THE HEALTH REVOLUTION: Surgery and Medicine in the Twenty-first Century - 1. Looking for Trouble
Figure 1. An X-ray photograph of a human skull.
Figure 2. A CAT scan in progress, showing the scanner and patient in the background and the radiographer working at the scanner's computer terminal in the foreground. An image of the patient's abdomen appear on the screen.
Figure 3. A patient being prepared for an MRI scan. The patient's head is surrounded by the coils of the scanner's magnet: the smaller device above the patient's head is a radio frequency receiver.
Figure 4. A three-dimensional reconstruction of a child's face obtained using CAT scan data, showing the skin surface on the left and the underlying skull on the right.
Figure 5. An ultrasound picture of a seven-month-old fetus in its mother's womb, showing the face and shoulders in profile.
Sixty years ago the main way to see inside a person's body without cutting them open involved taking an X-ray photograph. X-rays were discovered in 1895 by a German-born Dutch citizen, William Roentgen. X-rays pass fairly easily through the soft parts of the human body but are stopped by harder or more dense parts. So when a beam of X-rays is passed through a person's body and onto a photographic plate, solid objects such as bone show up in white on a negative print. This makes X-rays photos ideal for seeing details of fractures and dislocations without opening the body (see Figure 1).
However, bones are not the only body parts that X-rays can reveal. For example, if a special dye-like chemical is first introduced into soft tissues, such as an artery, the intestine, or the stomach, the chemical absorbs the X-rays and shows more clearly the shapes and positions of the soft structures. In this way, major blood vessels and organs can be observed to check for signs of damage or disease. Ordinary X-ray methods, however, are very limited in what they can show.
CAT's Eye View
In 1967 a British engineer, Geoffrey Hounslow, realized that computers were the key to providing a much better view of the inside of the human body. He devised a system called CAT (computerized axial tomography). This systems works by directing a series of narrow X-ray beams through the patient to produce detailed cross sections. A computer then combines the cross sections to build up a three-dimensional picture – a CAT scan – in which different tissues are distinguished by how well they block X-rays or let them through. The computer uses a range of colors to show up tissues and organs according to their density. Then it displays the processed image on a video monitor.
The earliest CAT scanners were fixed in position and took 30 seconds or so to produce the image of a single cross section. But as computers became more and more powerful, the performance of CAT scanners improved dramatically. The latest devices, which use rotating machinery and scan in a spiral pattern, can produce an image slice in less then a second. Such sophisticated, high-speed scanning allows doctors to track the passage of injected substances through the blood vessels and into organs and tumors.
The main drawback of a CAT scanner is its use of X-rays, which in large doses can be harmful to the patient. This is particularly true if the patient is a child. For this reason, although CAT scanners are still widely used at present and will remain important for studying organs such as the lungs, stomach, and intestines, they are likely to give way to other scanning methods in the future.
Scanning Without X-rays
The basic idea of the CAT scanner, to build three-dimensional pictures from many cross sections with the help of a computer, is now used in several other types of scanners. The PET (positron emission tomography) scanner, for instance, is used to identify and monitor problems with the brain. Patients are injected with a solution of glucose (a type of sugar) and a tiny amount of radioactive material. As the glucose reaches the brain, different areas of the brain take up varying amounts of the sugar, depending on the energy needs of those areas. How much energy a part of the brain needs depends on its level of activity. The PET scanner detects the radiation coming from the radioactive glucose and so allows a picture of brain activity to be built up. This enables doctors to look for the distinctive patterns of activity associated with specific brain disorders, such as Alzheimer's disease.
An even more advanced method of producing pictures of what is going on inside the body is MRI (magnetic resonance imaging). This technique is based on the fact that hydrogen atoms in the body give off radio waves at a special frequency after having been bombarded with other, high-frequency radio waves and subjected to a powerful magnetic field (see Figure 3).
As does a CAT scanner, the MRI machine enables doctors to view three-dimensional images of the body on a screen. But MRI can be used when X-rays might be considered too hazardous to the patient. It is especially useful for monitoring the progress of diseases such as muscular dystrophy (a muscle-wasting condition) and of patients who have undergone operations involving organ transplants.
MRI has also been applied to the study of muscle injuries. Even when injured muscles are no longer painful, MRI images show tissue repair continuing for up to three weeks. This suggests that before resuming heavy training and competition, athletes with muscle injuries should take a longer period to recover than previously believed necessary.
A great deal of research and development is going on to improve MRI technology, and it is likely that this method of scanning will become increasingly important in the future. MRI scanners will become less expensive, and as a result more hospitals will be able to afford them. At the same time, the equipment will become smaller and capable of producing higher-quality images. This will lead to the technique's being applied in new ways, such as the imaging of blood vessels and very small tumors.
Medicine by Numbers
Today, doctors are making more and more use of digital images – pictures built up from numbers that have been processed by a computer. Digital X-rays and pictures produced by various types of scanning machines will eventually make conventional X-ray film obsolete.
Much more can be done with picture information stored in a computer than with simple X-ray photographs (see Figure 4). A doctor can create a three-dimensional model of the a patient's organs, soft tissues, or bones and then rotate or slice through this model in various ways at a computer workstation. Output from the computer can be used to manufacture accurately shaped artificial body parts. Using the computer, surgeons can practice a new or difficult type of operation before carrying out the procedure on a patient. Medical students can study the layout of the body and the appearance of various diseases in a totally new way.
Another great benefit of digital images over conventional X-rays is that digital images can be produced using a much lower dose of radiation. This reduces the chance of any harmful side effects to the patient. Also, information in digital form can be sent easily from one computer to another. For example, detailed pictures of a patient in one location can be studied by a surgeon at a computer linked to the Internet in another city or country.
In the future, images produced by a variety of different devices, such as CAT and MRI scanners, will be combined to reveal even more details of the body's internal structure. This process will be extended to control the delivery of treatment. At a computer workstation, doctors will merge all the image information about a patient's condition with the most appropriate surgical procedure, including details of the instruments and techniques to be used. The computer will then generate all the information needed to direct needles, lasers, and even robotic surgical tools under the remote guidance of the surgeon.
An unborn baby, or fetus, in its mother's womb resembles a submerged submarine – a similarity that triggered an idea by a Scottish medical professor, Ian Donald, in the 1950s. To study the fetus, Donald and his colleagues borrowed a technique called sonar, developed by French, British, and American scientists during World War II to detect enemy vessels and other objects underwater. The medical professors realized that, when adapted, sonar could be used to check the progress of a fetus during pregnancy (see Figure 5).
Sonar is based on the fact that very high pitched sound waves, known as ultrasound, travel particularly well through water and similar liquids and send back clear echoes from any solid object they meet. The echoes can be converted into a picture of the object and displayed on a screen.
Great advances have been made in ultrasound methods over the pat 50 years, to the extent that roughly a quarter of all medical imaging carried out in hospitals now involves this technique. Because experience has shown that ultrasound is less harmful than X-rays and other types of radiation, it is used routinely to check on the health of fetuses. But it is also applied in many other ways. Ultrasound devices have been made in hundreds of specialized forms. Some scan organs from outside the body, and others are inserted through holes in the body to scan blood vessels and organs such as the heart, the stomach, and the intestines.
An adaptation of the ultrasound method, called Doppler scanning, can be used to measure how fast blood is slowing through a patient's arteries and veins and so help to detect any blockages in blood vessels.
With further progress in the miniaturization of equipment, the ultrasound technique will be applied in even more varied ways. As the scanning method of first choice when infants and young children are involved, it will be used increasingly in procedures such as heart and brain surgery in newborn babies. It will also be employed more and more frequently to monitor in adults those parts of the body, for example, the prostate gland, that are particularly sensitive to exposure to X-rays and other radiation.