will definitely show your entire chest. Let me first start out by saying that screening mammograms have been used for many years to help detect breast cancers in the hope that they can be treated early with improved outcomes in breast cancer patients. You probably already know this, but a mammogram
is an x-ray that can pick of masses if they are radio-opaque and large enough to be detected. An MRI (magnetic resonance imaging) is a completely different technology altogether. It relies on the fact that water has a very slight magnetic charge to it, and using a scanner with very large magnets can detect the water in different soft tissues and show a distinction in soft tissues that X-rays are not able to.
You can imagine however that MRIs are significantly more expensive to perform, take much longer, and are much more complicated to read. For this reason they are not typically used as the first line for screening purposes on a population level. However there are definitely situations in which the physician wants to get a better look at subtle changes within soft tissues, and they may order an MRI. Also as you mentioned since an MRI machine is a tube that you go through, not only can it look at your whole chest, it can look at your entire body if needed.
If your physician ordered an MRI, I would recommend going back to them and inquiring as to why. Or if you are just interested in more information on breast cancer screening
and imaging modalities I would recommend making an appointment to be seen at a local breast clinic. I hope that this is helpful.