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RAD413 > MRI sequences and image formation 1 > Flashcards

MRI sequences and image formation 1 Flashcards

1
Q

What is TE

A

echo time

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2
Q

the signal detected after the flip angle has no spatial information, to form an image, we need to localise the signal.

How is this done?

A
  • applying spatial magnetic field gradients (G) to temporarily vary the main magnetic field strength (B0)
  • this causes larmor frequency to vary depending on position
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3
Q

what are the purpose of gradient coils in the scanner

A
  • creates a magnetic strength gradient in x, y, z direction or combination of all 3
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4
Q

explain how the main coil in the machine generates a magnetic field

A
  • if a current runs through a wire, it creates a magnetic field around the wire (amperes law)
  • and when you run these wires in a coil, the magnetic fields will superimpose and forms a large single magnetic field (B0) that runs through the center of the coil
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5
Q

what 2 things affect the strength of B0

A
  • number of coils
  • strength of current running through coil
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6
Q

in order to get a strong magnetic field, a strong current is needed but this increases the resistance and hence increases the heat, hence what type of material wire is needed

A

superconducting

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7
Q

what material is the main coil usually made of

A

niobium titanium alloy

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8
Q

what circulates around the coil to keep the temps low enough for superconductivity of the wires

A

liquid helium

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9
Q

below what temperature is when superconductivity characteristic begin to work

A

4 kelvin

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10
Q

what is quenching

A

when the temperature within the MRI machine reaches above 4 kelvin, ‘deactivating’ the superconductive effect of the wire and hence causing increase in temp due to resistance.

this increase in temp causes the liquid helium to evaporate into gas and expands which leads to quenching where the helium gas is released into the room.

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11
Q

the stronger the magnetic field, the faster the procession of the hydrogen atoms along the main magnetic field

A
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12
Q

what are the purpose of shims along the main coil

A

adjust the magnetic field to make it more homogenous

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13
Q

explain how gradient coils work

A
  • 4 gradient coils sit perpendicular to each other in the x and y plane (whilst lying along the z plane)
  • we know that there is the main magnetic field going along the z axis causing the protons to spin at a certain speed
  • each gradient coil, can form its own magnetic field as long as a current is run through it
  • focusing on the far right gradient coil, it forms a magnetic field going from left to right that SUPERIMPOSES on the main magnetic field (so here it increases the strength of the B0)
  • the same can be done on the far left gradient coil, but instead it forms a magnetic field in the opposite direction to B0 (going right to left), this decrease the strength of B0
  • so you see the formation of 2 separate magnetic fields (by gradient coil) that overall influences the main magnetic field and causes a gradient to be formed (decreasing going left, increasing going right)
  • hence the protons depending on where they are along the z axis, will be affected by a different magnetic field strength due to the gradients and have different processional speeds
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14
Q

know that gradient coils DO NOT change the direction of magnetic field strength, they only change the strength of it

A
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15
Q

what is the isocentre

A

point in the magnetic field that is unchanged and has the same magnetic field strength as the b0 ( the field strength generated by the main coils)

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16
Q

as the 4 gradient coils are perpendicular to each other, this is how a gradient is formed in either the x or y axis

A
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17
Q

what is the role of the radio frequency coil

A
  • generates a magnetic field PERPENDICULAR to the main magnetic field (in the x, y plane)
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18
Q

how does a radio frequency coil, target specific hydrogen molecules

A
  • due to the gradient which has varying magnetic field strengths, it causes varying hydrogen percussion along this gradient
  • only when the RF is at the same speed of spin/frequency as a proton, will it be able to give it more and more energy (think of dad swimming analogy)
  • those with different frequency to the RF pulse will not be able to match and it cannot provide increasing energy to it
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19
Q

what 2 things happen when the RF pulse matches up to the perscession of a certain proton spin

A
  1. protons begin to fan out (more and more until it reaches a flip angle, reaching the transverse plate)
  2. protons become in phase with each other (was not in phase before)
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20
Q

when transverse magnetisation is gained, the RF pulse can be stopped.

what 2 independent process happen after the RF has been stopped

A
  1. T2 relaxation, loss of transverse magnetisation
  2. T1 relaxation, gain of longitudinal magnetisation
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21
Q

different tissues have different rates of t1 and t2 relaxation, this is what gives us contrast

A
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22
Q

what is slice selection

A

trying to figure out where along the z axis / longitudinal place, the signal has come from

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23
Q

what is used to select a slice from the z axis

A

slice selection gradient

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24
Q

know what all the protons along b0 are spinning at the same frequency known as the larmour frequency

the gyromagnetic ratio (42.5) and the size of the magnetic field (e.g 3T) will cause the protons to spin at a certain frequency

A
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25
Q

in order to select a slice along the z axis, the processing frequencies need to be different along the z axis, how is this done?

A

using the gradient coils

  • which means theres a difference in magnetic field from one end of patient to the other
  • because there are different field strengths along the z axis, we get different processional frequencies along the z axis (stronger to the right, weaker to the left)
26
Q

explain how a radio frequency pulse is used to select a slice

A

so we know that if we apply a RF pulse, it can cause matching processional protons to flip into their transverse plane.

we can apply an RF pulse to the entirety of the patient, and only the spins that match this RF pulse will exhibit nuclear magnetic resonance. (the other spinning protons will not flip into the transverse plane as their processional freq doesnt match)

The RF pulse is not at an exact frequency but rather has a slight range e.g for 60Mhz it is actually 55-65Mhz (this is known as the RADIOFREQUENCY BANDWIDTH)

So as RF pulse is applied, it matches up with certain processional frequencies within the patient. As this RF pulse has some ‘width’/ range, it matches to a ‘range’/area of processional frequencies, we will get nuclear magnetic resonance within that particular slice.

overall forming a slice

27
Q

slice selection is the ability to move this ‘slice’ along the z axis

A
28
Q

what are the 3 methods done to move the slice along the z axis (slice selection)

A
  1. change the RF bandwidth
  2. change the baseline strength of the gradient
  3. actually move the patient within the magnetic field
29
Q

what are 2 methods done to change slice thickenss

A
  1. increase radio frequency bandwidth ( so it covered larger range of processional frequencies)
  2. change the gradient (increase or decrease, by making the difference between the strongest and weaker magnetic field larger or smaller)
30
Q

as a result of slice thickenss, thee can be something known as slice phase, what is this

A
  • as you know, it is a radiorequency bandwidth that matches that the processional frequencies
  • the protons that matched with the lower frequency end of the bandwidth compared to the higher frequency end of the bandwidth are slightly out of phase with each other

this is slice phase

31
Q

how do you fix slice phase

A
  • you apply a rephrasing gradient after slice selection
  • you apply an equal and opposite gradient in the opposite direction along the z axis
  • this will now allow the protons to spin in phase with one another (as the spinning at the end with the lower frequency now gets an increased field strength and vice versa)
  • now all protons in that slice will spin in phase
32
Q

At the same time that RF is pulsed, the slice selection gradient is applied

A
33
Q

how does frequency encoding allow you to pin point a data point on the x axis of the slice plane

A
  • the receiver coil samples at multiple points during the TE (during the gain of transverse magnetisation)
  • (the ability of the receiver coil to sample the TE over a period of time is known as the receiver bandwidth)
  • this allows you to see the changes in the sample over a period of time
  • each sample is converted to an electrical signal from analogue that can be stored

-

34
Q

remember that the sampling size changes across TE

A
35
Q

know that, because all the protons in the slice plane are moving at the same FREQUENCY and all together produce a NET magnetisation, you cannot tease out a single data point from the x or y axis

A
36
Q

how does frequency encoding allow you to pin point a data point on the x axis of the slice plane

A
  • a gradient coil has 2 halves, a different current is applied to each half and this generate a magnetic field differential in the x axis
  • this hence applies a magnetic field strength along the x axis of the slice selected (frequency encoding gradient)
  • the varying MFS along the x axis, causes the frequency of the spins to differ along the x axis
  • and because the frequency now differer along there x axis, you can identify where a signal has come from, along the x axis
37
Q

when is a frequency encoding gradient applied

A

ONLY during read out,

  • so 90 degree flip, partial recovery, 180 degree flip, then apply the frequency encoding when net magnetisation vector in transverse plane is regained
38
Q

when you apply a frequency encoding gradient, it causes the spins to become out of phase and so you will lose net magnetisation in the transverse vector very fast.

How is this compensated for?

A
  • before frequency encoding gradient is applied, you apple an equal and opposite frequency encoding gradient prior to the read out
  • so the left hand side spins fast cuz of high MFS and the right hand side spins slower cuz of low MFS
  • then once the frequency encoding gradient is applied during readout, the spins will come back in phase with each other and the period of sampling will happen when the spins are much more in phase (but still at different frequencies)
39
Q

the different frequencies caused by the frequency encoding gradient is what produces a non-sinusoidal wave when you measure net magnetisation vector

A
40
Q

explain how different values are received during sampling of the TE with frequency encoding gradient applied

A
  • we know that application of a frequency encoding gradient forms a non-sinusoidal wave
  • as many samples are taken along the time fo echo ( across this non-sinusoidal wave), you can measured ranges of value
  • high value would be due to majority frequencies point towards the receiver coil vice versa (discrete values are formed from the amplitude of the net magnetisation vector at that point in time)
  • when they cancel out, you get a much lower value
  • these values are represented with greyscale BUT each ‘colour’ represents a numerical value (not a pixel in the image)
41
Q

understand that each sampled data of the non-sinusoidal wave/readout of TE with frequency encoded gradient applied, is the value of the net magnetisation vector for the WHOLE slice at a given point in time.

then you wait a bit and sample again to receive the net magnetisation vector for the whole slice. This data retrieved is what forms the non-sinusoidal wave/ data

hence it provides no data on its own as to exactly where in the slice this data is from

A
42
Q

because frequencies along the x axis are different (frequency encoding gradient), we can use a specific signal to figure out what frequencies are contributing to the signal

A
43
Q

what is the role of Fourier transformation in deciphering the non-sinusoidal wave formed from frequency encoded read out

A
  • it essentially uses a mathematical equation to figure out which unique combination of frequencies and amplitudes will give us a specific signal (data point) from the read out

(only one specific set of frequencies and amplitudes will give you the unique signal that youve read out)

44
Q

it is only when you compare all the different datasets along the non-sinusoidal read out, can you create a range of frequency values along the X axis of the slice, known as INVERSE Fourier transform

A
45
Q

the inverse Fourier transformation allows the readout of the NET magnetisation vector of the whole slice in the entirety of the x axis (columns) at different points in time

A
46
Q

remember that all of the X axis data from the application of the frequency encoding gradient can be acquired in a SINGLE TE

(where as phase encoding requires multiple TE to gather data)

A
47
Q

how many different times is the data acquisition sampled during frequency encoding application

A

128 - 256 times

48
Q

how is phase encoding gradient used/done to create different acquisition data in the y axis of the image

A
  • we know that along each point in the y axis, the protons spin at the same frequency forming a general superimposed sinusoidal wave (representing the net magnetisation)
  • a phase encoding gradient is applied to the Y axis of the plane to introduce differences in the based on y axis location
  • PEG is applied between the 90 and 180 degree flip, and the MFG happens in the Y axis of the slice using the gradient coils
  • the PEG increases the MFS towards the top of the plane and subtracts the MFS towards the bottom of the plane, whilst the central part of the image is unaffected and remains at the lamour frequency (main magnetic field) (known as the null point)
  • as the PEG is applied for a certain amount of time, it causes the spins dephase based on their y axis location
    (because of the gradient, there will be greater dephasing towards the periphery end compared to towards the null point)
  • then once the PEG has been switched off, the spins return to the frequency of the main magnetic field however because they had been affected individually by the PEG, spin at the same frequency but at different phases (phase change)
49
Q

what direction of dephasing is experienced by the spins above and below the null point / in area of increased or subtracted MFS

A
  • Above the null point ( experiencing increase in MFS) = clockwise dephasing
  • below null point (subtracted MFS) = anticlockwise dephasing
50
Q

how does the phase encoding gradient affect the net magnetisation vector

A

-we know that transverse magnetisation is due to the in phase spin of the protons

  • because PEG causes changing in phases of the spins whilst at the same frequency
  • the superimposition of the amplitudes decreases the net magnetisation vector as they are slightly off beat / out of phase which causes reduction in the maximum amplitude
  • the larger/longer the PEG, the more out of phase the spins are and so the increase in reduction of net magnetisation vector
51
Q

Phase encoding gradient causes de phasing in the y axis (but at the same frequency)

Frequency encoding gradient cases changes in frequency / precession speed along the x axis

A
52
Q

explain how phase encoding and frequency encoding gradient forms k-space

A
  • FEG produces data acquisition along the x axis in columns (hence we only need to apply the FEG a singular time during readout)
  • PEG is used several times at different magnitiudes (can be positive or negative) to acquire data at different positions along the y axis
  • PEG is repeated over and over again at different magnitudes until you have done the number of phase encoding steps that along you to reach the resolution you want on the y axis (e.g u want 516 x 516 resolution so u need to do PEG 516 times)
  • as the multiple PEG data is placed over the singular x axis column of FEG, you produced what is known as k-space
53
Q

remember that the greyscale of PEG and FEG do not form the image, rather it just represents a numerical value which is then processed by the Fourier transform to form the image

A
54
Q

what does each line of K space represent

A

net magnetisation vector change over a given period of time

55
Q

spin echo=

90 degree —> 180 degree ——> spin echo recieved

A
56
Q

gradient echo =

90 degree ——> gradient echo received

A
57
Q

why does spin echo have a larger signal reciveed that gradient echp

A
  • because 2 slice selective RF pulses are used ( both 90 and 180 degree)
  • gradient echo only uses one RF pulse (90 degree)
58
Q

why is using spin echo less susceptible to some artefacts

A

because the 180 degree pulse in spin echos reverses dephasing of protons making it less susceptible to artefacts such as metal

59
Q

which is faster gradient echo or spin echo sequence

A

gradient echo as there is a shorter TR (only 90 degree)

spin echo has longer time of reparation ( 90 +180 degree)

60
Q
A

RAD413 (24 decks)

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