Resonance Imaging (MRI)
does an MRI scanner work
A MRI (Magnetic Resonance Imaging) scanner consists
a large magnet housing, a table, coil(s), & associated
computers. The magnet housing consists of the magnet
proper & the gradient core. The table traverses
through the middle (core) of the magnet to place the
part of the body to be examined, at the center of
the magnet, where the magnet field is most homogeneous.
The Physical Basics of MRI
There are billions of atoms in a human being. The
nucleus of an atom spins, or precesses, on an axis.
One can think of the nucleus of an atom as a top spinning
off its axis.
In common MRI study, we are only interested in hydrogen
atoms in the human body. Each of these hydrogen atom
precesses along a certain random diection. However,
placing these hydrogen atoms in a strong magnetic
field changes the directions of these hydrogen atoms.
Each hydrogen atom tends to line up, either along
or against, the direction of the external magnetic
field. If we add up all the hydrogen atoms that precess
along the direction of the external magnetic view,
the total will almost equates the total of the hydrogen
atoms that precess against the direction of the external
magnetic field. However, there is still a few excess
hydrogen atoms that precess in one direction (the
number depends on the environment of these hydrogen
atoms & the strength of the magnetic field)
Inside the magnet housing, in addition to the main
magnet, there are three sets of gradient magnets.
They are turned on and off very rapidly so as to alter
the main magnetic field within a small volume of the
body. This is basis for imaging the body in different
Apart from the main magnet, the MR scanner also come
with coils. These coils work to transfer energy to
the precessing hydrogen atoms. They do it by applying
radiofrequency (RF) pulses that are specific only
to hydrogen atoms. The RF pulses change the net precession
direction of the hydrogen atoms. These RF pulses also
change the frequency of the net spin. That specific
frequency is called the Larmour frequency. When the
RF pulses are turned off, the hydrogen atoms return
to their natural (original) alignment within the magnetic
field and release their excess applied energy. These
energies are pick up by the same coils. These are
sent to the computer system, where the signals are
configured into images.
The Basis of MRI Medical Imaging
MRI is very effective in providing information on
the anatomy (structure) & pathology (abnormalities)
of the body. Hydrogen atoms are present in most type
of molecules within the human bodies. Hydrogen atoms
in different types of molecules precess differently.
These different types of hydrogen atoms will be presented
as difference in signal in a MRI image. Together with
the positions & the shapes of the organs, these
different signal appearances are the basic for anatomic
information of MRI. Similary, by using different applied
magnetic strength with the coil, the MRI system can
cause tissues in the body to take on different appearances.
This is the basic for recognition of pathology.
Similarly, by manipulating the volume, characteristics,
& strength of the applied local magnetic field,
MRI systems can also image flowing blood. This is
the basic for MR angiograms.
Precautions while inside the MR suite
Strict precautions should be observed at all times
within the MRI suite.
Within a strong magnetic field, metal objects can
become dangerous projectiles. The magnetic force exerted
on an object increases exponentially as it nears the
magnet. Prior to allowing a patient or support staff
member into the scan room, he or she is thoroughly
screened for metal objects
Metallic fragments in the eye can move within a strong
metallic field to cause eye damage or blindness.
Strong metallic fields can disable
or adversely affect the workings of pacemakers.
Aneurysm clips in the brain may
be displaced. This may tear the artery
Some dental implants are magnetic
Most orthopedic implants, even
though they may be ferromagnetic, are fine because
they are firmly embedded in bone
surgical staples in most parts
of the body are fine -- once they have been in a
patient for a few weeks (usually six weeks), enough
scar tissue has formed to hold them in place.
cochlear implants and ocular prostheses
Credit cards, bank cards, Octopus cards,
and anything else with magnetic encoding will be erased
by MRI system. .. Even metal
There are no known biological hazards to humans from
being exposed to magnetic fields of the strength used
in medical imaging today.
Most facilities prefer not to image pregnant women.
This is due to the fact that there has not been much
research done in the area of biological effects on a
developing fetus. The first trimester in a pregnancy
is the most critical because that is the time of the
most rapid cellular reproduction and division. The decision
of whether or not to scan a pregnant patient is made
on a case-by-case basis with consultation between the
MRI radiologist and the patient's obstetrician. The
benefit of performing the scan must outweigh the risk,
however small, to the fetus and mother. Pregnant MRI
technologists can still work in the department. In most
cases, they are simply kept out of the actual scan room
during their pregnancy.
Most imaging modalities use injectable contrast.
MRI contrast works by altering the local magnetic
field in the tissue being examined
• Diagnosing multiple sclerosis
• Diagnosing tumors of the pituitary gland and
• Diagnosing infections in the brain, spine
• Visualizing torn ligaments in the wrist, knee
• Visualizing shoulder injuries
• Diagnosing tendonitis
• Evaluating masses in the soft tissues of the
• Evaluating bone tumors, cysts and bulging
or herniated discs in the spine
• Diagnosing strokes in their earliest stages
The fact that MRI systems do not use ionizing radiation
is a comfort to many patients, as is the fact that
MRI contrast materials have a very low incidence of
side effects. Another major advantage of MRI is its
ability to image in any plane
There are many people who cannot
safely be scanned with MRI (for example, because
they have pacemakers), and also people who are too
big to be scanned.
There are many claustrophobic people
in the world, and being in an MRI machine can be
a very disconcerting experience for them.
The machine makes a tremendous
amount of noise during a scan. The noise sounds
like a continual, rapid hammering.
MRI scans require patients to hold
very still for extended periods of time. MRI exams
can range in length from 20 minutes to 90 minutes
or more. Even very slight movement of the part being
scanned can cause very distorted images that will
have to be repeated.
Orthopedic hardware (screws, plates,
artificial joints) in the area of a scan can cause
severe artifacts (distortions) on the images.
Diseases of the BRAIN
The brain is affected by a variety of disease processes.
The common diseases of the brain are
Due to the thick bone of the encasing skull, MR is the
most effective method to study the brain. However, in a
few circumstances, CT may be needed.
Stroke is the sudden loss of a brain function. It is caused
by a lack of blood flow to part of the brain. The flow of
blood through a blood vessel can be blocked in two ways.
One is by obstructing the vessel. The other is by interrupting
the vessel. Thus, there are two main types of stroke, ischemic
stroke and hemorrhagic stroke. Ischemic strokes, which account
for about 80 percent of all strokes, are caused by an obstruction
in an artery. The obstruction can arise from the wall of
the artery or a clot within the lumen of the artery. The
obstructive wall abnormality usually begins as an atherosclerotic
plaque. In atherosclerosis, fatty deposits build up on the
inner wall of an artery. As the fatty deposit grows, it
narrows the lumen through which blood can flow. A clot can
arise anywhere between the heart & the brain. In fact,
as atherosclerotic plaques enlarge, raw areas (ulcers) can
form on their surfaces. The fragments from these ulcers
may break loose & travel to a small vessel, where they
obstruct the flow of blood in that vessel
A special type of ischemic stroke is transient ischemic
attack (TIA). In a TIA, also known as a mini-stroke, stroke-like
symptoms develop but disappear within five minutes to 24
hours. A TIA occurs when a clot develops at the site of
an atherosclerotic deposit but dissolves right away, or
an embolism lodges in a narrowed vessel but is soon dislodged
on its own. Regardless of the cause, the oxygen deprivation
is not severe enough to kill brain cells, and the cells
are able to bounce back from their injury. About 10 percent
of ischemic strokes are preceded by TIAs.
MR is the method of choice to detect ischemic stroke. With
the use of modern medication, early ischemic stroke may
be cured, without leaving permanent problems.
Hemorrhagic strokes account for the remaining 20 percent
of all strokes. They occur when weakened blood vessels within
the brain rupture and bleed into the surrounding tissue.
Blood vessels can be weakened in several ways. An aneurysm
is a focal bulge in the wall of the blood vessel. With time,
this bulge expands, resulting in a balloon-like abnormality.
As it enlarges, its wall is progressively thinned &
weakened. It will eventually rupture. Another vessel abnormality
is an arteriovenous malformation (AVM). It is a cluster
of enlarged, structurally weak blood vessels that forms
at or before birth.
In hemorrhagic stroke, the escaped blood compresses nearby
blood vessels, cutting off blood flow and depriving the
surrounding tissue of oxygen. Though hemorrhagic strokes
occur less frequently than ischemic strokes, they tend to
affect larger areas of the brain. Symptoms of a hemorrhagic
stroke may be more sudden and more severe. The patient usually
complains of severe headache. These strokes also carry a
greater risk of death than ischemic strokes.
Early hemorrhagic stroke is best to detect with CT. However,
with aneurysm, MR can often detect these without use of
Bleeding can occur inside, as well as outside the brain.
The bleed that occurs outside the bleed is usually due to
subdural bleed. The bleeding is in the space between the
brain & the brain’s covering. Blood can slowly
accumulate, with progressive symptoms.
Subdural bleeds (hematomas) are most frequently the result
of a head injury. They can occur spontaneously in the elderly,
but this is less common. Subdural hematomas are seen in
approximately 15% of all head traumas. Those who are very
young or very old, those on anticoagulation therapy, &
those with chronic alcohol use are more at rik. Tiny "bridging
veins" that run between the surface of the brain and
its outer covering (dura) stretch and tear, allowing blood
to collect. Depending on the time of development of the
subdural hematoma, it is classified as acute, subacute,
and chronic. These patients usually complain of headache
& may have change of mental state.
Tumor may occur in the brain or its covering. These tumors
may arise from the brain or elsewhere. Again, MR is the
best method to detect these tumors. However, contrast may
Diseases of the Spine
There are many pathologic processes affecting the spinal
column. The common ones are
• Slipped disc
• Abnormal curvature
Slipped disc is a common cause for low back pain. Each
disc consists of a central gelatinous material surrounded
by layers of fibrous tissue. As one ages, these fibrous
layers develop cracks which extend from the central gelatinous
material to the periphery. The gelatinous material extends
along these cracks & drags the surrounding fibrous tissue
into the spinal canal or the intervertebral canal. The spinal
cord &/or the spinal nerves may be pinched. If a spinal
nerve is pinched, one may experience pain along the area
supplied by this nerve. Each spinal nerve supplies a certain
portion of the body. In sciatica, the L5 spinal nerve is
Radiographs cannot show discs. MR is the method of choice
to see & characterize abnormality of the disc.
Abnormal curvatures of the spinal column are common. These
conditions may be caused by weak ligaments, by poor posture
habits, by disease or congenital abnormalities of the spinal
column, by injury, or by spasm of the back muscles.
Hunchback is an abnormal hump of the thoracic spine. This
hump is the result of several abnormal curvatures. These
curvatures may be either a kyphosis, which is an accentuation
of the normal posterior curvature, or a combination of kyphosis
or extreme lateral curvature (scoliosis), which is known
as kyphoscoliosis. Hunchback may be due to one or many collapsed
vertebrae. Collapse may occur in elderly people, particularly
women, whose bones may become soft and brittle (osteoporosis),
causing a dorsal kyphosis. As a result of the spinal deformity
in hunchback, the ribs become contorted, compressing or
displacing the lungs and other structures within the chest
cavity and thrusting the collarbone and shoulder blades
into distorted positions. To compensate for such abnormal
curvatures, the body develops deformities in the hips and
other parts of the body in its effort to maintain balance.
Fortunately, one can detect osteoporosis before it results
in hunchback. There are many types of equipment that are
advertised to be able to detect osteoporosis. Many of these
claims are, at best, mis-representations. It is well established
among the medial specialists dealing with osteoporosis that
DEXA bone densitometers are the most reliable type of equipment
to assess bone density. Also, WHO has established that whole
body DEXA bone densitometer is the equipment of choice.