RT 4912 Review (C) Rex T. Christensen MHA RT (R) (MR) (CT) (ARRT) CIIP

MR Active Nuclei Nuclei that aligns its axis to a magnetic field. Nuclei that aligns its axis to a magnetic field. They do this because of electromagnetic induction: They do this because of electromagnetic induction: Angular momentum or spin. Angular momentum or spin. Posses an electrical charge (+) Posses an electrical charge (+) If two of these characteristics are present it induces the third - magnetism If two of these characteristics are present it induces the third - magnetism Called a magnetic moment Called a magnetic moment

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.

Precession Precession is the wobble of a top Precessional Frequency is the speed it wobbles. (Mhz)

The Larmor frequency is also called the Precessional frequency. Precession is a “wobble” of the hydrogen atom. Larmor Frequency:

Larmor Equation: MEMORIZE THIS!

Hydrogen Atoms before being placed in the MRI:

Hydrogen Atoms after being placed in the MRI:

Parallel vs. Anti-parallel: Anti-parallel = High energy states. Parallel = Low energy states. Anti-parallel Parallel

Net Magnetization:

Longitudinal and Transverse Magnetization Longitudinal 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: 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:

T2 Relaxation: This relaxation is also called: Dephasing Incoherence Lack of phase coherence T2 Decay Coherence: means sticking together.

T2 Relaxation:

Free Induction Decay (FID): Coils can be transmit and receive, but most are receive only. The external body coil is usually the transmitter.

Coils

Coils

Free Induction Decay (FID): The induction in reduced signal is called Free Induction Decay (FID)

TR = Time to Repetition Time (ms) between the beginning of one pulse sequence and the beginning of the succeeding pulse sequence.

TE = Time to Echo Time (ms) between the beginning of the 90-degree pulse and the center of the spine echo.

Image Contrast: MRI Made Easy T1 contrast = short TR and short TE. T2 contrast = long TR and long TE. PD contrast = long TR and short TE.

Spin Echo Contrasts T1 PDT2 TE TR

The net magnetization vector Longitudinal 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% T2 t Transverse Relaxation Time T2 Transverse Relaxation = Decay of magnetization by interaction between nuclei (Spin-Spin-Relaxation) Transverse Relaxation = Decay of magnetization by interaction between nuclei (Spin-Spin-Relaxation)

Mo Mz t 63% T1 Longitudinal Relaxation Time T1 Longitudinale Relaxation = Energy transfer between excited spins and Tissue (Spin-Lattice-Relaxation)  Reestablishing of longitudinal magnetization with time constant T1 Longitudinale Relaxation = Energy transfer between excited spins and Tissue (Spin-Lattice-Relaxation)  Reestablishing of longitudinal magnetization with time constant T1

MzMz 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: Westbrook, Catherine and Kaut, Carolyn (1998). MRI In Practice, 3rd Edition, Malden, MA: Blackwell Science, Inc.