rt 4912 review (c) rex t. christensen mha rt (r) (mr) (ct) (arrt) ciip
DESCRIPTION
MR Active Nuclei (cont.) Odd atomic number. Odd atomic number. (even proton, odd neutron or vice versa). Spinning mass (magnetic field-example earth). Spinning mass (magnetic field-example earth). Spinning mass with a charge. Spinning mass with a charge.TRANSCRIPT
RT 4912 Review RT 4912 Review (C)(C)
Rex T. Christensen MHA RT (R) (MR) (CT) (ARRT) Rex T. Christensen MHA RT (R) (MR) (CT) (ARRT) CIIPCIIP
MR Active NucleiMR Active Nuclei Nuclei that aligns its axis to a Nuclei that aligns its axis to a
magnetic field.magnetic field. They do this because of They do this because of
electromagnetic induction:electromagnetic induction: Angular momentum or Angular momentum or spinspin.. Posses an electrical Posses an electrical chargecharge (+) (+) If two of these characteristics are If two of these characteristics are
present it induces the third - present it induces the third - magnetismmagnetism
Called a Called a magnetic momentmagnetic moment
MR Active Nuclei (cont.)MR Active Nuclei (cont.) Odd atomic number.Odd atomic number.
(even proton, odd neutron or vice (even proton, odd neutron or vice versa).versa).
Spinning mass (magnetic field-Spinning mass (magnetic field-example earth).example earth).
Spinning mass with a charge.Spinning mass with a charge.
PrecessionPrecessionPrecession is the wobble of a top
Precessional Frequency is the speed it wobbles. (Mhz)
The Larmor frequency is also The Larmor frequency is also called the Precessional called the Precessional frequency. frequency.
Precession is a “wobble” of the Precession is a “wobble” of the hydrogen atom.hydrogen atom.
Larmor Frequency:Larmor Frequency:
Larmor Equation:Larmor Equation:MEMORIZE THIS!
Hydrogen Atoms Hydrogen Atoms beforebefore being placed in the MRI:being placed in the MRI:
Hydrogen Atoms Hydrogen Atoms afterafter being placed in the MRI:being placed in the MRI:
Parallel vs. Anti-parallel:Parallel vs. Anti-parallel:
Anti-parallel = High energy states.
Parallel = Low energy states.
Anti-parallel
Parallel
Net Magnetization:Net Magnetization:
Longitudinal and Longitudinal and Transverse Transverse
MagnetizationMagnetizationLongitudinal Magnetization is also known as:
• T1 Relaxation
• Spin-lattice Relaxation
• T1 recovery
Transverse Magnetization is also known as:
• T2 Relaxation
• Spin-spin Relaxation
• T2 Decay
Resonance: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.
T1 Relaxation:T1 Relaxation:
T1 Relaxation:T1 Relaxation:
T2 Relaxation:T2 Relaxation:This relaxation is also called:
•Dephasing
•Incoherence
•Lack of phase coherence
•T2 Decay
Coherence: means sticking together.
T2 Relaxation:T2 Relaxation:
Free Induction Decay Free Induction Decay (FID):(FID):
Coils can be transmit and receive, but most are receive only.
The external body coil is usually the transmitter.
CoilsCoils
CoilsCoils
Free Induction Decay Free Induction Decay (FID):(FID):
The induction in reduced signal is called Free Induction Decay (FID)
TR = TR = TTime to ime to RRepetitionepetition
Time (ms) between the beginning of one pulse sequence and the beginning of the succeeding pulse sequence.
TE = TE = TTime to ime to EEchocho
Time (ms) between the beginning of the 90-degree pulse and the center of the spine echo.
Image Contrast: Image Contrast: MRI Made Easy 21 MRI Made Easy 21 - 22- 22
T1 contrast = short TR and short TE.T1 contrast = short TR and short TE.T2 contrast = long TR and long TE.T2 contrast = long TR and long TE.PD contrast = long TR and short TE.PD contrast = long TR and short TE.
Spin Echo Contrasts
T1
PD T2
TE
TR
2500
2000
1500
1000
500
30 90
The net magnetization vectorThe net magnetization vectorLongitudinal and transverse componentsLongitudinal and transverse components
The spins precess at the Larmor frequency around the z-axis. The net magnetization points in the z-direction.
Unless we have applied an RF field, there is no net xy-magnetization.
We measure the xy-magnetization during recovery.
Mxy
37%
T2t
Transverse Relaxation Time T2
Transverse Relaxation = Decay of magnetization by interaction between nuclei (Spin-Spin-Relaxation)
MoMz
t
63%
T1
Longitudinal Relaxation Time T1
Longitudinale Relaxation = Energy transfer between excited spins andTissue (Spin-Lattice-Relaxation)
Reestablishing of longitudinal magnetization with time constant T1
Mz S
Tissue 1
Tissue 2
TR Short TE Medium TE Long TE
Longitudinal Relaxation Transverse Relaxation
Tissue 2
Tissue 1
Relaxation Times are Tissue Specific
References:References:Westbrook, Catherine and Kaut, Carolyn (1998). MRI In Westbrook, Catherine and Kaut, Carolyn (1998). MRI In
Practice, 3rd Edition, Malden, MA: Blackwell Science, Inc.Practice, 3rd Edition, Malden, MA: Blackwell Science, Inc.