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MRI final review
radt 411
| Question | Answer |
|---|---|
| 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
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