As a medical specialty, radiology can be classified into two subfields. Diagnostic radiology is concerned with the use of various imaging modalities to aid in the diagnosis of disease. Interventional radiology uses these imaging modalities to guide minimally invasive surgical procedures.
Radiation therapy uses radiation to treat diseases such as cancer and is commonly lumped together in the same class as Radiology. Most medical practitioners, however, make a clear distinction between the Radiologists and the Radiotherapists.
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.
Radiographs are often used to for quick evaluation of bony structures. Fluoroscopy is a special application of X-ray imaging, where an X-ray video camera allows the imaging of structures in motion. Often, contrast materials (chemicals which are absorb X-ray photons and can be easily seen on X-ray films) are injected, swallowed, or administered into the body in other ways to help visualize dynamic processes, such as the motion of the digestive tract.
CT imaging uses X-rays in conjunction with computing algorithms to image a variety of soft tissues in the body. X-ray contrast is often used with CT as well. CT can generate much more detailed images than plain X-rays, but exposes the patient to more ionizing radiation in the process.
Ultrasound imaging uses high-frequency sound waves to vizualize soft tissue structures in the body in real time. No radiation is involved, but this imaging modality is highly dependent on the skills of the person performing the exam.
MRI uses strong magnetic fields to align spinning hydrogen protons within body tissues, then disturbs the axis of rotation of these protons and measures the electrical currents generated as the protons return to their baseline states. MRI scans give the highest resolution and definition of all the imaging modalities. With advances in scanning speed and resolution and improvements in computer 3D algorithms and hardware, MRI appears to have the greatest potential for development in the next few years. The downside of it is that the patient has to hold still for long periods of time in noisy, cramped quarters while the imaging is performed.
Nuclear medicine imaging involves the administration into the patient of substances labelled with radioactive tracers which have affinity for particular tissues. The heart, lung, 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. As such, processes such as the growth of a tumor can often be monitored this way.