Female pelvic synthetic CT generation based on joint intensity and shape analysis

Abstract

Using MRI for radiotherapy treatment planning and image guidance is appealing as it provides superior soft tissue information over CT scans and avoids possible systematic errors introduced by aligning MR to CT images. This study presents a method that generates Synthetic CT (MRCT) volumes by performing probabilistic tissue classification of voxels from MRI data using a single imaging sequence (T1 Dixon). The intensity overlap between different tissues on MR images, a major challenge for voxel-based MRCT generation methods, is addressed by adding bone shape information to an intensity-based classification scheme. A simple pelvic bone shape model, built from principal component analysis of pelvis shape from 30 CT image volumes, is fitted to the MR volumes. The shape model generates a rough bone mask that excludes air and covers bone along with some surrounding soft tissues. Air regions are identified and masked out from the tissue classification process by intensity thresholding outside the bone mask. A regularization term is added to the fuzzy c-means classification scheme that constrains voxels outside the bone mask from being assigned memberships in the bone class. MRCT image volumes are generated by multiplying the probability of each voxel being represented in each class with assigned attenuation values of the corresponding class and summing the result across all classes. The MRCT images presented intensity distributions similar to CT images with a mean absolute error of 13.7 HU for muscle, 15.9 HU for fat, 49.1 HU for intra-pelvic soft tissues, 129.1 HU for marrow and 274.4 HU for bony tissues across 9 patients. Volumetric modulated arc therapy (VMAT) plans were optimized using MRCT-derived electron densities, and doses were recalculated using corresponding CT-derived density grids. Dose differences to planning target volumes were small with mean/standard deviation of 0.21/0.42 Gy for D0.5cc and 0.29/0.33 Gy for D99%. The results demonstrate the accuracy of the method and its potential in supporting MRI only radiotherapy treatment planning.

Dose evaluation of fast synthetic-CT generation using a generative adversarial network for general pelvis MR-only radiotherapy

“…When focusing on the pelvis area, many methods have been proposed and evaluated for radiotherapy of prostate cancer (Edmund andNyholm 2017, Johnstone et al 2017), showing dose deviations below 2% with respect to CT-based dose calculations. Only three contributions investigated the accuracy of MR-based dose calculations for locations other than prostate within the pelvic area (Kemppainen et al 2017, Liu et al 2017, Wang et al 2018. Moreover, no attention has been specifically dedicated to the time required to generate sCT, which should be of the order of minutes to allow daily replanning during MRgRT as, for example, underlined by Raaymakers et al (2017).…”

Dosimetric evaluation of an atlas‐based synthetic CT generation approach for MR‐only radiotherapy of pelvis anatomy

“…Various methods for generating synthetic CT images for pelvic anatomy have been introduced in the literature. [2][3][4][5][6][7][8][9][10] Among all these promising approaches, MRCAT (MR for Calculating ATtenuation) available on 3T Philips Ingenia platform 11 is one of the few commercial products 11,12 being used in our clinic for MR-only radiation therapy. 13 MRCAT CT is generated from a 3D mDixon fast field dual echo sequence by creation of three distinct images: water only, fat only, and in-phase MRI.…”

A novel anthropomorphic multimodality phantom for MRI‐based radiotherapy quality assurance testing

“…13,14 However, most commercially available phantoms do not mimic both MRI/CT tissue imaging characteristics for different human organs making it challenging to comprehensively test MRI-based processes such as synthetic CT (sCT) image generation. [15][16][17][18] Noncommercial multimodal MRI/CT phantoms have been developed to test MRI-based radiotherapy workflows.…”