Please have a look at:

http://www.via.cornell.edu/databases/

There are some databases listed containig CT data sets. Some also provide MRI data,

such as for example:

http://www.cancerimagingarchive.net/

https://wiki.cancerimagingarchive.net/display/DOI/Data+from+Head+and+Neck+Cancer+CT+Atlas

You may also have a look here:

http://plastimatch.org/data_sources.html

But public CT data at different radiation dose I do not know yet, sorry.

Just an additional remark:

To my opinion, CT scans at different radiation doses are not compatible with the principles of radiation protection, even in the case of radiation therapy. So I think you will not get any public data with respect to different doses; at least no data of human beings.

Doing the same scan over a range of exposures is pretty unethical. My suggestion would be to get in touch with someone who can take a certain patient scan at one radiation dose and who can simulate the images for other (lower) radiation doses. This is fairly common and easy to implement with a few lines of code. For example, see this paper that has done exactly this process: https://aapm.onlinelibrary.wiley.com/doi/pdf/10.1118/1.4789628%4010.1002/%28ISSN%292473-4209.FOC

hope that helps,

Jim