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Physics > MRI > Flashcards

MRI Flashcards

1
Q

What is the gyromagnetic ratio for H?

A

42.6 Mhz

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2
Q

What does the Larmor equation tell you?

A

The precessional frequency.

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3
Q

T1 relaxation is also known as?

A

Longitudinal relaxation

Spin-lattice relaxation

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4
Q

What is T1 relaxation defined as the time it take for longitudinal magnetization to recover how much?

A

63%

The greater the magnet the longer the T1 because there is more net magnetization to recover.

Also more energy is but into the molecule so longer to give it up.

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5
Q

What things are bright because of T1 relaxation?

A

Things with short T1 relaxation times will be bright on T1W images

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6
Q

What is T2 relaxation defined as?

A

The time at which the signal has decayed to 37% of its original value of transverse magnetization

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7
Q

What is T2 relaxation also known as?

A

Spin Spin - loss of energy due to inhomogeneities in the external field and local magnetic field.

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8
Q

How many gauss are in 1 Telsa? How many gauss is the earth’s magnetic strength?

A

There are 10,000 gauss in 1 Telsa

The earth magnet is 0.3-0.6 gauss

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9
Q

What is the difference in T2 relaxation and T2*?

A

T2 relaxation has a 180 refocusing pulse and therefore decays by spin spin interactions

T2* does not have a refocusing pulse and therefor decays due to spin spin and inhomogeneities. - Always faster. Free induction decay

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10
Q

Where is the 180 degree RF pulse placed or when do you deliver it?

A

1/2 the TE

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11
Q

In K-space, where is the contrast information and where is the resolution?

A

Contrast is in the middle

Resolution is around the edges

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12
Q

The fourier transform does what?

A

Change data from the time domain to the frequency domain.

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13
Q

Scan time duration depends on what in a 2D Spin Echo sequence?

A

TR x Number of phase encoding steps (phase matrix) x NEX

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14
Q

When is the freq encoding gradient turned on?

A

Read out.

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15
Q

A thin transmit bandwidth creates what in terms of slice size?

A

Steep gradient and thin slice

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16
Q

A large transmit bandwidth creates what in terms of slice size?

A

Bigger slice

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17
Q

Scan time duration depends on what in a 2D Turbo Spin Echo sequence?

A

1/Echo train length

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18
Q

Scan time duration depends on what in a 3D sequence?

A

TRxPE steps x NEX x # slices

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19
Q

TR is what?

A

The time between repetition of the same slice… multiple slices can be excited within that time.

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20
Q

What increases SNR in MRI?

A
  1. Stronger magnet
  2. Longer TR
  3. Big FOV
  4. Large Slices (Shallow gradient, large transmit bandwidth)
  5. More NEX
  6. Short TE
  7. Small Matrix
  8. Small reciever bandwidth (no sampling as often so not collecting as much noise)
  9. Appropriate coil size
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21
Q

Increase NEX from 2 to 4 double scan time but does what to SNR?

A

Square root of 2 = 1.4x

22
Q

A larger reciever bandwidth does what to SNR? Time of scan?

What artifacts does it affect and how?`

A

Decreases SNR

Decreases time of scan — Reciever bandwidth = 1/time

Decreases Chemical shift and magnetic susceptibility.

23
Q

1 TR fill how many lines of k-space?

24
Q

STIR = short inversion recovery… what is that in ms compared to FLAIR?

A

~200 STIR

~2000 FLAIR

25
Q

STIR is less susceptible to what artifact?

A

Metal and susceptibility.

26
Q

Gad is a what kind of molecule and what does it do?

A

It is a paramagnetic molecule that shortens T1 of the shit around it.

27
Q

Why does fat look lighter in FSE or TSE and not in conventional spin echos?

A

With fast or turbo spin echos the multi 180 degree pulses mess with the J coupling of the FAT molecules. J coupling is interactions between fat molecules that shortens their T2 time. By messing with the J-coupling the T2 is increased and therefore fat seems brighter on TSE.

28
Q

What is the difference between an Echo plannar image (HASTE/Single shot) vs a FSE?

A

Echo plannar tries to get as many phase encoding steps into one 180 refocusing pulse

FSE has a bunch of 180 pulses per 90 degree pulse.

The are both Spin echo sequences.

29
Q

What is the difference between a spoiled GRE sequence and a steady state GRE?

A
  1. Spoiled
    1. Get rid of the residual transverse magnetization (spoiling it)
    2. There for this incoherent or spoiled one is T1W but depends on the flip angle
  2. SS GRE or Refocused or Coherent
    1. Keeps the transverse mag
    2. No 180 degree pulse so it is basically a T2*
30
Q

Why are GRE sequences used? What are their disadvantages?

A

Time!!! they are really fast

Diadvantage is low SNR and more susceptibility artifact.

31
Q

DWI is what type of sequence?

A

Echo plannar or GRE

32
Q

What is the a DWI sequence of B0 called?

A

A poor man’s T2

33
Q

What are the three types of angiography (white blood) and how do they differ?

A
  1. TOF
    1. Inflow enhancement effect
      1. In short TR the longitudinal recovery has not happened before the next RF pulse. This causes a steady state to occur and therefore less signal from the tissues in the steady state.
      2. However, blood flowing to the slab has not recieved those multiple RF pulses and therefore is not in a steady state and therefore has more signal (enhancement)
    2. Outflow washout
      1. High veolcity signal loss occurs when flow is so fast that they miss both the 90 and 180 pulses thus have no signal
  2. Phase contrast
    1. Spin phase effect
      1. Flowing protons can gain or lose phase compared to stationary tissue. This change can be translated into an angiographic image
      2. Amount of phase shift is directly related to flow and therefore qualitative measurements can be made for flow velocity
  3. Contrast enhanced
    1. Gd
34
Q

What does black blood vs bright blood MRA mean?

A

Black blood suppresses blood signal

Bright blood enhances it.

35
Q

What is the fastest MRA sequence?

A

Contrast enhanced > TOF > PE

36
Q

What are the different fat suppression techniques?

A
  1. Inversion sequences (STIR)
    1. Using TI
  2. In Phase and out of phase
    1. Wate and fat go in and out of phase from eachother causing amplification and nulling of fat.
  3. Fat saturation
    1. Send a narrow bandwidth RF pulse at the frequency of fat. Once the regular pulse sequence occurs the fat is saturated (steady state) and has limited signal
    2. Requires a magnet higher than 0.3T becasue the higher the magnet the farther away fat and water precession freq are and therefore we can focus on one or the other without fucking up the other.
37
Q

What is Gadoxetate used for?

A

Liver imaging - 50% is uptake by the hepatocytes

38
Q

What is the advantage of a Low field magnet?

A
  1. Open design
  2. Lower money (everything)
  3. Lower fringe field - easier to sheild and don’t need to buy different anesthesia shit
  4. Reduction of MR artifacts
    1. Chemical shift
    2. Susceptibility (metal)
    3. Flow/motion
  5. Low energy deposition (SAR)
39
Q

Receiver bandwidth is proportional to the what?

A

Pixels in the frequency encoding direction or number of frequency encoding steps

40
Q

Reciever bandwidth is inversely porportional to what?

A

Time— narrow bandwidth = extended sampling time

SNR — narrow bandwidth = more SNR - Since the noise is evenly distributed across frequencies, limiting BW reduces the amount of noise mixed with the signal.

41
Q

What type of weighted sequences can be made with a spoiled GRE?

A

T1W, PD and T2*

Not T2.

42
Q

What type of weighted sequences can be made with a SS GRE?

A

Can be T2*/T2

43
Q

How does fat saturation (selective pulse) sequence work?

A

First you send a preparatory pulse only to FAT

Second you send a spoiler gradient (RF) that dephases the fat leaving them unable to send signal.

44
Q

What are the two types of chemical shift?

A
  1. Type 1: Bright rim on one side and dark on another…due to phase shift
    1. Increases with field strength
    2. Decreases with increased gradient strength
    3. Decreases with wide reciever bandwidth
    4. HAPPENS IN THE FREQUENCY ENCODING DIRECTION (like the only one)
  2. Type 2: India Ink/Black boundary
    1. GRE sequences (not SE)
    2. Same shit just different
45
Q

Where is gd eliminated?

A

Kidneys (think pseudolayering in the bladder).

46
Q

How can you fix aliasing and what direction does it occur in?

A
  1. Occurs in the PE direction- you always over sample in the freq direction cause it costs no time and therefore no aliasing happens
  2. Make FOV bigger
  3. Flip PE and FE
  4. Surface coils - wont detect outside signal
  5. Sat bands
  6. Increase the PE steps.
47
Q

What is the trunication artifact (ringing, gibbs, spectral leakage), what direction and how to fix it?

A
  1. Happens at high contrast interfaces due to data ripples during transformation - thus losing data (aka truncation)
  2. PE but less commonly FE direction
  3. Fix
    1. More matrix
    2. Decreasing pixel size
    3. Increasing PE steps
    4. Decreasing bandwidth
    5. Fixes are like that of poor spatial res.
48
Q

What is the motion artifact (pulsation), what direction and how to fix it?

A
  1. PE direction
  2. Fix
    1. Blade or propeller sampling (oversample the center of k-space)
    2. Switch PE/FE
49
Q

What color is blood on a GRE vs SE?

A

GRE is bright

SE is dark (misses the two pulses of the slice)

50
Q

What sequence cannot have cross-talk artifact?

A

3D images because the entire volume undergoes selection.

51
Q

When do eddy currents most commonly occur (what sequence)?

A

Diffusion waited

52
Q

What artifact is this and how is it fixed?

A

Crisscross or herringbone - reimage

Physics (10 decks)

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