Chapter 3 Flashcards
Which gradient is aka the readout gradient?
frequency
Match the timing of the slice select, phase encoding and frequency encoding gradients … slice select gradient
During the 90 and 180 in a spin echo, during excitation pulse of a gradient echo.
Match the timing of the slice select, phase encoding and frequency encoding gradients … phase encoding gradient
after the excitation pulse.
Match the timing of the slice select, phase encoding and frequency encoding gradients … frequency encoding gradient
during collection of the signal.
TRUE or FALSE: The top half of data in k-space is identical to the bottom half.
True
Match the portion of k-space to its respective amplitude and resolution - central portion
low resolution, high amplitude.
Match the portion of k-space to its respective amplitude and resolution - outer portion
high resolution, how aamplitude
Match the types of data acquisition … sequential
All the data from slice 1 then all the date from slice 2, etc.
Match the types of data acquisition … 2D
1 line of k-space for slice 1 then the same line for slice, 2, etc.
Match the types of data acquisition … 3D
Not slices. Acquires an entire volume
TRUE or FALSE: K-space is not the image.
True
TRUE or FALSE: The scan time is the time to fill k-space.
True
Match the polarity of each gradient to the direction traveled through k-space - positive frequence encoding gradient
left to right
Match the polarity of each gradient to the direction traveled through k-space - negative frequence encoding gradient
right to left
Match the polarity of each gradient to the direction traveled through k-space - positive phase encoding gradient
top half
Match the polarity of each gradient to the direction traveled through k-space - negative phase encoding gradient
negative half
How fast do we sample frequencies according to the Nyquist theorem?
2x/cycle
Match the slice select to their respective transmit bandwidth … thin slices
narrow transmit bandwidth
Match the slice select to their respective transmit bandwidth … thick slices
broad transmit bandwidth
Match these terms from Chapter 3 … FOV
Steepness of the slope of the frequence encoding gradient determines the size of the anatomy
Match these terms from Chapter 3 … sampling time
Duration of the readout gradient. Aka: acquisition window
Match these terms from Chapter 3 … FFT
mathmatical process that converts time into a frequency domain
Match these terms from Chapter 3 … aliasing
artefact caused by sampling too infrequently
Match these terms from Chapter 3 … range of frequencies we wish to sample during the readout
receive bandwidth
What is the mathematical process that converts time into a frequency domain?
Fast Fourier Transform
Match the gradients to their slice selection … X
sagittal
Match the gradients to their slice selection … Z
axial
Match the gradients to their slice selection … Y
coronal
Match the gradients to their slice selection … combo
oblique
what does the phase gradient do
encodes along the short axis of anatomy
what does the frequency gradient do
encodes along the long axis of anatomy
Which gradient determines the FOV?
frequence
How often must we sample frequencies, according to the Nyquist Theorem?
Twice per cycle
What is another term for the duration of the readout gradient?
Sampling time
Which gradient performs slice selection for coronal slices?
Y gradient
When is the slice select gradient switched on during a spin echo pulse sequence?
During the 90 and 180 degree pulses
Which gradient locates signal along the LONG axis of the anatomy?
Frequency encoding gradient
A data point contains both phase and frequency information
true
During a typical spin echo sequence, there are three gradient applications. In chronological order they are:
Slice select, Phase encoding, Frequency encoding
To achieve THIN slices would we use a narrow or broad transmit bandwidth?
Narrow
To achieve thin slices, do we need a STEEP or SHALLOW slice select slope?
STEEP
Which part of K space do shallow phase gradients fill?
Central lines of K space, both positive and negative
K space is not the image.
True
What is the unit of K space?
Radians per centimeter
Which part of K space do steep phase gradients fill?
Outer lines of K space, both positive and negative
What is another name for the frequency encoding gradient?
Readout gradient
What kind of information is contributed by data collected in the central lines of K space? (Select all that apply)
- Signal
* Contrast
What artifact occurs if we sample frequencies less than once per cycle?
Aliasing
Data in K space are symmetrical.
True
The scan time is the time to fill K space.
True
What controls the polarity of a gradient?
The direction of the current through the gradient coil
What does the slope of the frequency encoding gradient determine? (Select all that apply)
- The size of the anatomy covered along the frequency encoding axis
- The FOV
What does the phase encoding gradient do?
Encode data along the SHORT axis of the anatomy
Which gradient performs slice selection for sagital slices?
X gradient
When is the phase encoding gradient switched on in a pulse sequence?
Usually just before the application of the 180 degree rephasing pulse
Where are the data points collected by the system stored?
K space
The phase axis of K space is vertical and is centered in the middle of K space perpendicular to the frequency axis.
True
What kind of information is contributed by data collected in the outer lines of K space? (Select all that apply)
Resolution
Phase encoding gradient positive fills the bottom half of K space.
False
Phase encoding gradient positive fills the top half of K space - see page 96 in the text
Which gradient performs phase encoding for coronal slices?
Y gradient
If a 512 phase matrix is selected, which k-space lines are likely to be filled in a normal acquisition?
256, 0 to -255
Which gradient lies along the horizontal axis of a typical superconducting MRI system?
The X gradient
See page 61 in the text.
what is the unit of K space
radians/centimenter
what do data in central lines contribute to
signal and contrast
what do data in outer lines contribute to
resolution
