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

INTRODUCTION

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

AIM

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

METHOD

A T mapping phantom was manufactured to contain 9 T and T (T|T) tubes to mimic clinically relevant native and post-contrast T 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 and B uniformity using field maps, and temperature dependence. Baseline reference T|T 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|T was determined by repeated T|T mapping of the phantom at baseline and at 12 months.

RESULTS

The phantom embodies 9 internal agarose-containing T|T tubes doped with nickel di-chloride (NiCl) as the paramagnetic relaxation modifier to cover the clinically relevant spectrum of myocardial T. The tubes are surrounded by an agarose-gel matrix which is doped with NiCl 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 by balanced steady-state free precession T mapping compared to reference SE T with the exception of the post-contrast tube of ultra-low T 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 and B 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|T tubes. Excellent long-term (12-month) reproducibility of measured T|T was demonstrated across both field strengths (all coefficients of variation < 1.38%).

CONCLUSION

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