MRI Flashcards
What is the source and type of energy used in MRI?
Hydrogen
Electromagnetic radiation
What is a t1 weighted image?
Fluid black
Fat white
What is a t2 weighted image?
Fluid white
Fat black
What does a magnetic field exert to line up magnets?
Torque
What is the direction alignment?
In the direction of the magnetic field Btheta
What is torque proportional to?
Size of magnetic field
What is the typical magnetic field strength?
0.5-2 Tesla
Why is the magnet continuously rotating?
Superconducting magnet
Immersed in liquid nitrogen
at 77K no resistance
What happens when you put protons into a magnetic field?
Align with the magnetic field (lower energy state) or cancel go against (higher energy state) will oscillate between
slight excess will align with the field
What is precession?
Wobbling or rotation of a proton about the axis of Btheta
large gravity large magnetic field= fast precession
What is larmor ‘s equation?
Speed of precession=gyro magnetic ratio*magnetic field
1T=42 MHz/s
How do you tip protons out of alignment?
Apply RF pulse from RF coil energy depends on Larmor frequency
Causes protons to precess in phase and flips to transverse magnetisation
What happens when RF is turned off?
Absorbed RF energy is retransmitted
t1 Recovery-excited spins return to original orientation
T2 relaxation-protons dephase
What does a rotating magnetic field produce?
EM radiation- RF signal is retransmitted to MR signal
What is T1 relaxation?
Process of giving of RF energy as protons go from a high to low state energy
Some energy goes to heating up surrounding tissue
Relaxes back to longitudinal phase (along Z
What is T2 relaxation/ decay?
Decay or dephasing in transverse plane
Protons begin to interact with each other magnetic field (x)
What is quicker T2 decay or T1 relaxation?
T2
How is an image formed?
At a point TE/2 a 180 RF current is sent flipping protons into xy plane another TE/2 passes and at TE vectors have rephrased and signal produced.
What are the contraindications for MRI?
Pacemakers Projectiles Aneurysm clips Cryogen leak Defibs Foreign bodies
What are artefacts?
Zipper
RF entering room
Corduray
Motion
issues with MRI?
Expensive Noisy Claustrophopic Lack of electron density Geometric distortion
What are patient related artefacts?
Implants Surgical Bones Make Up Clothing
