MRI:2 foci of abnormal low signal on T1 & T2

Hello: I recently had a MRI & MRA of the brain and a MRI of the C-spine which came out fine.  They made the following note on the C-spine MRI: 2 foci of abnormal low signal within the T1 vertebral body on the T1 & T2 sequences, incompletely imaged.  Can anyone tell me what "foci"is and what type of foci give off a low signal on both the T1 and T2 sequences?  What type of test is usually used to follow up on this?  My GP said Cat scan does that sound right?  and what might a CAT scan show that an MRI didn't? Thank you for any help you may be able to offer. T