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.

Keywords: Phantom; Quality control; T1 mapping; T2 mapping.

Fig. 1

i Schematic (not to scale) showing the internal and external phantom structure. ii Phantom front view showing isocenter line and liquid crystal display thermometer. HDPE = high-density polyethylene; PVC = polyvinyl chloride; PC = polycarbonate

Fig. 2

i T₁ and T₂ (in ms) in the T₂ phantom (n = 1) as measured at 1.5 T and 3 T: slow scan reference T1 obtained using inversion recovery (IR) gradient echo (GRE) (purple) and reference T₂ using single echo (SE) (orange); T1 via modified Look-Locker inversion recovery (MOLLI) T₁ mapping (green) and T₂ via balanced steady state free precession (bSSFP) T₂ mapping (blue); T₂ obtained by the manufacturer in Australia using a 1.4 T Bruker minispec relaxometer at 22 °C (red). Tube arrangement is such that the more temperature-dependent and therefore unstable long-T₁ tubes are away from the corners and towards the middle of the 3 × 3 array. ii Exemplar T₂ and T₁ maps on a Siemens 3 T Prisma clinical CMR scanner. ID = tube identity

Fig. 3

i B₀ field homogeneity across the nine phantom compartments as a measure of off-resonance in Hertz (Hz) at 1.5 T (blue) and 3 T (green) are shown (bottom). The associated B₀ field maps with the field of view capturing the whole phantom at 1.5 T and 3 T are also presented (top–tube positions are overlaid in red). ii) B₁ field homogeneity across the nine phantom compartments as a measure of the FA (in degrees) at 1.5 T (red) and 3 T (blue) are shown (bottom). These represent small shifts in FA or frequency (e.g.,10 Hz = 0.08 ppm at 3 T) and should not be regarded as significantly different between the tube compartments. As expected, the variation of relative FA is larger at 3 T (0.590–0.656) compared to 1.5 T (0.849–0.866). The associated B₁ field maps of at 1.5 T and 3 T are also presented (top–tube positions are overlaid in red). FA flip angle. Other abbreviations as in Fig. 2

Fig. 4

Comparison of T₂ obtained by reference (long-TR) SE sequences (yellow) versus bSSFP T₂ mapping (grey) at 1.5 T (Siemens Aera, left) and 3 T (Siemens Prisma, right) on the final phantom (n = 1) at baseline. TR repetition time. Other abbreviations as in Fig. 2

Fig. 5

Temperature tests carried out at PTB–German Physikalisch-Technische Bundesanstalt (left)–using a 3 T Siemens Magnetom Verio (VB17) and a 12-channel head coil and at NIST–US National Institute of Standards and Technology (right)–using an Agilent 3 T small bore scanner. T₁ was measured by IRSE, and T₂ by SE. The measurements were performed on the final phantom (n = 1) at baseline. TE echo time. Other abbreviations as in Fig. 2

Fig. 6

Short-term reproducibility of T₂ at 1.5 T (left) and 3 T (right) acquired using T₂ mapping bSSFP repeated 3 times in each of the final prototypes #Ci (at a temperature of 22°) (n = 1) and #Cii (at 21°) (n = 1) manufactured months apart, from independent stock solutions. All these scans were performed on the same day with independent placement of phantom and shims. Coefficients of variation (CoV) of T₂ are shown per tube and were all < 1% in the absence of temperature correction. CoV for T₁ using 3 MOLLI repeats are not shown here but were also < 1% for both prototypes (1.5 T range: 0.13–0.94%; 3 T range: 0.03–0.38%)