Amide's a Medical Imaging Data Examiner (AMIDE) is a completely free tool for viewing, analysing, and registering volumetric medical imaging data sets. It's been written on top of GTK+ , and runs on any system that supports this toolkit (Linux, Windows, Mac OS X with fink, etc.).
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Rad is a DICOM workstation written in Cocoa for MacOSX. Using QuickTime, OpenGL, and open source databases iRad aims to provide an easy and efficient way to review medical images from CT, MRI, ultrasound, and other DICOM sources such as angiography.