Plain Radiographs

Very few indications remain for performing plain radiographs to evaluate the neuraxis in the acutely ill child in the emergency department. Plain radiographs, however, continue to have a role in documenting fractures of the skull and appendicular skeleton in the suspected child abuse victim, but this is usually done after transfer from the emergency department and therefore will not be discussed further in this review. Plain radiographs remain the primary method for visualizing the entire cervical spine in children with traumatic injuries. In most children, anteroposterior and lateral spine radiographs suffice to clear the spine following trauma. The false negative rate for the single cross-table lateral view ranges between 21% and 26%. A 3-view series can improve sensitivity to 94% in children. More recent studies suggest that the open-mouth odontoid view is routinely beneficial only in children older than 5 years.

Some investigators have argued that multislice CT with sagittal and coronal reformats may be required in the obtunded patient to clear the cervical spine. At our institution, we reserve focused CT and MRI to clarify areas of abnormality and to define further injuries.

Ultrasound

Ultrasound (US) has numerous imaging advantages, including ready access, portability, real-time and multiplanar capabilities, and reproducible results. Moreover, it is a noninvasive, radiation-free procedure that can be performed at the bedside, in the intensive care unit, or on an intubated, ventilated baby following delivery. Ultrasonography is also useful in evaluating hydrocephalus, extra-axial fluid collections, and large intracranial hemorrhages in the young infant who is too ill to be transported to CT or MRI. Further, transcranial Doppler techniques have been used to correlate resistive indices with elevated intracranial pressure in patients with head trauma.

Computed Tomography

The widespread availability and speed of image acquisition has made CT the neuroimaging modality of choice in the acutely ill child over the past 2 decades. CT can detect acute intracranial hemorrhage, cerebral edema, hypoxic-ischemic injury, infarction, hydrocephalus/shunt dysfunction, neoplasm, or abnormal collections. In addition, modern multidetector CT scanners can acquire submillimeter-thick images, which can be manipulated to produce multiplanar reformats and 3-dimensional (3D) images, thereby facilitating rapid detection of skull and facial fractures. When performed with iodinated contrast media, CT provides information about both inflammatory and infectious lesions and their resultant complications.

CT angiography (CTA) provides accurate information for a variety of arterial and venous abnormalities in the acute setting. CTA also compares favorably with MR angiography (MRA) in evaluating vertebral artery dissection, as shown in a number of adult studies. CT venography (CTV) is the initial study of choice in many centers for assessing cerebral venous sinus thrombosis. Further, in evaluating the child presenting after major trauma, multidetector CT (MDCT) has shown great utility in imaging the head, cervical spine, chest, abdomen, pelvis, and, where applicable, the appendicular skeleton.

The primary disadvantage of CT is that it requires the use of ionizing radiation, which can have potentially harmful effects on the tissues of the pediatric patient, particularly if used for multiple studies over the course of weeks, months, or years. It is, therefore, imperative that alternatives to CT be considered in making the diagnosis. When CT is chosen as the imaging modality for a child, the radiologist should follow the ALARA (as low as reasonably acceptable) principle, which entails using appropriate age- and weight-based dose adjustment parameters available on modern scanners. In addition, the scan should be strictly limited to the area of concern.

Magnetic Resonance Imaging

The multiplanar capability of MR to show superior anatomic detail and tissue contrast without the harmful effects of ionizing radiation makes it the imaging modality of choice for evaluating the neuraxis in children of all ages presenting with acute neurologic symptoms in the emergency department. MRI is superior to CT in evaluating the posterior fossa and in detecting early cerebral edema and microhemorrhages. Further, MRI can provide functional and physiologic information about the brain that cannot be generated by other modalities. Use of higher field strengths (most commonly 3-Tesla MRI scanners) has also demonstrated improved diagnostic accuracy for many conditions affecting the CNS.

Advanced MR imaging sequences, including susceptibility-weighted imaging (SWI), diffusion-weighted and diffusion-tensor imaging (DWI and DTI), magnetic resonance spectroscopy (MRS), and perfusion imaging, including arterial spin labeling (ASL), are being increasingly incorporated into acute pediatric neuroimaging protocols.

Susceptibility-weighted imaging

SWI imaging uses the paramagnetic property of blood products and increases visibility of microhemorrhages by accentuating signal dropout by rapid spin dephasing. It has been shown to be more sensitive than conventional MRI in detecting intraparenchymal blood products after traumatic brain injury in children ( Fig. 1 ). The technical aspects of SWI and its role in the detection of venous thrombosis, intracerebral hemorrhage, and vascular malformations have been described in a number of original studies and reviews. SWI has several advantages over conventional gradient-echo sequences, including its potential ability to differentiate between hemorrhage and calcium, as well as its ability to visualize vessel connectivity and microbleed location in relation to the vasculature and other structures in the brain. Methods for evaluating the arterial and venous systems simultaneously at higher field strengths (3 T and higher) using postcontrast SWI sequences have also been described. By optimizing the flip angle and echo times, the arterial and venous circulations can be separately visualized on maximum and minimum intensity projections. The value of this method has not yet been fully evaluated in children.

Traumatic axonal injury. 5-year-old child status-post blunt head trauma. Note the multiple foci of hemorrhage within the white matter depicted on SWI images ( A, B ).

Magnetic Resonance Angiography and Magnetic Resonance Venography

Magnetic resonance angiography (MRA) has shown considerable utility in the emergency setting for assessing aneurysms and arteriovenous malformations in patients presenting with intracranial hemorrhage, and in identifying dissections of the vertebral and internal carotid arteries following trauma. Recent studies have shown that, in some cases, 2D time-of-flight MRA may not be as sensitive as conventional angiography for detecting vertebral or internal carotid artery dissection in children; however, it is used successfully as an initial screening tool.

MRI with magnetic resonance venography (MRV) is the imaging study of choice in older children being evaluated for dural venous sinus thrombosis owing to their relatively clinically stable condition and their tolerance for longer imaging times compared with infants and toddlers. In addition, MRI/MRV can characterize associated parenchymal lesions without the use of ionizing radiation. One potential pitfall of MRV in neonates, however, is the increased flow gaps in the venous sinuses, particularly affecting the posterior aspect of the superior sagittal sinus, which can be attributed to the age-related smaller caliber of the sinus, smaller venous flow, and skull molding. At our institution, postcontrast, 3D, fast spoiled gradient-echo images (3D-FSPGR) have proved especially useful in cases where MRV is equivocal, or is affected by slow flow, in veins proximal to thrombus.

Acute Hydrocephalus

Hydrocephalus can be caused by either overproduction of cerebrospinal fluid (CSF) by a tumor such as choroid plexus papilloma, by an obstruction to normal CSF flow, or by decreased CSF absorption. The latter 2 problems result in “obstructive hydrocephalus.” Obstructive hydrocephalus can be further divided into communicating hydrocephalus, where there is obstruction to CSF flow or diminished CSF absorption, and noncommunicating hydrocephalus, where there is intraventricular obstruction to CSF flow. More recent classifications that are based on the effect of pressure-flow derangements in CSF dynamics within the circuit diagram of CSF flow are designed to serve as a template for the study, nomenclature, and treatment of hydrocephalus.

In the emergency setting, there are 2 common scenarios that often warrant imaging: (1) the child with shunted hydrocephalus, and (2) the acute presentation of noncommunicating hydrocephalus.

Child with Shunted Hydrocephalus

The child with known shunted hydrocephalus can present with nausea, vomiting, irritability, fever, altered level of consciousness, or increased seizure frequency. Papilledema, cranial nerve palsies, hyperactive reflexes, and ataxic gait may be found on examination. Infants may present with increased head circumference, a bulging fontanelle, or splayed cranial sutures. Neurosurgical consultation is mandatory if shunt malfunction is clinically suspected.

Ultrasound can be considered in the young infant with large, open fontanelles in this setting. However, in most cases, CT is the modality of choice because of its ready availability, ability to detect changes in ventricular size and configuration, easy identification of intracranial hemorrhage or infarction, shunt catheter discontinuity in the skull and upper neck, and catheter migration.

A low-dose CT technique can be used to minimize radiation risks in these patients, especially those in whom multiple CT scans are required over their life span. Several authors have shown that there is no significant loss in diagnostic accuracy when such techniques are used in this clinical setting.

At our institution, a limited axial T2-weighted MRI in an imaging plane similar to that used in prior CT studies is used. This approach offers an attractive imaging alternative for the stable, older child who can lie still for a few minutes in the MR scanner. With the increased availability of MRI, and the advent of faster pulse sequences and motion correction software, hydrocephalus may also be safely and reliably assessed in younger patients without the need for sedation, while being spared the potentially harmful effects of ionizing radiation.

When evaluating a CT in a child with hydrocephalus, comparison with a baseline study obtained after successful shunt placement and serial prior scans can help detect subtle evidence of shunt malfunction. A single shunt may fail in patients with multiple shunts, and comparison with prior studies is essential to determine which shunt is malfunctioning. Ultrasound or MRI examinations performed since the prior CT must be reviewed to avoid errors that result from a failure to recognize changes that may have occurred in the interval between the 2 CTs.

In the patients with suspected shunt malfunction, a plain radiograph shunt series is often performed to differentiate between obstruction and other causes of mechanical shunt malfunction including catheter fracture. It is important not to mistake the radiolucent areas between the intermittent radiopaque markers seen in some shunts for areas of discontinuity. Shunt mechanism disconnection occurs most commonly at the level of the valve where the proximal and distal tubing meet. The actual fracture of the catheter tubing, however, usually occurs in the neck, likely resulting from its increased mobility.

Abdominal ultrasound and/or CT may be useful in determining the presence of ascites or an intra-abdominal mass such as a large CSF pseudocyst. In some cases, shunt malfunction may not always cause absolute or relative ventriculomegaly. CT scans performed in children experiencing true shunt malfunction may be normal, have stable ventriculomegaly, or even show decreased ventricular size compared with prior studies. Neurosurgical consultation is mandatory if shunt malfunction is clinically suspected, despite normal imaging.

Acute Presentation of Noncommunicating Hydrocephalus

Noncommunicating hydrocephalus is caused by intraventricular obstruction of CSF flow. Obstruction sites are most commonly located where the CSF pathway is narrowest, namely at the foramen of Monro, cerebral aqueduct, and fourth ventricle and its outflow foramina. In the child, tumors are a common cause of such obstruction.

Tumors obstructing the foramina of Monro can involve the lateral ventricles (eg, choroid plexus tumors, astrocytomas, and ependymomas) or extend anteriorly from the third ventricle (eg, astrocytomas, choroid plexus tumors, and craniopharyngiomas) ( Fig. 2 ). Similarly, third ventricle tumors can extend posteriorly and inferiorly to obstruct the cerebral aqueduct. Most commonly, aqueductal obstruction is the result of a pineal region tumor including pineal origin tumors; germ cell tumors; astrocytomas arising from the tectal plate, thalamus, or quadrigeminal plate; or tentorial meningiomas and vein of Galen malformations. In patients with tuberous sclerosis, subependymal giant cell tumors originating in the region of the foramen of Monro can grow to obstruct the lateral ventricles.

Optic glioma. 2-year-old girl presenting with sixth nerve palsy. Axial noncontrast CT ( A ) shows obstructive hydrocephalus, and a large mass filling the third ventricle. On postcontrast T1-weighted MRI ( B ), the macrolobulated heterogeneously enhancing suprasellar tumor involves the optic pathway and hypothalamus.

Posterior fossa tumors in the pediatric population include the medulloblastoma, pilocytic astrocytoma, ependymoma, and, occasionally, cerebellar hemangioblastoma. Lesions of this type can cause fourth ventricle or aqueductal obstruction either by tumor extension into the ventricle, or by extrinsic compression of the ventricular walls.

MRI is the imaging modality of choice in these cases, although CT can identify many of these lesions as an initial screen in the emergency room and can demonstrate hydrocephalus. Though rare, primary and secondary (leptomeningeal metastases) tumors in the spine and spinal cord can cause hydrocephalus. The spine should therefore be imaged in all cases of new-onset unexplained hydrocephalus.

Non-neoplastic causes of noncommunicating hydrocephalus include arachnoid cysts, aqueductal stenosis, aqueductal webs, and congenital anomalies such as Chiari malformations. A discussion of these entities is beyond the scope of this article. In our experience, the 3D FIESTA (fast-imaging employing steady-state acquisition) sequence can play a useful role in evaluating new-onset hydrocephalus. This technique enables detection of arachnoid cysts and septations within the ventricles, and it can help confirm patency of the aqueduct. In some cases of Chiari 1 malformation, a phase contrast, velocity-encoded CSF flow study may also demonstrate CSF flow aberrations at the foramen magnum, eg, decreased flow posterior to the cerebellar tonsils.

Communicating Hydrocephalus (Extraventricular Obstruction to CSF Flow)

Extraventricular obstruction to CSF may result from intracranial hemorrhage, bacterial or granulomatous meningitis, CSF seeding of tumor, venous hypertension, or normal pressure hydrocephalus. Some of these entities are discussed in other parts of this article and in other articles in this issue: “Hemorrhagic Stroke and Nontraumatic Intracranial Hemorrhage,” “Central Nervous System Infections,” and “Intracranial Hypo- and Hypertension.”