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MRI flow and volume quantification with internal validation and correlation to Fick and thermodilution catheterization derived values in pulmonary hypertension1Radiology, Geneva University Hospital, Medicine Faculty of the University of Geneva, Geneva, Switzerland, 2Cardiology, Geneva University Hospital, Geneva, Switzerland, 3Pulmonary Medicine, Geneva University Hospital, Geneva, Switzerland, 4Pediatric Cardiology, Geneva University Hospital, Geneva, Switzerland
Synopsis
Cardiac output (CO) determination is mandatory for the diagnosis work-up of pulmonary hypertension (PH). It is classically obtained by invasive methods such as indirect Fick (Qfick) and thermodilution (Qthermo) performed during right heart catheterisation (RHC). However, non-contrast, non-invasive and reliable methods may be preferred for clinical routine and post-treatment monitoring of patients. This study compared QFlow derived parameters (stroke volumes and cardiac index/ pulmonary and aortic flow) measured non-invasively in PH patients for internal validation of MRI and for comparison to RHC.
Introduction
Patients with suspicion of pulmonary hypertension (PH) need cardiac output (CO) and pulmonary pressure measurement to determine pulmonary vascular resistance (PVR). Currently, such measurements are invasive using right heart catheterisation (RHC) according to the Fick principle (QFick) or thermodilution (QThermo) method (1).
With the complication of multiple possible conditions associated with PH, and difficulties to define reference values for both adults and paediatric patients (2), there has been significant interest in defining new hemodynamic parameters. Despite widespread acceptance as the clinical gold-standard, there remains discussion over the best RHC methods and a lack of standard consensus (3). New imaging techniques are emerging (4). Stroke volume from PA flow could have limited accuracy in PH patients (5).
This study compared MRI phase-contrast and RHC to measure flow and derived stroke volumes non-invasively in PH patients. Routine MRI phase contrast acquisitions included various vessels to compare pulmonary blood flow (Qp) and systemic blood flow (Qs) also allowing internal validation.
Methods
RHC and MRI measurement were compared in 30 patients (aged 58±18 years) with less than 3 weeks between exams. MRI used Siemens 1.5T AERA and 3T PRISMA with routine clinical phase-contrast protocol and SyngoVia for analysis. Flow and volume were measured in the major vessels: ascending aorta, pulmonary artery, right plus left pulmonary arteries, descending aorta plus superior vena cava; in addition to right and left ventricular cardiac index and stroke volumes (RV, LV) as part of a standard clinical protocol. RHC-derived flow and volume values were calculated by Fick and thermodilution methods as is standard practice.
Statistical analysis used SPSS (significance p<0.05). Inter- and intra-observer reproducibility was assessed for MRI measurements (ICC, n=10). Correlations between parameters expected to be equal had a set intercept=0 and the slope=1 was assessed using 95% confidence intervals. QFick was compared to QThermo, and internal validation of the MRI (Qp=Qs) compared all possible pairings. The 6 MRI values were compared to QFick and QThermo. All correlations were repeated with MRI-derived stroke volume and cardiac index/ pulmonary or aortic flow (ml and L/min, respectively) using the heart rate measured in the individual flow acquisition, and both were also scaled to body surface area.
Results
Inter- and intra-observer reproducibility (ICC) was excellent for all MRI flow quantification (flow L/min) ranging from 0.826 to 0.983 and 0.866 to 0.999, respectively, using semi-automated analysis. Internal validation showed that MRI measurements between anatomical sites were self-consistent and precise.
In these clinically stable patients there was a strong correlation of QFlow from MRI with QFick and QThermo with slopes not different from 1, and similarly for the internal validation (a few exceptions were within 0.03 of slope=1). Both stroke volume (ml) and cardiac index / pulmonary or aortic flow (L/min) performed well for the internal validation and correlation to catheter measurements with p<0.0001. Example correlations are illustrated in figures 1-5 with R2=0.74-0.88 for internal validation, R2=0.47-0.68 with QFick and 0.28-0.75 with QThermo. Correlations between MRI and QThermo were improved using volume in ml, whereas MRI vs QFick was slightly better in L/min (though not statistically different).
Values scaled to body surface area gave poorer correlations with R2 reduced by between 0.1 and 0.3 with some correlations (with QThermo) non-significant and others at p<0.05 (unlike the p<0.0001 above).
Discussion
In summary, flow and volumes derived from RHC measurements correlated with QFlow from MRI, and QFlow MRI with internal validation was robust. In our populations stroke volume and flow show equally good correlations. There was no significant difference in heart rate between the two exams which could lead to a difference in behaviour between flow and stroke volume. The different volume and index parameters were tested as it has been suggested that in patients with a low cardiac stroke volume (ml) the heart rate will increase to maintain the flow in L/min. Volumes scaled to body surface area, which are used to compare normal/abnormal values in patients of different size, reduced the range of values with subsequently worse correlation. Internal validation of the MRI measurements helps to validate the technique, but is also useful to verify flow values that are difficult to measure in, for example, the turbulent flow in the pulmonary artery.Conclusion
Non-contrast, non-invasive imaging methods are highly preferable for clinical routine and post-treatment monitoring of patients. In addition, despite widespread acceptance as the clinical gold-standard, there remains discussion over the best methods and a lack of consensus in RHC. Due to the precise, reproducible MRI values and strong correlation with QFick and QThermo methods, non-invasive MRI flow quantification can be proposed as a clinical alternative.Acknowledgements
References
[1] N. Galie, M. Humbert, J. L. Vachiery, S. Gibbs, I. Lang, A. Torbicki, et al., "2015 ESC/ERS Guidelines for the diagnosis and treatment of pulmonary hypertension: The Joint Task Force for the Diagnosis and Treatment of Pulmonary Hypertension of the European Society of Cardiology (ESC) and the European Respiratory Society (ERS): Endorsed by: Association for European Paediatric and Congenital Cardiology (AEPC), International Society for Heart and Lung Transplantation (ISHLT)," Eur Respir J, vol. 46, pp. 903-75, Oct 2015.
[2] K. L. Colvin, M. J. Dufva, R. P. Delaney, D. D. Ivy, K. R. Stenmark, and M. E. Yeager, "Biomarkers for pediatric pulmonary arterial hypertension - a call to collaborate," Front Pediatr, vol. 2, p. 7, 2014.
[3] S. Rosenkranz and I. R. Preston, "Right heart catheterisation: best practice and pitfalls in pulmonary hypertension," Eur Respir Rev, vol. 24, no. 138, pp. 642-52, Dec 2015.
[4] J. C. Grignola, "Hemodynamic assessment of pulmonary hypertension," World J Cardiol, vol. 3, no. 1, pp. 10-7, Jan 26 2011.
[5] G. J. Mauritz, J. T. Marcus, A. Boonstra, P. E. Postmus, N. Westerhof, and A. Vonk-Noordegraaf, "Non-invasive stroke volume assessment in patients with pulmonary arterial hypertension: left-sided data mandatory," J Cardiovasc Magn Reson, vol. 10, p. 51, Nov 05 2008.
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