Answer:
TEE Transesophageal Echocardiography
Explanation:
TEE is an ultrasound of the heart but instead of doing it from the chest it is done by inserting a small probe into the esophagus. Since the esophagus is very close to the heart, it can be seen much better than a conventional echocardiogram. The abbreviation comes from TransEsophageal Echocardiography (TEE).
It allows them to be flattened on a microscopic slide, so that the chromosomes of individual cells can be observed easily.
The circulatory system is the answer :)