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    MRI homogeneity and geometric distortion with continuous linac gantry rotation on an Elekta Unity MR-linac

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    Authors
    Jackson, Steven J
    Glitzner, M
    Tijssen, R
    Raaymakers, B
    Affiliation
    CMPE, Christie NHS Foundation Trust, 550 Wilmslow Road, Manchester, M20 4BX
    Issue Date
    2019
    
    Metadata
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    Abstract
    This work aimed to quantify any principal magnetic field (B 0) inhomogeneity and changes in MR image geometric distortion with continuous linac gantry rotation on an Elekta Unity MR-linac. This situation occurs for around a second between treatment beams during current image guided radiotherapy treatment and would occur frequently in foreseeable real-time adaptive radiotherapy treatment. Pixel by pixel maps of B 0 inhomogeneity were obtained via repeated high temporal resolution pulse sequences with the linac gantry static at 36 gantry angles spaced ten degrees apart, and in continuous rotation at both 1 and 2?rpm. Individual B 0 maps were subtracted from average maps across all data and the residual peak to peak inhomogeneity was calculated for each. The bulk geometric shift and change in physical extent of a 10?cm diameter spherical flood phantom during continuous linac gantry rotation at 1 and 2?rpm was compared to the static gantry case for two pulse sequences: the real-time clinical monitoring bFFE sequence and a non-clinical EPI sequence, chosen for its susceptibility to geometric distortion. The peak to peak inhomogeneity in the deviation-from-average ppm maps, plotted against gantry angle with the gantry in continuous rotation at 1 and 2?rpm were negligibly different from equivalent data obtained with the gantry static. The real-time clinical monitoring pulse sequence was shown to give negligible geometric distortion during continuous gantry motion, whilst a non-clinical EPI sequence showed bulk shifts of the order of one pixel and gantry angle dependent changes in extent, demonstrating the sensitivity of the chosen method. MR imaging on the Elekta Unity MR-Linac with the gantry in continuous motion is negligibly different from the static gantry case with current clinical pulse sequences. Real-time tracking and treatment plan adaptation using MR images obtained with the linac gantry in motion is possible.
    Citation
    Jackson S, Glitzner M, Tijssen RHN, Raaymakers BW. MRI B 0 homogeneity and geometric distortion with continuous linac gantry rotation on an Elekta Unity MR-linac. Phys Med Biol. 2019;64(12):12.
    Journal
    Physics in Medicine and Biology
    URI
    http://hdl.handle.net/10541/621895
    DOI
    10.1088/1361-6560/ab231a
    PubMed ID
    31108467
    Additional Links
    https://dx.doi.org/10.1088/1361-6560/ab231a
    Type
    Article
    Language
    en
    ae974a485f413a2113503eed53cd6c53
    10.1088/1361-6560/ab231a
    Scopus Count
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    All Christie Publications

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