EXAM #15 — PHYSICS UNIT 08

Question 1

a. signal.

Answer 1

a. signal. RF received in a coil that comes from the relaxation of excited protons

Question 2

b. noise.

Answer 2

b. noise. RF received in a coil that comes from background electrical interference and random thermal motion in a patient

Question 3

c. signal-to-noise ratio.

Answer 3

c. signal-to-noise ratio. a measure of image quality in MRI; the ratio of the average signal in an imaging volume and the average noise in the volume; signal intensity divided by standard deviation of noise

Question 4

d. contrast-to-noise ratio.

Answer 4

d. contrast-to-noise ratio. ratio between the SNR values of adjacent tissues or structures

Question 5

e. spatial resolution.

Answer 5

e. spatial resolution. ability to distinguish points as separate and distinct

Question 6

f. scan time.

Answer 6

f. scan time. time needed to complete all data acquisition for a sequence

Question 7

g. voxel.

Answer 7

g. voxel. 3 dimensional area of tissue that is imaged

Question 8

h. rectangular FOV.

Answer 8

h. rectangular FOV. a method whereby outer phase encoding steps are omitted and the FOVin the phase encoding direction is smaller than the frequency encoding direction

Question 9

define the following pulse sequence parameters and describe how a change in their values affects SNR in general:

a. TR. repetition time; increased TR = _____ SNR

Answer 9

define the following pulse sequence parameters and describe how a change in their values affects SNR in general:

a. TR. repetition time; increased TR = increased SNR

Question 10

define the following pulse sequence parameters and describe how a change in their values affects SNR in general:

b. TE. echo time; increased TE = _____ SNR

Answer 10

define the following pulse sequence parameters and describe how a change in their values affects SNR in general:

b. TE. echo time; increased TE = decreased SNR

Question 11

define the following pulse sequence parameters and describe how a change in their values affects SNR in general:

c. slice thickness. increased slice thickness= _____ SNR

Answer 11

define the following pulse sequence parameters and describe how a change in their values affects SNR in general:

c. slice thickness. increased slice thickness= increased SNR

Question 12

define the following pulse sequence parameters and describe how a change in their values affects SNR in general:

d. FOV. the total area that is imaged; increased FOV = _____ SNR

Answer 12

define the following pulse sequence parameters and describe how a change in their values affects SNR in general:

d. FOV. the total area that is imaged; increased FOV = increased SNR

Question 13

define the following pulse sequence parameters and describe how a change in their values affects SNR in general:

e. image matrix. number of phase and frequency encoding steps in an image, or relative number of pixels in an image;

finer matrix (more encoding steps, smaller pixels) = _____ SNR

coarser matrix (fewer encoding steps, larger pixels) = _____ SNR

Answer 13

define the following pulse sequence parameters and describe how a change in their values affects SNR in general:

e. image matrix. number of phase and frequency encoding steps in an image, or relative number of pixels in an image; finer matrix (more encoding steps, smaller pixels) = decreased SNR, coarser matrix (fewer encoding steps, larger pixels) = increased SNR

Question 14

define the following pulse sequence parameters and describe how a change in their values affects SNR in general:

f. NSA. number of signal averages; increased NSA = _____ SNR

Answer 14

define the following pulse sequence parameters and describe how a change in their values affects SNR in general:

f. NSA. number of signal averages; increased NSA = increased SNR

Question 15

4. explain the effect that voxel volume has on SNR and why. the larger the voxel volume, the _____ signal can be obtained from it because a larger voxel has a higher spin density

Answer 15

4. explain the effect that voxel volume has on SNR and why. the larger the voxel volume, the more signal can be obtained from it because a larger voxel has a higher spin density

Question 16

5. identify 3 sequence parameters that affect voxel volume.

Answer 16

5. identify 3 sequence parameters that affect voxel volume.

FOV, slice thickness, matrix size

Question 17

6. identify 3 sequence parameters that affect image contrast and SNR.

Answer 17

6. identify 3 sequence parameters that affect image contrast and SNR.

TR, TE, flip angle

Question 18

7. explain the relationship between voxel size and spatial resolution.

the larger th-e voxel size, the _____ the spatial resolution

Answer 18

7. explain the relationship between voxel size and spatial resolution. the larger th-e voxel size, the lower the spatial resolution

Question 19

8. identify 3 sequence parameters that affect scan time and SNR.

Answer 19

8. identify 3 sequence parameters that affect scan time and SNR.

TR, NSA, matrix size

Question 20

9. identify 2 sequence parameters that affect the number of slices available during each TR period.

Answer 20

9. identify 2 sequence parameters that affect the number of slices available during each TR period.

TE, receive bandwidth

Question 21

10. identify 3 sequence parameters that affect spatial resolution and SNR.

Answer 21

10. identify 3 sequence parameters that affect spatial resolution and SNR.

slice thickness, matrix size, FOV

Question 22

11. identify the amount by which the SNR is increased if the NSA is doubled.

Answer 22

11. identify the amount by which the SNR is increased if the NSA is doubled.

doubling NSA results in an increase in SNR by the square root of 2 = 1.44 or 44%

Question 23

12. explain how a narrower RF receive bandwidth increases the SNR.

Answer 23

12. explain how a narrower RF receive bandwidth increases the SNR.

a narrower (decreased) bandwidth allows less noise to be received relative to signal

Question 24

14. write and explain the formula for scan time for a spin echo pulse sequence.

scan time =

Answer 24

14. write and explain the formula for scan time for a spin echo pulse sequence.

scan time = (TR) x (number of phase encoding steps) x ( NSA)

Question 25

15. explain how slice encoding is done for 3D or volume imaging.

Answer 25

15. explain how slice encoding is done for 3D or volume imaging.

phase encoding steps are performed in the slice selection direction

Question 26

16. identify 4 advantages of volume imaging over 2D imaging.

Answer 26

16. identify 4 advantages of volume imaging over 2D imaging.

thinner slices possible;

no slice gap;

higher SNR;

ability to manipulate 3D volume into other imaging planes

Question 27

17. identify a disadvantage of volume imaging over 2D imaging.

Answer 27

17. identify a disadvantage of volume imaging over 2D imaging.

longer scan times