MRI Machine - Main, Gradient and RF Coils/ Magnets | MRI Physics Course | Radiology Physics Course#2

Understanding MRI Physics: The Main Coil

Introduction to MRI Topics

• The speaker welcomes the audience and reflects on the introductory talk in the MRI module, emphasizing that many important topics were covered superficially.

• The focus of this session is to delve deeper into each topic related to MRI physics, particularly the components of an MRI machine.

Structure of the MRI Machine

• The MRI machine consists of multiple layers, with each layer representing a different magnet.

• The outermost layer is known as the main coil, which generates a magnetic field when current flows through it.

Generating Magnetic Fields

• Ampere's law states that running current through a wire creates a magnetic field; when wires are coiled, these fields superimpose to create a larger magnetic field (B naught).

• The strength of B naught depends on two factors: the number of coils and the amount of current flowing through them.

Superconductivity in MRI Machines

• To achieve sufficient current for strong magnetic fields, superconductors are used; niobium titanium alloys are commonly employed.

• Liquid helium circulates around coils to maintain low temperatures (below 4 degrees Kelvin), essential for superconductivity.

Resistance and Critical Temperature

• As temperature increases in non-superconductors, resistance also increases; however, superconductors exhibit an abrupt drop-off in resistance at their critical temperature (around 4 degrees Kelvin).

• If temperatures exceed this threshold, resistance increases dramatically leading to heat generation and potential quenching—a safety feature where gaseous helium is released.

Functionality of Main Magnetic Coil

• The main coil applies B naught along the longitudinal axis (Z-axis), aligning hydrogen atoms within patients.

• Hydrogen protons precess at frequencies determined by their gyromagnetic ratio and the strength of B naught; stronger fields result in faster precession.

Manipulating Magnetic Field Strength

• Adjusting current within the main coil alters magnetic field strength, subsequently changing hydrogen proton precession frequency.

The Role of Shims in Achieving Homogeneity

Understanding Shims

• Shims are structures within the main coil designed to manipulate and improve homogeneity in the magnetic field.

Importance of Homogeneous Magnetic Fields

Understanding MRI Shimming Techniques

Types of Shims in MRI

• There are two main types of shims used in MRI: passive shims and active shims. Passive shims consist of magnetic sheets or ferromagnetic metals that passively adjust the magnetic field within the MRI bore.

• Active shims, on the other hand, have their own electricity supply and can actively manipulate the main magnetic field by adjusting the current running through their coils.

• The goal of using both types of shims is to create a more homogeneous magnetic field, which enhances signal localization during MRI imaging.

Gradient Coils Functionality

• The next component discussed is the gradient coils, which apply a gradient along the magnetic field. These coils are positioned perpendicular to each other across different planes (X, Y, Z).

• Gradient coils play a crucial role in spatial localization by creating varying magnetic fields that affect how hydrogen atoms process signals based on their position within the scanner.

• In clinical imaging, typical magnetic field strengths range from 1 to 3 Tesla. The gradient coils can generate additional magnetic fields that superimpose over the main B naught field.

Manipulation of Magnetic Fields

• By running current through gradient coils in opposite directions, distinct magnetic fields are created that either reduce or enhance the overall strength of the main B naught field at different ends of the scanner.

• This manipulation results in a differential magnetic field strength along the z-axis, affecting how protons respond based on their location within this gradient.

• As a result, protons experience different precessional frequencies depending on their position relative to these gradients; stronger gradients lead to faster processing frequencies for hydrogen protons.

Spatial Encoding with Gradients

• The primary function of gradient coils is to change or manipulate magnetic field strength along various axes (X, Y, Z), forming a foundation for spatial encoding in MRI imaging.

• The unchanged part of this system is known as the iso sensor, where there’s uniformity with respect to background B naught strength despite variations elsewhere due to gradients.

Role of Radio Frequency Coils

• Finally, we discuss radio frequency (RF) coils, which generate an alternating magnetic field perpendicular to the main static B naught field.

• RF pulses interact with hydrogen protons oscillating at specific frequencies determined by external fields and gradients applied previously.

Understanding Radio Frequency Pulses in MRI

The Interaction of Hydrogen Atoms and Radio Frequencies

• Children swinging at different frequencies illustrate how only hydrogen atoms resonating at the same frequency as a radio pulse gain energy, highlighting the importance of matching frequencies for effective resonance.

• When hydrogen atoms process at a specific frequency that matches the radio frequency pulse, they experience two key effects: increased energy absorption and protons fanning out more significantly.

• The radio frequency pulse causes protons to transition from being out of phase to becoming in phase with one another, leading to an increase in their flip angle as they fan out further.

• Prolonged application of the radio frequency pulse results in a larger flip angle, facilitating the measurement of signals by shifting net magnetization from the longitudinal plane into the transverse plane.

• By aligning the radio frequency pulse with the central processing frequency, we can achieve a 90-degree flip of net magnetization, enabling isolation of specific hydrogen atoms within a patient—a process known as slice selection.

Summary of Magnetic Field Manipulation Techniques

• The main magnetic field is generated along the longitudinal axis using active and passive shims to ensure homogeneity; this is crucial for accurate imaging.

• Gradient coils adjust magnetic field strength across various axes (Z, X, Y), allowing for precise control over imaging parameters and enhancing signal localization.

• Radio frequency coils are responsible for selecting specific hydrogen protons based on their processing frequencies and flipping them into the transverse plane to facilitate signal measurement necessary for generating MRI images.

Exploring Nuclear Magnetic Resonance