Unit 1 Physics Detailed Study 3.6 Chapter 15: Medical Physics

Section 15.3 Scanning techniques. The A-scan ∗ In an A-scan a short pulse of sound is sent out, in doing so a timer is started, and stoped when the reflected sounds is received by the detector. ∗ Given that we know the speed of sound through a given medium, and we have just determined the time taken to travel a certain distance, we can determine this distance to the reflecting surfaces. ∗ This can be used to determine the distances between multiple tissue boundaries. ∗ On the screen of an A-scan, the signal is seen as peaks on a screen.

Section 15.3 Scanning techniques. The B-scan ∗ A B-scan works similar to the A-scan, however the pulses show as dots on the screen rather than peaks. The larger the amplitude, the brighter the dot. ∗ The B-scan uses a line of transduces to create a 2D image as the transducer is tilted or rocked. ∗ The B-scan is commonly used in Obstetric imaging, though they are slowly being used less and less as new equipment can produce clear images.

Section 15.3 Scanning techniques. Three-dimensional images, real-time scanning and phase scanning ∗ Though the equipment used in modern ultrasounds has been improved greatly, they still rely on the same principles to form an image. ∗ New scanners can have hundreds of transducers in the probe. ∗ To create a real-time image, large stationary probes are placed on the patient and the synchronised transducers produce images moment by moment. ∗ By adding more transducers, the clarity of the image can be increased. Clarity can also be increased by analysing the phase that the wave is in, whether it is at a high/low pressure, or in between the two. ∗ 3D scanners are now available, however they are quite pricey. To do this a set of multi- element B-scans are taken, each from a slightly different perspective.

Section 15.3 Scanning techniques. Ultrasound therapies- heat treatments ∗ When used for diagnostic purposes, ultrasound uses low-intensity sound waves as these waves have minimal effect on the cells within the body. ∗ When used as a treatment method, ultrasounds require high-intensity sounds waves. This is because the sounds waves used in ultrasounds vibrate cells in the body, low-intensity waves barely vibrate the cells, while the vibrations caused by high-intensity waves create heat, however waves too high in intensity can cause cell damage. ∗ The heat created by high-intensity ultrasounds is thought to increase the metabolism of cells and therefore the healing process. ∗ Heat treatments are often used on muscular or joint injuries.

Section 15.3 Scanning techniques. Ultrasound therapies- Destructive effects of ultrasound ∗ When the intensity of a sound wave is high enough, ultrasounds can be used to destroy certain cells. ∗ The intense vibrations cause the cells to overheat and the cell membrane to rupture. ∗ This is used in a variety of different ways. It can be used to break apart blockages such as gallstones, or as a cauterisation tool when the beam is very narrow/concentrated. ∗ If the intensity of the soundwave is extremely high, ultrasounds can have the same effect as ionising radiation, though it is not fully understood why.

Section 15.3 Scanning techniques. Therapeutic uses of light ∗ Later in the year we will learn more about optical fibres in the study of light. For this study, however, it is only necessary to know a couple things in the context of endoscopes. ∗ An endoscope is a non-invasive device used to look inside the body. ∗ An endoscope contains between 5000 and optical fibres, some provide light to the area under investigation while the others are used to produce an image. In general, the more fibres, the clearer the image. ∗ The way light travels though the endoscope, and optical fibres in general, is though a process known as total internal reflection. In this process, light essentially bounces off the barriers within the optical fibre at an angle, which causes the light to continue to travel down the fibre.

Section 15.3 Scanning techniques. Therapeutic uses of light ∗ The development endoscopes have enabled keyhole surgeries to be conducted. ∗ In these surgeries, laser light of high intensity can be sent down the optical fibre. This high intensity laser can be uses to essentially vaporise cells. ∗ These lasers can also be used for a range of different procedures. Such as the removal of birthmarks and other cosmetic procedure, but also other major, more delicate surgeries such as the reshaping of the cornea in the eye. ∗ Eye surgeons use lasers to burn away a layer of cells on the cornea to alter the overall curvature.