K-Space (CH 6)
When the frequency matrix is selected, what is altered in k-space?
# of data points in each row
what is the scan time for the pulse sequence below? TR-500ms TE-15ms BW-150 mHz Averages-3 Phase- 512 frequency-320
12.8 minutes
What is the most common type of data aquistion?
2-D volumetric
If you select 20 slices, you have _____ K space areas in the array processor.
20
If we have selected 15 slices, a phase encoding matrix of 256 and a frequency encoding matrix of 256, how data points are stored in K space total?
983,040
Why does motion artefact only occur in the phase axis of k-space?
Because the sampling interval between data points in each column is much longer than the sampling interval between data points in each row.
The asymmetrical data in K space is termed conjugate symmetry and is used to reduce scan times. T/F?
FALSE.
-Data are symmetrical. -Scan time is the time to fill K space. -It is the image. -All data points contain information about the entire slice. ALL are true about K-space except...
It is the image.
This type of K space filling is performed when only part of the echo or signal is read during the application of the frequency encoding gradient.
PARTIAL ECHO IMAGING
As long as 60% or more than half of K space is filled during the acquisition, the system has enough data to produce an image in this type of imaging: K space filling options that apply are..
Partial echo, Fractional averaging, & 1/2 Fourier imaging.
The 3 ways to acquire data are:
Sequential, 3-D, & 2-D
what type of pulse sequence is listed below: TR-500 ms TE-15 ms Bandwidth-150mHz Averages-3 Phase-512 Frequency-320
T1
The scan time formula for a conventional spin echo pulse sequence is:
TR X NSA X PE
When all the lines of K space are filled the scan in sequence is complete. T/F?
TRUE
When we select the NSA in the scan protocol - what is going on behind the scenes in k-space?
The NSA is the number of times each line is filled with the same slope & amplitude of phase encoding gradient.
If the frequency matrix doubles & the receive bandwidth is unchanged what other parameter changes and why?
The TE increases bc more data points are required for the same sampling frequency. Therefore, the sampling window and TE increases.
What happens in k-space when the phase FOV is doubled?
The incremental step between each line is halved.
What is the digital sampling frequency?
The number of data points acquired per second during the sampling window.
When we place a certain number of slices through the patient's anatomy - what is going on behind the scenes in k-space?
The system allocates the same number of k-space areas as there are slices in the stack.
Why does the phase encoding gradient slope change every TR?
To ensure that the data looks different, every TR is placed in a different line of k-space. This ensures that there are multiple pixels in the phase direction of the image.
Logical gradients in the MR system are:
X (frequency), Y (phase) & Z (slice)
What happens if frequencies are insufficiently sampled?
alaising
In K space the frequency direction is oriented _____.
horizontal
_____ is defined as an area in the array processor where data about spatial frequencies are stored.
k-space
To produce high signal, there must be a ______________ transverse component of _______________transverse magnetization to induce a ______________signal in the coil.
large coherent large
If there is a phase matrix of 256 and a frequency matrix of 128, how many slices will result from this combination?
not enough information
Each line of K space is defined by:
phase encoding gradient
The number of lines of K space corresponds with the number of ____ encodings or _____ matrix.
phase/phase
K space is measured in:
radians per cm
______________ is the range of frequencies sampled during the readout gradient.
receive bandwidth
k-space is what shape?
rectangular
If the frequency encoding gradient has a negative polarity, how is K space traversed?
right to left
in which direction is k-space traversal when frequency encoding is negative?
right to left
The outer lines of k-space are filled by the _____ phase encoding gradients and have _____ signal amplitude.
steep, low
In a gradient echo pulse sequence the slice select gradient is turned on during:
the excitation pulse
The steep phase encoding gradient slope would fill data in which lines of K space?
the outer lines only
In K-space lower numbers are:
towards the center
In K space the phase direction is oriented ______.
vertical