Calcium reflects total plaque

People frequently ask, "Why measure coronary artery calcium? My doctor said that calcium only tells you if there's hard plaque, and that hard plaque is stable. He/she says that calcium doesn't tell you anything about soft plaque."

Is that true? Is calcium only a reflection of "hard" plaque? Is hard plaque also more stable, less prone to rupture and causes heart attack?

Actually, calcium is a means of measuring total plaque, both soft and hard. That's because calcium comprises 20% of total plaque volume. Within plaque, there may be areas that are soft (labeled "lipid pool" in the diagram). There are also areas made of calcium (shown in white arcs within the plaque). Even though this is just a graphic, it's representative of what is seen when we perform intracoronary ultrasound of a live human being's coronary artery. In other words, this cross section contains both "soft" (lipid pool) as well as "hard" (calcium) elements.

Is this artery "soft" or "hard"? It's both, of course. The artery compostion can vary millimeter by millimeter, having more soft or hard elements. The artery can also change over time in either direction. Thus, "soft" plaque may indeed be soft today, only to be "hard" in 6 months, and vice versa.

The essential point is that measuring just "soft" plaque provides limited information. What the CT heart scan does is provide a gauge of total plaque, soft and hard, and it does so easily, safely, precisely. If your score increases, the lengthwise volume of total plaque has also grown. If your score decreases, the total amount of plaque has also decreased.