Have 5-6 mm calcified lump in the posterior neck, doctor recommend imaging. should do mri with and without contrast or ct w/ contrast? benefits/risks?

Neither!: If you know the lump is calcified, you're best off not doing any imaging. If you want to know what it is, go to a surgeon or pathologist who can manage a simple biopsy, probably in his office, to figure out what it is. A FNA (fine needle aspiration) may work out for you. If you REALLY need to know if it is solid or a cyst, then ultrasound is better. CT involves radiation. MRIs are $$. Avoid them.