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MRI > MRI Pulse Sequences > Flashcards

MRI Pulse Sequences Flashcards

1
Q

What is a pulse sequence?

A

A series of RF pulses and gradient events, fired to excite the spins and induce a signal in the reeive coil, in a short time (TR)

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2
Q

What are the goals of imaging sequences?

A
  • To generate an RF signal perpendicular to static magnetic field
  • To generate good tissue contrast
  • To minimize artifacts
  • To maximize the SNR
  • To acheive all the above, in the shortest possible scan time
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3
Q

What is a Spin Echo?

A

The basic sequence (with good contrast)
-90-180 echo

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4
Q

The function of a spin echo is to:

A
  • generate T1, PD (proton density), and T2 weighted images
  • minimizes artifacts
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5
Q

List some advantages of spin echo imaging.

A
  • High signal to noise (SNR)
  • Least artifact prone sequence due to 180 refocusing pulse
  • Contrast mechanisms accurate and easy to understand
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6
Q

List some disadvantages of spin echo imaging.

A
  • High SAR (how much RF you put into the patient) because of 90 degree and 180 degree RF pulses
  • Long scan time due to single encoding per TR
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7
Q

Scan time Equation

A

Scan time = TR x phase resolution x averages (nex)

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8
Q

T1 weighted has a short/long TR & short/long TE.

A

short TR & short TE

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9
Q

T2 weighted was a short/long TR and short/long TE.

A

long TR and long TE

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10
Q

A PD weighted image has a short/long TR and short/long TE.

A

long TR and short TE

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11
Q

In a T2 weighted image, fluid appears _.

A

bright

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12
Q

What occurs during Fast/Tubo Spin Echo?

A
  • Multiple 180 pulse is used
  • For one excitation (90) there are many 180 pulses -> echoes
  • Multiple phase encodes per TR
  • Amplitude of echoes decrease as the time from 90 increases due to T2 decay
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13
Q

List some advantages of fast/turbo spin echo.

A
  • Much faster sequence than SE (spien echo)
  • Has nearly the same contrast as that of SE
  • Magnetic susceptibility effects are minimized
  • Artifact due to metallic implants are reduced (inhomogeneity reduced)
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14
Q

What is the scan time equation for fast/turbo spin echo?

A

Scan time = TR x phase resolution turbo factor x average (nex)

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15
Q

List some disadvantages of fast/turbo spin echo.

A
  • The weighting is not perfect due to multiple TEs -> it works better for T2 than T1 images
  • Multiple 180 pulses make fat look bright on T2 weighted images
  • Magnetic susceptibility is reduced so the sequences is not good for Hemorrhages or head trauma
  • Image blurring might occur (SNR of the last echoes that are filling the k-space border could be very low)
  • Very high SAR (due to multiple 180 pulses (high chance of burning)
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16
Q

What is Single Shot Fast Spin Echo (SS FSE)?

A

Fills half of k-space in one TR! Large echo trains, uses half Fourier to predict the rest of the k-space; very fast but has low SNR

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17
Q

What is a gradient echo?

A

the simplest sequence; also known as gradient recalled echo (GRE)

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18
Q

What are the three main parameters of a gradient echo and what are its effects?

A

3 main parameters: TR, TE, & flip angle
* low SAR
* Artifact prone
* weak signal (as compared to SE)

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19
Q

What is a partial flip angle?

A

flip/rotate a small amount of magnetization (no more than 90) = fast recovery

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20
Q

T2 decay occurs between the _ and the _ gradients.

A

dephasing; rephasing

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21
Q

Rephasing incompletely recovers the _.

A

signal

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22
Q

Signal loss is greater with longer/shorter TEs

A

longer

23
Q

Gradient Echo imaging cannot recover signal losses from:

A
  • magnetic field inhomogeneity
  • magnetic susceptibility
  • water-fat incoherence
24
Q

List some advantages of Gradient Echo.

A
  • Faster imaging-can use shorter TR and short TEs than SE
  • Low flip angle -> deposits less energy (more slices per TR than SE, decreases SAR)
  • Compatible with 3D acquisitions
25
Q

List some disadvantages of gradient echo?

A
  • Difficult to generate good T2 weighting
  • T2* relaxation influences contrast
  • Magnetic field inhomogeneity cause signal loss
    • worse with increasing TE times
  • -susceptibility effects
    • dephasing of water and fat photons
26
Q

What is an Inversion Pulse?

A

Initial 180 RF pulse which is followed by the excitation (90 pulse)

27
Q

What is Inversion T1 time?

A

time from inital 180 pulse to 90 pulse

28
Q

What are the advantages and disadvantages of inversion recovery (IR)?

A

Disadvantages: longer imaging time, fewer slices
Advantages: better T1 contrast

29
Q

How is suppression achieved?

A

by sampling, as the longitudinal magnetization of the tissue (a) is at the zero crossing

30
Q

What does STIR stand for?

A

Short Tau Inversion Recovery

31
Q

What does STIR do?

A

Short Tau Inversion Recovery:
nullifies fat signal; short T1; very useful in musculoskeletal and body imaging

32
Q

What does FLAIR stand for?

A

Fluid Attenuated Inversion Recovery

33
Q

What is FLAIR and what is it used for?

A

Fluid Attenuated Inversion Recovery:
A turbo-spin echo sequence with IR; routinely used for brain imaging; long TR and T1, dark CSF; bright tumors, fat and bleeds; very sensitive to parameter change

34
Q

True or false. Water and fat resonate at slightly different frequencies.

A

True

35
Q

A gradient echo image used in-phase looks _ and out-of-phase looks _.

A

bright; black outline around organs

36
Q

Ernst Angle

A

A defined flip angle less than 90 degrees used to optimize contrast of a given tissue or fluid

37
Q

What is SPGR?

A

Spoiled (incoherent) Gradient Echo- spoiling destroys the accumulated transverse magnetization; it is done by an additional de-phasing gradient; it can also be done by RF (phase detection)

38
Q

What is the advantage of Spoiled Gradient Echo (SPGR)?

A

it maximizes T1 contrast

39
Q

What is FLASH?

A

Fast Low Angle Shot (RF or Gradient Spoiled)

40
Q

FLASH
T1 Contrast has a short/long TE, short/long TR, large/small flip angle.

A

short TE, short TR, large flip angle

41
Q

Spoiled Gradient Echo T2* Contrast has short/long TE, short/long TR, and small/large flip angle.

A

long TE, long TR, small flip angle

42
Q

What is EPI?

A

Echo Planar Imaging- uses gradient echo

starts with 90 -> 180 -> gradient echo

43
Q

What are the advantages of EPI?

A

very fast sequence, all k-space in one TR, and 3D image in below a minute

44
Q

What are disadvantages of EPI?

A

low SNR, very noisy, and very susceptible to artifacts, specifically susceptibility

45
Q

Why are gradient echo images used?

A

due to short TR times

46
Q

In 3D imaging, an additional _ is added perpendicular to the imaging plane.

A

phase encoding gradient

47
Q

Partitions

A

planes of the 3D volume

48
Q

What is MPRAGE?

A

Magnetization Prepared Rapid Acquisition Gradient Echo- Fast 3D Gradient Echo with good T1 contrast that has an initial 180 inversion pulse followed by GRE sequence with rewinding gradient

49
Q

MPRAGE
-In order to produce 3D images, a _ occurs in the slice selection direction

A

secondary phase encoding

50
Q

What are the different k-space fillings?

A

linear, centric, spiral, and propellar/blade

51
Q

Which k-space filling is less motion sensitive?

A

spiral

52
Q

What is parallel imaging?

A

using multi-channel coils

53
Q

Which k-space filling corrects for motion?

A

propeller/blade

MRI (13 decks)

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