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Anatomical Imaging

Human Body

Anatomical imaging has revolutionized medical science. It has been estimated that during the past 20 years there has been as much progress in clinical medicine as in all the previous history of medicines combined, and anatomical imaging has made a major contribution to that progress. Anatomical imaging allows medical personnel to look inside the body with amazing accuracy and without the trauma and risk of exploratory surgery. Although most of the technology of anatomical imaging is very new, the concept and earliest technology are quite old.

X-rays, the magical, mysterious rays that can see inside the body, were first used in medicine by Wilhelm Roentgen in 1885. They were called x-rays because no one knew what they were. The rays, extremely short-wave electromagnetic radiation can pass through the body and expose a photographic plate to form a radiograph. Bones and radiopaque dyes absorb the rays and create underexposed areas that appear white on the photographic film. X-rays have been in common use for many years and have numerous uses. A major limitation of x-rays is that they give only a flat, two-dimensional (2-D) image of the body which is a three-dimensional (3-D) structure. Almost everyone has had an x-ray, either to visualize a broken bone or to check for a cavity in a tooth.

Ultrasound is the second oldest imaging technique. When it was first developed in the early 1950s as an extension of World War II sonar technology, it used high-frequency sound waves. The sound waves are emitted from transmitter-receiver placed on the skin over the area that is scanned. The sound waves strike internal organs and are reflected back to the receiver on the skin. Even though the basic technology is fairly old, the most important advances in the field occurred only after it became possible to analyze the reflected sound waves by computer. Once the computer analyzes the pattern of sound waves, the information is transferred to a monitor, where the result is visualized as an ultrasound image called a sonogram. One of the more recent advances in ultrasound technology is the ability of the more advanced computers to analyze changes in position through time and to display those changes as "real time" movements. Among other medical uses, ultrasound commonly is used to evaluate the condition of the fetus during pregnancy.

Computer analysis is also the basis of another major medical breakthrough in imaging. Computer tomographic (CT) scans, developed in 1972 and originally called computerized axial tomographic (CAT) scans, are computer-analyzed x-ray images. Allow-intensity x-ray tube is rotated through a 360-degree arc around the patient, and the images are fed into a computer. The computer then constructs the image of a "slice" through the body at the point at which the x-ray beam was focused and rotated. It is also possible with some computers to take several scans short distances apart and stack the slices to produce a 3-D image of a part of the body. Dynamic spatial reconstruction (DSR) takes CT one step farther. Instead of using a single rotating x-ray machine to take single slices and add them together, DSR uses about 30 x-ray tubes. The images from all the tubes are compiled simultaneously, rapidly producing a 3-D image. Because of the speed of the process, multiple images can be compiled to show changes through time, giving the system a dynamic quality. This system allows us to move away from seeing only static structure and toward seeing dynamic structure and function.

Digital subtraction angiography (DSA) is also one step beyond CT scans. A 3-D x-ray image of an organ such as the heart is made and stored in computer. A radiopaque dye is injected into the circulation, and a second x-ray computer image is made. The first image is subtracted from the second one, greatly enhancing the differences, with the primary difference being the presence of the injected dye. These computer images can be dynamic and can be used, for example to guide a catheter into a coronary artery during angioplasty, which is the insertion of a tiny balloon into a coronary artery to compress material clogging the artery.

Magnetic resonance imaging (MRI), which subjects a person to a large electromagnetic field and certain radio waves, also is based on principles that have been known for years but have been applied only recently to medicine. The magnetic field causes the protons of various atoms to align. Because of the large amounts of water in the body, the alignment of hydrogen atom protons is at present most important in this imaging system. Radio waves of certain frequencies, which change the alignment of the hydrogen atoms, then are directed at the patient. When the radio waves are turned off, the hydrogen atoms realign in accordance with the magnetic field. The time it takes the hydrogen atoms to realign is different for various tissues of the body. These differences can be analyzed by computer to produce very clear sections through the body. The technique is also very sensitive in detecting some forms of cancer and can detect a tumor far more readily than can a CT scan.

Positron emission tomographic (PET) scans are able to identify the metabolic states of various tissue. This technique is particularly useful in analyzing the brain. When cells are active, they are using energy. The energy they need is supplied by the breakdown of glucose (blood sugar). If radioactively labeled glucose is given to a patient, the active cells take up the labeled glucose. As the radioactivity in the glucose decays, positively charged subatomic particles called positrons are emitted. When the positrons collide with electons, the two particles annihilate each other, and gamma rays are given off. The gamma rays can be detected, pinpointing the cells that are metabolically active.

Whenever the human body is exposed to x-rays, ultrasound, electromagnetic fields, or radioactively labeled substances, there is potential risk. In the medical application of anatomical imaging, the risk must be weighed against the benefit. Numerous studies have been conducted and are still being conducted to determine the outcomes of diagnostic and therapeutic exposures.

The risk of anatomical imaging is minimized by using the lowest possible doses that provide the necessary information. For example, it is well known that x-rays can cause cell damage, particularly to the reproductive cells. As a result of this knowledge, the number of x-rays and the level of exposure are kept to a minimum, the x-ray beam is focused as closely as possible to avoid scattering of the rays, areas of the body not being x-rayed are shielded, and x-ray personnel are shielded. There are no known risks from ultrasound or electromagnetic field at the levels used in diagnosis.

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