Quantification of cardiac jets: theory and limitations

The Mitral/Aortic Flow Velocity Integral Ratio in Mitral Regurgitation

The development of echocardiography was driven, in part, by a need to diagnose mitral regurgitation in patients undergoing finger fracture commissurotomy in the 1950s. Decades later, color Doppler became the cornerstone for noninvasive evaluation of mitral regurgitation. The authors present 2 cases of calcific mitral stenosis in which reliance on color Doppler in transthoracic echocardiography resulted in erroneous conclusions as to the severity of coexisting mitral regurgitation. The possible application of the Mitral to Aortic Flow Velocity Integral Ratio in such cases as a possible adjunct to grading mitral regurgitation is also discussed.

Semiquantitative and Quantitative Color Flow Mapping Methods

Conventional pulsed and continuous-wave Doppler applications have been in use since the mid-1970s. The first color flow mapping system was a color m-mode instrument developed by a Swiss engineer named Brandistini. Stevenson and coworkers utilized this system for cardiac sonographic examinations in the late 1970s, but in general, there was very little interest in the method until the early 1980s, when a 2D color flow mapping system became available. With this new technology and the interest it generated, the development in color flow mapping Doppler technology moved at a rapid pace.

True shape and area of proximal isovelocity surface area (PISA) when flow convergence is hemispherical in valvular regurgitation

The proximal isovelocity surface area (PISA) method for quantifying valvular regurgitation uses an echocardiographic image with superimposed colour Doppler mapping to visualise the contours of velocity in the blood travelling towards the regurgitant orifice. The flux of blood through the regurgitant orifice is obtained as the product of the area of one of these (presumed hemispherical) contours and the speed of the blood passing through it. However, colour Doppler mapping measures the velocity component towards the echo probe (v cos theta;) rather than speed (v), so that the contours of equal Doppler velocity (isodoppler velocity contours) differ from isospeed contours. We derive the shape of the isodoppler contour surface obtainable by colour Doppler mapping, and show that its area is much less than that of the hemispherical isospeed contour. When regurgitant flux is derived from an appropriate single measure of contour dimension, an appropriate result may be obtained. However, if the true echocardiographic surface area is measured directly, the regurgitant flux will be substantially underestimated. Indeed, the conditions necessary for isodoppler velocity contours to be hemispherical are extraordinary. We should not therefore make deductions from the apparent shape for the convergence zone without considering the principles by which the image is generated. The discrepancy will assume practical significance when increased resolution of colour Doppler technology makes measurement of apparent surface area feasible. Assuming the flow contours are indeed hemispherical, a 'correction' factor of 1.45 would be required.

A rotating torus phantom for assessing color Doppler accuracy

A rotating torus phantom was designed to assess the accuracy of color Doppler ultrasound. A thin rubber tube was filled with blood analog fluid and joined at the ends to form a torus, then mounted on a disk submerged in water and rotated at constant speeds by a motor. Flow visualization experiments and finite element analyses demonstrated that the fluid accelerates quickly to the speed of the torus and spins as a solid body. The actual fluid velocity was found to be dependent only on the motor speed and location of the sample volume. The phantom was used to assess the accuracy of Doppler-derived velocities during two-dimensional (2-D) color imaging using a commercial ultrasound system. The Doppler-derived velocities averaged 0.81 +/- 0.11 of the imposed velocity, with the variations significantly dependent on velocity, pulse-repetition frequency and wall filter frequency (p < 0.001). The torus phantom was found to have certain advantages over currently available Doppler accuracy phantoms: 1. It has a high maximum velocity; 2. it has low velocity gradients, simplifying the calibration of 2-D color Doppler; and 3. it uses a real moving fluid that gives a realistic backscatter signal.

Evaluating isovelocity surface area flow convergence method with finite element modeling

Through numerical experimentation we investigated the isovelocity surface area flow convergence method used in estimating regurgitant valve flow rates. Recent advances in three-dimensional color Doppler flow imaging have created renewed interest in this method. Experimentation was based on the use of depth-averaged finite element models of the left heart. The heart models studied varied from "synthetic" representations to a model of a left heart traced from an actual echocardiographic image of a patient with a prolapsed mitral valve. The isovelocity surface area flow convergence method overestimated regurgitant flow rates throughout the Nyquist limits considered with a critical Nyquist limit in which this overestimation is minimized. The angle dependence of Doppler color flow imaging partially corrects for this overestimation. The isovelocity surface area flow convergence method is a viable alternative to methods currently in use. Through numerical experimentation, we have begun to shed light on the inaccuracies inherent in this flow convergence method.

Clinical application of transthoracic volume-rendered three-dimensional echocardiography in the assessment of mitral regurgitation

Two-dimensional echocardiography (2-DE) and Doppler methods are generally used for assessing mechanisms and severity of mitral regurgitation (MR). Recently, 3-dimensional echocardiography (3-DE) has been applied successfully in various cardiac disorders, but its value in evaluating the mechanism and the severity of MR are not known. We studied 30 patients with MR using 2-DE and 3-DE. Volume-rendered gray-scale 3-DE images of the mitral valve apparatus and MR jets were reconstructed. Maximal volume of the MR jet by 3-DE was compared with mitral regurgitant volume and fraction, regurgitant jet area and the ratio of jet area to left atrial area, and semiquantitative grading derived from 2-DE methods. Our results demonstrated that 3-DE aided in a better depiction of the mitral apparatus and its abnormalities in 70% of the patients. The origin, direction, and morphology of the MR jet were better delineated in 3-DE volumetric display. Quantitative analysis, however, showed only a weak to moderate correlation between 3-DE maximal MR jet volume and 2-DE mitral regurgitant volume (y = 0.5x + 11.4, r = 0.7), regurgitant fraction (y = 0.5x + 8.2, r = 0.65), mitral regurgitant jet area (y = 0.2x + 5, r = 0.51), jet area to left atrial area ratio (y = 0.53x + 7.6, r = 0.54), and semiquantitative grading of MR (y = 9.1x - 1.8, r = 0.74). In conclusion, 3-DE aids in a better understanding of the mechanisms of MR and morphology of the regurgitant jets. Its quantitative ability, when reconstruction of the jet alone is used, may be limited.

Aliasing-tolerant color Doppler quantification of regurgitant jets

Conservation of momentum transfer in regurgitant cardiac jets can be used to calculate the flow rate from color Doppler velocities. In this study, turbulent jets were simulated by finite elements; pseudocolor Doppler images were interpolated from the computations, with aliasing introduced artificially. Jets were also imaged by color Doppler in an in vitro flow system. To suppress aliasing errors, jet velocities were fitted iteratively to a fluid mechanical model constrained to match the orifice velocity (measured without aliasing by continuous-wave Doppler). At each iteration, the model was used to detect aliased velocities, which were excluded during the next iteration. Iteration continued until the flow rate calculated by the model and number of calculated nonaliased pixels were unchanged. The good correlations between measured and calculated flow rates in the experimental (R2 = 0.933) and computational studies (R2 = 0.990) suggest that this may be a clinically useful approach even in aliased images. Published by Elsevier Science Inc.

Effect of left ventricular outflow on flow convergence region on the left septal surface in ventricular septal defect

The corrected shunt flow rate (Fc) and corrected defect orifice area (Ac) were calculated by modified equation F = 2 pi R2 (NL-VLVOT x Sin theta) in 23 patients with single membranous ventricular septal defect, in order to correct the effect of left ventricular outflow on flow convergence region on the left septal surface. The results indicated that Fc was closely correlated with Qp-Qs, and Qp/Qs measured by pulsed wave Doppler (r = 0.95 and r = 0.81 respectively, P < 0.001). And the correlation between Ac and the diameter of defect (Dd) measured directly in two-dimensional views was better than that between uncorrected defect orifice area (A) and the Dd (r = 0.98 and 0.69, respectively, P < 0.001). The shunt flow rate calculated by ideal equation F = 2 pi R2 x NL overestimated the actual shunt flow rate in ventricular septal defect, especially in membranous type. Our study concluded that Fc can be used for a more accurate evaluation of the shunt severity of ventricular septal defect.

Stenotic lesions

Images

Detection and localization of early diastolic forces within the left ventricle from inflow jet dynamics. A comparison between normal subjects and patients with dilated cardiomyopathy

We studied the properties of the jet of blood entering the left ventricle from the left atrium during early diastole in 32 patients with dilated cardiomyopathy, and 24 normal subjects of similar age. The diameter of the jet was measured from the cross-sectional color Doppler image and its cross-sectional area (JA) was derived. Pulsed Doppler records of flow velocity were made at 1-cm intervals into the ventricle from the mitral ring. Peak (Vp) and mean (Vm) E wave velocity and time velocity integral (TVI) were determined. At any level in the ventricle, therefore, the early diastolic volume of blood remaining in the jet, i.e., the flow time integral, is given by JA.TVI; the local flow rate, Q, by JA.Vm; and jet momentum along the long axis of the ventricle by Q.Vp. In normals, the jet cross-sectional area fell from 5.9 (1.3) cm2 at the mitral ring to 4.9 (0.7) cm2 at 4 cm (P < 0.05), but the flow time integral fell proportionately more, from 46.0 (15.2) ml at the ring level to 15.9 (3.4) ml at 4 cm (P < 0.01). Axial momentum flux was 44 (13) x 10(2) cm4s-2 at the ring level, falling to 28 (10) x 10(2) cm4s-2 at 4 cm (P < 0.01). In dilated cardiomyopathy, the jet cross-sectional area was much smaller than normal, 1.9 (0.8) cm2 at the ring level, and it remained effectively constant, being 2.0 (0.9) cm2 at 6 cm (P < 0.01 vs normals).(ABSTRACT TRUNCATED AT 250 WORDS)