I've had one, to diagnose a mitral valve prolapse, my experience was that it was made clear my top half was to be without a bra, left to my own devices to remove all above the waist and was given a standard hospital gown to put on as a 'dressing gown' ( open part front facing as they need access). You can hold it in place for modesty but they do need to go under and around the breast.
First part I lay on my back on a bed next to the scanner, ultrasound gel was applied and the technician scanned relevant parts under the gown wherever possible- he was mostly looking at the screen tbh. He then asked me to turn to my side facing away. This was the worst bit (if you can even call it that) as at the end of the day what needs to be seen is under the breast so it was a little awkward and felt exposing but the technician was respectful and didn't have a need to touch my breast by any means than with the scanner trying to capture what was needed.
We chit chatted and just got it over with, he was very pleasant.Quite an odd situation as i suppose it feels intrusive but it is over quickly, my experience was totally fine. Took about 10 minutes if that. I went alone.
I've mentioned the technician was Male OP just in case this bothers you- it didn't me as I just wanted it done asap but I'm certain you could request a female technician for this, and are entitled to a chaperone.
No contrast is required and there are only electrode pads if they also requested an ECG (which you can keep a bra on for in my experience, especially if wearing non wired, they just tuck the pads under)
I got the results verbally at the time and an appt with a cardiologist a few weeks later to confirm and next steps.
It's a bit awkward but totally fine, and I found it uncomfortable at times but no severe pain. The discomfort is eg- pressing onto the chest/breast tissue to get a clear image.
I'd advise wearing easy to remove clothes OP, no fiddly stuff. Then you can get dressed quicker and feel normal again. Hope all goes well!