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Published online before print February 28, 2007, 10.1148/radiol.2431060477
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(Radiology 2007;243:70-79.)
© RSNA, 2007

Cardiac Imaging

Pulmonary Arterial Hypertension: Noninvasive Detection with Phase-Contrast MR Imaging1

Javier Sanz, MD, Paola Kuschnir, MD, Teresa Rius, MD, Rafael Salguero, MD, Roxana Sulica, MD, Andrew J. Einstein, MD, PhD, Santo Dellegrottaglie, MD, Valentin Fuster, MD, PhD, Sanjay Rajagopalan, MD and Michael Poon, MD

1 From the Zena and Michael A. Wiener Cardiovascular Institute and the Marie-Josée and Henry R. Kravis Center for Cardiovascular Health, Mount Sinai School of Medicine, Box 1030, One Gustave L. Levy Place, New York, NY 10029 (J.S., P.K., R. Salguero, R. Sulica, A.J.E., S.D., V.F., S.R.); Department of Cardiology, Centro Médico Teknon, Barcelona, Spain (T.R.); and Department of Cardiology, Cabrini Medical Center, New York, NY (M.P.). Received March 15, 2006; revision requested May 18; revision received June 13; final version accepted August 9. Supported in part by the Mount Sinai School of Medicine Consortium for Cardiovascular Imaging Technology, New York, NY. J.S. supported by a research grant from the Spanish Society of Cardiology ("Beca para la Formación en Investigación Post-Residencia"). S.D. supported by a research grant from the Italian Society of Cardiology. Address correspondence to J.S. (email: Javier.Sanz@mssm.edu).


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 References
 
Purpose: To retrospectively identify pulmonary arterial (PA) flow parameters measured with phase-contrast magnetic resonance (MR) imaging that allow noninvasive diagnosis of chronic PA hypertension (PAH).

Materials and Methods: The study was HIPAA compliant and was approved by the institutional review board; a waiver of informed consent was obtained. Fifty-nine patients (49 female patients; mean age, 46 years; range, 16–85 years) known to have or suspected of having PAH underwent breath-hold phase-contrast MR imaging and right-sided heart catheterization (RHC). The presence of PAH (mean pulmonary artery pressure [mPAP], >25 mm Hg) was confirmed in 42 patients. Parameters, including PA areas, PA strain, average velocity, peak velocity, acceleration time, and ejection time, were measured in each patient by investigators blinded to RHC results. These measurements were correlated with mPAP, systolic pulmonary artery pressure (sPAP), and pulmonary vascular resistance index (PVRI). The diagnostic ability of phase-contrast MR imaging to depict PAH was quantified. Statistical tests included Spearman {rho} coefficients, receiver operating characteristic curve analysis, and Bland-Altman plots.

Results: Results showed average velocity to have the best correlation with mPAP, sPAP, and PVRI (r = –0.73, –0.76, and –0.86, respectively; P < .001). Average velocity (cutoff value = 11.7 cm/sec) revealed PAH with a sensitivity of 92.9% (39 of 42) and a specificity of 82.4% (14 of 17). Sensitivity and specificity for the minimum PA area (cutoff value = 6.6 cm2) were 92.9% (39 of 42) and 88.2% (15 of 17), respectively.

Conclusion: The average blood velocity throughout the cardiac cycle is strongly correlated with pulmonary pressures and resistance.

© RSNA, 2007

Pulmonary hypertension (PH) is classified into five diagnostic categories: pulmonary arterial hypertension (PAH), PH with left-sided heart disease, PH associated with lung disease and/or hypoxemia, PH due to chronic thrombotic and/or embolic disease, and a miscellaneous group (1). Disorders leading to PAH are associated with a particularly ominous outcome (2). Right-sided heart catheterization (RHC) is the current reference standard for the diagnosis and assessment of the severity of PH (3,4), although it is invasive and carries a small but definite risk of severe complications. Doppler ultrasonography of the flow through the tricuspid and pulmonary valves is a widely available and well-established method for PH evaluation (5). Nonetheless, suboptimal acoustic windows may hamper adequate flow characterization in a subset of patients (6,7). Cardiac magnetic resonance (MR) imaging is an alternative modality for evaluation of PH (8–11). Phase-contrast MR imaging allows accurate quantification of blood velocity and flow (12,13), and results of prior research (14–20) suggest a potential role in the assessment of PH. Thus, the purpose of our study was to retrospectively identify pulmonary arterial (PA) flow parameters measured with phase-contrast MR imaging that allow noninvasive diagnosis of chronic PAH.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 References
 
Study Group
Our retrospective study was compliant with Health Insurance Portability and Accountability Act regulations and was approved by the institutional review board. A waiver of informed consent was obtained.

Patients known to have or suspected of having PH and referred between January 2003 and January 2005 to the PH program at Mount Sinai School of Medicine—a tertiary referral unit for patients with PH from causes other than left-sided heart disease—underwent cardiac MR imaging and RHC as part of their routine clinical evaluation. All patients or their legal representatives consented to the procedures. A total of 74 consecutive patients who were given a final diagnosis of PAH after a complete diagnostic workup and who underwent both procedures were retrospectively identified. Exclusion criteria comprised an interval between cardiac MR imaging and RHC of more than 2 weeks (23 patients), PH only with exercise (four patients), and changes in therapy between procedures (introduction of calcium blockers in one patient). Four patients with intracardiac shunts were also excluded because of elevated PA blood velocities even in the presence of PH (21) and abnormally increased or decreased pulmonary flow. Hence, 42 patients with PAH were included in the analysis: 34 female (81%) and eight male (19%) patients with a mean age of 45 years ± 14 (standard deviation) (age range, 16–85 years).

Seventeen consecutive additional patients suspected of having PH who underwent cardiac MR imaging and RHC within a 2-week interval but who had normal pressures both at rest and with exercise also were included. There were 15 women (88%) and two men (12%) with a mean age of 48 years ± 13 (age range, 30–80 years). Therefore, 59 patients comprised the final study group. For analysis purposes, they were categorized into four subgroups (1) with a similar number of individuals (Table 1).


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  		Table 1. Patient Subgroups according to Presence or Absence of PAH and Underlying Cause

 
Cardiac MR Imaging Protocol
Studies were performed with a 1.5-T clinical magnet (Magnetom Sonata; Siemens Medical Solutions, Erlangen, Germany) and were supervised by one of four investigators (J.S., S.D., S.R., M.P.). Contraindications included incompatible metallic implants, severe obesity, pregnancy, and claustrophobia. Studies were performed with patients in the supine position by using a phased-array surface coil as a receiver and retrospective electrocardiographic gating. At the examination, all patients were in sinus rhythm. Images were obtained during end-expiratory breath holds preceded by brief hyperventilation. After obtaining standard localizer views and contiguous short-axis cine views covering both ventricles, two double-oblique views oriented along the main axis of the pulmonary trunk were acquired with a standard steady-state free precession cine MR sequence (Fig 1). Both cine loops were used as the reference to prescribe a plane truly perpendicular to the main PA for the acquisition of phase-contrast MR images and to ensure that the imaging plane remained between the pulmonary valve and the PA bifurcation throughout the whole cardiac cycle.


Figure 1A
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  		Figure 1a: (a, b) Double-oblique steady-state free precession cine MR images of pulmonary trunk (repetition time msec/echo time msec, 3.2/1.6; flip angle, 60°; section thickness, 6 mm; matrix, 256 x 154). First, an image was prescribed perpendicular to a basal short-axis view that included the tricuspid and pulmonary valves, with the imaging plane intersecting both valves. This resulted in an image like the one shown in a. A second image like the one shown in b was obtained perpendicular to the former and aligned with right ventricular outflow tract and PA as indicated with black line. These two images were used simultaneously to plan phase-contrast plane orientation, which is represented by white line.

 

Figure 1B
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  		Figure 1b: (a, b) Double-oblique steady-state free precession cine MR images of pulmonary trunk (repetition time msec/echo time msec, 3.2/1.6; flip angle, 60°; section thickness, 6 mm; matrix, 256 x 154). First, an image was prescribed perpendicular to a basal short-axis view that included the tricuspid and pulmonary valves, with the imaging plane intersecting both valves. This resulted in an image like the one shown in a. A second image like the one shown in b was obtained perpendicular to the former and aligned with right ventricular outflow tract and PA as indicated with black line. These two images were used simultaneously to plan phase-contrast plane orientation, which is represented by white line.

 
Phase-contrast MR images were acquired with a segmented fast gradient-echo MR sequence (22), with velocity encoding perpendicular to the imaging plane and a predefined upper velocity limit of 100 cm/sec. If aliasing was noted, the velocity was progressively raised in 50-cm/sec steps until aliasing disappeared. Imaging parameters included the following: 7.5/3.1; flip angle, 15°; section thickness, 6 mm; field of view, 320–380 x 240–300 mm; matrix, 256 x 96 (typical in-plane resolution, 2.7 x 1.4 mm); number of signals acquired, one; number of segments, five to seven; temporal resolution, 75–105 msec; number of reconstructed cardiac phases, 20; bandwidth, 260 Hz/pixel. The typical breath-hold time ranged from 15 to 25 seconds. Patients were encouraged to hold their breath during the whole acquisition. Supplemental oxygen was administered as clinically indicated or if the patient experienced difficulties completing the period of apnea. If obvious breathing artifacts were noted, the acquisition was repeated.

RHC Protocol
RHC was performed by one of two investigators (M.P. and R. Sulica, with 19 and 5 years of experience with RHC, respectively) by using standard procedures. In brief, a Swan-Ganz catheter was placed through a 7.5-F introducer by using either an internal jugular or a femoral approach. Heart rate was monitored continuously. The body surface area was recorded. Zero-pressure calibration was performed at the level of the midaxillary line with the patient in the supine position. Baseline measurements included mean right atrial pressure, mean PA pressure (mPAP), systolic PA pressure (sPAP), pulmonary capillary wedge pressure, cardiac index (cardiac output obtained by the thermodilution method divided by body surface area), pulmonary vascular resistance index (PVRI) (calculated as [mPAP – PCWP]/CI, where PCWP is pulmonary capillary wedge pressure and CI is cardiac index), and mixed venous oxygen saturation.

In the case of normal baseline pressures, upper extremity exercise was performed for approximately 5 minutes or until the heart rate increased by 30% or more. PA and pulmonary capillary wedge pressures were then measured, and cardiac output was quantified in triplicate to confirm an increase by 30% or more. If this increase had not occurred, exercise was continued. If cardiac output did not increase by 30% or more, the test was considered inadequate. The presence of PH was defined as an mPAP of more than 25 mm Hg at rest or more than 30 mm Hg with exercise (3). Values of sPAP of more than 35 mm Hg or PVRI of more than 3 Wood units · meters squared were also considered abnormal (3,4,23).

Image Analysis
Phase-contrast MR images were postprocessed by using specialized software (Argus; Siemens Medical Solutions). The heart rate during cardiac MR imaging was recorded from the images. The contours of the main PA were automatically traced, with manual correction when necessary, simultaneously on magnitude and velocity-map images of all 20 reconstructed phases (Fig 2). The software then calculated the velocity in each of the voxels included within the contour and integrated the values over area and time to obtain the following parameters: peak velocity, average velocity, and minimum, maximum, and average areas. PA strain was then calculated as 100 · (MaA – MiA)/MiA, where MaA is maximum area and MiA is minimum area.


Figure 2A
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  		Figure 2a: (a, b) Double-oblique segmented gradient-echo phase-contrast MR images (7.5/3.1; flip angle, 15°; section thickness, 6 mm; matrix, 256 x 96) perpendicular to pulmonary trunk. (a) Magnitude and (b) velocity-map images were reconstructed from same acquisition and show traced contours of arterial perimeter in one phase of the cardiac cycle.

 

Figure 2B
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  		Figure 2b: (a, b) Double-oblique segmented gradient-echo phase-contrast MR images (7.5/3.1; flip angle, 15°; section thickness, 6 mm; matrix, 256 x 96) perpendicular to pulmonary trunk. (a) Magnitude and (b) velocity-map images were reconstructed from same acquisition and show traced contours of arterial perimeter in one phase of the cardiac cycle.

 
By using the flow values in each measured time point and the flow curve as references, the following additional parameters were quantified: acceleration time (AT), ejection time (ET), and AT/ET ratio. AT was defined as the time interval from the beginning of the anterograde flow upslope in systole to the peak systolic flow. ET was defined as the interval from the beginning of the systolic flow upslope to the horizontalization of the flow curve, presence of a new upslope, or crossing of the zero-flow line during protodiastole (Fig 3). By using only the images during these time periods, we subsequently obtained the average velocities during AT and ET.


Figure 3
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  		Figure 3: Flow curve of PA. AT and ET as quantified in our study are shown. Left dotted vertical line = time of peak systolic flow. Right dotted vertical line = time of end of ejection.

 
One investigator (J.S., with 5 years of experience in cardiac MR) analyzed all images unaware of RHC results. Twelve patients (three from each group) were randomly selected, and both the first investigator (14 months later on average; range, 6–23 months) and a second blinded investigator (A.J.E., with 1 year of experience with cardiac MR) independently repeated the measurements to assess inter- and intraobserver reproducibility. As a method of internal control, the results of phase-contrast MR cardiac output were compared with those determined from volumetric calculations from left ventricular contours (J.S.), as previously described (24).

Statistical Analysis
Categoric values are expressed as percentage, and continuous parameters are expressed as means ± standard deviations for normally distributed variables. Otherwise, values are expressed as medians with interquartile ranges. Ninety-five percent confidence intervals (CIs) are provided, if applicable. We calculated that the minimum sample size to detect a correlation of r = 0.60 between phase-contrast MR measurements and pulmonary pressures and resistance measurements with 80% power and a significance level of .05 would be 19 subjects. Therefore, our sample size of 59 patients would suffice for detection of such correlations. The value of r = 0.60 was based on results of a previous study on thromboembolic PH that evaluated the correlation between mPAP and peak PA flow velocity (11).

Departures from normality were determined with the Shapiro-Wilk statistic. For continuous variables, significant differences between patients without PH and patients with PAH were assessed with the Student t test or Mann-Whitney U test as appropriate. The subgroups of patients with PAH (groups 1, 2, and 3) were compared by using the Kruskal-Wallis test with the Dunn post hoc test or by using one-way analysis of variance with the post hoc Tukey test for multiple comparisons. The {chi}2 statistic was employed for comparison of percentages. Spearman {rho} correlation coefficients (r values) were obtained in an exploratory analysis to evaluate relationships between pulmonary pressures and resistance and all individual parameters measured on phase-contrast MR images (for the whole study group and for the PAH subgroups). Comparisons between r coefficients were performed with the z statistic. Further evaluation of relations between parameters was performed by using simple regression analysis. The runs test was employed to determine if the regression lines departed significantly from linearity. To determine the ability of phase-contrast MR imaging–derived parameters to depict the presence of PAH, receiver operating characteristic curve analysis was performed, and sensitivity and specificity were calculated. Areas under the curves for different parameters were compared by using the method described by Hanley and McNeil (25). Inter- and intraobserver variability were investigated with Bland-Altman tests. The analyses were performed by two investigators (J.S., R. Salguero) with statistical packages (SPSS for Windows, version 12.0, SPSS, Chicago, Ill; GraphPad Prism, version 3.02, GraphPad Software, San Diego, Calif; MedCalc for Windows, version 8.1.0.0, MedCalc Software, Mariakerke, Belgium). A P value of less than .05 was considered to indicate a significant difference.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCE IN KNOWLEDGE
 References
 
RHC Results
As expected, the mPAP, sPAP, mean right atrial pressure, PVRI, mixed venous oxygen saturation, and heart rate at RHC were significantly different between patients without PH and patients with PAH (Table 2). In addition, the cardiac index was lower in patients with PAH than in patients without PH, but this difference did not reach significance (P = .052). There were no significant differences in terms of age or sex distribution, body surface area, time interval between procedures, or pulmonary capillary wedge pressure. Among the PAH subgroups, the patients with collagen-vascular diseases were significantly older than patients in other PAH subgroups. No significant differences were noted among other patient characteristics or the severity of the disease (Table 2).


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  		Table 2. Demographic and Hemodynamic Data in Study Group and PAH Subgroups at RHC

 
The degree of PAH was not significantly different between female and male patients: median mPAP, 41.5 mm Hg (n = 31) versus 47.5 mm Hg (n = 22) (P = .41); median sPAP, 63.5 mm Hg (n = 47) versus 82.5 mm Hg (n = 34) (P = .18); and median PVRI, 9.2 Wood units (n = 11) versus 11 Wood units (n = 15) (P = .42), respectively. Similarly, there were no significant correlations between age and mPAP (r = –0.22, P = .10), sPAP (r = –0.18, P = .16), or PVRI (r = –0.09, P = .50).

Phase-Contrast MR Imaging Results
Correlation between phase-contrast MR imaging–derived and volumetric cardiac output was excellent (r = 0.91; 95% CI: 0.86, 0.95; P < .001), as was the agreement according to Bland-Altman analysis (mean bias, –0.15 L/min; 95% CI: 0.97, –1.27). All the phase-contrast MR imaging–derived parameters, with the exception of ET, were significantly different between patients without PH and those with PAH (Table 3). The largest significant differences between patients without PH and patients with PAH were observed in parameters involving average velocities, with mean reductions of 43%–56% in median values (38.4 to 16.9 and 15.6 to 8.9 cm/sec); pulmonary areas, with mean increases ranging from 66% to 102% (7.1 to 11.8 and 4.8 to 9.7 cm2); and PA strain, with a 65% reduction (49.1% to 17.4%). There were no significant differences at any level among subgroups according to the cause of PAH. Parameters in general showed good reproducibility (Table 4), with mean intraobserver biases of 1.5%–10.1% and interobserver biases of 1%–34% (expressed as percentages of the average measurement).


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  		Table 3. Phase-Contrast MR Imaging Data in Study Group and PAH Subgroups

 

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  		Table 4. Results of Bland-Altman Analyses of Intra- and Interobserver Variability of Phase-Contrast MR Imaging Measurements

 
Correlation between RHC and Phase-Contrast MR Imaging
Pulmonary pressures and resistance demonstrated significant positive correlations with PA areas and negative correlations with PA strain, blood flow velocities, AT, and AT/ET ratio (Table 5). Average velocity in the PA strongly correlated with mPAP (r = –0.73; 95% CI: –0.58, –0.83; P < .001), sPAP (r = –0.76; 95% CI: –0.62, –0.85; P < .001), and PVRI (r = –0.86, 95% CI: –0.77, –0.91; P < .001) (Fig 4). The age- and sex-adjusted r coefficients were –0.74 (P < .001) for all three variables. Restricting the quantification of velocity to specific intervals during the cardiac cycle (AT or ET) did not improve the correlations (Table 5). When PAH subgroups were analyzed separately, the correlations between average velocity and RHC-derived parameters were similar. The respective r coefficients between average velocity and mPAP for groups 1, 2, and 3 were –0.64, –0.56, and –0.60 (P = .76). The corresponding values were –0.53, –0.66, and –0.79 (P = .28) for sPAP and –0.88, –0.81, and –0.74 (P = .34) for PVRI. Graphic plots of average velocity versus RHC-derived parameters in patients with PAH (Fig 5) showed that two patients "behaved" differently from the rest of the study population in that they showed elevated pulmonary velocities despite the presence of PH. These two patients had PAH secondary to portopulmonary syndrome and substantially elevated cardiac indexes (5.6 and 6.3 L/min/m2). When the relation of average velocity to cardiac index was analyzed, a positive correlation was found (r = 0.59; 95% CI: 0.39, 0.73; P < .001). The average velocity/cardiac index ratio was then correlated with RHC-derived parameters in an attempt to account for the potential influence of cardiac output, but decreased correlations with mPAP (r = –0.36 [P = .006]), sPAP (r = –0.34 [P = .009]), and PVRI (r = –0.21 [P = .11]) were found.


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  		Table 5. Correlation Coefficients between Phase-Contrast MR Imaging Parameters and Pulmonary Pressures and Resistance

 

Figure 4A
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  		Figure 4a: Graph shows results of regression analysis in whole study group between pulmonary average velocity (AV) and (a) mPAP, (b) sPAP, and (c) PVRI. Curve fitting did not significantly depart from linearity (runs test). Dashed lines = 95% CIs of the mean.

 

Figure 4B
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  		Figure 4b: Graph shows results of regression analysis in whole study group between pulmonary average velocity (AV) and (a) mPAP, (b) sPAP, and (c) PVRI. Curve fitting did not significantly depart from linearity (runs test). Dashed lines = 95% CIs of the mean.

 

Figure 4C
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  		Figure 4c: Graph shows results of regression analysis in whole study group between pulmonary average velocity (AV) and (a) mPAP, (b) sPAP, and (c) PVRI. Curve fitting did not significantly depart from linearity (runs test). Dashed lines = 95% CIs of the mean.

 

Figure 5A
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  		Figure 5a: Graph shows results of regression analysis in patients with PAH between average velocity (AV) and (a) mPAP, (b) sPAP, and (c) PVRI. Dashed lines = 95% CIs of the mean. Results in two patients with high velocities despite presence of PH are highlighted (circles).

 

Figure 5B
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  		Figure 5b: Graph shows results of regression analysis in patients with PAH between average velocity (AV) and (a) mPAP, (b) sPAP, and (c) PVRI. Dashed lines = 95% CIs of the mean. Results in two patients with high velocities despite presence of PH are highlighted (circles).

 

Figure 5C
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  		Figure 5c: Graph shows results of regression analysis in patients with PAH between average velocity (AV) and (a) mPAP, (b) sPAP, and (c) PVRI. Dashed lines = 95% CIs of the mean. Results in two patients with high velocities despite presence of PH are highlighted (circles).

 
Detection of PAH with Phase-Contrast MR Imaging
Average velocities and minimum PA areas showed the best diagnostic performance at revealing the presence of PAH. There were no significant differences (P > .05) among the areas under the curves for average velocity (area under the curve = 0.90; 95% CI: 0.80, 0.96), average velocity during ET (area under the curve = 0.93; 95% CI: 0.84, 0.98), average velocity during AT (area under the curve = 0.94; 95% CI: 0.85, 0.98), or minimum PA area (area under the curve = 0.95; 95% CI: 0.86, 0.99) (Table 6, Fig 6).


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  		Table 6. Diagnostic Performance of Two Parameters for Detection of PAH at Phase-Contrast MR Imaging

 

Figure 6A
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  		Figure 6a: Receiver operating characteristic curves show ability of average velocity (AV) and minimum pulmonary area (Min PA area) to reveal PAH as defined by elevated values of (a) mPAP, (b) sPAP, and (c) PVRI. AUC = area under the curve.

 

Figure 6B
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  		Figure 6b: Receiver operating characteristic curves show ability of average velocity (AV) and minimum pulmonary area (Min PA area) to reveal PAH as defined by elevated values of (a) mPAP, (b) sPAP, and (c) PVRI. AUC = area under the curve.

 

Figure 6C
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  		Figure 6c: Receiver operating characteristic curves show ability of average velocity (AV) and minimum pulmonary area (Min PA area) to reveal PAH as defined by elevated values of (a) mPAP, (b) sPAP, and (c) PVRI. AUC = area under the curve.

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCE IN KNOWLEDGE
 References
 
Results of prior studies showed potential use of a variety of phase-contrast MR imaging–derived parameters in the evaluation of PH, including pulmonary regurgitant fraction (15), PA dimensions (16), PA strain (14,16), AT (14,16–18), acceleration volume (18), flow upslope (18), and pressure wave velocity (19). In our study, the minimum PA area was useful in detecting the presence of PAH (sensitivity and specificity of 92.9% and 88.2%, respectively). However, the use of PA areas had some limitations, including inconsistent performance (19) and more difficult contour tracing in diastole when proton inflow (and therefore blood signal intensity) was lower (20). An alternative approach, the measurement of AT and ET, has shown good correlation with pulmonary pressures and resistance in Doppler studies (7,26–29), although inconsistent results have also been reported (30). In our study, there were significant but weak correlations between AT and RHC measurements. This may be due in part to the limited temporal resolution of our protocol, which allowed for image acquisition within one breath hold. Although higher temporal resolution may help depict small changes in AT, which in turn correlate with PA pressures (16–18), a similarly limited performance was recently reported in a study with phase-contrast MR imaging with high temporal resolution (31).

In our study, the average velocity of PA flow was the most useful single parameter in the evaluation of PAH. The presence of slow pulmonary flow in patients with PH was observed in early investigations by using spin-echo MR techniques (32,33) or velocity mapping (14–16). In a group of 33 patients with chronic thromboembolic disease, PA peak velocity had a moderate correlation (r = 0.60) with mPAP (11). Our results in a larger group of patients with PAH confirm a significant though weaker correlation between peak velocity and pulmonary pressures and resistance, whereas average velocity had the best correlations with hemodynamic measurements (r ranging from –0.73 to –0.86). These findings suggest that as pulmonary pressures and resistance increase, the circulation of blood through the pulmonary vascular bed is globally hampered and progressively slows. A potential role for the quantification of average velocity at peak systole in PH evaluation has been previously suggested (16,20). However, to the best of our knowledge, this is the first investigation to evaluate average velocity during the complete cardiac cycle. The correlations did not improve significantly when specific phases of the R-R interval were evaluated, which obviated more complex and time-consuming image postprocessing. Advantages of averaging measurements from the complete PA cross section over a single point (ie, peak velocity) include the lack of assumption of a uniform PA flow profile, which is particularly heterogeneous in patients with PH (15,34), limitation of the impact of minor errors in contour tracing (12), and reduced measurement variability (35). These features may represent an advantage of cardiac MR imaging over pulsed Doppler techniques.

The correlations among PAH subgroups were similar, which suggests that the relationship between blood velocity and pressures exists in various clinical scenarios. Interestingly, there was abnormally high velocity in two patients with portopulmonary syndrome, which was probably due to the combination of elevated cardiac index and the hyperdynamic circulation characteristic of this syndrome (36,37). The utility of average velocity quantification may thus be lower in this context. From a practical perspective, the measurement of pulmonary average velocity appears clinically useful; a cutoff value of 11.7 cm/sec was used to differentiate between patients with PAH and those without PAH (sensitivity of 92.9%; specificity of 82.4%). Clinical scenarios in which this method can be useful include when patients are referred for cardiac MR imaging due to right ventricular abnormalities of unknown origin, when patients are suspected of having PH but have insufficient tricuspid regurgitation to enable the quantification of sPAP with Doppler echocardiography, and when patients with PH undergo cardiac MR imaging for evaluation of right ventricular function.

Only patients with PAH were included in our study. Whether these results hold true in other types of PH, such as pulmonary venous hypertension, requires further research. Similarly, all our patients had chronic conditions, and we are uncertain if equivalent correlations between PA pressures and average velocity occur in the acute setting. Not all the examinations were performed on the same day, and this fact might have influenced the strength (although probably not the direction) of the observed correlations. Ideally, simultaneous measurement of pressures and flow velocities would be desirable, but, although feasible (38), this is currently not possible in the majority of cardiac MR laboratories. This investigation was performed in a tertiary referral center, and, consequently, there is likely some referral bias, as shown by the high prevalence of PH in our study patients. Although receiver operating characteristic curve analysis is not affected by disease prevalence (39), extrapolation of our results to a population with lower pretest probability requires caution.

We acquired phase-contrast MR images during breath holding because of fewer motion artifacts and shorter acquisition time, whereas RHC was performed during free breathing. Our results might not apply exactly for images acquired during free breathing, because both pulmonary velocities and flow may change (40). Although gross motion artifacts were absent in this study, incomplete breath holds might have occurred in some instances. Similarly, the possibility of inadvertent Valsalva maneuvers by some of the patients, with additional influence on flow velocity, cannot be ruled out. Finally, the study was sufficiently powered to analyze correlations in the whole group, but the absence of differences among subgroups must be taken with caution because of sample size considerations.

In conclusion, in patients with chronic PAH, a variety of flow measurements in the pulmonary trunk evaluated with phase-contrast MR imaging correlate with the degree of hemodynamic disturbance as determined with the level of pulmonary pressures and vascular resistance. The average blood velocity throughout the cardiac cycle is strongly correlated with pulmonary pressures and resistance and appears to have consistent performance across different subgroups of patients. We believe this parameter can be used to noninvasively detect the presence or absence of chronic PAH and might offer important clinical use for the evaluation of clinical response after therapeutic intervention.


   ADVANCE IN KNOWLEDGE
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 MATERIALS AND METHODS
 RESULTS
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   FOOTNOTES
 

Abbreviations: AT = acceleration time • CI = confidence interval • ET = ejection time • mPAP = mean PA pressure • PA = pulmonary artery • PAH = pulmonary arterial hypertension • PH = pulmonary hypertension • PVRI = pulmonary vascular resistance index • RHC = right-sided heart catheterization • sPAP = systolic PA pressure

Authors stated no financial relationship to disclose.

Author contributions: Guarantors of integrity of entire study, J.S., M.P.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, J.S., P.K.; clinical studies, J.S., P.K., T.R., R. Sulica, S.D., S.R., M.P.; statistical analysis, J.S., R. Salguero; and manuscript editing, J.S., R. Salguero, A.J.E., S.D., V.F., M.P.


   References
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 References
 

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