= Coronary angiography
Coronary angiography images the <coronary arteries> using a contrast medium and <X-rays>. In conventional invasive coronary angiography, a <catheter> delivers contrast to the coronary circulation while the imaging system records the contrast-filled vessel paths. The contrast improves visibility of the blood-filled <lumen (anatomy)> against surrounding tissue.<Heart Tests by the National Heart, Lung, and Blood Institute>[Coronary angiography subsection, contrast and X-ray images]{ref}
The apparatus connects fluid delivery to imaging geometry. An X-ray source and detector record the contrast signal during its passage through the vessels; changing viewing angle changes how overlapping arterial segments project into the image. The result is primarily a description of the contrast-filled lumen. It is not a direct measurement of every feature of the arterial wall or of how much oxygen reaches the <myocardium>.
An original geometric example shows why one apparent width can be incomplete evidence. Suppose a local lumen has an elliptical area with diameters $3.0$ and $1.5\,\mathrm{mm}$. Its area is
$$
A=\pi(1.5)(0.75)\,\mathrm{mm^2}\approx3.53\,\mathrm{mm^2},
$$
half that of a circular $3.0\,\mathrm{mm}$ lumen. One viewing direction can nevertheless show the same $3.0\,\mathrm{mm}$ projected width as that circle, while an orthogonal view shows $1.5\,\mathrm{mm}$. A projected width does not uniquely determine lumen area without a shape and viewing model. These chosen dimensions are not a diagnostic threshold.
Timing and motion matter too: a moving vessel and changing contrast concentration can alter the recorded signal. A geometric narrowing, a pressure difference and downstream blood delivery are related observations but are not interchangeable. A physiological assessment needs evidence beyond one static projection.
Coronary CT angiography uses a different acquisition geometry: many X-ray views are reconstructed into a volume, commonly after contrast administration through a peripheral vein. The NHLBI account distinguishes cardiac <computed tomography> from the catheter-based angiographic description.<Heart Tests by the National Heart, Lung, and Blood Institute>[Cardiac CT scan and Coronary angiography subsections]{ref} Naming the method makes clear whether a claim concerns a projection sequence or a reconstructed three-dimensional image.
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