Unit 5 - Spatial Encoding Flashcards
Define voxel
finding something in three dimensions (volume)
Define pixel
found in two dimensions
Define spatial encoding
process of locating signals in three dimensions
Define gradients
copper winding that can add or subtract from the main magnetic field
Identify the main purpose of gradient coils
create a slope of magnetic field strength that is linear and predictable
Define polarity
direction of the magnetic field
Define magnetic isocenter
where the magnet is the most homogenous; where the three axis come together (dead center)
Define gradient amplitude
the steepness of the slope created
(the steeper = more current)
State the direction each gradient controls
X-gradient: right to left
Y-gradient: anterior/posterior
Z-gradient: long axis (head to foot)
{?} Explain how slice selection works
when a slice plane is selected on the MR protocol, it determines which gradient will be activated and performs slice-selection during the RF excitation pulse
When does slice selection occur in spin echo?
is turned on at the 90 degree RF excitation pulse and during the 180 degree rephasing pulse
When does slice selection occur in gradient echo sequences?
during the variable RF excitation pulse
Define transmit bandwidth
band of frequencies
Define what two ways a system controls slice thickness
1) the slice-select gradient amplitude (slope of the gradient)
2) bandwidth of the transmit RF excitation pulse
Explain how frequency encoding works
locates signal along the long axis of the image
{?} identify which axis in the image frequency typically controls
X-gradient: axial
Y-gradient: axial (head)
Z-gradient: coronal & sagittal
State another name for the frequency encoding gradient
readout gradient
Define sampling time
specific time frequency encoding gradient is switched on for
Define receive bandwidth
a range of frequencies
{?} Discuss how the frequency gradient controls field-of-view (FOV)
the amplitude of the frequency encoding gradient is one of the factors that determines FOV
Identify when the frequency gradient is on during the pulse sequence
during the echo of a pulse sequence
Explain how phase encoding works
locates signal along the short axis
{?} Identify which axis in the image phase typically controls
X-gradient: coronal, axial
Y-gradient: sagittal, axial
Z-gradient: long axis
Identify when the phase encoding gradient is on during a pulse sequence
it’s turned on after the RF excitation pulse
Define steep phase gradients
has a large phase shift (thin slice)
Define shallow phase gradients
has a small phase shift (thick slice)
Identify which gradient changes amplitude (slope) from one TR period to the next
phase encoding gradient
Define spatial frequency
the number of line pairs per unit length
