low dose ct scan

The ECG shows if the movement of blood volume is impeded, usually at a narrowing. There would almost certainly have been some narrowing revealed, but it was within a 'normal' range, or an average range for your sex and age. Note that, statistically, 'average' comprises all data points under a standard normal curve from about the 33%ile up to the 67%ile. That's a whopping size of the entire population distributed under the bell curve. Your cardiologist would want you treated if you are approaching the extremes of the 'average' or beyond for blockage.

A stress test (MIBI?) would also reveal blockages. Again, if the narrowings are less than about 60%, you might still qualify as 'severe coronary artery disease', but it's not dangerous..........yet. You can be blocked up even more before they intervene surgically with a bypass or stent. Meanwhile, you go on a statin to stabilize what deposition is already extant...already in place inside your coronary arteries.

This is my layman's understanding. You should ask your cardiologist to sit with you for three minutes and explain it to you in her words.

LDCT scan is related to checking for lung cancer?

Or, did you have a scan for arterial blockage? Or perhaps for calcium scoring?

Need quite a bit more info ... age, body mass, lipids, lifestyle, findings of the nuclear stress test, previous stress tests, meds, etc?

It was part of my annual check up because im a former smoker, im 57 BMI is 22 lipid panel was 190 total cholester, 78 HDL ,LDL 90 and TRI 111. Exercise five days a week. nuclear stress test normal.

The proper next step at this point would be to ask the cardiologist if they agree with the radiologist's finding of "severe coronary artery disease." Given your stats, that all other tests are normal, and you were asked to come back next year, it would seem that was not the case.
If you are still concerned, there is a higher dose CT with contrast that provids a more detailed picture, but the cardiologist probably doesn't think it is necessary.
Can you send a message the doctor through the patient portal, or phone them to get an answer?

I have moderate heart disease because my calcium score is 399
Also I am a former smoker and get a yearly low dose CT scan for prevention and also to follow up as I have benign nodules.
The CT scan shows the aorta and gives an accurate shape of any aneurysm. Mine is normal for my age, thus I don’t get an echocardiogram.
Because of my elevated calcium score, it is advisable that the LDL be at 55 or lower. I’m on a statin to prevent progression of calcification. It’s not about the total cholesterol but the LDL ( bad one)
I keep exercising and eat healthy.