Looking Inside the Body​—Without Surgery

THANKS to advances in computers, mathematics, and science, the scalpel is giving way to nonsurgical tools in the diagnosis of certain diseases. Besides X-ray imaging, now over 100 years old, the technologies include computed tomography (CT scans), positron-emission tomography (PET scans), magnetic resonance imaging (MRI), and ultrasound imaging, or sonography. * How do these techniques work? What are their health risks? And what are their advantages?

X-ray Radiography

How does it work? X-rays have a shorter wavelength than visible light and can penetrate body tissues. When a certain part of the body is x-rayed, dense tissues, such as bones, absorb the rays and appear as bright areas on the developed film, called a radiograph. Soft tissues appear in shades of gray. X-rays are commonly used to diagnose problems or disease involving teeth, bones, breasts, and the chest. To distinguish between adjacent soft tissues of the same density, a doctor may inject a radiopaque dye into the patient’s bloodstream to enhance the contrast. Nowadays, X-rays are often digitized and viewed on a computer screen.

Risks: There is a slight chance of damage to cells and tissues, but the risk is usually very low compared with the benefits. * Women who may be pregnant should inform their doctor before they submit to an X-ray. Contrast agents, such as iodine, may cause allergic reactions. So inform your doctor or technician if you have any allergies to iodine or to seafood, which contains this element.

Benefits: X-ray imaging is fast, generally painless, relatively inexpensive, and quite easy to perform. Hence, it is particularly useful in such areas as mammography and emergency diagnosis. No radiation remains in the body after the X-ray is administered, and usually there are no side effects. *

 Computed Tomography

How does it work? CT scans involve a more sophisticated and intense use of X-rays, along with special sensors. The patient lies on a table that slides into a tunnel in the machine. Images are produced by numerous narrow beams of radiation and detectors that rotate 360 degrees around the patient. The process has been compared to examining a loaf of bread by photographically cutting it into very thin slices. A computer reassembles the “slices,” providing a detailed cross-sectional view of the body’s interior. The latest machines scan the body in a helical, or spiral, fashion, thereby speeding up the process. Because CT scans provide much detail, they are often used for examining the chest, the abdomen, and the skeleton, and for diagnosing various cancers and other disorders.

Risks: CT scans usually involve higher doses of radiation than regular X-rays. The additional exposure carries a small but significant increased risk of cancer, and this should be carefully weighed against the benefits. Some patients have an allergic reaction to contrast agents, which commonly include iodine; and in certain patients, there may also be an element of risk to the kidneys. If a contrast fluid is used, nursing mothers may have to wait 24 hours or more before resuming breast-feeding.

Benefits: Painless and noninvasive, CT scans provide finely detailed data that can be digitally converted into three-dimensional images. Scans are relatively fast and simple, and they can save lives by revealing internal injuries. CT scanners do not affect implanted medical devices.

Positron-Emission Tomography

How does it work? For a PET scan, a radioactive substance is attached, or tagged, to a natural body compound, most commonly glucose, and injected into the body. The image results from the emission of positrons​—positively charged particles—​from the tissues. PET scans operate on the principle that cancerous cells use more glucose than normal ones do, thus attracting a larger amount of the radioactive substance. As a result, diseased tissues emit a greater number of positrons, which register as a variation in color or degree of brightness on the final image.

Whereas CT scans and MRI scans reveal the shape and structure of organs and tissues, PET scans show how they are functioning, thus revealing changes at an earlier stage. PET scans can be performed in combination with CT scans, the superimposed image enhancing the detail. PET scans may give false results, however, if patients have eaten within a certain time prior to the scan or if their blood sugar levels, perhaps because of diabetes, are outside the acceptable range. Also, because the radioactivity is very short-lived, timing is important.

Risks: Because the amount of radioactive substance used is very low and its radioactivity short-lived, radiation exposure is low. Still, it can pose a risk to a developing fetus. Hence,  women who may be pregnant should inform their doctor and the imaging staff. And women of childbearing age may be asked to give a blood or urine sample to test for pregnancy. If a PET scan is used in conjunction with a CT scan, then the risks associated with CT scans should also be taken into account.

Benefits: Because PET scans show not just the shape of organs and tissues but also how well they are working, these scans can uncover problems before changes in tissue structure can be seen with CT or MRI.

Magnetic Resonance Imaging

How does it work? MRI uses a powerful magnetic field along with radio waves (not X-rays) and a computer to produce highly detailed “slice-by-slice” pictures of virtually all internal structures of the body. The results enable physicians to examine parts of the body in minute detail and identify disease in ways that are not possible with other techniques. For example, MRI is one of the few imaging tools that can see through bone, making it an excellent tool for examining the brain and other soft tissue.

Patients must remain still during the imaging process. And because the scan takes place as the patient slides through a rather small tunnel in the machine, some people experience claustrophobia. In recent times, though, open MRI scanners have been developed for patients who are anxious or obese. Naturally, no metal objects such as pens, watches, jewelry, hairpins, and metal zippers as well as credit cards and other magnetically sensitive items are allowed into the examination room.

Risks: If a contrast fluid is used, there is a slight risk of allergic reaction, but the risk is less than that associated with the iodine-based substances commonly used with X-rays and CT scans. Otherwise, MRI poses no known risk to the patient. However, because of the effect of the strong magnetic field, patients with certain surgical implants or metal fragments from injuries may be unable to have an MRI. So if an MRI is recommended for you, be sure to tell your doctor and your MRI technologist if you have any of those things.

 Benefits: MRI does not use potentially harmful radiation, and it is particularly good at detecting tissue abnormalities, especially those that may be obscured by bone.

Ultrasound Imaging

How does it work? Also called ultrasound scanning, or sonography, this technology is essentially a form of sonar that uses sound waves above the range of human hearing. When the waves reach a boundary where there is a change in tissue density​—the surface of an organ, for example—​an echo results. A computer analyzes the echo, revealing two- or three-dimensional features of the organ, such as its depth, size, shape, and consistency. Low-frequency waves enable the imaging of deeper parts of the body; ultrahigh frequencies permit the study of surface organs such as the eyes and the layers of skin, perhaps assisting in the diagnosis of skin cancer.

In most instances, the examiner uses a handheld device called a transducer. After applying a clear gel to the skin, he rubs the transducer over the area of interest, and the resulting image immediately shows up on a computer screen. When necessary, a small transducer can be attached to a probe and inserted into a natural opening in the body to make certain internal examinations possible.

A technology called Doppler ultrasound is sensitive to movement and is used to reveal blood flow. This, in turn, can be helpful when making diagnoses involving organs and tumors, which tend to have an abnormally large amount of blood vessels.

Ultrasound imaging helps physicians to diagnose an array of conditions and to discern the underlying cause of symptoms, from heart-valve disorders to lumps in the breast or the status of an unborn infant. On the other hand, because ultrasound waves are reflected by gas, the technology has limitations when applied to certain parts of the abdomen. Also, the resolution may not be as high as that of other technologies, such as radiography.

Risks: Even though ultrasound is generally safe when used properly, it is a form of energy and can produce physical effects in tissues, including those of the unborn. Prenatal ultrasound, therefore, should not be considered risk free.

Benefits: The technology is widely available, noninvasive, and relatively inexpensive. It also provides real-time imaging.

Future Technologies

At present, the main thrust of research seems to be to improve technology that is already available. For example, researchers are developing MRI scanners that operate with a much weaker magnetic field than that of present devices, thus considerably reducing costs. A new technology under development is called molecular imaging (MI). Designed to detect changes within the body at the molecular level, MI promises very early detection and treatment of disease.

Imaging technology has reduced the need for many painful, risky, and even unneeded exploratory operations. And when imaging leads to early diagnosis and treatment of disease, the outcome may be much better. The equipment, however, is expensive​—some machines costing well over a million dollars.

Of course, the prevention of disease is better than its detection and cure. So try to stay healthy through proper diet, regular exercise, sufficient rest, and a positive mental outlook. “A heart that is joyful does good as a curer,” says Proverbs 17:22.

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 HOW MUCH RADIATION EXPOSURE?

Daily we are exposed to background radiation, whether from cosmic rays coming from outer space or from naturally occurring radioactive substances such as radon gas. The following comparison may help you to evaluate risks associated with certain medical tests. Measurements are averages in millisieverts (mSv).

A five-hour flight in a commercial airplane: 0.03 mSv

Ten days of natural background radiation: 0.1 mSv

One dental X-ray: 0.04-0.15 mSv

One regular chest X-ray: 0.1 mSv

One mammogram: 0.7 mSv

One CT scan of the chest: 8.0 mSv

If you require an examination, do not hesitate to ask your doctor or radiologist for specific information about radiation exposure levels or any other concern you may have.

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X-ray

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CT

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© Philips

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PET

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Courtesy Alzheimer’s Disease Education and Referral Center, a service of the National Institute on Aging

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MRI

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Ultrasound