Transcript MRI - Faculty

Equipment

Magnetic resonance imaging (MRI) scan
requires the use of a very strong
magnetic field.
• Unlike other devices used in radiology, MR
•
imaging uses no radiation.
The magnet is contained in the housing of the
scanner and this creates a magnetic field
oriented down the center of the magnet.
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The patient is placed within the magnetic field by
lying on a table which is placed through the center of
the opening of the magnet, similar to lying on a road
running through a tunnel.
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The strength of the magnetic field is
measured in units called gauss or Tesla:
• 10,000 gauss equals 1 Tesla.
• The earth's magnetic field is approximately 0.6
gauss.
• The strongest magnetic field permitted in MRI
scanning of humans is 1.5 Tesla (1.5T).
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Three types of magnets are available for use
in MRI.
• Most MRI scanners in use today are
•
superconductng magnets.
Resistive magnets are electromagnets, similar
to superconducting magnets, but they are air
cooled therefore have greater resistance to
current and create weaker magnetic fields.
• Permanent magnets are made of solid
magnetic material, similar to bar magnets,
and create the weakest magnetic fields.
• However, they can be arranged in a configuration
that doesn't require the patient to be surrounded by
the magnet and are used in Open MR scanners.
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The strongest is a superconducting
magnet.
• This is a type of electromagnet in which current
•
flowing in a circular direction in a coil of wire
creates a magnetic field oriented down the core of
the coil.
In superconducting magnets, the wire conducts
the current without significant resistance because
it is cooled to a temperature close to absolute
zero by being bathed in a jacket of liquid helium
and/or liquid nitrogen.

The picture shows the
actual magnet (the
outer container
resembles a thermos
and contains the
superconducting wire
surrounded by liquid
helium).
Creating an Image

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The physics of MRI are extremely
complex.
When a patient is placed within and MR
scanner, the protons in the patients
tissues (primarily protons contained in
water molecules) align themselves along
the direction of the magnetic field.

A radiofrequency electromagnetic pulse
is then applied, which deflects the
protons off their axis along the magnetic
field.
• As the protons realign themselves with the
•
magnetic field, a signal is produced.
This signal is detected by an antenna, and
with the help of computer analysis, is
converted into an image.
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The process by which the protons
realign themselves with the magnetic
field is referred to as relaxation.
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The protons undergo 2 types of
relaxtion:
• T1 (or longitudinal) relaxation and
• T2 (or transverse relaxation) relaxation.
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Different tissues undergo different rates
of relaxation, and these differences
create the contrast between different
structures, and the contrast between
normal and abnormal tissue, seen on
MRI scans.
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T1 weighted images emphasize the
difference in T1 relaxation times between
different tissues.
• In these images, water containing structures are
dark.
• Since most pathologic processes (such as tumors,
injuries, CVA's, etc.) involve edema (or water), T1
weighted images do not show good contrast
between normal and abnormal tissues.
• However, pathologic processes do demonstrate
excellent anatomic detail.
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T2 weighted images emphasize the
difference in T2 relaxation times between
different tissues.
• Since water is bright on these images, T2
weighted images provide excellent contrast
between normal and abnormal tissues,
although the anatomic detail is less then that
of T1 weighted images.
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Proton density images emphasize
neither T1 or T2 relaxation times, and
therefore produce contrast based
primarily on the amount of protons
present in the tissue.
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Intravenous contrast is often used to
improve the sensitivity of MR imaging,
• especially in the brain and spine.
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MR contrast agents contain gadolinium,
which increases T1 relaxation and
causes certain abnormalities to "light up"
on T1 weighted images.
• These agents contain no iodine, and allergic
reactions are extremely rare.
Image Orientation
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MRI images can be obtained in any
imaging plane without moving the
patient.
• However, three standard views are usually
used:
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Transverse (axial):
Imagine the patient
is lying on their
back and is sliced
across from right to
left.
•
You are viewing
from the patient's
feet.
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Coronal: Imagine the
patient is standing in
front of you and is
sliced across from
right to left.
• You are viewing from
the front of the
patient.
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Sagittal: Imagine
the patient is
standing sideways
and is sliced
across from front to
back.
• You are viewing
from the side of the
patient.