The PVA is planimetered directly from gate steady-state free precession (SSFP) images that are acquired as a series of thin, parallel slices. These slices are oriented perpendicularly to the PS jet. For accurate quantification, it is critical to select the correct image to planimeter: During the cardiac phase where the valve is most open, the measurement is made at the tips of the pulmonary valve cusps—and where the pulmonic valve opening is narrowest. The most most distal slice, where the pulmonary sinuses are clearly visible, is selected. Upon tracing the phase with the largest valve area, all blood (white) should be included; the calcified cusps (black) should be excluded (F1).
II. Continuity Equation:
The continuity equation is used as an indirect method for determining PVA and calculates it based on three parameters:
1. area of the right ventricular outflow tract (RVOT area, cm2);
2. peak velocity of blood flowing through the RVOT (VRVOT, cm/s); and
3. peak velocity of blood flowing through the pulmonic valve (Vp, cm/s).
Measurements are made from a series of thin, parallel phase-contrast images that are oriented perpendicularly to the right ventricular outflow tract (RVOT) and PS jet. The RVOT area is determined on a slice passing through the RVOT (F2). This slice should be closest to the pulmonic valve and must not contain partial volumes of blood averaging with the elevated blood velocities from the stenotic pulmonic valve. RVOT area may be acquired by placing a region of interest (ROI) on the phase contrast image (F2). The peak velocity of blood flowing through the RVOT (VRVOT) is determined on the magnitude image of the cardiac phase that has the highest systolic velocity. This ROI comprises the central portion of the RVOT (F2).
The peak velocity of the blood flowing through the pulmonary valve (VP) is determined at the tips of the pulmonic valve cusps. This 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 are excluded. A sizeable ROI is traced around the entire PV, and a histogram of a range of blood velocities through the it and throughout the cardiac cycle is displayed (F3). The highest, most reproducible blood velocity is selected as VP.
Once the above parameters (1.-3.) are acquired, the continuity equation is employed:
III. Pressure gradient:
Transvalvular pressure gradient also describes PS severity. To obtain the peak pressure gradient (PPG) and the mean pressure gradient (MPG) across the PV, the modified Bernoulli equation may be used:
The severity of PS is classified according to the table below (Tb.).
|Tb. Pulmonic stenosis severity||PV area (cm2)||PV peak pressure (mmHg)||PV mean pressure (mmHg)||PV peak velocity (m/s)|
Typically, classifications of PS severity agree: PS severity that is acquired by valve area and PS severity that is acquired by pressure gradient(s) often will correspond qualitatively. For instance, patients whose PS is determined “severe” by valve area often are found to have “severe” PS by pressure gradient analysis. Nevertheless, sometimes there are discrepancies between the two methods of analysis. In patients with diastolic dysfunction and small stroke volumes, the severity of PS by planimetry will exceed the severity by pressure gradient—due to a decrease in the volume of blood passing through the PV. Conversely, in patients with substantial pulmonic regurgitation, the severity of AS by pressure gradient will exceed the severity of AS by planimetry—due to an increase in the volume of blood passing through the PV.
One potential drawback to using MRI and the continuity equation to assess PS severity is that there is sometimes a loss of blood signal when PS is severe. As a result, the highest-velocity blood flow may go undetected, leading to an underestimation of the peak velocity(ies) 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.