Advanced Neuromonitoring Modalities on the Horizon: Detection and Management of Acute Brain Injury in Children

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• The basis of QEEG analysis is transformation of raw time-domain (TD) EEG waveforms into a frequency-domain (FD) distribution using the fast Fourier transform (FFT); the FFT may be used to assess the relative contributions of various frequency bands of electrical brain activity to overall power (spectrograms) [147]. Amplitude integrated EEG (aEEG), in which the EEG signal from each hemisphere is filtered, rectified, and displayed as peak-to-peak amplitude on a compressed time scale, is routinely used at the bedside of high-risk critically ill neonates. aEEG has demonstrated correlations with encephalopathy severity and can be used to screen for seizures

• MRS yields critical diagnostic information in neonates with perinatal asphyxia in which N-acetylaspartate (NAA) concentration, the lactate (Lac)/NAA ratio, and the Lac/choline ratio have been correlated with poor outcome [74–78]. Recent results of the MARBLE trial confirm the high predictive utility of reduced thalamic NAA concentration (AUC = 0.99) and elevated Lac/NAA ratio > 0.22 (AUC = 0.94), measured within 4–14 days after birth, for adverse neurodevelopmental outcomes at 2 years of age [

• Diffusion tensor imaging (DTI) is an advanced computational derivative of DWI that permits characterization of white matter structure and function [60]. Commonly reported voxel-specific DTI metrics are the fractional anisotropy (FA), reflecting the degree of directed diffusion, and the mean diffusivity (MD), the average magnitude of diffusion in all directions. With axonal injury, FA is commonly reduced and MD is elevated. Computational methods may be used to connect neighboring voxels with similar directed diffusion to reconstruct axonal fiber tracts. Advanced analysis of DTI data includes tract-based spatial statistics, tractography, and functional connectivity via application of graph theory

• In TBI, NAA level has been demonstrated as a measure of neuronal and axonal integrity, with reduction in the NAA level following TBI

• Microvascular imaging (MVI) is an advanced Doppler technique that permits visualization of slow flow in cerebral microvessels (Fig. 2). In MVI, an advanced adaptive wall filter is used to suppress tissue clutter or static noise while detecting low velocity flow (< 2.5 cm/s)

• Elastography is an advanced ultrasound technique that allows for the noninvasive assessment of brain tissue stiffness [129, 130, 131]. Brain tissue stiffness can change with many factors, including myelination, edema, and ICP. There are two primary ultrasound elastography methods used: strain-based and shear-wave-based elastography. It should be noted that strain elastography provides semiquantitative measures of brain stiffness whereas shear wave elastography provides quantitative measures of brain stiffness, such that the latter is most widely used in brain applications. In strain-based elastography, external pressure is applied by the operator, and Young’s modulus is calculated from tissue displacement from the applied external pressure.