DEAR DR. ROACH: My primary care physician scheduled me for a cardio stress test and ultrasonography with intravenous “microsphere” imaging. I’m slightly concerned about the term “microsphere.” Is this a safe procedure? Are there any complications stemming from this chemical/procedure? Should I be concerned about a stroke? How long do these microspheres stay in the body? Do they become a part of plaque? Are there other safer imaging methods?

— K.O.S.

ANSWER: Intravenous microbubble contrast agents are made from compressed gas that is contained in a phospholipid or protein, which are both components of normal cells. They are used in an ultrasound to make the blood “brighter,” making it much easier to see blood movement and allowing for a greater resolution of the images.

Microspheres don’t leak into the tissue, so strokes and plaque aren’t real concerns. They stay in the body from four to 10 minutes, then spontaneously break up, with the gas being dissolved in the blood and the shell getting removed by normal processes in the liver. Unlike traditional dyes used for CT and MRI scans, they aren’t toxic to the kidneys, and there’s a lower risk of allergic reactions.

DEAR DR. ROACH: I’m a 75-year-old woman. I have palpitations and underwent a stress echocardiogram a week ago. My cardiologist said that my heart is good, but I have atrial fibrillation (AFib). He told me that I need to take a blood thinner, so he gave me a sample box of Eliquis. I read about it, and the bleeding side effects are frightening. I cannot take this because I don’t know what my risk of bleeding is. What do you think?

— N.

ANSWER: AFib is a chaotic heart beat that can cause symptoms but also predispose people to clots inside the heart, which, in turn, can lead to a stroke. Every person with AFib should be considered for an anticoagulant, but you are quite right that we need to consider bleeding risks as well. Doctors want to avoid major bleeding events in their patients, but we also want to prevent strokes. It can be difficult sometimes to balance these risks.

There are two tools to help. One estimates your risk of a stroke, called the CHA2DS2-VASc score, and the other (HAS-BLED) estimates your bleeding risk. Your doctor probably checked both of these and determined that the benefit in reducing your stroke risk outweighed your bleeding risk. Just being a 75-year-old woman puts you at such a high stroke risk that you’d have to have a very high bleeding risk (like a major unexplained bleeding event in the past) for your doctor not to recommend an anticoagulant like Eliquis.

Readers may email questions to ToYourGoodHealth@med.cornell.edu.