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MRI PhYSicS > mri phsics > Flashcards

mri phsics Flashcards

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1
Q

what does mri look at?

A

function/ flow, differentiate bone and soft tissues

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2
Q

what type of radiation is used?

A

electromagnentic radiation (non-ionizing)

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3
Q

adv of mri?

A

excellent tissue contrast and resolution for visualization of pathology and anatomy

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4
Q

magnetism?

A

is the fundamental property of all matter

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5
Q

3 types of magnetic properties?

A

1-diamagnetic
2- paramagnetic
3- ferromagnetic

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6
Q

diamagnetic?

A

field lines go outwards/ non-magnetic (paper, plastic, gold)

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7
Q

paramagnentic?

A

field lines intensify near center (gadolinium)

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8
Q

ferromagnetic?

A

field lines greatly intensify/ highly magnetic (iron) so retains magentization

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9
Q

magnets are…

A

dipoles

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10
Q

what is a dipole?

A

a north and south end (two diff charges)

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11
Q

what is a field?

A

quantity that varies over a spatial region

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12
Q

what do magnetic fields exert in any given direction?

A

torque

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13
Q

direction of alignment is…

A

direction of magnetic field known as B sub 0

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14
Q

Tesla (T) is…

A

the unit of measurement for field strength

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15
Q

10,000 Gauss=?

A

1 T

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16
Q

what is Earth’s magnetic field?

A

0.00005 T

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17
Q

does a 1.0T MRI have a stronger magnetic field than Earth? if so how much?

A

yes, 20,000 x’s stronger

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18
Q

3 types of magnets used in MRI’s?

A

1- permanent
3- resistive
2-superconductive

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19
Q

permanent magnets?
lay down but sandwhich not tube

A

permanently magnetized iron
-2 bars; 1 above & 1 below
-1 bar twisted in C shape where poles are close
-vertical field

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20
Q

resistive magnets?
stand up in these

A

solenoid (magnet constructed from coil of wire)
-more turns the more I = stronger field
-Faraday’s law= current flowing in wire creates a magnet
-horizantal field (lay magnent on side)

21
Q

adv of resistive magnet?

A

-can turn off current which turns off magnet
-no liquid cyrogen
-relatively small fringe field

22
Q

superconducting magnet? mri tubeeeee

A

simlar to resistive + liquid cyrogen
-liquid helium to keep coils at 4 degrees K
-horizantal field
-tight coils
-no resistance
-allows current to flow with wires w/o being connected to external power source

23
Q

how much does a quench cost and what is it?

A

having to rapidly boil to turn liquid to gas which adds resistance and slows down electrons til it stops and is off. 500k

24
Q

why is a quench dangerous?

A

while rapidly boiling off liquid helium it turns to gas form, fans turn on and doors automatically open bc more oxygen is needed and windows will implode otherwise the patient will suffocate

25
Q

hydrogen is a mini magnet?

A

single rotating proton has magnetic moment acting like a dipole. this magnentic moment is the torque observed when in the presence of an external magnentic field

26
Q

what happens when hydrogen is placed in large external magnetic field?

A

either parrallel or anti parallel

27
Q

are hydrogen protons usually parallel or anti parallel?

A

usually parallel bc low energy so they go with the net magnetic force (Bsub0)

28
Q

molecules w low hydrogen presence= ?

A

weak net magnetization

29
Q

proton density is…

A

amount of hydrogen protons that a given tissue possesses

30
Q

how are RF’s used?

A

short burst of RF’s introduced to protons aligned in magnet. if right freq transfers energy to the atom, energy transfer will create a change in the net magnetization vector. as they relax they release energy.

31
Q

how does RF influence MR signal?

A

change in mag field induces a current. reciever coil is set at angle B0 and as magnetization moves through reciever coil, current or signal is induced.

32
Q

MR signal?

A

process in which nuclei when placed in an external mag field absorb and release energy.

33
Q

RF coils do what?

A

used for transmitting energy and recieving signals
-act like antenna (closer to source=better_

34
Q

gradient coils are used to…

A

vary field strength throughout magent because they turn on and off rapidly which ALLOWS localization of slices =noisy

35
Q

3 major slice selection gradient coils?

A

3 major coils (XYZ)
-3 diff planes
x coil=coronal
y coil= sagittal
z coil= axial
each coil creates is own magnet which can incr strength of mag field.

36
Q

how can we choose slice?

A

B applied to desired hyrodgen atoms and surrounding will be too slow or fast (b is perpendicular to protons)

37
Q

transverse relaxation

A

release of energy by excited nuclei as a result of interaction amongst themselves
-happens much faster than longitudinal

38
Q

longitudinal relaxation

A

natrual release if energy by excited protons to their general environment

39
Q

each tissue relaxes at different rate based on…

A

amount hydrogen

40
Q

T1 is

A

longitudinal relaxation

41
Q

T2 is

A

transverse relaxation

42
Q

what affects tissue contrast?

A

echo time (ET)

43
Q

echo time?

A

reps time in milli seconds between the application of the 90 degree RF pulse and the peak of the echo signal (signal picked up in coil)

44
Q

repetition time?

A

reps the amount of time that exists between successive RF pulses applied to the same location

45
Q

contrast control factors?

A

repetition time= longitudinal relaxation time
echo time= transverse relaxation

46
Q

in T1 weighted images, brighter area means….

A

stronger the signal

47
Q

in T2 weighted images, darker area means…

A

weaker signal

48
Q

when parameters of T1 and T2 are in midddle this is…

A

proton density weighted images (darker areas= lower signal)

49
Q
A

MRI PhYSicS (1 decks)

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