Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging (MRI)

• MRI is a medical imaging technique used in radiology to visualize detailed internal structures
• It uses the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body

Indications for MRI

• MRI visualizes detailed internal structures
• NMR is used to image atomic nuclei

Concept of MRI image

• The MRI image is a display of radio frequency signal intensities emitted by magnetized tissue during imaging
• Tissue becomes magnetized because it contains protons (magnetic nuclei of hydrogen atoms)
• When placed in a strong magnetic field, some protons align with the field, producing the magnetization of the tissue

Advantages of MRI

• No ionizing radiation, no short/long-term effects
• Variable thickness, any plane
• Better contrast resolution and tissue discrimination
• Various sequences to characterize abnormal tissue
• Many details without intravenous (IV) contrast

Disadvantages of MRI

• Very expensive
• Dangerous for patients with metallic devices within the body
• Difficult for claustrophobic patients (fear of enclosed spaces)
• Movement during scanning may cause blurry images
• RF transmitters can cause severe burns if mishandled
• Not easily available

Components of the MRI System

• Magnet:

  • A key element, integrated into the system
  • Includes RF and gradient systems
  • Superconducting magnet: heart and most expensive part of the MRI scanner
  • The magnetic field requirement in modern MRI scanners is limited to 3T
  • The main magnet is superconducting, cooled to LHe temperature(liquid Helium) and mounted in an efficient cryostat
  • Has a horizontal bore to accommodate the patient

• Gradient coils:

  • Used to produce deliberate variations in the main magnetic field

  • Usually three sets, one for each direction

  • The variation in the magnetic field permits localization of image slices, also phase encoding and frequency encoding.

  • The gradient coils for the z-axis are Helmholtz pairs, and the x and y axes have paired saddle coils -Generate secondary magnetic fields within the primary magnetic field

  • Arranged opposing one another for +ve and -ve pulses.

  • Cause variation in magnetic field, allowing spatial variation along the main field direction

  • Responsible along with RF pulses for slice and voxel formation

  • An extra magnetic field added to the main magnetic field

  • X coil creates a varying magnetic field from left to right

  • Y coil creates a varying magnetic field from top to bottom

  • Z coil creates a varying magnetic field from head to toe

• RF coils:

  • Act as antennas; transmitting and receiving RF signals.
  • Loops of wire (circular or rectangular).
  • Located inside the gradient coils
  • Produce the field that rotates spins by an angle dependent on the pulse sequence
  • Detect the signal emitted by spins inside the body
  • Coils start RF pulses (excitation- protons jump to higher energy levels by absorbing radiation)
  • Coils stop RF pulses (relaxation - protons return to original state emitting radiation)

• Other components:

  • Power supplies
  • Computer systems
  • Documentation systems
  • Cooling systems (for the superconducting magnet)
  • Receives RF signal then performs conversion from analog to digital. A digital signal representing the body part is stored temporarily in image space or case space, during data acquisition. Digital signal then sent to image processor where mathematical formula known as Fourier transformation is applied to display MRI scan on a monitor.

This quiz covers the fundamentals of MRI, including its techniques, indications, advantages, and disadvantages. Explore how MRI uses nuclear magnetic resonance to produce detailed images of internal structures without ionizing radiation. Test your knowledge on the principles and applications of this vital medical imaging technique.