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psc210 > intro > Flashcards

intro Flashcards

1
Q

why is the larmour equation important for MRI?

A

the lamer equation describes how Hydrogen atoms are excited in a magnetic field.Resonance frequency = gyromagnetic ratio(42.58 MHz/T) X the strength of the external magnetic field

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2
Q

What is Faraday’s law of induction?

A

A bar magnet moving in a loop of wire will induce A current in the wire.Changing the intensity of the magnetic field or moving a nearly magnet in any way will induce a current.

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3
Q

What are the 3 major parts of an MRI scanner?

A

RF coil, gradient coil & big magnet.

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4
Q

Approximately how much greater than the Earth’s magnetic field is a 1T magnet?

A

About 20,000 times

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5
Q

What is the advantage of a surface RF coil?

A

higher SNR (signal to noise ratio)?

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6
Q

What are the disadvantages of a surface RF coil?

A

smaller coverage Bless uniform distribution

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7
Q

What are the two advantages of a whole head birdcage coil?

A

More coverage & more uniform distribution

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8
Q

What are the advantages of phased array RF coils?

A

They are the best of both worlds- like a set of surface coils.They are basically a set of surface coils (essentially a set of small coils, each with higher SNR)

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9
Q

What types of imaging require phased array coils?

A

GRAPPA, SENSE, or multiband imaging

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10
Q

What is a major disadvantage of using phased-array coils for imaging?

A

The signal is inhomogeneous with high signal in superficial areas & drop out in deep/medial areas

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11
Q

At a basic level, how does MRI work?

A
  1. Stick someone in a large magnetic field. This causes hydrogen atoms in the body to align with the field. 2. Apply a radiofrequency pulse (another EMF) that knocks atoms out of alignment. 3. Precession of atoms back into alignment elicits tiny currents in the RF head coil. 4. Repeat steps 2-3 while fiddling with the magnetic gradients.
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12
Q

Approximately what percentage of protons are actually aligned in a 1 Tesla magnet?

A

About 0.0003% per Tesla of B

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13
Q

How is the static magnetic field (B) produced?

A

Superconducting magnet ( a little current goes a long way)

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14
Q

What does a gradient coil do?

A

Induces a field that varies in 1 of the 3 dimensions (x,y,z)

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15
Q

What are the critical parameters of a gradient coil?

A

Gradient strength and slew rate

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16
Q

What is a shim coil for?

A

A shim coil is used to adjust the homogeneity of B (main magnetic field)

17
Q

Name two potential risks of working with radiofrequency energy

A

Tissue heating(Power of RF pulse determined by weight and condition of patient) and burns(looped wire or cabling can act as an RF antenna)

18
Q

How is specific absorption rate calculated?

A

W/kg

19
Q

What risks do gradient changes have ?

A

Sometimes they can stimulate peripheral nerves (annoying to painful) The risk is greatly increased in conductive loops (crossed arms/legs)

20
Q

Can people have makeup and/or tattoos in the scanner?

A

Nope. Can cause swelling/tissue damage due to movements of oxides in the pigments.

21
Q

Name some risks associated with an MRI scanner

A

Acoustic noise (93-98dB EPI for 1.5T)
Burns
Tissue heating
motion/heating of implants/shrapnel/tattoos
peripheral nerve stimulation from gradient changes
claustrophobia(most common)

22
Q

Can a person with dental fillings enter the scanner?

A

Yes

23
Q

Describe what the contrast between brain tissue and CSF looks like with a really short TR during T1 imaging.

A

Low contrast since both signals are near baseline. Both are dark

24
Q

What TR time is optimal for T1 imaging (relative to the relaxsation curves of both brain tissue and CSF?

A

The TR time should be at the point where the slope of the signal on a TR vs. signal curve for brain tissue has begun to flatten out (about halfway between the origin and the end).

25
Q

Describe what the contrast between brain tissue and CSF would look like with a really long TR during T1 imaging.

A

low contrast. Both brain tissue and CSF would appear bright.

26
Q

Describe what the contrast would look like between brain tissue and CSF with a really short TE during T2 imaging

A

low contrast. Both brain tissue and CSF would appear bright since a short TE captures the beginning of the T2 relaxation.

27
Q

Describe why you would put the TE time in the middle of the curve for T2 relaxation. What tissues would be brightest?

A

You would put the TE time in the middle of the T2 relaxation curve because it is there that there is the greatest difference between relaxation times between tissues (brain tissue and CSF for example). For T2 imaging, since brain tissue has a faster relaxation time, signal from it will tend to be darker than the CSF.

28
Q

How can researchers activate protons at selective spatial locations (slice location)?

A

By manipulating B0 at different points in the z-axis (applying a z gradient) and sending RF pulses at different resonant frequencies.

29
Q

What is a fourier transform?

A

A fourier transform allows you to determine the magnitude of each frequency component that goes into a waveform.

30
Q

Where can I learn more about fourier transform?

A

Thefouriertransform.com

31
Q

How does a fourier transform transform images?

A

Images are decomposed into frequency components (sines and cosines). See page 74 of lecture 1

32
Q

How do I find out more about k-space?

A

radinfonet.com/cme/mistretta/traveler1.htm

33
Q

What is susceptibility?

A

Generation of extra magnetic fields in materials that are immersed in an external field. For example, adding a person or other non-uniform object to B0 will make the total magnetic field non-uniform.

34
Q

What size/type of inhomogeneities does ‘shimming’ usually compensate for?

A

Large inhomogeneities (10+ cm)

35
Q

Where are susceptibility artifacts likely to occur?

A

In the junctions between air and tissue (sinuses and ear canals for example). Spins become dephased so quickly that no signal can be measured. Susceptibility variations can also be seen around blood vessels where deoxyhemoglobin affects T2* in nearby tissue.

36
Q

What is T2* star imaging?

A

T2* relaxation is a result of the dephasing of the transverse magnetization due to both microscopic molecular interactions(T2) and spatial variations of the external main field (deltaB) (tissue/air and tissue/bone interfaces). T2* relaxes faster for higher B0

psc210 (6 decks)

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