In this paper, the effect on image quality of significantly reducing the primary electron energy of a radiotherapy accelerator is investigated using a novel waveguide test piece. The waveguide contains a novel variable coupling device (rotovane), allowing for a wide continuously variable energy range of between 1.4 and 9 MeV suitable for both imaging and therapy.Method:
Imaging at linac accelerating potentials close to 1 MV was investigated experimentally and via Monte Carlo simulations. An imagingbeam line was designed, and planar and cone beam computed tomographyimages were obtained to enable qualitative and quantitative comparisons with kilovoltage and megavoltage imaging systems. The imagingbeam had an electron energy of 1.4 MeV, which was incident on a water cooled electron window consisting of stainless steel, a 5 mm carbon electron absorber and 2.5 mm aluminium filtration. Images were acquired with an amorphous silicon detector sensitive to diagnostic x-ray energies.Results:
The x-ray beam had an average energy of 220 keV and half value layer of 5.9 mm of copper. Cone beamCTimages with the same contrast to noise ratio as a gantry mounted kilovoltage imaging system were obtained with doses as low as 2 cGy. This dose is equivalent to a single 6 MV portal image. While 12 times higher than a 100 kVp CBCT system (Elekta XVI), this dose is 140 times lower than a 6 MV cone beamimaging system and 6 times lower than previously published LowZ imagingbeams operating at higher (4–5 MeV) energies.Conclusions:
The novel coupling device provides for a wide range of electron energies that are suitable for kilovoltage quality imaging and therapy. The imaging system provides high contrastimages from the therapy portal at low dose, approaching that of gantry mounted kilovoltage x-ray systems. Additionally, the system provides low doseimaging directly from the therapy portal, potentially allowing for target tracking during radiotherapy treatment. There is the scope with such a tuneable system for further energy reduction and subsequent improvement in image quality.
This work is supported by Elekta and The Institute of Cancer Research. Work of the ICR radiotherapy physics group is partially supported by Cancer Research UK under programme Grant No. C46/A3970. The authors are grateful for information provided by Elekta and Perkin Elmer for the purposes of modeling the linac and detectors. They are extremely grateful to Kevin Brown, Alan Hitchings, Chris Knox, Andrew Lake, Terry Large, Carlos Sandin, and Abdul Sayeed from Elekta and to Craig Cummings, Clive Long, Nick Smith, Ellen Donovan, and Karen Rosser from the Royal Marsden for component manufacture, advice on the waveguide testing and setup, and advice on various aspects of this project and paper.
II.A. Waveguide test piece
II.B. Imagingbeam line
II.D. Dosimetry equipment
II.E. Monte Carlo model
III.A. Characterization of electron energy characteristics of waveguide
III.B. Experimental characterization of imagingbeam
III.C. Assessment of image quality
III.C.1. Imaging parameters
III.C.2. Planar imaging
III.C.3. Cone BeamCTCNR
IV.A. Characterization of waveguide test piece
IV.B. Imagingbeam characterization
IV.B.1. Planar contrast and CNR
IV.B.2. Cone beamCTcontrast to noise ratio
IV.B.3. CBCT spatial resolution
IV.B.4. Qualitative image quality
V. DISCUSSION AND CONCLUSION
- Medical imaging
- Medical image quality
- Medical X-ray imaging
- Cone beam computed tomography
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