A systematic review of synthetic CT generation methodologies for use in MRI-only radiotherapy

Publication date: Available online 8 September 2017
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): Emily Johnstone, Jonathan J. Wyatt, Ann M. Henry, Susan C. Short, David Sebag-Montefiore, Louise Murray, Charles G. Kelly, Hazel M. McCallum, Richard Speight
MRI offers superior soft tissue contrast as compared to CT, which is conventionally used for radiotherapy treatment planning (RTP) and patient positioning verification, resulting in improved target definition. The two modalities are co-registered for RTP, however this introduces a systematic error. Implementing an MRI-only radiotherapy workflow would be advantageous as this error would be eliminated, the patient pathway simplified and patient dose reduced. Unlike CT, in MRI there is no direct relationship between signal intensity and electron density, however various methodologies for MRI-only RTP have been reported. A systematic review of these methods was undertaken.The PRISMA guidelines(1) were followed. Embase and Medline databases were searched (1996-03/2017) for studies which generated synthetic CTs (sCT)s for MRI-only radiotherapy. 61 articles met the inclusion criteria.This review showed that MRI-only RTP techniques could be grouped into three categories: i]bulk density override ii]atlas-based and iii]voxel-based techniques, which all produce an sCT scan from MR image(s).Bulk density override techniques either used a single homogeneous or multiple tissue override. The former produced large dosimetric errors (>2%) in some cases and the latter frequently required manual bone contouring. Atlas-based techniques used both single and multiple atlases and included methods incorporating pattern recognition techniques. Clinically acceptable sCTs were reported, but atypical anatomy led to erroneous results in some cases. Voxel-based techniques included methods using routine and specialised MRI sequences, namely ultra-short echo time imaging. High quality sCTs were produced, however use of multiple sequences led to long scanning times increasing the chances of patient movement. Using non-routine sequences would currently be problematic in most radiotherapy centres.Atlas-based and voxel-based techniques were found to be the most clinically useful methods, with some studies reporting dosimetric differences of <1% between planning on the sCT and CT and <1mm deviations when using sCTs for positional verification.



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