Over the next few decades, X-rays grew into a widely used diagnostic tool. Since bones show up clearly as white objects against a darker background, Roentgen’s rays proved particularly suited for examining fractures and breaks, but they could also spot cancer tumors, respiratory diseases such as tuberculosis or black lung, and a variety of other tissue abnormalities. The great strengths of X-rays are their high resolution{SYMBOL 190 \f "Symbol"}with details as small as 0.1 millimeter{SYMBOL 190 \f "Symbol"}and their ease of use. On the other hand, X-rays do not distinguish well between tissues of similar densities. As a result, some body features are unclear. X-rays capture only a slice of the body’s three-dimensional structure. Furthermore, each X-ray exposes a person to potentially harmful radiation, although today’s minute doses are almost completely safe.

For more than half a century, the science of medical imaging grew steadily but slowly, as incremental improvements were made in the X-ray technique. But suddenly, in the early 1970s, that growth kicked into high gear with the appearance of a new imaging option: computerized tomography, or CT. By taking a series of X-rays, sometimes more than a thousand, from various angles and then combining them with a computer, CT made it possible to build up a three-dimensional image of any part of the body. Doctors could then instruct the computer to display two-dimensional slices from any angle and at any depth{SYMBOL 190 \f "Symbol"}just as if they had sliced through the body with a sharp knife and taken a picture of the result. The CT scan revolutionized medical diagnosis, allowing doctors, for instance, to see if a head injury had produced any bleeding in the brain or to make out the shape and extent of tumors in cancer patients.

CT was the first of what has proved to be a flood of new medical imaging tools, each revealing different information about the body. The tools work according to various physical principles, but they have one thing in common: unlike the X-ray, which was little more than a photograph using X-rays instead of light, the new machines all depend on computers to construct the images from a mass of data that is collected electronically instead of on film. These new techniques open up a wealth of possibilities, only some of which have been reaped, but they also demand that researchers solve a host of new problems in order to realize those possibilities.

In the early 1980s, CT was joined by a completely different way of taking pictures of the body’s interior: magnetic resonance imaging, or MRI. Instead of passing X-rays through the body, MRI relies on a strong magnetic field and a radio signal that together trigger atoms in the body to send out signals of their own. By collecting and analyzing these signals, it is possible to compute a three-dimensional image which, like a CT image, is normally displayed in two-dimensional slices.

MRI is Radiation-Free

If MRI had been just another way of capturing the same images that CT provides, there would have been little cause for celebration. True, MRI avoids any worries about harmful radiation by using a magnetic field and radio signal instead of X-rays, but that advantage is balanced by the fact that MRI images have less resolution than those from CT. What made MRI a valuable complement to CT is that it provides very different information about the body. X-rays and CT scans offer details that depend on the density of different structures in the body: the denser an object, the more X-rays it blocks and the whiter its appearance in a X-ray or a CT image. That’s why bones stand out so strongly in a standard X-ray{SYMBOL 190 \f "Symbol"}they are much denser than the surrounding tissue. MRI, on the other hand, responds to the prevalence of particular types of atoms in the body. Most MRIs look for hydrogen, usually found in the body as a component of water. Thus an MRI scan distinguishes among tissues based on their water content. Fatty tissues, which have little water, appear bright, while blood vessels or other fluid-filled areas are dark. MRI is particularly useful for seeing details in the brain. Gray matter has more fluid than white matter, making it easy to distinguish between the two.

In the past several years, two other imaging techniques have joined CT and MRI. In positron emission tomography, or PET, a patient ingests or is injected with a slightly radioactive substance that emits positrons, which can be monitored as the substance moves through the body. Where CT and MRI are mainly valuable in viewing the body’s internal structures, PET is also useful in tracking its metabolism. In one common application, for instance, patients are given glucose with positron emitters attached, and their brains are monitored as they perform various tasks. Since the brain uses glucose as it works, a PET image shows where brain activity is high. Brain researchers have used this technique to discover which parts of the brain a person uses to perform specific tasks, such as read a list of words. Closely related to PET is single-photon emission computed tomography, or SPECT. The major difference between the two is that instead of a positron-emitting substance, SPECT uses a radioactive tracer that emits high-energy photons.

Another technique is ultrasound, known by many for its pictures of babies in the womb. Ultrasound makes an image by bouncing sound waves off an object inside the body. The reflected sound waves are then converted into a picture.

An Array of Imaging Tools

This array of tools is changing the way medicine is practiced, giving doctors much more information than ever before to diagnose disease or injury. It begins with the structural information provided by CT and MRI{SYMBOL 190 \f "Symbol"}bones, tissues, tumors and so on{SYMBOL 190 \f "Symbol"}but it goes far beyond that. PET, for instance, with its ability to monitor the brain in operation, is proving to be useful in diagnosing a variety of brain disorders, such as Alzheimer’s disease and schizophrenia. A variant of MRI called MRS (for magnetic resonance spectroscopy) can monitor metabolism in cells and thus spot changes in metabolism produced by degenerative diseases. SPECT is valuable for diagnosing coronary artery disease, and already some 2.5 million SPECT heart studies are done in the United States each year. Ultrasound is widely used for measuring blood flow.

A Window Into the Brain

Not only doctors but medical researchers are putting the new tools to use. Neuroscientists have made a series of intriguing discoveries using MRI and PET. Research has shown, for instance, that as the brain learns to do a task, it becomes more efficient, expending less energy to accomplish the same result. Furthermore, the same parts of the brain are activated when a person imagines an object as when the object is actually viewed. This and other discoveries are offering new insights into how memory works.

As impressive as these payoffs are, doctors and researchers expect to reap even greater rewards as imaging technologies improve. After all, although X-rays are 100 years old and CT scans 25, most of the tools are still relatively new, with much of their potential still to be tapped.

Many of the advances will come in data processing. Because most of the images are constructed by computer from a collection of electronic data, image quality depends on the power of the computer and the caliber of its software{SYMBOL 190 \f "Symbol"}and there is plenty of room for improvement in both. PET and SPECT images could be sharper, for example. Both rely on mathematical calculations to determine the location from which a positron or photon was emitted. Imprecise calculations show up as fuzz in the picture. More accurate calculations would have the same effect as focusing a lens: the picture becomes sharper. Scientists know how to get better images from the data than are produced by current devices, but the necessary calculations take so long that they are impractical. A number of researchers, some funded by The Whitaker Foundation, are working on algorithms that will give more detailed and accurate images from PET and SPECT without significantly increasing the time needed to create them.

Many of today’s medical images are processed digitally. This opens up many new possibilities. Data can be manipulated to enhance image quality. The information for images can also be transmitted to other locations over telephone lines, and various images{SYMBOL 190 \f "Symbol"}even those from different types of machines{SYMBOL 190 \f "Symbol"}can be compared and fused into composites. All of this requires specialized computer software and hardware, which researchers are developing and refining.

In addition to improved data processing, many researchers are looking to enhance the ways data are collected. For example, both CT and MRI often use substances, called contrast media, that show up clearly in the images and highlight some part of the body. A doctor examining a patient’s digestive tract by X-ray often has the patient drink a barium compound, which clearly outlines the esophagus, stomach and intestines as it moves through them. Other contrast media are designed to attach themselves only to specific tissues, such as certain types of tumors. One challenge is to create contrast substances that will collect at a tumor or other target tissue for long enough to be imaged and then be quickly eliminated from the body. Many of these substances are mildly radioactive, and although they are not dangerous to patients over short exposures, doctors do not want them lingering in the body.

Downsizing MRI

The imaging machines themselves can be refined and improved, and scores of research groups are working toward that goal. Two Whitaker-sponsored electrical engineers at Stanford University, for instance, are developing a new type of MRI scanner that is smaller than existing machines and much cheaper. Typical MRI devices cost $2 million to buy and install, and they cost patients $1,000 or more per scan. Meanwhile, across the country at the Rochester Institute of Technology, scientists are hoping to improve ultrasound images by modulating the frequency of the sound. They may be able to both sharpen the images and increase the depth at which they can be taken without increasing the intensity of the ultrasound, which can heat up the body’s tissues if pushed too high.

Perhaps the advance that most excites doctors and medical researchers is the ability to use MRI not just to image static internal structures but to monitor body activities, such as blood flow and brain activity. So-called functional MRI can be performed with existing MRI machines, but it will be much more effective with the next generation of devices.

Another approach is to combine imaging techniques. At Yale University, for instance, researchers are linking MRI and SPECT to measure heart function and blood flow through the heart. Neither technique by itself is completely adequate, but they provide complementary information and the whole may be more than the sum of the parts. Scientists at Stanford are following a similar route in combining CT and MRI images of the brain to provide surgeons with much better guideposts for where to cut during surgery.

Putting New Tools to Work

Meanwhile, other researchers are looking for new ways to put imaging tools to work. MRI scanners, normally tuned to respond to hydrogen, can also report on the presence of other elements, including many that play important roles in the body. Information on sodium, for instance, should prove useful in evaluating kidney function, while data on phosphorus can help assess muscle function and tumor activity. One researcher has even proposed a way to determine the temperatures of internal organs, by using MRI to monitor perfluorocarbon liquids injected into the blood and carried to various parts of the body. At the Indiana University Medical Center, two scientists are trying to apply PET to differentiate breast cancer tumors and cysts, which can be nearly indistinguishable with normal imaging methods. And a number of people are attempting to perfect ultrasonic techniques to determine the strength and flexibility of bones.

The past two decades have seen a revolution in imaging that is profoundly affecting medicine and research. Doctors are able to provide more accurate diagnoses and prescribe more effective treatments. They are beginning to use these sophisticated images to plan surgical procedures in detail. Researchers are coming to grips with heart disease, cancer and other illnesses that are being revealed on film, through lenses and on computer screens. Physicians are on the threshold of being able to beam intricate medical images from small rural hospitals to major medical centers for collaborative diagnosis and expert consultation. It is easy to imagine a time in the near future when surgeons will watch an image on a screen rather than look directly at the patient on the operating table. The image will do a better job of guiding the surgeon’s hand by showing important details invisible to the naked eye. The research is under way.