Principles of Magnetic Resonance

Review

What is Larmor Frequency?

What is its purpose in image acquisition?

Larmor Frequency

It is the frequency at which the hydrogen precess

When hydrogen is near an externally applied magnetic field, an increase or decrease causes the precessional frequency or resonant frequency also increase or decrease.

Larmor equation

ω0 = B0*ƔExample:

If we have an MRI capable of having a magnetic strength of 1.5 Tesla. What is the precessional frequency of hydrogen?

Answer:▪ 1.5T x 42.6MHz/T = 63.9MHz

What happens when it precesses?

After the atom is “pushed” by an electromagnetic energy and the RF pulse is stopped.

Two things happens; They align with the applied magnetic field

Attain the normal rate of precession.

What happens…

The RF energy that was absorbed by the atom is released, this energy decays rapidly and is referred to as Free Induction Decay.

A single pulse will not provide enough information for imaging techniques, so this pulse is repeated multiple times.

An interval at which this pulse is repeated is called repetition time or TR and is usually in milliseconds.

Cont…

The repetition time determines how much longitudinal (T1) occurs. TR controls the T1 weighting of the

image.The FID signal needs to be

focused so a large signal appears at certain time. Echo, and the time it takes to

appear after the RF pulse is applied is called the echo time (TE).

Echo

Is produced from the nuclei remaining in the transverse plane. Dephasing and precession occurs

at different rates.A refocusing pulse is given at

a given time, usually 1800, at half the echo time (TE/2). It flips the entire system into its

mirror image.

Refocusing pulse effect on echo

• Those nuclei precessing at a faster rate, but are now going in opposite direction and now are behind the slower precessing nuclei.

• At the echo time, TE, all nuclei have reached the same point and the receiver coil detects a large signal, which refers to as echo.

A

B

C

RF Pulse

Refocusing Pulse

Refocusing pulse effect on echo

Relaxation TimeT1 and T2 relaxation time

Relaxation

The term returning to equilibrium is called relaxation and can be thought of as two-step process. T1 relaxation or Longitudinal relaxation

T2 relaxation or Transverse relaxation

Relaxation

T1 relaxation

It is controlled by a time constant referred to as T1.

It is the time it takes about 63% of the nuclei to realign with the external magnetic field.

After the magnetic moment is flipped 900 by the application of a pulse of RF energy, the pulse is turned off. This is followed by a gradual return to equilibrium along the z axis.

T2 relaxation

It occurs simultaneously with the T1 relaxation.

It involves the rate of precession of different nuclei.

When RF pulse is applied all atoms are precessing together, in phase, at the exact same rate. Once it stops, each atom finds itself in a slightly different magnetic environment.

Cont…

The time constant controlling how fast this process occurs is called the T2 relaxation.

As with T1 values, the T2 value is the time it takes for 63% of the nuclei to be out of phase with each other.

The T2 happens more rapidly than the T1.

Cont…

T2 value is much shorter than the T1 value.

Typical T1 for biological tissues range from 200 to 1,000ms, whereas T2 50 to 150ms.

At higher applied magnetic fields, T1 is increased while T2 are caused by local effects and are unaffected by field strengths.

T2* or T-2 star

There are Inhomogeneities in the local magnetic field around each nuclei which make the actual dephasing go faster.

Inhomogeneities; Variation in the applied magnetic

field Changes in magnetic

susceptibilities Chemical shifts Changes caused by spins of nuclei

moving in vascular tissues.

Spin-spin Relaxation

During T2 relaxation, all of the nuclide phase their spins by transferring energy to neighboring atoms, this process is referred as spin-spin relaxation.

Summary

During relaxation process, the atoms emit RF energy of exactly the same energy that they had absorbed; these atomic magnets return to the equilibrium state that they were in before. This energy that is emitted by the atoms is responsible for generating the MR image.

T1 is primarily affected by the magnetic field of the overall environment, while the T2 relaxation is predominantly affected by the magnetic field in the local vicinity.

The RF signal that is given off by the atoms returning to equilibrium is detected by an antenna and analyzed by computer for MR imaging.