MRI Terms Overview

MRI (Magnetic Resonance Imaging)

• Non-invasive imaging technique producing detailed internal body images.
• Based on nuclear magnetic resonance, originally a chemical analysis method.
• Images derived from NMR signals in specific body slices.

Magnets

• Defined as objects with a magnetic field that attracts iron or steel.
• A large static magnetic field, known as “Bo,” is necessary to capture MR signals.
• The magnetic field creates alignment within tissues enhancing imaging accuracy.

Magnetic Field

• Exists around magnets or electric currents, characterized by magnetic forces.
• Consists of north and south poles, affecting ferromagnetic and paramagnetic materials.
• Types include bipolar and dipolar magnets, always possessing poles.

Classification of Magnets

• Natural magnets, permanent magnets, and electromagnets based on magnetic origins.

Magnetic States of Matter

• Four categories of magnetic properties: nonmagnetic, diamagnetic, paramagnetic, and ferromagnetic.
• Nonmagnetic materials are unaffected by magnetic fields (e.g., glass, rubber, wood).
• Diamagnetic materials are weakly repelled (e.g., gold, diamonds, lead).
• Paramagnetic materials are weakly attracted and include gadolinium, an MRI contrast agent.
• Ferromagnetic materials can be strongly magnetized (e.g., iron, nickel).

Magnetic Field Units

• Measured in gauss (smaller unit) and tesla (1 tesla = 10,000 gauss).
• Earth's magnetic field strength is approximately 0.5 gauss.

Magnet Room

• Central component in MR systems with magnets surrounding the patient and necessary antennas.

Types of Magnets Used in MRI

• Resistive, superconductive, and permanent magnets are the main types utilized.

Resistive Magnets

• Large electromagnets requiring electric current passed through wire coils to generate a magnetic field.
• Field strength can reach up to 0.3 tesla; limited by heat generated.
• Advantages include being turn-off capable, while disadvantages relate to heat sensitivity and lower max strength.

Superconductive (Cryogenic) Magnets

• Utilize low temperatures with cryogens (liquid helium/nitrogen) to minimize resistance, allowing higher field strength (0.5 to 1.5 tesla).
• High signal-to-noise ratio (SNR) is a key benefit; however, high acquisition and maintenance costs are notable disadvantages.
• Sudden loss of superconductivity, known as "quench," results in a loss of magnetic field.

Permanent Magnets

• Consist of natural ferrous materials that maintain a constant magnetic field without electricity or cooling.
• Field strength ranges from 0.06 to 0.35 tesla, with field extension being less intrusive than other magnet types.

General Styles of Magnets Employed

• Vertical field (open MRI) and conventional horizontal field.

Key Questions

• Permanent magnets are characterized by consistent strength without electricity and minimal field expansion.
• Advantages include lack of power reliance; disadvantages are lower field strengths compared to other types.
• Resistive electromagnets are characterized by simple designs, limited by heat and maximum field strength.
• Advantages of resistive magnets include the ability to switch off; disadvantages involve temperature sensitivity and weaker fields.