SIMPLE MRI PHYSICS

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SIMPLE MRI Physics

Quantum Mechanics

Year -Researcher

1901 -Planck

1905 -Einstein

1913 -Bohr

1922 -Compton
1924 -Pauli

1925 -De Brodlie

1926 -Schroedinger

1927 -Heisenberg

1927 -Davison, Germer and Born

MRI-->Magnetic Resonance Imaging

M is for Magnetic

Ø Atomic structure

• Neutron
• Proton
• Electron
  

Ø SPIN

Motion within the atom

Ø MR active nuclei:

• H1
• C13
• F19
• P31
• N15
• O17

Ø Why H1?

1000 Billion Billion Atom in adult

Solitary Proton gives it a large magnetic moment

Ø What happen when placed in external magnetic field? (Quantum Theory)

Spin Down	Spin Up
High Energy NNSS	Low Energy NSNS

Ø "NMV"

Net Magnetization Vector

Ø PRECESSION

Ø Larmor Equation

Frequency α Magnetic Field

ω α β

Incoherent Magnetic moment of H is At different place in Precessional Path

Coherent Magnetic moment of H is Placed in the same place in Precessional Path

M Stands..........

Ø Atomic structure

Ø SPIN

Ø MR active nuclei H1

Ø What happen when placed in external Z magnetic field (Spin Up and Spin Down)

Ø NMV

Ø Precession

Ø Larmor Equation

Ø Coherent

R is for Resonance

Ø Resonance Definition:

Energy transition that occur when object is subjected to frequency the same as its own

Here;

Radio Frequency "RF"

Same Frequency of H nuclei

At 90 degree toB0

Ø 2 things happen at Resonance:

1- Energy Absorption

Increase number of High energy Spin Up nuclei

2- Phase Coherence

NMV precesses in transverse plane at Larmor Frequency

Ø

5

MR Signal

NMV rotates around transverse plane

It passes across Receiver Coil

Inducing voltage in it

RF Removed à Signal decreased à Amplitude of MR Signal decreased

Ø Free Induction Decay "FID":

Free (No RF Pulse)

ID (because of Decay of Induced signal in Receiver Coil)

R

Ø Radio Frequency "RF”

Ø 2 things happen at Resonance

Ø Energy Absorption

Ø Phase Coherence

Ø Free Induction Decay "FID"

Ø MR Signal

I is for Imaging

Ø Areas of High Signal

Ø Areas of Low Signal

Ø Areas of Intermediate Signal

Ø

NMV can be separated in to

Individual Vectors of tissue present in the patient

Such as Fat, CSF & Muscle

Low Signal	High Signal	Intermediate Signal
White	Black	Grey
Small transverse component of magnetization	Large transverse component of magnetization

Ø Gradient Magnets

Used to vary magnetic field in known manner

Each point has slightly different rate of precession & Larmor Frequency.

Variety of signal released by Protons returning to z-plane can used to determine the composition of exact location of each point.

Ø "Gradient Function"

1.Slice selection

2.Frequency encoding

3.Phase encoding

Ø Contrast Mechanisms

Relaxation Process

After removal of RF pulse

Signal induced in Receiver Coil decreased

Why?

1. NMV recovers and realign to B0this process called "T1 Recovery"
2. Nuclei loss Precessional coherence or dephase and NMV decay in the transverse planethis process called "T2 Decay"

Ø T1 time is

An intrinsic contrast parameter that inherent to tissue being imaged

Ø T2 Decay is

an intrinsic contrast parameter and is inherent to the tissue being imaged

Ø Repetition Time "TR"

Definition

Time from application of one RF pulse

To the application of the next

(It affects the length of relaxation period

after application of one RF excitation pulse

to the beginning of the next)

Ø Echo Time "TE"

Definition

Time between RF excitation pulse and

collection of signal

(It affects the length of relaxation period

after removal of RF excitation pulse

and the peak of signal received in receiver coil)

Ø

Flip Angle

Angle throw which the NMV moved as result of a RF excitation pulse

Ø Image contrast controlled by:

1. Extrinsic Contrast parameters:

TR, TE & Flip Angle

2. Intrinsic Contrast parameters:

T1 Recovery, T2 Decay, Proton Density, Flow & Apparent Diffusion Coefficient

Ø T1 Recovery

Caused by exchange of energy from

nuclei to their surrounding environment or lattice

"Spin Lattice Energy Transfer"

and realign in B0

this occur in exponential process

at different rates in different tissue

NB: Molecules are constantly in motion; Rotational and Transitional

T1 in Fat	T1 in Water
T1 is very short absorb energy quickly i.e. nuclei dispose their energy to surrounding fat tissue and return to B0 in very short time	T1 is longer inefficient at receiving energy i.e. nuclei take allot longer to dispose energy to surrounding water tissue

Ø Short TR --->T1 contrast (T1 Weighted)

TR 300-600 ms

TE 10-30 ms

Ø T2 Decay

Caused by exchange of energy from one nucleus to another

As result of intrinsic magnetic fields of nuclei interlacing with each other

This energy exchange à loss of coherence or dephasing

and as result NMV decay in transverse plane

T2 is exponential process

occur at different rates in different tissues

Contrast Mechanisms (T2 Decay)

Fat much better at energy exchange than Water

Ø Because this T2 depend on:

1-How closely molecular motion of atoms matches Larmor Frequency

2-Proximity of other spins

So;

Fat's T2 time is very short compared to water

Ø Long TE àT2 contrast (T2 Weighted)

TR 2000 ms

TE 70 ms

Ø Proton Density

Long TR --->Proton density

TR 2000 ms

TE 10-30 ms

Ø Contrast Media

· As Gadolinium

· Local magnetic field fluctuation occur near Larmor frequency

· T1 Relaxation times of nearby protons are reduced

· So they appear brighter in T1 weighted Image

I

Ø Areas of High Signal, Low Signal, & Intermediate Signal

Ø Gradient Magnets

Ø T1 Recovery

Ø T2 Decay

Ø Contrast parameter

Ø T1, T2&PD Imaging

Ø Contrast Media

ARUN A
MRI TECHNOLOGIST
DR SHAJI'S MRI &
MEDICAL RESEARCH CENTER (P) Ltd.