Developing a medical device-grade T2 phantom optimized for myocardial T2 mapping by cardiovascular magnetic resonance

Abstract

Introduction

A long T₂ relaxation time can reflect oedema, and myocardial inflammation when combined with increased plasma troponin levels. Cardiovascular magnetic resonance (CMR) T₂ mapping therefore has potential to provide a key diagnostic and prognostic biomarkers. However, T₂ varies by scanner, software, and sequence, highlighting the need for standardization and for a quality assurance system for T₂ mapping in CMR.

Aim

To fabricate and assess a phantom dedicated to the quality assurance of T₂ mapping in CMR.

Method

A T₂ mapping phantom was manufactured to contain 9 T₁ and T₂ (T₁|T₂) tubes to mimic clinically relevant native and post-contrast T₂ in myocardium across the health to inflammation spectrum (i.e., 43–74 ms) and across both field strengths (1.5 and 3 T). We evaluated the phantom’s structural integrity, B₀ and B₁ uniformity using field maps, and temperature dependence. Baseline reference T₁|T₂ were measured using inversion recovery gradient echo and single-echo spin echo (SE) sequences respectively, both with long repetition times (10 s). Long-term reproducibility of T₁|T₂ was determined by repeated T₁|T₂ mapping of the phantom at baseline and at 12 months.

Results

The phantom embodies 9 internal agarose-containing T₁|T₂ tubes doped with nickel di-chloride (NiCl₂) as the paramagnetic relaxation modifier to cover the clinically relevant spectrum of myocardial T₂. The tubes are surrounded by an agarose-gel matrix which is doped with NiCl₂ and packed with high-density polyethylene (HDPE) beads. All tubes at both field strengths, showed measurement errors up to ≤ 7.2 ms [< 14.7%] for estimated T₂ by balanced steady-state free precession T₂ mapping compared to reference SE T₂ with the exception of the post-contrast tube of ultra-low T₁ where the deviance was up to 16 ms [40.0%]. At 12 months, the phantom remained free of air bubbles, susceptibility, and off-resonance artifacts. The inclusion of HDPE beads effectively flattened the B₀ and B₁ magnetic fields in the imaged slice. Independent temperature dependency experiments over the 13–38 °C range confirmed the greater stability of shorter vs longer T₁|T₂ tubes. Excellent long-term (12-month) reproducibility of measured T₁|T₂ was demonstrated across both field strengths (all coefficients of variation < 1.38%).

Conclusion

The T₂ mapping phantom demonstrates excellent structural integrity, B₀ and B₁ uniformity, and reproducibility of its internal tube T₁|T₂ out to 1 year. This device may now be mass-produced to support the quality assurance of T₂ mapping in CMR.