MRI Coils Types

MRI Coils

• Gradient coils produce controlled variations in the main magnetic field (B0) for spatial localization of signals and position-dependent variation of signal frequency.
• Radiofrequency coils receive and/or transmit the RF signal.
• Shim coils provide supplementary magnetic fields to compensate for inhomogeneities in the main magnetic field.

Coil Safety

• Cables must consist of a conductive material to deliver RF power and send signals to the image processor.
• Coils should have the capacity to transmit heat during normal operation.
• Coil safety considerations include:
  • Avoiding looped or touching cables with the patient or magnet bore.
  • Keeping cables away from other wires or cables, such as ECG leads.
  • Regularly inspecting coil cables and not using them if protection is damaged.

Radiofrequency Coils

• RF coils are critical for image optimization and the selection of the right coil for the area under examination is essential.
• Large coils provide large coverage but relatively poor SNR due to noise from the entire imaging volume.
• Small coils provide small coverage but relatively good SNR due to reduced noise.
• Phase array coils combine the benefits of small and large coils, providing optimal SNR and coverage.

Pulse Control Unit

• The pulse control unit synchronizes the application of gradients and RF pulses in a pulse sequence.

Coil Types

• Large coils:
  • Provide a large area of uniform signal reception.
  • Increase the likelihood of aliasing with small FOV.
  • Are less critical for patient positioning.
  • Have lower SNR, allowing only for lower resolution.
  • Are used in examinations of the torso, such as the chest and abdomen.
• Small coils:
  • Provide a small area of signal reception.
  • Are less likely to produce aliasing artefacts.
  • Require critical patient and coil positioning.
  • Have high SNR, allowing for higher resolution.
  • Are used in examinations of small body parts, such as the wrist, spine, and knee.

Phased Array Coils

• Combine the advantages of small surface coils (increased SNR and resolution) with a large FOV for increased anatomy coverage.
• Consist of multiple coils and receivers (up to four) grouped together to increase longitudinal coverage or improve uniformity across a whole volume.
• Individual signals are combined to create one image with improved SNR and increased coverage.

Operator Interface

• Consists of:
  • Operator control panel for controlling all system functions and image viewing.
  • A computer with specified capabilities.
  • Array processor for Fourier transformation.
  • Image processor that takes data from the array processor to form the image.
  • Hard disc drives for storage of raw data and pulse sequence parameters.
  • Power distribution mechanism to distribute and filter the alternating current.