Next: Selecting a Slice or
Up: Imaging in More Dimensions
Previous: Imaging in More Dimensions
  Contents
- think of a 3D data set being made up of
points
where for now
- due to transverse relaxation, historically have only had
time to collect a single line of data (
points)
following each excitation. (IN-CLASS QUESTION)
- repeat the experiment
times to get all data.
- sequence diagram describes how we move about to collect these
points
-
![\includegraphics[height=4.0in]{figs/imbf56}](img427.gif)
- area under each gradient determines location in
-space
- if apply identical gradients again, will collect same line
of
-space data.
- in order to collect different
-space data need to vary
the area of
or
gradients
- gradients which are turned off before ADC comes on for
sampling are generally referred to as ``phase encode'' gradients
because they impart some phase that is constant during sampling
- common name for phase encode gradient applied in the plane of
the resulting image called phase encode, or primary phase
encoding gradient
- common name for phase encode gradient applied perpendicular
to the plane of
the displayed image called partition encode, or secondary phase
encoding gradient
-space step size in phase encode directions is
conventionally determined by how much we change the PE gradient
amplitude between repetitions of the experiment
- refer to gradient that is on during sampling as the read
or frequency encoding gradient. Think of sampling in terms of
time points we pick off at certain interval given by
,
that relates to
-space spacing given by
- each time the experiment is repeated, the phase encode
gradients are changed to cover different lines of
-space.
- should always identify the PE and RD directions
in images that you view
- under ideal conditions (no noise, no motion, no relaxation,
no field issues, no frequency aliasing)
phase encoding and frequency encoding are identical processes, but
practically, this is not the case!
- shorthand for altering phase encode gradients is to draw them with
many lines on a single sequence diagram(IN-CLASS QUESTION)
-
- How long does experiment take??? (IN-CLASS QUESTION)
Next: Selecting a Slice or
Up: Imaging in More Dimensions
Previous: Imaging in More Dimensions
  Contents
Michael Thompson
2003-11-21