Transformer-based synthesis of 7 Tesla-like T1 contrast from routine clinical magnetic resonance imaging.

Abstract

Background: Ultra-high-field 7 Tesla (7T) magnetic resonance imaging (MRI) provides improved resolution and signal-to-noise ratio (SNR) over standard clinical field strengths (1.5T, 3T). However, 7T scanners are costly, scarce, and introduce additional challenges such as susceptibility artifacts.

Purpose: We propose an efficient transformer-based model (7T-Restormer) to synthesize 7T-like MRI (quantitative T1 MP2RAGE maps) from routine 1.5T or 3T T1-weighted (T1W) images.

Methods: The proposed method leverages an efficient restoration transformer backbone and spatial attention layers to capture long-range dependencies and generate high-quality 7T-like images. Our model was validated on an institutional dataset, comprised of 35 1.5T and 108 3T T1w MRI paired with corresponding 7T T1 maps of patients with confirmed multiple sclerosis (MS). A total of 141 patient cases (32 128 slices) were randomly divided into 105 (25 1.5T and 80 3T) training cases (19 204 slices), 19 (5 1.5T and 14 3T) validation cases (3476 slices), and 17 (5 1.5T and 12 3T) test cases (3145 slices). The synthetic 7T T1 maps were evaluated by comparing their similarity to the ground truth volumes using peak signal-to-noise ratio (PSNR), structural similarity index measure (SSIM), and normalized mean squared error (NMSE), and were compared against ResViT and ResShift. In addition, a blinded reader study was performed in which three clinicians scored the diagnostic image quality of each method on a 5-point Likert scale (1 = non-acceptable, 5 = excellent). Statistical significance (α = 0.05) was assessed using two-sided paired t-tests with Holm correction for multiple comparisons and Wilcoxon signed-rank tests with Holm correction for reader scores. Effect sizes (Cohen's $d_z$ for paired data) were also computed to quantify practical significance. We interpreted $\mid d_z\mid \approx 0.2,0.5,$ and $0.8$ as small, medium, and large effects.

Results: For 1.5T inputs, 7T-Restormer achieved an NMSE of 0.018 ± 0.003, PSNR of 25.0 ± 0.6 dB, and SSIM of 0.870 ± 0.019; for 3T inputs, the corresponding values were 0.019 ± 0.006, 24.5 ± 1.2 dB, and 0.874 ± 0.027. Across both field strengths, 7T-Restormer provided the lowest NMSE and highest PSNR and SSIM among the three methods, reducing NMSE by approximately 5% and 25% relative to ResShift and ResViT at 1.5T and by 14% and 24% at 3T, respectively, and increasing PSNR by 0.5-1.5 dB (Holm-adjusted p < 0.05, paired Cohen's $d_z$ = 1.37-6.84). Training on mixed 1.5T+3T data improved performance for 1.5T inputs compared with 1.5T-only training (e.g., NMSE 0.018 vs. 0.019; p = 0.001, $d_z=-4.29$ ), without degrading performance for 3T inputs (p = 0.630, $d_z=-0.14$ for NMSE). In the blinded reader study, 7T-Restormer received higher diagnostic quality scores (3.50 ± 0.28) than ResShift (3.00 ± 0.27) and ResViT (2.33 ± 0.27), corresponding to mean improvements of 0.5-1.2 points on the 5-point scale (Holm-adjusted p = 0.014 and 0.004; paired Cohen's $d_z=1.27$ and 3.24, respectively).

Conclusion: We propose a novel method for predicting quantitative 7T MP2RAGE maps from 1.5T and 3T T1W scans with higher quality than existing state-of-the-art methods. Future development and application of our approach may enhance diagnostic accuracy, treatment planning, and facilitate downstream tasks by making the benefits of 7T MRI more accessible to standard clinical workflows.