X-Rays Flashcards
Describe in simple terms how X-rays are produced.
High speed electrons emitted from a heated filament hit a tungsten anode. They decelerate when they hit the anode, and some of their kinetic energy is transformed into X-ray photons.
Describe the nature of X-rays.
They are electromagnetic, transverse waves that travel a the speed of light in a vacuum and are ionising.
How is overheating avoided in x-ray production?
By rotating the tungsten anode.
Describe the photoelectric effect.
An X-ray photon with around 30MeV of energy is absorbed by an electron, which is ejected from its atom. The gap in the electron shell is filled by another electron, which emits a photon.
Describe the Compton effect.
A photon with around 0.5-5MeV of energy knocks an electron out of an atom, which causes the photon to lose energy and be scattered.
Describe pair production.
A photon with high energy (>1,1MeV) decays into an electron and a positron.
Define intensity of an X-ray.
Power per unit cross-sectional area.
In what fashion does the intensity of an X-ray decrease once it enters a medium?
Exponentially.
What equation do we use to show that the intensity of an X-ray beam varies with the thickness of a medium, and what condition must be in place for this to be true?
I=I0e^(ux)
The X-ray beam must be collimated.
Describe the use of X-rays in imaging internal body structures.
A collimated beam of X-rays are fired at the patient from a distance, and a detection plate very close to the other side of the patient picks them up. Between the patient and the film lies a lead grid, which stops scattered X-rays fogging the image and reducing the contrast.
Explain why we use an image intensifier when taking X-rays, and how it works.
An image intensifier consists of crystals that fluoresce - they absorb X-rays and re-emit the energy as visible light, which develops the photograph more quickly. This means a shorter exposure time is needed, reducing the patient’s radiation dose.
Explain how soft tissues like the intestines can be imaged.
The tissues in the intestines have similar attenuation coefficients, so we use a contrast media, such as barium, to line the tissue. Barium will show up on the X-ray image because it has a high atomic number, so can be traced.
Describe the operation of a computerised axial tomography (CAT) scanner.
An X-ray producer describes a circular path around the patient, being picked up by many detectors opposite it. It creates lots of images of two-dimensional slices of the body. A computer then puts all these images together to show a 3-dimensional image.
Describe the advantages and disadvantages of a CAT scan compared with an X-ray image.
They produce a 3-dimensional image rather than a 2-dimensional one, so we can see exactly how deep anything that shows up is. The image is of a much higher quality. However, it does mean a longer exposure time for the patient.
