Chapter 3 Flashcards
The gradient subsystem consists of
the gradient coils and gradient amplifiers which provide power to the coils
Gradient
defined as a slope and also as a measure of change in a physical quantity over distance
gradient magnetic field
a magnetic field that varies in intensity (or strength) over distance
the gradient magnetic fields are produced by
the gradient coils superimposed over the main static magnetic field
What is the primary purpose of gradient fields/coils?
to provide for the spatial encoding and localization of the MR signals
There are how many sets of gradient coils?
3
-each coil produces a gradient magnetic field oriented along 1 of the 3 orthogonal axes of the body
the gradient coil which produces a gradient field along the head to foot direction of the body is referred to as the
Z gradient coil
the gradient coil which produces a gradient field along the anterior to posterior direction of the body is referred to as the
Y gradient coil
the gradient coil which produces a gradient field along the right to left direction of the body is referred to as the
X gradient coil
The gradient fields not only vary in strength over distance but also vary in
time.
they are switched on and off very rapidly during the imaging process and are responsible for the sounds heard during a scan
whether the gradient is positive or negative is referred to
as it’s polarity
What is maximum amplitude?
the maximum slope to which the gradient field rises
High amplitude gradient magnetic fields are required for:
thin slices and small FOV
What is rise time?
the time required for the gradient to reach its maximum amplitude
What is Slew Rate?
a way to express both the maximum amplitude and the rise time is the slew rate which is the max amplitude divided by the rise time
Slew Rate formula
Slew rate = max amp / rise time
(T/m/sec)
rise time is expressed in
units of microseconds
max amplitude is expressed in
units of millitesla per meter (mT/m)
slew rate is expressed in
units of tesla per meter per second
T/m/sec (Tesla per metre per second)
What is one of the loudest MRI sequences?
Echo Planar Imaging (EPI)
can easily reach and exceed levels of 120 dB
Gradient acoustic noise is greater the ____the gradients switch
faster
Faraday’s Law of Induction states
if you move or change a magnetic field over time in presence of a loop of conductive material you will induce a current in that conductive loop
As the gradient magnetic fields change rapidly over time,
they can produce significant current in loops of conductive material.
Care should be taken not to form ________ with surface coil wires or have the wire touch the patient
large loops
The patient should be positioned so that they do not:
cross or hold their hands nor should they cross their ankles.
this prevents skin to skin contact but will also eliminate the formation of loop. this is done to prevent peripheral nerve stimulation
The faster the gradients switch, the ____possibility of PNS
greater
True or false: FDA guidelines state: No painful nerve stimulation
True
-avoid large conductive loops (surface coil wires or ECG leads)
-avoid patient loops (grasping wrist/fingers, crossing ankles, or wires touching patients)
