How are imaging and radiology related?
Before the advent of radiology, physicians were forced to rely on visual clues, palpation (examination by touch), and exploratory surgery to discover what lay beneath a patient’s skin. X-rays provided the first path around this limitation and allowed physicians to peer beneath the skin without surgery. X-rays are electromagnetic radiation similar to visible light, but they have much higher energies than visible light, which makes them more penetrating.
Circumstances under which a doctor needs to see inside the body and might use x-rays to do so include those in which a patient has fallen and experiences localized pain and swelling in the leg (possible broken bone); a patient with severe pain in the lower back passes blood in the urine (possible kidney stone); a patient experiences intermittent and squeezing chest pain, nausea, and shortness of breath (possible constricted arteries in the heart); and a patient has a history of localized stomach pain and heartburn, also called acid reflux (possible inflamed esophagus, hiatal hernia, or ulcer).
All medical imaging devices require a source of radiation, a detector, and a way to position the patient. For x-rays, the patient stands in front of a film holder or lies on a table over a film holder while x-rays from an x-ray tube pass through the patient and cast the patient’s shadow onto the film. The film must be as large as the body part being examined. Dense constituents such as bones show up as white on x-ray negatives, while various other body tissues show up as shades of gray depending upon the fraction of the x-rays that they block.
X-ray film has been developed to have excellent resolution and contrast, and an x-ray is a permanent record. It is not easy to take several x-rays in quick succession, however, as might be done to examine the upper gastrointestinal tract. An alternative is to use fluoroscopy, in which a screen coated with special compounds is used in the place of film. These compounds fluoresce when struck by x-rays. Advantages are that the image is immediate (no film to develop) and moving images can be viewed. The image can be made brighter by an image intensifier—a television-camera-like device that amplifies the image—and a permanent record can be made by photographing this final image.
An ultrasound unit adapts sonar to medical purposes. The unit consists of a probe; electronics to generate, receive, and analyze signals; and a monitor to display the results. Such a device is relatively inexpensive compared with other types of scanners, and at the intensities used, ultrasound waves do not harm the body. The ultrasound probe contains a transducer, a crystal that can change electrical signals into sound waves and can also act as a receiver changing sound waves into electrical signals. During the examination, a gel is spread over the patient’s skin in the area of interest. The gel provides good acoustical coupling between the probe and the patient’s body tissue. To create an image of structures as small as 1 millimeter, ultrasound frequencies between 3 and 7 megahertz are used. The crystal inside the probe emits sound waves while rapidly rocking back and forth, producing a wedge of sound pulses. The crystal then becomes a receiver and picks up sound echoes that form as the sound passes from one type of tissue into another. Distance can be determined from the time elapsed between sending a pulse and receiving the echo. Combining this distance information with the direction and intensity of the echo allows a computer to build an image of the structures inside the body.
This image is displayed on a monitor screen and can also be printed for a permanent record. The probe is moved around to provide the least obstructed view of the target. Since ultrasound waves pass easily through soft tissues such as the liver and fluid-filled organs such as the gallbladder or the uterus during pregnancy, ultrasound is particularly useful to examine these organs. Ultrasound does not penetrate bone or gas, and the latter property makes it difficult to examine the digestive tract with ultrasound.
Like ultrasound, magnetic resonance imaging (MRI), uses no potentially harmful radiation, but it is the most expensive of the imaging techniques. MRI uses a very large, superconducting magnet. Once the magnetic field is established, superconducting magnets require very little electrical power. To be superconducting, the magnet is wound with niobium-titanium wire and maintained at liquid helium temperature (about five kelvins above absolute zero). The magnet itself is in the form of a very large, hollow cylinder into which the patient can be inserted. The magnetic field inside the cylinder is uniform and very strong, typically 0.5 to 2.0 tesla; for comparison, the strength of the earth’s magnetic field is only about 0.00005 tesla near the surface. There are also three sets of much smaller (.0018 to .0027 of a tesla) “gradient” magnets, which are used to tweak the magnetic field at pinpoint locations during the scan. Hydrogen atoms are among the most abundant atoms in soft tissue. In a strong magnetic field, the spin axis of hydrogen nuclei (protons) precesses about magnetic field lines. The exact frequency with which it precesses depends on the proton’s surroundings, or the type of tissue in which it is found.
The patient rests on a table that moves into the tunnel formed by the magnet. The magnetic field causes no noticeable sensation, but claustrophobic patients may become anxious. To combat this effect, newer machines have wider tunnels or have one side open. An insulated wire coil may be placed on the patient over the area to be examined, or coils mounted on the magnet may be used. A brief pulse of radiofrequency waves is broadcast from the coil at the same time that the gradient magnets adjust the magnetic field to the right value in a narrow slice of the patient. If the radiofrequency is at the precession frequency, then those protons with which it resonates will absorb energy. After the radiofrequency pulse, the protons produce an echo that radiates energy back to the coil. Analysis of this energy allows the identification of the type of tissue, which, along with location information from the gradient magnets, allows a computer to construct an image of the internal tissues of the patient. The image can be either two-dimensional or three-dimensional, as needed. Two-dimensional slices can be vertical, horizontal, or in any plane desired.
A scan of the brain and spinal column can detect areas of damage to the myelin (insulating sheath) surrounding nerve fibers that is the hallmark of multiple sclerosis. MRI scans are also used in diagnosing tumors, infections of the brain, strokes, torn ligaments, and many other conditions. As with x-rays, contrast agents may be injected into the patient to make certain tissues or systems stand out. For example, the blood vessels in the heart may be examined in this fashion. Depending upon the size of the region to be scanned, the procedure takes from twenty to ninety minutes, and the patient must remain as still as possible during this time. Currents in the gradient magnets produce loud thumping sounds. These may be masked by providing the patient with music through earphones during the examination. Physicians now depend heavily on MRI investigations, and the 2003 Nobel Prize in Medicine or Physiology was awarded to Paul C. Lauterbur and Sir Peter Mansfield for their work leading to the development of the MRI technique.
A nuclear medicine scan, also called nuclear radiology or isotope study, uses radioactive pharmaceuticals that have been injected into or ingested by the patient. The radiation dose to the patient is strictly controlled and is the least possible for the procedure to be effective. The radioactive isotopes used emit gamma rays—essentially high-energy x-rays—which easily pass through the body and are observed by an array of gamma detectors. The patient lies on a table while the detector array is positioned near the target organ. In some applications, the array slowly scans a region. A computer receives information on the location and intensity of the detected gamma rays and constructs an image from this information. The image is fuzzy and lacks the fine detail that can be seen with x-rays or ultrasound, but it shows directly how well the target organ functions.
To be useful for this purpose, radioactive isotopes must emit penetrating gamma rays, have a short half-life, and either be taken up by the target organ or be capable of being attached to a molecule that is taken up by the target organ. A short half-life is desirable to ensure that the isotope will cease to be radioactive not too long after it has served its purpose. Technetium-99m is frequently used because it is taken up by the thyroid and salivary glands (if those are the targets), it can be used to tag other biologically active molecules, and its half-life is only 6.01 hours.
For a thyroid study, the patient may be given a drink or a pill containing iodine-123 (13.1 hour half-life). About twenty-four hours later, the gamma emission from the thyroid is measured. This procedure shows how well the thyroid works in general. The patient may then be injected with technetium-99m and several scans made over the next hour. This provides another measure of the thyroid’s functioning. It also allows a measure of the thyroid’s size and shape. Any area of inactivity suggests a blockage or damage, while an unusual area of activity suggests an abnormally high tissue growth rate such as a tumor. In general, a region of rapidly growing tissue shows up as a hot spot on a nuclear medicine scan, which makes it possible to see a healing bone fracture that an x-ray might miss.
A positron emission tomography (PET) scan is a special type of nuclear medicine scan that capitalizes on a property of the gamma rays emitted during positron annihilation. Positrons are positively charged and are the electron’s antiparticle. Since they are oppositely charged, positrons and electrons are strongly attracted to each other, and as soon as they touch, they mutually annihilate, converting their energy into two gamma rays emitted at precisely 180 degrees to each other.
A gantry houses a large ring of gamma ray detectors arranged like a doughnut on its side. The patient rests on a movable table that carries the patient into, and perhaps through, the detector ring. Depending upon the purpose of the examination, one of several radioactive pharmaceuticals is administered. For example, fluorine-18 (half-life 109.8 minutes) is attached to glucose molecules and used to monitor the brain’s metabolism. Since active areas of the brain draw the most glucose, a PET scan can watch the brain at work. When a fluorine nucleus emits a positron, the positron encounters an electron almost at once and is annihilated. The gamma ray detectors continually register hits from natural background radiation, but if two detectors located 180 degrees around the ring from each other record gamma rays of the right energy (0.511 million electron volts) and within a few nanoseconds of each other, it is virtually certain that they originated from a positron annihilation in the patient. A computer then draws a straight line between the two detectors and through the patient. Where two or more lines intersect is the location of the activity.
A single photon emission computed tomography (SPECT) scan is similar to a PET scan, but it detects only one photon at a time instead of a pair. To define the direction of a gamma ray photon, a collimator is placed in front of the detector array. The collimator is a slab of five-centimeter-long lead straws pointing at the patient so that a gamma ray can reach the detector only by traveling up a straw. Such a collimator and detector array is called an Anger camera. A ring of cameras around the patient allows a computer to construct a three-dimensional image of the target organ. SPECT combines the organ function information of nuclear medicine with some of the resolution of a computed tomography (CT) scan.
The CT apparatus consists of a movable table on which the patient rests and a gantry that houses the x-ray equipment. A doughnut hole in the gantry is large enough for the patient and table to pass through. An x-ray tube is mounted in the doughnut, and an array of x-ray detectors are mounted in the doughnut directly opposite from the x-ray source. As the doughnut rotates all the way around, the x-ray beam is sent through a narrow “slice” of the patient. Instead of the normal x-ray silhouette of the patient as illuminated from one direction, silhouettes from all directions are obtained, but only as numbers, not yet as images.
The information available consists of where the detector was when it picked up the signal and how strong the signal was. As the scan continues, the computer uses this information to construct an image of that slice of the patient. While the doughnut carries the x-ray tube and detectors around and around the patient, the table advances slowly and continuously until the whole body (or the desired segment) has been scanned. Blood clots and ruptured vessels in the brain are easily detected, as well as tumors in soft tissues such as the liver.
Bones, bone breaks, and some tumors show up well on an x-ray, but soft tissue images may require additional techniques. For examination of the upper gastrointestinal tract, the patient is given a barium solution to drink while standing in front of the x-ray detector. Barium blocks x-rays and coats the esophagus and stomach lining, thereby outlining them in detail on the x-ray. In the “barium swallow,” a quick series of x-rays is taken to follow the progress of the barium into the stomach. Constrictions in the esophagus, the action of the stomach valve, and the presence of a hiatal hernia (the upper part of the stomach bulging through a weakened diaphragm) become visible. As the barium proceeds, a stomach ulcer may stand out in outline.
To check for kidney stones and to examine the kidneys, ureters, and urinary bladder, an intravenous pyelogram (IVP) is performed. First, a contrast solution (based on iodine, which blocks x-rays) is injected into the patient’s vein. The patient may feel a brief warm flush as the body reacts to the iodine, but this reaction passes quickly. The solution used soon passes through the patient’s system: through the kidneys, through the ureters, and to the bladder. A series of x-rays is then taken in which kidney stones may be revealed; any swelling of a kidney or blockage of a ureter will show up, if present.
The angiogram, a similar but more extensive procedure, is used to examine the blood vessels of the heart or other location. In this procedure, a catheter (fine plastic tube) is inserted into the femoral artery at the groin or into a blood vessel in the upper arm. The patient is given medication for pain and anxiety. Using a fluoroscope monitor, the physician maneuvers the catheter to the desired location and then injects the contrast solution through the catheter. Several x-rays are taken from various angles, and any blockages or constrictions are generally apparent. The catheter is then withdrawn, and the patient is required to rest.
CT scanning represents a major step forward in x-ray technology. A patient might suffer a sudden loss of muscle control and feeling in part of the body (possible stroke). The doctor would like to x-ray the brain for signs of a ruptured or blocked blood vessel, but contrast agents are not useful for the brain. Without them, internal organs and tissues appear only as faint ghosts in an x-ray. A CT scan uses computer technology to convert special x-ray images into clear views of the organs.
While prudence dictates that radiation exposure be kept to a minimum, the medical benefits of procedures using radiation generally outweigh the risks. Provided that a radiation dose is not overwhelmingly massive, the body has amazing recuperative properties and is able to repair most radiation damage. Everyone is exposed daily to background radiation from naturally occurring trace amounts of radioactive elements and from cosmic rays. None of the typical imaging procedures generates radiation above allowable limits, but it should be noted that a medical procedure may expose the target organ to many times the average body dose.
A typical x-ray image shows different types of tissue as various shades of gray. Advances in medical imaging include digitizing any image and using powerful computer programs to enhance the subtle differences between different types of tissue, even coloring them so that they are immediately obvious to the physician. Three-dimensional images can be constructed and rotated so that they can be examined from every angle. A physician can see exactly what problems will be encountered in removing a tumor. Computers can scan x-rays used to screen for breast cancer, called mammograms, and draw the radiologist’s attention to any questionable regions. Finally, computers can merge images from complementary techniques such as PET (shows functionality of the organ) with MRI (shows high-resolution detail) to give a more complete view.
On November 8, 1895, Wilhelm Conrad Röntgen made an astounding and disquieting discovery. Many scientists of the day, including Röntgen, were studying cathode rays. The required apparatus was an evacuated glass tube with electrodes sealed inside at either end. When a high voltage was placed across the electrodes, cathode rays (electrons) streamed from the cathode, and where they struck the glass tube, the glass fluoresced.
Röntgen covered the tube with black paper and darkened the room, but still a fluorescent screen some distance away from the tube glowed whenever he operated the tube. Since cathode rays cannot travel far through air, Röntgen deduced that some unknown type of radiation must be coming from the tube. He called it X radiation (X for unknown). Placing bits of wood or metal between the tube and the screen cast shadows on the screen, some darker and some lighter. The key moment came when Röntgen held up a small lead disk. The expected shadow of the lead appeared, but holding the disk, Röntgen saw the shadows of his finger bones. He opened his hand, and the shadowy skeleton hand opened. Röntgen wondered if he could be hallucinating. In those days, some people believed that dreaming about or imagining seeing a skeleton was a premonition of one’s impending death. Röntgen was concerned that others might think him insane if he were to describe what he had seen, so he needed proof.
In the ensuing weeks, Röntgen made an intense study of the properties of x-rays. Three days before Christmas, he placed photographic film in a black paper package and had his wife, Bertha, place her hand on it while he exposed it with x-rays for fifteen minutes. The photograph showed the bones of Bertha Röntgen’s hand, with a ring on her finger. Röntgen sent a copy of the photograph along with an explanation to a friend, but it soon found its way into newspapers all over the world. Only four months after the public announcement, Thomas Edison’s company began marketing “complete outfits for x-ray work.” While some considered looking at skeletons while people were still using them revoltingly indecent, the medical usefulness of x-rays was immediately apparent. The first Nobel Prize in Physics ever awarded went to Röntgen in 1901 for his discovery of x-rays.
In spite of the obvious medical advantages, relatively few doctors used x-rays at first, but the public found them a source of entertainment. Edison built a fluoroscope for use at amusement parks. Looking through a hooded visor, spectators placed their hands between an x-ray tube and a fluorescent screen and watched their bones wiggle as they moved their hands. Casual exposure to x-rays continued into the 1950s with the use of fluoroscopes by shoe stores to see how well shoes fit.
The widespread use of medical x-rays was hampered by the lack of trained operators and proper equipment. This situation changed with World War I, when x-ray teams were trained and equipped by the hundreds. Marie Curie was a prime force in establishing such teams for the French and Belgian militaries. From this modest beginning, the various imaging techniques have evolved to provide the physician with “magic eyes” that can peer inside the body.
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