Chapter 9 Flashcards
Magnetic field strength can be expressed in units of Tesla (T) or gauss (G), where:
1 T = 10 000 G
Ohm’s law states:
V = I (R)
What rule determines the direction of current flow through a coil, and the direction of the magnetic field that the current produces?
Right hand thumb rule
To maintain the magnetic field of a resistive magnet, the following should be applied to the magnet coils:
Current
Fourier transform (FT) is a mathematical process that is used to convert MR signal from a FID into a spectrum. FT occurs in the:
Array processor
Passive magnetic shielding can be accomplished by lining the scan room walls with:
Steel
Which cryogen is commonly used to super cool wires in MRI?
Liquid helium
What is the maximum field strength allowed for clinical imaging of adults in the USA?
8 T
In a typical superconducting magnet, the direction of the magnetic field is:
Horizontal
The homogeneity of a magnetic field is expressed in units of:
ppm (parts per million)
Which material is most commonly used to make superconducting wires?
Niobium-titanium alloy
The SNR is enhanced when using local coils. (T or F)
true
What is the most common material used to make permanent magnets
Alnico
Why do we use shimming
To make the field even or homogenous
The magnetic field strength in superconducting magnets is maintained with cryogens at a temperature of:
4 K
The purpose of magnetic field shimming is to reduce the size of the fringe field. (T or F)
false
The purpose of magnetic field shimming is to correct for field inhomogeneity. The purpose of SHIELDING is to reduce the fringe field.
The law of electromagnetic induction indicates that whenever a charge particle such as an electron exhibits motion, a corresponding magnetic field is induced.(T or F)
true
Gradient strength is expressed in units of all of the following EXCEPT:
w/kg
Once a ferromagnetic material is exposed to an externally applied magnetic field, it becomes magnetized (a permanent magnet). Permanent magnets are said to be:
Bipolar
Select all of the following that determine the strength of an electromagnet:
The amount of current passed through the windings
The number of windings in the coil
The distance or spacing between the windings
The diameter of the wire used in the windings
Which correctly describes the right hand rule?
right fingers wrapped around wire, thumb points to current, fingers point in the MF
Which is true concerning superconducting magnets?
1.5-3.0T
Bo is horizontal
Which of the following are true concerning the temperature of superconductive magnets?
-269 C
-450 F
4 K
What are the factors that change the strength of an electromagnet?
current passing thru the windings
# of windings
diameter of the wire
spacing between the windings
