Flashcards in Module 7 Deck (46)
Why is a stronger magnet in MRI better ?
1. more spins in low energy states therefore greater spin excess
2. higher signal
3. better image quality
What is the purpose of a shim coil?
Maintain magnetic homogeneity
What is the purpose of a magnetic filed gradient?
Spatial location of RF signals
What is the process called by which a FID is converted to a spectrum?
Which of the following converts signals into images?
c. Image processor
d. Computer system
c. image processor
What are the four types of magnetic behaviour related to?
Related to the configuration of orbiting electrons
Which of the following has the highest magnetic susceptibility ?
True or False: paired electrons means a large magnetic moment?
False - means no magnetic moment when there is no external magnetic field
What happens when a diamagnetic substance is exposed to an external magnetic field?
They have a small magnetic moment that opposes the external applied field so they are repelled.
Which of the following is false about paramagnetic substances:
a. They have unpaired orbital electrons
b. They have a small magnetic moment
c. When an external field is applied the add to it
d. they have a high positive susceptibility
d. is false paramagnetic substances have a low positive susceptibility
Fill in the blank
Superparamagnetic substances have a __________ magnetic susceptibility
Can a ferromagnetic substance be dangerous in an MRI environment and why or why not?
Yes it can be dangerous because it can become a projectile
Why do ferromagnetic substances have a very high positive magnetic susceptibility?
They have a half filled electron shell which results in strong attraction and alignment to an external magnetic field.
What happens to a ferromagnetic substance once an external magnetic field is applied and then removed?
It becomes a permanent magnet
What is the limit of the stray fringe field of a MRI magnet that is publicly accessible?
5 Gauss - called the '5 Gauss line'
What is the range of magnetic field strength of most clinical MRI machines?
What is the purpose of shimming ?
To minimize field inhomogeneity
What are advantages of permanent magnet MRI machines?
- low purchase and operating cost
- minimal fringe field
- open MRI less claustrophobic
True or false: when multiple wires are carrying current in the same direction the magnetic fields are additive
What determines the strength of the magnetic field in a solenoid electromagnet?
Amount of loops and current and distance between the loops
What is an electromagnet called at room temperature ? Why?
It is called a resistive magnet because there is resistance because it is subject to Ohm's law
Why is the magnetic field low in resistive magnets ?
Magnet would be destroyed because the wires would have to much resistance and would result in overheating
What are some advantages of superconducting
- low power requirements
- only one that has high field strengths
- low operational costs
Why do ultra high field systems have small bores?
It is easier to maintain homogeneity
What is involved in ramping up a superconducting magnet ?
- running current through main superconducting coil
What are Niche magnets used for?
Used to image specific parts of the body
What are two types of shielding used in MRI?
Passive or active
Why is passive shielding not used often?
- not really necessary unless ultra-high field magnet
- expensive to surround room with steel
- need special foundations
How does active shielding work?
Current is run in the opposite direction from main magnet results in cancellation