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

IntroductionA 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.AimTo fabricate and assess a phantom dedicated to the quality assurance of T-2 mapping in CMR.MethodA 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.ResultsThe phantom embodies 9 internal agarose-containing T-1|T-2 tubes doped with nickel di-chloride (NiCl2) 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 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 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 degrees 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%).ConclusionThe 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.