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Impact of the MLC on the MRI field distortion of a prototype MRI-linac
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1.
1. C. Plathow, M. Schoebinger, C. Fink, H. Hof, J. Debus, H. Meinzer, and H. Kauczor, “Quantification of lung tumor volume and rotation at 3D dynamic parallel MR imaging with view sharing: Preliminary results,” Radiology 240(2), 537545 (2006).
http://dx.doi.org/10.1148/radiol.2401050727
2.
2. Y. Suh, S. Dieterich, B. Cho, and P. Keall, “An analysis of thoracic and abdominal tumour motion for stereotactic body radiotherapy patients,” Phys. Med. Biol. 53(13), 36233639 (2008).
http://dx.doi.org/10.1088/0031-9155/53/13/016
3.
3. A. Sawant, K. Pauly, M. Alley, S. Vasanawala, B. Loo, S. Joshi, J. Hinkle, and P. Keall, “Real-time MRI for soft-tissue-based IGRT of moving and deforming lung tumors,” Med. Phys. 37, 3424 (2010).
http://dx.doi.org/10.1118/1.3469379
4.
4. T. Bortfeld, K. Jokivarsi, M. Goitein, J. Kung, and S. B. Jiang, “Effects of intra-fraction motion on IMRT dose delivery: Statistical analysis and simulation,” Phys. Med. Biol. 47(13), 22032220 (2002).
http://dx.doi.org/10.1088/0031-9155/47/13/302
5.
5. D. Verellen, M. De Ridder, N. Linthout, K. Tournel, G. Soete, and G. Storme, “Innovations in image-guided radiotherapy,” Nat. Rev. Cancer 7(12), 949960 (2007).
http://dx.doi.org/10.1038/nrc2288
6.
6. D. Verellen, M. D. Ridder, and G. Storme, “A (short) history of image-guided radiotherapy,” Radiother. Oncol. 86(1), 413 (2008).
http://dx.doi.org/10.1016/j.radonc.2007.11.023
7.
7. B. Raaymakers, J. Lagendijk, J. Overweg, J. Kok, A. Raaijmakers, E. Kerkhof, R. van der Put, I. Meijsing, S. Crijns, and F. Benedosso, “Integrating a 1.5 T MRI scanner with a 6 MV accelerator: Proof of concept,” Phys. Med. Biol. 54(12), N229N237 (2009).
http://dx.doi.org/10.1088/0031-9155/54/12/N01
8.
8. B. Fallone, M. Carlone, B. Murray, S. Rathee, T. Stanescu, S. Steciw, K. Wachowicz, and C. Kirkby, “Development of a Linac-MRI system for real-time ART,” Med. Phys. 34, 2547 (2007).
http://dx.doi.org/10.1118/1.2761342
9.
9. J. Dempsey, D. Benoit, J. Fitzsimmons, A. Haghighat, J. Li, D. Low, S. Mutic, J. Palta, H. Romeijn, and G. Sjoden, “A device for realtime 3D image-guided IMRT,” Int. J. Radiat. Oncol., Biol., Phys., Suppl. 63, S202S202 (2005).
http://dx.doi.org/10.1016/j.ijrobp.2005.07.349
10.
10. D. Constantin, R. Fahrig, and P. Keall, “A study of the effect of in-line and perpendicular magnetic fields on beam characteristics of electron guns in medical linear accelerators,” Med. Phys. 38(7), 41744185 (2011).
http://dx.doi.org/10.1118/1.3600695
11.
11. J. St. Aubin, S. Steciw, C. Kirkby, and B. G. Fallone, “An integrated 6 MV linear accelerator model from electron gun to dose in a water tank,” Med. Phys. 37, 22792288 (2010).
http://dx.doi.org/10.1118/1.3397455
12.
12. J. St. Aubin, S. Steciw, and B. G. Fallone, “Waveguide detuning caused by transverse magnetic fields on a simulated in-line 6 MV linac,” Med. Phys. 37, 47514754 (2010).
http://dx.doi.org/10.1118/1.3480481
13.
13. J. Yun, J. Aubin, S. Rathee, and B. Fallone, “Brushed permanent magnet DC MLC motor operation in an external magnetic field,” Med. Phys. 37, 21312134 (2010).
http://dx.doi.org/10.1118/1.3392165
14.
14. D. Santos, J. S. Aubin, B. Fallone, and S. Steciw, “Magnetic shielding investigation for a 6 MV in-line linac within the parallel configuration of a linac-MR system,” Med. Phys. 39, 788797 (2012).
http://dx.doi.org/10.1118/1.3676692
15.
15. J. St. Aubin, S. Steciw, and B. G. Fallone, “Magnetic decoupling of the linac in a low field biplanar linac-MR system,” Med. Phys. 37, 47554761 (2010).
http://dx.doi.org/10.1118/1.3480482
16.
16. B. Fallone, B. Murray, S. Rathee, T. Stanescu, S. Steciw, S. Vidakovic, E. Blosser, and D. Tymofichuk, “First MR images obtained during megavoltage photon irradiation from a prototype integrated linac-MR system,” Med. Phys. 36, 20842088 (2009).
http://dx.doi.org/10.1118/1.3125662
17.
17. B. Burke, M. Lamey, S. Rathee, B. Murray, and B. Fallone, “Radio frequency noise from clinical linear accelerators,” Phys. Med. Biol. 54(8), 24832492 (2009).
http://dx.doi.org/10.1088/0031-9155/54/8/015
18.
18. B. Burke, A. Ghila, B. Fallone, and S. Rathee, “Radiation induced current in the RF coils of integrated linac-MR systems: The effect of buildup and magnetic field,” Med. Phys. 39, 50045014 (2012).
http://dx.doi.org/10.1118/1.4737097
19.
19. S. Crijns, B. Raaymakers, and J. Lagendijk, “Proof of concept of MRI-guided tracked radiation delivery: Tracking one-dimensional motion,” Phys. Med. Biol. 57(23), 78637872 (2012).
http://dx.doi.org/10.1088/0031-9155/57/23/7863
20.
20. J. Yun, K. Wachowicz, M. Mackenzie, S. Rathee, D. Robinson, and B. Fallone, “First demonstration of intrafractional tumor-tracked irradiation using 2D phantom MR images on a prototype linac-MR,” Med. Phys. 40, 051718 (12pp.) (2013).
http://dx.doi.org/10.1118/1.4802735
21.
21. J. M. Galvin, “The multileaf collimator: A complete guide,” in Proceedings of the AAPM Annual Meeting, Nashville, Tennessee, 1999.
22.
22. J. R. Brauer, What Every Engineer Should Know about Finite Elem Anal 2e (CRC, Xi'an, China, 1993), Vol. 31.
23.
23. Y. Shahbazi, K. Niayesh, and H. Mohseni, “Finite element method analysis of performance of inductive saturable-core fault current limiter,” in 2011 1st International Conference on Electric Power Equipment-Switching Technology (ICEPE-ST), 2011 (IEEE, CRC Handbook of Chemistry & Physics, Taylor and Francis Group, LLC, 2011), pp. 352355.
24.
24. L. Liu et al., “Flanged-edge transverse gradient coil design for a hybrid linac-MRI system,” J. Magn. Reson. 226, 7078 (2013).
http://dx.doi.org/10.1016/j.jmr.2012.11.017
25.
25. A. Raaijmakers, B. Raaymakers, and J. Lagendijk, “Magnetic-field-induced dose effects in MR-guided radiotherapy systems: Dependence on the magnetic field strength,” Phys. Med. Biol. 53(4), 909923 (2008).
http://dx.doi.org/10.1088/0031-9155/53/4/006
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/40/12/10.1118/1.4828792
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Figures

Image of FIG. 1.

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FIG. 1.

Implemented model of Varian Millennium 120 MLC. MLC leaves were simplified to rectangular shape and interleaf gaps removed. MLC motors and drive screws are represented through two mass-equivalent steel blocks. Inner air cavities are used to increase the outer extent of the steel blocks to better approximate the real distribution of ferromagnetic material in space. (a) Realistic MLC model and (b) simplified MLC model.

Image of FIG. 2.

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FIG. 2.

BH curves for the heavy tungsten alloy and 1010 steel, as implemented in the COMSOL model. For comparison, the curves of vacuum and 1008 steel are shown.

Image of FIG. 3.

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FIG. 3.

(a) Magnetic field ( ) magnitude map of the Agilent MRI design (fill plot) and COMSOL match (contour plot). Current coils are shown in shaded rectangles and field values < or > are shown as white. Our COMSOL results are in excellent agreement with the Agilent data. (b) Histogram of inside the DSV. For the 30 and DSV volumes, the spread in is 6.73 and , respectively. This comfortably matches the manufacturer specification of < for the DSV.

Image of FIG. 4.

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FIG. 4.

Comparison of the models of mass-equivalent blocks and 60 square motors placed in a uniform ( ) background field. (a) Magnetic field with MLC motors. (Top) Field maps are in good agreement in the far-field regime where the DSV distortion is evaluated. (Bottom) Magnetic field component along the CAX is shown. (b) Field distortion over DSV as a function of SID; for , the difference between the two models is <3%.

Image of FIG. 5.

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FIG. 5.

Field inhomogeneity due to the MLC banks for different s. The dashed line shows the DSV outline. The inline orientation: , (b) , and (c) ; the perpendicular orientation: (d) , (e) , and (f) . In the inline (perpendicular) orientation, the MLC is positioned in positive direction ( direction), gradually lifting from left to right (top to bottom) across the DSV. The field homogeneity improves with increasing in both orientations.

Image of FIG. 6.

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FIG. 6.

Peak-to-peak distortion Δ versus . MLC leaves and motors contribute to Δ in similar order. The total Δ (dotted curve) lies below for and in inline (a) and perpendicular (b) orientation, respectively. Due to a steeper falloff of the fringe field, the perpendicular orientation is favorable in terms of field distortion.

Image of FIG. 7.

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FIG. 7.

Dynamic change in peak-to-peak distortion with treatment field size. Field sizes from 0 × 0 to were simulated. At any given , Δ is maximum for the field and decreases with increasing field size. The differences in Δ caused by the field sizes become smaller with increasing , meaning geometric details such as the MLC leaf positions have less impact. (a) Inline and (b) perpendicular.

Image of FIG. 8.

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FIG. 8.

Peak-to-peak distortion Δ versus SID for the MLC in field configuration and different magnetic field strengths = 0.5, 1.0, and . The saturation of the ferromagnetic materials below results in an almost identical absolute field distortion at magnetic field strengths of 1.0 and . Thus, a gain in image quality could be obtained without increasing the difficulty of shimming the MRI-linac assembly at . (a) Inline and (b) perpendicular.

Tables

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TABLE I.

Characteristics of the MRI-guided radiotherapy systems currently under development.

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TABLE II.

Maximum spatial difference in ( ) within DSV for various field sizes with respect to field and of in (a) inline and (b) perpendicular orientation. Differences are classified as (italics), (bold), and (underlined). Based on the use of a frequency-encoding gradient, these limits together with remaining distortion after passive shimming correspond to geometric distortions of , when summed linearly (italics) or in quadrature (bold), and (underlined). Geometric distortions up to can be tolerated for our purposes. Thus, passive shimming may be sufficient for (perpendicular) and (inline).

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/content/aapm/journal/medphys/40/12/10.1118/1.4828792
2013-11-08
2014-04-23

Abstract

To cope with intrafraction tumor motion, integrated MRI-linac systems for real-time image guidance are currently under development. The multileaf collimator (MLC) is a key component in every state-of-the-art radiotherapy treatment system, allowing for accurate field shaping and tumor tracking. This work quantifies the magnetic impact of a widely used MLC on the MRI field homogeneity for such a modality.

The finite element method was employed to model a MRI-linac assembly comprised of a split-bore MRI magnet and the key ferromagnetic components of a Varian Millennium 120 MLC, namely, the leaves and motors. Full 3D magnetic field maps of the system were generated. From these field maps, the peak-to-peak distortion within the MRI imaging volume was evaluated over a diameter sphere volume (DSV) around the isocenter and compared to a maximum preshim inhomogeneity of . Five parametric studies were performed: (1) The source-to-isocenter distance (SID) was varied from 100 to , to span the range of a compact system to that with lower magnetic coupling. (2) The MLC model was changed from leaves only to leaves with motors, to determine the contribution to the total distortion caused by MLC leaves and motors separately. (3) The system was configured in the inline or perpendicular orientation, i.e., the linac treatment beam was oriented parallel or perpendicular to the magnetic field direction. (4) The treatment field size was varied from 0 × 0 to , to span the range of clinical treatment fields. (5) The coil currents were scaled linearly to produce magnetic field strengths of 0.5, 1.0, and , to estimate how the MLC impact changes with .

(1) The MLC-induced MRI field distortion fell continuously with increasing SID. (2) MLC leaves and motors were found to contribute to the distortion in approximately equal measure. (3) Due to faster falloff of the fringe field, the field distortion was generally smaller in the perpendicular beam orientation. The peak-to-peak DSV distortion was below at (perpendicular) and (inline) for the design. (4) The simulation of different treatment fields was identified to cause dynamic changes in the field distribution. However, the estimated residual distortion was below geometric distortion at (perpendicular) and (inline) for a frequency-encoding gradient. (5) Due to magnetic saturation of the MLC materials, the field distortion remained constant at .

This work shows that the MRI field distortions caused by the MLC cannot be ignored and must be thoroughly investigated for any MRI-linac system. The numeric distortion values obtained for our magnet may vary for other magnet designs with substantially different fringe fields, however the concept of modest increases in the SID to reduce the distortion to a shimmable level is generally applicable.

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Scitation: Impact of the MLC on the MRI field distortion of a prototype MRI-linac
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/40/12/10.1118/1.4828792
10.1118/1.4828792
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