MRI instrumentation and MRI safety
After reading this chapter, you should be able:
- To present the different types of main magnet, their advantages and drawbacks
- Describe the role of liquid Helium and its safety implications (quench)
- Characterize a magnetic field gradient, its specifications and performance
- Describe the Eddy current effects
- Explain the origin of acoustic noise during MRI scanning
- Specify the components of the radiofrequency channel and the different types of antenna
- List the materials at risk and the precautions prior to an MRI examination
- Explain the origin of peripheral nerve stimulation during an MRI examination
- Present the factors affecting SAR and how to reduce it
In an MRI device at 1.5 T, the magnet is superconducting, cooled by liquid Helium. Quench corresponds to liquid Helium evaporation after heating. The large volume of gaseous Helium thus given off causes a risk of thermal burns and asphyxia. An adapted evacuation system and safety arrangements in the examination room should prevent such risks in any MRI installation.Gradients are characterized by amplitude, slew rate and linearity. Switching gradients cause induced currents (Eddy currents) and the acoustic noise of the MRI. They can provoke peripheral nerve stimulation, especially in echo-planar sequences.The quality of the radiofrequency system and coils is crucial. There are coils for each type of exploration and organ studied.Ferromagnetic materials carry a projectile risk effect inside the MR scanner.Intra-ocular metallic foreign bodies, intracranial aneurysm clips, cardiac pacemakers and cochlear implants are generally counter-indicated for MRI. It is advisable to check their MR compatibility and for any counter-indications prior to the examination.The SAR limit must not be exceeded, and sequences must be adapted accordingly (coil, TR, number of slices, flip angle, echo train).
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