Chapter 5 Spatial Encoding Flashcards
RF pulse is applied at
90 degree angle from B0
- at the processional frequency of the magnetic moments of hydrogen nuclei (within tissue)
magnetization is created in
the transverse plane and the individual magnetic moment of hydrogen line up into phase
voltage/ signal is created in
transverse plane of the receiver coil
signal in an alternating voltage equal to
Larmor frequency
magnetic moments precess at the same frequency
so all their signals are oscillating at the same frequency
- so the system cannot spatially locate them
to create an image the system needs to
calculate how much signal is coming from each #d location in the paitent
voxel
3D
1st step is to
select a slice within the tissues that is gonna be examined
pixel
2D
- there is a signal within
spatial encoding
process of locating the signal
spatial encoding is a function of
gradients
all hydrogen precesses at the same frequency when
exposed to the same magnetic field strength
within the main magnetic field there are
inhomogenities
to locate the individual signals
the B0 is altered so it slopes in a linear and predictable way
graded (sloped) magnetic fields are generated by
cylinder electro magnets that are in the warm bore of the cryostat
- coils called gradient coils
warm bore
inside the cylinder of cryostat
- contains patient bore and the components of MR system
- operates at room temp
gradient coil
current is passed through each of the coils
current is passed through each coil at
a specific time during the pulse sequence
Faraday’s law
when a current is passed through a gradient, a magnetic field is created
polarity depends on
which end of the gradient magnetic field is higher than B0 and which ones is lower
Isocenter
center of the bore of the magnet in all planes
- this means the middle axis of the gradient stay at the same field strength as B0 even when they are switched on
Larmor equation
magnetic moments increase or decrease based on strength of the magnet
magnetic field strength increases
hydrogen nuclei speed up
magnetic field strength decreases
hydrogen nuclei slow down
change in the magnetic field caused by gradient is
small
- frequency of the magnetic moment spin located in the gradient is less than 1%
gradient amplitude determines
rate of change of the magnetic field strength along the gradient axis
steep gradient slopes=
large differences in precessional frequency
shallow gradient slopes
small differences in precessional frequency
what happens in the gradient coil that determines the polarity of the gradient?
the direction of the current through the coil
what happens in a gradient that determines the amplitude of a gradient?
amplitude of the current applied through the gradient coil.
to perform spatial localization
MR signal must be spatially encoded in 3 directions
- use 3 gradient magnetic fields
3 axes
X, Y , Z
the order that these gradients are activated
- slice select
- phase encoding
- frequency/ readout encoding
any one of these 3 gradients can perform
- slice select
- phase encoding
- frequency/ readout encoding
each one of these gradient magnetic fields are assigned
- one orthogonal direction
- one function of spatial encoding
Z axis
always parallel to B0
Y axis
always perpendicular (90 degrees) to B0
- Ceiling to floor
X gradient
left to right
slice planes
- axial
- sagittal
- coronal
when the slice is selected
gradient for that slice is automatically chosen
orientation of the phase and frequency gradients are logical
if choosing one direction, the other is automatically chose
Z gradient
alters the Long axis
- head to foot of patient
Y gradient
alters vertical/short axis
- front to back
X gradients
alters horizontal axis
- left to right
slice selection
locates the slice within the scan plane
frequency encoding
locates the signal along the long axis of the slice
phase encoding
locates the signal along the short axis of the slice
Oblique slices
combination of two or more gradients
gradient magnetic fields are
superimposed on B0
different areas of the body can imaged in cross sectional volumes known as
slice
