I was told by the cardiologist who ordered both scans that MR results are very precise, while ultrasound is more of an "orientation scan" and small variations in how the doctor holds the probe and interprets the picture can add up to milimeters of difference. If there is a discrepancy, MR is the gold standard for decisions about treatment.

This was at IKEM in Prague, one of the best cardio units around, so I trust their judgment.

https://www.ikem.cz/en/