Magnetic Resonance Imaging (MRI)

Introduction

Magnetic Resonance Imaging (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. It is based on principles of nuclear magnetic resonance (NMR), a spectroscopic technique used to obtain microscopic chemical and physical data about molecules.

Basic Principle of MRI

The basic principle of MRI involves the alignment of nuclear magnetization using a strong magnetic field. The main magnetic field is denoted by . When a secondary, oscillating magnetic field (radio frequency pulse) is applied, the nuclear magnetization vector is deflected. Once the field is turned off, the nuclear magnetization vector returns to its equilibrium position, a process called relaxation. This relaxation process produces a detectable signal that can be used to construct an image.

Larmor Frequency

The precession of nuclear spins in the magnetic field occurs at a specific frequency, known as the Larmor frequency, which is given by:

where is the Larmor frequency, is the gyromagnetic ratio (which is specific for each type of nucleus), and is the magnetic field strength.

Spin Echo and Image Formation

A technique known as spin echo is used to generate the signal for MRI images. A sequence of radiofrequency (RF) pulses and gradient fields is applied to the patient, and the response signal is recorded. The signal is then Fourier transformed to produce the image.

T1 and T2 Relaxation

Two different types of relaxation are typically measured in an MRI scan: T1 (spin-lattice) and T2 (spin-spin) relaxation.

T1 relaxation is the time it takes for the longitudinal magnetization to recover to 63% of its initial value after a 90-degree pulse. It is given by:

T2 relaxation is the time it takes for the transverse magnetization to decay to 37% of its initial value due to dephasing among spins in the transverse plane. It is given by:

where and are the longitudinal and transverse components of magnetization at time , is the initial magnetization, and and are the relaxation times.

MRI Contrast Agents

Contrast agents are often used in MRI to enhance the contrast between different tissues. Gadolinium-based contrast agents (GBCAs) are the most commonly used, as gadolinium ions affect both T1 and T2 relaxation.

Functional MRI (fMRI)

Functional MRI (fMRI) is a type of MRI that measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area of the brain is in use, blood flow to that region also increases.

Safety and Limitations of MRI

Despite the many advantages of MRI, such as its non-invasiveness and its ability to produce images in any plane, it also has limitations. Patients with certain implants, such as pacemakers, cannot be scanned with MRI due to the strong magnetic field. Additionally, MRI scans are time-consuming compared to other imaging techniques.

Do you prefer video lectures over reading a webpage? Follow us on YouTube to stay updated with the latest video content!

Want to study more? Visit our Index here!