Week 4 Flashcards
what is used to support superconductivity of MRI
Vacuum tank with 500 – 1500L of liquid helium at 4K as cryogen
___ of current increases slowly during ramping phase
500 - 1000A
passive shimming is done using __
Small pieces of iron placed at specific locations inside scanner to correct field distortion such that only body tissue disrupts it
active shimming is done using __
Special coils used to distort main field specifically for area to be scanned
magnetic field must be homogenous to __ variation
< 3.5 ppm
what are the use of gradients
Localizes signal & affects speed & resolution of scan; Combining gradients in directional pairs allows for oblique imaging
gradients uses large currents within magnetic fields to create __
lorentz forces
what are Lorentz forces
gradients vibrate and knock against each other causing loud acoustic noise in MRI
key parameters of gradients include
- max amplitude
- siew rate
- gradient linearity
- duty cycle
transmit coils are powered by __
RF power amplifier
transmit coils transmits ___
Radio waves transmitted at specific resonant frequency to excite nuclei, creating B1 field
transmit coils dissipate heat within patient measured at __
specific absorption rate (SAR)
receiver coils detects __
weak signals given off from nuclei > pre-amplified > digitised
the smaller the coil, the ___ signal, ___ depth penetration & coverage
higher; lower
receiver coils are placed ___ to B0
perpendicular
passive shielding uses __
Large amounts of ferromagnetic materials in the walls to contain fringe magnetic flux
restricts magnetic field from sensitive equipment
active shielding uses __
Shielding coils outside windings of main magnetic field
RF shielding AKA __
RF cage / Faraday’s Cage
RF shielding is made of ___
copper / galvanized steel to enclose MRI room
RF shielding blocks __
stray external RF signals from contaminating MR signal, causing artifacts
Wires & tubes must pass through __ made of specific diameter & length to filter unwanted wavelengths
waveguide
dynamic switching fields and gradients causes __
peripheral nerve stimulation & acoustic noise
what should patients do to prevent dangerous current loop burns
- Parts of the patient’s body should not touch each other
- RF coils must not touch body
- Skin does not make contact with tunnel lining
how do dynamic switching fields cause peripheral nerve stimulation
rapid gradient switching causes currents in body = nerve depolarization; harmless but uncomfortable
how to manage acoustic noises
hardware = absorptive materials, rigid mounting, force compensation
software = modulated gradients to reduce volume & zero TE sequences are nearly silent
what patient care should be done for everyone
hearing protection
time varying B0 field is created by __
low frequency subject movement in static/fringe field
time varying B0 field induces __
low electrical currents in body causing sensory effects
- cranial nerves
- retina
- metallic taste
light flashes experienced are due to __
stimulated magneto-phosphenes
what causes metallic taste
electric current in tongue
what should be considered when imaging patients with implanted leads
- check implant compatibility
- risk vs benefit
- updated info
- manufacturer’s guidelines
- third party validation
superconducting magnet can be switched off using __
helium
what moves during scans
gradient coils
what is the difference between active shimming & active shielding
Active shimming used to reduce B0 inhomogeneity / active shielding is to prevent electrical interference
Peripheral nerve stimulation is dependent on __
scan orientation
