Specific Imaging Findings

Global cerebral anoxia (hypoxic-ischemic injury, HII) findings vary depending on the duration and severity of insult, brain maturity, and type and timing of imaging studies with different presentations in premature neonates, full-term neonates, and older children and adults. In all three scenarios the lesions are bilateral and predominantly symmetrical. The spectrum of mild to severe findings in mature brains on non-contrast CT are: (1) poor differentiation of the cortical gray matter from subcortical white matter with sulcal effacement, predominantly affecting the arterial boundary zones; (2) loss of deep gray and white matter definition; (3) relative hyperdensity of the subarachnoid space (pseudosubarachnoid hemorrhage); (4) relatively hyperdense cerebellum (bright cerebellum); (5) hypodense brainstem; and (6) herniations. MRI shows hyperintense T2 (and low T1) signal of the affected areas, especially cortical, when CT findings may be absent. Diffusion MRI is the most sensitive technique with bright lesions on DWI and low ADC values involving deep gray matter, cortex, and cerebellum in the early stages. Periro-landic and deep gray matter involvement is observed in term neo-nates. Profound HII in older children and adults affects the deep gray matter nuclei, cortices, hippocampi, and cerebellum. Areas of reduced diffusion shift from the gray matter to the white matter in the subacute phase, when laminar enhancement of the cortex and patchy enhancement of the deep gray matter may be present on post-contrast images. This is followed by T1 bright cortical laminar necrosis in the late subacute phase and cortical atrophy and progressive white matter changes in the chronic stage.