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Introduction to Magnetic Resonance Imaging

درس مقدمه ای بر فیزیک پزشکی

Sahand University of Technology

Faculty of Electrical Engineering

M. Shamsi

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Magnetic Resonance Imaging

• Non-invasive medical imaging method, like ultrasound and X-ray.

• Clinically used in a wide variety of specialties.

Abdomen Spine Heart / Coronary

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Magnetic Resonance Imaging

Advantages:• Excellent / flexible contrast• Non-invasive• No ionizing radiation• Arbitrary scan plane

Challenges:• New contrast mechanisms • Faster imaging

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Topics

• MRI Physics

• Hardware

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Topics

• MRI Physics

• Hardware

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Magnetic Resonance

• Certain atomic nuclei including 1H exhibit nuclear magnetic resonance.

• Nuclear “spins” are like magnetic dipoles.

1H

MRI Physics

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Polarization

• Spins are normally oriented randomly.

• In an applied magnetic field, the spins align with the applied field in their equilibrium state.

• Excess along B0 results in net magnetization. No Applied Field Applied Field

B0

MRI Physics

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MRI Physics

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Static Magnetic Field

Longitudinal

Transverse

B0

z

x, y

MRI Physics

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Precession

• Spins precess about applied magnetic field, B0, that is along z axis.

• The frequency of this precession is proportional to the applied field:

B

MRI Physics

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Relaxation

• Magnetization returns exponentially to equilibrium:• Longitudinal recovery time constant is T1

• Transverse decay time constant is T2

• Relaxation and precession are independent.

Precession Decay Recovery

MRI Physics

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Signal Reception• Precessing spins cause a change in flux () in

a transverse receive coil.

• Flux change induces a voltage across the coil.

Signal

y

x

B0

z

MRI Physics

Origin of the MR Signal

Before Excitation

After Excitation

Excitation tips the net magnetization (M) down into the transverse plane, where it can generate current in detector coils.

During Excitation (to)

During Excitation (t1)

The amount of current oscillates at the (Larmor) frequency of the net

magnetization.

MRI Physics

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Topics

• MRI Physics

• Hardware

MRI hardware

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x 60,000 =

The Big MagnetMain field = B0

• Continuously on

• Very strong : Earth’s magnetic field = 0.5 Gauss / 1 Tesla (T) = 10,000 Gauss

3 Tesla = 3 x 10,000 0.5 = 60,000 x Earth’s magnetic field

B0

Robarts Research Institute 3T

MRI hardware

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MRI hardware

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MRI hardware

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MRI hardware

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MRI hardware

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MRI hardware

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At $2 million, the most expensive equipment in the hospital…

MRI hardware

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Superconducting Magnet

MRI hardware

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Resistive Electromagnet

MRI hardware

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Permanent MRI System

MRI hardware

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MRI hardware

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MRI hardware

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MRI hardware

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MRI hardware

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MRI hardware

Head CoilHead Coil

31Head CoilHead Coil

MRI hardware

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RF Receiver Coils

MRI hardware

Three Sets of Gradient Coils

MRI hardware

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GRADIENTS

MRI hardware

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MRI hardware

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MRI hardware

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MRI hardware

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MRI hardware

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MRI hardware

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MRI hardware

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MRI hardware

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MRI hardware

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43Surface CoilsSurface Coils

MRI hardware

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MRI hardware

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MRI hardware

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Exclusion of Various Objects

MRI hardware

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Exclusion of Ferromagnetic Objects

MRI hardware

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Shielding of MRI Room

MRI hardware

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49•Delivery of 1.5T Superconducting MagnetDelivery of 1.5T Superconducting Magnet

MRI hardware

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50• 0.2 T Open MRI with permanent magnet (11 tons)0.2 T Open MRI with permanent magnet (11 tons)

MRI hardware

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51• 0.7 T Open MRI with superconducting magnet0.7 T Open MRI with superconducting magnet

MRI hardware

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52Body Array and Head Array CoilsBody Array and Head Array Coils

MRI hardware

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MRI hardware

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