Module 4: Encoding and Image Formation

Question 1

What are Gradients composed of?

Answer 1

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

Question 2

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

Answer 2

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).

Question 3

What is the isocentre?

Answer 3

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

Question 4

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

Answer 4

True

Question 5

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

Answer 5

Steep gradients.

Question 6

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

Answer 6

Steep gradients

Question 7

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

Answer 7

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.

Question 8

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

Answer 8

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.

Question 9

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

Answer 9

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.

Question 10

What is Transmit bandwidth?

Answer 10

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

Question 11

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

Answer 11

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.

Question 12

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

Answer 12

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.

Question 13

What is the Slice Gap?

Answer 13

Distance between slices (typically expressed as a percentage).

Question 14

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

Answer 14

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

Question 15

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

Answer 15

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

Question 16

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

Answer 16

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).

Question 17

What is the long axis for a coronal image?

Answer 17

Z-axis

Question 18

What is the long axis for a Sagittal image?

Answer 18

Z-axis

Question 19

What is the long axis for a axial image?

Answer 19

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

Question 20

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

Answer 20

increases

Question 21

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

Answer 21

steep

Question 22

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

Answer 22

shallow

Question 23

The short axis of the image is usually encoded via ___

Answer 23

phase encoding.

Question 24

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

Answer 24

x-axis

Question 25

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

Answer 25

Y-axis

Question 26

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

Answer 26

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

Question 27

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

Answer 27

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).

Question 28

What is the Sampling Interval?

Answer 28

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

- Sampling interval = 1 / sampling frequency

Question 29

What theorem is the Sampling frequency is determined by?

Answer 29

Nyquist theorem

Question 30

What does the Nyquist theorem state about sampling frequency?

Answer 30

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

Question 31

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

Answer 31

• Nyquist frequency

- sampling frequency = 2 x Nyquist frequency

Question 32

What is the receive bandwidth?

Answer 32

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

Question 33

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

Answer 33

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.)

Question 34

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

Answer 34

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

Question 35

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

Answer 35

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