Doctors make diagnoses based on what they can see. For the earliest recorded physicians, this meant what they could detect with their own sense organs: the color of the skin, tongue, and fingernails; the smell of the breath and urine; the feel of the skin temperature and pulse rate; and the sound of a cry of pain when they touched a tender spot.
For centuries, physicians relied on little more than their senses and their experience to make diagnoses. Not only were the inside workings of the body a mystery, but also the inside parts of the body. Physicians could feel hard bones, elastic muscles, and soft organs. But they had little knowledge of what lay beneath the skin. Only in the 16th century, when doctors in Italy are rumored to have stolen corpses from the gallows and dissected them, did physicians begin to understand the internal parts and workings of the human machine. In 1543, the same year as the publication of Copernicus' heliocentric theory of the solar system, Andreas Vesalius produced the first essentially correct book of anatomy.
But dead bodies and living ones are two very different things. More than three centuries passed before a tool to look inside the living human body was developed. This tool was the X ray. It was first applied to studying the human body in 1895, four days after its discovery was announced.
X Rays—The First Imager
Wilhelm Konrad Roentgen, a German physicist, was studying a device called a Crookes tube. This tube contained a partial vacuum and had electric wires connected to two metallic electrodes inside the tube. When a high-voltage current was applied to the electrodes, the tube glowed with a phosphorescent light. In one experiment, Roentgen had the tube enclosed in a black paper cover. In his darkened laboratory, he noticed a greenish glow several feet away whenever he turned the tube on. Investigating, he found that the glow came from a paper screen coated with barium platinocyanide. Further experiment showed that the screen continued to glow even when it was shielded from the tube by several layers of cardboard and paper, and that a photographic plate near the tube was blackened. Roentgen came to the conclusion that some kind of invisible rays must be coming from the tube. He dubbed them X rays, the "x" being the common mathematical symbol for an unknown.
Further experimentation led Roentgen to discover that X rays travel through some, but not all, materials. In time, he experimented with his own body. In his first public lecture on the new rays, he called for a volunteer to be X-rayed. He placed the volunteer's hand between the Crookes tube and the photographic plate. When he developed the plate, he found the image of the hand. Although the X rays could pass through the hand, they were blocked in different amounts by different tissues. The dense bones appeared relatively light in color because they resisted the passage of X rays to the plate. Softer surrounding tissues appeared darker because they were less dense and offered less resistance to the passage of X rays. The image on the photographic plate was essentially a negative on the basis of tissue density. Medical personnel now read the negative image obtained directly on photographic film to evaluate the X-ray findings.
In a Crookes tube, electrons are propelled through a partial vacuum. Electrons are subatomic particles that carry a negative charge of electricity. The glow that Roentgen saw in the tube was light waves produced when the electrons bounced against the gas molecules remaining in the tube. The electrons that hit the glass walls produced the X rays.
Today we know that X rays and light are different wavelengths of the same kind of electromagnetic radiation. Electromagnetic radiation is produced whenever electrons are deflected from their paths. The high energy of the electrons in the Crookes tube and the sharp deflection on meeting molecules of glass produce X rays. The more moderate deflection of the electrons on meeting gas molecules in the tube produces light.
X rays gave physicians their first easy look inside the living body. The procedure required no surgery and caused no pain. The patient just held still the body part being photographed until the plate was exposed.
X rays provided a great deal of information not available before. Physicians could study the living skeletal system and organ placements and get a clear picture of the broken bones, which they could previously feel only through the skin. They could also identify small, hairline fractures that tactile examinations could not pick up, and they could detect tumors in some body parts and displacements of certain bones and organs.
Today X-ray pictures, or radiographs, are still the most widely used images in medical diagnosis. They are almost routine when there is a limb or joint injury or a blow to the head. They also show lung tumors and enlarged organs.
But radiographs have their limitations. They give only a two-dimensional view of the body. Layers of body organs can make X-ray images difficult to interpret. X rays often cannot show structures deep within a tissue. Furthermore, X-ray overexposure can damage tissues and cause cancer. That is why doctors limit the amount of X rays that any one person receives. It is also why dentists usually leave the room while X rays are taken of the teeth. The accumulation of exposure to stray X rays could cause a dentist to develop cancer.
A new tool emerged in the 1950s that revolutionized diagnostic imaging. This tool is the computer. The coupling of computers and imaging technologies enables physicians to obtain cross-sectional images of the body that eliminate X-ray shadows of structures in front of and behind the desired sections. The computer can store information, manipulate it, and show it as sequential images in motion. In other words, the computer can give moving pictures of the body at work. Doctors can now see internal organs at work, blood coursing through vessels, and even metabolic changes in active cells.
Some of the new imaging devices rely on X rays, while others exploit totally new medical technologies. The CAT (computerized axial tomography) scan—also called the CT (computed tomography) scan—is a technology that has been used medically since the early 1970s. It is derived from X rays, but with a powerful difference. The source of X rays is in a rotating doughnut in which the patient lies during examination. X rays are shot out in a fan shape. Sensors on the opposite side of the doughnut detect the amount of radiation passing through the body, and send this information to a computer in which the data are stored. The X-ray source moves a few degrees and fires another burst of radiation. The sensors pick up these readings and add these data to the computer. This process continues all the way around the body.
The computer then analyzes the information based on preprogrammed instructions and creates an image on a monitor. This image is a cross section of the body at the level of the scan. If the scan is taken of the head, the image will show a cross section of the brain. Likewise, a scan of the chest will show the lungs and heart.
Additional scans in an area provide a series of images of the internal structure of an organ. The images also show the changes in organs as they work, such as the pumping of the heart, the flow of blood through the vessels, the churning of the stomach. These images provide insight about the health of an organ.
DSR and DSA Scans
Dynamic spatial reconstructor (DSR) is a more sophisticated type of CAT scan. The DSR machine isolates a working organ, photographically slicing through the organ from any desired angle. In the time it takes a CAT apparatus to make one scan, the DSR machine can make up to 75,000. Using computer storage and manipulation, the DSR machine combines the information from these scans into a single three-dimensional image. Like the CAT machine, it can take images of organs as they are moving, but because it displays the organs in three dimensions, the image can provide more information to the trained eye.
Digital subtraction angiography (DSA) also uses X rays, but in a different way than CAT or DSR. An X-ray image is taken of the area under study, such as the heart. Then a substance that does not allow the passage of X rays is injected into the area of interest, say the coronary arteries. A second X-ray image is taken. The computer compares the two images, and subtracts the information in the first image from the information in the second image. The result is an image that highlights that area, using the substance that is opaque to X rays.
Dual-energy X-ray absorptiometry (DEXA), also known as dual X-ray absorptiometry (DXA), is an effective method of testing bone density. Using photons at two different energy levels given off by X rays, DXA is able to measure the amount of photon absorption by the bone. The degree of photon absorption is related to the mineral content of the bone itself. DXA provides both an X-ray-like image of the bone and a measure of the bone's density. The process is faster than other diagnostic methods, taking from 2 minutes for a femur scan to 10 minutes for a total body scan.
Bone-density measurement is an extremely important factor in the early detection of osteoporosis, a disease in which bone mass is lost progressively, resulting in brittle, easily fractured bones. DXA can be targeted at people who are especially at risk for osteoporosis (such as women who are postmenopausal). Early detection often allows for more effective treatments.
Not all of the new tools that extend the physician's senses are based on X rays. One that has been in use for many years employs sound to "see" inside the human body. The apparatus for sonography, or ultrasound, consists of three specific components: a monitor, a computer, and a probe. The probe is both the transmitter and the receiver of ultrasound waves, sound in the range too high for human ears to hear.
Sonography works on the same principle as sonar—echoes from objects are used to locate the objects. Higher pitches give a more precise location, since sound waves become shorter as the pitch is raised. Bats, which use sonar to locate and zero in on small flying insects, use pitches that are in the ultrasound range.
The technician moves the probe in a circular motion over the area under study. Sound waves sent out from the probe penetrate the body and reflect off internal structures. The probe captures the returning echoes and transmits them to the computer. The echoes are analyzed and constructed into an image on a monitor. The process is so rapid that the technician can reposition the probe to get more information as the imaging occurs.
Sonography is widely used to study fetuses in utero; it can identify birth defects prenatally, particularly in women who are at high risk for pregnancy problems. Also, it is often used to determine the fetus' age when the time of conception is uncertain, to detect twins, and to guide invasive procedures, such as amniocentesis. Increasingly, sonography is also being used to detect and treat heart disease, as well as vascular diseases that can lead to stroke.
Sonography is also applied in the Doppler color flow imaging technique. By measuring sound waves reflected from blood flowing in vessels, an ultrasound machine can produce a picture of a patient's vascular system. The blood moving within the human body is shown in color, enabling doctors to pinpoint such heart-threatening problems as clots, cholesterol deposits, and vessel narrowing.
Magnetic resonance imaging (MRI) is a powerful type of imaging technology that began to be used in hospitals in the early 1980s. Also referred to as nuclear magnetic resonance (NMR), this technique uses the body's water molecules as the basis for its images.
The theoretical basis for this technology is that a powerful magnetic field turned on and off can cause the nuclei of certain atoms to line up in a particular direction. The hydrogen atoms in water are among the ones affected by magnetism in this way. The MRI machine also releases a burst of radio waves, which, like light and X rays, are a form of electromagnetic radiation. Just as the electrons in a Crookes tube interact with the molecules of glass, radio waves of the proper frequency can be used to change the alignment of the hydrogen atoms that have been "organized" by the magnetic field. When the magnetic field is turned off, the atoms move back into alignment, producing a burst of electromagnetic radiation of their own. Each atom produces a different signal. These signals can be received, compared, and used to produce an image. With the human body, this is accomplished with giant magnets.
The MRI machine itself looks like a tunnel. Its outer walls contain magnets that can be switched on and off. The magnets produce a magnetic field so powerful that it can pull iron objects into the machine from across the room. Physicians, nurses, technologists, and patients all have to be careful not to wear iron belt buckles or use other iron objects when the machine is going to be in operation. Other metal obects—such as pens, keys, or scissors—must be removed from the room. The patient is placed on a stretcher within the tunnel, and the magnetic field is turned on. The nuclei of hydrogen atoms in water molecules in the body line up parallel to the direction of the magnetic force. This is similar to aligning iron filings with a bar magnet. The radio waves then briefly shift the atoms, after which the atoms are allowed to return to their former alignment. In this brief period of changing position, the nuclei emit radio waves that can be recorded and analyzed.
The diagnostic images produced by MRI can help determine both stroke-ravaged regions of the brain and, through tumor imaging, the efficacy of cancer treatment. Another device, called the echo planar MR scanner, is able to pinpoint the area of a stroke at its earliest stages—even while it is happening—by measuring the rate of water diffusion in the brain. Water movement slows down considerably during a stroke. This technique may lead to advances in limiting the brain damage caused by strokes.
Radioactive Tracers and Pet Scans
Radioactive tracers are substances that include a radioactive isotope in place of a normal nonradioactive element. An isotope of an element is chemically the same as the element; however, it has a different mass because of a difference in the number of neutrons in its atoms. The number of neutrons in an atom can affect its stability, so many isotopes of nonradioactive elements are radioactive. Tracers often contain radioactive isotopes of carbon, oxygen, or nitrogen, all of which are elements found in high concentrations within the body.
Radioactive tracers are injected or inhaled into the body. Their interaction with the body can be tracked by instruments sensitive to radioactive emissions. Radioactive tracers can be used with CAT imaging as well as with simpler systems.
When the radioactive tracer is a nutrient such as glucose, physicians can study how it is used by the healthy body and by the diseased body. They can follow its uptake, distribution, and consumption within specific body tissues. When the radioactive tracer used is a drug or a poison, the doctors can study where the substance becomes concentrated, where it is metabolized, where it is stored, and how it is eliminated from the body.
Sometimes radioactive tracers are used because they attach to or become part of a tissue of interest. For example, carbon monoxide has an affinity for certain blood cells. By having a person inhale small amounts of radioactive carbon monoxide, the course of blood through the body can be traced.
Positron emission tomography (PET) is an imaging technique developed in the 1980s that uses a special type of radioactive tracer. This technique traces the path of specific radioactive molecules through the body as they enter cells or are metabolized by cells.
Some radioactive materials give off tiny particles called positrons. Positrons are subatomic elements that are exactly like electrons in size and amount of charge. However, positrons are positively charged—not negatively charged, as electrons are. In fact, a positron is the antiparticle of the electron—its mirror image. When a particle meets its antiparticle, both particles vanish, and only electromagnetic radiation is left. In PET, it is more convenient to think of the electromagnetic radiation as particles, or photons, than as waves. As the radioactive substances travel through the body, they act much as an electronic signaling device does. They leave a photon trail that the PET scanner can recognize and record.
The PET apparatus is shaped much like the CAT scanner. It has a ring or doughnut shape in which the patient lies. The ring is positioned along the area to be studied. The patient is given an appropriate radioactive substance, one that is normally used by the body and that has been made radioactive by incorporating isotopes into it. Usually, the physician "administers" the radioactivity intravenously via radioactive sugar or protein products, or has the patient inhale radioactive gases. As the radioactive substance is absorbed by and interacts with the area under study, photons leave the body and strike the encircling ring. The pattern they make is analyzed by computer and projected as an image on a monitor.
What is truly unique about PET is that it allows physicians to watch body processes as they occur. Oxygen uptake by the brain can be observed; the rate of sugar use in various cells under a given set of conditions can be compared. This technology not only aids in diagnosis, it also provides information on the relationships between structures and normal metabolic processes. It has proven to be valuable in studying brain functions, including memory, and observing the brain as words are spoken or read, and so on.
Recently, radiologists have begun to test multimodality machines that perform two types of nuclear imaging: PET and single photon emission computed tomography (SPECT). The latter uses radioisotopes to measure blood flow in small vessels and is particularly well-suited to imaging the brain. Multimodality machines appear to offer distinct advantages over devices that only perform PET or SPECT. The resolution is better, and the machine permits a greater range of applications. PET has proven to be a safe and efficient method for imaging various forms of cancer, heart disease, and a number of neurological disorders, including Alzheimer's disease.
Thermography uses differences in body temperature to create images. These differences arise because different areas of the body have different concentrations of blood vessels, and because cells that are metabolically active produce more heat than those that are less active.
Like all of the forms of imaging discussed, except sonography and radioactive tracers, thermography uses electromagnetism to see inside the body. In this case the waves are infrared, just a little longer than light waves. The image, therefore, is fairly sharp, although not as sharp as images based on X rays.
Thermography is a technique that is especially useful in locating cancerous tumors because these tumors are quite active metabolically. Thermography can also be used to identify areas of the body where there is reduced blood flow.
Better Images and Diagnoses
The images produced by CAT, DSR, DSA, MRI, and PET are initially produced in black and white. To the trained eye, they speak volumes about the tissues being observed, but these images can be enhanced further by the addition of color. By programming the computer to assign specific colors to specific signals, simulated color images can be produced. Not only is this more attractive visually, but it can be of real assistance to medical personnel using the images during surgery or other treatment. If, for instance, a brain tumor is found, and the course of treatment is to be radiation, color images that highlight sensitive structures nearby, such as the eyesor optic nerves, can be of immeasurable help to the physician as he or she plans the next course of action. Since the images are so precise, the physician administering the treatment has more information.
People who suffer from heart and circulatory diseases have benefited greatly from the new imaging techniques, several of which show the human heart in action and the flow of blood through vessels. Irregularities in the heart's beating, and faulty performance of the valves within the heart, can be seen; degeneration in the strong muscular ventricles identified; blockages in blood vessels detected; and ballooning of overstretched vessel walls in an aneurysm spotted.
These same techniques can be used to study the heart under stress. By injecting a patient with a heart stimulant, the heart can be made to race while a series of images are taken. By comparing images of the stressed and unstressed heart, physicians can determine even more about the health and overall functioning of this vital muscle.
The brain is the second body organ for which imaging technology has become especially important. CAT scans, PET scans, and MRI scans have made it easier to detect brain tumors, even when the tumors are quite small. MRI scans are especially useful because they can clearly distinguish between white and gray matter based on their different water content.
PET scans are good for detecting the locations of epileptic activity. During a seizure, these areas are more active than are surrounding tissues, and therefore consume more glucose. Once identified, these small areas sometimes can be surgically removed to control the epilepsy.
Brain scans conducted for a specific reason sometimes detect silent conditions. A small aneurysm or a congenital defect that has produced no symptoms may be identified. Treatment can prevent later problems caused by these conditions.
Early detection of cancer is another payoff of the new imaging devices. Cancer cells have a high metabolic rate that lends them to detection by PET and MRI. Tumors can be spotted much earlier with these techniques than with others.
Mammography, a technique used to identify breast cancer, can detect tumors of less than 0.5 inch (1.25 centimeters). X rays are sent through the breast. If tiny calcium deposits are present, they show up on the X rays. Such deposits are often associated with cancerous tumors and indicate that additional testing needs to be done. Sonography can also be used in breast-cancer detection. It can detect only those tumors larger than 0.4 inch (1 centimeter) in diameter, but it can distinguish between malignant and certain benign tumors. In addition, CAT scans and radioactive tracers can be used to identify breast cancer. Cancers of the breast take up iodine, so radioactive iodine is used as a tracer in this procedure. A baseline CAT scan is performed; then the patient is injected with radioactive iodine. After a waiting period, a second CAT scan is made. A comparison of the two helps doctors locate any cancerous growths that might be present.
The CAT scan is useful in producing images of bones and surrounding tissues. Bone misalignment, joint malformations, and traumatic damage to areas involving bones and nerves can be clearly imaged with CAT scans. An emerging area for CAT-scan use is in reconstructing damaged or deformed bones. The images from CAT can be used as the basis for creating artificial bones and joints. The information from these scans allows the artificial-bone makers to construct an almost perfect replacement—one that is a good fit with the person's own healthy bones.
Advanced imaging technologies improve our understanding of the human body, simplify the diagnosis of illnesses and conditions that were once difficult to identify, enhance the safety of surgery, and reduce the need for exploratory surgery. In addition, imaging technologies may detect certain conditions before symptoms appear, making preventive intervention possible. Nonetheless, the full potential of these technologies has yet to be realized. For example, in some cases, techniques may be more powerful in combination. Current research is evaluating PET/MRI in the study of pain, as well as CAT/MRI in determining the location and composition of tumors.
The information provided should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed physician should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies.
Copyright Information: Public domain information with acknowledgement given to the U.S. National Library of Medicine.