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Abstract
Background
Pulmonary arterial hypertension (PH) is a progressive disease of increased resistance
to flow through the lungs, leading to right ventricular (RV) failure [1]. MRI is increasingly
used to assess right ventricular (RV) function in PH. RV stroke work (SW) based on
invasive pressure and volume measurements, is used to assess ventricular work. Determining
RV work from MRI could enable a more complete characterization of RV and PA interactions
in PH. The purpose of this study was to non-invasively estimate RV work from simultaneously
acquired RV volume (VRV) and pulmonary artery flow (QPA) using a 4D flow-sensitive
MRI sequence in a canine model.
Methods
After IACUC approval, hemodynamic measurements were performed prior to and following
induction of acute PH by injection of embolizing micro-beads; details are available
elsewhere [2]. Pre- and post-embolization right heart catheterization (RHC) was performed
to measure hemodynamic changes in the RV and PA. 4D flow MRI (Phase Contrast with
Vastly undersampled Isotropic Projection Reconstruction - PCVIPR) was performed on
3T clinical scanners (MR750, GE Healthcare, Waukesha, WI) after the intravenous administration
of gadolinium-based contrast agents. PCVIPR parameters: FOV=32x32x22cm, isotropic
1.3mm spatial resolution, TR/TE=6.3/2.1ms, Venc=150cm/s, scan time: ~10min using adaptive
respiratory gating of bellows and retrospective ECG gating [3]. Post-processing was
done using Mimics (Materialise, Ann Arbor, MI) for the segmentation of the VRV from
dynamic magnitude images and Ensight (CEI, Apex, NC) for quantification of QPA. RV
pressure - VRV loops were generated to assess SW by calculating the area inside the
loop. QPA - VRV loops were generated and their area calculated for comparison to the
SW calculations (Fig 1). Direct comparison was used for the analysis of the results
and a student ttest was used to compare the two methods.
Figure 1
Visualization and quantification of pulmonary arterial flow and right ventricular
volume using 4D Flow MR images.
Results
In all cases embolization induced an increase in SW (180 ± 140 vs 374 ± 210mmHg*cm3).
Similarly the calculated area of the QPA - VRV loops increased for all the cases (369
± 210 vs 785 ± 486 s-1) (Fig 2). No significant difference was found between the percent
increase of SW and the QPA - VRV loops area (53 ± 15 vs 52 ± 12%, p = 0.95).
Figure 2
Direct comparison of P-V loop area and QMPA-V loop area before and after pulmonary
embolization.
Conclusions
QMPA - VRV loop area estimated noninvasively using 4D flow MRI can be used to evaluate
right ventricular stroke work. The results from this study indicate that 4D flow-sensitive
MRI with PC VIPR can also be used to estimate right ventricular work, complementing
the analysis of alterations in flow patterns in the heart and pulmonary arteries in
patients with cardiopulmonary disease, however more studies need to be done for validation
of the model.
Funding
NIH R01HL072260, NIH R01HL086939, and the Department of Radiology.
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