Chapter 16

#### High-Yield Facts

• With currently existing fast magnetic resonance imaging (MRI) techniques, it is possible to evaluate for shunt malfunction in less than 90 seconds.

• CT findings of ischemic stroke lag far behind diffusion-weighted MRI findings, which can detect edema within the first hour after the event.

• MRI is the imaging modality recommended by the American College of Radiology for identifying appendicitis in pregnant patients who have a nondiagnostic ultrasound.

• Whole-body MRI is used in metastatic cancer staging and has a higher sensitivity and specificity over scintigraphy in this application. Image acquisition times of 30 to 45 minutes are similar.

• 3.0T magnets are becoming more widely available and provide thinner-slice, higher-resolution images in a shorter amount of time than the current standard 1.5T magnet machines.

#### Magnetic Resonance Imaging Basics

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 super conduction. 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.

#### MR Precautions

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, metallic clothing should be taken off. There is also the danger of magnetic attraction of nearby devices and equipment, which can lead to missiles 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, ...

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