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 ...