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A cardiac long axis is a base-to-apex direction used to describe the heart or a specified ventricular chamber. In left-ventricular imaging it is commonly planned through the cardiac apex and the center of the mitral valve. It is a chamber-based anatomical reference, not necessarily the vertical direction of a patient's body or an image display.

The primary cardiovascular magnetic resonance imaging protocol specifies connected two-chamber, four-chamber and short-axis planning from anatomical landmarks.^[ref] A long-axis image plane contains the chosen cardiac long axis; the corresponding cardiac short axis describes a transverse direction or view. Different planes can contain the same long-axis line while displaying different walls and neighboring chambers.

An original vector construction makes the distinction precise. Let $a$ be the apex position and $b$ a chosen basal reference point. Thenis the axial unit vector. A displacement $r$ has axial component $(r\cdot \mathbf{u})\mathbf{u}$ and transverse component $r-(r\cdot \mathbf{u})\mathbf{u}$. The line alone does not determine how the transverse coordinates are rotated around it.

This geometry matters for cardiac image foreshortening. In a simple projection model, a length $L$ tilted by $\theta$ out of the display plane projects to $L\cos \theta$. A 10 cm length tilted by $30^{\circ }$ appears about 8.66 cm long. This is a geometrical illustration, not a correction formula for a tomographic slice: a slice may miss the true apex rather than project it. Consistent landmark selection is therefore part of meaningful comparisons of cavity shape, motion and volume.

A cardiac short axis is a transverse reference direction or imaging view relative to a specified cardiac long axis. For left-ventricular imaging, a short-axis plane is planned across the chamber rather than through its base-to-apex length. The chamber and anatomical landmarks must be specified; a patient's horizontal body plane is not automatically a cardiac short axis plane.

A primary cardiovascular magnetic resonance imaging protocol plans short-axis images using linked long-axis views and covers the ventricle from base to apex.^[ref] A cardiac short-axis stack turns successive cavity outlines into volumetric information.

For an original geometrical construction, let $\mathbf{u}$ be the unit vector along the chosen cardiac long axis. A transverse plane through $r_0$ satisfiesParallel planes at different axial positions sample different portions of the ventricle. With measured cavity areas $A_i$ and represented slice thicknesses $\Delta z_i$, a simple volume estimate is $V\simeq {\sum }_i{A}_i\Delta z_i$.

An oblique slice also illustrates a useful caution. An ideal circular cylinder cut at angle $\theta$ away from its perpendicular section has an elliptical cross-sectional area $A_0/\cos \theta$. At $30^{\circ }$ this is about $15.5\%$ larger. This does not prove that an entire consistently acquired oblique stack overestimates volume: integrating all areas with the correct normal spacing and complete coverage still recovers volume. Orientation, spacing and coverage must be treated together. The major practical question is whether the chosen stack and image segmentation represent the complete intended cavity, not whether one slice happens to look circular.