Imaging Modalities

• If this episode doesn't fascinate you, I don't know what will!!

• Radiographs
  • Ionizing radiation hits a photosensitive film to produce an image
  • Digital radiography: photosensitive plate processed by electronic reader to be stored digitally
  • PACS system: picture archiving, communications, and storage system
  • Plain films = X-ray = radiograph = without contrast material
  • Advantages: availability, less expensive
  • Disadvantages: limited detail, uses radiation (caution in pregnancy)
  • Uses: chest x-ray, abdomen x-ray, bone visualization

• Densities
  • Air is black (absorbs the least), bone is white (calcium), metal appears whitest, fat and soft tissues appear in between as gray 

• CT scanners
  • Uses x-ray machine that rotates around patient to create different planes and produce a large series of 2D images slices
  • Important to understand positioning of patient from image (you're look up from the bottom of their feet, with patient's right on the left of the screen)
  • Hounsfield units: -1000 to +1000, water is zero, air is -1000, bone is +400-600, fat is -40 to -100, soft tissues is 20-100
  • Windowing: range of densities to most optimally view certain structures
  • Advantages: greater detail
  • Disadvantages: more radiation, more expensive 
  • Uses: non-contrast head CT for stroke, traumas, 3D reformats 

• Ultrasound
  • Uses high frequency sound waves emitted from a probe
  • Advantages: less expensive, availability, no ionizing radiation
  • Disadvantages: operator dependent, low resolution 
  • Uses: great for pregnancy, gallstones, breast masses, thyroid nodules
  • Doppler flow: red towards probe, blue away from probe

• MRI
  • Uses magnetic fields and radio waves to affect hydrogen items 
  • Advantages: great resolution, no radiation
  • Disadvantages: expensive, time-consuming, special precautions (pacemakers)
  • Uses: brain imaging (MS), soft tissues like muscles, tendons, ligaments, herniated discs, spinal cord pathologies 

• Fluoroscopy
  • Uses ionizing radiation in real-time 
  • Can give barium to a patient which will show up black
  • Uses: esophagrams, voiding cystourethrograms, interventional radiology (angiography)
  • Advantages: mobile, procedure guidance, dynamic
  • Disadvantages: higher dose of radiation

• Nuclear medicine
  • Uses radioactive substances (elements that emit radiation as they decay) and couples them with drugs that will accumulate in certain tissues
  • Different types of scans: positron emission tomography (PET) scans use radioactive glucose (fluorodeoxyglucose, FDG), single photon emission computed tomography (SPECT) uses a gamma camera to acquire images from many angles to create a 3D map
  • Different organs use different substances; the brain loves glucose and the thyroid loves iodine 
  • Uses: cancer imaging, assessing for metastases
  • Less radiation than CT scans, but must use caution with radiation exposure via shielding and appropriate timing 

References: Herring's Learning Radiology, Radiopaedia, Mandell's CORE Radiology