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MRI axis
- Z - axial - longitudinal
- X - sagittal
- Y - coronal
- X+Y - transverse
- OBL - 2 simultaneously
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Larmor EQ
- F = P = W0 = y B0
- F - frequency =
- P - precession = 42.6mhz/1T
- W0 - resonance frequency
- y - helium
- B0 - strength of the magnet
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NMR
- Nuclear Magnetic Resonance (WRONG b/c no ionization)
- NMV - Net Magnetic Vector - (RIGHT!)
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Nuclear spin
- ability of P to create (NMV = GMR) = AM + MM
- AM - Angular Momentum
- MM - Magnetic Moment
-
3 necessaries for MRI:
- 1) MR active nuclei - H1
- 2) Static magnetic field - Main magnet - B0
- 3) RF - Radiofrequency
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MR active nuclei
- Proton (P) - subatomic nuclei
- H1 - element
- most abounded - - has property of spin - AM (Angular Momentum)
- - MM (Magnetic Moment) of its own
- H1 is not only one:
- 1) Phosphorus-31
- 2) Fluorine-19
- 3) Sodium-21
- 4) Carbon-13
- 5) Nitrogen-15
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What nuclei form MRI images
- with odd atomic number
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Main Magnet
- B0:
- function (2):
- criteria (3):
- materials:
- B0 - same value, never OFF
- Function:
- 1) Polarizes body tissue
- 2) Possess strong attraction and deflection
- Criteria:
- 1) Stable
- - cryogenic materials (Helium, Nitrogen)
- - conductive under -469F, -269C, 4K
- 2) Homogeneous - shim coils
- - active - always ON and has it's own power supply
- - passive - Sheets of iron cores stacked together
- 3) Large - up to 3T
- Materials:
- - Iron, Cobalt, Nickel
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Equilibrium
- - body introduced to the magnet
- - in a few sec magnet polarizes body
- - week oriented parallel, strong - untiparallel
- - NMV = Weak - Strong
- - stronger magnet - stronger NMV - stronger signal and better image
-
Magnetic susceptibilities
- 1) Diamagnetic - weakly attracted (gold, silver)
- - no contribution to MRI
- 2) Paramagnetic - slightly attracted (GPTA - Gadolinium Pentacetic Acid)
- - major contribution
- - very usefull
- 3) Ferromagnetic - strongly attracted
- - catastrophic, deadly
- - depends on distance and orientation (position)
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Cryogenic materials
- to maintain...
- materials...
- T
- most modern...
- - to maintain stability
- - Helium, Nitrogen
- - conductive under extremely low T
- -469F, -269C, 4K
- - most modern MRI use Helium
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Resonance
Exchange of energy between two systems at a specific frequency
-
net magnetization
- macroscopic magnetization
- NM = Weak - Strong
- has a longitudinal component (along the Z axis) aligned with B0
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B1
- RF coils
- second order of magnetism
- detect and transmit MRI signal
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B0
- Main magnet
- static magnetic field - nonfluctuating
- never OFF
- cannot be adjusted
- as ↥ B0 as T1 quicker
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Radiofrequency
- - Criteria of magnetism
- - Low level electromagnetic radiation
- - Lowest energy wave in the electromagnetic spectrum (Radio wave)
- - Applied to the human body by RF coils
- - RF coils detect and transmit MRI signal
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Excitation
- Radiofrequency applied by RF coils - B1 - second order of magnetism.
- proton jumps from low to high energy state
- from parallel to anti-parallel
- gives magnitude (FOV) and direction (scanning plans)
-
Longitudinal magnetization due to...
...a difference in the number of spins in parallel and anti-parallel state
-
Transverse magnetization due to...
...spins getting more or less into phase
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Relaxation
- radiofrequency OFF
- P going back to equilibrium of net magnetization giving E to environment
- electromagnetic energy is retransmitted (NMR signal)
-
Longitudinal relaxation
- corresponds to...
- due to...
- ?-? relaxation
- % ?
- corresponds to longitudinal magnetization recovery
- due to energy exchange between the spins and surrounding lattice
- spin-lattice relaxation
- 63% of energy recovered - T1
-
Transverse relaxation
- corresponds to...
- result from...
- ?-? relaxation
- % ?
- - corresponds to transverse magnetization decay
- - results from spins getting out of phase
- - spin-spin interaction
- - 37% of energy left - T2
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T1 Conventional MRI scan (SE)
- term
- TR-TE
- what primarily controls the contrast?
- NMV in what direction
- ?-?
- energy...
- ... time contrast
- shows...
- what is bright
- relation to field strength
- - time to get to a point where longitudinal magnetization has returned to 63% of its final value
- - TR↧ - TE↧
- - TR primarily controls the contrast
- - NMV in longitudinal (Z) direction
- - spin-lattice relaxation
- - energy given to surrounding tissue (other lattice)
- - tissue-specific time constant
- - shows anatomy (bone)
- - fat is bright
- - longer at higher field strengths
-
T2 Conventional MRI scan (SE)
- term
- TR-TE
- what primarily controls the contrast?
- NMV in what direction
- ?-?
- energy is exchanged to...
- ... time contrast
- shows...
- what is bright
- relation to field strength
- - time to get to a point where transverse magnetization decay to 37% of its original value
- - TR↥ - TE↥
- - TE primarily controls the contrast
- - NMV in transverse (X-Z) plane
- - spin-spin interaction
- - energy is exchanged to other spin of the same lattice
- - tissue-specific time contrast and is always shorter than T1
- - shows pathology (fluid)
- - fluid is bright
- - unrelated to field strength
-
Time to get to a point where longitudinal magnetization has returned to 63% of its final value
T1
-
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Time to get to a point where transverse magnetization decay to 37% of its original value
T2
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spin-lattice relaxation
T1
-
-
-
-
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longer at higher field strengths
T1
-
unrelated to field strength
T2
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GPTA
- Gadolinium Pent-acetic Acid
- Paramagnetic - slightly attracted
- major contribution, very usefull
- Any spinal surgery must be done w gado. Must start immediately w 20 min to see diff b/w scar tissue and disk space.
- QT - what enhances quicker? - scar tissue
-
If BBB is disrupted - contrast will pass from what compartment to what
From intravascular to interstitial
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BBB
- Blood-Brain Barrier
- dynamic interface that separates the brain from the circulatory system and protects the central nervous system from potentially harmful chemicals .
-
FID
- - Free Induction Decay
- - Eddy Current
- - occur before T1 and T2 (leakage current)
- - caused by MF inhomogeneous
- - decays along transverse plan
- - when reach X-Y plan, becomes - T2*
- - dephasing the P spin
- - best for nerve roots
 
-
T2*
- product of FID
- when FID reach X-Y plan, becomes - T2*
- FID - Free Induction Decay
- Eddy Current
- occur before T1 and T2 (leakage current)
- caused by MF inhomogeneous
- dephasing the P spin
- best for nerve roots
- decays along transverse plan X-Y
-
Best for nerve roots
FID - Free Induction Decay - T2*
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Spin Echo PS
- SE Pulse Sequence:
- - has 2 RF - starts w 90, ends w 180
- - FID - Free Induction Decay
- - TR - Time to repeat
- - TE - Time to echo
- - T1, T2, PD - SE
-
TE
- Time to echo
-
TR
- Time to repeat
-
Brightest signal in MRI
PD Conventional MRI scan
-
PD Conventional MRI scan
- PD - Proton Density
- Brightest signal in MRI
- Intermediate Pulse Sequence
- TR↥ - TE↧
- grayer in appearance
- b/c 15% tissue differentiation
- Pathology and anatomy
-
TR primarily controls the contrast in...
T1 (TR↧ - TE↧)
-
TE primarily controls the contrast in...
T2 (TR↥ - TE↥)
-
Good for anatomy
T1 (TR↧ - TE↧)
-
Fat bright
T1 (TR↧ - TE↧)
-
Good for pathology
T2 (TR↥ - TE↥)
-
Fluid bright
T2 (TR↥ - TE↥)
-
Good for pathology and anatomy
PD - Proton Density (TR↥ - TE↧)
-
Grayer in apperance
PD - Proton Density (TR↥ - TE↧)
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FSE Sequence
 - Fast Spin Echo
- start w 90 followed by a train of 180
- quicker scanning times
- causes blurring as MRI is a trade-off b/w parameters
- blurriness is solved by keeping ETL as low as possible
- ETL - Echo Train Length - # of 180 pulses after the 90 pulse
-
- FID - Free Induction Decay - T2*
- SE - Spin Echo Signal
- TE - Time to echo
- TR - Time to repeat
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Intermediate Pulse Sequence
- PD - Proton Density Contrast mechanism
- TR↥ - TE↧
- grayer in appearance
- b/c 15% tissue differentiation
- Pathology and anatomy
-
IR Sequence
 - Inversion Recovery
- start w 180 followed by 90
- 180° RF inversion wave
- 90° RF excitation wave
- TI - Time to invert (delay between 180° and 90°)
- TI controls contrast
- Bone black - if white - pathology
-
Good for all bone work
STIR sequences
-
Good for osteomyelitis
STIR sequences
-
STIR sequences
- Short TAU IR
- TAU - The interpulse times (time between the 90° and 180° pulse, and between the 180° pulse and the echo)
- Good for:
- - osteomyelitis
- - all bone work
-
Good for CSF flow
FLAIR sequences
-
Good for multiple sclerosis
FLAIR sequences
-
FLAIR sequences
- Fluid Attenuated IR
- Good for:
- - multiple sclerosis
- - whenever CSF flow
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T1 GRE
- T1 Grade and Echo
- TR↧ - TE↧
- Flip Ang ↥ <90o
- the absence of a 180° RF rephasing pulse (GRE instead)
- MRA - Angiography
- Blood is bright
-
How Pulse Sequence identified?
By Flip angle
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T2 GRE
- T2 Grade and Echo
- TR↥ - TE↥
- Flip Ang ↧ <90o
- the absence of a 180° RF rephasing pulse (GRE instead)
- Blood imaging
-
Gradient GRE echo
 - the flip angle usually below 90°
- the absence of a 180° RF rephasing pulse (GRE instead)
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Motion artifact
- - Ghosting
- - phase encoding direction (mismapping the image
-
Diamagnetic
- - weakly attracted (gold, silver)
- - no contribution to MRI
-
Paramagnetic
- - slightly attracted
- - major contribution
- - major contribution
- - GPTA - Gadolinium Pentacetic Acid)
-
Ferromagnetic
- - strongly attracted
- - catastrophic, deadly
- - depends on distance and orientation (position)
-
Spin start w 90 followed by a train of 180
Fast Spin Echo
-
Spin with quicker scanning times
Fast Spin Echo
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Why Fast Spin causes blurring?
causes blurring as MRI is a trade-off b/w parameters
-
How to solve blurriness in Fast Spin Echo?
- - by keeping ETL as low as possible
- - ETL - Echo Train Length - # of 180 pulses after the 90 pulse
-
Spin start w 180 followed by 90
Inversion Recovery
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What kind of wave is 180/90 in Inversion Recovery?
- - 180° RF inversion wave
- - 90° RF excitation wave
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What is Time to invert?
- delay between 180° and 90° in Inversion Recovery
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What controls contrast in Inversion Recovery?
TI - Time to invert (delay between 180° and 90°)
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In what spin if bone black - pathology?
- Inversion Recovery
- Bone black - if white - pathology
-
In what spin the flip angle usually below 90° with the absence of a 180° RF rephasing pulse (GRE instead)
Gradient GRE echo
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