Complex Cyst Right Kidney

The ultrasound report demonstrates a 1.6 cm cystic area in the right kidney with a septation, which was not appreciated on the previous imaging a year ago.  Followup with CT or MRI with contrast is suggested.
What is the best imaging test to obtain more information about this cyst?  Is 1.6 cm with a septation considered a small or large cyst?  What is the likelihood this is malignant?  Thanks for any information you can give me.

[David Rudnick, M.D.]

This is small and repeat imaging with a contrast enhanced CT or even ultrasound would be the standard way to follow this over time.