Optimization of a novel large field of view distortion phantom for MR‐only treatment planning

Price, Ryan; Knight, Robert A.; Hwang, Ken Pin; Bayram, Ersin; Nejad-Davarani, Siamak P.; Glide-Hurst, Carri K

doi:10.1002/acm2.12090

Cited by 23 publications

(57 citation statements)

References 28 publications

(43 reference statements)

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“…Our measurements in the MR‐linac resulted in outcomes similar to those measured by our group in other MR scanners. GNL distortion across three different MRI platforms was found to be <1 mm within 10 cm of magnet isocenter along all three axes, which is comparable to the distortion measured in the 0.35T MR‐linac. Similarly, distortion along the LR and SI directions started to increase above 1 mm in all scanners as the radial distance from the isocenter increased; however, for the 1.5T and 3T scanners the AP distortions stayed almost within 1 mm.…”

Section: Discussionsupporting

confidence: 65%

“…In this work, we characterized GNL and total distortions of a 0.35T MR‐linac over the course of 1 yr to assess the necessity of geometric distortion correction in MR‐IGRT applications, as well as to explore the temporal stability of geometric distortions. For measuring GNL distortions, we used an in‐house large FOV modular phantom to isolate these distortions in the LR‐RL, AP‐PA, and SI‐IS directions. Our measurements in the MR‐linac resulted in outcomes similar to those measured by our group in other MR scanners.…”

Section: Discussionmentioning

confidence: 99%

“…An in‐house modular large FOV phantom was configured to fit the magnet bore. The details of phantom design and implementation have been previously reported . Briefly, the phantom is constructed of layers of low‐density polyurethane foam boards (6 lbs/ft 3 , 2.5 cm thick) with each board consisting of linear arrays of 6 mm diameter paintballs spaced 2.5 cm apart for signal generation.…”

Section: Methodsmentioning

confidence: 99%

“…[19][20][21] Several phantom-based methods have been developed for measurement and correction of GNL effects. [21][22][23][24] While we have previously reported characterization of GNL at varied MRI field strengths using a modular in-house large field of view (FOV) phantom, 25 the characterization and understanding of GNL for MR-IGRT is currently limited. This work sought to characterize the GNL distortion across a large FOV for a 0.35T MR-linac, with the overarching goal of quantifying the GNL to assess the necessity of correcting GNL-based distortions for MRI-only planning in an MR-IGRT clinical workflow.…”

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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Section: Discussionsupporting

confidence: 65%

Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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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“…The study of distortion corrections is of particular interest nowadays, especially in the domain of radiotherapy planning . The visualization (and thus the assessment) of the vendor distortion correction could be of interest for a number of applications, which include: (a) software packages dedicated to the distortion characterization and (b) the development of improved distortion correction strategies, such those using higher degree spherical harmonic models of the gradient system, for instance. It should also be pointed out that, for this specific application, the role of the virtual (grid) phantom is not to replace a physical phantom, but to provide information that is complementary to that of the physical phantom.…”

Section: Discussionmentioning

confidence: 99%

Virtual phantom magnetic resonance imaging (ViP MRI) on a clinical MRI platform

2017

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Purpose: The purpose of this study was to implement Virtual Phantom Magnetic Resonance Imaging (ViP MRI), a technique that allows for generating reference signals in MR images using radiofrequency (RF) signals, on a clinical MR system and to test newly designed virtual phantoms. Methods: MRI experiments were conducted on a 1.5 T MRI scanner. Electromagnetic modelling of the ViP system was done using the principle of reciprocity. The ViP RF signals were generated using a compact waveform generator (dimensions of 26 cm 9 18 cm 9 16 cm), connected to a homebuilt 25 mm-diameter RF coil. The ViP RF signals were transmitted to the MRI scanner bore, simultaneously with the acquisition of the signal from the object of interest. Different types of MRI data acquisition (2D and 3D gradient-echo) as well as different phantoms, including the Shepp-Logan phantom, were tested. Furthermore, a uniquely designed virtual phantom -in the shape of a gridwas generated; this newly proposed phantom allows for the investigations of the vendor distortion correction field.Results: High quality MR images of virtual phantoms were obtained. An excellent agreement was found between the experimental data and the inverse cube law, which was the expected functional dependence obtained from the electromagnetic modelling of the ViP system. Short-term time stability measurements yielded a coefficient of variation in the signal intensity over time equal to 0.23% and 0.13% for virtual and physical phantom, respectively. MR images of the virtual grid-shaped phantom were reconstructed with the vendor distortion correction; this allowed for a direct visualization of the vendor distortion correction field. Furthermore, as expected from the electromagnetic modelling of the ViP system, a very compact coil (diameter~cm) and very small currents (intensity~mA) were sufficient to generate a signal comparable to that of physical phantoms in MRI experiments. Conclusions: The ViP MRI technique was successfully implemented on a clinical MR system. One of the major advantages of ViP MRI over previous approaches is that the generation and transmission of RF signals can be achieved with a self-contained apparatus. As such, the ViP MRI technique is transposable to different platforms (preclinical and clinical) of different vendors. It is also shown here that ViP MRI could be used to generate signals whose characteristics cannot be reproduced by physical objects. This could be exploited to assess MRI system properties, such as the vendor distortion correction field.

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Geometric accuracy of the MR imaging techniques in the presence of motion

Torfeh

Hammoud

Kaissi

et al. 2018

J Applied Clin Med Phys

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Magnetic Resonance Imaging (MRI) is increasingly being used for improving tumor delineation and tumor tracking in the presence of respiratory motion. The purpose of this work is to design and build an MR compatible motion platform and to use it for evaluating the geometric accuracy of MR imaging techniques during respiratory motion. The motion platform presented in this work is composed of a mobile base made up of a flat plate and four wheels. The mobile base is attached from one end and through a rigid rod to a synchrony motion table by Accuray® placed at the end of the MRI table and from the other end to an elastic rod. The geometric accuracy was measured by placing a control point‐based phantom on top of the mobile base.In‐house software module was used to automatically assess the geometric distortion. The blurring artifact was also assessed by measuring the Full Width Half Maximum (FWHM) of each control point. Our results were assessed for 50, 100, and 150 mm radial distances, with a mean geometric distortion during the superior–inferior motion of 0.27, 0.41, and 0.55 mm, respectively. Adding the anterior–posterior motion, the mean geometric distortions increased to 0.4, 0.6, and 0.8 mm. Blurring was observed during motion causing an increase in the FWHM of ≈30%. The platform presented in this work provides a valuable tool for the assessment of the geometric accuracy and blurring artifact for MR during motion. Although the main objective was to test the spatial accuracy of an MR system during motion, the modular aspect of the presented platform enables the use of any commercially available phantom for a full quality control of the MR system during motion.

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Optimization of a novel large field of view distortion phantom for MR‐only treatment planning

Cited by 23 publications

References 28 publications

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

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

Virtual phantom magnetic resonance imaging (ViP MRI) on a clinical MRI platform

Geometric accuracy of the MR imaging techniques in the presence of motion

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