Module 4: Encoding and Image Formation Flashcards Preview

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Flashcards in Module 4: Encoding and Image Formation Deck (35)
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1
Q

What are Gradients composed of?

A

Coils of wire within the scanner and are used to alter the main magnetic field when current is passed through them.

2
Q

What are the three gradient coils that alter the main magnetic field in each of their corresponding planes?

A

i. (Gz) Z gradient alters the magnetic field along the Z-axis (long axis of the scanner).
ii. (Gy) Y gradient alters the magnetic field along the Y-axis (vertical axis of the scanner).
iii. (Gx) X gradient alters the magnetic field along the X-axis (horizontal axis of the scanner).

3
Q

What is the isocentre?

A

the magnetic center point of all 3 gradient axes. It is also the center point of the scanner’s magnet.

4
Q

T/F: The isocenter is always at the main magnetic field strength (e.g. 1.50T for a 1.50T scanner).

A

True

5
Q

What affects the magnetic field strength between two points more, steep gradients or shallow gradients?

A

Steep gradients.

6
Q

What alters the precessional frequency of nuclei between two points more, steep or shallow gradients?

A

Steep gradients

7
Q

What types of encoding do gradients do when perform slice selection? Describe these types of encoding.

A

i) Frequency Encoding :
- they spatially locate (encode) the signal along the long axis of the anatomy being imaged.

ii) Phase Encoding:
- and they spatially locate (encode) the signal along the short axis of the anatomy being imaged = phase encoding.

8
Q

What are the 4 types of slice selection and what type of slices do they select?

A

i. Z gradient selects axial slices.
ii. Y gradient selects coronal slices.
iii. X gradient selects sagittal slices.
iv. Oblique slices are selected by using 2 gradients in combination.

9
Q

What pulses are the slice selection gradient turned on for in spin echo sequences? When are they turned on in gradient echo pulse sequences?

A

the slice selection gradient is turned on during both the 90° and 180° pulses in spin echo sequences, and during ONLY the excitation pulse in gradient echo pulse sequences.

10
Q

What is Transmit bandwidth?

A

the range of RF frequencies transmitted to excite a particular slice.

11
Q

What slice select and transmit bandwidth should be applied for a thin slice?

A

steep slice select slope and/or a narrow transmit bandwidth is applied.

Note: the MRI system automatically calculates the appropriate gradient slope and transmit bandwidth according for the slice thickness required.

12
Q

What slice select and transmit bandwidth should be applied for a thick slice?

A

a shallow slice select slope and/or a broad transmit bandwidth is applied.

Note: the MRI system automatically calculates the appropriate gradient slope and transmit bandwidth according for the slice thickness required.

13
Q

What is the Slice Gap?

A

Distance between slices (typically expressed as a percentage).

14
Q

How large would the gap between slices be for two 4mm thick slices with a 25% slice gap?

A

There would be 4mm x 25% = 1mm gap between the slices that is not imaged.

15
Q

T/F: Slice gap is not important for reducing image artifacts.

A

False. It is important (Will learn in later module)

16
Q

Frequency encoding typically encodes (i.e. spatially locates the signal) along the _____ axis of the anatomy using a frequency encoding gradient.

A

long

Note: that frequency and phase directions can easily be swapped on the scanner, but as you will learn later on, there are important reasons why frequency is typically chosen to be the long axis (along with significant advantages and disadvantages to changing these axes in certain situations).

17
Q

What is the long axis for a coronal image?

A

Z-axis

18
Q

What is the long axis for a Sagittal image?

A

Z-axis

19
Q

What is the long axis for a axial image?

A

X-axis in the body, Y-axis in the head

20
Q

The precessional frequency of magnetic moments of hydrogen nuclei ____ when the magnetic field increases.

A

increases

21
Q

To achieve a small FOV in the frequency direction a _____ frequency-encoding gradient should be applied.

A

steep

22
Q

To achieve a large FOV in the frequency direction a ____ frequency-encoding gradient should be applied.

A

shallow

23
Q

The short axis of the image is usually encoded via ___

A

phase encoding.

24
Q

What is the phase encode direction (short axis of the anatomy) for a coronal image?

A

x-axis

25
Q

What is the phase encode direction (short axis of the anatomy) for a Sagittal image?

A

Y-axis

26
Q

What is the phase encode direction (short axis of the anatomy) for a axial mage?

A

Y-axis in the torso, X-axis in the head

27
Q

What are the 5 steps of image encoding and when should they be applied? (Remember the image)

A

1) 90° RF Excitation Pulse – flips the NMV into the transverse plane. Recall that the FID is the decay that occurs as the magnetization in the transverse plane decreases back to zero (as the nuclei realign to the main magnetic field).
2) Slice Selection Gradient is turned on to create a linear fluctuation in the magnetic field. A slice is excited by transmitting RF with a band of frequencies matching the Larmor frequencies of those spins.
3) Phase Encoding Gradient is turned on to localize the signal along the short axis of the anatomy within the slice. This occurs after the FID has decayed, and before the 180° refocusing pulse.
4) 180° RF Refocusing Pulse – recall that dephasing rapidly depletes the MRI signal, so we need to regenerate (rephase) the signal. Spin echo sequences use a 180° pulse, gradient echo pulse sequences use an additional gradient.
5) Frequency Encoding Gradient is turned on to localize the signal along the long axis of the anatomy within the slice. This gradient is on during readout (i.e. during the dephasing and rephrasing of the spin echo signal).

28
Q

What is the Sampling Interval?

A
  • The time interval between each sample (or data point)

- Sampling interval = 1 / sampling frequency

29
Q

What theorem is the Sampling frequency is determined by?

A

Nyquist theorem

30
Q

What does the Nyquist theorem state about sampling frequency?

A

sampling frequency must be equal to or greater than twice the highest frequency in the signal

31
Q

What is the name of the maximum frequency that can be accurately sampled?

A
  • Nyquist frequency

- sampling frequency = 2 x Nyquist frequency

32
Q

What is the receive bandwidth?

A
  • the range of frequencies we sample during readout, determined by applying a filter to the frequency encoding gradient.
  • receive bandwidth = 2 x Nyquist frequency
33
Q

When the Nyquist theorem is obeyed exactly how does the sampling frequency compare to the receive bandwidth?

A

sampling frequency = 2 x Nyquist frequency

receive bandwidth = 2 x Nyquist frequency

Therefore:
receive bandwidth = sampling frequency

(Note that receive bandwidth is a parameter than can be set on the MRI scanner, not sampling frequency.)

34
Q

In spin gradient-echo pulse sequences when is the slice select gradient applied?

A
  • It is switched on with the 1st RF excitation pulse. This is the only time it is switched on,
35
Q

In spin-echo pulse sequences when is the slice select gradient applied?

A
  • It is switched on with the 1st RF excitation pulse and again with every RF rephasing pulse.