Magnetic Resonance Imaging (MRI) Introduction

Magnetic Resonance Imaging (MRI)

• MRI is a medical imaging modality that uses magnetic fields and resonance to produce images of the body.

Unit Overview

• The unit will be delivered onsite over 4 sessions with additional learning resources available on CANVAS.
• A Question and Answer session will be held on the 31st October, and a revision session will be facilitated on the 30th January 2024.

Learning Outcomes

• Differentiate between MRI and Computed Tomography (CT).
• Describe the basics of image generation in MRI.
• Correctly identify, view, and orientate an MR image.
• Identify common pulse sequences used in MRI.

Physical Principles of MRI

• MRI sounds different from CT sounds.
• MRI measures different things than CT.
• MRI developed into an important imaging modality from 1978.
• MRI utilizes the fact that magnetic nuclei in a static magnetic field exhibit a characteristic resonance frequency proportional to the field strength.

Generating the MRI Signal

• MRI primarily maps the distribution of hydrogen nuclei within the body.
• The medical imaging community has recognized MRI's unique ability to image soft tissue, differentiate between benign and malignant tissue, and provide multi-planar capabilities.

MRI Terminology

• MRI stands for Magnetic Resonance Imaging.
• MRA stands for MR Angiography.
• MRS stands for MR Spectroscopy.
• fMRI stands for Functional MRI.

MRI Hardware

• Gradient coils have evolved over time.
• Inside an MRI scanner, there are strong magnetic fields, gradient coils, and RF coils.

MRI Field Strengths

• High field systems: 1.0 to 3.0 Tesla (standard clinical systems).
• Low field systems: 0.2 – 0.5 Tesla (some clinical sites, but mainly research or spectroscopy).
• Up to 7 Tesla (high field systems).

Principles of Magnetism and Resonance

• MRI relies on the principles of magnetism and resonance to produce an image.
• The hydrogen proton is targeted because of its abundance in the body.
• A spinning proton produces a magnetic charge.

Atomic Structure

• Atoms are made up of electrons, protons, and neutrons.
• Protons and neutrons have a property of spin.

Nuclear Spin

• Protons and neutrons have a net spin that can be aligned with an external magnetic field.
• In most nuclei, they cancel each other out, leaving no net spin.

Hydrogen Atoms

• Hydrogen atoms are abundant in the human body.
• They have a strong magnetic moment.

Magnetic Moment

• A moving (or rotating) particle with electric charge has an associated magnetic field.
• Protons can be thought of as a tiny bar magnet.

Magnetic Distribution

• Magnetic moment in an external magnetic field becomes aligned with the field.
• Alignment is not exact because of the proton's spin.

Energy States

• Hydrogen atoms have two stable orientations: spin up (lower energy) and spin down (higher energy).
• There is always a small excess of spins in the lower energy state, producing a net magnetization M0 parallel to the direction of B0.

Net Magnetization Vector

• M0 represents the average behavior of all protons.
• M0 can be separated into components: longitudinal (z-axis) and transverse (x and y axes).

Vector Components

• In MRI, the z-axis is defined by the direction of B0.
• There is no difference between x and y axes, which are both perpendicular to B0.

Review

• MRI is based on the interaction between hydrogen atoms and a magnetic field.
• The magnetic moment of the protons precesses around the magnetic field at a particular frequency.

Generating the MR Signal

• Magnetic Resonance Imaging (MRI) requires a scanner to generate a strong magnetic field (B0) and radiofrequency (RF) pulses to excite the signals.
• Gradients are used to localize the signals by changing the magnetic field strength in a defined way, causing protons to change their resonant frequencies to match.
• The signal is optimized by exciting it multiple times and adjusting the signal-to-noise ratio (SNR).

Generating B0

• The main magnet is a cylindrical bore with a diameter of approximately 60cm, and a field strength of 1.5-3 Tesla.
• The magnet is superconducting, and it is surrounded by gradients and body RF coils.
• The gradients and body coil are used to localize the signal and improve the signal-to-noise ratio.

Localizing MR Signal

• The Larmor frequency is used to localize the signal, which depends on the strength of the magnetic field and the gyromagnetic ratio.
• The frequency of the signal changes depending on the location within the magnetic field.
• The isocentre is the point where the magnetic field strength is at its maximum.

Slice Selection

• Slice selection is achieved by applying a gradient to the magnetic field, which causes the resonant frequency to change.
• The frequency of the signal changes depending on the location within the slice.
• The slice selection gradient is used to select the desired slice.

Signals

• The signal is composed of many frequencies, which are added together to form a complete signal.
• The signal is a mixture of frequencies, which are used to create an image.

Total Signal and Fourier Transform

• The total signal is composed of all the frequencies, which are added together.
• The Fourier transform is used to convert the time-domain signal into a frequency-domain signal.
• The frequency-domain signal is used to create an image.

Displaying an Image

• The image is created by taking the Fourier transform of the signal and displaying it as a function of frequency.
• The image is a representation of the spatial distribution of the signal.

Image Matrix

• The image matrix is a 2D array of pixels, which represents the spatial distribution of the signal.
• The image matrix is used to create a visible image.

Review

• Gradients are used to modify the frequency of the signals in a controlled way.
• The frequency and amplitude of the signals are used to localize the signals within the magnet.
• Pulse sequences are used to create an image with different contrast.

Pulse Sequences

• A pulse sequence is a set of changing magnetic gradients and RF pulses.
• Each pulse sequence has a number of parameters, and multiple sequences are grouped together into an MRI protocol.
• Pulse sequences are used to produce images with different contrast.

Spin Echo Pulse Sequence

• The spin echo pulse sequence is a basic sequence that produces a high signal-to-noise ratio (SNR) and resolution.
• The sequence consists of a 90° RF pulse, followed by a 180° RF pulse, and a data collection period.

Spin Echo Pulse Sequence Parameters

• The time to echo (TE) is the time between the 90° RF pulse and the data collection period.
• The time to repeat (TR) is the time between successive 90° RF pulses.

Rephasing

• Rephasing is the process of re-aligning the magnetic moments of the protons.
• Rephasing is used to refocus the signal and improve the signal-to-noise ratio.

Image Contrast

• The contrast in the image depends on the TR and TE.
• The TR controls the amount of T1 relaxation, which affects the contrast.
• The TE controls the amount of T2 relaxation, which affects the contrast.

T1 and T2 Relaxation

• T1 relaxation is the process of longitudinal magnetization recovery.
• T2 relaxation is the process of transverse magnetization decay.
• T1 and T2 relaxation times are tissue-dependent and affect the contrast in the image.

T1 Relaxation Times

• Typical T1 relaxation times for different tissues:
  • Muscle: 630 ms
  • Fat: 190 ms
  • Grey matter: 825 ms
  • White matter: 685 ms
  • CSF: 1200 ms

T2 Relaxation Times

• Typical T2 relaxation times for different tissues:
  • Muscle: 40 ms
  • Fat: 110 ms
  • Grey matter: 110 ms
  • White matter: 105 ms
  • CSF: 800 ms

A review of image generation in MRI, covering the basics of the unit and its applications. This quiz is designed for students in the medical field.