If you go into the hospital complaining of pain and find out that you;re having a heart attack, can they refuse to give you a cat scan because of no insurance?

Medical malpractice is negligence.  Negligence is the failure to exercise due care (that degree of care that a reasonable medical practitioner in the community would have exercised under the same or similar circumstances to prevent foreseeable harm).

In CA, you can't be denied medical treatment for inability to pay and a sign is usually posted in the hospital stating that.  However, the issue here is whether or not a CT scan is a medical procedure which a reasonable medical practitioner in the community would have performed in the case of a heart attack.  If it is not, then there is no malpractice claim.  It would be advisable to get a second opinion from another doctor, who can review your medical records, and determine whether or not a CT scan should have been performed.  That second doctor's report will determine whether or not there is any basis for a medical malpractice claim.