Other Imaging
Techniques In addition to X rays,
radiology involves the use of a variety of other newly developed imaging
techniques using different types of energy. Magnetic resonance imaging (MRI) produces two-dimensional visual images of internal structures in any plane using strong electromagnetic fields. In MRI the patient is placed inside a powerful magnet that aligns the hydrogen atoms in the body tissues. A radio signal is directed to the body part being examined, temporarily disrupting this alignment. When the radio signal stops, the hydrogen atoms return to alignment, but not all at the same time, since different body tissues align at different rates. A computer measures the change in realignment and converts the data into an image in any desired plane. MRI is used to evaluate injury and disease in all the organs of the body. MRI can image, in motion, the heart, its valves, and moving blood in the veins and arteries. This
technique can evaluate brain function based on blood flow. Metabolic activity in organs can be measured by MR spectroscopy.
Ultrasound produces images using high-frequency sound waves. When these sound waves come into contact with structures within the body, part of the sound energy is reflected back to the body surface, where it is converted electronically into a picture. This technique is commonly used during pregnancy to determine if multiple fetuses are present, or the position and age of a fetus. An echocardiogram is an ultrasonic technique used to examine patients with congenital or acquired heart conditions, and Doppler ultrasound detects blood flow in the veins and arteries.
The field of radiology also includes nuclear medicine, in which radioisotopes are introduced into the body. The isotopes are monitored as they travel through the blood vessels to determine if specific organs are functioning properly. Special cameras measure the intensity of the radiation released by the isotope and allow visualization of parts of organs not usually seen by X rays. Radioisotopes are also used to send a therapeutic dose of radiation to a specific site to kill cancer cells.
Positron emission tomography (PET) uses an isotope that emits a radioactive particle called a positron. These isotopes attach to a variety of chemicals, such as glucose, that are found in the body's metabolism. This procedure not only detects the anatomy of an organ, but also the status of its metabolic function, such as brain activity and blood flow. It is also useful in diagnosing cancer and Alzheimer disease.
Single photon emission computed tomography (SPECT) is a recently developed technique that is just beginning to have medical applications. Like PET, SPECT tracks a radioactive isotope and a computer translates the data into an image showing details about metabolic function. SPECT is proving particularly successful in monitoring heart function.
Interventional Radiology
Interventional radiology uses imaging
techniques to guide thin tubes called catheters through veins, arteries, and organs of the body to diagnose and treat a variety of conditions. These techniques are often outpatient procedures and are generally less invasive, safer, and less costly than surgical procedures. Balloon angioplasty is a common interventional technique designed to open blocked or narrowed arteries, and if necessary, an internal stent or graft may be left in position to hold the vessel open. Guided by fluoroscopy, a balloon-tipped catheter is inserted into the blocked or narrowed blood vessel. The balloon is inflated to open the blocked or narrowed section of the vessel. Other procedures are used to block the arteries to stop them from bleeding or supplying nutrients to cancerous tumors. Abscesses deep in the body can be drained and biopsies of internal organs can be obtained using these techniques. High doses of anticancer drugs may be supplied directly to tumors via their blood supply.