Last week, we had two cases, showing left atrial thrombi
The first was a lady who presented with a left atrial mass without risk
factors -
http://bhavin.typepad.com/cardiac_images/2006/05/large_left_atri.html
The second was a lady with mitral stenosis and atrial fibrillation -
http://bhavin.typepad.com/cardiac_images/2006/05/mitral_stenosis.html
Both these were diagnosed using MRI.