Module 7 Flashcards
To learn material from module 7
Why is a stronger magnet in MRI better ?
- more spins in low energy states therefore greater spin excess
- higher signal
- 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?
Fourier transformation
Which of the following converts signals into images?
a. Software
b. Receiver
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 ?
a. Diamagnetism
b. Paramagnetism
c. Ferromagnetism
d. Superparamagnetism
c. Ferromagnetism
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
moderately positive
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?
0.2T- 4T
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
True
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
magnets ?
- 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
Are passive and active shimming used together ?
Yes - passive shimming maximizes homogeneity of the scanner and active optimizes for each patient and scan
Why is ramp sampling faster ?
Sampling is done while frequency encoding gradient is changing
True or false : Coils provide high resolution.
False - coils provide high signal which is traded to get high resolution
What are some safety measures taken with coils?
Do not loop the cables that come with the coils
do not let the cables touch the patient
do not position cables around each other
What does the Faraday cage consist of ?
Copper lining walls , windows and door frame (zipper)
What are the components of a gradient coil?
Gradient coil is composed of 3 electromagnets supplied by 1-2 amplifiers
Which of the following does not affect the strength of an electromagnet ?
a. The amount of current
b. The voltage that powers the magnet
c. Space between the windings of the coil
d. The diameter of the wire
e. number of windings in the coil
b. the voltage that powers the magnet
What happens when a third terminal is placed in the middle of an electromagnet coil?
This creates two magnetic fields equal but opposite in direction. Gradient goes from positive to negative.
Define slew rate.
Slew rate is the time it takes for a gradient to reach its maximum amplitude.
When is the maximum amplitude or strength of a gradient important ?
The maximum amplitude is important when good spatial resolution is required.
What happens to amount of allowable slices when duty cycle and slew rate increases ?
Number of allowable slices decreases
Which of the following is not a type of volume coil?
a. birdcage coil
b. body coil
c. solenoid coil
d. helmholtz coil
d. helmholtz coil
Why does SNR not double when 2 coil elements are used ?
There is increased noise however it does boost the signal
True or false : Volume coils have higher SNR than other coil types.
False they have lower SNR than other coil types
Which of the following statements is incorrect ?
a. Linear coils have low SNR
b. Quadrature detection can increase SNR in linear coils
c. Closer anatomy is to the coil the higher SNR
d. Surface coils are small and placed on or near anatomy
Linear coils have high SNR so a is incorrect
What is the sensitivity profile of a surface coil?
The sensitivity profile is a half sphere extending down to 75% of the coil’s diameter
How do phased array coils work?
Each coil receives its signal from its own small FOV each coil output is separately received and processed and then combined
