Myocardial T2 quantitation in patients with iron overload at 3 Tesla

Abstract

Purpose: To investigate the feasibility of measuring myocardial T2 at 3 Tesla for assessment of tissue iron in thalassemia major and other iron overloaded patients.

Materials and methods: A single-breathhold electrocardiogram-triggered black-blood multi-echo spin-echo (MESE) sequence with a turbo factor of 2 was implemented at 3 Tesla (T). Myocardial and liver T2 values were measured with three repeated breathholds in 8 normal subjects and 24 patients. Their values, together with the T2 values measured using a breathhold multi-echo gradient-echo sequence, were compared with those at 1.5T in the same patients.

Results: At 3T, myocardial T2 was found to be 39.6 +/- 7.4 ms in normal subjects. In patients, it ranged from 12.9 to 50.1 ms. "T2 and T2(*) [corrected] were observed to correlate in heart (rho = 0.93, P [corrected] < 0.0001) and liver (rho = 0.95, P < 0.0001). Myocardial T2 and T2 at 3T were also highly correlated with the 1.5T measurements. Preliminary results indicated that myocardial T2 quantitation was relatively insensitive to B1 variation, and reproducible with 3.2% intra-exam and 3.8% inter-exam variations.

Conclusion: Myocardial T2 quantitation is feasible at 3T. Given the substantially decreased T2 and increased B0 inhomogeneity, the rapid myocardial T2 measurement protocol demonstrated here may present a robust alternative to study cardiac iron overload at 3T.

Figure 1

a: First echo MESE image acquired by the single-breathhold T2 (=1/R2) mapping sequence. b: Relationship between the measured R2 values and MnCl₂ concentrations. The 11 large phantom tubes contain varying MnCl₂ concentrations from 0 to 1 mM with 0.l mM increment, as indicated by the legends. Measurement ROIs are also shown. Linear dependency was clearly seen with R = 0.999 (P < 0.0001).

Figure 2

a: First echo MESE images acquired during the three consecutive breathholds or trials from an iron overloaded patient. Heart and liver measurement ROIs are delineated in yellow. b: Corresponding echo signal decays in the septal myocardium. c: Corresponding echo signal decays in the liver. d,e: Echo signal decays are also plotted in logarithmic scale in the septal myocardium (d) and liver (e). Note that liver ROI is typically chosen to exclude large blood vessels.

Figure 3

Myocardial T2 measurement reproducibility observed in two normal subjects. Intra-exam variations among the three repeats (during the same exam) and inter-exam variations among the three exams (performed on different days within a week) are shown.

Figure 4

Measured myocardial T2 values versus the different RF flip angle scaling factors (with 1.0 corresponding to the automatic scanner calibration based on the total MR signal within imaging slice).

Figure 5

Representative MESE heart images observed in a patient with mild iron overload. Echo time (TE) is given in ms. The corresponding T2 map is shown in Figure 6.

Figure 6

Typical T2 (left column) and T2* (right column) maps from a normal subject (top row), and patients with mild (middle row) and severe (bottom row) iron overload in heart. Display grayscales have a unit of ms.

Figure 7

a: Myocardial T2* and T2 at 3T in 18 iron overloaded patients and 2 normal subjects. b: Liver T2* versus T2 in 16 patients and 2 normal subjects. c: The 3T T2* versus 1.5T T2* in heart in 18 patients. d: The 3T T2 versus 1.5T T2 in heart in 19 patients. The dashed lines indicate the 95% limits.