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

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.

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.

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.

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.

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