Radiography

X-rays

How do x-rays produce an image?

  1. Electric current heats cathode
  1. Electrons are emitted + strike anode
  1. X-rays exit window, penetrate patient and scatter
  1. The denser a tissue, the fewer x-rays pass through it

Strengths of x-rays

  • Good at showing things surrounded by air
  • Good at showing things surrounded by bone
  • Good at showing things that destroy white bone (e.g. cancer)

Weaknesses of x-rays

  • Abnormality must be of very different density to surrounding tissue in order to be visible
  • Radiation can be dangerous

Contrast study

  • Liquids containing dense elements poured/injected into tissue, so x-rays could show the tissue outline

CT scan

How does a CT scan produce an image?

  • Combines a series of X-ray images taken from different angles around the patient and uses computer processing to create cross-sectional images

Strengths of CT

  • Superior contrast resolution to x-ray – can show tissues of similar density
  • Images are not superimposed like with x-rays

Weaknesses of CT

  • Give large doses of ionizing radiation
  • Cannot distinguish soft tissues of similar density
  • Cannot detect bone marrow disease

Ultrasound

How is an ultrasound image produced?

  1. Piezoelectric crystals convert electrical energy into acoustic energy; crystals are housed in a transducer which has an electrical supply
  1. US machine transmits sound pulses into patient
  1. Some sound waves get reflected, some travel further until they reach another boundary and get reflected
  1. Machine calculates the distance from the probe to structure, and forms a 2D image on the screen

Strengths of US

  • No ionizing radiation
  • High spatial resolution – can show organs, muscles and tendons

Weaknesses of US

  • Bone and gas block US
  • Difficult to perform on fat patients

MRI

How does an MRI produce an image?

  1. Scanner creates strong magnetic field
  1. Scanner produces a burst of radio signal which energizes the body’s protons
  1. Scanner listens for return signal produced by protons and uses signal to produce image

Strengths of MRI

  • No ionizing radiation
  • Can assess acute muscle and tendon tears, especially in deep tissues
  • Interior of joints can be imaged
  • Most complete assessment of the brain

Weaknesses of MRI

  • Expensive
  • Can be harmful
  • Some patients may find the process claustrophobic
  • Scans take a long time