7 The impact of water exchange on estimates of extracellular volume calculated using contrast enhanced T1 measurements in patients with severe aortic stenosis

Introduction

Extracellular volume (ECV) is an independent predictor of mortality and outcome in patients with severe aortic stenosis (AS).¹ SCMR guidelines recommend measurement of ECV using T1 maps taken before and 10–30 minutes post contrast.² These measurements use a conventional model (CM) which assumes rapid water exchange (WX) between the ECV and the myocytes which can underestimate ECV.³ The shutter speed model (SSM), which incorporates WX, requires T1 measurements at multiple time points post-contrast.⁴ The aim of this study was to investigate whether WX influences estimates of ECV in patients with severe AS.

Materials and Methods

25 patients with severe AS referred for AVR were recruited. T1 measurements were made on a 3T Siemens system using mSASHA prototype⁵ (a) before contrast, (b) 4 mins after a 0.05 mmol/kg Gadovist injection and (c) 4, (d) 10 and (e) 30 minutes after an additional 0.1 mmol/kg dose. Three CM-based ECV estimates were made using T1 measurements (a & b), (a & d) and (a & e) and were compared to SSM ECV estimates made using all 5 T1 measurements.

Results

Median (IQR) age was 69 (63–73) years with a male:female ratio of 14:11. Median ECV estimated using the CM at 4 minutes post-0.05 mmol/kg was 25 (23–27)%. ECV estimated using the CM at 10 minutes after the additional 0.1 mmol/kg was 21 (20–24)% and at 30 minutes was 22 (21–26)%. ECV estimated using the SSM was 25 (24–30)%.

Discussion

CM ECV estimates increased between 10 and 30 mins following an accumulated dose of 0.15 mmol/kg Gadovist (p = 0.0001) but were highest 4 mins after the initial low dose. This suggests it is unnecessary to wait 10–30 mins for a contrast agent steady state to develop (this is established after only 4 mins); CM ECV after a 0.15 mmol/kg dose is underestimated because of limited WX.³

Conclusion

Measurements made using current guidelines (and 0.15 mmol/kg Gadovist) may underestimate ECV (by up to 12%). In the absence of multiple T1 measurements, allowing application of a SSM, measurement of T1 following a low dose of contrast agent may produce more reliable ECV estimates.

Acknowledgements

This work was supported by the BHF (award PG/20/10008). The authors thank the clinical staff of the Advanced Imaging Centre at Leeds General Infirmary and LICAMM, University of Leeds, for their assistance in scanning and collecting data for this study.

References

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