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Purpose: To develop a Dixon‐based B0$$ {\mathrm{B}}_0 $$ self‐navigation approach to estimate and correct temporal B0$$ {\mathrm{B}}_0 $$ variations in radial stack‐of‐stars gradient echo imaging for quantitative body MRI. Methods: The proposed method estimates temporal B0$$ {\mathrm{B}}_0 $$ variations using a B0$$ {\mathrm{B}}_0 $$ self‐navigator estimated by a graph‐cut‐based water‐fat separation algorithm on the oversampled k‐space center. The B0$$ {\mathrm{B}}_0 $$ self‐navigator was employed to correct for phase differences between radial spokes (one‐dimensional [1D] correction) and to perform a motion‐resolved reconstruction to correct spatiotemporal pseudo‐periodic B0$$ {\mathrm{B}}_0 $$ variations (three‐dimensional [3D] correction). Numerical simulations, phantom experiments and in vivo neck scans were performed to evaluate the effects of temporal B0$$ {\mathrm{B}}_0 $$ variations on the field‐map, proton density fat fraction (PDFF) and T2∗$$ {\mathrm{T}}_2^{\ast } $$ map, and to validate the proposed method. Results: Temporal B0$$ {\mathrm{B}}_0 $$ variations were found to cause signal loss and phase shifts on the multi‐echo images that lead to an underestimation of T2∗$$ {\mathrm{T}}_2^{\ast } $$, while PDFF mapping was less affected. The B0$$ {\mathrm{B}}_0 $$ self‐navigator captured slowly varying temporal B0$$ {\mathrm{B}}_0 $$ drifts and temporal variations caused by respiratory motion. While the 1D correction effectively corrected B0$$ {\mathrm{B}}_0 $$ drifts in phantom studies, it was insufficient in vivo due to 3D spatially varying temporal B0$$ {\mathrm{B}}_0 $$ variations with amplitudes of up to 25 Hz at 3 T near the lungs. The proposed 3D correction locally improved the correction of field‐map and T2∗$$ {\mathrm{T}}_2^{\ast } $$ and reduced image artifacts. Conclusion: Temporal B0$$ {\mathrm{B}}_0 $$ variations particularly affect T2∗$$ {\mathrm{T}}_2^{\ast } $$ mapping in radial stack‐of‐stars imaging. The self‐navigation approach can be applied without modifying the MR acquisition to correct for B0$$ {\mathrm{B}}_0 $$ drift and physiological motion‐induced B0$$ {\mathrm{B}}_0 $$ variations, especially in the presence of fat. [ABSTRACT FROM AUTHOR]

Purpose: This work aims at investigating intensity corrected cone-beam x-ray computed tomography (CBCT) images for accurate dose calculation in adaptive intensity modulated proton therapy (IMPT) for prostate and head and neck (H&N) cancer. A deformable image registration (DIR)-based method and a scatter correction approach using the image data obtained from DIR as prior are characterized and compared on the basis of the same clinical patient cohort for the first time., Methods: Planning CT (pCT) and daily CBCT data (reconstructed images and measured projections) of four H&N and four prostate cancer patients have been considered in this study. A previously validated Morphons algorithm was used for DIR of the planning CT to the current CBCT image, yielding a so-called virtual CT (vCT). For the first time, this approach was translated from H&N to prostate cancer cases in the scope of proton therapy. The warped pCT images were also used as prior for scatter correction of the CBCT projections for both tumor sites. Single field uniform dose and IMPT (only for H&N cases) treatment plans have been generated with a research version of a commercial planning system. Dose calculations on vCT and scatter corrected CBCT (CBCT cor ) were compared by means of the proton range and a gamma-index analysis. For the H&N cases, an additional diagnostic replanning CT (rpCT) acquired within three days of the CBCT served as additional reference. For the prostate patients, a comprehensive contour comparison of CBCT and vCT, using a trained physician's delineation, was performed., Results: A high agreement of vCT and CBCT cor was found in terms of the proton range and gamma-index analysis. For all patients and indications between 95% and 100% of the proton dose profiles in beam's eye view showed a range agreement of better than 3 mm. The pass rate in a (2%,2 mm) gamma-comparison was between 96% and 100%. For H&N patients, an equivalent agreement of vCT and CBCT cor to the reference rpCT was observed. However, for the prostate cases, an insufficient accuracy of the vCT contours retrieved from DIR was found, while the CBCT cor contours showed very high agreement to the contours delineated on the raw CBCT., Conclusions: For H&N patients, no considerable differences of vCT and CBCT cor were found. For prostate cases, despite the high dosimetric agreement, the DIR yields incorrect contours, probably due to the more pronounced anatomical changes in the abdomen and the reduced soft-tissue contrast in the CBCT. Using the vCT as prior, these inaccuracies can be overcome and images suitable for accurate delineation and dose calculation in CBCT-based adaptive IMPT can be retrieved from scatter correction of the CBCT projections.

The ability to perform dose recalculation on the anatomy of the day is important in the context of adaptive proton therapy. The objective of this study was to investigate the use of deformable image registration (DIR) and cone beam CT (CBCT) imaging to generate the daily stopping power distribution of the patient. We investigated the deformation of the planning CT scan (pCT) onto daily CBCT images to generate a virtual CT (vCT) using a deformable phantom designed for the head and neck (H & N) region. The phantom was imaged at a planning CT scanner in planning configuration, yielding a pCT and in deformed, treatment day configuration, yielding a reference CT (refCT). The treatment day configuration was additionally scanned at a CBCT scanner. A Morphons DIR algorithm was used to generate a vCT. The accuracy of the vCT was evaluated by comparison to the refCT in terms of corresponding features as identified by an adaptive scale invariant feature transform (aSIFT) algorithm. Additionally, the vCT CT numbers were compared to those of the refCT using both profiles and regions of interest and the volumes and overlap (DICE coefficients) of various phantom structures were compared. The water equivalent thickness (WET) of the vCT, refCT and pCT were also compared to evaluate proton range differences. Proton dose distributions from the same initial fluence were calculated on the refCT, vCT and pCT and compared in terms of proton range. The method was tested on a clinical dataset using a replanning CT scan acquired close in time to a CBCT scan as reference using the WET evaluation. Results from the aSIFT investigation suggest a deformation accuracy of 2-3 mm. The use of the Morphon algorithm did not distort CT number intensity in uniform regions and WET differences between vCT and refCT were of the order of 2% of the proton range. This result was confirmed by proton dose calculations. The patient results were consistent with phantom observations. In conclusion, our phantom study suggests the vCT approach is adequate for proton dose recalculation on the basis of CBCT imaging.