Physics Ch 10 MRI

Question 1

T1 –> represents what?

Answer 1

recovery of longitudinal magnetization: time it takes for proton to normalize (align back to longitudinal magnetization) after knocked down by RF pulse

Question 2

T1 –> definition?

Answer 2

time at which long magnetization is 63% of final value

Question 3

“intrinsic T1 shortening” –> T1 signal?

Answer 3

bright

Question 4

inc magnetic field strength –> what happens to T1?

Answer 4

longer T1

Question 5

T2 –> represents what?

Answer 5

decay of transverse magnetization

Question 6

T2 –> definition?

Answer 6

time at which signal has decayed 63% of original value of transverse magnetization

Question 7

what is time to repetition (TR)?

Answer 7

time bw initiation bw 2 successive RF pulses

Question 8

inc time to echo (TE) –> what happens to T2 effects?

Answer 8

inc

Question 9

short TR, short TE –> T1 vs T2 vs PD weighting?

Answer 9

• max long magnetization
• min trv mag

==> T1 weighting

Question 10

long TR, long TE –> T1 vs T2 vs PD weighting?

Answer 10

• min long mag
• max trv mag

==> T2 weighting

Question 11

long TR, short TE –> T1 vs T2 vs PD weighting?

Answer 11

• min long mag
• min trv mag

==> PD weighting

Question 12

k-space –> center vs periphery –> contains what kind of info?

Answer 12

• center: tissue contrast

- periphery: spatial resolution

Question 13

phase encoding vs freq encoding –> which takes longer?

Answer 13

phase encoding

Question 14

encoding spatial information in vertical direction –> describes phase or freq encoding?

Answer 14

phase encoding

Question 15

encoding spatial information in horizontal direction –> describes phase or freq encoding?

Answer 15

freq encoding

Question 16

thin slices:

• small vs lrg slice selection gradient (SSG)?
• small vs lrg transmit bandwith?

Answer 16

• lrg SSG

- small transmit bandwith

Question 17

thick slices:

• small vs lrg slice selection gradient (SSG)?
• small vs lrg transmit bandwith?

Answer 17

• small SSG

- lrg transmit bandwith

Question 18

table time –> formula?

Answer 18

time = (TR) x (#Phase encoding steps) x (#excitations)

Question 19

better spatial resolution w…:

• small vs lrg voxel
• small vs lrg field of view
• small vs lrg matrix
• thin vs thick slice

Answer 19

• small voxel
• small field of view
• lrg matrix
• thin slice

Question 20

bigger voxel –> what happens to…:

• spatial resolution
• signal to noise ratio

Answer 20

• spatial resolution –> worse

- signal to noise ratio –> improve

Question 21

better signal to noise ratio w…:

• small vs lrg voxel
• small vs lrg field of view
• small vs lrg matrix
• thin vs thick slice
• weaker vs stronger magnetic field
• short vs long TR
• less vs more #excitations
• short vs long TE
• small vs lrg receiver bandwith

Answer 21

• lrg voxel
• lrg field of view
• small matrix
• thick slice
• stronger magnetic field
• long TR
• more #excitations
• short TE
• lrg receiver bandwith

Question 22

Nyquist ghosting –> occurs w what phase seq?

Answer 22

EPI

Question 23

Nyquist ghosting –> cause?

Answer 23

too high demand for gradient system –> imperfections in gradient performance

Question 24

Nyquist ghosting –> appearance?

Answer 24

ghost –> shifted by 1/2 FOV

Question 25

Nyquist ghosting --> remedy? (4)

Answer 25

- apply eddy-current correction - re-shimming - reduce echo train length - lower phase-enc resolution

Question 26

truncation --> remedy? (2)

Answer 26

- inc matrix --> inc resolution | - apply pre- or post- reconstruction image filtration

Question 27

spike (herringbone) artifact --> appearance?

Answer 27

dark/bright stripes --> not necess aligned w phase/freq direction

Question 28

spike (herringbone) artifact --> cause? (2)

Answer 28

- readout process --> corrupted --> falsely detect lrg signal 1 or more times during readout - RF arcs get produced in scan room (loose electrical connections, vibrating metal) ==> spike in raw k-space data

Question 29

spike (herringbone) artifact --> remedy?

Answer 29

call field engineer to check for vibrating metal or loose wires

Question 30

zipper --> cause?

Answer 30

detect extraneous RF signal during readout

Question 31

zipper --> remedy? (3)

Answer 31

- make sure scanner room door is closed - scan room equipment --> check for RF emissions - call physics staff or field engineer to find RF leak in shielding

Question 32

susceptibility artifact --> common in what seq? (3)

Answer 32

- T2* - GRE - EPI

Question 33

susceptibility artifact --> remedy? (6)

Answer 33

- remv metal - use non-gradient echo seq - inc bandwith - inc matrix - dec slice thick - EPI --> use parallel imaging

Question 34

surface coil artifact --> appearance?

Answer 34

anatomy near surface coil --> stronger signal

Question 35

surface coil artifact --> remedy? (2)

Answer 35

- shimming | - use bird cage coil

Question 36

magic angle artifact --> remedy?

Answer 36

lengthen TE

Question 37

chem shift artifact type 1 --> occur in phase or freq-enc direction?

Answer 37

freq

Question 38

chem shift artifact type 1 --> remedy? (1)

Answer 38

inc receiver bandwidth

Question 39

fat saturation failure --> cause?

Answer 39

resonant freq of fat --> not uniform in image: - poor shimming - anatomy is distant from isocenter

Question 40

fat saturation failure --> remedy? (2)

Answer 40

- shim region of interest | - mv anatomy closer to isocenter

Question 41

dielectric effect --> remedy? (1)

Answer 41

use dielectric pads

Question 42

motion artifact --> occur in phase or freq-enc direction?

Answer 42

phase

Question 43

motion artifact --> remedy? (5)

Answer 43

- use flow compenstation or apply saturation bands to lessen flow - switch freq & phase enc to rotate the artifact - ask patient to be still - use motion resistant seq (SSFE, partial NEX, PROPELLER, BLADE, MULTI-VANE) - use resp-gating or breath-holding

Question 44

parallel imaging sequences --> undersample data during readout --> what artifact?

Answer 44

aliasing in image center

Question 45

susceptibility artifact --> worst in which pulse seq?

Answer 45

EPI

Question 46

chemical shift artifact type 2 --> occurs with spin echo vs GRE vs both?

Answer 46

GRE only