Quantifying right ventricular (RV) stroke volume
Accurately quantifying the right ventricular stroke volume (RVSV) is essential to a complete analysis of cardiac function. RVSV is the volume of blood ejected from the right ventricle (RV) during one heart contraction. RVSV may be quantified, either manually or automatically, using software. This datum is derived from a series of parallel short axis slices or from one or more long axis slices (using geometric assumptions). A common method for determining RVSV, also used when quantifying left ventricular stroke volume and discussed in greater detail below, involves the manual segmentation of contiguous short axis slices. This analysis proceeds in four basic steps (Fig.1): First, end-diastolic and end-systolic phases are selected from all phases of a complete cardiac cycle. Second, the locations of the bases at each respective end phase are established. Third, RV segments are traced at end systole and at end diastole. Fourth, software sums the total RV blood volume at these two phases, the total end-systolic volume is subtracted from the total end diastolic volume, and the RVSV is determined.
I. Determining end diastole & end systole
End-diastolic and end-systolic phases that are selected when quantifying the RVSV should be the same as those chosen when quantifying the left ventricular stroke volume. Generally, phase determinations should be made at the papillary muscle tips of the left ventricle (Figs.2A&B). (For additional details, see “Stroke Volume” under “Left Ventricle” in the Analysis Guide.) For electrocardiogram(ECG)-gated images, end diastole is usually the last cardiac phase (Fig. 2A), and end systole is approximately 40% of the way through the cardiac cycle (Fig.2B). However, which cardiac phase uniquely matches end diastole and/or end systole may differ between individuals. Moreover, variations in ECG gating, heart rate, and/or other parameters might influence the exact phase designation of end diastole and end systole. Careful analysis involves ensuring that the aforementioned phases are correctly defined: Mismatching cardiac phases can lead to artificially low stroke volumes and ejection fractions.
Sometimes, in the same individual the end-systolic phase may appear different at the base than at the apex. However, a single end-systolic time point—the phase when the RV is smallest—should be selected and held constant. Because the base contributes a greater blood volume than the apex, the selection will be influenced to a greater extent by the timing of end systole at the base.
Furthermore, an advantage of selecting the same respective phases for all end-systolic and end-diastolic slices is that, if difficulties (due to artifacts, arrhythmias, and/or limited time for analysis) arise when drawing contours on a given slice(s), software may interpolate, estimating the RV volume without including the problematic slice(s).
II. Establishing the location of the RV base at end diastole and at end systole
It is necessary to define the RV base at end diastole and at end systole, so that accurate RV contours are constructed properly. Determining the RV basal location from the short axis images alone is sometimes difficult. Even though the presence of a clearly defined RV chamber is used as a marker for the basal extent of the RV, relying on this characteristic alone might be problematic due to ill-defined RV boundaries and potential RV-overlap with the right atrium (Fig.3A). Thus, incorporating positional information from long axis images might be required (Fig.3A).
Analysis of the RV’s excursion also may facilitate defining the precise location of the base. A straight line is drawn across the tricuspid valve at end diastole and at end systole (Fig.3B). The displacement of the valve (and right atrium) may be measured and divided by the distance between the slices (Fig.3B), producing a number that approximately corresponds to the difference in the number of slices between the basal locations at the end-diastolic and end-systolic phases (Figs.3A&3B).
III. Drawing accurate, consistent RV contours
Using manual segmentation the endocardial contours are drawn as smooth, triangular regions of interest; traces include all RV blood and trabeculations (Fig.4A). However, this may artificially both increase the RV-end-diastolic and end-systolic volumes and lower the RV ejection fraction. To minimize the probability of these errors, endocardial contours should encompass as small an area as possible, while still including all of the RV blood.
To obtain an accurate RVSV and ejection fraction, all trabeculations included in a trace at end diastole also must be included at end systole; that is, the total number of trabeculations should be constant in both phases. The best way to appreciate this is by reviewing the endocardial contours while playing the images in a cine loop (Fig.4B).
IV. Right ventricular stroke volume is calculated by software
Once all traces are placed, software then sums the total RV blood volume in each phase, subtracts the total end-systolic volume from the total end-diastolic volume, and produces the RVSV.