Abstract
PurposeMagnetic resonance imaging (MRI) is the primary modality for targeting brain tumors in radiotherapy treatment planning (RTP). MRI is not directly used for dose calculation since image voxel intensities of MRI are not associated with EDs of tissues as those of computed tomography (CT). The purpose of the present study is to develop and evaluate a tissue segmentation‐based method to generate a synthetic‐CT (sCT) by mapping EDs to corresponding tissues using only T1‐weighted MR images for MR‐only RTP.MethodsAir regions were contoured in several slices. Then, air, bone, brain, cerebrospinal fluid (CSF), and other soft tissues were automatically segmented with an in‐house algorithm based on edge detection and anatomical information and relative intensity distribution. The intensities of voxels in each segmented tissue were mapped into their CT number range to generate a sCT. Twenty‐five stereotactic radiosurgery and stereotactic ablative radiotherapy patients’ T1‐weighted MRI and coregistered CT images from two centers were retrospectively evaluated. The CT was used as ground truth. Distances between bone contours of the external skull of sCT and CT were measured. The mean error (ME) and mean absolute error (MAE) of electron density represented by standardized CT number was calculated in HU.ResultsThe average distance between the contour of the external skull in sCT and the contour in coregistered CT is 1.0 ± 0.2 mm (mean ± 1SD). The ME and MAE differences for air, soft tissue and whole body voxels within external body contours are −4 HU/24 HU, 2 HU/26 HU, and −2 HU/125 HU, respectively.ConclusionsA MR‐sCT generation technique was developed based on tissue segmentation and voxel‐based tissue ED mapping. The generated sCT is comparable to real CT in terms of anatomical position of tissues and similarity to the ED assignment. This method provides a feasible method to generate sCT for MR‐only radiotherapy treatment planning.
Highlights
Magnetic resonance imaging (MRI) is the modality of choice for defining brain tumor volume in precision radiotherapy techniques such as stereotactic radiosurgery (SRS) and stereotactic ablative radiotherapy (SABR) because MRI provides high soft‐tissue contrast, functional information, and high resolution, which is superior to what can be provided by a planning computed tomography (CT)
An in‐house algorithm developed with IDL8.7 (ITT Visual Information Solutions) calculated the statistic mean and standard deviation of MR intensity values of the air, generated an air mask by subtracting soft tissues enclosed in the air regions
Bone segmentation in this study mainly focused on trabecular bone because the volume of trabecular bone can be quite large for some patients due to aging and pathological effect
Summary
Magnetic resonance imaging (MRI) is the modality of choice for defining brain tumor volume in precision radiotherapy techniques such as stereotactic radiosurgery (SRS) and stereotactic ablative radiotherapy (SABR) because MRI provides high soft‐tissue contrast, functional information, and high resolution, which is superior to what can be provided by a planning CT. MRI alone is not sufficient in radiotherapy treatment planning (RTP) due to its lack of electron density (ED) information in its image voxel intensity values for radiation dose calculation. The radiation dose has to be calculated using CT images which are registered with MR images. This image registration process will introduce errors in defined tumor volumes.[1] these errors are usually small due to the rigid structure of the skull, image registration errors can be significant in some cases when MR and CT scans have differences in positions or setup.[2] This may lead to geometrical miss of target volumes as the target volumes are usually small and increase in dose to nearby critical organs. MR‐only treatment planning will eliminate image registration error, minimize patient setup error, and reduce unnecessary radiation to patients from multiple CT scans
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