Introduction

Gradient recalled echo (GRE) based magnetic resonance elastography (MRE) is a robust technique.1 When results are suboptimal, most of these technical failures are attributable to low MR signal intensity within the liver, most commonly due to iron overload, which leads to T2*-mediated signal loss.2 This occurs even more frequently at 3.0 T, where sensitivity to susceptibility is higher. This is an important issue, because many patients with chronic liver disease have iron overload. To reduce failure rates in the presence of iron, a single-shot spin-echo echoplanar (SE-EPI) based MRE acquisition can be used in place of the conventional GRE-based technique.

Purpose

To compare 2D GRE and 2D SE-EPI-based MRE sequences in the liver at 3.0 T in terms of quantitative liver stiffness (LS) measurement and image quality among patients with mild to severe iron overload.

Methods

This IRB-approved, HIPAA-compliant prospective study involved 20 subjects (12 men, 8 women, mean age 54 years) with mild to severe iron overload (mean R2* 114 sec-1, range 78 – 265 sec-1), who underwent liver MRI at 3.0 T (MAGNETOM Prisma, Siemens Healthcare, Erlangen, Germany), including MRE with both GRE (TR, 50 msec; TE, 22 msec; slice thickness, 5 mm; gap, 1 mm; matrix 77 x 128; phase partial Fourier; flip angle, 25 degrees; breath hold, 18 sec) and SE-EPI prototype (TR, 1600 msec; TE, 49 msec; slice thickness, 5 mm; gap, 1 mm; matrix, 128 x 128; echo train length, 47; flip angle, 90 degrees; breath hold 18 sec) sequences on the same day. R2* values were calculated using a semi-automated multi-echo Dixon technique (Liver Lab, Siemens). Image quality scores (IQS) were evaluated by two independent observers based on wave propagation and right hepatic lobe coverage on the confidence mask (0-15), based on a previously described technique.3 The scoring system is described in Figure 1. One of the two observers also measured LS on stiffness maps (in kilopascal, kPa) for each sequence. Mean LS, region of interest sizes (based on confidence mask) and IQS were compared using paired nonparametric Wilcoxon test. Inter-reader reliability for IQS was assessed using Cohen’s kappa, and IQS were correlated with R2*, using Spearman correlation coefficient.

Results

There were two cases of failure with GRE and one with both GRE and SE-EPI. All failures consisted of anatomical mask failure coincident with very low liver signal on the source images, leading to no liver included on the confidence mask. Mean R2* among these three cases was 171 sec-1. LS measurements were not significantly different between the two sequences (2.29 + 0.46 vs 2.18 + 0.52, P = 0.93). There was a trend toward higher region of interest (ROI) size for SE-EPI (median, 12,523 mm2 [IQR 6493-18966] vs 10,487 mm2 [IQR 3252-14402], P = 0.08). IQS were significantly higher for SE-EPI for both readers (9.2 + 4.2 vs 6.9 + 4.2, P < 0.01, Reader 1; 9.8 + 4.5 vs 7.2 + 4.5, P < 0.02, Reader 2). IQS was significantly correlated with R2* for GRE and SE-EPI (r2 = -0.72, P < 0.0004 and r2 = -0.58, P < 0.004, respectively). Inter-reader reliability was good (κ 0.6 – 0.66).

Discussion

SE-EPI-based MRE techniques provide a good alternative in cases of iron overload, because the 180 degree refocusing pulse makes them inherently insensitive to T2* effects. This improvement is suggested by the significantly higher IQS with SE-EPI and the trend toward increased coverage on the confidence mask in our cohort.

Conclusion

SE-EPI-based MRE may be a better option than conventional GRE for imaging patients with iron overload, especially at 3.0 T.

Acknowledgements

No acknowledgement found.