Imaging in an emergency setting demands assessment in the shortest possible time. While computed tomography (CT) scan has been the mainstay of cross-sectional imaging in emergency radiology for its rapid image acquisition, magnetic resonance imaging (MRI) can be useful in the emergency setting as well. MRI, with its inherent superior soft-tissue contrast, is highly sensitive for detection of abnormal fluid or edema, thus obviating the need for contrast agents. Lack of ionizing radiation makes it an obvious choice in the abdominal evaluation in pregnant women, and in the pediatric population. Gadolinium based contrast agents are safer than iodinated contrast agents used in CT.1,2 Furthermore, current refinements in MRI technology have resulted in shorter scanning times, which are particularly suited for emergency indications. MRI is therefore not only being increasingly used as a problem-solving tool, but also as a first-line modality. This chapter outlines the indications for MRI as well as the basic sequences and protocols used with an emphasis on the diagnostic approach to emergencies.
BASIC PRINCIPLES AND INSTRUMENTATION
MRI makes use of the fact that water-containing hydrogen nuclei (protons) forms a major constituent of the human body. Every spinning hydrogen proton has a small magnetic field around it. The hydrogen nuclei, to begin with, are randomly placed along their own magnetic field. For the orientation in space, conventionally, Z-axis is considered as the long axis of the patient as well as bore of the magnet. Once inside the magnetic field of the scanner, some of the protons align themselves along the direction of the magnetic field and others anti-parallel to it. When protons align, not only do they rotate or spin around themselves, but also their axis of rotation moves (which is termed “precession”). There are always more protons spinning parallel to the Z-axis than opposite to it. Forces of these protons add to form a magnetic vector along the Z-axis, a process called “longitudinal magnetization.” A radiofrequency current, known as “resonance frequency,” is then applied, which causes the protons to flip. Some of these protons go to a higher energy level and start precessing in an anti-parallel direction, which reduces the magnitude of longitudinal magnetization. The energy of the protons then adds to form a new magnetic vector in the transverse (X-Y) plane. This is called “transverse magnetization.” When the current is switched off, the protons realign. During this period of relaxation, receiver coils measure the generated signal, and then a computer converts the signal to images via a complex process called Fourier transformation (Figure 5–1).
Illustration showing mechanism of MRI and basic terminologies like TR (repetition time) and TE (time to echo). [(Illustration courtesy of Lakshmanan Subramanian-Coimbatore, India.)
Magnet: Magnet strengths of 1.5 Tesla (T) ...
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