Transcript MRI

MRI
Physics
Dr Mohamed El Safwany, MD.
MRI
Magnetic
Resonance
Imaging
Intended learning outcome
► The
student should learn at the end of this
lecture MRI physics .
M is for Magnetic
►Atomic
 Proton
 Neutron
 Electron
structure:
M is for Magnetic
►Motion
within the atom:
SPIN
M is for Magnetic
►MR
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H1
C13
F19
P31
N15
O17
active nuclei:
M is for Magnetic
►Why
H1?
 1000 Billion Billion Atom in adult
 Solitary Proton gives it a large magnetic
moment
M is for Magnetic
"NMV"
Net Magnetization Vector
M is for Magnetic
Precession
Larmor Equation
Frequency α Magnetic Field
ωαβ
M
► Atomic
►SPIN
► MR
structure
active nuclei H1
► What happen when placed in external
magnetic field (Spin Up and Spin Down)
► NMV
► Precession
► Larmor Equation
R is for Resonance
Def:
Energy transition that occur when
object is subjected to frequency
the same as its own
R is for Resonance
Here;
Radio Frequency "RF"
Same Frequency of H nuclei
At 90 degree to B0
R is for Resonance
► RF
Removed  Signal decreased 
Amplitude of MR Signal decreased
I is for Imaging
►Areas
of High Signal
►Areas of Low Signal
►Areas of Intermediate Signal
I is for Imaging
NMV can be separated in to
Individual Vectors of tissue present in
the patient
►Such
as Fat, CSF & Muscle
I is for Imaging
High Signal
Low Signal
Intermediate
White
Black
Grey
I is for Imaging
Gradient Magnets
► Used
to vary magnetic field in known
manner
► Each point has slightly different rate of
precession & Larmor Frequency.
► Variety of signal released by Protons
returning to z-plane can used to determine
the composition of exact location of each
point.
Contrast Mechanisms
High Signal
Low Signal
Intermediate
White
Black
Grey
Contrast Mechanisms
(Relaxation Process)
Relaxation Process
after removal of RF pulse
Signal induced in Receiver Coil decrease
Contrast Mechanisms (Definitions)
Repetition Time "TR"
Time from application of one RF pulse
To the application of the next
(it affects the length of relaxation period
after application of one RF excitation pulse
to the beginning of the next)
Contrast Mechanisms (Definitions)
Echo Time "TE"
Time between RF excitation pulse and
collection of signal
(it affects the length of relaxation period
after removal of RF excitation pulse
and the peak of signal received in receiver
coil)
Contrast Mechanisms (Definitions)
Flip Angle
Angle throw which the NMV moved as result
of a RF excitation pulse
Contrast Mechanisms (Parameters)
Image contrast controlled by:
1- Extrinsic Contrast parameters:
TR, TE & Flip Angle
2- Intrinsic Contrast parameters:
T1 Recovery, T2 Decay, Proton Density, Flow
& Apparent Diffusion Coefficient
Contrast Mechanisms
(T1 Recovery)
Short TR T1 contrast
(T1
► TR
300-600 ms
► TE 10-30 ms
Weighted)
Contrast Mechanisms (T2 Decay)
Fat much better at energy exchange than Water
Because this T2 depend on:
1-How closely molecular motion of atoms matches
Larmor Frequency
2-Proximity of other spins
So;
Fat's T2 time is very short compared to water
FAT
WATER
Contrast Mechanisms (T2 Decay)
Long TE T2 contrast
(T2 Weighted)
► TR
2000 ms
► TE 70 ms
Contrast Mechanisms
► Proton
(Proton Density)
Density
Long TR Proton density
► TR
2000 ms
► TE 10-30 ms
Contrast Mechanisms
►Contrast
► as Gadolinium
► local
(Contrast Media)
Media
magnetic field fluctuation occur near
Larmor frequency
► T1 Relaxation times of nearby protons are
reduced
► So they appear brighter in T1 weighted
Image
Text Book
► David
Sutton’s Radiology
► Clark’s Radiographic positioning and
techniques
Assignment
► Two
students will be selected for
assignment.
Question
► Define
echo time TE in MRI?
Thank You
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