 Cardiac short-axis stack

A 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.^[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 isAll 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.