Use of Waves in Human Organ Diagnosis

When a wave is sent out, the wave that comes back can tell us what it struck and how far away that thing is. An ultrasonic ruler sends out high frequency sound, times how long the wave takes to bounce off a wall and return, and works out the distance from that time. A fathometer fitted to a ship does the same job at sea. It sends sound towards the sea bed and finds the depth of the water from the time the echo takes to come back. Every one of these instruments rests on a single idea: a reflected wave carries information about the object it struck.

Absorbing sound is another way of putting sound to work. The walls of recording studios and meeting halls are lined with perforated acoustic boards. These boards absorb sound, so no echo can form and outside noise cannot get in.

Doctors use this same reflection principle inside the human body. Three main wave based techniques let them build pictures of the internal organs without cutting the body open: ultrasonography, X-ray imaging and the CT scan.

This technique uses a device called a transducer. The transducer produces ultrasound of very high frequency, and this ultrasound is aimed into the body. As the waves travel through the body, the different layers inside reflect them back, and the same transducer receives the returning waves. Signals carrying the return time and frequency of these waves go to a computer, and from that data the computer builds up a picture of the structure inside.

Ultrasound gives good images of the body's soft tissues and fluids, so it is used in many different kinds of examination.

• Finding stones in the kidneys, the gall bladder and similar organs
• Detecting tumours in the brain, the pancreas and other organs
• Checking liver disease and problems of the intestines
• Examining problems of the uterus in women
• Checking the size, position and organ movements of a baby growing in the womb, and spotting any abnormal condition

With three dimensional, or 3D, ultrasonic technique the condition of the baby can be observed even more clearly. Ultrasound is also used to examine the beating of the heart, the condition of its valves and how well they are working.

An X-ray is an electromagnetic wave. Unlike sound it needs no medium, and it can pass right through many parts of the body.

X-ray photography obtains an X-ray photo from the shadows that the internal organs form when X-rays are sent through the body. In the traditional technique, the person is placed at a fixed distance in front of the X-ray machine. When the machine is switched on, the X-rays that pass through the person's body fall on an X-ray film placed on the other side and form a shadow there. From that shadow, the X-ray photo is prepared from the film. On the photo, the parts in shadow appear white and the rest appears black. In the modern technique an electronic recording device is used instead of photographic film, so the shadows of the internal organs can be observed directly on a computer screen.

The bones in our body absorb more of the X-rays, so on an X-ray photo the shadow of bone appears dense white. Other tissues, such as muscle and fat, let most of the X-rays pass through and absorb only a little, so these parts show only a very faint shadow. Parts of the body that hold air, such as the lungs, let X-rays through easily, so they appear black on the photo. X-rays are used to check broken or cracked bones, the roots of teeth, infections, tumours, blockages in blood vessels and similar problems.

For a CT scan, the person is passed into a doughnut shaped machine. The X-ray source inside the machine can travel right around it, through a full angle of 360°. Directly opposite the source sits the X-ray detector, and the machine is built so that as the source turns, the detector turns with it.

During the scan, a powerful narrow X-ray beam from the source is aimed at the part of the body that holds the organ to be examined. As the source is swung around the body, it is aimed at the same slice of the body from a different direction at every interval of 45°, and the data is collected through the computer. The computer processes this data and prepares two dimensional, or 2D, cross sectional pictures, called tomographic images. For a detailed picture of the structures inside, the data from the X-ray slices is combined to build a three dimensional, or 3D, image.

A tomographic image holds far more detailed information than a traditional X-ray photo. Even the state of the soft tissues inside the body can be observed in it. The 3D image makes it easy to identify the parts inside the body and to pick out any change or abnormal condition in the internal structures. CT scans are used to examine injuries inside the body, clotted blood, tumours, the condition of the brain, the condition of the lungs and similar problems.

Which technique to choose depends on what has to be examined. A baby in the womb is checked with ultrasonography, because X-rays are harmful radiation while ultrasound does not harm cells. For a broken bone, an ordinary X-ray is enough. When a detailed three dimensional examination of an organ such as the brain is needed, a CT scan is done.