imaging week 10 Flashcards
what is an X ray
an electromagnetic energy with a short wavelength between 0.1-10 nm
how does X ray tube work
- electrons accumulate on filament on cathode with heat applied to it
- then the electrons emit which is called thermionic emission
- electrons move from cathode to rotating anode
- X rays generated as photons and released through small window with filter on it
what are X rays interactions with matter
the X rays can either be absorbed, transmitted or deflected
- deflection causes scattered radiation
what is attenuation
the process by which radiation loses power as it travels through matter and interacts with it
- increases with atomic number, density and thickness
order of densities for X rays (starting most dense to least)
- contrast agents/metal implants
- bone
- muscle
- fat
- air
what is X ray film made up of
3 layers:
luminescent screen
film emulsion
luminescent screen
how does X ray film work
when the photons come through and strike the luminescent film this results in light emission onto the film emulsion and therefore image is generated
emulsion contains silver halide, clumps form after exposure to light generated on luminescent screens
what is digital radiography
- same X ray tube generation
- digital acquisition
- scintillation counters detect the photons and count them and based on how many they count will result in the image that’s seen on the screen through the PAC system (picture archiving communication system)
how does the ultrasound probe work
- the probes have generation crystals and detection crystals in one device and dependent on the tissue they encounter you get various patterns returned back to the probe and converted into an image that is seen on the screen
what contrast agent is used for outlining GI tract
barium
- it’s high atomic number absorbs more X ray photons than surrounding tissues do
what is barium used for in clinical investigation
swallow and meal - oesophagus, stomach and duodenum
small bowel study - small bowel
enema - large bowel
(barium is in declining use)
what are the complications of barium use
- bowel disturbance (common)
- colonic perforation at enema (very rare)
what is a CT scan
- rotating X ray tube
when do you get given an iodinated contrast
- can be given this for CT scan
- can be given orally or intravenously
- although IV contrast can lead to kidney failure but benefits outweigh risks
what are CT scans used for
- trauma
- cancer staging and response to treatment
- guidance for procedures
what unit is used for describing the absorption of radiation in the body
sieverts
what is ionising radiation
a particle with sufficient energy to ionise a neutral atom or molecule and leave them with either a positive or negative charge
what are the biological effects of ionising radiation
leads to cell damage which can lead to;
- repair
- cell death
- transformation (mutation)
how do you minimise exposure to radiation
- keep a distance
- use a shield
- minimise time exposed
what techniques can be used for diagnosing and staging cancer
- plain radiographs
- barium studies
- CT
- MRI
- PET
what are conventional diagnostic modalities
- don’t require computers
- endoscopy and biopsy
- barium examinations
what is the fluoroscopic examination machine used for
barium examinations
what specific barium study has CT replaced
barium enema used to look at large bowel
what is barium follow-through
- used to look at small bowel
- AKA small bowel study
how is CT colonography done
- insufflating carbon dioxide gas into the colon and taking CT scan images and then visualising them through special software
what imaging techniques are cross sectional
- CT
- MRI
- PET
what is cross sectional imaging used for
- initial diagnosis and staging of disease
- biopsy of lesions
- monitoring response to treatment
- recognition of compications to treatment
- when there is concern for disease relapse
- sometimes treatment
how does CT scan work
- while the CT scanner rotates it produces photons every time
- every time the photons cross the tissues some get lost
- the computer at the other end of the detectors find out how much is lost
what are pixels in CT
the 2D depiction of the structure that you normally see on the screen
what is a voxel in CT
the 3D component which actually contributes to the 2D pixel
what are the CT contrast agents
oral - dilute iodine based contrast (gastrografin) can be given to outline the GI tract
intravenous - iodine based contrast (omnipaque) injected into the veins to demonstrate blood vessels or the vascularity of different tissues
what is diagnosis and staging with CT
- position of tumour
- depth of penetration of tumour
- relationship to adjacent structures (T)
- involvement of regional lymph nodes (N)
- presence of distant metastases (M)
what kind of tumours can CT diagnose
lung, pancreatic, renal, adrenal, brain, retroperitoneal (behind lining of abdominal wall)
what is ablation of tumours
- treatment
- cryoablation or renal tumours
- microwave ablation of liver metastases
- radio frequency/microwave ablation of lung tumours
- protein electrophoresis of pancreatic tumours
what is ALARA principle
as low as reasonably achievable
- ensure whether examination is necessary
- adequate clinical info is essential for appropriate protocol
- avoid repeat examinations
how does MRI imaging work
- radio frequency pulse displaces protons and image is created by displaying time taken for protons to ‘relax’ back to original alignment
- so resting state of proton is aligned parallel to magnetic field
- energy sent and now proton alignment perpendicular to magnetic field
- time taken to relax again differs if different tissues and this is how image is created
do heavier or lighter molecules take a longer time to relax in MRI
lighter take longer to go back to original alignment
what can you get images of with MRI
- excellent bone soft tissue detail
- vessels
- brain, spine and musculoskeletal
- abdomen and pelvis
- cardiac imaging
(vessels shown without contrast)
disadvantages of MRI
- claustrophobic and noisy
- motion artefact (moving organs e.g. bowel cannot be pictured because patient has to be still)
- cannot image patients with pacemakers, aneurysm clips, etc.
what are some MRI contrast agents
- gadolinium DPTA is intravenous contrast which causes changes in local magnetic field and so alters tissue signal
- vascular lesions and some tumours can be seen more easily
what are the benefits of screening
- diagnose disease at an earlier stage before symptoms start
- cancer is easier to treat and more likely to be curable
what are some NHS screening programmes
- breast
- bowel
- cervix
what are the WHO principles of screening
- the condition should be an important health problem
- there should be a latent stage of disease
- there should be a test or examination for condition
- the test should be accessible to the population
- there should be a treatment
- facilities for diagnosis and treatment should be available
what are the rules for the test in screening
- test should detect disease at early stage
- test should cause no harm
- test should have high sensitivity and specificity
- benefits should outweigh costs
what is artificial intelligence
programming systems to perform tasks which usually require human intelligence
what is machine learning
training algorithms to solve tasks by pattern recognition instead of specifically programming them how to solve the task
what is deep learning
training algorithms by using deep neural networks with multiple layers
what is molecular imaging
- radionuclide imaging
- PET
- MRI
- optical imaging
how does radionuclide imaging work
radiopharmaceutical injected into patient, that produces gamma rays, patient is now radioactive and the gamma rays are detected by lead collimator connected to gamma camera and an image is formed that is sent to PAC system
what is a radiopharmaceutical
radioactive element attached to a pharmaceutical element
what are the properties of an ideal isotope
- half-life similar to length of examination
- gamma emitter (rather than alpha or beta)
- energy of gamma rays should be 50-300 keV
- radionuclide should be readily available at hospital site
- easily bound to pharmaceutical component
- radiopharmaceutical should be simple to prepare
- radiopharmaceutical should be eliminated in similar time to half life
how does gamma camera work
patient injected with pharmaceutical
they’re emitting gamma rays
gamma rays pass through a collimator that will improve the spatial resolution and then image crystal produces flash of light
image is boosted by photomultiplier tubes and computer applies algorithm to produce image
what is SPECT
- single photon emission computed tomography
- nuclear medicine version of CT
- gamma cameras rotate around area of interest
- commonly used for brain and cardiac studies
what is PET scan
- positron emission tomography
- uses radionuclides that decay by positron emission
- can be used to image biologically interesting processes
- can be used for absolute quantitation but requires arterial sampling
- all scanners now PET CT
what is a positron
positively charged electron
proton converts to neutron and emits positron
how does PET scan work
- proton converts into neutron and emits positron
- when positron comes into contact with electron they annihilate each other producing energy and two high energy gamma rays are sent out in opposite directions and these are detected by PET machine
what is a cyclotron
enables us to produce proton to neutron decay for PET
how do you detect cancer in PET
areas that are highly metabolic will have accumulation of fluorine 18 (pharmaceutical)
