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MRI final review

radt 411

biological effects of primary magnet elevated t-waves
biological effects of secondary magnet involuntary muscle contractions, heat
FDA guidelines 1. imaging up to 4T 2. 102 decibel for one hour of continuous noise 3. limit when patient is uncomfortable
quench boiling off liquid helium
side effects of a quench confusion, asphyxiation, frostbite, hypothermia
when do quenches occur installation, deinstallation, intentional, non-intentional
MRI zones 1. general public 2. patients and family (supervised) 3. locked/ restricted, work area 4. MRI suite
how do you prevent MRI burns no conductive loops, pads (don't touch bore of magnet), no ferromagnets, ice packs
Faraday law of magnetic induction magnetic field create a charge
Faraday's cage line walls with copper
What do we image in MRI? hydrogen protons in fat and water
longitudinal and transverse magnetization (what are they in relationship to net magnetization vector) vector= direction and magnitude
Free induction decay weak signal, losing energy after RF is shut off
Gauss general public= 5 gauss line
active shielding opposite opposing magnets
passive shielding concrete or steel lines walls
active shimming coil of wire
passive shimming ferromagnetic materials, plates of metal on cylinder
RF shielding walls lined with copper
Larmor equation F= (y/2pi)B
what does each part of the Larmor equation mean; F, y/2pi, B F= larmor frequency (precession) Y/2pi= gyromagnetic ratio (constant for all elements) B=magnetic field in Tesla
pulse sequence grouping of 2 or more RF pulses
diamagnetic weak, unaffected by mag field, gold, copper, silver, mercury, carbon, and water
paramagnetic weakly magnetized, gadolinium, aluminum, platinum, and magnesium
ferromagnetic magnetized when in mag field, iron, steel, nickle, and cobalt
TR time to repition, time from the start of one pulse to the time of start if next pulse (1 pulse)
TE time to echo, time from initial pulse to the collection of the echo (shorter than TR)
B0 static magnetic field, longitudinal magnetization (Z)
B1 RF, transverse magnetization (XY), secondary magnet, excites hydrogen protons
isocenter most accurate to T, strongest part, most homogeneous
RF coils perpendicular to XY plane (4 max signal), transmit only, receive only, and receives and transmits
transmit only transmit RF into pt only, main body receives
receive only receives signal from pt only, heats up pt more, main body transmits, majority
Helmholtz pair= pairing RF coils and using out same time (ex: TMJ studies)
local coils (surface coils) small FOV, either transmits and receive or receive only, over side of interest
Linear local coil 1st in MRI, not used now
Quadrature local coil limited to area of coverage, 2 sets of receivers, receives back and forth= more signal, increased SNR-40%
Phased array local coil multi channel, head coils (larger coverage), good SNR
gradient coils varies magnetic field
physical notation in gradient coils (x, y, z) x= left to right (sagittal) y= front to back (coronal) z= head to foot (axial) has own power supply always know physical notation
logical notation x, y, and z changes
T/F: phase tells freq which spot to collect true
slice select gradient RF goes into pt
freq encoding gradient row/ columns read out gradient, collect signal back from pt
phase encoding gradient rows/ columns, encoding after slice before freq
gradient= steepness of slope
amplitude= thinner slice
steeper the slope= smaller slices= more slices= smaller FOV= shorter echo
flip angle when net mag has reached the achievable angle from a RF plan from the z-axis
T/F: stop RF when reached desired flip angle true
common flip angles 15, 45, 60, 90, 180
permanent magnet always on/ open MRI, bricks, lower field strength (0.2-0.6T), cons: weight, poor homogeniety
resistive low field strength, small fringe field, high cost, can turn off, con: constant current= high heat
superconducting 95% of MRI systems, quench, large field strength (1.5-3T), closed, large fringe field, ferromagnetic material attracted
secondary magnets RF coils, gradient coils, shim coils; can be turned off
duty cycle percent of time a gradient is applied at a specific amplitude 100%= gradient turn on/off simultaneously 80%= gradients turned on/ off when it needs time to rest
amplitude steepness of slope of magnetic field, plays role in slice thickness
phase coherence hydrogen protons spinning in same direction, collect signal when there is coherance
dephasing of protons 1. magnetic field in homogeitis 2. magnetic susceptibility 3. chemical shift
magnetic field in homogeneitis spinning at difference speeds at points of MRI
magnetic susceptibility certain tissues magnetize easier, spin out different speeds
chemical shift spin faster in water than in fat, 220 Hz slower in fat than water
hydrogen= 42.6 mHz/1T 63.9 mHz/1.5T 128.18 mHz/3T
T/F: majority of hydrogen protons align with B1 false, B0
net mag for every 1 low energy state take out 1 high energy state
90 degree precess spin echo
180 degree precess inversion recovery
Created by: meechthebeech91