Chapter 5: Spatial Encoding Flashcards
Spatial encoding is a process in which signal is located and pinpointed, and it is performed by what?
Gradients
Graded or sloped magnetic fields are generated by cylindrical electromagnets situated in the warm bore of the cryostat. These coils are called?
Gradient coils
If current flows clockwise through the gradient coil, then the magnetic field induced around the coil _____ to Bo.
adds; This increased the magnetic field strength relative to Bo.
If current flows anticlockwise through the gradient coil, then the magnetic field induced around the coil _____ from Bo.
subtracts; The magnetic field is decreased relative to Bo.
The middle of the axis of the gradient remains at the field strength of the main magnetic field even when the gradient is switched on. This is called the what?
Magnetic isocenter
The changes in field strength and therefore frequency imposed by gradients are small. They usually vary the magnetic field and frequency of magnetic moments of spins located along them by less than ____ percent
one
What determines the rate of change of the magnetic field strength along the gradient axis?
Gradient amplitude
Which type of gradient slope alter the magnetic field strength between two points more, steep or shallow?
Steep
The _____ of current passing through the coil determines the amplitude, strength, or slope of the gradient.
amount
The _____ of the current passing through the coil determines its polarity.
direction
What spatially locates or encodes signal depending on where it is located along these three gradients.?
Gradients
Gradients spatially locate (encode) signal along the long axis of the slice, this is called ______ encoding.
frequency
Gradients spatially locate (encode) signal along the short axis of the slice, this is called _____ encoding.
phase
In SE pulse sequences, when is the slice-select gradient switched on?
During the application of the 90 degree RF excitation pulse and during the 180 degree RF rephasing pulse.
In gradient-echo sequences, when is the slice-select gradient switched on?
Only during the RF excitation pulse.
The amplitude of any gradient is determined by how much ______ passes through it when it is switched on.
current
To achieve ______ slices, a steep slice-select slope and/or ______ transmit bandwidth is applied.
thin; narrow
To achieve ______ slices, a shallow slice-select slope and/or ______ transmit bandwidth is applied.
thick; broad
What 2 things determine the range of nuclei that resonate on either side of the center frequency?
Transmit bandwidth and gradient slope
The center frequency is called the _____ frequency
carrier
The transmit bandwidth is a small range of frequencies on either side of the carrier frequency that typically has a range of ___ to ____ KHz.
1 to 2
What 2 things determines the gap between slices?
gradient slope and the thickness of the slice
Thin slices are produced by passing a ______ current through the slice-select gradient.
large
Thick slices are produced by passing a ______ current through the slice-select gradient
smaller
A large current passing through the slice-select gradient coil produces a ______ or _____ gradient.
high-amplitude or steep
Once a slice is selected, signal coming from it is located along both axes of the image. Signal is usually located along the long axis of the anatomy by a process known as _______.
frequency encoding
The frequency-encoding gradient is switched on when signal is received (i.e. when the echo occurs) and is often called the _____ or ______ gradient.
readout or measurement
The frequency-encoding gradient is switched on for a very specific time. This is called the ______ time/window.
sampling
A steep frequency-encoding gradient produces a ____ FOV dimension in the frequency axes of the image.
small
A shallow frequency-encoding gradient produces a _____ FOV dimension in the frequency axes of the image.
large
Signal is located along the remaining short axis of the anatomy, and this localization of signal is called ______.
phase encoding
The number of steps in the ladder or how many times the phase-encoding gradient is switched on to a different amplitude or polarity during the sequence is determined by the ________ we select in the scan protocol.
phase matrix
The steepest slope of phase-encoding gradient, either positive or negative, determines ________ in the phase direction of the image.
pixel size
To achieve a small pixel, the highest or lowest step in the ladder is selected using a ______ phase-encoding gradient.
steep
To achieve a large pixel, the highest or lowest step in the ladder is selected using a ______ phase-encoding gradient.
shallow
______ is the math that gets us from frequencies in the echo to the image.
FFT (Fast Fourier Transform)
