Radiotherapy Beam Alignment in MR-Guided Linacs
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Solution Overview
Problem
Conventional radiotherapeutic beam alignment techniques are inadequate for MR-guided linear accelerators due to restricted space within the MR scanner bore, inability to account for mechanical deviations, and inability to align both the beam and imaging device accurately, especially in achieving beam symmetry.
Innovation Solution
A method using a fiducial phantom with visible markers to align the beam by rotating it and the imaging device, obtaining images from multiple angles, adjusting the trajectory to position the isocentre centrally, and calculating the centre of rotation to account for mechanical deviations and align ancillary equipment like the collimator, without requiring laser systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional beam alignment techniques using lasers or phantoms are used, then alignment can be performed, but the space requirements exceed the restricted bore space of MR scanners
Solution Approach 1:
The patent extracts the alignment functionality from external equipment (lasers, large phantoms) and integrates it into the existing MR scanner bore space by using the MR scanner's own imaging capabilities and a compact fiducial marker system that fits within the limited space
Solution Approach 2:
The MR scanner's imaging system is used for multiple purposes: both for its primary diagnostic function and for beam alignment verification. The fiducial markers serve dual purposes as both alignment references and visible targets in MR images, eliminating the need for separate alignment equipment
2Measurement precision
If conventional phantom-based alignment is used, then isocentre location can be determined, but the phantom markers are not visible to MR scanners
Solution Approach 1:
The fiducial markers are designed with specific local properties (MR-visible materials such as ferromagnetic or paramagnetic substances) that enable them to be detected by the MR scanner, while maintaining their function as precise alignment references for beam positioning
Solution Approach 2:
The patent changes the material parameters of the fiducial markers from X-ray visible (conventional phantom materials) to MR visible materials, altering their magnetic properties to enable detection by the MR scanner while preserving their geometric function for alignment
3Loss of time
If simple alignment methods are used, then alignment can be performed quickly, but mechanical deviations such as sagging and deflection are not accounted for
Solution Approach 1:
The system uses MR images of the fiducial markers at multiple gantry positions to provide feedback on the actual beam position and trajectory, allowing calculation and correction of mechanical deviations such as sagging and deflection that occur during gantry rotation
Solution Approach 2:
The alignment procedure performs preliminary measurements at multiple gantry positions to characterize mechanical deviations before final alignment is established, enabling the system to pre-calculate correction factors for subsequent treatments
4Measurement precision
If separate alignment procedures are used for beam and imaging device, then each can be aligned, but achieving beam symmetry and precise relative alignment is difficult
Solution Approach 1:
The patent combines the alignment of the beam and imaging device into a single integrated procedure by using the same fiducial markers for both alignment tasks and calculating the relative geometry of the beam-imaging system from the same set of MR images
Data Source
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AI summary
A method of aligning the radiation beam in a radiotherapy system comprising a source for producing a beam of radiation and a device for imaging from the beam, both mounted so as to be rotatable about an axis using a fiducial phantom between the source and the device, the method comprising: rotating the beam and device in a trajectory about the axis while obtaining a plurality of images of the fiducial phantom from a plurality of different angles, determining from each image of the fiducial phantom the position of the source at the rotational position the image was obtained, and calculating the centre of rotation of the positions of the source to define the isocentre of the system. The invention enables the alignment of a collimator with the source of radiation, the reduction of asymmetry in beam intensity, and is also applicable to magnetic resonance-guided radiotherapy.