magnetic field of B₀ is altered by gradient coils, so that the magnetic field strength and therefore the precessional frequency experienced by nuclei along the axis of the gradient can be predicted
Gradients-
alterations to the magnetic field and are generated by coils of wire, inside the bore of the magnet, through which current is passed.
Nucleus along a gradient can be identified according to it's ___ ___
precessional frequency
The 3 gradient coils in the bore are named according to ___
axis
Z gradient-
alters magnetic field along Z (long) axis
Through the bore
Y gradient-
alters magnetic field along the Y (vertical) axis
X gradient-
alters magnetic field along X (horizontal) axis
Magnetic isocenter-
where all 3 axis meet
Isocenter magnetic field is:
always the same as B₀,
Gradients have 3 main tasks in encoding
slice selection
frequency encoding
phase encoding
Slice selection-
locating slice along scan plane
Frequency encoding
spatially locating signal along the long axis of anatomy
Phase encoding-
spatially location signal along the short axis of anatomy
Scan plain determines which _____ preforms slice selection during excitation pulse
gradient
axial slices preformed by ___
A. Z gradient
sagittal slices performed by ___
B. X-gradient
coronal slices preformed by ___
C. y-gradient
Oblique slices are preformed by ___
C. combination gradients
Steep slope=
large difference in frequencies in a slice
Shallow slope =
small differences in frequencies in a slice
Bandwidth -
RF pulse for the slice must contain a range of frequencies to match the difference in precessional frequency
Transmit Bandwidth-
RF being transmitted for particular slice
how is a Thin slice made?
-steep slice select slope
-narrow transmit bandwidth
how is a thick slice made?
-shallow slice select slope
-broad transmit bandwidth
how are axial images different on Head imaging?
long axis lies vertical so Y-gradient preforms frequency encoding
Read out gradient-
frequency encoding gradient is switched on when the signal is received
FOV- (Field of view) in frequency encoding gradient
steepness of the slope of the frequency gradient determines the size of the anatomy covered along the frequency encoding axis during scan
Magnetic moments at isocenter don't change in Phase encoding.
A. True
B. False
A. True
Magnetic moments at isocenter do change in Frequency encoding
A. True
B. False
B. false
Phase encoding Coronal imaging-
short axis is horizontal
X-gradient
short axis is vertical, Y-gradient for phase encoding
Sagittal imaging-
short axis is vertical, Y-gradient for phase encoding
Axial imaging -
Readout gradient-
system reads frequency present on the signal from frequency encoding gradient and samples/ digitizes them
Sampling time / acquisition window-
duration of the readout gradient
Sampling rate / frequency-
rate at which frequencies are sampled during readout gradient
Data point-
how sampling is stored
Aliasing-
artifact from not sampling once per cycle
Sampling time is ___ ___ to the sampling frequency and to the received bandwidth
inversely proportional
K Space-
where data points are stored
K Space Frequency axis=
horizontal and centered on the middle of several horizontal lines
K Space Phase Axis-
vertical and centered in the middle of the frequency axis
what is measured in Radians per cm
K-Space
What happens If phase encoding gradient is not changed?
the same line is filled every TR
Steep gradients select ___ lines of K space
A. inner
B. outer
B. outer
Shallow gradients select ___ lines of K space
A. inner
B. outer
A. inner
TR determine how many ___ are permitted
slices
K-space is the image.
A. True
B. False
B. false
Fast Fourier Transform (FFT)-
math used to produce image from data points
Result of FFT-
pixels have a gray-scale color corresponding to the amplitude of frequency coming from the spatial location represented by the pixel
Conjugate Symmetry-
Frequencies in upper K space is symmetrical to the lower K space just in reverse polarity
Data acquired in central lines of K space contribute to:
signal and contrast
Data acquired in outer lines of K space contribute to:
resolution
Scan time is the time to:
fill the K space
Repetition Time (TR)-
the timing between exciting a particular slice and then exciting it again to fill another line of its K space
(Number of Excitation) NEX-
instead of changing TR every time keep it the same and fill more data in a single line on K space for better signal to noise ratio
Partial or Fractional Echo-
If only ½ the signal is read the computer can make a mirror image of the rest
Partial Echo Imaging-
when only part of the signal or echo is read during application of frequency encoding gradient
Options that Fill K Space: (9)
Rectangular FOV
Anti-aliasing
Fast Spin Echo Sequences
Keyhole Imaging
Respiratory compensation
Parallel imaging
Single shot and echo planar imaging
Partial Echo Imaging
partial/ fractional averaging or half Fourier
TE can be reduced when ___ is performed
partial echo imaging
Partial, Fractional Averaging or ½ Fourier
Since K space is symmetrical as long as either the top half or bottom half is filled the computer can mirror image the other side
3 ways of acquiring data
2D
3D
Sequential
Sequential
Get all data from 1 slice then moves to the next slice (most common)
3D volumeteric
(Volume imaging)- acquired data from whole tissue rather than separate slices
Slice encoding
used at the end of Volume imaging (3D)
when switched on separates the slices according to their phase value along the gradient
what is an advantage of using 3D volumetric imaging?
Many can be obtained without a slice gap
What controls the polarity of a gradient?
direction of current through a coil
What factor does the frequency encoding gradient slope control?
FOV
What operator function alters the number of data points in k-space?
frequency matrix
what are the units of K-space?
radians per cm
what happens if frequency's are insufficiently sampled?
aliasing
Which area of K space contributes to irresolution?
outer
Which direction is k space traversal when frequency encoding is positive?