Medical Physics - Medical Applications

In senior high school, you are required to know about X-rays and ultrasound for scanning, and about using light for image formation with lenses and endoscopes.

You could be tested on your knowledge of the structure of the eye and how lenses can be used to correct sight defects; you may have to draw and interpret ray diagrams to work out what type of image is formed; you might be challenged to compare the medical uses of any of these applications or even evaluate the advantages and disadvantages of different applications, so make sure you have revised them well.

X-rays are high energy electromagnetic waves that can pass easily through soft tissue like skin, the heart and lungs, however, it is harder for them to pass through solids like bones and teeth. This makes X-rays very useful for dentists, also for doctors who are treating broken bones as they can see exactly what needs doing. Soft tissue isn't completely transparent to X-rays and the method of Computerized Tomography provides enough data to produce a detailed view of soft tissue as well as bones. During a CT scan, many images are taken from different angles and combined using a powerful computer. Since the images are computer generated, they can be used to give three dimensional images of the internal organs. Doctors can then look at any of the organs in great detail from any angle. This is extremely useful to surgeons operating on cancer tumors as they can see the exact size and shape of a tumor before operating.

The disadvantage of X-rays is that they are ionising radiation and therefore could harm the patient and radiographers (medical staff who operate X-ray machinery). Applications using light or ultrasound are non-ionising, ultrasound is safe enough to use on pregnant women to check on the progress of the baby. Light cannot penetrate the body deeply enough to see beyond the skin but it is used for endoscopes. An endoscope is used to look inside body cavities and is used to examine the esophagus, inside of the stomach, the intestines and trachea.

Endoscopes have also enabled surgeons to develop keyhole surgery. This is so-called because the surgeon makes only a small incision (cut) and works using an endoscope. In normal surgery, a large incision is made and the surgeon looks directly at what they are doing. The biggest advantage of keyhole surgery is that it is less damaging to the patient who will therefore recover faster. Light is shone down the endoscope for illumination. The light reflects off the surrounding tissue and travels back up to a camera, through which the surgeon can see what they are doing. The endoscope is constructed from a bundle of optical quality glass fibers which transmit the light very efficiently by total internal reflection.