Weber State University
Radiologic Technology 4603
MRI Physics and Instrumentation
_____________________________________________
Instructor: Rex T. Christensen MHA R.T. (R) (MR) (CT)
Water Molecule H O
2
• Atom + Atom = molecule
• A water molecule consists of 2
hydrogen atoms and 1 oxygen atom.
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Water Molecule H O
2
•
Hydrogen in the body is found
predominately in three forms:
1. Free water in body fluids (CSF)
2. Water bound to large molecules
(proteins)
3. Hydrogen atoms within fat.
Hydrogen Atom
•Has the simplest atomic
structure.
•Contains one proton and one
electron.
•The nucleus of a hydrogen
atom contains only a proton.
•Therefore, hydrogen atoms
are often referred as “protons”
•Abundant in the body.
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MR Active Nuclei
• Nuclei that aligns its axis to a magnetic
field.
• They do this because of electromagnetic
induction:
• Angular momentum or spin.
• Posses an electrical charge (+)
• If two of these characteristics are present
it induces the third - magnetism
• Called a magnetic moment
MR Active Nuclei:
Naturally occurring nuclei with an odd number of
Protons or Neutrons.
Gyromagnetic ratio is a constant for each
type of MR active nuclei.
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Precession
Precession is the wobble of a top
Precessional Frequency is the speed it
wobbles. (Mhz)
Larmor Frequency:
The Larmor frequency is also called the
Precessional frequency.
Precession is a “wobble” of the hydrogen
atom.
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Larmor Equation:
MEMORIZE THIS!
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Larmor Equation:
So What?
It calculates the operating frequency of
the MRI magnet.
42.57 Mhz @ 1 Tesla
What would it be at 1.5T? 3.0T?
1.5T = 63.9 Mhz
3.0T = 127.7 Mhz
What is a Vector?
A quantity (magnitude) that has direction.
http://www.e-mri.org/nmr/excitation-nmr.html
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Cartesian Coordinates
Frames of reference:
Stationary Frame: Looking at a moving
object from a stationary position.
Rotating Frame: Looking at a moving object
while on the moving object.
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Hydrogen Atoms before
being placed in the MRI:
Hydrogen Atoms after being
placed in the MRI:
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Parallel vs. Anti-parallel:
Anti-parallel
Parallel
Anti-parallel = High energy states.
Parallel = Low energy states.
Net Magnetization:
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Hydrogen and Field Strength
• As field strength
increases the number
of Hydrogen atoms
becomes more
visible.
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Longitudinal and Transverse
Magnetization
Longitudinal Magnetization is
also known as:
•
T1 Relaxation
•
Spin-lattice Relaxation
Transverse Magnetization is
also known as:
•
T2 Relaxation
•
Spin-spin Relaxation
Resonance:
When an RF pulse is turned
on and its frequency matches
that of the frequency of the
hydrogen atom energy is
transferred. This tips the
hydrogen atoms down into
the transverse plane (90
degrees).
The magnitude of the flip
angle (i.e. 90 degrees)
depends upon the amplitude
and duration of the RF pulse.
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Hard/Soft RF Pulses:
Hard Pulse: Strong, very short RF pulse.
Soft Pulse: Weaker, longer RF pulse.
The duration of the pulse rarely exceeds 10 ms.
T1 Relaxation:
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T1 Relaxation:
T2 Relaxation:
This relaxation is also called:
•Dephasing
•Incoherence
•Lack of phase coherence
•T2 Decay
Coherence: means sticking
together.
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T2 Relaxation:
Free Induction Decay (FID):
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Free Induction Decay (FID):
The induction in
reduced signal is
called Free Induction
Decay (FID)
Coils
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Coils
Relaxation Times:
T1 Relaxation Time:
63 % relaxation of
the tissue along the
longitudinal (z) axis.
T2 Relaxation Time:
63% decay of the
tissue along the
transverse (xy) axis.
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TR = Time to Repetition
Time (ms) between the beginning of
one pulse sequence and the
beginning of the succeeding pulse
sequence.
TE = Time to Echo
Time (ms) between the center
of the 90-degree pulse and the
center of the spine echo.
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Image Contrast:
T1 contrast = short TR and short TE.
T2 contrast = long TR and long TE.
PD contrast = long TR and short TE.
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