16 Proximal Isovelocity Surface Area and Flow Convergence Methods

The proximal isovelocity surface area (PISA)/flow convergence technique is an accepted quantitative measure of both valvular regurgitation and stenosis. Although it can be applied to any valve, subvalvular lesion,¹ valve prosthesis,² or any other structure with an orifice (e.g., a ventriculoseptal defect³), the PISA technique is used principally to assist in determining the severity of mitral regurgitation (MR), mitral stenosis (MS), and aortic insufficiency (AI) when other methods are less concordant and appear less sound.

The shortcomings of color Doppler flow mapping to determine the severity of valvular insufficiency are numerous and have been repeatedly characterized.^4–6 Although MR color Doppler jet size (area and length) predict angiographic grade, they exhibit a weak correlation with regurgitant volume (R_Vol) and do not predict hemodynamic dysfunction.⁷ In some lesions, such as functional/ischemic MR, color Doppler flow mapping tends to systematically overestimate the severity of mitral insufficiency; in fact, most jets larger than 8 cm² do not correspond to severe MR, advancing the concept of the need for quantitative determination of the severity of mitral insufficiency.⁸ Eccentric jets of MR correlate much less well with severity of MR⁹ due to complex spatial redistribution and loss from frictional forces.¹⁰ The effect of general anesthesia on the severity of mitral insufficiency is profound: more than half (51%) of patients with moderate to severe MR improved by at least one severity grade when assessed by transesophageal echocardiography under general anesthesia.¹¹ In the postoperative state, PISA determination of grade of MR correlates far better with angiographic grade of MR (r = 0.89 and 0.92, P < 0.001) than does color Doppler flow mapping determination of severity (r = 0.44, P < 0.1).¹² Given essentially perfect specificity (100%, positive predictive value: 100%),¹³ the finding of upper vein pulmonary venous flow reversal is the single most useful parameter to determine that MR is severe, but is limited by imperfect transthoracic sampling (reducing sensitivity: 82%),^13,14 and occasionally by the effect of highly eccentric jets or massive atrial compliance. The single most common scenario in which the PISA technique is applied is in describing the severity of MR when color flow mapping is confounded by severe jet eccentricity and the pulmonary venous spectral tracings are confounded by poor quality.

The PISA method arises from the suitability of color Doppler flow mapping to depict the hemodynamic phenomenon of flow convergence as fluid is pushed toward and through a restrictive orifice (one that imparts a pressure gradient). As blood is forced toward, and then through, a restrictive stenotic or regurgitant orifice it accelerates progressively toward its maximal velocity within its tightest stream—the vena contracta. The phenomenon of flow convergence, coupled with the versatility of color Doppler flow mapping, lends itself to the depiction of volumetric flow across a restrictive orifice, because by color Doppler flow mapping, a series of concentric “isovelocity” rings or hemispheres are depicted over the area of convergence. The greater the flow rate/volume and the smaller the orifice, the larger the flow convergence and acceleration.

Flow acceleration occurs within a hemisphere before the orifice, largely independently of the shape of the orifice, which eliminates one of the most common variables encountered in valve disease. The greater the flow volume, the larger the hemisphere of flow acceleration and the greater dimension of the concentric isovelocity rings. Hence, the dimension of the isovelocity rings depicts the flow rate: a large PISA is consistent with a large flow rate. Optimal hemispheric depiction by color Doppler occurs when the contour velocity is approximately 5% to 10% of the orifice velocity.¹⁵

The hemisphere of flow acceleration is oriented in line with the orifice; the base of the hemisphere sits on the orifice. As many orifices are oblique to the valve structure, the hemisphere may be oblique or very oblique to the angle of imaging, which engenders difficulty in recording accurate peak velocity and velocity time integral (VTI), which are needed for subsequent calculations.

In many cases, the full hemisphere of flow acceleration cannot form because physical structures are so close to the orifice that they deny (“constrain”) the formation of a geometric hemisphere. Isovelocity mapping constraint occurs commonly: in organic MR, as with mitral valve prolapse and flail leaflets; in mitral stenosis, should the diastolic shape of the valve leaflets yield a cone, as invariably happens when subvalvar disease predominates; or in aortic stenosis, as the walls of the left ventricular outflow tract confine the isovelocity rings. In such cases, applying the usual PISA method yields less accurate or inaccurate results. “Angle correction” has been proposed as a remedy for cone-shaped orifices, which are common in mitral stenosis (the orifice area calculation is multiplied by the oblique angle of the orifice [in degrees] divided by 180). The correction often is feasible for MR and mitral stenosis, but less so for aortic stenosis. Without angle correction, in the presence of constraining walls, there is significant overestimation of flow when a hemispheric model is used.¹⁶

The extent of convergent flow is readily depicted and described using color Doppler flow mapping, by measuring the dimension of the hemisphere formed from the blood flow. As the blood accelerates toward the orifice, velocity aliasing occurs and a distinct two-color (mostly red-blue) interface occurs at the boundary of the shell. At this interface the velocity is equivalent to the aliasing velocity, which is represented by the color scale. The ability to select color Doppler flow mapping parameters, such as the aliasing velocity, affords the ability to optimize the depiction of the hemisphere of flow convergence, and thereby the parameters needed to calculate aliasing flow velocity and the dimension at which flow velocity aliases. The ready means to adjust the baseline aliasing velocities makes it possible to optimize the velocities of the PISA concentric isovelocity rings (usually by lowering the aliasing velocity) by shifting the baseline down, but also to allow somewhat of a constant over the aliasing limit for the mathematical calculation of the descriptors of severity of regurgitation—the effective regurgitant orifice (ERO) and the R_Vol. The same technique can be used to determine the orifice of a stenotic lesion.

PISA method parameters needed for the equations that determine the oriface area and R_Vol include the following:

Measuring the radius of the first aliasing hemisphere is the single most difficult aspect of the PISA method, and should be the focus of attention and time. As the measurement is squared, error compounds rapidly; hence, optimizing the image and measurement is critical. Identification of valve plane (by two-dimensional echocardiography) is critical because the PISA measurement is from the aliasing velocity to the valve orifice.

Proximal Isovelocity Surface Area Scanning Parameters

The parameters used in PISA calculations to describe flow through a restrictive orifice are discussed in the following sections.

Two-Dimensional and Grayscale Imaging

The level of the orifice is one means by which it is possible to delineate the inner aspect of the PISA hemispheric shell and its radius. Neither the structure with the orifice nor the orifice itself is readily seen on the color Doppler flow-mapped image. A separate grayscale image is needed. Some software allows the color Doppler flow mapping to be toggled off and on from the image, or side-by-side display of the image with and without color flow mapping, which is very useful. Zoom views are needed.

Often the plane of tissue in which the orifice resides has a dimension of depth. Usually, the orifice is at the deeper rather than the most superficial aspect of the tissue.

Color Doppler Measurements

Proximal Isovelocity Surface Area Hemisphere

In the PISA hemisphere, or outer radius, the half moon (as depicted by planar imaging) that is created as flow accelerates or convects to move through a restrictive orifice. The use of color Doppler flow mapping depicts the outer margin of the PISA hemisphere. Specifically, the outer margin of the first aliasing radius (the inner hemisphere shell) is the outer aspect of the radius.

The following technical issues may arise:

For the PISA radius measurement, one should attempt to measure the radius in line with the angle of insonation.

Use the largest zoom of the flow at the mitral tips.

Move the color aliasing baseline between 20 and 40 cm/sec. However

• In high-flow states, move it closer to 40 cm/sec

• In low-flow states, move it closer to 20 cm/sec

Use of lower aliasing velocities may engender greater variation in the delineation of the outside of the shell.

Measure the radius of the internal isovelocity hemisphere (the first aliased hemisphere)—this is invariably encoded as the color opposite the direction of flow.

Measure at the average distance of the color boundary, not the maximal distance.

Color Doppler Aliasing Velocity

The actual velocity is assigned to the first aliasing velocity (usually the interface of blue to red). If looking at mitral regurgitation from the apical four-chamber view, the aliasing baseline is shifted in the direction of the jet, to between 20 and 40 cm/sec, allowing for a better defined visual demarcation of the PISA hemisphere. This allows for a more accurate measurement of the radius of the flow, as the PISA is larger and less prone to measurement error.

There are no technical issues to be aware of. The aliasing velocity is displayed on the scale and can be used as is.

Radius of the Proximal Isovelocity Surface Area Hemisphere

The radius of the PISA hemisphere is measured as the dimension from the top of the hemisphere to the waist of the vena contracta. The dimension of the vena contracta itself is an index of flow volume, but in the case of PISA calculations, the level of the formation of the vena contracta determines the innermost extent of the PISA hemisphere, and helps yield the diameter of the hemisphere.

The following technical issues may arise:

The outer margin of the hemisphere may or may not be uniform, and may or may not lend itself to measurement.

The average outer margin, rather than the greatest outer margin, probably should be used.

The ideal measurement of the innermost aspect of the hemisphere is made to the base of the waist of the apparent vena contracta.

Vena Contracta

The vena contracta is the narrowest portion of the orifice through which flow occurs (the level of the orifice). The dimension of the vena contracta itself is an index of flow volume, but in the case of PISA calculations, the level of formation of the vena contracta determines the innermost extent of the PISA hemisphere, and helps yield the diameter of the hemisphere.

The following technical issues may arise:

The vena contract remains a more conceptual/flow chamber notion than a reliable clinical imaging finding. Until the advent of anti-aliasing filters it is likely that it often will remain ambiguously depicted.

The proximal end of the vena contracta, as depicted by color Doppler flow mapping, does not always co-register with the level of the orifice as seen on two-dimensional grayscale imaging.

Continuous Wave Doppler Measurements

It is critical that the angle of insonation be within 20 degrees of parallel to ensure that measurement error is within 10%:

where f_D is the Doppler shift of reflected ultrasound, f₀ is the transmitted frequency, v is the blood or tissue velocity, c is the sound velocity in tissue, and α is the isonation angle between the ultrasound beam and the direction of motion (velocity vector).

The peak velocity of the continuous wave spectral Doppler flow profile is obtained by measuring with calipers. Determ ination of the peak velocity requires sufficient signal and adequate signal-to-noise ratio to have the actual peak depicted. Often the true peak is unclear or not depicted unless the amount of signal is adequate. Hence, for mild and moderate grades of insufficiency, continuous wave (CW) parameters may be weak.

The VTI of the continuous wave spectral Doppler flow profile is obtained by edge-tracing the spectral “envelope.”

Incomplete spectral profiles are a common problem for mild and moderate grades of insufficiency.

Proximal Isovelocity Surface Area Equations

Derivations of the Proximal Isovelocity Surface Area

Aliasing velocity is calclulated in the direction of flow at the radial distance r:

VFR, or simply flow (cc/sec, mL/sec) = 2π ×

× V_a(cm/sec)

 

 

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1. The Aortic Valve
2. Aortic Insufficiency
3. Coronary Artery Disease: Stress Echocardiography
4. Mitral Insufficiency

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