ACCELERATED ANKLE MRI PROTOCOL USING COMBINED VARIABLE DENSITY UNDERSAMPLING OF K-SPACE WITH ECHO SPACING OPTIMIZATION le Density

Abstract

PURPOSE

To compare image quality and acquisition times for a commercial deep-learning reconstruction technique with a new technique that combines a variable density undersampling of k-space with echo spacing optimization for 3-fold acceleration in ankle MR.

METHODS

In this prospective study, 28 patients underwent ankle MR exams as part of standard clinical care using our institution’s routine protocol on a GE Premier 3T magnet with a protocol employing a commercialized DL Recon algorithm (AIR™ Recon DL, GE HealthCare), which is based on a Convolutional Neural Network (CNN) trained to remove ringing, reduce noise, and increase sharpness. Each patient underwent an additional 3-fold accelerated MRI exam which combined 2 features: a vendor-supplied prototype known as Sonic DLTM 2D, which is an acquisition and reconstruction technique that combines highly accelerated variable density undersampled data acquisition with an unrolled DL-based reconstruction and echo spacing optimization, which allows for longer echo train lengths (ETL) and SAR reduction. Commercial AIR Recon DL images were compared to the accelerated images. Reviewers evaluated lateral ankle ligaments, as well as tibiotalar articular cartilage and subchondral bone for pathology. Reviewers also performed a subjective evaluation of image quality, assessment of main pathology, and diagnostic confidence in commercial and accelerated images. Image-quality Likert scores were compared between protocols using Wilcoxon signed-rank tests; inter-reader agreement was quantified with Gwet's AC2 for image quality and diagnostic confidence, and with weighted Cohen's κ for lesion assessment.

RESULTS

Mean acquisition time was 705.5 ± 91.5 s for the standard protocol versus 471.1 ± 52.1 s for the accelerated protocol (33% reduction). Image-quality Likert scores did not differ between protocols for any reader (Wilcoxon signed-rank, p = 0.21, 0.10, 0.65). Inter-reader agreement (Gwet's AC2) for image-quality was excellent in both protocols (standard 0.90 [95% CI 0.81–0.99]; accelerated 0.83 [0.73–0.93]); diagnostic confidence agreement was good to excellent (0.79 [0.67–0.92] vs 0.77 [0.62–0.92] respectively). For diagnostic lesion assessment, inter-reader agreement evaluated using Cohen’s kappa was consistently good to excellent across all structures. For standard sequences, kappa values ranged from 0.72 (95% CI: 0.54–0.90) for calcaneofibular ligament (CFL) injury to 0.93 (CI: 0.84–1.00) for subchondral edema, with anterior talofibular ligament (ATFL) injury (κ = 0.84 CI: 0.71–0.97), cartilage (κ = 0.86 CI: 0.70–1.00), and subchondral cyst (κ = 0.89, CI: 0.76–1.00) also demonstrating high agreement. For accelerated sequences, kappa values ranged from 0.80 (CI: 0.65–0.95) for CFL to 0.97 (CI: 0.89–1.00) for subchondral cysts, with ATFL (κ = 0.87, CI: 0.75–0.98), cartilage (κ = 0.87, CI: 0.71–1.00), and subchondral edema (κ = 0.93, CI: 0.84–1.00).

CONCLUSION

This accelerated protocol shortens ankle MRI by approximately 4 minutes while preserving image quality and inter-reader diagnostic agreement, enhancing patient comfort and MRI throughput.