Rationale and Objectives
To investigate four-dimensional (4D) phase-contrast (PC) magnetic resonance (MR) in the evaluation of intracardiac shunts by simultaneous assessment of pulmonary (Q P ) and systemic (Q S ) flows in a pilot study and to compare results to through-plane two-dimensional (2D) PC MR.
Materials and Methods
Institutional review board approval and written informed consent were obtained. Nineteen patients with suspected intracardiac shunts underwent cardiac MR at 1.5T. Assessments of Q P and Q S were performed using free-breathing retrospectively gated 2D PC gradient recalled echo (GRE; 1.6 × 1.6 × 5 mm 3 ) imaging with one-dimensional through-plane velocity encoding gradient (v enc = 150 cm/s) in consecutive measurements for the main pulmonary artery (MPA) and ascending aorta (AA), respectively. A prospectively triggered 4D PC GRE technique (2.4 × 1.8 × 3 mm 3 ) with three orthogonal v enc directions was also used with volume coverage of both MPA and AA.
Results
Q P and Q S assessed by 4D PC correlated with 2D PC acquisitions ( r = 0.92 and r = 0.67 respectively; P < .0001 for both) but demonstrated significant underestimation of individual flow volumes (−21.9 ± 12.2 mL; P < .0001 and −10.7 ± 13.1 mL; P = .0023, respectively). Calculated Q P :Q S ratios demonstrated high correlation ( r = 0.78; P < .0001) and no significant differences between 4D PC and 2D PC acquisitions (−0.09 ± 0.24, P = .14). Image acquisition times for 2D PC assessment of Q P and Q S were 2.98 ± 0.52 and 2.84 ± 0.50 minutes, respectively ( P = .038), whereas time to acquire 4D PC images was significantly longer, 18.75 ± 4.58 minutes ( P < .001).
Conclusions
Four-dimensional PC MR imaging allows for accurate assessment of Q P :Q S ratios in the evaluation of intracardiac shunts while absolute flow volumes demonstrate offsets. Further refinement of the technique with improvement in acquisition times may be required before widespread clinical implementation.
Atrial septal defect (ASD), ventricular septal defect (VSD), and patent ductus arteriosus (PDA) are among the most common congenital cardiac abnormalities . Shunt volumes and the ratio of flows in the pulmonary (Q P ) and systemic (Q S ) circulations are important measures in the evaluation of left-to-right shunts .
Multiple studies have demonstrated reliability of two-dimensional (2D) phase-contrast (PC) magnetic resonance imaging (MRI) in the assessment of Q P :Q S . Good agreement of 2D PC MRI with invasive oximetry, radionucleotide angiocardiography, and ventricular stroke volumes obtained by cine MRI have been previously reported . However, standard 2D PC magnetic resonance (MR) assessment of Q P :Q S performed with consecutive evaluation of the ascending aorta (AA) and main pulmonary artery (MPA) may be limited in accuracy by interval changes of hemodynamics such as heart rate (HR) that occur between separate PC measurements. Operator-related nonoptimized slice orientation may result in additional offsets for a single velocity encoding direction (along the slice selection direction-through-plane orientation) because of misalignment of slice encoding versus actual flow direction . As in other MR angiography techniques, special attention must be made during prescription of imaging planes .
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Materials and methods
Study Population
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Cardiac MRI
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Data Postprocessing
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Statistical Analysis
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Results
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Table 1
Phase-contrast Measurements ( n = 19)
Flow Correlation 2D PC 4D PC_P_ value_r_ value Q S (mL) 77.4 ± 19.1 66.7 ± 22.9 .001 0.67 Q P (mL) 96.4 ± 34.5 74.5 ± 25.7 .001 0.92 Q P :Q S 1.28 ± 0.51 1.19 ± 0.51 .001 0.78
Q P , pulmonary flow (main pulmonary artery); Q S , systemic flow (ascending aorta); 2D PC, two-dimensional phase-contrast acquisition; 4D PC, four-dimensional phase-contrast acquisition.
Data are mean ± standard deviation unless otherwise noted.
Correlation was assessed by Pearson correlation coefficient.
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Table 2
Two-by-Two Table Comparing Shunt Identification by 4D PC to 2D PC Acquisition Using a Q P :Q S Cutoff of ≥1.5
Q P :Q S Cutoff ≥1.5 Total 2D PC + 2D PC − 4D PC + 3 (15.8) (TP) 0 (0.0) (FP) 3 (16) 4D PC − 1 (5.3) (FN) 15 (78.9) (TN) 16 (84) Total 4 (21) 15 (79) 19 (100)
FN, false negative; FP, false positive; TP, true positive; TN, true negative.
Data are represented as number of patients with percentages in parentheses.
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Discussion
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Acknowledgments
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