Measurement of pulmonary arterial pulse wave reflection from single-slice phase-contrast and steady-state free precession MRI
Journal of Cardiovascular Magnetic Resonance volume 14, Article number: W35 (2012)
Pulmonary arterial hypertension (PAH) is associated with elevated pulmonary vascular resistance, resulting in increased reflection of pressure and flow waves from distal vessels1. The gold standard for assessing PAH is right heart catheterization, an invasive procedure that carries a 5% risk of major complications2. We validate a noninvasive method for quantifying pulmonary arterial reflection using phase-contrast (PC) and steady-state free precession (SSFP) sequences acquired in a single slice.
An arterial segment approximates a hydraulic transmission line terminated distally by a reflection site that partially reflects forward-traveling pressure and flow (q) waves back toward the heart3. Due to finite pulse wave velocity (PWV), backward-traveling waves are minimal in early systole (Figure 1); since arterial cross-sectional area (a) increases roughly linearly with pressure, PWV = ∂q(t)/∂a(t). Combining this with the water hammer equation yields an expression for the backward flow wave4:
<center>qb(t) = [qmeas(t) - PWV×a(t)]/2,</center>from which arterial reflection magnitude (R) can be computed in the frequency domain:
<center>R(ω) = Qb(ω)/Qf(ω).</center>
The right pulmonary artery in three healthy adult volunteers was imaged on a 1.5 T MR system (Siemens, Germany) using retrospectively ECG-gated cine PC and SSFP sequences to quantify blood velocity and vessel cross-section, respectively. PC and SSFP images were co-registered in MATLAB (The MathWorks, USA). The arterial lumen was outlined semi-automatically using Segment (Medviso, Sweden), yielding flow and area time series that were resolved into forward and backward flow waves in MATLAB. The frequency-domain ratios of backward to forward flow waves yielded estimates of R which were then averaged over the fundamental heart frequency and the next two harmonics3 and compared to literature values using a two-tailed Student's t-test.
The single-slice MRI method reliably resolved forward and backward flows in vivo (Figure 2), enabling noninvasive measurement of normal right pulmonary arterial reflection magnitudes, R (SD) = 0.34 (0.05), statistically equivalent (p = 0.74) to invasively measured literature values5 of R (SD) = 0.33 (0.13).
The feasibility of single-slice MRI measurement of pulmonary arterial reflection in healthy adults motivates follow-up studies in adult and pediatric patient populations and lays the groundwork for noninvasive assessment of pulmonary hypertension.
This study was supported by the Canadian Institutes of Health Research (App #199854).
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Leimbigler, P., van Amerom, J., Grosse-Wortmann, L. et al. Measurement of pulmonary arterial pulse wave reflection from single-slice phase-contrast and steady-state free precession MRI. J Cardiovasc Magn Reson 14 (Suppl 1), W35 (2012). https://doi.org/10.1186/1532-429X-14-S1-W35