Fast T2 mapping of short-T2 tissues in knee using 3D radial dual-echo balanced steady-state free precession

Abstract

Background: T₂ mapping of short-T₂ tissues in the knee (meniscus, tendon, and ligament) is needed to aid the clinical MRI knee diagnosis, which is hard to realize using traditional clinical methods.

Purpose: To accelerate the acquisition of T₂ values for short-T₂ tissues in the knee by analyzing the signal equation of balanced steady-state free precession (bSSFP) sequence in MRI.

Methods: Effect of half-radial acquisition on pixel bandwidth was analyzed mathematically. A modified 3D radial dual-echo bSSFP sequence was proposed for 0.53 mm isotropic resolution knee imaging with 2 different TEs at 3 T, which alleviated the problem of off-resonance artifacts caused by traditional half-radial acquisition scheme. A novel pixel-based optimization method was proposed for efficient T₂ mapping of short-T₂ tissues in the knee given off-resonance values. Simulation was conducted to evaluate the sensitivity of the proposed method to other parameters. Phantom results were compared with 2D spin-echo (SE), and in vivo results were compared with SE and previously studies.

Results: Simulation showed that the proposed method is insensitive to T₁ and B₁ variations (estimation error < 1% for T₁/B₁ error of ±90%), avoiding the need for separated T₁ and B₁ scans. High isotropic resolution knee imaging was achieved using the modified dual-echo bSSFP. The total scan time was within 3.5 min, including a separate off-resonance scan for T₂ measurement. Measured mean T₂ values for phantoms correlated well with SE (R² = 0.99), and no significant difference was observed (P = 0.45). In vivo meniscus T₂ measurements and ligament T₂ measurements agreed with the literature, while tendon T₂ measurements were much lower (31.7% lower for patellar tendon, and 13.5% lower for quadriceps tendon), which might result in its bi-component property.

Conclusions: The proposed method provides an efficient way for fast, robust, high-resolution imaging and T₂ mapping of short-T₂ tissues in the knee.

Keywords: 3D radial; Knee; Short-T(2) tissue; T(2) mapping; bSSFP.