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Why is MRI not routinely used in Radiotherapy?

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


What is the MRI localisation decribed by?

wG = gamma.(B0+G.x)


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

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


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

B0 = u0 . n . I


What is the compromise in bore length?

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


How is the homogeneity of a scanner described?

ppm over a diameter spherical volume (DSV)


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

wG = gamma(B0(r)+G.x)
The frequency is no longer linearly related to the location so get geometric distortion


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

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


Why is high gradient uniformity needed?

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


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

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


How is linearity checked for gradients?

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


What are the switching effects for the gradient coils?

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


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

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


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

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


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

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


What is the origin of eddy currents?

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


What effect do Eddy currents have?

They alter the gradient waveform during and after the pulse


Hiw can the effect of the eddy currents be measured?

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


How can the effects of eddy currents be minimised?

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


What are the RF coils used for?

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


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

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


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

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


What are the 2 major patient-scanner interactions?

Magnetic susceptibility
Dielectric effects


What is the cause of the magnetic susceptibility effects?

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


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

B = u0(1+Chim).H


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

The object size and shape
The object orientation relative to the main field


What do the magnetic susceptibility factors vary between

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


What is the cause of dielectric effects?

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


What problems do dielectric effects cause?

Get local signal voids
Get local heating (SAR) effects


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

Depends on the magnetic field strength
Use low field strengths for RT