Unit 8

Question 1

What is signal?

Answer 1

RF received from relaxation of excited protons

Question 2

What is noise?

Answer 2

RF received from background electrical interference and thermal motion in a patient

Question 3

What is SNR?

Answer 3

signal to noise ratio; measure of image quality

Question 4

What is contrast to noise ratio?

Answer 4

ratio between SNR of adjacent tissues

Question 5

What is spatial resolution?

Answer 5

ability to distinguish points as separate and distinct

Question 6

What is crosstalk?

Answer 6

mixing of signal from adjacent slices

Question 7

What is cross excitation?

Answer 7

transmitted RF that leaks over from adjacent slices

Question 8

What is respiratory compensation?

Answer 8

method whereby shallow phase encoding are acquired between breaths

Question 9

What is phase/frequency oversampling?

Answer 9

method whereby extra encoding steps are acquired, but discarded prior to image reconstruction

Question 10

Why is respiratory compensation used?

Answer 10

reduce respiratory motion artifact

Question 11

Why is phase/frequency oversampling used?

Answer 11

reduce wrap-around

Question 12

How is SNR affected by TR?

Answer 12

proportionally

Question 13

How is SNR affected by TE

Answer 13

inversely proportional

Question 14

How is SNR affected by slice thickness

Answer 14

proportionally

Question 15

How is SNR affected by FOV?

Answer 15

proportionally

Question 16

How is SNR affected by image matrix?

Answer 16

inversely proportional

Question 17

How is SNR affected by Number of Signal Avgs (NSA)?

Answer 17

proportionally

Question 18

How is SNR affected by slice gap?

Answer 18

proportionally

Question 19

How is SNR affected by receiver bandwidth?

Answer 19

inversely proportionally

Question 20

Describe phase mismapping/ghosting artifact

Answer 20

ghosting/blurring of image along phase econding axis

Question 21

What causes phase mismapping/ghosting?

Answer 21

motion - resp, blood, swallowing

Question 22

Describe wrap around artifact

Answer 22

anatomy is placed in wrong posistion in image

Question 23

What causes wrap around?

Answer 23

FOV too narrow in phase direction

Question 24

Describe partial volume avg artifact

Answer 24

blurred edges of structures

Question 25

What causes partial volume avg?

Answer 25

large voxel size

Question 26

Describe Chemical shift artifact

Answer 26

dark edge on one side of structures, and bright on the opposite in frequency direction

Question 27

What causes chemical shift?

Answer 27

the different precessional frequencies of fat and water

Question 28

Describe chemical misregistration artifact

Answer 28

dark lining around tissue interfaces on GE

Question 29

What causes chemical misregistration?

Answer 29

fat and water precessions being out of phase

Question 30

Describe truncation/Gibbs artifact

Answer 30

low intensity bands that run parallel to tissue interface in phase direction

Question 31

What causes truncation/gibbs?

Answer 31

undersampled tissue interfaces

Question 32

Describe data clipping artifact

Answer 32

signal loss of image; exaggerated/inverted contrast

Question 33

What causes data clipping?

Answer 33

image data is beyond the range of the receiver

Question 34

What parameters affect voxel volume?

Answer 34

FOV, slice thickness, matrix size

Question 35

What parameters affect contrast and SNR?

Answer 35

TR, TE, flip angle

Question 36

What parameters affect scan timeand SNR?

Answer 36

TR, NSA, matrix size

Question 37

What parameters affect number of slices available during each TR?

Answer 37

TE, receive bandwidth

Question 38

What parameters affect resolution and SNR?

Answer 38

slice thickness, matrix size, FOV

Question 39

How much does SNR increase by doubling NSA?

Answer 39

44%

Question 40

How does slice gap affect SNR?

Answer 40

greater gap = less noise

Question 41

How is scan time calculated?

Answer 41

=TR x #phase encoding steps x NSA

Question 42

How is slice encoding done for 3D imaging?

Answer 42

phase encoding steps are performed in the slice direction

Question 43

What are advantages to 3D imaging?

Answer 43

thinner slices, no slice gap, higher SNR, manipulate into different orientations

Question 44

What is a disadvantage to 3D imaging?

Answer 44

longer scan time

Question 45

How do you compensate for motion/ghosting artifact?

Answer 45

swap phase and frequency direction, presatureation, resp comp, flow comp, gating

Question 46

How do you compensate for wrap-around?

Answer 46

oversample, presaturate

Question 47

How do you compensate for chemical shift?

Answer 47

increase bandwidth, decrease FOV, decrease voxel size

Question 48

How do you compensate for chemical misregistration?

Answer 48

SE squences, GE w/ TE when fat and water in phase

Question 49

Why is GE more susceptible to chemical misregistration?

Answer 49

there isn't a refocusing pulse

Question 50

How do you compensate for truncation?

Answer 50

inc phase encoding steps

Question 51

How do you compensate for cross-excitation?

Answer 51

increase slice gap, interleaved slice excitation