Pediatric MRI of the Brain: A Primer

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Because of recent advances in magnetic resonance imaging (MRI) techniques, pediatricians should be aware of the different modalities and their unique advantages and appropriateness in different clinical situations.After completing this article, readers should be able to:Magnetic resonance imaging (MRI) is based on the absorption and emission of radiofrequency energy by hydrogen protons whose spin is influenced by changing magnetic fields (0.3 to 1.5 T). Unlike computed tomography (CT), there is no radiation exposure.T1-weighted images cause fat (eg, myelin in white matter) to appear bright and water (eg, cerebrospinal fluid [CSF] or edema) to appear dark on this sequence. The gray-white interfaces of the brain are well depicted on these sequences, especially if with the images are thinly sliced. T2-weighted images cause water (eg, CSF and edema) to appear bright and fat to appear dark. The MRI-based intravenous contrast agents (eg, gadolinium) are frequently used in T1-weighted images (Fig 1A and B) to make serum appear bright. The blood-brain barrier typically serves to limit the passage of many molecules out of the blood vessels. If disease processes break down this barrier (such as infection, tumors, or inflammation), intravenous contrast agents can cross into the brain, causing areas of contrast entry to appear very bright.MRI and CT are complementary diagnostic tools with mutually distinct advantages and disadvantages. CT can be performed quickly and is preferred in cases of trauma and emergency circumstances. CT is more sensitive for detecting calcification and better delineates cortical bone. CT angiography has a better resolution compared with magnetic resonance angiography; however, the latter has the advantage of not absolutely requiring the use of contrast agents. MRI cannot be performed in claustrophobic patients and those with ferromagnetic medical devices, such as pacemakers. Further, MRI takes longer to perform and might require sedation, precluding its use in emergency situations. However, for evaluating posterior fossa disease, white matter disease, temporal lobe epilepsy, and vascular diseases, MRI is preferable to CT. In this article, we present some common pediatric case vignettes that illustrate the role of brain MRI to acquaint the reader with the common modalities of MRI.A 7-year-old girl with a history of migraine headaches presented with a head tilt to the left and worsening headaches. Optic disc edema was found on ophthalmoscopy. A brain MRI was performed, which revealed a large heterogeneous intermediate signal mass that filled and obstructed the fourth ventricle on the axial T2-weighted images at the level of the fourth ventricle (Fig 2). She underwent surgical resection, and the histologic diagnosis was medulloblastoma.Apart from sparing the effects of radiation, MRI is superior to CT in delineating tumor extent, spread, mass effect, vascularity, necrosis, and edema. T2-weighted sequences are sensitive for the detection of tumor and edema.A 6-year-old girl presented with swelling of her right eye. She had a history of sinusitis. Her right eye was swollen and erythematous, she had difficulty opening her right eye, and she had double vision with all gazes. Clinical findings included proptosis, ptosis, restriction of ocular motility, ocular pain, and chemosis. Laboratory tests revealed neutrophilic leukocytosis and elevated C-reactive protein. Head MRI was performed. Coronal postcontrast T1-weighted imaging revealed inflammation in the right orbit with diffuse stranding in orbital fat, enhancing myositis of the inferior and medial rectus muscles (Fig 3).Orbital cellulitis is an infection of the soft tissue posterior to the orbital septum, whereas preseptal cellulitis affects anterior to the orbital septum. The former is distinguished from the latter by the presence of proptosis, chemosis, ophthalmoplegia, or decreased visual acuity.MRI is superior to CT when there is suspicion of intracranial extension, optic nerve involvement, and cavernous sinus thrombosis because MRI is better for discerning soft tissue disease. Gadolinium is a paramagnetic contrast agent that prolongs the spin of water protons, resulting in postcontrast enhancement of areas of inflammation on T1-weighted imaging. The most sensitive technique for demonstrating orbital infection is postgadolinium, fat-suppressed T1-weighted imaging.A 16-year-old girl presented to the emergency department with a chief symptom of “vision loss in right eye.” She denied eye pain, diplopia, photophobia, or headache. Her medical history was unremarkable. Visual acuity was normal in the left eye (20/20); however, it was decreased in the right eye (20/200). Her right pupil constricted in response to consensual but not to direct light (ie, deafferented pupil). Bilateral fundi appeared normal. MRI of the brain revealed a normal-appearing left optic nerve. The left optic nerve was round with distinct borders and was appropriately surrounded by CSF as demonstrated by T2-weighted fluid-attenuated inversion recovery (FLAIR) imaging. The right optic nerve, however, had poorly defined borders, suggesting an inflammatory process. The inflamed right optic nerve also enhanced with gadolinium administration (Fig 4).Given the patient’s visual disturbance and the evidence of optic nerve inflammation on MRI, optic neuritis was suggested as a diagnosis. Other MRIs obtained revealed demyelination in the pons and cerebellum. The presence of CSF oligoclonal bands supported multiple sclerosis as the diagnosis.FLAIR is an extremely useful technique in brain imaging. Like conventional T2-weighted imaging, edema appears bright, but this technique nulls (or makes dark) CSF signal. FLAIR is a sensitive technique for displaying demyelination within the brain, thus clearly revealing lesions in proximity to CSF, such as periventricular plaques in multiple sclerosis. The technique is accomplished via a relatively long inversion time to allow the longitudinal magnetization of CSF to return to the null point preceding the conventional spin echo imaging. It also has a tremendous role in early detection of cortical gray matter infarcts. The cortical gray matter is vulnerable to ischemia because of its high metabolic activity. However, cortical gray matter immediately adjacent to CSF within the sulci makes infarction hard to delineate when this area undergoes conventional imaging sequences that emphasize fluid signal. FLAIR suppresses the CSF signal and makes the cortical or periventricular area more conspicuous.A 3-month-old boy was admitted to the pediatric intensive care unit for bilateral subdural hematomas (SDHs) and concern for intentional trauma. Per his parents, he had emesis for 24 hours and had been unable to keep formula down. He also had 2 episodes of arm stiffening and breath holding followed by agitation and crying. Neurologic examination revealed an enlarged head circumference, dilated scalp veins, and a bulging anterior fontanel. He had brisk tendon reflexes. Brain MRI FLAIR images revealed localizing tissue loss in the right parietal region from an older injury and 2 SDHs of different densities, with the more acute-appearing SDH on the left and the more subacute SDH on the right (Fig 5).The presence of SDHs of varying ages and of skull fractures, which are depressed or multiple or diastatic or involve multiple or nonparietal bones, is a key neuroimaging finding that is consistent with the diagnosis of intentional trauma. CT is the modality of choice to detect acute hemorrhages and skull fractures. MRI is superior to CT in detecting extra-axial hemorrhages, diffuse axonal injury, and early recognition and prognostication of parenchymal injury. SDHs over the falx, posterior fossa, and tentorium are more characteristic of intentional trauma. (1) T2-weighted gradient echo MRI enhances the sensitivity for recognizing acute bleeds and old shear bleeds. Diffusion-weighted imaging (DWI) is also helpful

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