J Overweg

Integrating a 1.5 T MRI scanner with a 6 MV accelerator: proof of concept

At the UMC Utrecht, The Netherlands, we have constructed a prototype MRI accelerator. The prototype is a modified 6 MV Elekta (Crawley, UK) accelerator next to a modified 1.5 T Philips Achieva (Best, The Netherlands) MRI system. From the initial design onwards, modifications to both systems were aimed to yield simultaneous and unhampered operation of the MRI and the accelerator. Indeed, the simultaneous operation is shown by performing diagnostic quality 1.5 T MRI with the radiation beam on. No degradation of the performance of either system was found. The integrated 1.5 T MRI system and radiotherapy accelerator allow simultaneous irradiation and MR imaging. The full diagnostic imaging capacities of the MRI can be used; dedicated sequences for MRI-guided radiotherapy treatments will be developed. This proof of concept opens the door towards a clinical prototype to start testing MRI-guided radiation therapy (MRIgRT) in the clinic.

Eight-channel transmit/receive body MRI coil at 3T.

Multichannel transmit magnetic resonance imaging (MR) systems have the potential to compensate for signal‐intensity variations occurring at higher field strengths due to wave propagation effects in tissue. Methods such as RF shimming and local excitation in combination with parallel transmission can be applied to compensate for these effects. Moreover, parallel transmission can be applied to ease the excitation of arbitrarily shaped magnetization patterns. The implementation of these methods adds new requirements in terms of MRI hardware. This article describes the design of a decoupled eight‐element transmit/receive body coil for 3T. The setup of the coil is explained, starting with standard single‐channel resonators. Special focus is placed on the decoupling of the elements to obtain independent RF resonators. After a brief discussion of the underlying theory, the properties and limitations of the coil are outlined. Finally, the functionality and capabilities of the coil are demonstrated using RF measurements as well as MRI sequences. Magn Reson Med 58:381–389, 2007.

Installation of the 1.5 T MRI accelerator next to clinical accelerators: impact of the fringe field.

In the UMC Utrecht a prototype MRI accelerator has been installed to investigate the feasibility of real-time, MRI guided radiotherapy. The system consists of a 6 MV Elekta (Crawley, UK) accelerator and a 1.5 T Philips (Best, The Netherlands) MRI system. The system is installed in a standard radiotherapy bunker. The bunker is at the corner of a block of six bunkers, so there are three neighbouring clinical Elekta accelerators. During ramping of the magnet, the magnetic fringe field in the two nearest bunkers was measured as a function of the magnetic field strength of the MRI magnet. At 8 m, a maximum increase of 1.5 G was measured, at 12 m, 0.6 G. This is up to three times the earths magnetic field. The clinical accelerators are needed to be re-calibrated in order to operate in such an external magnetic field. The resulting radiation field flatness of the clinical accelerators was measured and was similar to the situation before ramping the magnet.

MO-E-J-6B-03: In Room Magnetic Resonance Imaging Guided Radiotherapy (MRIgRT)

Precise, soft‐tissue based, on‐line position verification and treatment monitoring is a prerequisite for image guided radiotherapy(IGRT). The system being developed is a 1.5 T MRI scanner integrated with a 6 MV radiotherapy accelerator. Basically, the design is a modified 1.5 T Philips Achieva MRI scanner with a small, single energy (6 MV) accelerator rotating around it. The technical feasibility of simultaneous irradiation and MRimaging will be discussed. The magnetic interference between the MRI system and the linear accelerator has been solved by modifying the active shielding of the magnet, minimising the magnetic field at the components of the accelerator and completely nullifying the magnetic field at the location of the accelerator gun section. The MRI system is being homogenized and the radiation thickness has been minimised to allow beam transmission through the midplane of the closed bore MRI system. A special gradient coil set has been designed with a radiation window allowing a 24 cm field size in the caudal cranial direction for every gantry angle. The dose deposition kernel in the presence of a transverse magnetic field has been investigated. The kernel has been quantified and special effects like the electron return effect (ERE) around air cavities, have been investigated. It is shown that integrating MRI functionality with a radiotherapy accelerator is technically feasible. Ultimately the system may be used for daily treatment optimisation by providing the imaging information for on‐line treatment planning. On‐line MRI may also provide treatment monitoring and treatment response assessment required for further biological optimisation.

A high-performance gradient insert for rapid and short-T2 imaging at full duty cycle

The goal of this study was to devise a gradient system for MRI in humans that reconciles cutting‐edge gradient strength with rapid switching and brings up the duty cycle to 100% at full continuous amplitude. Aiming to advance neuroimaging and short‐T2 techniques, the hardware design focused on the head and the extremities as target anatomies.

WE‐D‐BRC‐04: MR‐XRT at 1.5T, the UMC Utrecht Hybrid MRI Linac System

At the UMC Utrecht, the Netherlands we have constructed a prototype MRI accelerator. The design is a 6 MV Elekta (Crawley, U.K.) Compact accelerator combined with a modified 1.5 T Philips Achieva (Best, The Netherlands) MRI system. The systems run according to our specifications. The simultaneous operation is shown by performing diagnostic quality 1.5 T MRI with the radiation beam on. As designed the interference between the two systems is completely suppressed, no interference was found. The integrated 1.5 T MRI system and radiotherapy accelerator allow simultaneous irradiation and MR imaging. Both systems operate independent and no synchronization is required. The full diagnostic imaging capacities of the Philips MRI can be used, dedicated sequences for MRI guided radiotherapy treatments will be developed. This proof of concept opens the door towards a clinical prototype to start testingMRI guided Radiotherapy (MRIgRT) in the clinic. Acknowledgements: Both Elekta and Philips participate in this research program

MO-F-CAMPUS-J-04: Radiation Heat Load On the MR System of the Elekta Atlantic System

Purpose: The Elekta Atlantic system combines a digital linear accelerator system with a 1.5T Philips MRI machine.This study aimed to assess the energy deposited within the cryostat system when the radiation beam passes through the cryostat. The cryocooler on the magnet has a cooling capacity which is about 1 Watt in excess of the cryogenic heat leak into the magnet’s cold mass. A pressure-controlled heater inside the magnet balances the excess refrigeration power such that the helium pressure in the tank is kept slightly above ambient air pressure. If radiation power is deposited in the cold mass then this heater will need less power to maintain pressure equilibrium and if the radiation heat load exceeds the excess cryocooler capacity the pressure will rise. Methods: An in-house CAD based Monte Carlo code based on Penelope was used to model the entire MR-Linac system to quantify the heat load on the magnet’s cold mass. These results were then compared to experimental results obtained from an Elekta Atlantic system installed in UMC-Utrecht. Results: For a field size of 25 cm x 22 cm and a dose rate of 107 mu.min-1, the energy deposited by the radiation beam led to a reduction in heater power from 1.16 to 0.73 W. Simulations predicted a reduction to 0.69 W which is in good agreement. For the worst case field size (largest) and maximum dose rate the cryostat cooler capacity was exceeded. This resulted in a pressure rise within the system but was such that continuous irradiation for over 12 hours would be required before the magnet would start blowing off helium. Conclusion: The study concluded that the Atlantic system does not have to be duty cycle restricted, even for the worst case non-clinical scenario and that there are no adverse effects on the MR system. Stephen Towe and David Roberts Both work for Elekta; Ezra Van Lanen works for Philips Healthcare; Johan Overweg works for Philips Innovative Technologies

TH‐E‐BRA‐10: The 1.5 T MRI Accelerator for MRI during Radiation Delivery: Status Report

Purpose: We have developed and built a prototype ring based accelerator around a 1.5 T MRI system for MRI guided radiation therapy.Methods: The design is a closed bore cylindrical 1.5 T MRI with a ring mounted 6 MV accelerator in the transversal mid‐plane with a 7 mm non‐rotating MLC without back‐up collimators and flatness filter. The ring allows continuous rotation in both directions. The magnetic and RF interference between the MRI and the accelerator is overcome by active shielding and redesign of the Faraday cage respectively. The beam passage through the magnet and gradient coils is through a circumferential beam portal which is homogeneous and contains the equivalent of approximately 10 cm of aluminium.Results: After installation of the ring gantry around the MRI at zero field, radiation from the rotating accelerator was shown and arbitrary segments can be delivered. Recently the MRI is brought up to 1.5T and performance testing at 1.5 T is in progress. Radiation delivery was shown. For static gantry positions MRI can be performed, without striking image deterioration, substantiation is work in progress. Gantry rotation in both directions can be performed, but this does affect the MRI B0 field. These distortions are mapped using a standard field camera and a combination of active and passive corrections as presented by Amthor et al. (2011) seems feasible and is ongoing. Conclusion: The MRI accelerator is close to full functionality, that is, diagnostic 1.5T MRI during IMRT for delivering stereotactic precision radiation under MRI guidance. Amthor T. and Overweg J. Compensation of magnetic field perturbations for combined MRI/radiotherapy system. Eposter 526 in proc ESMRMB 2011Leipzig, Germany. Work is supported by Elekta, U.K. and Philips, Best, The Netherlands

INTEGRATED 1.5 T MRI AND ACCELERATOR: PROOF OF CONCEPT FOR REAL-TIM E MRI GUIDED RADIOTHERAPY

INTEGRATED 1.5 T MRI AND ACCELERATOR: PROOF OF CONCEPT FOR REAL-TIME MRI GUIDED RADIOTHERAPY B. Raaymakers1, J. Lagendijk1, J. Overweg2, J. Kok3, A. Raaijmakers1, E. Kerkhof1, R. van der Put3, I. Meijsing1, S. Crijns1, F. Benedosso3, M. van Vulpen1, N. de Graaff3, J. Allen4, K. Brown4 1 UMC UTRECHT, Department of Radiotherapy, Utrecht, Netherlands 2 PHILIPS, Hamburg, Germany 3 UMC UTRECHT, Utrecht, Netherlands 4 ELEKTA UK LIMITED, Crawley, West Sussex, United Kingdom

SU‐GG‐J‐60: Constructing a 6 MV Radiotherapy Accelerator with Integrated 1.5T MRI functionality: Status Report

Radiotherapy is increasingly dependent on image data for treatmentpreparation, delivery, response assessment and follow‐up. In collaboration with Elekta, Crawley, UK and Philips, Best, The Netherlands, we are constructing a hybrid 1.5T MRIradiotherapy system. This system facilitates real‐time, soft‐tissue based image guidance during delivery as well as treatment response assesment. The preceeding technical feasibility study led to a design in which the 6MV accelerator can rotate in a ring around the MRI in the mid‐transversal plane. The magnet design is adjusted in order to minimise the magnetic interference and to minimise the absorption of the beam. The prototype is planned for autumn 2008. The aim is to demonstrate MRI guided radiation with sub‐mm precision. The prototype will initially be static: the accelerator will be in a fixed lateral position. The treatment room preparation will be discussed including: creation of entrance route in bunker, faraday cage, cooling and electrical connections for in‐room MRI peripherals and passive magnetic room‐shielding to minimise magnetic interference with neighbouring clinical accelerators.The current Geant4 Monte Carlo simulations on the accelerator output and radiation dosimetry will be verified in the prototype. Geant4 simulations were also used to find suitable IMRT solutions in the presence of a 1.5T field, these will be evaluated experimentally. System related issues addressed will be the radiofrequency interference between the MRI and the accelerator, the geometric correction of images dedicated for this system and the geometrical coupling of the MRI and accelerator coordinate system. Also the automatic, on‐line target definition on MRI and the clinical benefit of MRI guidance will be discussed.