Some people have called CT heart scans the "mammogram of the heart." The analogy contains a lot of wisdom.
First of all, both--mammograms and CT heart scans--are screening tests, one for cancer, of course, the other for coronary atherosclerotic plaque. Both are performed in specific age groups, mammograms in women 40 years and over (generally), heart scans in women 50 years and over (generally).
Mammograms: Left, normal; right, a small mass. (Courtesy Nat'l Institutes of Health and Wikipedia.)
Both are also meant to be repeated periodically when normal as a surveillance process.
Both use low quantities of radiation of about 0.3-0.4 mSv (the most real-life measure of total body exposure), a modest quantity of radiation.
Both are good for their purposes, though not perfect. Can a mammogram performed properly miss a small cancerous mass? Sure it can, but it's still unusual. Can a CT heart scan miss the non-calcified plaque prone to rupture? Sure it can, but this is also unlikely (<5% probability).
Given the exorbitant costs of medical tests, both are quite inexpensive. On the flip side, they are both also quite unprofitable for the centers providing the tests. Unfortunately, this means that mammography centers and heart scan centers come and go because of the difficulties of the profit-side of these services.
Both tests initially struggled to gain acceptance among the medical community. In 1960, for instance, mammograms were performed on standard x-ray devices, the same as that used to perform chest x-rays--low precision, high radiation back then. In 1969, dedicated mammography devices made the scene. However, it took over 10 years for even these new dedicated devices to become widely used. Use of mammograms has gradually increased over the ensuing 20 years. In other words, 47 years have passed since the introduction of mammography.
CT heart scans, of course, have had a shorter history of approximately 20 years, since engineer, Dr. Douglas Boyd, first invented the "ultra-fast" EBT devices, the first devices with sufficient scanning speed to scan the heart and coronary arteries.
One interesting difference between the two: In a woman between the age of 50 and 60, the likelihood of detecting cancer is 1 in 237. The likelihood of detecting coronary atherosclerotic plaque? About 1 in 4. Coronary disease eventually kills 1 in 3 females, hugely overshadowing breast cancer in frequency.
Progress on both fronts, one in cancer detection, the other in atherosclerotic coronary plaque detection. But still lots more progress to go.