Overview of imaging modalities


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Modality Terminology Contrast medium Radiation Prep Contraindications/Problems
X-ray Opacity vs Lucency Iodine (e.g. IV pyelogram) Yes - Pregnancy (relative)
CT Attenuation/Density Iodine Yes Hydration (low eGFR) Renal impairment, Pregnancy (relative)
MRI Signal intensity Gadolinium No Remove piercings Metals, Electronics, Claustrophobia
Ultrasound Echogenicity Air (‘microbubbles’) No Full bladder (gynae scans) Body habitus, Operator skill
Nuclear Uptake Radioactive labelled ‘tracer’ Yes - Pregnancy, Breastfeeding
Fluoroscopy Filling defect Barium/Air/Gastrografin Yes NBM/Bowel prep Poor mobility

 

X-rays, or plain films / plain radiographs, are used as a first-line imaging investigation in most situations due to their low radiation dose and easy availability.

Nuclear imaging measures the uptake of various labelled radioactive isotopes. Bone scans use an IV tracer that concentrates in areas of high bone activity, including inflammation and infection. They are mainly used to show bone metastases (multiple myeloma is an exception which may not show). Arthritis will also show as hot spots. The renal collecting system (kidneys and bladder) show high uptake as they excrete tracer.

Anterior and posterior images are taken. Other applications include V/Q scans and PET scans (PET-CT = PET overlaid over CT).

Fluoroscopy is the use of X-rays to obtain live moving images. Common uses include barium studies, angiography, and interventional procedures.

Barium studies:

  • Swallow = Oesophagus
  • Meal = Stomach
  • Follow-through = Small bowel
  • Enema (double-contrast if air is also used) = Large bowel

Interventional radiology performs procedures under imaging guidance. Examples include the insertion of PICC and Hickman lines. Foams and coils are used to embolise blood vessels that are bleeding or supplying a tumour. Biopsies are done under CT and ultrasound guidance. Percutaneous coronary intervention (PCI) which involves angiography and stenting is an intervention that is now being carried out by cardiologists.  Learn more about interventional radiology.

CT, MRI and Ultrasound are cross-sectional imaging modalities - these will be discussed further in the following pages.


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