Medical Applications of Physics: X-rays and Ultrasound

Medical Applications of Physics: X-rays and Ultrasound

1. X-rays

   • Properties of X-rays
     • X-rays, being electromagnetic, transfer energy and are transverse waves
     • X-rays have a very short wavelength
     • They cause ionisation
     • They affect photographic film in the same way as visible light in that they turn it black
     • They are absorbed by bone and metal
     • They are transmitted through healthy body tissue
   • Medical Diagnosis
     • Because X-rays turn photographic paper black, and because they go through body tissue and not bone, X-rays can be shot through the body onto photographic paper to produce an image of the patient's bone structure
       • The white bits on an X-ray are where the x-rays haven't got through
       • This is used to detect broken bones or to diagnose dental problems
         • X-ray images can be formed electronically using charged coupling devices (CCDs); silicon chips divided into millions of identical pixels which detect X rays and produce electronic signals which are used yo form high resolution images
     • Computerised Tomography (CT) scans
       • These take x-ray images from various positions which are built up to make a 3D image which can be manipulated to see structures at different layers and from different point of view
   • Cancer Treatment
     • Cancer is a disease where cells divide uncontrollably to form tumors
       • One method of treating this is to direct high energy x-rays at the tumours causing damage which kills the cells
         • This called radiotherapy
   • Precautions
     • People who work with x-ray machines and CT scans (radiographers) need to take precautions to minimise their x-ray dose
       • They wear lead aprons, stand behind a lead screen or leave the room
         • Also, to protect the patient, lead is used to shield areas of the body not being scanned and the exposure time is always kept at a minimum

2. Ultrasound

   • Properties of Ultrasound
     • Human hearing can only hear from a frequency of 20Hz to 20KHz
       • Ultrasound is anything above this range so we cannot hear it
   • Reflections
     • When an ultrasound wave passes from one medium to another, some of the wave is reflected off the boundary between the two media, and is transmitted; this is partial reflection
       • This means that you can point a pulse of ultrasound at an object, and wherever the boundaries between one substance and another, some of the ultrasound gets reflected back
         • The time it takes fir the reflections to reach a detector can be used to measure how far away the boundary is; this is how ultrasound imaging works
           • The calculation to determine the distance of the boundary is s = v × t
             • Where S is distance (m), V is speed (m/s), and t is time (s)
           • Pre Natal Scanning
             • This is done to produce a video image of a foetus
   • Kidney Stones
     • Kidney Stones are hard masses that can block the urinary tract
       • An ultrasound beam can concentrate high energy waves at the kidney stone and turn it into smaller particles which are then passed out of the urine
         • This is beneficial to the patient as it means the patient does not have to have surgery and it is relatively painless

3. Comparing Ultrasound and X-rays

   • X rays produce much higher resolution images than ultrasound scans which means they have greater detail and clarity which is vital for detecting fractures
     • CT scans produce even better picture quality and allow greater analysis for the doctor
     • However, X-rays are ionising which means that X-rays damage cells
       • Therefore it is not right to use x-rays on a developing foetus but ultrasound waves are non-ionising so they are safe to use for pre-natal scanning