MRI102 week5 6-3 Ch5 - part 3

Flashcards about MRI slice thickness, phase encoding, and frequency encoding based on a lecture transcript.

How do you obtain thin slices in MRI?

To obtain thin slices, the slice select gradient must have a steep slope, meaning the gradients have to work harder.

How do you obtain thick slices in MRI?

To obtain thick slices, the slice select gradient must have a shallow slope, meaning it's easier to obtain thicker slices.

What is bandwidth?

The range of frequencies in an RF pulse.

What is transmit bandwidth (TBW)?

The range of frequencies transmitted into the patient.

What do you need for thin slices regarding slice select gradient and transmit bandwidth?

Steep slice select gradient slope and/or a narrow transmit bandwidth (smaller range of frequencies).

What do you need for thick slices regarding slice select gradient and transmit bandwidth?

Shallow slice select gradient slope and/or a broad transmit bandwidth (wider/larger range of frequencies).

What does the phase encoding gradient do?

It changes the phase of the nuclei every TR and is applied along the shorter length of the anatomy.

When is the phase encoding gradient activated?

Turns on at some point after excitation.

How does the phase encoding gradient change over time?

The phase encoding gradient changes amplitude every TR to ensure that the echo collected every TR has different phase information, meaning it has a different slope every TR.

What does a steep phase encoding gradient slope cause?

It causes larger phase shifts, meaning more dephasing.

What does a shallow phase encoding gradient slope cause?

Causes smaller phase shifts, meaning less dephasing.

What does phase encoding mean?

It creates a difference in phase in the nuclei to help identify their location in the slice.

What does frequency encoding do?

It changes the precessional frequency of the nuclei along the long axis or longer length of the anatomy.

How does the precessional frequency change along the frequency encoding gradient?

Nuclei at the high end of the gradient experience a large magnetic field strength and precess faster (higher frequency), while nuclei at the low end have a lower frequency because they precess slower.

When is the frequency encoding gradient activated?

When the echo is received (during echo collection); also called the readout gradient.

What is the field of view (FOV)?

The size of the image.

What kind of frequency encoding gradient is applied if you want a small FOV in the frequency direction?

A steep frequency encoding gradient must be applied.

What kind of frequency encoding gradient is applied if you want a large FOV?

A shallow frequency encoding gradient must be applied.

What does spatial encoding mean?

The gradients locate the signal in three dimensions.