MRI Image Formation Flashcards
(21 cards)
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)
