MRI physics overview | MRI Physics Course | Radiology Physics Course #1

Introduction to MRI Physics

Overview of the Course

• The course consists of multiple talks, each focusing on specific topics within MRI physics.

• Learning MRI physics is likened to building a large puzzle; understanding individual pieces is essential before seeing the whole picture.

• The aim is to provide a conceptual understanding of how MRI physics operates by breaking down complex ideas into manageable sections.

Structure of the MRI Machine

• A 3D model of an MRI machine illustrates its layered structure, with each layer representing different magnets used for imaging.

• Unlike X-ray and CT imaging, MRI generates signals from within the patient, necessitating localization techniques.

Understanding Coordinate Systems in MRI

Cartesian Plane in Imaging

• The image can be divided into three axes: longitudinal (Z-axis), transverse (X-Y plane), and axial planes.

• These planes are crucial concepts that will be explored further in upcoming discussions.

Nuclear Magnetic Resonance Basics

Role of Hydrogen Atoms

• Nuclear magnetic resonance (NMR) utilizes hydrogen atoms due to their abundance and non-zero spin properties.

• Hydrogen atoms act as tiny bar magnets with a magnetic moment, which contributes to image generation in MRI.

Magnetic Field Influence

• When exposed to a magnetic field, hydrogen protons align and precess around their axis, similar to a spinning top under gravity.

• The precession frequency is proportional to the strength of the magnetic field; stronger fields result in higher frequencies.

Magnetic Moments and Net Magnetization

Alignment and Energy States

• Hydrogen atoms can align either parallel or anti-parallel to the magnetic field; more atoms align parallelly at lower energy states.

Net Magnetization Vector

Understanding MRI Imaging and the Net Magnetization Vector

The Role of the Net Magnetization Vector in MRI

• MRI imaging focuses on the net magnetization vector, which is influenced by changing magnetic fields within the MRI machine. This vector represents the collective behavior of hydrogen atoms in a patient's body.

• Direct measurement of this net magnetization vector along the longitudinal direction is not feasible due to interference from strong main magnetic field strength. Instead, it must be moved perpendicular to this field for effective measurement.

Application of Radio Frequency Pulses

• A secondary magnetic field, known as a radio frequency pulse, is applied perpendicularly to the main magnetic field. This pulse alternates at a frequency matching that of protons' precessional frequency.

• When the radio frequency pulse matches the precessional frequency of hydrogen atoms, two outcomes occur: protons fan out to align more perpendicularly with the main magnetic field and their precessional frequencies synchronize (process in phase).

Generating Transverse Magnetization

• The application of a radio frequency pulse results in transverse magnetization (in the XY plane), flipping the net magnetization vector by an angle known as the flip angle; typically set at 90 degrees.

• With protons processing in phase after being flipped, they induce a current within a receiver coil when they move, allowing for image generation based on this induced signal.

Signal Generation and Free Induction Decay

• The movement of this net magnetization vector generates signals measured in the transverse plane. However, once we stop applying the radio frequency pulse, these vectors begin to lose coherence.

• As coherence diminishes post-pulse cessation, individual net magnetization vectors become out-of-phase leading to reduced signal strength over time—a phenomenon referred to as free induction decay or T2* decay curve.

Tissue-Specific Characteristics and Contrast Generation

• Different tissues exhibit unique T2* curves; for instance, water has a slow free induction decay while bone or fat decays much faster. These differences are crucial for generating contrast in MRI images.

Understanding Transverse and Longitudinal Magnetization

The Dynamics of Transverse Magnetization

• Transverse magnetization loss occurs due to protons going out of phase with one another, leading to free induction decay (T2 star), which happens more rapidly than the regaining of longitudinal magnetization.

Regaining Longitudinal Magnetization

• Over time, longitudinal magnetization increases while transverse magnetization is completely lost; protons regain some longitudinal alignment but remain out of phase.

Independent Processes: T1 Recovery and T2 Star Decay

• Longitudinal relaxation (T1 recovery) and free induction decay occur independently; knowing one does not allow for calculation of the other. Different tissues exhibit varying rates for these processes, crucial for image contrast.

Measuring Signal in MRI

• Signal measurement is challenging for longitudinal magnetization since it must be flipped perpendicular to the magnetic field to be detected.

Time Parameters in MRI Imaging

• Two key parameters are used in imaging: echo time (TE) and repetition time (TR). TE measures signal generation after a 90-degree RF pulse, while TR is the interval between two RF pulses.

Contrast Generation in MRI

Echo Time and Tissue Contrast

• As time progresses post-RF pulse, differences between tissue signals increase while overall signal strength decreases; this trade-off affects image quality versus contrast.

Longitudinal Relaxation Rates

• Different tissues gain longitudinal magnetization at different rates over time. If sufficient waiting occurs, full net longitudinal vectors can be achieved before flipping again with a second RF pulse.

Short Repetition Times Impact on Signal

• A short TR results in incomplete T1 recovery; fat exhibits longer longitudinal magnetization compared to water. This discrepancy leads to stronger transverse signals from fat when measured post-RF pulse.

Understanding T1 Differences

• Short TR times highlight differences in T1 recovery rather than T2 differences. This concept is essential for generating specific contrasts within images based on tissue properties.

Future Learning Directions

Understanding MRI Signal Generation and Imaging Techniques

The Basics of T2 Relaxation

• The subcutaneous fat exhibits a high signal in MRI due to its long time to repetition, allowing tissues to regain magnetization before the 90-degree flip, which is essential for generating T2 images.

Differences Between Water and Fat Signals

• The distinction between water and fat signals arises from their differing rates of dephasing in the transverse plane; water maintains a high signal while fat's signal diminishes quickly due to spin-spin interactions.

Implications of T2 Relaxation on Imaging

• Fat dephases faster than water, resulting in dark signals in fat-coated axons within white matter and bright signals in cerebrospinal fluid (CSF), highlighting the importance of T2 relaxation differences.

Pulse Sequences in MRI

• Generating MRI images involves complex processes beyond basic principles, focusing on echo time and repetition time. Different pulse sequences like spin echo, inversion recovery, and gradient echo are crucial for image localization.

Advanced Imaging Techniques

• This module will cover advanced imaging techniques including MR spectroscopy and various types of angiography. It will also address different MRI artifacts along with considerations for image quality and safety.

Data Encoding with K-Space

• To create images from generated signals, data is stored using k-space encoding. Understanding how to fill k-space is vital for constructing scrollable images from stacked data.

Learning Pathway Through the Module

• Each subsequent talk will delve into specific components discussed here, helping learners piece together a comprehensive understanding of MRI physics as they progress through the module.

Assessment Resources