EXAM #14 — PHYSICS UNIT 07

Question 1

Define: a. gradient magnetic field.

Answer 1

magnetic fields that change linearly over the magnet bore

Question 2

Define: b. logical gradients.

Answer 2

slice select, frequency encoding, and phase encoding gradients

Question 3

Define: c. spatial localization.

Answer 3

the process of locating each voxel of tissue in an imaging volume, and positioning it correctly in the image

Question 4

Define: d. gradient slope.

Answer 4

the amount of change in strength of a gradient magnetic field

Question 5

Define: e. magnet isocenter.

Answer 5

the center of the primary magnetic field in all three planes

Question 6

Define: f. slice selection gradient.

when is it switched on?

Answer 6

the logical gradient that localizes tissue in the imaging plane, is switched on during RF transmission

Question 7

Define: g. frequency encoding gradient.

when is it switched on?

Answer 7

logical gradient that localizes tissue perpendicular to the imaging plane- is switched on during signal reception

Question 8

Define: h. readout gradient.

Answer 8

same as frequency encoding gradient

Question 9

Define: 1. phase encoding gradient.

when is it switched on?

Answer 9

logical gradient that localizes tissue perpendicular to the imaging plane- is switched on before the refocusing pulse

Question 10

Define: j. RF transmit bandwidth.

Answer 10

the range of frequencies of transmitted RF

Question 11

Define: k. RF receive bandwidth.

Answer 11

the range of frequencies of received RF

Question 12

Define: 1. slice thickness.

Answer 12

the amount of tissue that is imaged with each slice, affects spatial resolution

Question 13

Define: m. sampling rate.

Answer 13

the frequency at which a waveform is sampled, or how many times per second the MR signal is sampled

Question 14

Define: n. sampling time.

Answer 14

the amount of time during which a waveform is sampled

Question 15

Define: o. k-space.

Answer 15

the frequency- and phase- encoded raw data map of an MR image

Question 16

Define: p. image matrix.

Answer 16

the number of frequency and phase encoding steps used to spatially localize an image

Question 17

Define: r. partial or fractional echo imaging.

Answer 17

a method of imaging whereby only half of k­ space in the frequency encoding axis is filled during acquisition, and the other half is calculated by the MR system

Question 18

Define: s. partial or fractional averaging.

Answer 18

a method of imaging whereby only half of k­ space in the phase encoding axis is filled during acquisition, and the other half is calculated by the MR system

Question 19

Define: u. 2D k-space filling.

Answer 19

a method of imaging whereby the first line of k-space for each slice is filled, then the second line for each slice is filled, and so on until all lines are filled

Question 20

Define: v. 3D k-space filling.

Answer 20

a method of imaging whereby an entire volume of tissue is excited by the RF transmission, and slices are divided up after image acquisition

Question 21

Define: w. centric ordered k-space filling.

Answer 21

filling the central phase encoding steps of k­ space first, then filling toward the outer edges

Question 22

Define: x. Fourier transformation.

Answer 22

the mathematical calculation that converts waveforms into discrete values that describe signal strength at each frequency

Question 23

Define: y. field of view (FOV).

Answer 23

the area of the volume to be imaged

Question 24

Define: z. slice dephasing.

Answer 24

the dephasing of spins within a slice that is caused by the gradient magnetic field

Question 25

Define: aa. slice profile.

Answer 25

the shape of the transmitted RF pulse

Question 26

Define: aa. slice profile.

Answer 26

the shape of the transmitted RF pulse

Question 27

explain the relationship between gradient magnetic fields and the local magnetic field experienced by tissues along the gradient.

Answer 27

where the gradient add to the primary magnetic field, the tissues experience a higher local magnetic field, where the gradient subtracts from the primary field, the tissues experience a lower local field strength

Question 28

explain the relationship between gradient magnetic fields and the precessional frequencies of spins along the gradient.

Answer 28

where the gradient add to the primary magnetic field, the tissues experience a higher frequency, where the gradient subtracts from the primary field, the tissues experience a lower frequency

Question 29

explain "steep" and "shallow" gradient slopes and how they are used to describe the change in magnetic field strength over the length of the gradients.

Answer 29

a steep gradient is one in which the magnetic field strength changes a lot over the length of the gradient, a shallow gradient is one that changes little

Question 30

5. identify which physical gradient coil pair(s) perform(s) slice selection during axial, sagittal, coronal, and oblique imaging in a superconducting MRI system.

Answer 30

axial- slice select is the Z gradient sagittal- slice select is the X gradient coronal- slice select is the Y gradient oblique- slice selection is performed by more than one physical gradient

Question 31

6. describe how the slice select gradient slope affects slice thickness.

Answer 31

a steep slice select gradient produces thinner slices, a shallow slice gradient produces thicker slices

Question 32

7. describe how the RF transmit bandwidth affects slice thickness.

Answer 32

a narrower (decreased) transmit bandwidth produces thinner slices, a wider (increased) bandwidth produces thicker slices

Question 33

8. identify the logical gradient that is on during RF transmission.

Answer 33

slice select

Question 34

9. identify the logical gradient that is on during MR signal reception.

Answer 34

frequency encoding or read-out

Question 35

10. identify the logical gradient that is on before the refocusing pulse.

Answer 35

phase encoding

Question 36

11. state the Nyquist theorem.

Answer 36

a waveform must be sampled at least twice per cycle to represent it accurately

Question 37

12. explain the relationship between RF receive bandwidth and sampling rate that is necessary to satisfy the Nyquist theorem.

Answer 37

they must change proportionally- e.g. as bandwidth increases, sampling rate must increase in order to sample accurately

Question 38

13. explain the relationship between sampling time and sampling rate that is necessary to satisfy the Nyquist theorem.

Answer 38

they are inversely proportional- e.g. as sampling time decreases, sampling rate must increase; as sampling rate decreases, sampling time must increase, etc.

Question 39

14. explain the relationship between sampling time and RF receive bandwidth that is necessary to satisfy the Nyquist theorem.

Answer 39

they are inversely proportional- e.g. as bandwidth increases (wider), sampling time may decrease, as bandwidth decreases (narrower), sampling time must increase. etc.

Question 40

15. describe a "fine" and "coarse" image matrix.

Answer 40

a fine matrix divides the image matrix into smaller pixels; a coarse matrix divides the image matrix into larger pixels

Question 41

16. explain how the numbers of phase and frequency encodings affect the image matrix.

Answer 41

a fine matrix contains many frequency and/or phase encoding steps, a coarse matrix contains fewer frequency and/or phase encoding steps

Question 42

17. identify the number of lines of k-space that are phase encoded during each TR period.

Answer 42

one

Question 43

18. describe how a change in the number of phase encoding steps affect the imaging time of a sequence. as the number of phase encoding steps increases, the scan time _____ proportionally

Answer 43

18. describe how a change in the number of phase encoding steps affect the imaging time of a sequence. as the number of phase encoding steps increases, the scan time increases proportionally

Question 44

19. identify the area of k-space in which the highest amplitude of MR signal is placed.

Answer 44

the center of k-space, or the shallow phase encoding steps

Question 45

20. identify the area of k-space that determines image resolution.

Answer 45

the outer edges, or the steep phase encoding steps

Question 46

21. identify the area of k-space that determines tissue contrast.

Answer 46

the center, or the shallow phase encoding steps

Question 47

22. explain how RF transmit bandwidth and sampling time affect slice profile.

Answer 47

the narrower (lower) the bandwidth the squarer the slice profile; the longer the sampling time the squarer the slice profile

Question 48

23. explain how changes in RF transmit bandwidth affect the duration of the RF pulse.

Answer 48

a narrower (lower) bandwidth requires a longer RF pulse duration, a wider (higher) bandwidth allows a shorter RF pulse duration.

Question 49

24. explain how changes in RF receive bandwidth affect the minimum TE and TR of a pulse sequence.

Answer 49

an wider (higher) receive bandwidth makes a shorter TE and TR possible, a narrower (lower) receive bandwidth makes a longer TE and TR necessary

Question 50

25. explain how changes in RF transmit bandwidth affect the minimum TE and TR of a pulse sequence.

Answer 50

a wider (higher) transmit bandwidth makes a shorter TE and TR possible, a narrower (lower) transmit bandwidth makes a longer TE and TR necessary

Question 51

26. explain how changes in sampling rate affect the minimum TE and TR of a pulse sequence.

Answer 51

a faster sampling rate makes a shorter TE and TR possible, a slower sampling rate makes a longer TE and TR necessary

Question 52

27. describe the effect that the frequency encoding gradient and RF receive bandwidth have on the field of view (POV).

Answer 52

the FOV can be decreased by using a steeper frequency encoding gradient or a narrower (lower) receive bandwidth (or both); the FOV can be increased by using a shallower frequency encoding gradient or a wider (higher) receive bandwidth (or both)

Question 53

28. identify which ai: as of k-space are filled with steep and which are filled with shallow phase encoding gradients.

Answer 53

the outer edges are filled with steeper phase encoding gradients; the center of k-space is filled with shallow phase encoding gradients

Question 54

29. calculate pixel size of an image when given the POV, number of Frequency Encoding points, number of Phase Encoding steps, and slice thickness.

Answer 54

formula [POV (FE)/# FE points] x [POV (PE)/# PE steps] x [5mm slice thickness] example [256mm / 256] x [256mm / 256] x [5mm] = 1mm x 1mm x 5mm

Question 55

30. recognize the following on a pulse sequence diagram: a. frequency encoding gradient. b. rephasing lobe of the frequency encoding gradient. c. phase encoding gradient. d. slice select gradient. e. rephasing lobe of the slice select gradient.

Answer 55

30. recognize the following on a pulse sequence diagram: a. frequency encoding gradient. b. rephasing lobe of the frequency encoding gradient. c. phase encoding gradient. d. slice select gradient. e. rephasing lobe of the slice select gradient.