Unit 1: Exam 2 Flashcards
How do we create a slope, or have one side of the magnet stronger than the other?
Gradients
What is the point in which all three axis meet?
magnetic isocenter
What gradient is turned on first in a spin echo sequence?
Slice select
When is slice select turned on for the second time in a spin echo sequence?
at the 180 degree rf pulse
What kind of slice does a steep slope create?
Thin slices
What kind of slice does a shallow slope create?
Thick slices
Which gradient spatially locates along the long axis?
Frequency encoding gradient
Frequency encode gradient is switched on for a specific time called sampling time, which is also known as?
Sampling window or acquisition window
When specifically in the pulse sequence is the frequency gradient turned on?
During the signal
What is the process of turning the data into an image called?
Fast Fourier transform
In the chest of drawers analogy what is the chest?
Slices
In the chest of drawers analogy, what are the drawers?
Phase encode steps
In the chest of drawers analogy, what are the socks that fill the drawers?
Frequency data points
This states that the highest frequency in the modulated frequencies must be sampled at least twice
Nyquist theorem
What determines the number of data points collected during the sampling window?
Frequency matrix
What do the central lines of kspace contribute to?
Contrast and signal
What do the outer lines of kspace contribute to?
Resolution
What is the scan time formula?
TR x Phase x NEX/NSA
What happens to the image when the phase matrix increases?
-Scan time increases
-signal decreases
-resolution increases
What are the three things to consider when creating an image?
-high SNR
-high spatial resolution
-short scan time
This controls the amount of recovery of longitudinal magnetization
TR
This controls the amount of coherent transverse magnetization decay
TE
What controls the amount of transverse magnetization?
flip angle
What has better signal, spin echo or gradient echo?
spin echo
This determines how many times a line of kspace is filled with data
NSA/NEX
What’s another term for aliasing?
foldover; wrap
What artifact produces a banding artifact at the interface of high and low signal?
truncation artifact (or Gibs)
Before calling your engineer, what should you check for that causes a zipper artifact?
burnt lightbulb
The use of foam pads or water bags between the coil or patient to minimize contact reduces what kind of artifact?
shading artifact
What is anti-aliasing also called?
anti-foldover
Where are data points stored?
kspace
The position of the nuclei along a gradient can be identified according to its what?
-precessional frequency
-resonant frequency
or
-Larmor frequency
The number of times data is collected with the same amount
NEX/NSA
What is a disadvantage to large voxels?
decrease in resolution
This determines the number of data points collected during readout
frequency matrix
Once the slice is selected, the signal is located, or _______ along both axes of the image
encoded
In large voxels, individual signal intensities are averaged together and are not represented together as distinct within the voxel. This results in ___________
partial voluming
Do the edges of kspace have low amplitude or high amplitude?
low amplitude
Does the center of kspace have low amplitude or high amplitude
high amplitude
