Bin Dong1,2, Gary P. Liney1,2,3,4, Kevin K. Zhang1,4, Lois Holloway1,2,3,4,5, Peter E. Metcalfe1,2, and Paul J. Keall1,5
1Ingham Institute for Applied Medical Research, Liverpool, Australia, 2Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, Australia, 3Department of Medical Physics, Cancer Therapy Centre, Liverpool Hospital, Liverpool, Australia, 4South Western Sydney Clinical School, University of New South Wales, Liverpool, Australia, 5Radiation Physics Laboratory, Sydney Medical School, The University of Sydney, Camperdown, Australia
Synopsis
Hybrid
systems combining an MRI with a linear accelerator for image guided radiation treatment
are being implemented world-wide. As part of our own development we
investigated the effect of increasing magnetic field on the performance of a
multi-leaf collimator (MLC) motor and encoder from the treatment head of a
linear accelerator. Measurements were made at various locations in the fringe
field of a 3T scanner with the MLC positioned to replicate both in-line and
perpendicular treatment beam to B0 directions. Results show a static field threshold
and speed reduction which is position dependent which can be improved
significantly with shielding.Purpose
One of the challenges for hybrid MRI-Linac systems is to overcome or minimize the influence of the magnetic fringe field on the Linear Accelerator components. A multi-leaf Collimator (MLC), used to shape the treatment beam, has a large number of permanent magnet motors and magnetic encoders which can be expected to be affected by the magnetic field.
1 Previous research shows that variations in MLC speed and acceleration affects treatment delivery accuracy.
2 This work evaluated the performance of a single leaf unit in two alignment orientations over a wide range of postions within the fringe magnetic field and investigated possible solutions to reduce the impacts for MRI-Linac.
Method
In this work, a Single Leaf Test Platform (SLTP) was built that can simulate the MLC leaf motor/encoder working conditions. The SLTP consists of the same motor/encoder used for the Varian Millennium 120 MLC and the SLTP is driven and controlled by a Raspberry Pi single board computer. It was placed in different positions of the fringe magnetic fields of a Siemens Skyra 3T MRI system. The measurements were taken in different orientations to simulate the currently proposed MRI-Linac systems, i.e. SLTP at 90 degrees to B0 (to replicate in-line B0 beam orientation) and parallel to B0 (to replicate perpendicular treatment design)1,3,4. Previous work by Yun et al1. showed that the Hall Effect encoder is particularly sensitive, thus an additional test was performed with the unit rotated a further 90 degree in the in-line orientation to account for an arbitrary-encoder observed in a real clinical treatment head. The MLC motor was driven by a 3.3V 10kHz Pulse Width Modulation (PWM) signal with variable duty cycle. Figure 1 shows the setup of SLTP on the patient table of the MRI scanner. The performance was evaluated by testing motor speed response, shielding response and magnetization response over a wide range of magnetic fields typical of prototype MRI-Linac configurations. The magnetic field was independently measured at each position using a AlphaLab VGM gaussmeter.
The speed response of the motor in terms of rotation speed and time for change is measured from the counts output of the encoder which is translated into speed. The PWM driver was set at a fix duty cycle, the variation of motor speed reflects the effects due to the fringe magnetic field.
The effect of shielding was measured by repeating the speed response tests with the motor/encoder enclosed in a close fitting metal box made of approximately 0.5mm steel thickness.
The possible magnetization response was measured by leaving the SLTP at the 300 Gauss position for more than 14 hours. The motor speed response was measured again and compared with previous test results.
Results
The results of motor/encoder performance in terms of speed response are shown in Figure 2. Leaf motion speed is shown for each orientation both with and without shielding.
In the magnetic field at which the motor was not operational, the motor shaft was rotated manually to simulate working conditions. Readings from the encoder were still observed by the Raspberry Pi.
Discussion and Conclusion
The results indicate that the MLC motor speed reduces as fringe field increases, there is up to 30% (parallel setup) or 20% (perpendicular setup) speed decrease as the fringe field increases. In each orientation there is a threshold field when the motor experiences sufficient Lorentz force to fail. The inline position setup of the MRI-Linac has an advantage in terms of MLC motor tolerance and performance as it has less speed reduction when the field increases and the cut-off field is 600+/-60 Guass in contrast with the perpendicular setup at 300 Gauss.
Even though the motor stops working, by rotating the motor key manually, the Hall Effect encoders were found to still be working at magnetic fields as high as 800 Gauss but this is orientation dependent. No sign of malfunction or false readings were observed during the test.
Preliminary shielding tests show that the half millimetre ferrous material can significantly improve the MLC performance in terms of tolerance and stability in the magnetic.
No significant of performance changes were observed due to short-term magnetisation in low-middle fringe fields, but long term effects need to be monitored.
Acknowledgements
We would like to thank Bill Morrison and Herb Cattell of Varian Medical Systems for their fantastic technical support throughout. References
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