Scanners in Real Life Flashcards
What factors affect MRIs slow uptake for the use in Radiotherapy?
- Resolution: can be as low as 1.4x1.4x3mm
- Geometric Accuracy: Is a function of the manufacturing process, the interaction between B0-field and patient, and the imaging sequence.
- Compatibility with Treatment systems: Image intensity is qualitative.
- Availability: Not all centres have them
What are the advantages of CT over MRI in RT planning?
- Intensity calibrated units: relate directly to tissue density.
- Good resolution: 1x1x2mm
- Geometric accuracy is a function of the scanner set up.
How does the magnet design affect image accuracy?
- End effects of solenoid depend on length and diameter of the bore. Small thinner bore, fewer end effects, but worse patient comfort and compliance.
- Inhomogeneity from end effects leads to geometric distortion and B0 is now a function of r.
How does the bore length, width and field strength affect B0 homogeneity?
Shorter bore - worse B0 homogeneity.
Wider bore - worse B0 homogeneity.
Higher field - worse B0 homogeneity.
What other problems do MRI systems have for RT compatibility?
MRI systems usually have curved beds, RT systems have flat beds.
What are the advantages and disadvantages of open bore systems in RT compatibility?
+ Wide patient access, allows the patient to be scanned in the treatment position.
- Lower B0 field, reduced SNR, worse resolution.
How does gradient design affect the image accuracy?
High gradient uniformity required, if the gradient is a function of distance then the frequency is no longer linearly related to the location and there will be geometric distortion.
What determines the linearity of a gradient?
The accuracy of the gradient windings. Typically gets worse the further off axis you are, causing end effects.
How can gradient non-linearity be corrected?
Scan a test object with a uniform structure. Use software to calculate the correction required to shift the points back to the correct place.
How does inductance affect gradient timing?
Higher inductance requires a longer time to reach the maximum gradient and also the longer it takes the gradient field to fall to zero afterwards.
How can we overcome inductance effects in gradient coils?
- Design a coil with minimal inductance.
- Use a high voltage to drive the current rapidly.
- Drive the gradient coils with a trapezoidal waveform.
- Rise rate is deliverable in a linear fashion with the capacity of the gradient amplifiers.
How will gradient heating effect image quality?
Significant heating can cause thermal expansion which in turn causes geometric changes.
Heating of the local environment can cause more frequency changes and more geometric effects.
How are eddy currents formed by gradient switching?
- Turning gradient on and off causes rate of change of magnetic flux (dB/dt).
- This magnetic field extends inside and outside the coil.
- The coil is surrounded by conducting surfaces (e.g. scanner bore tube, internal magnet structures, Rf coil structures)
- Lenz’s law applies: switching gradients induced a current flow in those surrounding structures, that current creates a magnetic field which opposes the change.
How can we change the gradient waveform to reduce the effect of eddy currents?
- Shield the gradient coils with secondary coils designed to cancel distant field (i.e wound in opposite directions)
- Pre-emphasise the waveworm: drive the gradient with the opposite function to the eddy current.
How effective is gradient shielding on removing Eddy currents?
Can reduce eddy current effect to 1% of un-shielded. This can still lead to significant distortions.
