 MAGNETIC RESONANCE IMAGING CLINICAL APPLICATIONS
 What is Magnetic Resonance Imaging (MRI)

MRI uses a computer and the physical properties of magnetic fields and radio waves to
produce high quality sectional images of the inside of the body

MRI produces images of the anatomy without the use of radiation found in x-ray or CT
scanning
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It is the fastest and safest way to get the clearest pictures of the human anatomy

MRI can see right through bone to image structures, tissues, organs, and fluids such as
blood
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To date, MRI has been particularly valuable in scanning the brain and spine

Through brain and spine scans, MRI can detect multiple sclerosis in its earliest stages,
tumors, brain and spine disease, and fluid in the skull

Heart scans can show plaque build-up in arteries

It can detect cancer and other diseases in the kidneys, ovaries, uterus and liver
 Brief History of MRI

MRI utilizes a physics phenomenon discovered in the 1930s called nuclear magnetic
resonance (NMR) in which magnetic fields and radio waves, both harmless, cause atoms
to give off tiny radio signals

It wasn't until 1970, however, that Raymond Damadian, a medical doctor and research
scientist, discovered the basis for using magnetic resonance as a tool for medical
diagnosis

The "nuclear" was dropped off in the late 1970's because of the negative connotations
associated with the word

The future of MRI seems limited only by our imaginations
•
This technology, comparatively speaking, is still in its infancy
•
It has been in widespread use about 20 years (compared with over 100 years for
x-rays)
 "Magnetic"

Magnetic fields
•
The strength of the magnetic field relates to the pull or force from the magnet
and measures in units called gauss or tesla
•
10,000 gauss equals 1 tesla
•
The main magnetic field of a 1.5T magnet is about 30,000 times the strength of
the earth's magnetic field, which is approximately 0.5 gauss

•
MAGNETIC RESONANCE IMAGING CLINICAL APPLICATIONS
The strongest magnetic field permitted in MRI of humans is 1.5 tesla (1.5T)
 Low field magnet less than 0.5T
 Mid-field magnet: between 0.5 and 1.0T
 High-field magnet: between 0.75 and 1.5T


•
The strength of electromagnets used to pick up cars in junkyards is about the
field strength of MRI machines (1.5 to 2.0 Tesla)
•
The scientific community uses MRI with a field strength as high as 4.0T for nonhuman testing
Three types of magnetic properties of matter
•
Diamagnetic substances have a negative interaction or negative magnetic
susceptibility inside a magnetic field
•
Paramagnetic substances also exhibit no magnetic properties outside a
magnetic field
•
In the magnetic field, these substances exhibit a slight positive interaction
•
Ferromagnetic materials generally contain iron, nickel, or cobalt
•
These materials have a large positive magnetic susceptibility and have the ability
to remain magnetized when removed from a magnetic field
Magnet types available for use in MRI
•
Super-conducting magnets
 The most common
 Made from coils of wire producing a horizontal field
 Use liquid helium to keep the magnet wire at 4 degrees Kelvin (-268.8
Celsius) where there is no resistance
 Keep the coil and the liquid helium in a large dewar or jacket
 This dewar is typically surrounded by liquid nitrogen (77.4Kelvin) dewar
that acts as a thermal buffer between the room temperature (293K) and
the liquid helium
 It can also be a helium-only design with a refrigeration system to keep
the helium boil off at a minimum
•
Resistive magnets
 Constructed from a coil of wire
 The more turns to the coil, and the more current in the coil, the higher
the magnetic field
 Resistive magnets are similar to superconductors
 Air-cooled resistive magnets have a greater resistance to current and
create weaker magnetic fields
 The magnets are lower in cost to construct than a super-conducting
magnet, but require huge amounts of electricity (up to 50kw) to operate
because of the natural resistance of the wire

Permanent magnets
 Sometimes referred to as a vertical field magnet
 Made from two ferromagnetic plates forming a north and south pole
 The patient lies on a scanning table between these two plates
 No need for electricity or cryogenic liquids to maintain the magnetic
fields
 Are the weakest magnetic fields

The major drawback is that these magnets are extremely heavy - many tons in
weight at the 0.4T level
 "Resonance"

The "resonance" part of MRI

The MRI machine applies an RF (radio frequency) pulse that is specific only to
hydrogen


The system directs the pulse toward the examination area of the body

The pulse causes the protons in that area to absorb the energy required, making
them spin in a different direction

The RF pulse forces the protons to spin at a particular frequency, in a particular
direction

The Larmour frequency is the specific frequency of resonance

Base the calculation on the image of the particular tissue and the strength of the
main magnetic field
RF Coils
•
RF coils are the "antenna" of the MRI system that broadcasts the RF signal to
the patient and/or receives the return signal
•
There are several varieties of coils
 Receive-only, in which case the coil is used as a receiver
 Transmit-only, in which case the coil is used as a transmitter
 Transceiver, transmitter-receiver
•
Detect the weak MR signals in the presence of background RF signals from local
television and radio stations
•
MRI scanners are normally enclosed in a copper or stainless steel shield known
as a Faraday shield to filter out extraneous noise
•
Use gradient coils are used to produce deliberate variations in the main
magnetic field
•
There are usually three sets of gradient coils, one for each direction (x, y and zaxis)
•
The variation in the magnetic field permits localization of image slices as well as
phase encoding and frequency encoding
 How it works

The physics involved are extremely complex

Patients undergoing scans of their brain, heart or other body parts lie flat on a scanning
table in a magnet field created by a large magnet

Millions of negatively and positively charged atoms compose the human body

The hydrogen nucleus (a single proton) is abundant in the body due to the high water
content of non- bony tissues

The protons in the patient's tissues align themselves along the direction of the magnetic
field when placed within a MRI scanner

This is a physical phenomenon known as gyroscopic precession

Excitation occurs by applying short RF pulses causing the hydrogen protons to absorb
energy making them spin perpendicular to the magnetic field

The radio frequency power produced matches that of many small radio stations (15-20
kW)

Relaxation occurs when the radio signal stops, the protons relax back into alignment
with the magnetic field releasing energy signals, which the RF coil receives and acts as
an antenna

The receiver coils record these changes in excitation and relaxation and then a
sophisticated super-computer mathematically reconstructs them into 2D and 3D images
that vividly represent conditions of health, disease or injury

Transfer the constructed images onto film
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The science is different from x-ray

MRI is the absorption and emission of energy in the form of radio waves within the
electromagnetic spectrum, whereas radiographs (x-ray) are the absorption of x-ray
energy

Perform scans for multiple body parts without repositioning the patient. The entire
procedure takes 15 to 45 minutes
 How safe is MRI

It is a non-invasive procedure of which there are no significant biological hazards
demonstrated because of exposure to patients from the magnetic fields or radio
frequency pulses used in magnetic resonance imaging

More of an annoyance than a safety problem is the ability of the magnetic field of a MRI
machine to erase the information contained on the magnetic strip on ATM and credit
cards. This may occur a short distance inside of the scanner room of a MRI machine

The magnetic field can pull certain metal objects like watches, hairpins, writing
pens, phones, pagers, etc., away from the body when entering an MRI room

It is strong enough to pull heavy-duty floor buffers and mop buckets into the bore of the
magnet, pull stretchers across the room and turn oxygen bottles into flying projectiles
 Important patient considerations

Patients with various medical implant devices should avoid MRI

The MRI magnetic field attracts ferromagnetic (metallic iron) objects

Do not scan patients with cardiac pacemakers
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Cerebral aneurysm clip: most patients who have had surgical repair of a cerebral (brain)
aneurysm cannot have MRI scans (the clip might move and bleeding could occur)

The magnetic field can damage implanted electromagnetic devices: medication or
insulin pumps, bio-stimulators, and neuro-stimulators; therefore, do not scan patients
with these devices

Prosthetic heart valve: Most are safe but check with physician and staff

Do not scan patients with magnetically activated or supported implants (cochlea
implant, some dental and ocular implants)

The patient and staff should address other devices not noted

Metal workers or individuals exposed to ocular metallic injuries or having metallic
foreign objects in soft tissue should be examined before undergoing MR

Patients with ferromagnetic shrapnel or bullet fragments should be imaged with
another modality (most bullet fragments are non-ferromagnetic)

If the patient's scan is to include a contrast medium, any known allergies should be
discussed with the physician prior to the test. (Gadolinium is used as a contrast to
enhance images of patients undergoing MRI)
 Advantages

Unlike MRI, some imaging studies require the patient to change position during the
examination

The MRI is able to generate images in the sagittal (left/right), coronal (front/back), axial
(head/toe), and oblique (slanted) planes without moving the patient

MRI is able to produce vivid complex images in 256 levels of gray characterizing
relationships between vertebrae, intervertebral discs, the spinal cord, and nerve roots

May also eliminate the need for painful diagnostic procedures with serious side effects
such as:
•
Myelography
•
Arthrography
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Soft tissue differentiation is excellent giving display and boundary contrast between
anatomical structures with unprecedented clarity

Its high sensitivity to early pathological changes makes early detection possible
 Disadvantages
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MRI is not usually recommended for pregnant patients, particularly in the 1st Trimester

Large units and high magnetic fields produce a banging noise that is often frightening to
patients
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Because patients must lie quietly inside a narrow tube, MRI may raise anxiety levels in
the patients, especially those with claustrophobia
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It has a longer scanning time than CT, which makes it more sensitive to motion artifacts
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Higher cost compared to a regular x-ray or CT scan
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CT scan is better at looking at the bones than MRIFIXED