Physics of Magnetic Resonance Imaging
The body is largely composed of water molecules. Each water molecule has two
hydrogen nuclei or protons. When a person goes inside the powerful magnetic
field of the scanner, the magnetic moments (the measure of its tendency to align
with a magnetic field) of some of these protons changes, and aligns with the
direction of the field.|
In a magnetic resonance imaging (MRI) machine a radio frequency (Rf) transmitter
is briefly turned on, producing an electromagnetic field. The photons of this
field have just the right energy, known as the resonance frequency, to flip the
spin of the aligned protons in the body. As the intensity and duration of
application of the field increase, more aligned spins are affected. After the
field is turned off, the protons decay to the original spin-down state and the
difference in energy between the two states is released as a photon. It's these
photons that produce the electromagnetic signal that the scanner detects. The
frequency the protons resonate at depends on the strength of the magnetic field.
As a result of conservation of energy, this also dictates the frequency of the
released photons. The photons released when the field is removed have an energy,
and therefore a frequency, due to the amount of energy the protons absorbed
while the field was active.
It's this relationship between field-strength and frequency that allows the use
of nuclear magnetic resonance for imaging. Additional magnetic fields are
applied during the scan to make the magnetic field strength depend on the
position within the patient, in turn making the frequency of the released
photons dependent on position in a predictable manner.
Position information can then be recovered from the resulting signal by the use
of a Fourier transform. These fields are created by passing electric currents
through specially-wound solenoids, known as gradient coils. Since these coils
are within the bore of the scanner, there are large forces between them and the
main field coils, producing most of the noise that is heard during operation.
Without efforts to dampen this noise, it can approach 130 decibels (dB) with
strong fields as discussed further down on acoustic noise.
An image can be constructed because the protons in different tissues return to
their equilibrium state at different rates, which is a difference that can be
detected. By changing the parameters on the scanner, this effect is used to
create contrast between different types of body tissue or between other
properties, as in diffusion MRI.
Contrast agents may be injected intravenously to enhance the appearance of blood
vessels, tumors, or inflammation in the case of multiple sclerosis (MS).
Contrast agents are typically used for MS patients to help determine if there is
any active disease progression. Unlike computed tomography (CT), MRI uses no
ionizing radiation and is generally a very safe procedure. Nonetheless the
strong magnetic fields and radio pulses can affect metal implants, including
cochlear implants and cardiac pacemakers. In the case of cardiac pacemakers, the
results can sometimes be lethal, so patients with such implants are generally
not eligible for MRI.
MRI is used to image every part of the body, and is particularly useful for
tissues with many hydrogen nuclei and little density contrast, such as the
brain, spinal cord, muscle, connective tissue and most tumors.
Death and injuries have occurred from projectiles created by the magnetic field,
although few compared to the millions of examinations administered. MRI makes
use of powerful magnetic fields that, though not known to cause direct
biological damage, can interfere with metallic and electromechanical devices.
Additional (small) risks are presented by the RF systems, components or elements
of the MRI system's operation, elements of the scanning procedure and
medications that may be administered to facilitate MRI imaging.
There are many steps that the MRI patient and referring physician can take to
help reduce the remaining risks, including providing a full, accurate and
thorough medical history to the MRI provider.
Implants and foreign bodies
Pacemakers are generally considered an absolute contraindication towards MRI
scanning, though highly specialized protocols have been developed to permit
scanning of select pacing devices. Several cases of arrhythmia or death have
been reported in patients with pacemakers who have undergone MRI scanning
without appropriate precautions. Other electronic implants have varying
contraindications, depending upon scanner technology, and implant properties,
scanning protocols and anatomy being imaged.
In the case of pacemakers, the risk is thought to be primarily RF induction in
the pacing electrodes/wires causing inappropriate pacing of the heart, rather
than the magnetic field affecting the pacemaker itself.
Ferromagnetic foreign bodies such as metal fragments, or metallic implants such
as surgical prostheses and aneurysm clips are also potential risks, and safety
aspects need to be considered on an individual basis. Interaction of the
magnetic and radio frequency fields with such objects can lead to trauma due to
movement of the object in the magnetic field, thermal injury from Rf induction
heating of the object, or failure of an implanted device. These issues are
especially problematic when dealing with the eye. Most MRI centers require an
orbital x-ray to be performed on anyone suspected of having metal fragments in
their eyes, something not uncommon in metalworking.
Because of its non-ferromagnetic nature and poor electrical conductivity,
titanium and its alloys are useful for long term implants and surgical
instruments intended for use in image-guided surgery. In particular, not only is
titanium safe from movement from the magnetic field, but artifacts around the
implant are less frequent and less severe than with more ferromagnetic materials
such as stainless steel. Artifacts from metal frequently appear as regions of
empty space around the implant that are frequently called a "black-hole
artifact". For example, a 3 mm titanium alloy coronary stent may appear as a 5
mm diameter region of empty space on MRI, whereas around a stainless steel
stent, the artifact may extend for 10–20 mm or more.
Switching of field gradients causes a change in the Lorentz force experienced by
the gradient coils, producing minute expansions and contractions of the coil
itself. As the switching is typically in the audible frequency range, the
resulting vibration produces loud noises (clicking or beeping). This is most
marked with high-field machines and rapid-imaging techniques in which sound
intensity can reach 120 dB(A) (equivalent to a jet engine at take-off). As a
reference, 120 dB is the threshold of loudness causing sensation in the human
ear canal — tickling, and 140 dB is the threshold of ear pain. Since decibel is
a logarithmic measurement, a 10 dB increase equates to a 10-fold increase in
intensity — which, in acoustics, is roughly equal to a doubling of loudness.
The use of ear protection is essential for anyone inside the MRI scanner room
during the examination.
The most commonly used intravenous contrast agents are based on a chemical
compound of Gadolinium (gd). In general, these agents have proved safer than the
iodinated contrast agents used in X-ray radiography or CT. Anaphylactoid
reactions are rare, occurring in approximately 0.03–0.1%. Of particular interest
is the lower incidence of nephrotoxicity, compared with iodinated agents, when
given at usual doses — this has made contrast-enhanced MRI scanning an option
for patients with renal impairment, who would otherwise not be able to undergo
Although Gadolinium agents have proved useful for patients with renal
impairment, in patients with severe renal failure requiring dialysis there is a
risk of a rare but serious illness, nephrogenic systemic fibrosis, that may be
linked to the use of certain gadolinium-containing agents. The most frequently
linked is gadodiamide, but other agents have been linked too. Although a causal
link has not been definitively established, current guidelines in the United
States are that dialysis patients should only receive gadolinium agents where
essential, and that dialysis should be performed as soon as possible after the
scan to remove the agent from the body promptly.
No known effects of MRI on the fetus have been demonstrated. In particular, MRI
avoids the use of ionizing radiation, to which the fetus is particularly
sensitive. However, as a precaution, current guidelines recommend that pregnant
women undergo MRI only when essential. This is particularly the case during the
first trimester of pregnancy, as organogenesis takes place during this period.
The concerns in pregnancy are the same as for MRI in general, but the fetus may
be more sensitive to the effects. However, one additional concern is the use of
contrast agents; gadolinium compounds are known to cross the placenta and enter
the fetal bloodstream, and it's typically recommended that their use be avoided.
Despite these concerns, MRI is rapidly growing in importance as a way of
diagnosing and monitoring congenital defects of the fetus because it can provide
more diagnostic information than ultrasound and it lacks the ionizing radiation
Claustrophobia and discomfort
Due to the construction of some MRI scanners, they can be potentially unpleasant
to lie in. Older models of closed bore MRI systems feature a fairly long tube or
tunnel. The part of the body being imaged must lie at the center of the magnet,
which is at the absolute center of the tunnel. Because scan times on these older
scanners may be long (occasionally up an hour for the entire procedure), people
with even mild claustrophobia are sometimes unable to tolerate an MRI scan
without management. Modern scanners may have larger bores and scan times are
shorter. This means that claustrophobia is less of an issue, and many patients
now find MRI an innocuous and easily tolerated procedure.
Alternative scanner designs, such as open or upright systems, can also be
helpful where these are available. Though open scanners have increased in
popularity, they produce inferior scan quality because they operate at lower
magnetic fields than closed scanners. However, commercial 1.5 tesla open systems
have recently become available, providing much better image quality than
previous lower field strength open models.