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CORE > Physics Ch 10 MRI > Flashcards

Physics Ch 10 MRI Flashcards

1
Q

T1 –> represents what?

A

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

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2
Q

T1 –> definition?

A

time at which long magnetization is 63% of final value

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3
Q

“intrinsic T1 shortening” –> T1 signal?

A

bright

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4
Q

inc magnetic field strength –> what happens to T1?

A

longer T1

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5
Q

T2 –> represents what?

A

decay of transverse magnetization

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6
Q

T2 –> definition?

A

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

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7
Q

what is time to repetition (TR)?

A

time bw initiation bw 2 successive RF pulses

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8
Q

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

A

inc

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9
Q

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

A
  • max long magnetization
  • min trv mag

==> T1 weighting

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10
Q

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

A
  • min long mag
  • max trv mag

==> T2 weighting

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11
Q

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

A
  • min long mag
  • min trv mag

==> PD weighting

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12
Q

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

A
  • center: tissue contrast

- periphery: spatial resolution

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13
Q

phase encoding vs freq encoding –> which takes longer?

A

phase encoding

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14
Q

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

A

phase encoding

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15
Q

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

A

freq encoding

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16
Q

thin slices:

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

- small transmit bandwith

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17
Q

thick slices:

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

- lrg transmit bandwith

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18
Q

table time –> formula?

A

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

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19
Q

better spatial resolution w…:

  • small vs lrg voxel
  • small vs lrg field of view
  • small vs lrg matrix
  • thin vs thick slice
A
  • small voxel
  • small field of view
  • lrg matrix
  • thin slice
20
Q

bigger voxel –> what happens to…:

  • spatial resolution
  • signal to noise ratio
A
  • spatial resolution –> worse

- signal to noise ratio –> improve

21
Q

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
A
  • lrg voxel
  • lrg field of view
  • small matrix
  • thick slice
  • stronger magnetic field
  • long TR
  • more #excitations
  • short TE
  • lrg receiver bandwith
22
Q

Nyquist ghosting –> occurs w what phase seq?

A

EPI

23
Q

Nyquist ghosting –> cause?

A

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

24
Q

Nyquist ghosting –> appearance?

A

ghost –> shifted by 1/2 FOV

25
Q

Nyquist ghosting –> remedy? (4)

A
  • apply eddy-current correction
  • re-shimming
  • reduce echo train length
  • lower phase-enc resolution
26
Q

truncation –> remedy? (2)

A
  • inc matrix –> inc resolution

- apply pre- or post- reconstruction image filtration

27
Q

spike (herringbone) artifact –> appearance?

A

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

28
Q

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

A
  • 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

29
Q

spike (herringbone) artifact –> remedy?

A

call field engineer to check for vibrating metal or loose wires

30
Q

zipper –> cause?

A

detect extraneous RF signal during readout

31
Q

zipper –> remedy? (3)

A
  • 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
32
Q

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

A
  • T2*
  • GRE
  • EPI
33
Q

susceptibility artifact –> remedy? (6)

A
  • remv metal
  • use non-gradient echo seq
  • inc bandwith
  • inc matrix
  • dec slice thick
  • EPI –> use parallel imaging
34
Q

surface coil artifact –> appearance?

A

anatomy near surface coil –> stronger signal

35
Q

surface coil artifact –> remedy? (2)

A
  • shimming

- use bird cage coil

36
Q

magic angle artifact –> remedy?

A

lengthen TE

37
Q

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

A

freq

38
Q

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

A

inc receiver bandwidth

39
Q

fat saturation failure –> cause?

A

resonant freq of fat –> not uniform in image:

  • poor shimming
  • anatomy is distant from isocenter
40
Q

fat saturation failure –> remedy? (2)

A
  • shim region of interest

- mv anatomy closer to isocenter

41
Q

dielectric effect –> remedy? (1)

A

use dielectric pads

42
Q

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

A

phase

43
Q

motion artifact –> remedy? (5)

A
  • 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
44
Q

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

A

aliasing in image center

45
Q

susceptibility artifact –> worst in which pulse seq?

A

EPI

46
Q

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

A

GRE only

CORE (33 decks)

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