Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

RA6007: Radiation Physics > Magnetic Resonance Imaging (MRI) All Notes > Flashcards

Magnetic Resonance Imaging (MRI) All Notes Flashcards

You may prefer our related Brainscape-certified flashcards:
Physics 101 flashcards AP® Physics flashcards
1
Q

Name the 4 components to an MRI scanner

A

the magnet, the radiofrequency system, the gradient system, the host computer

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What has the most impact on quality of the magnet?

A

magnetic field homogeneity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

name the three types of magnets

A

superconducting magnets, permanent magnets and resistive magnets.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Name the two properties of superconducting magnets that make them the most popular choice.

A

high field strength (better SNR) and good homogeneity.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

how is the magnet made superconducting?

A

by cooling it to -274 degrees Celsius

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what is meant by superconducting?

A

when an electric current is applied it will keep flowing with virtually no loss of energy

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

what is the superconducting magnet made of?

A

Solenoid (coil of wire)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

what is used to cool the magnet?

A

helium

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

what is the helium surrounded by to maintain the low temp

A

liquid nitrogen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What are some advantages for permanent MRI magnets?

A

can be used to make open magnets, magnetic field can be positioned horizontally or vertically, do not use cryogens or electricity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

what are the disadvantages of permanent magnets?

A

magnetic field is unstable if ferrite not kept at the same temperature, cannot be quenched, very sensitive to external disruption by cars, elevators, etc.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

what are resistive MRI magnets?

A

electrical coils wrapped around an iron core

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

what are the advantages of resistive MRI magnets

A

they can be designed as an open magnet and can be switched off in the evening

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What is the radiofrequency system?

A

the RF system is needed to transmit RF into the patient at an appropriate Larmour frequency and to detect and gather the very tiny RF echo that the patient emits afterwards/

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

what does the host computer do

A

the computer will calculate the RF pulses for the sequence and these pulses are amplified along the RF transmitting chain

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Describe (very basic) RF transmission

A

Pt in strong magnetic field, pump radiowaves into them, turn off radiowavse, pt sends radiowaves back out, frequency we send in is the same as what we get back

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is the relationship between field strength and RF requirement

A

as field strength increases, there is an increased need for RF power.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

what is the gradient system

A

the gradients are used to alter the main magnetic field of the scanner in order to get spatial information about the patient being scanned

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

what causes the noise in an MRI scanner

A

the switching of the gradient coils

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

how is the digitised data from the patient echoes processes?

A

using the fast fourier transforms (FFFT)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What happens when an external magnetic field B0 is applied?

A

the proton spins that are normally randomly orientated with no net magnetisation vector now line up with or against the magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

what has has higher energy; the spins with or against the magnetic field?

A

The spins antiparallel to the magnetic field have higher energy

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What happens when a proton drops to a lower energy level?

A

it must give off energy in the form of an RF signal at the resonant frequency

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What does the strength of the MR signal depend on?

A

Low temperature (cannot lower temp in vivo)
large magnetic dipole moment
Large N (1H is abundant)
B0 (this is the only thing we can control!!)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

What is T1 =

A

Spin-lattice relaxation time

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

what is T2 =

A

Spin-spin relaxation time

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

what is T2* =

A

Actual transverse relaxation time, taking into account magnetic field inhomogeneities

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

What is the free induction decay (FID) signal?

A

the signal produced after a 90 degree pulse

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

what time for M0 to return to equillibrium?

A

4-5 x T1

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

T1 is similar to T2 for ____. T1&raquo_space; T2 for _____.

A

T1 is similar to T2 for liquids. T1&raquo_space; T2 for solids.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

what is the magic angle?

A

NB in MSK shoulder and knee imaging as when a tendon makes this angle with the field, we get decreased signal and this mimics a tear in the tendon.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

what is T1 relationship with B0

A

T1 is dependent on B0

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

what is T2 relationship with B0

A

T2 is independent of B0

34
Q

what is the main contrast agent used in MRI?

A

gadolinium

35
Q

What are the three sequences that most sequences used in modern scanners are now based on?

A
  1. Spin-Echo sequence
  2. Inversion Recovery sequence
  3. Gradient Echo sequence
36
Q

What is the tip angle?

A

the term used to describe the angle that the RF pulse tilts the NMV. EG: A 90 degree RF pulse would tilt the NMV through 90 degrees from the Z-axis to the XY plane

37
Q

What is the echo generated from

A

the echo is generated from the rephasing spins

38
Q

how many minimum echoes to build an image?

A

256 echoes

39
Q

What is Tr

A

time between successive repetitions of a pulse sequence

40
Q

what is Te

A

echo time is the time between the first RF pulse and the middle of the echo

41
Q

what does matrix size determine

A

no of pixels in an image

42
Q

what does FOV mean

A

the field of view is the area you wish the image to cover

43
Q

What is slice selection in MR based on?

A

that the resonant frequency depends on magnetic field strength, at the isocentre, strength is B0, its a little over B0 at the head, a little less than B0 at the feet

44
Q

what is the slice thickness determined by in MR?

A

The bandwidth of the RF pulse (range of frequencies) and the steepness of the slice selection gradient

45
Q

what is the relationship between gradient strength and slice thickness?

A

the stronger the gradient, the narrower the slice

46
Q

what effect does a bigger bandwidth have on slice thickness?

A

bigger bandwidth, thicker slice, we want slice thin as possible so bandwidth small as possible!!

47
Q

what is the data acquisition time?

A

the time it takes for a sequence to run fully (depends on matrix size, Tr, and NEX)

48
Q

Briefly describe spin echo sequence (know diagram also)

A

B0 magnetic field lines up proton spins with or against the magnetic field.
Sum of the magnetic dipole moments of protons gives the Net Magnetisation Vector (NMV) pointing in the Z direction.
We apply a 90 degree RF pulse to rotate the NMV to the XY plane.
RF pulse switched off, spins begin to relax.
Spins are in phase, then lose spin coherence because of T2* effects and also due to T1 and T2 relaxation
180 degree pulse applied this is the refocusing pulse. flips the spins through 180 degrees, some spins were fast some were slow, after refocussing pulse, slow spins now ahead of the fast spins. eventually, fast spins catch up to the slow spins and the echo is produced!

49
Q

what is the fate of the echo once produced?

A

the echo (very weak RF signal) is detected by the MR coil, amplified and sampled by ADC and stored in the computer, image then formed by FFT.

50
Q

T1 Weighting spin echo: Tr and Te

A

T1 weighting has a short Tr and short Te

51
Q

T2 weighting spin echo: Tr and Te

A

Long Tr and long Te

52
Q

What are some advantages to spin echo

A

not as sensitive to inhomogeneity of magnet, T2* effects eliminated by the refocussing pulse, hence, larger echo.
different types of contrast achieved by the same sequence (T1, T2, etc.)

53
Q

Disadvantage of spin echo

A

takes a long time, esp T2 weighted images, can lead to motion blurring and reduced throughput in the scanner.

54
Q

Give a reason for using IR instead of spin echo

A

if strong T1 weighted imaging is required
or if we need to suppress fat or CSF signal.

55
Q

what is the main difference between the spin echo and IR sequences

A

IR has a 180 degree pulse before the 90 degree pulse

56
Q

In inversion recovery sequence, what is meant by Ti?

A

the time between the initial 180 and 90 degree pulse is known as the inversion time (Ti)

57
Q

In IR, what determines how strongly T1 weighted the sequence will be

A

Ti and Tr

58
Q

What is the main purpose of the IR sequence’s 180 degree initial pulse?

A

180 RF pulse will tip the spins and NMV into the opposite direction to the main magnetic field, B0

59
Q

Where does T1 relaxation occur in IR?

A

During the Ti

60
Q

How can we reduce T2 effects in IR?

A

keeping the Te as short as possible.

61
Q

How to suppress fat signal?

A

using a STIR sequence

62
Q

what are the main advantages of IR sequences

A

can produce heavily T1 weighted images
can selectively suppress fat signal (STIR) or CSF (FLAIR).

63
Q

disadvantages of IR

A

Long scan times
not possible to get as many slices per Tr as with other sequences
slice profile poor due to use of three RF pulses

64
Q

Why were gradient echo sequences designed?

A

to reduce scan times

65
Q

what are gradient echo sequences used for?

A

orthopaedic and angiography

66
Q

what do gradient echo sequences require?

A

a very good magnetic field homogeneity

67
Q

How is scan time reduced in gradient echo?

A

elimination of the 180 RF pulse and use of an RF pulse <90 degrees.

68
Q

How does gradient echo refocus spins?

A

gradient echo refocuses by gradient switching

69
Q

What is FLASH?

A

Fast Low Angle Shot is the most basic form of gradient echo, uses low tip angle between 10 and 70 degrees.

70
Q

effect of GE sequences on SNR

A

GE has reduced SNR

71
Q

name three parameters influencing image contrast in GE

A

flip angle, Tr, Te

72
Q

Why was fast spin echo developed?

A

to speed up conventional spin echo

73
Q

how is time saving achieved in fast spin echo

A

by acquiring more than one echo after the 90 degree pulse

74
Q

below what T have no significant biological effects been seen

A

below 2.5 T no biological effects of significance have been recorded

75
Q

What is the Echo Train Length

A

no of 180 degree pulses determines the number of echoes

76
Q

what minor effects have been seen

A

10% change in nerve conduction at 24T
nausea, vertigo at 4T
Magnetophosphenes - flashes of light induced in rods >4T

77
Q

what effect in the aorta and above what magnetic strength?

A

elevated T waves on ECG have been seen on ECG in magnets over 0.1T

78
Q

what is the projectile effect

A

forces on ferromagnetic objects inside and outside of the patient

79
Q

size of current depends on:

A

electrical resistance, cross sectional area and rate of change of the magnetic field

80
Q

What is the specific absorption rate?

A

Specific Absorption Rate (SAR) is the RF power absorbed per kg of tissue

81
Q

what can be caused by escaped helium during accidental quenching of the magnet?

A

asphyxiation

RA6007: Radiation Physics (6 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions