and depletion of normal GFAP+ astrocytes but not with astrocyte or myelin swelling. We conclude that FA is associated with cytotoxic edema in astrocytes and oligodendrocyte processes as well as myelin injury at the cellular level. MD can detect glial cell death and loss, but it may not discern subtle pathology in swollen astrocytes, oligodendrocytes, or myelin. This study provides a cytopathologic basis for interpreting DTI in the neonatal brain after HI.

Localized Modification of Water Molecule Transport After Focused Ultrasound-Induced Blood–Brain Barrier Disruption in Rat Brain

“…However, the diffusion of water through the BBB increased diffusivity, which may have affected the decrease in FA. A recent report suggested that a low FA is also related to swollen astrocytes stained with AQP4 and GFAP after neonatal hypoxia-ischemia (HI) ( Lee et al, 2021 ). In this study, we observed that group-0.8 MPa induced significant decreases in the FA signal at 24 h and 48 h ( Figure 5 ).…”

Multi‐Parametric Evaluation of Cerebral Hemodynamics in Neonatal Piglets Using Non‐Contrast‐Enhanced Magnetic Resonance Imaging Methods

“…Prior to imaging, each piglet underwent one of three procedures: excitotoxic brain injury induced by stereotaxic quinolinic acid (QA) injection into the striatum, global HI injury, or sham procedure. The piglet preparation, procedural anesthesia, and brain injury protocols were previously published 17,18 . Briefly, piglets were anesthetized and intubated for mechanical ventilation.…”

“…Excitotoxic brain injuries were induced in six anesthetized piglets 17,18 . Bilateral cranial burr holes 2–3 mm in diameter were drilled 10 mm anterior to the bregma suture and 10–12 mm lateral to midline.…”

“…classified and counted neuronal cell bodies in 9–10 non‐overlapping fields in the caudate and 10 non‐overlapping fields in the putamen at 400× magnification. Neurons were assigned to one of three categories: 1) normal; 2) degenerating and undergoing irreversible cell death; or 3) injured with uncertain fate and without overt degeneration according to published criteria 17,18 . A second investigator with 36 years of neuropathology experience (L.J.M.)…”

“…Piglets have similar brain growth patterns and cerebral anatomical structures as human neonates, and the regional distribution of neuropathology in piglet HIE resembles that in human newborns 15 . MRI in piglet models of HI and excitotoxicity, an integral cytotoxic molecular cascade that contributes to neonatal HIE, 16 can also provide salient information relevant to human neonatal brain injuries 17,18 . However, so far, neonatal piglets with brain damage have been primarily interrogated using structural imaging, such as T 1 ‐weighted, T 2 ‐weighted imaging, and diffusion imaging techniques 17,18 .…”