We greatly appreciate the author of the fourth edition chapter Jannet J. Lee-Jayaram, MD. This fifth edition chapter is an update of her previous chapter.

Magnetic resonance (MR) imaging is a technique that uses an external magnetic field and its specific effect on various atomic nuclei to produce detailed images of different tissues in the body. The magnet in the MR machine is large and cylindrically coiled, surrounded by a cooling agent such as liquid helium to allow for magnet superconduction. Computers generate the pulse sequences, collect the data, and transform the data into interpretable images. MR contrast agents are typically gadolinium-based agents. The major advantages of MR imaging (MRI) are the excellent detail in various planes when imaging areas require soft-tissue differentiation and the lack of ionizing radiation to the body. Greater awareness of the risks of ionizing radiation, including the increased risk of cancer and cognitive harm, has also led to expanding the applications of MRI to the evaluation and management of pediatric disease.^1,2 While CT imaging depends solely on the density gradients of tissues, MRI is a complex function of small differences in the tissues’ excitability from the applied magnetic field. MRI provides more flexibility for radiologists with the use of various sequences to elucidate fine details in differences of the tissues depending on the area of the body to be imaged. Areas of the body poorly visualized by CT due to artifact, such as the posterior fossa of the brain, are well defined on images obtained by MRI.

There are no established biologic effects associated with exposure to current medical standard MRI; however, few precautions do exist with respect to the powerful magnet in the machine,³ such as implanted or embedded electric or ferromagnetic devices. A thorough screening of each patient should take place for such devices or objects and all removable items such as jewelry, hair clips, and metallic clothing should be removed. There is also the danger of magnetic attraction of nearby devices and equipment, which can result in projectiles that may injure the patient and staff. Most resuscitation equipment cannot be brought in close proximity to the magnet, making clinically unstable patients unsuitable for MRI. Other effects include the heating of tissues, peripheral nerve stimulation, thermal injury caused by heating of applied patches or adhesives, and claustrophobia.⁴ Compared with agents used for CT, MR agents have typically fewer side effects and anaphylactoid reactions.⁵ However, a rare and possibly fatal nephrogenic systemic fibrosis that is related to concomitant severe renal dysfunction and the administration of gadolinium-based contrast agents⁶ should be included in the informed consent process for those patients receiving contrast for MRI.

When ordering an MRI, consultation with a radiologist facilitates the determination of the most optimal sequences that need to be obtained. Although a few sequences are discussed in the following section, these are neither comprehensive nor detailed and are for the purpose of general familiarity for the nonradiologist emergency medicine provider.

Fast MRI techniques such as single-shot fast spin echo (SSFSE) provide 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 rapid techniques may not yet be applied for evaluation of demyelination or migration disorders or for evaluating acute hemorrhage.^7,8

HYDROCEPHALUS

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 230⁹ in addition to the potential for cumulative cognitive harm on the developing brain in young children.¹ Elimination of CT scans in this population could have a significant impact on radiation-associated harm. 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.^10–17 Figure 17-1 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.

OTHER BRAIN

MRI has the potential to replace CT scans for emergency department (ED) brain imaging for other causes as well. Recent American College of Radiology (ACR) appropriateness criteria publications recommend MRI as the initial study for children with chronic headache, signs of increased intracranial pressure, abnormal neurologic signs, or when brain tumor is suspected.^18,19 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, rapid MR techniques were noted to perform well with respect to detection of acute ischemia, infection, hydrocephalus, and tumor,⁸ but they did not perform as well as standard MR sequences with respect to congenital malformations, specifically those with cortical abnormalities and detecting subarachnoid hemorrhage (SAH).²⁰ Rapid MRI was noted to superior to CT in detecting more abnormalities in the posterior or middle cranial fossae.⁷ The abnormalities missed by CT scan included ischemia/infarction, encephalitis, mastoiditis, thrombosis, parenchymal hemorrhage, and contusions. If there is a suspicion for SAH, then gradient echo (GRE), susceptibility-weighted imaging (SWI), and fluid-attenuated inversion recovery (FLAIR) sequences should be included,¹⁹ although CT is classically utilized 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 rapid MRI techniques appear to adequately screen for these or at least determine the need for traditional MRI sequences.

Ataxia can be a manifestation of stroke, infection, demyelination, degenerative processes, or masses, all of which are detected with great detail by MRI.

TRAUMATIC BRAIN INJURY

ACR’s appropriateness criteria for head trauma in children states:

Other reports further describe strategies to reduce radiation exposure using MRI instead of CT in children.^22–25

STROKE

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.^25,26 Figure 17-2 demonstrates this contrast in a 16-year-old baseball player with sudden-onset collapse, dense right hemiparesis, and aphasia. The addition of diffusion-weighted imaging sequences in addition to standard sequences can be used to detect acute ischemia, are highly resistant to motion artifact, and can be obtained in less than 2 minutes.²⁶ 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. Early MRI to follow closely after normal CT or to replace CT in stable patients would add more information to the diagnosis, specifically with additional information about early ischemia; however, its limited immediate availability still requires the frequent use of CT.^27–31 MR angiography could also be obtained at the same time if vascular malformation is suspected.