Physics of magnetic resonance imaging

Introduction

Modern 3T MRI scanner

The physics of magnetic resonance imaging (MRI) involves the principles of nuclear magnetic resonance (NMR) to create detailed images of the human body. MRI is a non-invasive imaging technology that produces three-dimensional detailed anatomical images. It is often used for disease detection, diagnosis, and treatment monitoring.

Basic Principles

MRI is based on the principles of NMR, which involves the interaction of nuclear spins when placed in a magnetic field. The primary components of MRI include a strong magnetic field, radiofrequency (RF) pulses, and gradient fields.

Magnetic Field

The main magnetic field, denoted as B₀, aligns the nuclear spins of hydrogen atoms in the body. This alignment is crucial for the subsequent steps in the imaging process.

Radiofrequency Pulses

MRI 2D Fourier Transform Spin Echo Pulse Sequence

RF pulses are used to perturb the alignment of the nuclear spins. When the RF pulse is turned off, the spins return to their equilibrium state, emitting RF signals in the process. These signals are detected and used to construct images.

Gradient Fields

Gradient fields are used to spatially encode the positions of the spins. By varying the magnetic field linearly across the body, the frequency of the emitted signals can be used to determine the location of the spins.

Image Formation

The process of image formation in MRI involves several steps:

Signal Detection

The emitted RF signals are detected by receiver coils. These signals are known as free induction decay (FID).

Fourier Transform

The detected signals are processed using a mathematical technique called the Fourier transform, which converts the time-domain signals into frequency-domain data.

Image Reconstruction

The frequency-domain data is used to reconstruct images of the body. This involves complex algorithms that take into account the spatial encoding provided by the gradient fields.

Artifacts and Limitations

MRI images can be affected by various artifacts, which are distortions or errors in the images. Common artifacts include motion artifacts, susceptibility artifacts, and chemical shift artifacts.

Advanced Techniques

MRI scanner schematic

Advanced MRI techniques include functional MRI (fMRI), diffusion-weighted imaging (DWI), and magnetic resonance spectroscopy (MRS). These techniques provide additional information about the physiological and biochemical processes in the body.

Safety Considerations

MRI is generally considered safe, but there are important safety considerations due to the strong magnetic fields and RF energy. Patients with metallic implants or devices must be carefully screened before undergoing an MRI scan.

Related Pages

• Nuclear magnetic resonance
• Functional magnetic resonance imaging
• Diffusion MRI
• Magnetic resonance spectroscopy