MRI Image Formation Flashcards
What do gradient coils do?
generate spatially varying magnetic fields so that spins at different location precess at frequencies unique to their location
What is a Fourier Transform and what does it do?
It’s how we process the signal; it changes the frequency
What occurs during the generation of transverse magnetization (in terms of your selected slice)?
Only the slice that is selected gets excited; signal of that slice is produced
Changing the slope of the slice selection gradient, changes the slice _.
thickness
Changing the center frequency of the RF results in changing the slice _.
Position
Generally, increasing the gradient = a thinner/thicker slice.
thinner slice
Generally decreasing the gradient = a thinner/thicker slice
thicker slice
What are the steps in 3D localization?
- Excite and receive magnetic field in a thin (2D) slice of the subject (Reduce dimension from 3D to 2D)
- Make magnetic field strength B depend on location within slice
- Make RF signal phase depend on location within slice
The spatial information of the proton pools contributing to MR signal is determined by the _ and ___ of their magnetization.
spatial frequency; phase
During readout (image acquision) period, turning on gradient field is called _.
frequency encoding
Before readout (image acquision) period, turning on gradient field is called _.
phase encoding
What is the k-space?
the raw MR data space
When there are more data points in the k-space, resolution increases/decreases.
increases
When the gradient increases, there are more/less data points, resulting in a increase/decrease in spatial resolution
more; increase
True or false. Spins occur with phase gradient first, then w/o phase gradient, and then frequency gradient.
True
The center of the K-space mainly contains:
intensity information
The outer area of the K-space mainly contains:
The resolution information (sharpness)
Changing the FOV of the k-space, changes the __ of the data points.
distance
increased FOV = increased data point distance
A change in the pixel size, causes a change in the _.
matrix size
increase pixel size = decrease matrix size = decrease resolution
Voxel
volume element (3D)
Pixel
Picture element (2D)
