This paper evaluates CycleGAN and Pix2Pix models to synthesize missing T1/T2 brain MRI across HCP, PPMI and an MS cohort, showing Pix2Pix often outperforms CycleGAN and that synthesized T1 images can be used for lesion detection, volumetry, and texture analysis in MS.

Direct therapeutic relevance to Parkinson's is limited, but the demonstrated ability to synthesize missing MRI contrasts (validated on PPMI data) can help recover incomplete imaging datasets and enable quantitative biomarker or longitudinal imaging studies in PD research.

BACKGROUND: Secondary quantitative analysis of brain magnetic resonance imaging (MRI) can provide valuable information for many neurological diseases, including multiple sclerosis (MS), but it demands complete datasets that are often unavailable clinically. We investigated how image synthesis via deep learning using cycle-consistent generative adversarial networks (CycleGANs) compared with Pix2Pix as a related method, based on T1-weighted and T2-weighted brain MRI in MS, following verification on two streamlined datasets. The synthesized images were also evaluated against the source data. METHODS: The streamlined datasets involved 1,113 healthy participants from the Human Connectome Project (HCP) and 318 participants from the Parkinson's Progression Markers Initiative (PPMI). The MS cohort in this study included 105 participants scanned with different protocols. Image synthesis was bidirectional between T1- and T2-weighted MRI using CycleGAN with and without spectral normalization, as well as Pix2Pix. Utility testing focused on T1-weighted MRI that was most often unavailable in MS, and that involved lesion detection, brain volumetry, and lesion texture analysis. RESULTS: All CycleGAN models performed competitively, while Pix2Pix performed better, mostly with streamlined datasets (p < 0.001). The average peak signal-to-noise ratio ranged from 24.860-28.570 versus 28.520-31.100, and the structural similarity index ranged from 0.838-0.901 versus 0.924-0.943. With spectral normalization, CycleGAN improved in PPMI but not in HCP and generally not in MS (p < 0.001). Furthermore, the synthesized images showed high similarity to the source data in utility tests, although Pix2Pix T1 images appeared more heterogeneous in lesion texture than source T1 images. CONCLUSION: CycleGAN without spectral normalization appeared feasible for synthesizing common clinical brain MRI, including T1-weighted images usable for subsequent quantitative analysis in MS.