Fast MRI techniques such as single-shot fast spin echo (SSFSE) provides T2-weighted images within 20 seconds and echo planar imaging (EPI) provides T1-weighted images within 90 seconds. These techniques can image the pediatric brain without the need for sedation; however, there is poorer gray–white differentiation with SSFSE and artifact around the skull base with the EPI sequences.7 Due to these limitations, the techniques may not yet be applied for evaluation of demyelination or migration disorders, as well as for evaluating acute hemorrhage.7,8
Although CT has been the common method of assessing ventriculoperitoneal shunt function, at two CT scans per year to age 20 years, the accumulated radiation exposure has an attributable risk of developing a fatal cancer of 1 in 230.9 Elimination of CT scans in this population could have a significant impact on radiation-associated disease. Using fast MRI techniques, visualization of the size and configuration of ventricles has been demonstrated, although there may be poorer visualization of the shunt catheter and the detection of hemorrhage.10,11 Figure 16-1 demonstrates shunt malfunction in a 7-year-old boy with shunted hydrocephalus due to aqueductal stenosis using a T2-weighted SSFSE image obtained in 30 seconds. Some institutions already have in place a protocol for rapid MRI upon suspicion of shunt malfunction using these techniques.11 Image acquisition is fast enough such that they are performed without sedation.
T2-weighted SSFSE sequence obtained in 30 seconds demonstrating ventriculomegaly from shunt malfunction in a 7-year-old boy with shunted hydrocephalus from aqueductal stenosis presenting with vomiting, headache, and lethargy. (Used with permission from Dr. Bogna Targonska.)
MRI has the potential to replace CT scans for emergency department (ED) brain imaging for other causes as well. The most recent American College of Radiology (ACR) appropriateness criteria from 2012 recommend MRI as the initial study for children with chronic headache or headache with signs of increased intracranial pressure and neurologic signs, assuming MRI availability.12 CT is recommended first-line evaluation technique only for those presenting with sudden-onset severe headache concerning for vascular rupture, due to its superior ease of detecting acute hemorrhage. When compared with standard MR sequences, these fast MR techniques were noted to perform well with respect to detection of acute ischemia, infection, hydrocephalus, and tumor.8 These fast sequences did not perform as well as standard MR sequences with respect to congenital malformations, specifically those with cortical abnormalities and detecting subarachnoid hemorrhage (SAH).13 When compared with CT, these fast techniques were noted to be clearly superior, detecting 38% of abnormalities, mainly in the posterior or middle cranial fossae, that CT did not.7 The abnormalities missed by CT scan included ischemia/infarction, encephalitis, mastoiditis, thrombosis, parenchymal hemorrhage, and contusions. Standard MR sequences performed well at detecting subacute hemorrhage on gradient echo T2* and fluid attenuatedinversion recovery (FLAIR) sequences, especially with elucidating associated ischemia. CT still remains the standard for imaging acute SAH. Often, the need for brain imaging in the ED, if not concerned about SAH, is to evaluate for possible mass effect or hydrocephalus to plan ongoing therapeutic management and evaluation. These fast techniques appear to adequately screen for these or at least determine the need for traditional MRI sequences.
For ataxia, ACR criteria recommends first-line evaluation with MR, except in the setting of acute trauma,14 since the posterior fossa is poorly visualized by CT. Ataxia can be a manifestation of stroke, infection, demyelination, degenerative processes, or masses, all of which are detected with great detail by MRI. With the advent of the fast techniques mentioned previously, the ataxic child presenting to the ED may be able to have a MR image which is much more sensitive at detecting lesions in the area of concern.
CT is still the mainstay of evaluation of an acutely head-injured child with evidence of significant trauma or neurologic deficit due to its superior ability to detect acute hemorrhage (especially SAH), bony injury, and its immediate availability. However, there may be a role for MRI in the evaluation of the stable, subacutely injured child, including those who may present for evaluation of nonaccidental trauma. In the stable child, without evidence of significant acute injury and nonfocal neurologic examination, the patient may benefit more from having a MRI if immediate neurosurgical intervention is unlikely.15 MRI is more sensitive than CT in evaluating parenchymal injury including contusions, diffuse axonal injury, edema, and early ischemia. It is also superior at differentiating chronic subdural hemorrhage from benign collections and further defining small subdural collections close to the calvarium which may not be detected by CT.16 The extra information provided by MRI may impact the forensic evidence in the case of proven inflicted injury, as well as suggesting prognosis and need for ongoing therapeutic treatments for the injured child.17
Pediatric stroke is due to both arterial ischemic insufficiency and hemorrhage, with a very small percentage due to sinus venous thrombosis. Congenital abnormalities, infections, hematologic abnormalities, vascular malformations, congenital heart disease, hypercoagulable states, and vasculopathies are some causes of arterial ischemic pediatric stroke. Sickle cell disease and congenital heart disease are the underlying etiology in the majority of cases. The presentation of arterial ischemic stroke in children may be subtle, with focal neurologic deficits, seizures, altered mental status, and headache. CT findings of ischemic stroke lag far behind diffusion-weighted MRI findings which can detect the early effects of cytotoxic edema from ischemia as early as 30 to 45 minutes after the event.18 Figure 16-2 demonstrates this contrast in a 16-year-old baseball player with sudden-onset collapse, dense right hemiparesis, and aphasia. CT is also inferior to MRI at detecting sinus venous thrombosis. The addition of diffusion-weighted imaging sequences in addition to standard sequences can be used to detect acute ischemia and are highly resistant to motion artifact and can be obtained in less than 2 minutes.18 CT has advantages for hemorrhagic stroke (including intraparenchymal and SAH), which accounts for half of pediatric stroke, and is heralded by abrupt clinical onset and subsequent neurologic deterioration. However, early MRI to follow closely after normal CT or to replace CT in stable patients (if MRI is immediately available) would add more information to the diagnosis, specifically with additional information about early ischemia. MR angiography could also be obtained at the same time if vascular malformation is suspected. ACR appropriateness criteria on cerebrovascular disease almost universally recommend MRI over CT if available, with the exception of suspected SAH.19 Although traditional sequences have not been able to reliably identify SAH, more recent experience with alternative sequences such as T2* gradient echo and FLAIR have allowed for improved recognition and characterization of intracranial blood, including SAH.20
CT head and comparable MRI DWI sequence for a 16-year-old baseball player diagnosed with left middle cerebral artery (MCA) ischemic stroke who presented with sudden collapse, right hemiparesis, and aphasia. CT head is essentially normal, whereas MRI with DWI demonstrates restricted diffusion in the distribution of the left MCA. (Used with permission from Dr. Bogna Targonska.)