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Effects of MR imaging time reduction on substitute CT generation for prostate MRI-only treatment planning

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The introduction of MRI linear accelerators (MR-linacs) and the increased use of MR imaging in radiotherapy, requires improved approaches to MRI-only radiotherapy. MRI provides excellent soft tissue visualisation but does not provide any electron density information required for radiotherapy dose calculation, instead MRI is registered to CT images to enable dose calculations. MRI-only radiotherapy eliminates registration errors and reduces patient discomfort, workload and cost. Electron density requirements may be addressed in different ways, from manually applying bulk density corrections, to more computationally intensive methods to produce substitute CT datasets (sCT), requiring additional sequences, increasing overall imaging time. Reducing MR imaging time would reduce potential artefacts from intrafraction motion and patient discomfort. The aim of this study was to assess the impact of reducing MR imaging time on a hybrid atlas-voxel sCT conversion for prostate MRI-only treatment planning, considering both anatomical and dosimetric parameters. 10 volunteers were scanned on a Siemens Skyra 3T MRI. Sequences included the 3D T2-weighted (T2-w) SPACE sequence used for sCT conversion as previously validated against CT, along with variations to this sequence in repetition time (TR), turbo factor, and combinations of these to reduce the imaging time. All scans were converted to sCT and were compared to the sCT from the original SPACE sequence, evaluating for anatomical changes and dosimetric differences for a standard prostate VMAT plan. Compared to the previously validated T2-w SPACE sequence, scan times were reduced by up to 80%. The external volume and bony anatomy were compared, with all but one sequence meeting a DICE coefficient of 0.9 or better, with the largest variations occurring at the edges of the external body volume. The generated sCT agreed with the gold standard sCT within an isocentre dose of 1% and a gamma pass rate of 99% for a 1%/1 mm gamma tolerance for all but one sequence. This study demonstrates that the MR imaging sequence time was able to be reduced by approximately 80% with similar dosimetric results.

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Data availability

Due to the nature of this research, participants of this study did not agree for their data to be shared publicly, so supporting data is not available.


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Authors and Affiliations



TY: conceived and designed the analysis, collected the data, contributed data or analysis tools, performed the analysis, wrote the paper. JD: contributed data or analysis tools, manuscript review. RR: conceived and designed the analysis, collected the data, manuscript review. GL: conceived and designed the analysis. PG, DT, LH: conceived and designed the analysis, manuscript review.

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Correspondence to Tony Young.

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Informed consent was obtained from all individual participants included in the study.

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The authors affirm that human research participants provided informed consent for publication of data.

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Volunteer data obtained for this study was acquired with local ethics committee approval (HREC/15/LPOOL/506).

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Young, T., Dowling, J., Rai, R. et al. Effects of MR imaging time reduction on substitute CT generation for prostate MRI-only treatment planning. Phys Eng Sci Med 44, 799–807 (2021).

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