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
What technique superseded helical scanning, which is now used in modern CT scans?
Helical MSCT:
- Multi-slice CT
- Faster scan
- More coverage per rotation
Describe 4 uses of CT scanning.
- Acute/urgent diagnosis - e.g. brain haemorrhage or blood clot?
- Monitor disease progression - e.g. by looking at tumour or lymph node size
- Monitor response to therapy
- Treatment planning - where to target the beam to spare healthy tissue
In nuclear emission imaging, what does functional imaging mean?
Imaging where the image depends on the metabolism of the tracer
Nuclear medicine is therefore a functional modality
What are the different types of nuclear emission imaging?
Gamma camera:
- Uses single photon emitting radionuclides
- Can operate in 2D - planar, or 3D - SPECT
PET:
- Positron emission tomography
- Uses positron emitting radionuclides
- Always 3D
Name 3 tracers used in gamma camera imaging.
- Tc-99m MDP - bone scans
- Tc-99m DTPA - kidneys
- Tc-99m White Cells - infection/inflammation
Tc-99m is the most common radionuclide used, with a half-life of 6 hours
What is β+ particle radiation and what is it used for?
- Positron (β+ particle) collides with electron
- Positron and electron are annihilated
- 2 gamma rays are emitted at 180 degrees to eachother
- This is how gamma rays are produced in PET
How is a SPECT “Datscan” used to distinguish Parkinson’s disease from essential tremor?
- Ioflupane binds to dopamine transporters (DAT) on the neurons in the putamen
- Reduced ioflupane uptake indicates Parkinson’s rather than essential tremor
What is FDG and how is FDG metabolism involved in PET scanning?
- FDG is a glucose analogue that enters cells in the same way as glucose
- Good reflection of the distribution of glucose uptake and phosphorylation by cells
- Phosphorylated FDG - FDG-6P - is metabolically trapped, unlike glucose
- FDG is a positron emitting molecule that reflects metabolic activity on a PET scan
- Tumours e.g. melanoma + lymphoma are very metabolically active so are detected
How can PET be used to diagnose Alzheimer’s disease and Pick’s disease?
- PET measures gamma rays given off by FDG positron emission (colliding with electrons)
- FDG reflects metabolic activity
- Alzheimer’s - hypometabolism in temporal and parietal regions
- Pick’s disease (frontotemporal dementia) - frontotemporal hypometabolism
