Radiotherapy planning of the pelvis using distortion corrected MR images: the removal of system distortions

Tanner, Steven F.; Finnigan, David J.; Khoo, Vincent; Mayles, Philip; Dearnaley, David P.; Leach, Martin O.

doi:10.1088/0031-9155/45/8/305

Cited by 57 publications

(59 citation statements)

References 12 publications

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“…Our previous work 3 has shown limitations of one such commercial diagnostic phantom to adequately determine distortion across all 3 imaging planes, hence missing regions where patient anatomy may need to be imaged for RTP. A number of studies have developed their own distortion phantoms, for specific applications [24][25][26][27][28] .…”

Section: Introductionmentioning

confidence: 99%

Continuous table acquisition MRI for radiotherapy treatment planning: Distortion assessment with a new extended 3D volumetric phantom

et al. 2015

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(2015). Continuous table acquisition MRI for radiotherapy treatment planning: Distortion assessment with a new extended 3D volumetric phantom. Medical Physics, 42 (4), 1982Physics, 42 (4), -1991 Continuous table acquisition MRI for radiotherapy treatment planning: distortion assessment with a new extended 3D volumetric phantom AbstractPurpose: Accurate geometry is required for radiotherapy treatment planning (RTP). When considering the use of magnetic resonance imaging (MRI) for RTP, geometric distortions observed in the acquired images should be considered. While scanner technology and vendor supplied correction algorithms provide some correction, large distortions are still present in images, even when considering considerably smaller scan lengths than those typically acquired with CT in conventional RTP. This study investigates MRI acquisition with a moving table compared with static scans for potential geometric benefits for RTP. Methods: A full field of view (FOV) phantom (diameter 500 mm; length 513 mm) was developed for measuring geometric distortions in MR images over volumes pertinent to RTP. The phantom consisted of layers of refined plastic within which vitamin E capsules were inserted. The phantom was scanned on CT to provide the geometric gold standard and on MRI, with differences in capsule location determining the distortion. MRI images were acquired with two techniques. For the first method, standard static table acquisitions were considered. Both 2D and 3D acquisition techniques were investigated. With the second technique, images were acquired with a moving table. The same sequence was acquired with a static table and then with table speeds of 1.1 mm/s and 2 mm/s. All of the MR images acquired were registered to the CT dataset using a deformable B-spline registration with the resulting deformation fields providing the distortion information for each acquisition. Results: MR images acquired with the moving table enabled imaging of the whole phantom length while images acquired with a static table were only able to image 50%-70% of the phantom length of 513 mm. Maximum distortion values were reduced across a larger volume when imaging with a moving table. Increased table speed resulted in a larger contribution of distortion from gradient nonlinearities in the through-plane direction and an increased blurring of capsule images, resulting in an apparent capsule volume increase by up to 170% in extreme axial FOV regions. Blurring increased with table speed and in the central regions of the phantom, geometric distortion was less for static table acquisitions compared to a table speed of 2 mm/s over the same volume. Overall, the best geometric accuracy was achieved with a table speed of 1.1 mm/s. Conclusions: The phantom designed enables full FOV imaging for distortion assessment for the purposes of RTP. MRI acquisition with a moving table extends the imaging volume in the z direction with reduced distortions which could be useful particularly if considering MR-only planning. If utilizing MR image...

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Section: Introductionmentioning

confidence: 99%

Continuous table acquisition MRI for radiotherapy treatment planning: Distortion assessment with a new extended 3D volumetric phantom

et al. 2015

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“…Two early papers by Schad et al (15,16) discussed the "pincushion" effects seen with 2D phantoms. Subsequent schemes include using a stereotaxic frame as a reference marker (11,14), comparing phantom images from CT and MRI (7,19), and imaging specific MRI phantoms (1,13,17,18) that typically consist of an array of tubes filled with a suitable contrast agent. Mizowaki et al (12) alluded to an important problem with phantom studies: after they assessed the reproducibility of their own phantom-based correction, the authors concluded that such a correction may only be applicable in limited situations.…”

mentioning

confidence: 99%

Use of spherical harmonic deconvolution methods to compensate for nonlinear gradient effects on MRI images

Janke

Zhao

Cowin

et al. 2004

Magnetic Resonance in Med

140

191

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Spatial encoding in MR techniques is achieved by sampling the signal as a function of time in the presence of a magnetic field gradient. The gradients are assumed to generate a linear magnetic field gradient, and typical image reconstruction relies upon this approximation. However, high-speed gradients in the current generation of MRI scanners often sacrifice linearity for improvements in speed. Such nonlinearity results in distorted images. The problem is presented in terms of first principles, and a correction method based on a gradient field spherical harmonic expansion is proposed. In our case, the amount of distortion measured within a typical field of view (FOV) required for head imaging is sufficiently large that without the use of some distortion correction technique, the images would be of The quality of an MR image is dependent upon the accuracy with which the physical position is spatially encoded. Since MRI data are now used routinely for stereotaxy, longitudinal studies of atrophy, and functional studies, it is critical to ensure that images have no distortion or inhomogeneity. The principal machine-dependent sources of this inhomogeneity are eddy currents, gradient nonlinearity, and B 0 and B 1 inhomogeneity (14). In this study we present an analytical approach to calculating and removing the effects of nonlinear gradients only. We chose to focus on a gradient-only solution because of the recent interest in short-bore, high-speed gradients. Although peripheral nerve stimulation is a limiting feature of short rise times, such gradients have been found to be useful in high-speed echo-planar imaging (EPI) of the heart and diffusion tensor imaging (DTI) of the brain. To achieve short rise times and avoid peripheral nerve stimulation, designers have restricted the length of gradients and limited the number of turns in those gradients. Although these constraints are suitable for the implementation of pulse sequences with the desired speed, they have the undesired consequence of increased nonlinearity.Nonlinear pulsed field gradients induce image distortions due to incorrect spatial encoding of the signal. If we assume that the field gradients are linear, it follows that k-space is sampled linearly, and thus the fast Fourier transform (FFT) is suitable for reconstruction. However, any deviation from linearity in the gradients results in nonlinear data sampling and subsequent errors in image spatial encoding. A nonlinear FT would allow these data to be correctly transformed to an image. Unfortunately, nonlinear FT greatly increases computation time by N/log 2 N, relative to an FFT, which makes real-time image generation computationally prohibitive. Here we present a general analytical solution to correct image distortions induced by gradient nonlinearity. The method is applicable to any gradient configuration. It is robust and, more importantly, is based upon an approach whereby the FFT is maintained for image reconstruction.The notion of mapping and correcting such MRI distortions is not new. Two early pa...

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“…Magnetic resonance imaging (MRI) prosimulation routine, we have chosen 154 Hz/pixel in the vides superior image quality for soft-tissue delineation over frequency encoding direction; therefore, the effects caused computed tomography (CT) and is widely used for target by chemical and susceptibility are considered negligible. and organ delineation in radiotherapy for treatment planning For system-related distortions, we have used gradient dis- (9)(10)(11). The prostate volume on CT appears much larger tortion correction (GDC) software to correct the MRI disthan on MRI (12).…”

Section: Discussionmentioning

confidence: 99%

MR Imaging Based Treatment Planning for Radiotherapy of Prostate Cancer

Chen¹

2007

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Radiotherapy planning of the pelvis using distortion corrected MR images: the removal of system distortions

Cited by 57 publications

References 12 publications

Continuous table acquisition MRI for radiotherapy treatment planning: Distortion assessment with a new extended 3D volumetric phantom

Continuous table acquisition MRI for radiotherapy treatment planning: Distortion assessment with a new extended 3D volumetric phantom

Use of spherical harmonic deconvolution methods to compensate for nonlinear gradient effects on MRI images

MR Imaging Based Treatment Planning for Radiotherapy of Prostate Cancer

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