MRI Magnet System, Shimming & Quenching Explained (Easy Guide)

Introduction

Magnetic Resonance Imaging (MRI) works on a very powerful and precise magnetic field.
To understand MRI properly, every student and technologist must clearly know the Magnet System, Cryogen System, Quenching, and Shimming.

In this article, we will explain:

MRI Magnet System
Superconducting Magnet
Cryogen System
What is Quenching?
Shimming System (Passive & Active)
Viva and exam-oriented points

MRI Magnet System, Shimming & Quenching

1. MRI Magnet System – The Heart of MRI

The Magnet System is the most important component of an MRI scanner.

Function of Magnet System

Produces a strong static magnetic field (B₀)
Aligns hydrogen protons in the human body
Allows MRI signal generation

Common MRI Magnetic Field Strengths

0.5 Tesla
1.5 Tesla (Most commonly used)
3 Tesla
7 Tesla (Research use)

2. Superconducting Magnet

Most modern MRI scanners use a Superconducting Magnet.

Construction

Made of Niobium–Titanium (NbTi) coils
Cooled using Liquid Helium
Operating temperature: –269°C (4 Kelvin)

Why Superconductivity?

At extremely low temperatures:

Electrical resistance becomes zero
Current flows continuously
A strong and stable magnetic field is created

This makes superconducting magnets ideal for clinical MRI.

3. Cryogen System

The Cryogen System keeps the magnet coils at superconducting temperature.

Components

Liquid Helium
Cryostat tank
Vacuum insulation

Role

Maintains low temperature
Prevents heat entry
Ensures stable magnetic field

4. What is Quenching?

Quenching is an emergency condition in MRI.

Definition

Quenching is the sudden loss of superconductivity, causing:

Rapid collapse of the magnetic field
Sudden release of liquid helium as gas

Causes of Quenching

Helium leakage
System failure
Emergency manual quench

Why Quenching is Dangerous?

Oxygen displacement risk
Frostbite hazard
Loud noise and pressure release

Modern MRI rooms have quench pipes to safely vent helium gas outside.

5. Shimming System

A strong magnetic field is not enough — it must be uniform.

Why Magnetic Field Uniformity is Important?

MRI signal frequency depends on the magnetic field.

Larmor Frequency ∝ Magnetic Field Strength

If the field is non-uniform:

Signal mismatch occurs
Image quality decreases
Artifacts appear

This problem is solved by Shimming.

6. Types of Shimming

1. Passive Shimming

Uses small metal plates
Done during installation
Fixed correction

2. Active Shimming

Uses electromagnetic shim coils
Computer controlled
Automatically adjusts field uniformity
Used in modern MRI scanners

7. Artifacts Due to Poor Shimming

If shimming is not proper, the following artifacts may appear:

Signal voids
Chemical shift artifact
Image distortion
Blurring

Poor shimming mostly affects abdominal MRI.

Saturday, March 7, 2026