= Cardiac short-axis stack
A \b[cardiac short-axis stack] is a series of images across a cardiac chamber at successive positions approximately transverse to its <cardiac long axis>. In ventricular volumetry, the series must cover the intended cavity from its basal boundary toward the <cardiac apex>. A primary cardiovascular <magnetic resonance imaging> protocol specifies linked long-axis planning and short-axis coverage.<Standardized cardiovascular magnetic resonance imaging (CMR) protocols: 2020 update by Christopher M. Kramer and others>[General techniques, Left ventricular structure and function; Figures 1–2]{ref}
The stack supplies a spatial sum rather than a single diameter. Let $A_i(t)$ be the segmented cavity area in slice $i$ at phase $t$, and let $\Delta z_i$ be the represented distance along the stack normal. A basic approximation is
$$
V(t)\simeq\sum_iA_i(t)\Delta z_i.
$$
All areas in a given sum must refer to a compatible phase of the <cardiac cycle>. <Image segmentation>, slice coverage and phase matching are separate inputs to the volume estimate.
An original example uses four representative areas of 2, 6, 10 and 6 cm$^2$, each assigned 1 cm of axial extent. Their represented volume is 24 cm$^3$, or 24 mL. Omitting the basal 6 cm$^2$ contribution reduces the result to 18 mL without changing the tracing quality of the other slices. If slice thickness is 0.8 cm but center spacing is 1 cm, the numerical integration convention must also account for the intervening extent; treating thickness and represented spacing as interchangeable changes the estimate.
Phase mixing causes a different error. An area from one slice near end-filling and another near end-ejection do not generally describe a cavity that existed at one time. A stack can therefore be complete spatially yet inconsistent temporally. With correctly matched phases, the derived <end-diastolic volume> and <end-systolic volume> can be used for <stroke volume> and <ejection fraction>; those formulas do not repair missing anatomy or mismatched acquisition phases.
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