Magnetic Resonance Imaging
(MRI)
by Alex Kiss
Introduction
• 1946: MRI science was developed
independently by Felix Bloch and Edward
Purcell
• 1952: both awarded Nobel Prize
• By the late seventies the name was
changed from Nuclear Magnetic
Resonance (NMR) to MRI because
“nuclear” carried a negative connotation
• 1977: First MRI exam on a human
Basic Purpose
• Best type of Imaging Modality, especially
in brain scans
• Used in the diagnosis of many injuries and
conditions, exam can be tailored to answer
the particular medical question asked
• Creates a detailed view inside human
body by mapping tissues point by point (a
point can be a 0.5 mm cube)
Structure
• Usually has an outer box (2m high x 2m
wide x 3m long)
• Patient lies inside a large hollow cylinder
• In the cylinder is several kilometers of wire
wrapped around in a coil
• When current is passed through the wire,
a magnetic field (0.5 – 2.0T) is generated,
especially in the center (bore) of the
cylinder
A fully loaded pallet jack has been
sucked into the bore of the MRI
system
Magnets
• Three types of magnets are used:
• Resistive: already mentioned, require up to
50kW to maintain due to the high resistance of
the wires
• Permanent: need no electricity, extremely heavy
(many tons)
• Superconducting: most commonly used, same
as resistive except wires are soaked in -452.4°F
liquid helium to lower resistance to zero
Protons
• Hydrogen nuclei (single protons) have a
strong tendency to line up with the
direction of the magnetic field because of
their large magnetic moments (spin)
• Some line up toward head, some toward
feet of patient; 2 protons with opposite
spin pair up to cancel each other out
• Only a couple hydrogen nuclei out of a
million are not canceled out
Radio Frequency (RF)
• A radio frequency pulse specific to
hydrogen is applied from a coil toward the
area of body being examined
• Each unmatched proton absorbs the
energy of a photon and undergoes a
transition from the lower energy state to a
higher energy state, effectively switching
the spin and alignment of the proton in the
magnetic field
• A single proton with
its magnetic moment
vector (spin)
symbolized by
magnetic poles
• When an external
magnetic field is
activated the spin
vector of the proton
aligns itself with the
external field
• When the proton is hit
with exactly the right
amount of energy
(equal to the
difference in energy
states, E=h*f, where f
is the resonance
frequency), the spin
of the proton flips to
that of a high energy
state
Imaging
• When the radio frequency is turned off, the
protons slowly return to their original
alignment within the magnetic field and
release their excess stored energy
• The signal is picked up by the coil and
sent to the computer system
• This mathematical data is converted to a
picture
Used to Diagnose or Evaluate:
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Multiple Sclerosis
Tumors
Infections of the brain, spine, and joints
Torn ligaments
Shoulder injuries
Tendonitis
Strokes
Masses in the soft tissue
Bone Tumors, Cysts, and Bulging or Herniated
Disks
Knee
Lower Back
Head