Medical Imaging: Illuminating the Body

Before the light of technology shed itself to mankind, we can only imagine how do our body works inside a living person. How the blood circulates inside the body? How babies develop inside the womb? What does brain looks like when a person is thinking? these and other questions has bothered doctors and scientists for many years, until today.

By bombarding the body with energy, new scanning techniques can reveal the structures of internal organs, and wring out information about the private and, until now, secret working of their molecules. these new imaging techniques are changing the face of medical diagnosis.

Until about 50 years ago the magical but murky X ray the only means of peeping into a living body. What X ray did and still do best was visualize hard and bony structures and locate abnormally dense structures (tumors, tuberculosis nodules) int the lungs. The 1950's saw the birth of nuclear medicine, which uses radio isotos to scan the body, and ultrasound techniques. In the 1970's, CT, PET, and MRI scanning techniques were introduced.

The best known of new imaging devices is computed tomography (CT) (formerly called computer axial tomography [CAT]), a refined version of X ray. A CT scanner confines its beam to a thin slice of the body andends the confusion resulting from images of overlapping structures seen in conventional X rays. CT's clarity has all but eliminated exploratory surgery As the patient is slowly moved through a doughnut-shaped CT machine, its X-ray tube rotates around the body. Different tissues absorb the radiation in varying amounts. The device's computer translates this information into a detailed, cross-sectional picture of the body region scannedt. CT scans are at the forefront in evaluating most problem that affects the brain, abdomen, and calcification of the coronary arteries in those at the elevated risk for heart disease. Special ultrafast CT scanners have produced a technique called dynamic spatial reconstruction (DSR), which provides three-dimentional images of body organs from any angle. it also allows their movements and changes in their internal volumes to be observed at normal speed, in slow motion and at specific moment in time. Although DSR can be used to evaluate the lung and certain other mobile organs, its greatest value has been to visualize the heart beating and the blood flowing through vessels. This allows heart defects, constricted blood vessels, and the status of coronary bypass grafts to be assessed.

Another computer-assisted X ray technique is digital subtraction angiography (DSA) (angiography = vessel pictures). This technique provides an unobstructed view of diseased blood vessels. Conventional radiographs are taken before and after a contrast medium is injected into an artery. Then the computer substracts the "before" image from he "after" image, eliminating all traces of body structures that obscure the vessel. DSA is often used to identify blockage in the arteries that supply the heart wall and brain.



Just as X ray spawned "new technologies" so did the nuclear medicine in the form of positron emission tomography (PET). PET excels in observing metabolic processes. PET's greatest clinical value has been its ability to provide insights into brain activity in those affected by mental illness, Alzheimer's disease and epilepsy. One of its mmost exciting uses is to determine which area of the healthy brain are most active during certain tasks (speaking, listening to music, and so on). The patient is given an injection of short-lived radioisotopes that have been tagged to biological molecules (such as glucose)and then positioned in the PET scanner. As the radioisotopes are absorbed by the most active brain cells, high-energy gamma rays are produced. The computer analyze the gamma emission and produce a picture of the brain's biochemical activity in vivid colors.



Ultrasound imaging, or ultrasonography, has distinct advantage over the approaches described so far: the equipment is inexpensive and it employs high-frequency sound waves(ultrasound) as its energy source. Ultrasound, unlike ionizing form of radiation, ahs no harmful effect on living tissues (as far as we know). The body is probed with pulses of sound waves, which causes echoes when reflected and scattered by body tissues. The echoes are analyze by computer to construct visual image of body organs of interest. Because of its safety, ultrasound is the imaging technique of choice for obstetricst, that is, for determining fetal age and position and locating the placenta. Because sound waves have very low penetrating power and are rapidly scattered in air, ultrasonography is of little value for looking at air-filled structures (the lungs) or of those surrounded by bone (the brain and spinal cord).



Another technique that depends on non-ionizing radiation is Magnetic Resonance Imaging (MRI) which uses magnetic fields up to 60,000 times stronger than the earth's to pry information from the body's tissues. The patient lies in a chamber within a huge magnet. Hydrogen molecules spin like tops in the magnetic field, and their energy is enhanced by radio waves. When the radio waves are turned off, the energy is released and translated by computer into a visual image. MRI is immensly popular because it can do many things a CT scan cannot. Dense structures do not show up in MRI, so bones of skull and/or vertebral column do not impair the view of soft tissues such as brain. MRI is also particularly good at detecting degenerative disease of various kinds. Multiple sclerosis plaques, for examples, do not shpw up well in CT scans but are dazzingly clear in MRI scans. A key issue being investigated by MRI studies is how brain development and behavior change with growth or experience.



A newer variation of MRI called Magnetic Resonance Spectroscopy (MRS) maps the distribution of elements other than hydrogen to reveal more about how disease changes by chemistry. In 1992, MRI technology leaped forward with the development of the functional MRI, which allows tracking down of blood flow into the brain in real time. Until then, matching thoughts, deeds and disease to corresponding brain activity has been the sole domain of PET. Because functional MRI does not require injections of tracer elements, it provides another, perhaps more desirable alternatives to such studies. despite its advantages, the powerful magnets of MRI present some problems. For example, the can suck metal objects such as implanted pacemakers and loose tooth filings through the body. Also there is no convincing evidence that such magnetic fields are risk free.

As you see, modern medical science has many remarkable tools and its disposal. CT and PET scan are accounted for about 25 percent of all imaging. Ultrasonography, because of its safety and low cost is the most wide spead of the new techniques. Conventional X rays remains the work horse of diagnostic imaging techniques and still accounts for morethan half of all imaging currently done.