Here is a quite comprehensive database of journals with listed IF:

http://www.medical-journals-links.com/radiology-journals-nuclear-medicine-imaging.php

Circulation Cardiovascular Imaging, JACC Cardiovascular Imaging

And you also have to consider: Theranostics (IF=7.8)

Radiology hands down.  Most number of cites.  RED journal higher but specialized.  Cardiac Imaging and Brain Mapping higher but specialty audiences.

Also depends on the topic.  If Cancer the RED, or one of the Os; JCO

, JTO, etc

RAS

As it has been said before, it depends on the topic, and also if it is more methods/application driven. All the above are good suggestions, but I would forward you to the following website where you can perform a more comprehensive search!

http://about.jcr.incites.thomsonreuters.com/

Radiology should be included probably on the first place comparing IF and prestige. 

Radiology is probably the first impact factor for medical imaging.

see the png attached

try this journal

http://www.journals.elsevier.com/medical-image-analysis/

http://www.scimagojr.com/journalrank.php?category=2741