A high resolution 2D array detector system for small-field MRI-linac applications

A portable magnet for radiation biology and dosimetry studies in magnetic fields

BACKGROUND AND PURPOSE

In the current and rapidly evolving era of real-time MRI-guided radiotherapy, our radiation biology and dosimetry knowledge is being tested in a novel way. This paper presents the successful design and implementation of a portable device used to generate strong localized magnetic fields. These are ideally suited for small scale experiments that mimic the magnetic field environment inside an MRI-linac system, or more broadly MRI-guided particle therapy as well.

MATERIALS AND METHODS

A portable permanent magnet based device employing an adjustable steel yoke and magnetic field focusing cones has been designed, constructed and tested. The apparatus utilises two banks of Nd₂ Fe₁₄ B permanent magnets totalling around 50 kg in mass to generate a strong magnetic field throughout a small volume between two pole tips. The yoke design allows adjustment of the pole tip gap and exchanging of the focusing cones. Further to this, beam portal holes are present in the yoke and focusing cones, allowing for radiation beams of up to 5 x 5 cm² to pass through the region of high magnetic field between the focusing cone tips. Finite element magnetic modelling was performed to design and characterise the performance of the device. Automated physical measurements of the magnetic field components at various locations were measured to confirm the performance. The adjustable pole gap and interchangeable cones allows rapid changing of the experimental set-up to allow different styles of measurements to be performed.

RESULTS

A mostly uniform magnetic field of 1.2 T can be achieved over a volume of at least 3 x 3 x 3 cm³ . This can be reduced in strength to 0.3 T but increased in volume to 10 x 10 x 10 cm³ via removal of the cone tips and/or adjustment of the steel yoke. Although small, these volumes are sufficient to house radiation detectors, cell culture dishes and various phantom arrangements targeted at examining small radiation field dosimetry inside magnetic field strengths that can be changed with ease. Most important is the ability to align the magnetic field both perpendicular to, or inline with the radiation beam. To date, the system has been successfully used to conduct published research in the areas of radiation detector performance, lung phantom dosimetry, and how small clinical electron beams behave in these strong magnetic fields.

CONCLUSIONS

A portable, relatively inexpensive, and simple to operate device has successfully been constructed and used for performing radiation oncology studies around the theme of MRI-guided radiotherapy. This can be in either inline and perpendicular magnetic fields of up to 1.2 T with x-ray and particle beams.

High resolution silicon array detector implementation in an inline MRI-linac

PURPOSE

Dynamic dosimaging is a concept whereby a detector in motion is tracked with magnetic resonance imaging (MRI) to validate the amount and position of dose in a radiation therapy treatment on an MRI-linac. This work takes steps toward the realization of dynamic dosimaging with the novel high resolution silicon array detector: MagicPlate-512 (M512). The performance of the M512 was assessed in a 1.0 T inline MRI-linac, without simultaneous imaging and then during an imaging sequence, both during dosimetry. MR images were acquired to determine the effect of the detector and its components on image quality.

METHODS

Beam profiles were measured using the M512 on the Australian MRI-Linac and a comparison made with Gafchromic EBT3 film to investigate any intrinsic magnetic field effects in the silicon. The M512 has 512 sensitive volumes, each 0.5 × 0.5 × 0.037 mm³ in dimension, organized in a two-dimensional array. Small field sizes up to 4.2 × 3.8 cm² were investigated in both solid water and then solid lung phantoms. Beam profiles taken at 1.0 T were compared to 0 T conditions, and also to profiles taken during a gradient echo (GRE) imaging sequence. Differences in 80%-20% penumbral width and full width at half maximum (FWHM) were investigated. Localizer MR images were acquired of the detector adjacent to a water phantom.

RESULTS

Good agreement was observed between the M512 and film, with average differences in penumbral width and FWHM of <1 mm in the absence of the imaging sequence. Concurrent imaging widened the penumbra by up to 1.2 mm due to RF noise affecting the detector; film profiles were unchanged. Magnetic resonance images were affected by noise, in particular, due to the large amount of aluminum present, as well as from the USB cable, which acted as an antenna. Unfortunately, due to these issues, suitable dynamic dose imaging was not achieved with the current M512/phantom configuration and the MRI-linac. However, progress was made toward achieving this goal for future work.

CONCLUSIONS

The M512 silicon array detector successfully measured high-resolution beam profiles in agreement with Gafchromic film to within an average of <1 mm on the first MRI-linac in Australia. More effective noise reduction will be required for the achievement of dynamic dosimaging in the future.

Experimental characterization of magnetically focused electron contamination at the surface of a high-field inline MRI-linac

PURPOSE

The fringe field of the Australian MRI-linac causes contaminant electrons to be focused along the central axis resulting in a high surface dose. This work aims to characterize this effect using Gafchromic film and high-resolution detectors, MOSkin^TM and microDiamond. The secondary aim is to investigate the influence of the inline magnetic field on the relative dose response of these detectors.

METHODS

The Australian MRI-linac has the unique feature that the linac is mounted on rails allowing for measurements to be performed at different magnetic field strengths while maintaining a constant source-to-surface distance (SSD). Percentage depth doses (PDD) were collected at SSD 1.82 m in a solid water phantom positioned in a low magnetic field region and then at isocenter of the MRI where the magnetic field is 1 T. Measurements for a range of field sizes were taken with the MOSkin^TM , microDiamond, and Gafchromic^® EBT3 film. The detectors' relative responses at 1 T were compared to the near 0 T PDD beyond the region of electron contamination, that is, 20 mm depth. The near surface measurements inside the MRI bore were compared among the different detectors.

RESULTS

Skin dose in the MRI, as measured with the MOSkin^TM , was 104.5% for 2.1 × 1.9 cm² , 185.6% for 6.1 × 5.8 cm² , 369.1% for 11.8 × 11.5 cm² , and 711.1% for 23.5 × 23 cm² . The detector measurements beyond the electron contamination region showed agreement between the relative response at 1 T and near 0 T. Film was in agreement with both detectors in this region further demonstrating their relative response is unaffected by the magnetic field.

CONCLUSIONS

Experimental characterization of the high electron contamination at the surface was performed for a range of field sizes. The relative response of MOSkin^TM and microDiamond detectors, beyond the electron contamination region, were confirmed to be unaffected by the 1-T inline magnetic field.

A feasibility study for high-resolution silicon array detector performance in the magnetic field of a permanent magnet system

PURPOSE

Magnetic field effects on dose distribution and detector functionality must be well understood. The detector utilized to investigate these magnetic field effects was the DUO silicon array detector; the performance of this high spatial resolution detector was assessed under these conditions. The results were compared to Gafchromic EBT3 film to highlight any intrinsic magnetic field effects in the silicon. The results were also compared to previously published MagicPlate-512 (M512) data. The DUO has an improved spatial resolution (200 µm) over the M512 (2 mm).

METHODS

A permanent magnet named Magnetic Apparatus for RaDiation Oncology Studies (MARDOS) paired with a standard linear accelerator (linac) enables either transverse (1.2 T) or inline (0.95 T) orientations of the magnetic field with respect to the radiation beam. A 6 MV Varian 2100C Linac provided the radiation component for the measurements. The DUO detector has 505 sensitive volumes (each volume measuring 800 × 40 × 100 µm³ ) organized in two orthogonal, linear arrays. The DUO was embedded in a solid water phantom in the first set-up and then a solid lung phantom in the second set-up and placed between the magnet cones. Beam profiles were compared under the magnetic field conditions and 0 T. Small field sizes from 0.8 × 0.8 cm² up to 2.3 × 2.3 cm² were investigated. The size of the air gap above the sensitive volumes of the DUO was investigated in the transverse orientation to assess the anticipated magnetic field effects. Full width at half maximum (FWHM), 80-20% penumbral widths and maximum dose differences between detectors and between the presence/absence of a magnetic field were investigated. Symmetry was also assessed for investigation of profile skewness under the transverse field.

RESULTS

The penumbral widths measured by the DUO detector demonstrated good agreement with film and the M512 to within an average of 0.5 mm (within uncertainty: ±1 mm). The static inline magnetic field had minimal effect on the profiles in solid water. As expected, the lower density of solid lung meant that this material was more susceptible to demonstrating magnetic field effects in the dose deposited. The greatest penumbral narrowing due to the inline field (0.7 mm) occurred in lung. Central axis dose increase was greatest in lung (maximum: 9%). The transverse field widened penumbra, most notably in the solid lung phantom, by a maximum of 2.3 mm. The largest asymmetry due to the transverse field (4.6%) was also in solid lung. When the air gap above the DUO was filled with bolus, the dose maximum measured by the DUO was within 1.4% of film.

CONCLUSIONS

The DUO detector has been shown to be successful in accurately describing the dose changes for small field sizes to within a 200-µm resolution in an environment resembling that of an MRI-linac. The DUO measurements were in agreement with both film and the M512 measurements, and therefore the DUO was found to be an appropriate alternative to the M512, with improvement in terms of its higher spatial resolution. MARDOS provided a suitable environment for these preliminary tests before progressing to the MRI-linac.