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If tissue has small transverse component of coherent magnetization at time TE
- amplitude received by the coil is ___
- image is ___
- - amplitude received by the coil is small
- - image is dark
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If tissue has large transverse component of coherent magnetization at time TE
- amplitude received by the coil is ___
- image is ___
- - amplitude received by the coil is large
- - image is bright
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Bright area on image
- - tissue has large transverse component of coherent magnetization at time TE
- - amplitude received by the coil is large
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Dark area on image
- - tissue has small transverse component of coherent magnetization at time TE
- - amplitude received by the coil is small
-
NMV can be separated into individual vectors
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What is number of mobile Hydrogen protons per unit volume of the tissue?
Proton Density
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If PD is high what happened to signal?
- PD ↑ - more signal available form tissue
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Proton Density
- - number of mobile Hydrogen protons per unit volume of the tissue
- - PD ↑ - more signal available form tissue
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Intrinsic contrast parameters?
- list
- - cannot be changed (inherent to the body tissue)
- - T1
- - T2
- - Proton density
- - Flow
- - ADC – Apparent Diffusion Coefficient
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Extrinsic contrast parameters?
- list
- - can be changed
- - TR
- - TE
- - Flip Angle
- - Turbo Factor / Echo Train Length
- - B value
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Factors that affect image contrast
- 1) Intrinsic contrast parameters
- - cannot be changed (inherent to the body tissue)
- - T1
- - T2
- - Proton density
- - Flow
- - ADC – Apparent Diffusion Coefficient
- 2) Extrinsic contrast parameters
- - can be changed
- - TR
- - TE
- - Flip Angle
- - Turbo Factor / Echo Train Length
- - B value
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T1 and T2 depend on 3 factors:
- 1) Inherent Energy of tissue
- 2) How closely packed the molecules are
- 3) How well the molecular tumbling rate matches the Larmor Frequency of H
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What factor is impotent in T1 relaxation (spin-lattice)?
Inherent Energy of tissue
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Inherent Energy of tissue is a factor ___
- __ factor that affect T1 and T2
- - if Inherent Energy is low, then molecular lattice is more able to absorb energy from H during relaxation
- - important in T1 relaxation (spin-lattice)
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What factor is impotent in T2 relaxation (spin-spin)
How closely packed the molecules are
-
How closely packed the molecules are, is a factor ___
- __ factor that affect T1 and T2
- - the closer molecules are, there is more efficient interaction b/w neighboring H nuclei
- - important in T2 relaxation (spin-spin)
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How well the molecular tumbling rate matches the Larmor Frequency of H is a factor ___
- __ factor that affect T1 and T2
- - if it is a good match, energy exchange b/w H and the molecules of lattice is efficient
-
large molecules closely packed together in ?
Fat
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low inherent energy is in?
Fat
-
molecular tumbling rate is slow in?
Fat
-
carbon doesn’t take electrons form around of H nucleus, protecting nucleus from the effect of B₀ in?
Fat
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Larmor Frequency of H is ↓ in ?
Fat
-
Fat
- molecules characteristic
- inherent energy
- tumbling rate
- effect of B₀
- Larmor Frequency of H is
- - large molecules closely packed together - C54 H108 O6
- - low inherent energy
- - molecular tumbling rate is slow
- - carbon doesn’t take electrons form around of H nucleus, protecting nucleus from the effect of B₀
- - Larmor Frequency of H is ↓
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T1 Recovery in Fat
- - Spin-lattice Relaxation
- - T1 time is short
- - Fat has low inherent energy and can easily absorb energy into its lattice form H nuclei
- - molecular tumbling rate matches the Larmor Frequency
- - molecular mobility is low, so recovery process is relatively rapid
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T2 Decay in Fat
- - Spin-spin Relaxation
- - T2 time is short
- - molecules are packed closely, so interaction b/w H nuclei is efficient
- - spin dephase quickly and loss of transverse magnetization is rapid
-
molecules spaced apart in?
water
-
high inherent energy is in?
Water
-
molecular tumbling rate is fast in?
Water
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Larmor Frequency of H is ↑ in ?
Water
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oxygen steals electrons away from around the H nucleus, so it’s more available to the effect of B₀ in?
Water
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Water
- molecules characteristic
- inherent energy
- tumbling rate
- effect of B₀
- Larmor Frequency of H is
- - molecules spaced apart - H2 O
- - high inherent energy
- - molecular tumbling rate is fast
- - oxygen steals electrons away from around the H nucleus, so it’s more available to the effect of B₀
- - Larmor Frequency of H is ↑
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T1 time is long in?
Water
-
T2 time is long in?
Water
-
-
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T1 Recovery in Water
- - Spin-lattice Relaxation
- - T1 time is long
- - Water has high inherent energy and cannot easily absorb energy into its lattice form H
- - molecular tumbling rate does not match the Larmor Frequency
- - molecular mobility is high, so recovery process is less efficient
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T2 Decay in Water
- - Spin-spin Relaxation
- - T2 time is long
- - molecules are placed apart, so interaction b/w H nuclei is less efficient
- - spin dephase slowly and loss of transverse magnetization is gradual
-
-
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Contrast in PD
- Contrast in PD depends on differences in signal intensity b/w tissues with different relative number of mobile H protons per unit volume
-
- - high PD – brain is bright
- - low PD – bone is dark
-
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Contrast with TR ad TE
- T1 Weighting
- ↓TR ↓TE
-
- ↑T1 Contrast
- ↓T2 Contrast
-
- - TR must be short enough so that neither fat nor water returns to B₀ and recover their longitudinal magnetization fully
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Contrast with TR ad TE
- T2 Weighting
↑TR ↑TE
- ↓T1 Contrast
- ↑T2 Contrast
-
- - TE must be long enough to give both fat and water time to decay
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Contrast with TR ad TE
- PD Weighting
↑TR ↓TE
- ↓T1 Contrast
- ↓T2 Contrast
-
- - effects of T1 and T2 must be diminished (уменьшать)
- - TR is longer than T1 times of tissue, so both fully recovered before next RF
- - transverse component for both depends only on their individual PD
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T2* Decay
- - when RF removed, the relaxation and decay processes are faster than T2
- - in area w lower magnetic field strength, the precessional frequency slows down
- - in area w higher magnetic field strength, the precessional frequency speeds up
- - this inhomogeneity causes immediate dephasing and produces an FID (Free Induction Decay)
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What sequence has Additional 180˚ RF
Spin Echo Pulse Sequence
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2 ways to produce image w good contrast:
- 1) Spin Echo Pulse Sequence
- - Additional 180˚ RF
- - 1 or 2 echoes
- 2) Gradient Echo Pulse Sequence
- - Gradient
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Spin Echo using 1 echo
- T1
-
- - ↓TR
- - water vectors are not fully recovered so, difference in T1 dominate
- - contrast will depend on differences in longitudinal magnetization recovery (T1)
- - ↓TE
- - 180˚ RF pulse and subsequence echo occurs early so little T2 decay occurs
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Spin Echo using 2 echoes
- PD & T2
- - First Echo PD (↑TR ↓TE)
- - ↓TE – T2 differences b/w the tissues are minimized
- - Second Echo T2 (↑TR ↑TE)
- - ↑TE – T2 differences b/w the tissues are maximized
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In clinical practice TE:
TE is always shorter than TR
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In clinical practice Short TR:
- = approximately equal to the average T1 value
- - usually lower than 500 ms
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In clinical practice Long TR:
- = 3 times the short TR
- - usually greater than 1500 ms
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In clinical practice Short TE:
- usually lower than 30 ms
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In clinical practice Long T:
- = 3 times the short TE
- - usually greater than 90 ms
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Typical values of TR and TE in PD?
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Typical values of TR and TE in T1?
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Typical values of TR and TE in T1?
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What is GRE Pulse Sequence?
- - RF that flips the NMV through any angle (usually below 90˚)
- - the absence of a 180° RF rephasing pulse (gradient instead)
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what information GRE contains?
GRE contains T1 and T2 information
-
what controls amount of T1 in GRE?
TR and Flip Angle (FA) control the amount of T1
-
what controls amount of T2*?
TE controls the amount of T2*
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Gradient Echo (GRE) Pulse Sequence
- what is RF?
- rephasing pulse?
- what information GRE contains?
- what controls amount of T1?
- what controls amount of T2*?
- - RF that flips the NMV through any angle (usually below 90˚)
- - the absence of a 180° RF rephasing pulse (gradient instead)
- - GRE contains T1 and T2 information
- - TR and Flip Angle (FA) control the amount of T1
- - TE controls the amount of T2*
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T1 weighting in Gradient
- - ↓TR and ↑FA
- - to avoid full recovery for fat and water
- - ↓TE
- - so neither fat or water has had time to decay
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T2 weighting in Gradient
- - ↑TR and ↓FA
- - to permit full recovery for fat and water
- - ↑TE
- - so fat and water have had enough time to decay
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Gradients Echo advantages
TR can be reduced b/c Gradient rephase faster than 180˚ RF
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Gradients Echo disadvantages
- Gradient Echo is very suspectable to magnetic fields inhomogeneities and contains Magnetic Susceptibility Artifact
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