= Cardiac long axis
A \b[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.<Standardized cardiovascular magnetic resonance imaging (CMR) protocols: 2020 update by Christopher M. Kramer and others>[General techniques, Left ventricular structure and function, items 3 and 6; Figures 1–3]{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. Then
$$
\boldsymbol u=\frac{a-b}{\lVert a-b\rVert}
$$
is the axial unit vector. A displacement $r$ has axial component $(r\cdot\boldsymbol u)\boldsymbol u$ and transverse component $r-(r\cdot\boldsymbol u)\boldsymbol u$. 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.
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