Physics · MU Sofia

19. Magnetic resonance imaging (MRI)

Explains how MRI produces three-dimensional images of soft tissues, joints and organs using static magnetic fields and radio waves, based on nuclear magnetic resonance of hydrogen atoms. Covers spin alignment, resonance and relaxation, and why MRI avoids ionising radiation. Also notes contrast dyes, the effect of metal implants, and the Larmor equation linking precession frequency to the magnetic field strength.

Inside: 1 section · 2 labelled pictures · 7 flashcards · 3 min

Topic 20 of 42 in the MU Sofia syllabus for Physics (Dental Medicine, Year 1). The free AceMed note covers it in 1 section, about 387 words (3 min of reading), with 7 flashcards.

In this note

  1. Introduction

Introduction

Summary

  • Magnetic Resonance Imaging (MRI) utilizes static magnetic fields and high-frequency electromagnetic (EM) waves to generate detailed images of internal structures, particularly effective for soft tissues, joints, and internal organs. The technique is based on the nuclear magnetic resonance phenomenon where hydrogen atoms in the body, when exposed to a magnetic field, absorb radio waves, change their spin, and then release these waves upon relaxation. This emission is detected to create a three-dimensional image. MRIs are advantageous as they do not involve ionizing radiation and can be enhanced with contrast dyes for clearer visibility.

Principles of MRI Diagnostics

  • Static Magnetic Fields: Generated either by permanent magnets or DC electricity.
  • Nuclear Magnetic Resonance: Selective absorption of high-frequency EM radio waves by atomic nuclei, crucial for image formation.
  • Applications: Particularly useful for diagnosing injuries in soft tissues, joints, the brain, and internal organs.

Spin Dynamics in MRI

  • Spin Orientation: Atomic spins are randomly oriented without a magnetic field.
  • Alignment: Under a magnetic field, spins align with the field's direction.
  • Resonance and Relaxation:
  • Exposure to radio waves changes the spin orientation, causing resonance.
  • Post-resonance, spins realign with the magnetic field during relaxation, releasing radio waves detectable by MRI technology.
  • Mainly involves hydrogen atoms in the human body.

MRI Imaging and Safety

  • Image Formation: Hydrogen atoms, aligned by a strong magnet and reoriented by radio waves, emit detectable radio waves during relaxation to form a 3D image.
  • Safety: MRI does not use ionizing radiation, making it a safer option compared to other imaging modalities.
  • Use Cases: Effective for visualizing tumors, torn ligaments, and infections, particularly in the brain.
  • Contrast Enhancement: Contrast dyes may be injected to produce clearer images.
  • Considerations with Metal: MRI imaging may be hindered by metal implants due to interference with the magnetic field.
  • Equation to know :
  • The Larmor equation is primarily used to describe Larmor precession, which is the precession of the magnetic moments of charged particles in response to an external magnetic field. This precession is an important concept in nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), and in understanding atomic and molecular magnetic fields. w=yB
  • where :
  • ω (omega) is the Larmor frequency,
  • γ (gamma) is the gyromagnetic ratio, a property of the particle being considered, which relates the magnetic moment and angular momentum,
  • B is the magnitude of the external magnetic field.

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