Chapter 3 Encoding And Image Formation Flashcards
Def of gradients
Electromagnets or winding coils creating a linear slope in MF strength across the imaging volume in a particular direction. There are 3 sets of gradient coils x y z
Shape and function of gradients
Shape- are in pairs.: the pairs of gradients are of equal and opposite polarity
Function- each either increase or decrease the MF by a specific amt
What makes the loud sounds in MRI
The gradients
What 7 things does the gradients CONTROL
Matrix size FOV Min TE Slice thickness Slices TR Ability to obtain oblique images ETL echo spacing
What are the 4 characteristics of gradients
Amplitude
Duty cycle
Rise time
Slew rate
What does a gradient do and what are the 3 planes
Creates a plane or environment and isolates the Hs in a specific area
AP-y
SI- z
LR- x
Is B1 always parallel or perpendicular to B0
Perpendicular
What is the units to measure gradient strength?
B0/distance G/cm or T/m
1G/cm = 10mT/m
Why is it important to have higher amplitude in gradients? What’s a good range?
Allows us to do thinner slices, use a smaller FOV and provides a higher resolution
Good range is 10-60 mT/m
15 is good but 33 is best
in magnet the difference and strength allows for faster acquisition for the thinner slices as well as wider range of capabilities
What is rise time and how is it measured
It’s how long for gradients to go from zero to maximum amplitude. It is measured in microseconds. The smallest number means less time and is fastest
What is the duty time? How was it expressed?
It’s expressed as a percentage. It is the percent of time the gradient works at maximum amplitude. It allows the gradients to combined a small fov with a high matrix, thin slice, flow comp.
The higher a percentage usage the less slices can be done
What is slew rate
It is a marketing term. It is the speed and strength of gradients and measured in mT/m/s
The rate the gradients may be turned on or off
The faster the slew rate the shorter the TEs
What happens if there is inhomogeneity induced by the gradient in slice selection?
Causes spins to resonate at different frequencies. Only proton spinning at the same frequency as an applied RF pulse will respond
What must be in first…. B1 or SS and why?
SS just a nanosecond before to B1
BC SS will select the plane/environment, select slice thickness and adjust the bandwidth… then do the RF pulse
When in equilibrium and out of phase where is M? Where is M if protons are in phase?
Longitudinal if out of phase
Transverse if protons are in phase.
What is the function of slice thickness (2)
Slope and bandwidth
Steep versus shallow slopes and narrow versus wide bandwidth
Steep will provide thin slices, smaller range btwn two pts….however requires more current and gradient strength
Shallow gives thicker slices and large range
Narrow=thin slices
Wide= thick slices
Bandwidth is the size of B1
What is a voxel?
It is a three dimensional boxes that make up the FOV. DxWxH
Will affect the resolution of scan. The smaller the FOV the better. So need more voxels within FOV
The location of spins in a particular row is determined by what?
Phase shift. Phase shift between spins is directly proportional to their location
it helps determine the rows of a matrix
What is phase encoding
It’s the magnetic moment of hydrogens along the gradient in ROWS
When choosing a PE do you want the longest or shortest to go along with SS
Shortest
When do we apply a frequency encoding (FE) gradient and what does it help determine?
Right before the PE gradient to negate the effect of the SS gradient
Also applied again after the rephasing 180 during read out
Helps determine the Columns of the matrix and the FOV
What determines the location of hydrogens along the gradient in columns
FE
Frequency encoding
Long axis
Is M parallel or perpendicular with the protons are in phase
Perpendicular
What is the FE and PE for the following:
High
Standard
Coarse
High- 512 512-256
Standard- 256 246-128
However the best is 192-256 PE
Coarse 128 128-64
What should be higher FE or PE
Frequency
If you have a higher PE what does it do to scan time and resolution
Increases
When is PE gradient switched on
After the excitation pulse
When is the FE gradient switched on
During the collection of the signal….. the echo
What is acquisition window
Also known as sampling time
The duration of the readout gradient.
What is K space and where is it collected at?
It is raw data formed by collections of echoes as gradient amplitudes change for every TR
The data is collected in the array processor
What is the unit of K space
Radians per centimeter
What is another name for SS
Physical gradient or logical gradients
Short axis? Long axis?
Short is PE
Long is FE
Other names for SS
Physical and logical
What is a transient MF
Gradients, superpulse electromagnetic field
What is M?
Bo
B1
NMV
External MF
RF pulse
What is Bandwidth
Range of frequency
Narrow or wide
B1 field
