• Anaesthesia in an MRI
    Link:www.aagbi.org/sites/default/files/mri02.pdf
    • MRI Machine
      • Unpaired photons poses spin which produces a magnetic field
      • Their alignment is random
      • Magnetic field aligns the nuclei (0.5-4T)
      • They are then flipped out their alignment by pulsed RF energy
      • This energy is then released as they relax, the amplitude of which depends on tissue properties and the pulse sequence used
      • T1 & T2 relaxation times
        • T1 - CSF is dark
          T2 - CSF is bright
      • Gadolinium used to increase signal intensity on T1 weighted scan
      • Gradient magnetic fields in x, y & z planes allows spatial information to be gathered
    • 1T = 10, 000 Gauss
      Earth's field = 0.5 - 1 Gauss
      Magnetic field falls exponentially
      Safety line at 5 Gauss (PPM fail inside this)
      30 Gauss line - infusion pumps fail
      50 Gauss line - attractive forces can propel objects
    • Anaesthetic factors
      • Monitoring
        • MR compatible
          Field may induce currents
          Carbon/fibre optic cables used
          Column of fluid as in aorta can induce currents and lead to ECG changes resembling hyperkalaemia => short braided ECG leads used & narrow triangle on patients chest
      • Dedicated room for induction
      • LMA or ETT
        TIVA vs Inhalational
        MRI compatible machine/laryngoscopes
        Staff aware of evacuation plan
        ETT spring taped down
        Infusions with extensions so pumps outside 30 Gauss line
        Risk of microshock
      • Tell radiologist if using 100% O₂ as it can affect T2 weighted images
      • Contrast reactions
        • Patients with renal failure at risk of developing nephrogenic systemic fibrosis
    • Staff factors
      • Remove bleeps
        Stethoscopes
        Ear protection
        Emergency quenching -> hypoxaemia
        Pregnant - shouldn't work in MRI
    • Patient factors
      • Jewellery
        Hearing aids
        Transdermal patches
      • Surgical clips/joint prostheses/sternal wires/ heart valves generally ok
      • Hearing protection (85db in MRI)
      • Patients may be children, learning difficulties, raised ICP
      • Access to patient limited
      • PPM is risk benefit but best viewed as a CI
      • Programable shunts - need technician to verify settings pre & post-MRI
      • Neurostimulators
        • Probably not safe
    • CEACCP
      Link:ceaccp.oxfordjournals.org/content/early/2012/02/24/bjaceaccp.mks002.full.pdf