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Emily Imaging > MRI I and II > Flashcards

MRI I and II Flashcards

1
Q

What are some negatives of MRI

A 
Long acquisition time ~10s of minutes
high equipment and personnel costs
patient claustrophobia
image artifacts
scan complexity
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2
Q

When did we start seeing MRIs used in clinic?

A

1970’s

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3
Q

What is the strength of the main magnets used?

A

usually 1.5T, sometimes 0.5T or 3T

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4
Q

What temperatues do the magnets need to be at and how is this accomplished?

A

4K and liquid Helium

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5
Q

What kinds of coils are in an MRI machine?

A

There are the main coils which create a uniform magnetic field.

There are also gradient coils, one positive and one negative, which add together to form a nice linear gradient along the length of the scanner.

There are also active and passive shim coils which even everything out.

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6
Q

How large of a change does the gradient coils introduce?

A

around 5mT

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7
Q

What is magnetic susceptibility?

A

The extent to which a material becomes magnetized in a magnetic field.

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8
Q

What do Diamagnetic, Paramagnetic, and Ferromagnetic materials do in a magnetic field?

A

Diamagnetic: has paired electron spins and they align to oppose B

Paramagnetic and Ferromagnetic: unpaired electron spins (net spin can be non-zero) and align to B

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9
Q

What is often used as a contrast in MRI? is is Para or Diamagnetic?

A

Gadillidium, Paramagnetic

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10
Q

How many nuclei does it take to produce a measurable signal?

A

10^15

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11
Q

What makes a nuclei medically useful?

A
  • spin quantum number is non-interger
  • abundant in body (or can be injected as contrast)
  • should have a high magnetic moment
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12
Q

What is a typical MRI voxel size?

A

1mm x 1mm x 1mm

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13
Q

Are there more spins in the parallel or antiparallel state? How many more? Is this state higher or lower energy?

A

3p/million more are in the parallel state because this is lower energy

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14
Q

How many protons are in a typical voxel?

A

10^21

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15
Q

What is the gryomagnetic ratio of hydrogen?

A

42.58 MHz/T

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16
Q

What is the Larmor Equation?

A

Larmor Frequency = (gyromag ratio)(B0)

17
Q

How do we flip more spins into the anti-parallel state?

A

apply energy (RF pusle) at the precession (larmor) frequency so that the energy is absorbed. resonance. Like pushing somebody on a swing.

18
Q

What are the components of the net magnetization?

A 
Mz = Longitudinal magnitization
Mxy = transverse magnetization
19
Q

How do we actually get an MRI signal?

A

The information we get comes from the rate at which energy is released (after exciting and aligning protons)

20
Q

Why do we choose the work in the proton’s reference frame?

A

“Spinning top” because stationary

21
Q

Explain Two common flip angles

A

90: pulse makes all Mz become Mxy (Nup = Ndown)
180: larger pulse makes all Mz becomse -Mz

22
Q

What is Free Induction Decay? What is the equation for it?

A

It causes the spins to dephase - Spin-Spin Relaxation, rate at which Mxy dephases

Mxy = M0*exp(-t/T2)

23
Q

What kind of things affect T2?

A

molecule structure, characteristics of bound water

24
Q

Does a large molecule size mean a longer or shorter T2??

A

Shorter - they dephase faster ?????NOT POSITIVE ON THIS ONE

25
Q

Which affects the dephasing time of Mxy? T1 or T2?

26
Q

Which affects the dephasing time of Mz? T1 or T2?

27
Q

Which generates a MR signal directly, Mxy or Mz?

A

Mxy does. We need to be sneaky to measure Mz (and T1)

28
Q

How do we measure T1?

A

90 degree pulse, delay time, another 90. You do this for different delay times and get T1 from plotting those vs amount of signal (comes from amount of Mz that was able to recover in that delay time)

29
Q

What governs T1?

A
  • rate of energy exchange back to the lattice (ie surrounding material affects T1)
  • hydration layer and physical characteristics of tissue
  • Matching of larmor frequency
30
Q

By changing B0, can we change T1 or T2?

A

T1! T1 depends on larmor frequency which depends on B0! You can get MORE signal from a specific material by choosing B0 appropriately

31
Q

What do contrast agents do?

A

Create a hydration layer and change T2

32
Q

What’s longer? T1, T2, T2*?

A

T1 > T2 > T2*

33
Q

How much longer/shorter is T1 than T2?

A

T1 = 5-10 x T2

34
Q

would a viscous material (like CSF) have a large of small T1?

A

Loooong T1 - takes a long time to transfer energy back to the lattice

35
Q

What is time of repetition? How long is it?

A

time between Rf excitation pulses. ms to 10000ms

36
Q

What is time of Echo?

A

time from excitation pulse to echo

Emily Imaging (6 decks)

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