Chapter e299

MRI has several major advantages over other imaging modalities.

First, in contrast to CT and other x-ray methods, which may have long-term carcinogenic effects owing to ionizing radiation, MRI itself does not have known short- or long-term adverse consequences. (The use of gadolinium is a separate issue.) Given its better safety profile, MRI is often preferable to CT, especially for pediatric and childbearing female patients.

Second, MRI provides better contrast resolution and tissue discrimination, and scanning parameters can be adjusted to create pictures with tissue contrast best suited to the clinical rationale for imaging. Thus, MRI is often the modality of choice for visualizing the brain, spinal cord, bone marrow, muscles, ligaments, tendons, heart, vessels, and solid abdominal and pelvic organs.

Third, MRI produces variable-thickness, two-dimensional slices in any orientation through the body region of interest, enabling optimal evaluation of the anatomic relationships between structures.

MRI is based on the principle of nuclear magnetic resonance, in which energy emissions from nuclei in the presence of an applied, external magnetic field are detected after the nuclei have been stimulated by radio waves with the same, or resonant, frequency as the nuclei themselves. Though the descriptor “nuclear” was abandoned early in the development of MRI due to negative connotations among a Cold War era public, the technique fundamentally involves probing nuclei, exploiting differences in their response to a perturbing radiofrequency (RF) pulse based on their variable molecular environments within the body. Radiography and CT, in contrast, rely on interactions between x-rays and the cloud of electrons external to the nucleus. The atom of paramount interest in MRI is hydrogen, based on its sheer abundance within the body, numbering more than 1020 nuclei per cubic centimeter of tissue. As hydrogen nuclei do not contain any neutrons, they are often referred to simply as protons.

Protons have a magnetic axis and are often likened to small bar magnets with positive and negative poles. When placed in an external magnetic field, protons align themselves and precess, or spin, around an axis parallel to the field at a frequency directly proportional to the field strength, their so-called resonant frequency. In so doing, they generate secondary magnetic fields, collectively referred to as net magnetization. At equilibrium, net tissue magnetization is aligned with, and dwarfed by, the main magnetic field. To make use of net magnetization to produce images, it is necessary to excite the protons by delivering an RF pulse at their resonant frequency, thereby knocking net magnetization out of the longitudinal plane (parallel to the main field) into the transverse plane (perpendicular to the main field).

As the protons return to their equilibrium state, a phenomenon termed relaxation, they emit their own RF signal, an “echo” of the excitatory RF pulse. By providing a window into the particular molecular environment of the resonating protons, the data collected from these echoes ...

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