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Measurement of pulmonary arterial pulse wave reflection from single-slice phase-contrast and steady-state free precession MRI

  • Peter Leimbigler
  • Joshua van Amerom
  • Lars Grosse-Wortmann
  • Shi-Joon Yoo
  • Chris Macgowan
Open Access
Workshop presentation

Keywords

Pulmonary Arterial Hypertension Pulse Wave Velocity Flow Wave Water Hammer SSFP Sequence 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Summary

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.

Background

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>
Figure 1

Illustration of single-slice wave separation. Before the onset of backward waves, flow and area are related by the constant factor PWV. Backward-traveling flow and area waves are equal in form but opposite in sign, and are proportional to the difference between PWV × area and flow.

Methods

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.

Results

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).
Figure 2

Flow wave separation in the right pulmonary artery of Subject 1 calculated from an initial MR study and a repeat scan 36 days later. Reflection magnitudes and standard deviations are tabulated for all subjects. Scan parameters were TR/TE = 9.8/2.2 ms, 192 × 192 matrix, 28 × 28 cm2 FOV, 2 averages, flip angle = 10° (PC) and 45° (SSFP). The root-mean-variances of the forward, measured and backward flow waves between scans were 11, 16 and 8 ml/s, respectively.

Conclusions

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.

Funding

This study was supported by the Canadian Institutes of Health Research (App #199854).

References

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    Weinberg, et al : Circulation. 2004, 110 (17): 2609-17.CrossRefPubMedGoogle Scholar
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    Carmosino, et al : Anesth Analg. 2007, 104 (3): 521-7.PubMedCentralCrossRefPubMedGoogle Scholar
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    Milnor : Hemodynamics. 1989, 2Google Scholar
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    Parker : Med Biol Eng Comp. 2009, 47 (2): 175-88.CrossRefGoogle Scholar
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    Laskey, et al : J Am Coll Cardiol. 1993, 21 (2): 406-12.CrossRefPubMedGoogle Scholar

Copyright information

© Leimbigler et al; licensee BioMed Central Ltd. 2012

This article is published under license to BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Authors and Affiliations

  • Peter Leimbigler
    • 1
  • Joshua van Amerom
    • 1
    • 2
  • Lars Grosse-Wortmann
    • 2
    • 3
  • Shi-Joon Yoo
    • 2
    • 3
  • Chris Macgowan
    • 1
    • 2
  1. 1.Medical Biophysics & Medical ImagingUniversity of Toronto & Hospital for Sick ChildrenTorontoCanada
  2. 2.Diagnostic ImagingHospital for Sick ChildrenTorontoCanada
  3. 3.Labatt Family Heart CentreTorontoCanada

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