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MRI > Scanners in Real Life > Flashcards

Scanners in Real Life Flashcards

1
Q

Why is MRI not routinely used in Radiotherapy?

A

Resolution is not always as good as CT
Geometric accuracy not as good and depends on more factors than CT
Intensity is not calibrated to HU or tissue density
It is not as widely available as CT
It may not be supported by all TPS options

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2
Q

What is the MRI localisation decribed by?

A

wG = gamma.(B0+G.x)

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3
Q

What type of magnet is most common in MRI? Why?

A

Closed bore superconducting

It has a high B0 field and stability - therefore high SNR and good resolution

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4
Q

What is the equation for finding the magnetic field of an infinite solenoid?

A

B0 = u0 . n . I

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5
Q

What is the compromise in bore length?

A

Short bore is better for patient compliance but has worse homogeneity, long bores are the opposite

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6
Q

How is the homogeneity of a scanner described?

A

ppm over a diameter spherical volume (DSV)

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7
Q

How does the localisation of an MRI signal change if B0 is inhomogeneous?

A

wG = gamma(B0(r)+G.x)

The frequency is no longer linearly related to the location so get geometric distortion

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8
Q

What options are there for imaging patients in the Radiotherapy treatment positions?

A

Use a shorter bore, wider bore - worse overall B0 homogeneity
Use an open bore - better patient access, permanent magnet with cryogenic systems - lower B0 field and stability therefore lower SNR and worse resolution - longer scan times

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9
Q

Why is high gradient uniformity needed?

A

It will result in G being replaced by Gr) in the localisation equation

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10
Q

How are gradient coils manufactured to make the uniformity as high as possible?

A 
Resistive coils are laminated to the coil structure
The winding (plate) geometry is used to generate the required gradient field
The accuracy of the windings determines the linearity
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11
Q

How is linearity checked for gradients?

A

Check with a uniform structure to measure the distortion - it typically gets worse the further off axis you are - end effects

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12
Q

What are the switching effects for the gradient coils?

A

The gradients are switched on and off to encode te signal location - they have resistance R and inductance L
The minimum TE is determined by the gradient rise and fall times - if they are slow get T2 loss so reduced SNR

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13
Q

What are the equations for the applied voltage across a coil and the current?

A 
V = iR + Ldi/dt
i = V/R(1-e^(-tL/R))
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14
Q

How is the inductance overcome to minimise the gradient rise/fall times?

A

Design the coils with minimal L
Use high voltage to drive the current rapidly
Drive the gradient with a trapezoidal waveform: has a rise rate which is deliverable in a linear fashion - within the capacity of gradient amplifiers

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15
Q

What is the side effect when high current and voltage are used to overcome the inductance?

A

There is significant heating - gives thermal expansion of gradient coils resulting in geometric changes
Heating of the local environment (shims) results in a frequency change and therefore geometric effects

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16
Q

What is the origin of eddy currents?

A

The gradient coils are switched on an off during a pulse sequence with a rate of change of the magnetic field dB/dt
This creates a magnetic gradient field inside and outside the coil
The coil is surrounded by the magnet structure
According to Lenz’s law this change in magnetic flux will induce an EMF so current will flow to oppose the change in magnetic field
Switching the gradient induces a current flow in the surrounding structures
The eddy current flow is proportional to dB/dt and decays exponentially depending on the resistance of the material

17
Q

What effect do Eddy currents have?

A

They alter the gradient waveform during and after the pulse

18
Q

Hiw can the effect of the eddy currents be measured?

A

The gradient induces an Eddy current, need to wait a certain delay for the Eddy current to evolve, measure the scanner frequency
Repeat for a set of time delays

19
Q

How can the effects of eddy currents be minimised?

A

Shield the gradient coils - use a coil system design with 2 layers - primary and secondary to cancel the distant field - this reduces the Eddy currents to 1% - still significant
Use pre-emphasis - drive the gradient with the precise opposite function to the Eddy current - nulls the effect

20
Q

What are the RF coils used for?

A

Detect the patient signal via magnetic induction - principle of reciprocity applies
Use localised coils close to the body to minimise r - use array coils with large number of small loops
The intensity varies according to B1 field distribution equating to the element distance

21
Q

How does the RF sensitivity change with depth and how can this effect be remedied?

A

Sensitivity falls with 1/r^2 relationship with distance from each element - SNR is lower at greater depth
Software intensity correction done by scanner

22
Q

What is the problem with mounting coils on the patient for Radiotherapy planning? What does mounting the coils away from the patient result in?

A

Get a distortion of the soft tissue which can effect the body contouring
This results in lower SNR

23
Q

What are the 2 major patient-scanner interactions?

A

Magnetic susceptibility

Dielectric effects

24
Q

What is the cause of the magnetic susceptibility effects?

A

Human tissues have a range of magnetic susceptibility constant which define the relationship between the applied magnetic field b and the magnetic field strength H within an object
Different tissues have a different B established by the applied field so there will be field gradients in adjacent tissues
Therefore there are inhomogeneities in B even when the magnet is very uniform
This causes distortions

25
Q

What is the equation for the B field in a human tissue?

A

B = u0(1+Chim).H

26
Q

What does the geometry of a B field in tissue depend on?

A

The object size and shape

The object orientation relative to the main field

27
Q

What do the magnetic susceptibility factors vary between

A

Patient to patient
Body area to body area
Scan type to scan type

28
Q

What is the cause of dielectric effects?

A

Tissue has a high dielectric constant - reduces the propagation speed and wavelength of the RF to ~30cm
Get standing wave effects as the wavelength is comparable to the size of the patient - dielectric resonance effect
The RF wave must be continuous at boundries between free space and tissue

29
Q

What problems do dielectric effects cause?

A

Get local signal voids

Get local heating (SAR) effects

30
Q

What is the dielectric effect magnitude dependent on? What effect does this have for the choice of field strength in Radiotherapy?

A

Depends on the magnetic field strength

Use low field strengths for RT

31
Q

What are the two methods to counteracting scan imperfections?

A

Avoid/minimise distortions through the careful selection of scan parameters
Correct distortion in post processing

32
Q

How is MRI data collected and transformed from analogue to digital data?

A

The signal is demodulated and sampled
Need to stay within the Nyquist criterion otherwise the signal will alias
A readout gradient is applied in the x direction with strength Gx(mT/m)
At either side of the FOV the manetisation precesses at w+ = gamma.Gx.x and w- = -gamma.Gx.x

33
Q

What is the frequency range which is sampled?

A

2.gamma.Gx

34
Q

What can be used to reduce the noise?

A

Filter to restrict frequencies to only MRI frequencies

Use a low sampling bandwidth

35
Q

What effect does using a low sampling bandwidth have on the scan?

A

It takes a long time so the T2 decay will reduce during sampling

36
Q

How can sampling be used to control distortion?

A

Sampling bandwidth influences the SNR
Bandwidth affects the distortion
As the signal is sampled and digitised it has a specific frequency range
The image will consist of discrete pixels so they have an effective bandwidth too
The selection of the sampling bandwidth can control distortion
The distortion is the ratio of local field variation and pixel bandwidth: deltaf/N.deltaB0

37
Q

What does the bandwidth define?

A

The SNR and therefore sensitivity

The degree of geometric image distortion - use the highest possible bandwidth

38
Q

What is the intensity correction factor and how is it used in post processing?

A

Collect a reference scan to reveal the spatial variation in sensitivity
The inverse of the sensitivity reference is the applied to intensity flatten the profile

MRI (3 decks)

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