MRLinac, Hardware & Method Development
MRI has entered radiotherapy about 30 years ago. The benefits of MRI, i.e. the wide variety of excellent endogenous soft-tissue contrast, facilitate contouring of the target volumes to treat and the organs at risk to spare. In addition, it offers the possibility to image biomarkers for tumor characterization and response assessment.
A hybrid machine, that enables real-time imaging during treatment, allows gating and tracking of the target. However, combining a linear accelerator and an MRI is challenging due to the basics of the technology. Therefore, the first clinical MRI guided radiotherapy was given using a Cobalt-60 source in combination with low field superconducting MRI (0.35T) in 2014 [1]. In 2018, three different linear accelerators combined with an MRI (0.35T, 0.5T and 1.5T) were clinically introduced [2-4]. The Australian MRLinac project is still in an experimental stage [5].
The challenges of the integration of an MRI and a linear accelerator include:
· Magnetic interference
· Radio-frequency (RF) interference
· Beam transmission through the MRI
The different systems have solved the challenges in different ways:
Magnetic interference:
For Cobalt-60 systems this is no issue, but for the systems using a linear accelerator, the magnetic field needs to be actively or passively shielded at the location where the accelerator is located. The passive shielding and ferrous components of the accelerator can perturb the magnetic field of the MRI. Therefore shimming of shield and rotating components is necessary [6].
RF interference
To not introduce noise and artefacts in the images by RF generated by the linac, the MRI and the linac need to be separated by an RF cage. The solution here is to shield the linac parts form the MRI with RF reflecting copper or RF absorbing carbon.
Beam transmission
Two solutions are implemented. In the first, the beam travels through the MRI. Here, a homogeneous and stable beam attenuation is necessary. The second is the use of a split or open MRI system. In the design, also the orientation of the photon beam in relation the magnetic field lines is an important deceision. viewRay [2].
A parallel or perpendicular orientation of the radiation beam relative to the magnetic field direction, affects the dose deposition. In a parallel set-up, the effect of the electrons that are released in the tissue and will travel along the magnetic field lines will be limited [6] In a perpendicular set-up, the electrons will rotate around the magnetic field lines due to Lorentz forces and by that impact the dose deposition. [8, 9] This effect is magnet field strength dependent.

MRimaging
For scanning, also a transmit RF coil, the gradient coils and the receive coils are needed. The RF coil and gradient coils are mostly in a split design, to avoid deterioration of the coils by the photon beam and to avoid beam attenuation and release of stray electrons. The receive coils need to be radiolucent to avoid dose built up in the patient’s skin and all electronic components should be placed outside the radiation beam.
Besides good image quality based on SNR and contrast the geometric accuracy is of utmost importance for the treatment of radiotherapy patients. The geometry of images is dependent on machine parameters, i.e. the gradient performance and the B0 homogeneity and patient variables, i.e. susceptibility variations. The gradient linearity and the applied corrections are tested using body size geometry phantoms specially designed for radiotherapy [10-13]. Volumetric image acquisition (3D FSE, 3D SPGR or 3D balanced SSFP) is used to allow 3D gradient imperfection correction. To reduce the effect of susceptibility effects, first order B0 shimming is used and high bandwidth in the frequency encoding direction are used.
During radiation dose delivery, cine MR images can be acquired to gate the beam when the target is outside of the treatment field [14]. This also requires fast imaging, reconstruction and registration of the cines to stop the beam if necessary. This will prolong treatment times. The ultimate goals is to track the target during treatment. This will reduce treatment fields, reduce complications and will not prolong treatment times [15]. For moving organs in the abdomen and thorax, motion characterisaion using MRI can also be useful to know the extent of the moving organ. For this, retrospective phase rebinning strategies are available or softgating such as MRRiddle [16, 17].
The daily imaging also allows for DW MRI. As the gradient systems have a lower maximum and a shallower slope, DWI is preferred to perform at lower b-values than on a conventional MR system to maintain SNR and diffusion time [18].

Acknowledgements

No acknowledgement found.

References

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Reviews:

Lagendijk JJ, Raaymakers BW, Van den Berg CA, Moerland MA, Philippens ME, van Vulpen M. MR guidance in radiotherapy. Phys Med Biol. 2014 Nov 7;59(21):R349-69.

Liney GP, Whelan B, Oborn B, Barton M, Keall P. MRI-Linear Accelerator Radiotherapy Systems. Clin Oncol (R Coll Radiol). 2018 Nov;30(11):686-691