Physics OCR A A-Level: Medical Imaging: Module 6: Particles and Medical Physics: Questions with Complete Solutions

Why are X-rays used in medical imaging often referred to as ‘soft X-rays’? Because they have energies that are lower than gamma rays. How are X-rays produced? By rapidly accelerating or decelerating charged particles – their kinetic energy is transferred into highenergy photons How can you differentiate between X-rays and Gamma rays? X-rays and gamma rays have frequencies that overlap, so you cannot distinguish them by their wavelengths. Instead you have to use their method of production – gamma rays come from radioactive decay or particle collisions with a mass defect, whereas X-rays are produced by accelerating charged particles. Describe the general structure of an X-ray tube. Heated filament (cathode) and tungsten anode with a potential difference between them of up to 200 kV and sealed in a vacuum tube. How does an X-ray tube work? Electrons are emitted from the heated filament via thermionic emission and drawn towards the anode. They collide with the anode and some of their E k is released as X-rays in all directions (the rest is transferred to heat energy within the anode). Why does the X-ray tube need a vacuum? To prevent electrons from colliding with molecules of air before they gain enough energy to release Xrays. How is the anode prevented from overheating? By either rotating it so that a new section of it is in contact with the X-rays all the time, or by using water as a coolant, circulating it through the anode. How are the X-rays focused into one beam? The vacuum tube is encased in a material that is thinner in one area, so only X-rays that pass through that section are released from the tube. They then pass through a collimator– a series of straight, parallel tubes that absorb any rays that are not travelling parallel to the axis of the tubes. Why is it better for X-rays to be in a beam rather than emitted in all directions? Because it allows them to be directed at specific areas (like a broken bone) and minimises the patient’s exposure to them. What is X-ray attenuation? When a material absorbs X-rays, decreasing the intensity exponentially. Explain the process of taking an X-ray of a patient. X-rays are directed at an area of a patient’s body and pass through the bone and soft tissue. Since bone has a higher attenuation coefficient, it absorbs more X-rays than soft tissue does. If photographic film is placed behind the patient, the areas where the bone is will not blacken as much as the areas of soft tissue, creating an image of the inside of the patient’s body. However, nowadays, digital detectors are used in place of photographic film. The greater the attenuation (absorption) coefficient, ____________. the more the material will absorb incident X-rays