1. Medical Physics Unit Summary

2. Diagnostic Techniques  Ultrasound  Endoscopes (Fibrescopes)  X-ray imaging  Computed Axial Tomography (CT)  Magnetic Resonance Imaging (MRI)  Diagnostic imaging with radioisotopes  Positron Emission Tomography (PET)

3. Therapeutic Techniques  X-ray radiotherapy  Radioisotope emplacement Some diagnostic techniques also have therapeutic applications. These include...  Ultrasound  Endoscopes (fibrescopes)

4. Ultrasound  Ultrasound is sound at frequencies above 20000 Hz which is higher than can be detected by the human ear.  Sound below 20 Hz also cannot be detected by the human ear and is called Infrasound.  Ultrasonography is the use of ultrasound waves in medical and industrial applications.

5. Sound Waves  Sound is a longitudinal wave. It carries energy but not matter.

6. Measuring Sound  The energy carried by a sound wave is measured in Joules.  The rate at which the energy is carried is measured in Joules per second or...... Watts  The area over which the energy is spread defines the intensity (Wm -2 )

7. Ultrasound in Medical Practice  Ultrasound is produced by supplying an electric current to a crystal that changes shape. This is called the piezoelectric effect.  The sound bounces off the internal structures of the human body and is detected (using the piezoelectric effect in reverse).  An image is created by measuring the time taken for the sound to return and the intensity of that sound.

8. Attenuation & Acoustic Impedance  As a sound wave travels through a medium some of its energy will be absorbed by the surrounding material.  This results in a reduction of the intensity of the sound and is called attenuation.  The amount of attenuation depends on the Acoustic Impedance of the material.

9. Reflection Coefficient  At the boundary between two mediums...  Some sound is reflected  Some is transmitted into the new medium  Some is absorbed

10. Reflection Coefficient  How much sound is reflected depends on the acoustic impedances of the two materials.  This can be calculated using the reflection coefficient. When the acoustic impedances are similar there will be little sound reflected.

11. Advantages of Ultrasound  By measuring the doppler shift in the reflected sound we can monitor blood-flow.  At the sound frequencies used, diagnostic ultrasound is very safe and so is the preferred method of monitoring pregnancies.

12. Therapeutic Ultrasound  High frequency sound waves cause heating in bones and denser muscles. This is used to speed up metabolism and healing of sporting and other injuries.  Very high intensity sound waves (~10 5 Wm -2 ) can be used to break up gallstones and kidney stones and treat some tumours.  An advantage of this method is that the heat can cauterise (seal) wounds to prevent bleeding and speed up healing.