There is no interrelation between your options. To take up USMLE, you should me a Medical Student and should be officially enrolled in a US or a Canadian Medical School programme that leads to an MD degree. You can also be a Medical graduate.In short, USMLE is for Doctors and its a license exam for Medical Students. Ph.D in Cardiology is not a Medical Degree. It is a doctoral degree after you complete either your MS or MD and not after M.Sc Cardiac Care Technology. M.Sc in CCT involves study of invasive cardiacvascular procedures like Angioplasty, Cardiac catheterisation, devise closure and electrophysiology. Candidates with a Masters degree in CCT can only assist Cardiologists but cannot act as Cardiologists themselves. This course helps you to work in Operating rooms of Cardiac Catheterisations, non invasive analysis, etc but does not help you become a Cardiologist at any given point. This course only teaches you to operate the equipment but not treat the human heart. To become a Cardiologist, you need to study MBBS, pursue MS/MD and then appear for USMLE to practise abroad.