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VetRad Boards prep > MRI physics > Flashcards

MRI physics Flashcards

1
Q

What are advantages and disadvantages of Superconductive magnets?

A

Advantages:

  • High field strength and homogeneity
  • Low power consumption
  • High SNR
  • Fast scanning

Disadvantages:

  • High capital cost and cryogen cost
  • Acoustic noise
  • Motion artifacts
  • Techical complexity
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2
Q

What is diamagnetic susceptibility and what are examples?

A
  • Slightly negative susceptibility and opposes the applied magnetic feild
  • Materials:
    • calcium
    • water
    • most organic materials (C,O)
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3
Q

What is paramagnetic suscptibility and what are examples?

A
  • Slightly positive susceptibility and enhances local magnetic field, but no measurable self-magnetism
  • Examples:
    • Molecular O2
    • some blood degredation products
    • Gadolinium
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4
Q

What is ferromagnetic susceptibility and what are examples?

A
  • Supramagnetic augmenting the external magnetc field and can exhibit self-magnetism
  • Examples:
    • iron
    • cobalt
    • nickel
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5
Q

What is the gyromagnetic ratio for H?

A

H = 42.58 MHz/T

(1.5T = 63.87 MHz)

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6
Q

What is the Larmor equation?

A
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7
Q

At equilibrium, what do Mz and Mxy equal?

A

Mz = M0

Mxy = 0

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8
Q

What is the difference between T2* and T2 decay/relaxation?

A
  • T2* depends on both intrinsic (spin-spin) and extrinsic (field inhomogeneities) relaxation factors
  • T2 depends only on intrinsic spin-spin interactions
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9
Q

What is the T2 decay constant?

A

Time after the 90-degree RF pulse (time 0) over which the signal decays to 37% of the maximal transverse magnetization

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10
Q

What causes T2 shortening?

A
  • Slow molecular tumbling rates
  • Rigidly bound molecules (tumbling slower than the Larmor frequency)
  • Large macromolecules
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11
Q

Arrange from long to short the T2 relaxation times of:

Grey matter, CSF, white matter

A

CSF (long) > grey matter > white matter (hypo)

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12
Q

How does magnetic field strength affect T2 relaxation?

A

Magnetic field strength has no effect on T2 relaxation (unlike T1)

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13
Q

What causes T1 shortening (hyperintensity)?

A
  • intermediat molecular tumbling (close to Larmor), size and protein binding
    • fat stores in adipose and marrow tightly bound and close to Larmor
    • protein binding in fluids (mucin) close to Larmor)
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14
Q

Arrange tthe following from short to long T1 relaxation times:

CSF, fat, grey matter, white matter

A

fat (hyper) < white matter < grey matter < CSF (hypointense) Mmm

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15
Q

How does B0 impact T1 relaxation?

A

Larger B0 can cause increased T1 relaxation times - due to less overlap of lattice with precessional frequencies

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16
Q

What is the T1 relaxation constant?

A

Time needed to recover 63% of the longitudinal magnetization (Mz) (after the 90 degree RF pulse)

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17
Q

What are the typical TR and TE for T1?

A

TR = 400-600, TE = 2-20

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18
Q

What molecules can cause T1 shortening?

A

Gd-DTPA and methemoglobin

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19
Q

T2 shortening?

A

Gd-DTPA, deoxyhemoglobin and intracellular methemoglobin

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20
Q

What is the difference in resonance frequencies of the protons in water and fat?

A

224 Hz (3.5 ppm)

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21
Q

WHat are the TR and TE in PD weighting?

A

TR = 2000-4000, TE = < 40

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22
Q

What is the typical TI for STIR?

A

TI = 140-180 ms, TR = 2500

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23
Q

WHat is the typical TI and TR of FLAIR?

A

TI = 2400, TR = 7000

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24
Q

How can you make a GRE sequence more T1W? more T2W or PD?

A
  • T1W GRE: Short TE, large FA (70-90)
  • PD GRE: Short TE, medium FA (40-50)
  • T2W GRE: Longer TE, small FA (5-30)
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25
Q

What determines slice thickness?

A

the RF bandwidth (range of frequencies used, Hz) and the SEG field strength (slope, Hz/cm)

26
Q

What determines the thickness of a voxel?

A

slice encoding gradient strength and RF frequency bandwidth

27
Q

What information is provided by the center and periphery of k-space?

A
  • Center: signal and contrast - contains low frequencies
  • Periphery: spatial resolution - high frequencies
28
Q

How do you calculate FOV?

A

FOV = sample bandwidth ÷ gradient strength

29
Q

What is the trade off of having a smaller/narrower bandwidth?

A

Increased SNR at the expense of increased TE with more T2 decay and increased chemical shift artifact

30
Q

What is the trade off with increasing NEX?

A
  • Increased SNR
  • reduces artifacts due to signal averaging
  • increases scan time (in order to double SNR, NEX = 4, quadruples scan time)
31
Q

In regard to the ACQUISITION matrrix, what occurs when you increase it?

A

decreases voxel size, thus:

  • reduces SNR
  • increases spatial resolution
  • increases scan time in MPEG is increased (more k-space to fill)
32
Q

What three parameters define the spatial resolution of an MR image?

A
  • Dimension of the FOV
  • Slice thickness
  • Size on the image matrix
33
Q

What is one way to increase resolution?

A
  • Increase the matrix size, this will decrease the pixel size
34
Q

What is the relation ship between k-space line spacing and FOV?

A
  • ∆k inversely related to FOV
    • E.g. wider ∆k, smaller FOV
35
Q

In basic terms, how does phase-wrap artifact (or aliasing) occur?

A
  • An object containing protons outside the prescribed FOV but within the bore will e subject to the same PE gradient
  • This Fourier transformation erroneously calculates the signal as arising from within the prescribed FOV, wrapped around to the opposite side of the image
36
Q

What is the relationship between FOVFE and the GFE and receiver bandwidth

A
  • If GFE increases, then FOVFE decreases
  • Narrowing rBw decreases FOVFE
37
Q

In basic terms, how does frequency wrap -around, or aliasing occur?

A
  • Sampling frequency determines the maximum frequency (fFEmax) in the echo that can be sampled accurately (similar to Doppler US and PRF)
  • Max freq we wish to record is determined by total rBW, so equal to rBW/2
  • If higher frequencies in the signal from outside the rBW/FOVFE range, will be sample insufficiently and wrap around.
38
Q

How does TR relate to T1 weighting?

A
  • TR determines how much longitudinal magnetization difference there is between tissues and emphasizes these differences when TR is short
  • Short TR = good T1 contrast
39
Q

What is the relationship of TE and T2?

A
  • TE determines how much transverse magnetization was allowed to decay prior to the signal read out
  • Shorter the TE, the less difference in residual transverse magnetization between tissues
  • Long TE maximizes differences in tissues
  • Long TE = good T2 contrast
40
Q

What combination on TR and TE are used for proton density weighted images and why?

A
  • Long TR and short TE
  • Minimizes both T1 and T2
41
Q

What is the relationship between SNR and spatial resolution?

A

inversely related

42
Q

What controllable factors influence SNR?

A
  • VOXEL size
    • Larger voxels generate more signal
  • Receiver bandwidth
    • SNR inversely related to the square root of rBW
  • Number of excitations (NEX)
    • When NEX doubled, SNR increased by a factor of root 2
  • Number of PE steps (# of pixels in the PE direction [NPE])
    • SNR is a function of the square root of NPE
      • increase in NPE causes some increase in SNR
    • However, if NPE is doubled there is a net decrease in SNR (0.7 factor)
      • SNR divided by two since pixels two times smaller in PE direction
  • Longer TR = more signal
  • Longer TE = less signal
  • Flip angle <90, lower SNR (lower amplitude of transverse magnetization)
  • Type of coil
  • Decreasing acquisition time decreases SNR
43
Q

In regard to T1 weighted imaging, how does gadolinium affect the nearby protons?

A
  • The strong paramagnetic effect of gadolinium shortens the T1 relaxation time of these protons, thereby causing a dramatic increase in signal on T1W images
44
Q

What is the major benefit of FSE/TSE and roughly how is this accomplished?

A
  • Faster scan times
  • More 180 RF pulses within a TR, fills k-space faster
45
Q

Why does fat tend to be brighter on FSE/TSE in comparison to standard SE?

A
  • The multiple 180 RF pulses used reduce the spin-spin interactions in fat (j-coupling), thereby strengthening its T2
46
Q

How is the SS-FSE accomplished in regard to k-space?

A
  • Only a little over half of the lines of k-space are filled within 1 TR after a single 90 RF
47
Q

Why is STIR more efficient than fat sat techniques for nulling fat?

A
  • STIR is not sensitive to magnetic field inhomogeneities
48
Q

Why can’t STIR sequences be used with gadolinium?

A
  • The T1 of enhancing tissue is shortened and closer to that of fat, leading to suppression of signal from enhancing tissues on STIR images
49
Q

Motion artifacts occur in what direction?

A

phase encoding

50
Q

Why does entry slice phenomenon occur?

A

blood in the center of the stack has experienced more excitations than blood at the entry slice

51
Q

What causes the flow artifact?

A

Fast flowing blood will not have experienced the 180 degree rephasing pulse, thus will not have signal in spine echo sequences

52
Q

How can you correct for TOF and entry slice phenomenon?

A

saturation band adjacent to the FOV or in the slice direction

53
Q

How can you correct flow related intravoxel dephasing?

A

flow compensation, aka gradient moment rephasing

54
Q

In what direction does the zipper artifact occur?

A

Frequency encoding

55
Q

What causes the spike/herring bone artifact?

A
  • loose electrical connection or build up os static electricity
  • occurs due to production of spike noise, resulting in a bad data point in the raw data (k-space)
56
Q

How can cross-excitation be prevented?

A

using an interslice gap of at least one-third of the slice thickness

57
Q

In what direction does wrap around artifact occur?

A

phase encoding, hence the term “phase wrap”

58
Q

How can you correct truncation/Gibb’s artifact?

A

due to undersampling →increase number of phase encoding steps (increase matrix and voxel size)

59
Q

In what direction does chemical shift artifact occur?

A

frequency encoding direction

60
Q

How can you minimize chemical shoft artifacts?

A

increase reciever bandwidth or use fat suppression techniques

61
Q

On what type of sequence do phase cancellation/black boundry artifacts occur?

A

gradient echo sequences - when fat and water are exactly 180 degress out of phase

62
Q
A

VetRad Boards prep (11 decks)

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