Magnetic Resonance Imaging

Part 1The Science Bit

Lynn Graham DCR MscClinical Specialist in MRI

Particle Physics!

OUTLINE ( part 1) History + Local origins of MRI

Fundamental Physics of MRI

Tissue contrast + Versatility

History Lesson

• Carl Fredrich Gauss. (1777-1855)• German Physicist• Findings led to a knowlegdge of magnetism and its

quantifiation

• Gauss- unit of measurement of magnetism

• Nikola Tesla (1856 –1943)

• Serbian Electrical Engineer

• Work in electromagnetic induction

• Tesla –unit of measurement for Magnetic Field strength

Sir Joseph Larmor FRS.MA.DSCMathematician + Physicist

1857-1942

• Born 11th July 1857 at Magheragall, Co Antrim

• Educated at RBAI, Belfast• Graduated from Queens 1877• Appointed Professor @ St Johns

College Cambridge 1903• Knighted 1909

The Larmor Equation

0BL

Key to Nuclear Magnetic Resonance

Clinical Magnetic Resonance Imaging

Nuclear Magnetic Resonance

NUCLEAR ATOMIC SPIN :

• +ve electric charge• Intrinsic spin/ Precession• nuclear magnetic moment

PRECESSION

NNuclear Magnetic Resonance

• MAGNETIC MOMENTS ALIGN WITH B0

+

No B0: random motion B0 : alignment

NNuclear Magnetic Resonance

Out of phase In phase

“OUT OF” PHASE

“IN” PHASE

The Larmor Equation

0BL L = Larmor frequency (MHz)

B0 = magnetic field (Tesla)

= gyromagnetic ratio

0 toalproportion BL

Key to Nuclear Magnetic Resonance

NMR – CLINICAL MRI

C

H

H

C

H

H

C

H

H

C

H

HFatty acid chain

H H

O

Water

Fat + Water = 99% body tissue

H+: ALIGNMENT + PRESCESSION

NMR – CLINICAL MRI

• Apply the Larmor equation

0BL

H1 @ 1 T : = 42.58 MHz T-1

@ 1.5 T Larmor frequency = 63.87 MHz

Electromagnetic Spectrum

Precessional Frequency of H+

Resonance + Excitation

• Energy in the form of an RF pulse

• Leads to misalignment with B0: antiparallel

• Also leads to phase coherence.

This is Excitation

Resonance / Excitation

B0 RF pulse

Relaxation

• Remove the RF and the spins will loose their energy.

• Realign with B0 : “relax”

• Loose phase coherance : “decay”

Energy loss is variable

Relaxation / Decay

B0

Excitation + Relaxation = MR Signal

+

Current induced in RF coil due to

alternating B field = MRI signal

Bo

NMV

90 RF pulse

Particle Physics : almost done!

Image Formation

Y

X

Z

Spatial Localisation

Gradient B0

B0 -

B0 +

Image Formation

Phase

Frequency

Slice

XX

Y Z

Pixel Mapping

Each line of data is stored as the Image is built up gradually

Fourier transform decodes data + forms the image

Phase

Frequency

X

Y

Z

Resolution

Few pixelsShort scan time

Many pixelsLong scan time

The MR effect!

Differing MR Images

• T1• Fluid dark

• T2• Fluid bright

Relaxation : Free Induction Decay (FID)

• The spins will loose their energy in two ways:

Energy decays slowlyRelaxing back to B0

Loose phase coherance

T1 Recovery

T2 decay

T1 Recovery

B0 Realign with

T2 Decay

In phase

Out of phase

Loose phase

Brownian Motion FAT : • Large , slow molecules• Lots of bumps • Fast energy loss • Short T1 + Short T2

WATER : • Small , fast molecules• Fewer bumps • Slow energy loss • Long T1 + long T2

Mr Blobby Vs Speedy Gonzalez!

Typical T1 + T2 values for tissues (@1.5T)

Tissue T1 value T2 valueDistilled water

Cerebro Spinal Fluid

Gray Matter

White Matter

Fat

Muscle

Liver

Kidney

3000

2400

900

780

260

750

500

760

3000

160

100

90

80

50

40

30

Pulse Sequences

• Pre-set sequences of excitation, relaxation and signal organization that vary tissue contrast and image quality.

T1 and T2 weightings

• Sag spine T1W• Fluid dark

• Sag spine T2W• Fluid bright

Tissue differentiation• > 99% body tissues produce MR signal

• Each tissue has unique properties

- molecular structure

- number of H+ ions

- moving/stationary

• Each tissue behaves differently in the MR environment

Unique MR signals from normal + abnormal tissues

Excellent disease diagnosis.

Tissue contrast : Versatility of MRI

T1 SE T2 SE

GE brainT1 SE+ gad

Tissue contrast : Versatility of MRI

FLAIR Fat sat orbits

Black blood

Angio

Coming up Next !!!Clinical Applications of MRI

MRI Equipment Safety issues of MRI Advantages + Disadvantages of MR MRI vs Other imaging modalities ( CT/ USS) Clinical Images