A quick check under google suggests that in the UK we use the original word "Echo..." (after all the principle is based on use of echoes), whereas in the USA they seem to have dropped the "h".
It all depends therefore on your target language, though as a purist (and a Brit) the version without "h" makes me feel quite ill......

"eco" is wrong wrong wrong :-)

Just remember "sounds echo" - echocardiography uses ultrasound echoing off structures

http://www.nlm.nih.gov/medlineplus/ency/article/003869.htm
Definition

Echocardiogram is a test that uses sound waves to create a moving picture of the heart. The picture is much more detailed than x-ray image and involves no radiation exposure.

How the test is performed

A trained sonographer performs the test, then your physician interprets the results. An instrument that transmits high-frequency sound waves called a transducer is placed on your ribs near the breast bone and directed toward the heart. The transducer picks up the echoes of the sound waves and transmits them as electrical impulses. The echocardiography machine converts these impulses into moving pictures of the heart.