Heard pet scan expose equal radiation as ct.what will be the radiation for pet +ct which are using nowadays combined?is it double the ct radiation?!

Not double: Pet scan delivers radiation from a radioactive injection given before the test. It fades very quickly. Ct scan delivers beams of xrays. In general, the pet injection delivers about 50% of the radiation dose from a ct scan. Numbers may vary depending on what particular body part is being scanned. So the total is about 1.5 times the ct dose alone.

Variable: The exact radiation you get from ct and pet/cts varies based on the used protocol. Roughly speaking a diagnostic ct gives you a rad dose of 10-15 msv. Pet/cts are often done with low-dose ct, so instead of 10-15 msv you get 1-2 msv from the ct. For the pet portion is approximately 8-12 msv. So a regular pet/ct is approx 12-14 msv and if you add a diagnostic ct you get 10-15 msv more.

Not usually: Pet radiation dose is approximately 7 msv for an injection of 10 mci of f-18 fdg. The ct component radiation dose is as high as 25 msv. However, dose reduction techniques are being developed, and may locations have reduced the combined pet/ct dose to approximately 14 msv. At that level, the ct images are of reduced quality, but provide excellent attenuation correction and are still useful.