4D flow MR imaging for differentiation of pulmonary arterial hemodynamics in pre-capillary pulmonary hypertension
Hideki Ota1, Koichiro Sugimura2, Haruka Sato2, Yuta Urushibata3, Yoshiaki Komori3, Hiroaki Shimokawa2, and Kei Takase1

1Diagnostic Radiology, Tohoku University Hosipital, Sendai, Japan, 2Cardiovascular Medicine, Tohoku University Hosipital, Sendai, Japan, 3Research&Collaborations, Siemens Japan KK, Tokyo, Japan

Synopsis

Etiologies of pre-capillary pulmonary hypertension may be associated with pulmonary arterial hemodynamics. This study included 64 patients (pulmonary arterial hypertension [PAH], 25, chronic thromboembolic pulmonary hypertension [CTEPH], 39) who underwent 4D flow and cardiac MR imaging. Backward flow ratio and forward flow eccentricity in the pulmonary trunk as assessed by 4D flow MR and several cardiac MR parameters were different between two diseases. After controlling for age and mean pulmonary arterial pressure, backward flow ratio was the strongest differentiator of PAH from CTEPH. 4D flow has a potential to visualize different pulmonary arterial hemodynamics according to etiologies in pulmonary hypertension.

Introduction

Pulmonary hypertension (PH) is defined as an increase in mean pulmonary arterial pressure (mPAP) ≥25mmHg at rest. A clinical classification is established in order to individualize different groups of PH sharing similar pathological findings1. Among them, both pulmonary arterial hypertension (PAH) and chronic thromboembolic pulmonary hypertension (CTEPH) show pre-capillary PH. However, their image findings such as ventilation/perfusion scintigraphy implies difference of occluded vasculatures and pulmonary circulation between the two diseases. Past study demonstrated the presence of vortex flow visualized by 4D flow MR imaging in the pulmonary trunk in PH2. However, there have been sparse data showing differences of PA flow patterns between PAH and CTEPH.

Purpose

We aimed to evaluate whether PA flow patterns imaged with 4D flow MR imaging are different between PAH and CTEPH.

Methods

This retrospective study included 64 consecutive patients with pulmonary hypertension (PAH, 25, CTEPH, 39). All patients were imaged with a 3.0T whole-body scanner (Magnetom Trio A Tim System, Siemens Healthcare, Erlangen, Germany). Scan protocols included standard cardiac cine MR imaging and prototype 4D flow MR imaging of the pulmonary trunk. 4D flow MR imaging was acquired using the following parameters: 3 dimensional phase-contrast MR imaging with 3-directional velocity encoding in transverse slab orientation; ECG gating; respiratory gating using a navigator; TR/TE, 52.4ms/3.43ms; flip angle, 15 degrees; VENC, 50-110cm/sec; voxel size, 2.4mm x 1.8mm x 3.5mm; the number of slices, 30.

Using cine MR images, we measured left-ventricular ejection fraction (LVEF), LV stroke volume, LV cardiac index, right-ventricular ejection fraction (RVEF), RV stroke volume, RV cardiac index, RV end-diastolic and end-systolic volume index (RVEDVI and RVESVI) and pulmonary trunk diameter to ascending aortic diameter ratio (PA/AA ratio). 4D flow images were analyzed with a standalone prototype software (4D Flow Demonstrator ver. 2.3, Siemens Healthcare, Erlangen, Germany). On 4D flow MR imaging, two parameters indicating the degree of vortex flow in the pulmonary trunk were measured in the end-systolic phase. 1) Backward flow ratio: a cross-section that contained the largest vortex flow was extracted; a ratio of area with backward flow to total cross-sectional area was calculated (backward flow ratio). 2) Forward flow eccentricity: on the same cross-section with the largest vortex flow, a distance between the center of gravity of forward flow and the center of gravity of total flow was calculated; the distance was divided by the mean of long and short axis diameters of the cross-section (forward flow eccentricity).

MRI parameters, mPAP as assessed by right heart catheterization and patients’ demographics were compared between the two groups with PAH and CTEPH using unpaired t-test. Multivariable logistic regression analysis using a stepwise backward selection method (p>0.10 for removal from model) was used to evaluate significant difference of PA flow patterns controlling for potential confounding factors. P < 0.05 was used to designate statistical significance.

Results

The mean age in the PAH group was significantly lower than that in the CTEPH group (39.4±13.7 years vs. 66.1±13.0 years, p<0.01). The mean of mPAP was not significantly different between the two groups (42.5±12.3 mmHg vs. 37.6±9.5 mmHg, p<0.08).

Vortex flow in the pulmonary trunk was observed in all patients on 4D flow MR imaging. Significant differences between the PAH and CTEPH groups were observed in the following MR parameters: LVEF (56.6±9.8% vs. 63.4±9.7%, p<0.01), RVEDVI (124.4±59.9 ml/m2 vs. 96.9±30.0 ml/m2, p=0.01), RVESVI (79.2.4±55.3ml/m2 vs. 57.24±23.9ml/m2, p=0.03), PA/AA ratio (1.4±0.3 vs, 1.1 ± 0.2, p<0.01), backward flow ratio (0.31±0.12 vs. 0.22±0.09, p<0.01) and forward flow eccentricity (0.28±0.10 vs. 0.20±0.13, p=0.01).

In a multivariable logistic regression analysis controlling for age and mPAP as potential confounders, only backward flow ratio remained significant; when CTEPH was used as the reference, the adjusted odds ratio for 0.1 increase of backward flow ratio was 1.1 (95% confidence interval, 1.04, 8.37, p=0.04). The other MR parameters were removed by the backward stepwise selection.

Discussion

Vortex flow in the pulmonary trunk may be a characteristic finding of PH. Among various parameters derived by cardiac MR imaging, backward flow ratio was the strongest differentiator between PAH and CTEPH after controlling for age and mPAP. CTEPH mainly differs from PAH by the proximal location of pulmonary artery obstruction. Occlusion in the proximal portion may lead to production of pressure wave reflections and decrease vascular compliance3. These factors may induce difference of flow patterns in the pulmonary trunk between CTEPH and PAH as identified by 4D flow MR imaging.

Conclusions

Backward flow area ratio was the strongest differentiator between PAH and CTEPH. 4D flow MR imaging has a potential to visualize the difference of PA hemodynamics according to etiologies of PH.

Acknowledgements

This study was conducted using the prototype sequence and prototype post-processing software providedby Andreas Greiser and Aurelien Stalder, Siemens Healthcare GmbH.

References

1. Simonneau G, Gatzoulis MA, Adatia I, et al. Updated Clinical Classification of Pulmonary Hypertension. Journal of the American College of Cardiology. 2013;62(25, Supplement):D34-D41.

2. Reiter G, Reiter U, Kovacs G, et al. Magnetic Resonance–Derived 3-Dimensional Blood Flow Patterns in the Main Pulmonary Artery as a Marker of Pulmonary Hypertension and a Measure of Elevated Mean Pulmonary Arterial Pressure. Circ Cardiovasc Imaging. 2008;1(1):23-30.

3. Delcroix M, Vonk Noordegraaf A, Fadel E, Lang I, Simonneau G, Naeije R. Vascular and right ventricular remodelling in chronic thromboembolic pulmonary hypertension. Eur Respir J. 2013;41(1):224-232.

Figures

4D flow MR images in end-systolic phases of pulmonary arterial hypertension (PAH) and chronic thromboembolic pulmonary hypertension (CTEPH) with the same mean pulmonary arterial pressure (mPAP). Vortex flow and backward flow ratio in the pulmonary trunk is larger in PAH than CTEPH.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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