X-Ray, CT, PET Flashcards
Name the 2 types of ionising radiation used in medical imaging.
Gamma rays and X-rays
Contrast the direct and indirect mechanisms of ionising radiation action on cells.
- Direct - acts on cells, causes mutations
- Indirect - acts on water, producing free radicals and hydrogen peroxide (H2O2) - these are highly reactive and may also induce mutations in cells
Compare the risks of direct and indirect ionising radiation damage.
Direct:
- Only at high radiation dose - not noticed at usual diagnostic doses
- Threshold effect - e.g. erythema and hair loss
Indirect:
- Risk of cancer induction
- Risk of genetic change in subsequent population
- No threshold, effect is proportional to radiation dose - all radiation has risk
A PET scan has a dose of 10mSv (millisieverts). What is the risk of fatal cancer induction?
- Risk of fatal cancer induction = 5% per Sievert = 0.005% per millisievert (mSv)
- 10mSv > risk = 0.05%
What is the average annual radiation dose in the UK?
2.2 mSv
What types of radiation are produced in radionuclide therapy, PET scanning and gamma camera imaging (SPECT)?
- Radionuclide therapy - alpha (2 neutrons + 2 protons +ve) and beta particles (electrons -ve)
- PET scanning - positrons - positive electrons interact with matter to create gamma rays
- Gamma camera imaging - gamma rays - penetrating radiation
- Alpha + beta particles, positrons and gamma rays all emitted following radioactive decay of an unstable nucleus, unlike X-rays which are artificially produced in an X-ray tube
X-ray attenuation varies with what parameters of tissue? What is the significance of this?
- Attenuation is the degree to which X-rays are absorbed - or the intensity of the X-rays are reduced - when they pass through matter
- X-ray attenuation increases with higher atomic number and density
- X-rays effectively provide an attenuation map
Compare and contrast transmission imaging with emission imaging.
Transmission imaging:
- Radiation is directed through patient
- Transmission map is an attenuation map
- Good at showing structure, especially between tissues of different atomic number or density
Emission imaging:
- Radiation administered in form of a tracer
- Emitted radiation detected outside of patient
In an X-ray tube, what controls the energy and amount of X-rays?
- Energy - controlled by voltage
- Amount - contolled by current
Describe the 3 types of detector for planar X-ray.
- Film hardcopy - film processor with tanks of chemicals, high resolution
- Computed radiology computer copy - phosphor plate, special laser scanner or CR reader that reads and digitises image, digital enhancemant and archiving
- Digital radiology (DR) - flat panel detector, fully digitised system
Name 4 diagnostic uses of X-rays.
- Fractures
- Squamous cell lung cancer - chest X-ray
- Pulmonary embolism - wedge-shaped defect - only large PE seen on chest X-ray
- Mammography screening for breast cancer
Why are X-rays used for mammography screening?
- High resolution
- Compression plate used to reduce breast thickness - improves resolution, lowers radiation dose
Explain how real time X-ray fluoroscopy works.
- A catheter is fed inside an artery and radio opaque dye is injected
- Real time imaging
- Shows blood flow inside vessels and can be used to assist in interventions
Explain how coronary angiography is carried out.
- An example of real time fluoroscopy
- A cardiac catheter is fed into the aorta
- Radio-opaque contrast agent injected and used to identify areas of occlusion
- Treatment may be balloon angioplasty or insertion of a stent
Describe the 2 main limitations of planar X-ray, and how this was partially solved in the past.
- Cannot distinguish between overlying tissue
- Tissues other than those being observed reduce contrast in the image
Historically partially solved by moving the film cassette and X-ray relative to the patient to blur out overlying tissues - tomography
Compare the dose, cost, resolution and contrast of the 5 imaging techniques that use ionising radiation.
