Quantifying the Effect of 3T Magnetic Resonance Imaging Residual System Distortions and Patient-Induced Susceptibility Distortions on Radiation Therapy Treatment Planning for Prostate Cancer

Adjeiwaah, Mary; Bylund, Mikael; Lundman, Josef; Thellenberg‐Karlsson, Camilla; Jönsson, Johan; Nyholm, Tufve

doi:10.1016/j.ijrobp.2017.10.021

Cited by 35 publications

(46 citation statements)

References 29 publications

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“…In MRI‐guided radiotherapy treatment, geometric distortions can translate into dosimetry uncertainties . The clinical dosimetry accuracy is normally evaluated by using the steepest dose–response curves for normal tissue complication probability and for tumor control probability (TCP) .…”

Section: Discussionmentioning

confidence: 99%

“…In MRI-guided radiotherapy treatment, geometric distortions can translate into dosimetry uncertainties. 42 The clinical dosimetry accuracy is normally evaluated by using the steepest dose-response curves for normal tissue complication probability and for tumor control probability (TCP). 43 The geometric inaccuracy of the target will potentially decrease the tumor control probability or overlap regions with the adjacent healthy structures, which could be detrimental for patients in terms of normal tissue complications.…”

Section: Discussionmentioning

confidence: 99%

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Geometric distortion characterization and correction for the 1.0 T Australian MRI‐linac system using an inverse electromagnetic method

et al. 2020

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Purpose The magnetic resonance imaging (MRI)‐Linac system combines a MRI scanner and a linear accelerator (Linac) to realize real‐time localization and adaptive radiotherapy for tumors. Given that the Australian MRI‐Linac system has a 30‐cm diameter of spherical volume (DSV) with a shimmed homogeneity of ±4.05 parts per million (ppm), a gradient nonlinearity (GNL) of <5% can only be assured within 15 cm from the system's isocenter. GNL increases from the isocenter and escalates close to and outside of the edge of the DSV. Gradient nonlinearity can cause large geometric distortions, which may provide inaccurate tumor localization and potentially degrade the radiotherapy treatment. In this study, we aimed to characterize and correct the geometric distortions both inside and outside of the DSV. Methods On the basis of phantom measurements, an inverse electromagnetic (EM) method was developed to reconstitute the virtual current density distribution that could generate gradient fields. The obtained virtual EM source was capable of characterizing the GNL field both inside and outside of the DSV. With the use of this GNL field information, our recently developed “GNL‐encoding” reconstruction method was applied to correct the distortions implemented in the k‐space domain. Results Both phantom and in vivo human images were used to validate the proposed method. The results showed that the maximal displacements within an imaging volume of 30 cm × 30 cm × 30 cm after using the fifth‐order spherical harmonic (SH) method and the proposed method were 6.1 ± 0.6 mm and 1.8 ± 0.6 mm, respectively. Compared with the fifth‐order SH‐based method, the new solution decreased the percentage of markers (within an imaging volume of 30 cm × 30 cm × 30 cm) with ≥1.5‐mm distortions from 6.3% to 1.3%, indicating substantially improved geometric accuracy. Conclusions The experimental results indicated that the proposed method could provide substantially improved geometric accuracy for the region outside of the DSV, when comparing with the fifth‐order SH‐based method.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Geometric distortion characterization and correction for the 1.0 T Australian MRI‐linac system using an inverse electromagnetic method

et al. 2020

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“…Based on their simulation and using vendor‐corrected MR images, they found that MR distortions can lead to <4% of the target volume coverage loss and potential increased doses to organs at risk of 5–6 Gy. In another recent study, Adjeiwaah et al estimated <0.5% difference in the PTV dose in prostate cancer patients arising from residual system and patient induced susceptibility distortions . Data are still emerging to determine the acceptable magnitude of geometric distortions for MRgRT.…”

Section: Discussionmentioning

confidence: 99%

“…In another recent study, Adjeiwaah et al estimated <0.5% difference in the PTV dose in prostate cancer patients arising from residual system and patient induced susceptibility distortions. 13 Data are still emerging to determine the acceptable magnitude of geometric distortions for MRgRT. More specifically, recommendations for GNL distortion accuracy are not currently well defined for MRgRT and are currently being addressed in AAPM TG-284 for MR simulators and MRIs sited in Radiation Oncology.…”

Section: Discussionmentioning

confidence: 99%

“…The major sources of distortion in MRI arise from static field inhomogeneity, magnetic susceptibility, chemical shift, and spatial‐encoding gradient nonlinearity (GNL) . Subject‐specific distortions are mainly due to the chemical shift and magnetic susceptibility while GNL and field inhomogeneity are system‐specific causes of distortion. Several factors such as type and field strength of the MR scanner, imaging sequence, and receiver bandwidth can influence the magnitude of distortions from one or more of these sources .…”

Section: Introductionmentioning

confidence: 99%

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Large field of view distortion assessment in a low‐field MR‐linac

Nejad-Davarani

Kim

et al. 2019

Medical Physics

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Purpose MR‐guided radiation therapy (RT) offers unparalleled soft tissue contrast for localization and target tracking. However, MRI distortions may be detrimental to high precision RT. This work characterizes the gradient nonlinearity (GNL) and total distortions over the first year of clinical operation of a 0.35T MR‐linac. Methods For GNL characterization, an in‐house large field of view (FOV) phantom (60 × 42.5 × 55 cm3, >6000 spherical landmarks) was configured and scanned at four timepoints with forward/reverse read polarities (Gradient Echo sequence, FA/TR/TE = 28°/30 ms/6 ms). GNL was measured in Anterior‐Posterior (AP), Left‐Right (LR), and Superior‐Inferior (SI) frequency‐encoding directions based on deviation of the auto‐segmented landmark centroids between rigidly registered MR and CT images and assessed based on radial distance from magnet isocenter. Total distortion was assessed using a 30 × 30 cm2 grid phantom oriented along the cardinal axes over >1 year of operation. Results The scanner's spatial integrity within the first ~10 months was stable (maximum total distortion variation = 10/6/8%, maximum distortion = 1.41/0.99/1.56 mm in Axial/Coronal/Sagittal planes, respectively). GNL distortions measured during this time period <10 cm from isocenter were (−0.74, 0.45), (−0.67, 0.53), and (−0.86, 0.70) mm in AP/LR/SI directions. In the 10‐20 cm range, <1.5% of the distortions exceeded 2 mm in the AP and LR axes while <4% of the distortions exceeded 2 mm for SI. After major repairs and magnet re‐shim, detectable changes were observed in total and GNL distortions (20% reduction in AP and 36% increase in SI direction in the 20–25 cm range). Across all timepoints and axes, 38–53% of landmarks in the 20–25 cm range were displaced by >1 mm. Conclusions GNL distortions were negligible within a 10 cm radius from isocenter. However, in the periphery, non‐negligible distortions of up to ~7 mm were observed, which may necessitate GNL corrections for MR‐IGRT for treatment sites distant from magnet isocenter.

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Design and construction of a customizable phantom for the characterization of the three‐dimensional magnetic resonance imaging geometric distortion

Torfeh

Hammoud

Paloor

et al. 2021

J Applied Clin Med Phys

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One of the main challenges to using magnetic resonance imaging (MRI) in radiotherapy is the existence of system-related geometric inaccuracies caused mainly by the inhomogeneity in the main magnetic field and the nonlinearities of the gradient coils. Several physical phantoms, with fixed configuration, have been developed and commercialized for the assessment of the MRI geometric distortion.In this study, we propose a new design of a customizable phantom that can fit any type of radio frequency (RF) coil. It is composed of 3D printed plastic blocks containing holes that can hold glass tubes which can be filled with any liquid. The blocks can be assembled to construct phantoms with any dimension. The feasibility of this design has been demonstrated by assembling four phantoms with high robustness allowing the assessment of the geometric distortion for the GE split head coil,the head and neck array coil,the anterior array coil,and the body coil. Phantom reproducibility was evaluated by analyzing the geometric distortion on CT acquisition of five independent assemblages of the phantom. This solution meets all expectations in terms of having a robust, lightweight, modular, and practical tool for measuring distortion in three dimensions. Mean error in the position of the tubes was less than 0.2 mm. For the geometric distortion, our results showed that for all typical MRI sequences used for radiotherapy, the mean geometric distortion was less than 1 mm and less than 2.5 mm over radial distances of 150 mm and 250 mm, respectively. These tools will be part of a quality assurance program aimed at monitoring the image quality of MRI scanners used to guide radiation therapy.

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Quantifying the Effect of 3T Magnetic Resonance Imaging Residual System Distortions and Patient-Induced Susceptibility Distortions on Radiation Therapy Treatment Planning for Prostate Cancer

Cited by 35 publications

References 29 publications

Geometric distortion characterization and correction for the 1.0 T Australian MRI‐linac system using an inverse electromagnetic method

Geometric distortion characterization and correction for the 1.0 T Australian MRI‐linac system using an inverse electromagnetic method

Large field of view distortion assessment in a low‐field MR‐linac

Design and construction of a customizable phantom for the characterization of the three‐dimensional magnetic resonance imaging geometric distortion

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