Introduction to Medical imaging and the role of radiography Flashcards
What are the main imaging modalities?
- X-ray
- PET scan
- CT scan
- MRI
- Ultrasound
- Interventional/Fluoroscopy
Describe an X-ray
- Small doses of ionising radiation
- Hours to days’ worth of background radiation
- Readily accessible (24/7 with A&E)
- Portable imaging available
- Quick image acquisition time
- Good for bone detail, helpful for soft tissue info
How do X-ray photons interact with human tissue?
Attenuation:
- Type 1 - Deflect with no energy loss (Coherent)
- Type 2 - Deflect with loss of energy (Compton effect)
- Type 3 - Absorption into an atom (Photoelectric effect)
Penetration/transmission:
Type 4 - Passes through with no loss of energy
Describe a CT scan
- Very high doses ionising radiation
- Months to years’ worth of background radiation
- Readily accessible for emergency cases (24/7 in A&E)
- Department only imaging available
- Limited portable heads scans in neuro units
- Quick image acquisition time
- image reformats available including 3D
- High sensitivity and specificity
- Requires iodinated contrast for best results
- Best for bone detail, good soft tissue information
What is a CT image made of?
Grey scale image of varying densities (levels of grey)
Describe a fluoroscopy
- High doses of ionising radiation
- Months to years’ worth of background radiation
- Elective lists with emergency cover
- Resource dependent
- Time to complete case can vary
- Diagnostic and/or therapeutic
- Limited bone detail, contrast usage gives good soft tissue information
Describe an Ultrasound
- Does not use ionising radiation
- Long waiting lists, emergency slots normally available
- Department imaging preferred, portable possible in extreme cases
- Short acquisition times
- Dynamic scan, accurate clinical details reduces scan time and improves sensitivity and specificity
- Bone detail limited, good soft tissue detail
Describe an MRI
- Does not use ionising radiation
- Magnetic field poses significant risk
- Long waiting lists, limited emergency availability
- Department only imaging available
- Long image acquisition time
- Contrast scans available
Describe Nuclear medicine scans and describe gamma rays
- High doses ionising radiation
- Months to years’ worth of background radiation (not as high as CT)
- Elective lists only
- Resource dependent - radioactive isotopes are made to order
- Time to complete cases can vary
- Usually hour, some measured over days
- Department only, strictly controlled areas
Gamma rays - Produced by radionuclide (atom with unstable nucleus), try to become stable, loses energy by emitting gamma ray, gamma ray identical to x-ray but originates from atomic nucleus, not electron shells
Describe a PET scan
- Fluorodeoxyglucose (FDG- 18)
- Tomographic nuclear imaging so cross-sectional images produced
- Functional imaging technique that is used to observe metabolic processes in the body as an aid to the diagnosis of disease using positron emission
What is the most suitable imaging modality for bone detail?
X-ray - less radiation exposure than CT, though CT provides more detailed results
What is the most suitable imaging modality for soft tissue?
Ultrasound - no radiation and provides good imaging of soft tissue
What contrast is used for Fluoroscopic Gastro-intestinal investigations? Why is this used?
- Barium 56 (Barium sulphate)
- Provides thorough visualisation of structures, especially mucosa, without extensive local adverse effects
- Produce significant artifact on CT, not usually preferred method for this modality
- Cannot be dissolved in water, so cannot be used intravenously as would block blood vessels
What contrasts are used for Ultrasounds and why?
- Octafluoropropane - prepared in microsphere (emulsion of gas). Enhances left myocardial ventricular cavity while improving visualisation of endocardial borders during echocardiography
- Perflutren - similar design to Octafluoroporpane, but made from lipids
These are used as they have acoustic properties that enhance the Ultrasound image
What contrasts are used for MRI?
- Gadolinium 64 - Rapidly distribute into intracellular and extravascular spaces, particularly good at viewing CNS lesions
- Gadopentetate salts used in renal, hepatic, myocardial and MSK imaging
- Iron 26 - Contain iron particles that are highly paramagnetic, stay in blood long periods
- Used to demonstrate hepatic and lymphoid tissue lesions
- Manganese 25 - Highly paramagnetic, high affinity for hepatic cells
- Primarily used for identifying cancers with hepatocytes in their matrix
