Computational analysis of quantitative echocardiographic assessments of functional mitral regurgitation: Proximal Isovelocity Surface Area (PISA) methods
Authors: Qin Tongran , Caballero Andrés , Hahn Rebecca T. , McKay Raymond , Sun Wei 
Published: September 2021

While proximal isovelocity surface area (PISA) method is one of the most common echocardiographic methods for quantitative mitral regurgitation (MR) assessment, accurate MR quantification remains challenging. This study examined the theoretical background of PISA, performed virtual echocardiography on computer models of functional MR, and quantified different sources of errors in PISA. For regurgitant flow rate measurement, the conventional 2D hemispherical PISA caused significant underestimation due to underestimation of PISA area, the multiplane 2D hemiellipsoidal and hemicylindrical PISA provided improved accuracy with better assumptions on PISA contour shape. With the direct capture of PISA area, the 3D-PISA was found to be the most accurate. However, it should be noted that PISA method is subject to systematic underestimation due to the Doppler angle effect, and systematic overestimation due to the “flow direction angle” between the regurgitant flow direction and the PISA contour normal direction. For regurgitant volume quantification, integrated PISA, when performed properly, was able to capture the dynamic MR and therefore was more accurate than peak PISA. In specific, integrated PISA using the sum of regurgitant flow rates is recommended.

Objectives

The aim of this study was to evaluate the accuracy of different proximal isovelocity surface area (PISA) methods, examine their theoretical background, and quantify multiple sources of error in functional mitral regurgitation (MR) assessment.

Background

While PISA method is one of the most common echocardiographic methods for MR severity assessment, it is associated with multiple sources of errors, and accurate MR quantification remains challenging.

Methods

Five functional MR (FMR) computer models were created, validated and treated as phantom models. The phantom models have fully resolved and detailed flow fields in the left atrium (LA), left ventricle (LV) and cross the mitral valve, from which the reference values of mitral regurgitant flow rate and regurgitant volume can be obtained. The virtual PISA measurements (i.e., 3D and 2D PISA) were performed on the phantom models assuming optimal echo probe angulation and positioning. The results of different PISA methods were compared with the reference values.

Results

For regurgitant flow rate measurements, compared to the reference values, excellent correlations were observed for 3D-PISA (R = 0.97, bias -24.4 ± 55.5 ml/s), followed by multiplane 2D hemicylindrical (HC)-PISA (R = 0.88, bias -24.1 ± 85.4 ml/s) and hemiellipsoidal (HE)-PISA (R = 0.91, bias -55.7 ± 96.6 ml/s), while weaker correlations were observed for single plane 2D hemispherical (HS)-PISA with large underestimation (PLAX view: R = 0.71, bias -77.6 ± 124.5 ml/s; A2Ch view: R = 0.69, bias -52.0 ± 122.0 ml/s; A4Ch view: R = 0.82, bias -65.5 ± 107.3 ml/s). For regurgitant volume (RV) quantification, integrated PISA presented improved accuracy over peak PISA for all PISA methods. For 3D-PISA, the bias in RV improved from -12.7 ± 7.8 ml (peak PISA) to -2.1 ± 5.3 ml (integrated PISA).

Conclusions

In FMR, conventional single plane 2D HS-PISA significantly underestimated MR, multiplane 2D PISA (HE-PISA and HC-PISA) improved the accuracy, and 3D-PISA is the most accurate. To better capture the dynamic feature of MR, integrated PISA using the sum of regurgitant flow rates is recommended.