MRI Image Formation Principles
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What is the net magnetization vector (NMV)?
Total number of excess protons aligned with the magnetic field
What is the NMV proportional to?
Magnetic field strength
What is FID signal?
the signal produced immediately following an RF pulse
What is the process that digitizes the MR signals?
2D acquisitions usually require an _____
The overall SNR will _____ in 3d volume imaging when there is an increase in either ____ and/or and increase in the _____
- number of slices
What is MPR?
the ability to generate images in scan planes in addition to the acquisition plane
The ETL is a scan time acceleration factor, but it also can lead to what?
increased thermal induction in patients due to the increased RF power
What is a gradient?
a small change in the magnetic field along a particular axis
A narrow receiver bandwidth increases ______
Lower receiver bandwidth = _____SNR and _____chemical shift
A wide receiver bandwidth generates _____SNR
A wide receiver bandwidth should be applied in anatomical regions that contain
high fat and water interfaces
Decreasing receiver bandwidth has what effect on chemical shift?
Increases chemical shift
What is transmit bandwidth?
The RF excitation pulse required for slice selection
What is proportional to RF bandwidth?
What happens to slice thickness when pulse bandwidth is lowered?
Slice thickness is reduced
What is receive bandwidth?
the range of frequencies accepted by the receiver to sample the MR signal
What is the relationship between receive bandwidth and SNR?
- Direct relationship
- Decreasing bandwidth by factor of 2 increases SNR by square root of 2
What are the advantages of increasing receiver bandwidth?
- Shorter echo spacing (less blurring)
- Reduced susceptibility artifacts
- less chemical shift
- fewer metal artifacts
- increased available ETL
What are the disadvantages of increasing receiver bandwidth?
- Lower SNR
- larger minimum required FOV
What are advantages of decreasing receiver bandwidth?
- Higher SNR
- Smaller minimum required FOV
What is needed in order to generate an MR image?
the slice select gradient must be turned on during the RF energy application
What does the slice selection gradient determine?
- slice orientation
- slice thickness
- slice position
If a steep slice gradient slope is used, what kind of slices are there?
How are thin slices are achieved?
- steep slice gradient
- high amplitude
- narrow transmit bandwidth
What is the frequency encoding (readout) gradient?
The gradient that is on during the production of the echo
When is the readout gradient usually turned on?
During the sampling or readout peak echo and also during frequency encoding
What is sampling time?
the time to sample the entire k-space line
What controls sample time?
Formula for sampling time
Frequency matrix / Receive bandwidth
What is sampling rate?
Signal sampling frequency
sampling rate must be twice that of the sampling frequency in order to avoid aliasing in our image
When does aliasing occur?
In the phase direction
What is gradient moment nulling?
- Flow compensation
- compensates for first order motion (protons moving with constant velocity) and slow flowing vessels
What affects the flip angle?
strength and duration of the RF field
Increasing the FOV would _______spatial resolution because of _______
- increase in pixel size
What happens to SNR when the flip angle is increased?
SNR increases to a point called the Ernst angle
What is the Ernst angle?
the Optimal flip angle that yields the maximum signal for a particular spin in the least amount of time
What is Rectangular FOV?
a technique reduces scan time by not filling as many lines of space in the phase direction while maintaining spatial resolution
Why are flip angles used in gradient echo sequences?
to control saturation effects
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