mri phsics

Question 1

what does mri look at?

Answer 1

function/ flow, differentiate bone and soft tissues

Question 2

what type of radiation is used?

Answer 2

electromagnentic radiation (non-ionizing)

Question 3

adv of mri?

Answer 3

excellent tissue contrast and resolution for visualization of pathology and anatomy

Question 4

magnetism?

Answer 4

is the fundamental property of all matter

Question 5

3 types of magnetic properties?

Answer 5

1-diamagnetic
2- paramagnetic
3- ferromagnetic

Question 6

diamagnetic?

Answer 6

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

Question 7

paramagnentic?

Answer 7

field lines intensify near center (gadolinium)

Question 8

ferromagnetic?

Answer 8

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

Question 9

magnets are…

Answer 9

dipoles

Question 10

what is a dipole?

Answer 10

a north and south end (two diff charges)

Question 11

what is a field?

Answer 11

quantity that varies over a spatial region

Question 12

what do magnetic fields exert in any given direction?

Answer 12

torque

Question 13

direction of alignment is…

Answer 13

direction of magnetic field known as B sub 0

Question 14

Tesla (T) is…

Answer 14

the unit of measurement for field strength

Question 15

10,000 Gauss=?

Answer 15

1 T

Question 16

what is Earth’s magnetic field?

Answer 16

0.00005 T

Question 17

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

Answer 17

yes, 20,000 x’s stronger

Question 18

3 types of magnets used in MRI’s?

Answer 18

1- permanent
3- resistive
2-superconductive

Question 19

permanent magnets?
lay down but sandwhich not tube

Answer 19

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

Question 20

resistive magnets?
stand up in these

Answer 20

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)

Question 21

adv of resistive magnet?

Answer 21

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

Question 22

superconducting magnet? mri tubeeeee

Answer 22

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

Question 23

how much does a quench cost and what is it?

Answer 23

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

Question 24

why is a quench dangerous?

Answer 24

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

Question 25

hydrogen is a mini magnet?

Answer 25

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

Question 26

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

Answer 26

either parrallel or anti parallel

Question 27

are hydrogen protons usually parallel or anti parallel?

Answer 27

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

Question 28

molecules w low hydrogen presence= ?

Answer 28

weak net magnetization

Question 29

proton density is...

Answer 29

amount of hydrogen protons that a given tissue possesses

Question 30

how are RF's used?

Answer 30

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.

Question 31

how does RF influence MR signal?

Answer 31

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.

Question 32

MR signal?

Answer 32

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

Question 33

RF coils do what?

Answer 33

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

Question 34

gradient coils are used to...

Answer 34

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

Question 35

3 major slice selection gradient coils?

Answer 35

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.

Question 36

how can we choose slice?

Answer 36

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

Question 37

transverse relaxation

Answer 37

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

Question 38

longitudinal relaxation

Answer 38

natrual release if energy by excited protons to their general environment

Question 39

each tissue relaxes at different rate based on...

Answer 39

amount hydrogen

Question 40

T1 is

Answer 40

longitudinal relaxation

Question 41

T2 is

Answer 41

transverse relaxation

Question 42

what affects tissue contrast?

Answer 42

echo time (ET)

Question 43

echo time?

Answer 43

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)

Question 44

repetition time?

Answer 44

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

Question 45

contrast control factors?

Answer 45

repetition time= longitudinal relaxation time echo time= transverse relaxation

Question 46

in T1 weighted images, brighter area means....

Answer 46

stronger the signal

Question 47

in T2 weighted images, darker area means...

Answer 47

weaker signal

Question 48

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

Answer 48

proton density weighted images (darker areas= lower signal)