Quantifying Aortic Stenosis
Aortic stenosis or aortic valve stenosis (AS) is defined as the presence of an increase in pressure across the aortic valve (AV). It is important to distinguish AS from aortic valve sclerosis, which is the narrowing of the AV without a transvalvular pressure gradient. MRI analysis defines AS severity by aortic valve area (AVA) and/or transvalvular pressure gradient. AVA is quantified directly by planimetry and/or indirectly by the continuity equation.
The AVA is directly planimetered from gated steady-state free precession (SSFP) images that are acquired as a series of thin parallel slices, which are oriented perpendicularly to the AS jet. Determining the correct image to planimeter is critical for accurate quantification. The measurement must be made at the tips of the aortic cusps where the aortic valve is narrowest during the cardiac phase where the valve is most open. To determine the proper slice location, it is necessary to identify the most distal slice where the 3 sinuses of Valsalva are clearly visible. When tracing the phase with the largest valve area, all the white blood should be included, and the black boundary, representing the calcified cusp, should be excluded (F1).
II. Continuity equation:
The continuity equation is often used as a second, indirect method for determining the AVA. Analysis by continuity equation calculates the AVA from three parameters:
i. area of the left ventricular outflow tract (LVOT area, cm2)
ii. peak velocity of blood flow through the LVOT (VLVOT, cm/s or m/s)
iii. peak velocity of blood flow through the aortic valve (VA, cm/s or m/s)
Measurements are made from a series of thin, parallel phase-contrast images that are oriented perpendicularly to the left ventricular outflow tract (LVOT) and AS jet. The LVOT area should be determined on the slice passing through the LVOT. This slice also should be closest to the aortic valve and must not contain partial volumes of blood averaging with the elevated blood velocities from the stenotic aortic valve. LVOT area may be acquired by placing an ROI on the phase contrast image (F2). The peak velocity of blood flow through the LVOT (VLVOT) should be determined on the magnitude image of the cardiac phase that has the highest systolic velocity; this ROI should comprise the central portion of the LVOT (F2).
The peak aortic valve velocity (VA) is determined at the tips of the aortic valve cusps. The slice location is the same as for planimetry (described above). The single pixel with the highest reliable velocity is used, and pixels with excessive noise must be excluded (F3). A large ROI is placed around the entire aortic valve, and a histogram of velocities is displayed (F3). An envelope of displayed velocities helps to assess which velocity values are reliable.
Once the above parameters (i.-iii.) are obtained, the continuity equation may be employed:
III. Pressure gradient:
The transvalvular pressure gradient also describes AS severity. To obtain the peak pressure gradient (PPG) and the mean pressure gradient (MPG), the modified Bernoulli equation may be used:
The severity of AS is classified according to the table below (Tb.).
|Tb. Aortic stenosis severity scale||AV area (cm2)||AV peak pressure (mmHg)||AV mean pressure (mmHg)||AV peak velocity (m/s)|
The classifications of AS severity that are acquired by valve area and pressure gradients typically will correspond qualitatively. That is, patients whose AS is determined to be severe by valve area also often have severe AS by pressure gradient analysis. However, in some cases there are discrepancies between the two methods of analysis. For example, in patients with diastolic dysfunction and small stroke volumes, the severity of AS by planimetry will exceed the severity by pressure gradient. Conversely, in patients with substantial aortic regurgitation, the severity of AS by pressure gradient will exceed the severity of AS by planimetry. This is because the amount of blood passing through the aortic valve is increased in patients with aortic regurgitation.
One potential drawback to using MRI and the continuity equation to assess AS severity is that there is sometimes a loss of blood signal when AS is severe. Consequently, the highest velocity blood flow may go undetected, leading to an underestimation of peak velocities and a corresponding underestimation of the severity of the transvalvular pressure gradient. In these circumstances, it may be useful to employ an alternative approach for determining the transvalvular pressure.
Cardiac Output Valve Area (COVA) is a novel method that may be used for quantifying the aortic transvalvular pressure gradient. Phase contrast images are not required. However, this method is reliable only in the absence of bradycardia, tachycardia, shunts and/or regurgitation. COVA is based on the Hakki equation:
Based on frame-by-frame changes in the left ventricular (LV) systolic volume, pressure gradients are computed by cardiac output (CO) and AVA. CO is a function of heart rate (HR) and stroke volume (SV). Each of these three variables is acquired by software through the autosegmentation and quantification of LV blood volume changes. To determine CO, HR, and SV, the range of LV autosegmentation is defined from the most apical, end systolic slice at one end point on the short-axis series. The other end point is defined by drawing a line across the mitral valve on the 2-chamber, long-axis view, marking the end systolic base (F4A). Software then propagates the contours across the defined range, completing the autosegmentation and acquiring an LV volume vs. time graph, and CO, HR, and SV readings (F4B). Along with the AVA (obtained by planimetry), these parameters also may be employed in the Haaki equation (above) to obtain the mean pressure gradient (MPG) and corresponding AS severity. The peak pressure gradient (PPG) may be acquired from a pressure gradient vs. time graph (Fig.4C).
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