I am not convinced that the CAC score actually went down. There is always a standard deviation or error band on any measurement. You are correct that the scores were performed in different facilities and different scanners.

However, my conclusion is that the dramatic increase from a my 35 CAC score in 2001 to the 1907 score in 2019 (performed on different scanners) was real. The "decrease" in my 2020 CAC score was hopefully caused by the lifestyle (I lost 25% of my body weight), dietary and medication changes over the 1-1/2 year period between CAC scores.

I also use CIMT to track plaque progression in my carotid arteries. During the 2019-2020 period, my CIMT did not change significantly. The analysis showed some plaque progression in the carotid arteries but "all" was deemed to be calcified and stable. This is somewhat contradictory of the CAC score stabilization.

There are a number of "real" reductions in plaque volume measured with Q-CCTA (Quantitative CCTA- CLEERLY), presumably caused by interventions between the Q-CCTAs. In most of these cases the calcified plaque increased as the dangerous non-calcified plaque decreased.

I am endeavoring to have my 2020 CCTA analyzed by CLEERLY and then compared to a new CLEERLY Q-CCTA later this year to see what has happened to my plaque during the 2-1/2 year lifestyle, medication and dietary interventions.