Transcript MRI Overview

Magnetic Resonance Imaging
Astounding
The MRI magnet and patient table
The operator’s console
Most MR magnets are
Superconductors
Liquid helium is the
primary cryogen
Boiling Point @ 1 atm:
-452.1°F (-268.9°C, 4oK)
Once ramped up, MR
magnets remain on
all the time
Housed in an 18 wheeler, mobile MR machines use
permanent or resistance coil magnets.
Damadian’s apparatus
1972 patent did not describe imaging
Lauterbur built the first imaging machine, but Damadian made
The first for human imaging.
Hydrogen: 55-60% of the body is water
2/3 of water is hydrogen
An MRI scan of the
head and brain
Created from imaging
hydrogen
A spinning, charged particle, such as a hydrogen proton,
has a magnetic north and south pole.
It will align to an external magnetic field like a
compass needle aligns to the
earth’s magnetosphere
South
North
Magnetic poles of H are randomly oriented in
the human body
Unpaired spinning
protons create a net
magnetization
vector
Aligned to the MRI
magnet roughly half
spin up, and half
spin down
Opposing spins cancel
each other’s force
The protons that are used to
create the MR signal are the
unpaired ones
In a sample of 20 million protons,
about 7 are usable.
Hydrogen protons in the
patient’s body do not align
fully parallel to the magnetic
field. They precess (wobble).
The rate of precession is
called the larmor frequency.
It is 42.5 MHz at one tesla
42.5 MHz is in the
range of radio frequencies
RF energy
A H proton will absorb RF
when it has the same
frequency. It resonates.
90o flip
RF energy
180o flip
When the RF is turned off,
the proton returns to alignment
with the MRI magnet. It relaxes.
As it does it gives off the
energy it absorbed from the
RF in a process called free
Induction decay (FID)
The net magnetization vector
scans coils of wire, inducing
a flow of electrical current
which is the MR signal.
X
Z
Gradient coils vary the magnetic field
strength thoughout the x, y, z planes
causing every H proton to precess at
a different frequency
42.5 42.6
43.0 43.1
43.5 43.6
44.0 44.1
44.5 44.6
Y
42.7
43.2
43.7
44.2
44.7
42.8
43.3
43.8
44.3
44.8
42.9
43.4
43.9
44.4
44.9
43.7 MHz
The net magnetization vector
scans coils of wire, inducing
a flow of electrical current
which is the MR signal.
Surface Coils
QD Head Speeder
Functional MRI fMRI
Is that the truth, the whole truth, and nothing but the truth?
Mammography
Radiologic Science
Advanced Registry
Modalities
www.arrt.org
Magnetic Resonance Imaging
(MRI)
Cardiovascular Interventional
Technology (CIT)
Computed Tomography (CT)