Radiology is the branch of medical science dealing with medical imaging. It may use x-ray machines or other such radiation devices. It also uses techniques that do not involve radiation, such as MRI and ultrasound.
  As a medical specialty, radiology can refer to two sub-fields, diagnostic radiology and therapeutic radiology.

Diagnostic radiology is concerned with the use of various imaging modalities to aid in the diagnosis of disease. Diagnostic radiology can be further divided into multiple sub-specialty areas. Interventional radiology, one of these sub-specialty areas, uses the imaging modalities of diagnostic radiology to guide minimally invasive surgical procedures.

Therapeutic radiology—or, as it is now called, radiation oncology uses radiation to treat diseases such as cancer using a form of treatment called radiation therapy.
  Commonly used imaging modalities include plain radiography, computed tomography (CT), magnetic resonance imaging (MRI), ultrasound, and nuclear imaging techniques. Each of these modalities has strengths and limitations which dictate its use in diagnosis.

Radiography
Radiographs are image created with X-rays, and used for the evaluation of many bony and soft tissue structures. Fluoroscopy and angiography are special applications of X-ray imaging.

Fluoroscopy is a technique where a fluorescent screen or image intensifying tube is connected to a closed-circuit television system to image internal structures of the body. Angiography uses methods to demonstrate the internal structure of blood vessels, highlighting the presence and extent of obstruction to the vessel, if any. In medical imaging, contrast media are substances that are administered into the body, usually injected or swallowed, to help delineate the anatomy of blood vessels, the genitourinary tract, the gastrointestinal tract, etc. Contrast media, which strongly absorb X-ray radiation, in conjunction with the real-time imaging ability of fluroscopy and angiography help to demonstrate dynamic processes, such as the peristalsis of the digestive tract or blood flow.

CT scanning
CT imaging uses X-rays in conjunction with computing algorithms to image tissues in the body. Imaging is usually performed in the axial plane; however, computer reconstructions can be rendered in other planes or to produce 3D images. Contrast media is often used to delineate anatomy and allows 3D reconstructions of structures, such as arteries and veins. Although the resolution of radiographs is higher for imaging of the skeleton, CT can generate much more detailed images of soft tissues. CT exposes the patient to more ionizing radiation.

Sonography
Medical ultrasonography uses ultrasound (high-frequency sound waves) to visualize soft tissue structures in the body in real time. No radiation is involved, but the quality of the images obtained using ultrasound is highly dependent on the skill of the person performing the exam. Ultrasound procedures are best used for ante natal checkups. It is not harmful to fetus nor to the mother.

MRI Magnetic Resonance Imaging
MRI uses strong magnetic fields to align spinning atomic nuclei (usually hydrogen protons) within body tissues, then disturbs the axis of rotation of these nuclei and observes the radio frequency signal generated as the nuclei return to their baseline status. MRI scans give the best soft tissue contrast of all the imaging modalities. With advances in scanning speed and spatial resolution, and improvements in computer 3D algorithms and hardware, MRI has the potential for great development in the next few years. One disadvantage is that the patient has to hold still for long periods of time in a noisy, cramped space while the imaging is performed.

Nuclear medicine
Nuclear medicine imaging involves the administration into the patient of substances labelled with radionuclides or radiopharmaceuticals which have affinity for particular tissues. Generally speaking, Technecium-99m (half-life 6.02 hours) is the radionuclide used. The heart, lungs, thyroid, liver, gallbladder, and bones are commonly evaluated for particular conditions using these techniques. While anatomical detail is limited in these studies, nuclear medicine is useful in displaying physiological function. Processes such as the growth of a tumor can be monitored this way, even when the tumor cannot be adequately visualized using any of the other imaging modalities. Nuclear medicine also involves the therapeutic administration of isotopes labeled to antibodies or other substances, therefore delivering a high dose of radiation to specific targets such as tumors or the thyroid gland.