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MRI > 5: Pulse Sequence Design > Flashcards

5: Pulse Sequence Design Flashcards

1
Q

Pulse sequence

A

the process of acquiring patient data by disorienting net magnetization of protons with radio-frequency (RF) pulses, gradient variations, and data collection

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2
Q

Traditional standard pulse sequences are: (4)

A
  1. Spin Echo (SE)
  2. Inversion Recovery (IR)
  3. Gradient Echo (GE)
  4. Echo Planar Imaging (EPI)
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3
Q

Pulse Sequence Timing Diagram:

A

visualizes detailed timing and duration of RF pulses and gradient variations

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4
Q

Echo time (TE)

A

the time it takes to complete an entire pulse sequence

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5
Q

Events recorded by the Pulse Sequence Timing Diagram: (5)

A
  1. RF Transmission (RF-t)
  2. RF Receive (RF-r)
  3. Slice select Gradient
  4. Readout
  5. Phase
  • Each event can initiate or pause another event*
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6
Q

Musical Note Analagy

A
  • imagine each different pulse sequence being its own unique string of musical notes
  • in music, there are different sequences of musical notes that have different overall sound and rhythm combinations
  • in MRI, there are different sequences of events that create different tissue characteristics and contrast combinations
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7
Q

Step 1: Radio Frequency Transmitter (RF-t)

A

Excitation event, Applied RT pulse, the beginning of any pulse sequence.

The events that excite or flip net magnetization into the transverse direction.

Begins with a 90 degree initial pulse followed by a 180 degree pulse (Double the strength). 1/2 of a TE period

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8
Q

Step 2: RF-Receive (RF-r)

A

Dephasing step, begins simultaneously during the 90 degree and 180 degree pulses (RF Transmission)

known as the dephasing event because once the net magnetization is disoriented by the 90 degree pulse, the signal immediately begins to dephase or weaken.

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9
Q

Dephasing

A

Progressive weakening of the net magnetization

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10
Q

Dephasing is also known as:

A

Free Induction Decay (FID)

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11
Q

Step 3: Slice Select Gradient:

A

Also turns on during RF Transmission similar to RF Receive

allows a certain slice to be singled out (spatial localization), allowing us to attain an image of a specific slice.

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12
Q

Step 4: Readout Gradient

A

Sampling gradient

As RF Transmit, RF Receive, and Slice selection occur, the readout gradient reads or samples the signal through (Frequency encoding)

Turned on during frequency encoding and during the resultant echo. Reads during the echo.

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13
Q

Step 5: Phase Gradient

A

Turned on between the 90 degree and 190 degree pulses, storing the information.

the phase gradient is solely determined by the number of phase encoding lines in a slice. Ex: 256 (frequency) x 192 (phase) = 192 phase encodes

higher the matrix, longer the time : directly correlated.

Phase encoding: during the echo

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14
Q

Two types of Spin Echo

A
  1. Conventional Spin Echo (CSE) (older)
  2. Fast Spin Echo (FSE) (modern)
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15
Q

Conventional Spin Echo

A

Traditional (not used)

T1, T2, or Proton Density Weighted

Contain ONE echo per TE period

Due to scan time length being long, CSE’s have been innovated and are rarely used.

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16
Q

Fast Spin Echo (FSE)

A

Aka Turbo Spin Echo (TSE) Siemens

Innovated and faster than CSE due to the added Echo Train Length (ETL)

T1, Td, or Proton Density Weighted

FSE’s ETL is chosen MANUALLY by the technologist during pulse sequence manipulation

More ETL, more echo phases, but progressively gets smaller/weaker as you move away from the initial 90 degree pulse

Shorter ETL, stronger the signal, longer the scan time.

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17
Q

Inversion Recovery (IR)

A

T1 environment (relaxation) by flipping the net magnetization with a 180 degree pulse

IR is a pulse sequence that contains an additional 180 degree pulse before the 90 degree pulse

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18
Q

Inversion time (TI)

A

time from the initial 180 pulse to the 90 pulse, First 180 then 90. Take longer

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19
Q

Echo Train Length (ETL)

A

FSE’s ETL is chosen MANUALLY by the technologist during pulse sequence manipulation, during Fast Spin Echo (FSE)

More ETL, more echo phases, but progressively gets smaller/weaker as you move away from the initial 90 degree pulse

Shorter ETL, stronger the signal, longer the scan time.

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20
Q

Two types of Inversion Recovery (IR)

A
  1. STIR (Short Tau (time) Inversion Recovery)
  2. FLAIR (FLuid Attenuated Inversion Recovery)
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21
Q

TI

A

the time before the initial 90 degree pulse

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22
Q

180 degree pulse causes…
short T1 tissues to relax (quickly or slowly)
long T1 tissues to relax (quickly or slowly)
Fat (bright or dark)
Water ( bright or dark)

A

short T1 tissues to relax quickly
long T1 tissues to relax slowly
Fat - dark
water- bright

*Contrast between short and long T1 tissue is created BEFORE the pulse sequence begins

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23
Q

STIR

A

Short Tau Inversion Recovery

widely used as a fat suppression technique. to suppress fat content and focus on high water content such as fractures and infection

Long TI 10-150 ms

Contrast agents DO NOT work or highlight in STIR imaging due to similar short T1 tissue property of fat and gadolinium

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24
Q

How does STIR suppress fat?

A

applying 90 degree pulse as Short T1 tissue (fat) returns to net magnetization.

we only image Long T1 tissue (fluid, edema from fracture in bone) and subtract fat tissue.

10-150 ms

25
Q

How will fat look on a STIR? what anatomy?

A

saturated black

bone, bone marrow, tendon, muscle

26
Q

How will water look on a STIR? what anatomy?

A

bright

Blood, edema, infection

27
Q

FLAIR

A

Fluid Attenuated Inversion Recovery

suppress CSF and highlight diseased periventricular tissue such as infarction (stroke), brain/cord lesions (MS), subdural hemorrhage, and meningitis. Past Blood brain barrier (BBB)

Long TI time ( ~ 2000 ms)
Long TR time ( ~ 8000 - 9000ms )

28
Q

How will CSF look on FLAIR?

A

saturated black or “null”

29
Q

How will diseased tissue look on FLAIR?

A

Bright

Stroke, MS, etc.

30
Q

If you have a patient MRI Brain who may not cooperate, what sequence should be first?

A

FLAIR followed by Diffusion Weighted Imaging Sequence

31
Q

Difference between FLAIR and STIR?

A

Much longer TI in FLAIR, longer pulse sequence

32
Q

4 Types of Gradient Echo

A

Gradient Recall Eco (GRE)
Spoiled Gradient Echo (Spoiled GE)
Steady State Gradient Echo (SS-GE)
Fast Gradient Recall Echo (Fast GRE)

33
Q

Conventional Gradient Echo (GE) used to show 3 things…

A

Blood in brain, ligaments, cartilage

34
Q

GRE imaging is completely different from FSE’s and IR’s for three reasons…

A
  1. RF Flip Angles are applied first, NOT initial 90 degree pulses
  2. Gradient pulses are used to refocus decay, NOT 180 degree pulses
  3. Slice and Readout gradients are used to balance the phase gradient
35
Q

Spoiled Gradient Echo’s (Spoiled GE) differ from Gradient Recall Echo (GRE) mainly due to…

A

the addition of a spoiler pulse after the readout of a gradient echo

36
Q

A spoiler pulse can be either…. or …

A

A - strong RF pulse
B - strong gradient pulse

37
Q

** What is the reason for a spoiler pulse?

A

To cancel or terminate any remaining magnetization still in the transverse plane

38
Q

Steady State Gradient Echo (SS-GE) or Coherent differ from Spoiled Gradient Echo’s (Spoiled GE) because

A

Steady state gradients want to preserve remaining transverse magnetization

39
Q

Steady State Gradient Echo’s (SS-GE) preserve remaining transverse magnetization by _____, which allows…..

A

rephrasing, allows both relaxed and remaining net magnetization to both be present for the next initial RF flip angle.

40
Q

What is the opposite of Steady State Gradient Echo? (SS-GE)

A

Spoiled Gradient Echo (Spoiled GE)

41
Q

Another name for Coherent Echo

A

Steady State Gradient Echo (SS-GE)

42
Q

What is the result of Steady State Gradient Echo (SS-GE)?

A

T2* Weighted Images

43
Q

Fast Gradient Echo (Fast GE) requires 3 things…

A
  1. Short TR
  2. Short TE
  3. Smaller flip angle
44
Q

Fast GRE gains contrast properties by…

A

establishing the contrast before the initial flip angle by applying RF pulses

45
Q

T1 weighting Fast GRE: What degree of pulse is used?

A

180 degree pulse is used

46
Q

T2 weighting Fast GRE: what pulse degree?

A

90 degree, 180 degree, 90 degree pulse combination is used

47
Q

What is used at the end of a Fast GRE to accelerate the relaxation process?

A

A Spoiler

48
Q

Echo Planar Imaging (EPI)

A
  • The fastest and loudest MRI technique

GE - Fiesta

49
Q

Echo Planar Imaging is good for…

A

IAC

50
Q

The readout portion of the pulse sequence reads or ________ the signal data through ____________ encoding

A

Samples, Phase

51
Q

Which pulse sequence has the largest TVMF (Time Varying Magnetic Field) effects?

A

Echo Planar Imaging (EPI)

52
Q

The general public is limited to a ________ gauss magnetic field.

A

5 Gauss

  • In some MR rooms, they will have a “5 Gauss Line” which shows how far you can proceed before significant pull from the fringe field. *
53
Q

If a fast spin echo sequence has parameters set as: 224 x 256, 3 NEX, with a FOV of 24. How many phase encoding steps have to be filled in order to correctly produce a FSE?

A

224

  • Remember * some vendors read matrix (frequency/phase) or (phase/frequency). Remember that the phase encoding steps will ALWAYS be the lower value and it can never be higher than frequency encoding. *
54
Q

MRI Techs should consistently employ ______ asepsis cleaning techniques after each patient and at the end of every night.

A

Medical Asepsis

55
Q

When positioning patient’s it is important not to loop coil cables thus reducing the chances of ________.

A

Decoupling

56
Q

When scanning EKG leads and RF coils should be placed:

A

straight throughout the MR scanner to reduce decoupling.

57
Q

Which pulse sequences employ an initial 180 RF pulse followed by a 90 RF pulse and another 180 RF pulse?

A

STIR and FLAIR

58
Q
A

MRI (13 decks)

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