CT scan involves x ray radiation and can be harmful to the body. Unless it is essential for any diagnostic procedure one should not undergo repeated CT scans on annual basis for diagnosis of certain cancers. On the other hand ultrasound and MRI are safe and carry no radiations. Experts have said that MRI and CT scan are not interchangeable diagnostic procedures. Contrast imaging which was earlier used with x ray is now available for ultrasound and echocardiography. Thirty percent of patients do not have a good heart window of routine echocardiography. In these cases using special ultrasound contrast can prevent angiography and other cath procedures.