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.|
Magnetism is a property of matter that is a result of the orbiting electrons
in atoms. The orbiting electrons cause the atoms to have a magnetic moment
associated with an intrinsic angular momentum called 'spin'.
Magnetic field strengths are measured in units of gauss (G) and Tesla (T).
One Tesla is equal to 10,000 gauss. The earth's magnetic field is about 0.5
gauss. The strength of electromagnets used to pick up cars in junk yards is
about the field strength of MRI machines (1.5-2.0T).
The echo time refers to time between the application of radiofrequency
excitation pulse and the peak of the signal induced in the coil. It's measured in
milliseconds. The amount of T2 relaxation is controlled by TE.
The repetition time or (TR) is the time from the application of an
excitation pulse to the application of the next pulse. It determines how much longitudinal
magnetization recovers between each pulse. It's measured in milliseconds.
The Bo in MRI refers to the main magnetic field and is measured in Tesla.
The majority of MRI systems in clinical use are between 1.5T and 3T. Altering the field
strength will affect the Larmour frequency at which the protons precess.
Resonance and Radiofrequency
Protons in a magnetic field have a microscopic magnetisation and act like tiny
toy tops that wobble as they spin.The rate of the wobbling or precession is the
resonance or Larmor frequency. In the magnetic field of an MRI scanner at room
temperature, there is approximately the same number of proton nuclei aligned with
the main magnetic field B0 as counter aligned. The aligned position is slightly
favored, as the nucleus is at a lower energy in this position. For every one-million
nuclei, there is about one extra aligned with the B0 field as opposed to the field.
This results in a net or macroscopic magnetisation pointing in the direction of the
main magnetic field. Exposure of individual nuclei to radiofrequency (RF) radiation
(B1 field) at the Larmor frequency causes nuclei in the lower energy state to jump
into the higher energy state.
On a macroscopic level, exposure of an object or person to RF radiation at the Larmor
frequency, causes the net magnetisation to spiral away from the B0 field. In the
rotating frame of reference, the net magnetisation vector rotate from a longitudinal
position a distance proportional to the time length of the RF pulse. After a certain
length of time, the net magnetization vector rotates 90 degrees and lies in the
transverse or x-y plane. It is in this position that the net magnetisation can be
detected on MRI. The angle that the net magnetisation vector rotates is commonly
called the 'flip' or 'tip' angle. At angles greater than or less than 90 degrees
there will still be a small component of the magnetisation that will be in the x-y
plane, and therefore be detected.
MRI image contrast is influenced by several characteristics of tissues and other
materials including: T1, T2 and T2* relaxation as well as spin density,
susceptibility effects and flow effects.
Relaxation is the process in which spins release the energy received from a
T1 and T2 relaxation rates affect the SNR in an image. Improvement in the
SNR is seen when the TR is increased significantly to about 3-5 T1 times.
Changing the TR time will also affect the T1 weighting of the image and the
acquisition time. T1 weighting occurs in a short TR spin echo sequence because
of incomplete recovery of longitudinal magnetization.
The T1 relaxation time, also known as the spin-lattice relaxation time, is a
measure of how quickly the net magnetisation vector (NMV) recovers to its ground
state in the direction of B0. The return of excited nuclei from the high energy
state to the low energy or ground state is associated with loss of energy to the
surrounding nuclei. Nuclear magnetic resonance was originally used to examine
solids in the form of lattices, hence the name "spin-lattice" relaxation. Two
other forms of relaxation are the T2 relaxation time (spin-spin relaxation)
and T2* relaxation.
Now for all of you science and/or math geeks out there, and you know who you are,
the length of the net magnetisation vector for a spin echo sequence is given by
the following equation:
Mt = Mmax
Where Mt is the magnetisation at time =
t, the time after the 90o pulse,
Mmax is the maximum magnetisation at
Another term that you may hear is the T1 relaxation rate. This is merely the
reciprocal of the T1 time (1/T1). T1 relaxation is fastest when the motion of
the nucleus (rotations and translations or "tumbling rate") matches that of the
Larmor frequency. As a result, T1 relaxation is dependent on the main magnetic
field strength that specifies the Larmor frequency. Higher magnetic fields are
associated with longer T1 times.
T1 weighted images can be obtained using an inversion recovery sequence or by
setting short TR (<750ms) and TE (<40ms) values in conventional spin echo
sequences. In gradient echo sequences, T1WI can be obtained by using flip angles
over 50o and setting the TE to less than 15 ms.
T2 relaxation refers to the progressive dephasing of spinning dipoles following
the 90° pulse as seen in a spin-echo sequence due to tissue-particular characteristics,
primarily those that affect the rate of movement of protons, most of which are found
in water molecules. This is alternatively known as spin-spin relaxation.
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. 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.
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 (clicking or
beeping)that is heard during operation. This is most marked with high-field
machines and rapid-imaging techniques in which sound intensity can reach 120
decibels (dB) (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.
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.
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.