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.