Developing a medical device-grade T 2 phantom optimized for myocardial T 2 mapping by cardiovascular magnetic resonance.

Introduction: A long T ₂ relaxation time can reflect oedema, and myocardial inflammation when combined with increased plasma troponin levels. Cardiovascular magnetic resonance (CMR) T ₂ mapping therefore has potential to provide a key diagnostic and prognostic biomarkers. However, T ₂ varies by scanner, software, and sequence, highlighting the need for standardization and for a quality assurance system for T ₂ mapping in CMR.
Aim: To fabricate and assess a phantom dedicated to the quality assurance of T ₂ mapping in CMR.
Method: A T ₂ mapping phantom was manufactured to contain 9 T ₁ and T ₂ (T ₁ |T ₂ ) tubes to mimic clinically relevant native and post-contrast T ₂ 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, B ₀ and B ₁ uniformity using field maps, and temperature dependence. Baseline reference T ₁ |T ₂ 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 T ₁ |T ₂ was determined by repeated T ₁ |T ₂ mapping of the phantom at baseline and at 12 months.
Results: The phantom embodies 9 internal agarose-containing T ₁ |T ₂ tubes doped with nickel di-chloride (NiCl ₂ ) as the paramagnetic relaxation modifier to cover the clinically relevant spectrum of myocardial T ₂ . The tubes are surrounded by an agarose-gel matrix which is doped with NiCl ₂ 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 T ₂ by balanced steady-state free precession T ₂ mapping compared to reference SE T ₂ with the exception of the post-contrast tube of ultra-low T ₁ 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 B ₀ and B ₁ magnetic fields in the imaged slice. Independent temperature dependency experiments over the 13-38 °C range confirmed the greater stability of shorter vs longer T ₁ |T ₂ tubes. Excellent long-term (12-month) reproducibility of measured T ₁ |T ₂ was demonstrated across both field strengths (all coefficients of variation < 1.38%).
Conclusion: The T ₂ mapping phantom demonstrates excellent structural integrity, B ₀ and B ₁ uniformity, and reproducibility of its internal tube T ₁ |T ₂ out to 1 year. This device may now be mass-produced to support the quality assurance of T ₂ mapping in CMR.
(© 2023. The Author(s).)