Introduction 1 2 4 Pathophysiology of meningitis 2 5 5 5 6 7 9 10 11 2 2 3 5 12 13 14 Escherichia coli Listeria monocytogenes Klebsiella Citrobacter 15 Sonographic features of meningitis 3 16 22 17 3 3 17 23 24 23 We recommend cranial sonography as a baseline study in every infant who has an adequate size fontanel if the diagnosis of bacterial meningitis is suspected clinically. Because the major advantage of sonography is its ability to be safely repeated, a second study should be performed if any clinical deterioration occurs, such as increasing head circumference, occurrence of new neurological findings, and lack of response to therapy. However, in patients with complicated bacterial meningitis whose clinical situation is sufficiently stable to leave the nursery, MRI should be the next study of choice. In children, MRI is widely accepted for its safety and high soft-tissue resolution. It is superior to sonography in showing the existence and extension of the complications, especially in the posterior fossa. It has the further advantage of not using ionizing radiation. 25 26 Meningeal and extra-axial findings 27 1 3 16 20 28 2 5 3 16 20 Fig. 1 a arrows asterisk b c b arrows c  Fig. 2 arrowheads  3 16 19 20 28 3 3 5 17 24 28 29 4 30 31 Fig. 3 a b  Fig. 4 a E. coli arrows b arrows  25 32 33 34 35 5 Fig. 5 blue vessels asterisk  Intraventricular findings E. coli 3 36 6 37 21 28 38 7 38 Fig. 6 a–c a b c E. coli arrows d  Fig. 7 E. coli arrow  5 13 3 16 20 28 16 28 38 39 20 28 37 39 Parenchymal findings Abnormal parenchymal echogenicity 3 20 28 3 17 40 3 2 3 16 21 28 41 42 43 44 Citrobacter koseri C. diversus 43 45 45 46 43 46 46 2 8 3 21 16 21 47 48 49 50 Fig. 8 a b a b c arrow d e d e  Brain edema 5 28 51 20 23 28 50 52 9 10 53 57 35 58 Fig. 9 Brain edema. Coronal sonogram in a 7-day-old infant with group B streptococcus meningitis shows generalized edema with compression of lateral ventricles and sulci and diffuse increased heterogeneity of the cerebral hemispheres  Fig. 10 a b arrows c  Diffuse cerebral atrophy, multicystic encephalopathy, and porencephaly might be seen as the end-stage of complications of bacterial meningitis. Hydrocephalus 16 21 28 16 11 59 Fig. 11 a b  53 54 57 60 Conclusion Cranial sonography continues to be an excellent front-line imaging modality in the initial diagnosis and monitoring of infants with acute bacterial meningitis and its potential complications. Its utility is enhanced by the use of alternative scanning approaches and Doppler techniques for identification of ventriculitis, extra-axial fluid collections, and hemodynamic alterations. Spinal sonography might also play a role in assessing the risk of progressive hydrocephalus.