X-Ray, CT and PET Scan Flashcards
What is ionising radiation?
Ionising radiation is radiation that causes ionisation (esentially takes or add electrons off and become charged particles) when it interacts with matter
Why do we use ionising radiation?
Ionising radiation is used as they can penetrate through the body through different mediums/tissue) and be captured on our imaging detectors
What are the two ways ionising radiation can act?
- Indirect action (radiation will break up water and produce free radicals)
- Direct action (directly impact DNA inside a cell)
Describe the indirect action of ionising radiation
- Majority of our body is water
- Ionising radiation can break up water and result in the production of free radicals and hydrogen peroxide
- H• OH• HO2• H2O2
- Free radicals are highly reactive and break down DNA
Describe the direct action of ionising radiation
Direct impact of the DNA inside our cells
Direct effect of radiation damage
- Threshold effect (very high level of radiation exposure/ doses needed, beyond the threshold level, in order to see those effects)
- E.g Erythema and hair loss
What are the indirect effects of radiation damage?
- Indirect effects can increase risk of the cancer induction
- Risk of genetic change in subsequent population
- Effect is proportional to radiation dose, no threshold
- All radiation has risk (the higher the radiation the higher the risk that radiation will cause cancer)
What are positrons?
Posotively charged electrons which interact with matter to create gamma rays
How are positrons formed?
They are emitted following the radioactive decay of an unstable nucleus
What are positrons used in
PET scanning (Positron Emission Tomography)
What are gamma rays
Are a type of penetrating electromagnetic radiation emitted following the radioactive decay of an unstable nucleus
What are gamma rays used in?
Gamma camera imaging e.g SPECT
What are X-rays?
Spectrum of electcomagnetic radiation
How are X-rays produced?
They are produced artificially in an X-ray tube
What are X-rays used in?
X-ray imagine
CT and radiographs
What does X-ray attenuation mean?
As the high-energy photons pass through material, their energy is decreasing
Essentially there is a reduction in the intensity of an X-ray beam as it traverses matter
What factors affect attenuation?
Attenuation increases with
- Higher atomic number
- Higher density
What is used for transmission imaging and what is used for emission imaging?
Transmission Imaging → X-Ray
Emission Imaging → Gamma Rays
What is transmission imaging?
- A patient will lie between an X-ray tube and detector
- Radiation will be directed through the patient
- A transmission map is collected (attenuation map)
- Maps all the tissues that either block the signal or not
- Good at showing structure, especially between tissues of different densities or atomic number
Describe emission imaging?
- Radiation is administered to the patient in the form of a tracer (radioactive material injected)
- Patient will emit radiation (e.g gamma rays)
- These will be detected outside the patient
What is an X-Ray tube composed of?
- Filament, Target (anode), tube (evacuated), Voltage difference between two terminals (filament and target) to accelarate electrons
How does an X-ray work? How are the X-rays produced?
- A current will flow through a heating circuit which will cause electrons to be fired from the filament to the target (anode)
- The electrons will accelerate through the voltage and hit the target (anode)
- Upon hitting the target (metal tungsten), those electrons will produce a beam of X-rays
How do we have the abillity to control X-rays?
- Operators can control the X-rays by changing the voltage setting
- X-rays are only produced when the tube is in action (can be switched on/off)
- High voltage and current can be used to control the amount and energy of X-rays produced
What is the effect of X-ray on bone?
Bone has a high atomic number and high density so therefore blocks most of the X-ray signal (high attenuation) hence appears white
Soft tissue has a lower atomic number and lower density so therefore allows more X-rays through = blacker appearance
Explain some uses of X-rays
- Fractures
- Dental radiology
- Chest X-ray (look for pulmonary embolisms)
- Mammography = X-ray of breast tissue
- Uses a compression plate to reduce breast thickness. Thinner breast tissue = better resolution
- Used to detect benign micro-calcifications, tumours
What is fluoroscopy?
Real-time X-ray
A method of viewing the X-ray directly in real time (moving images of the interior of an object). e.g pumping of the heart, motion of swallowing
Can be used for coronary angiographies to identify areas of stenosis and occlusion
How does fluoroscopy work?
- A catheter will be fed inside an artery for example and radio opaque dye is injected
- This will show blood flow inside vessels
- A radiologist will inject dye with a high atomic number int the patient and can be seen more visually using X-rays
What are the limitations of planar X-ray?
- Cannot distinguish between overlying tissues
- E.g skull and below
- Tissues other than those being observed can 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 called ‘tomography’
- Superseded by Computed Axial Tomography (now abbreviated to CT)
Why does CT supersede X-ray?
The CT scan is an X-ray image, but you can decipher all different tissues within the image
What are the key elements of a CT scanner?
- X-ray tube
- Detectors (transmission image)
- Aperture/ Bore (where patient sits)
- Fan beam (rotates around the patient)
- Patient will go into the CT scanner
- When positioned the X-ray tube will rotate around the patient in order to derive slices through the patient while the patient bed slowly moves through the gantry
- Has the abillity to show us 3D slices
What is the benefit of helical MSCT compaired to helical CT?
- Multi-slice
- Faster scan
- More coverage for each rotation
What is the clinical significance of CT scans?
-
Acute Diagnosis
- We can use a CT scan to identify between a haemorrhage/blood clot (drugs for these are very different and can counteract eachother, giving the wrong drug could be detrimental )
-
Disease Progression
- Monitor response to therapy
-
Treatment planning for radiotherapy
- CT is used to define the area to be treated and the direction of radiotherapy beams that are used to minimise damage to normal tissue
Describe emission imaging
- Patient will be administered with a radioactive material in the form of a tracer
- Emitted radiation (gamma rays) will be detected outside the patient by scanner
- We are making an image on the distribution of the radioactive tracer. Nuclear medicine will only show function
What does emission imaging depend on?
Metabolism of the tracer (not on anatomy)
FUNCTIONAL IMAGING
What are the types of emission imaging?
Gamma Camera and PET (positron emission tomography)
What is the difference between a gamma camera and PET?
-
Gamma camera = uses single photon emitting radionuclides
- Can operate ibn 2D or 3D (SPECT)
-
PET = uses positron emitting radionuclides
- Always 3D
What has to be considered when injecting a radioactive tracer?
When we inject a radioactive tracer we have to consider the radioactive decay
The tracer will decay according to its HALF LIFE. Half life is the time it takes for the radioactivity to be reduced to 50%
What is the most common radionuclide tracer used in gamma camera imagine?
Tc-99m (T½ = 6 hours)
Radioactive tracers will be coupled to a biological marker to asses the function of specified organs
- Tc-99m MDP (bone scans)
- Tc-99m DTPA (kidneys)
- Tc-99m White Cells (infection/inflammation)
Gamma camera in cancer
- Cancer has a higher metabolic rate than normal tissues therefore they will take up more of the tracer
- This allows the radiation to be concentrated in them areas of cancer
- Allows us to image function and not anatomy
- This allows the radiation to be concentrated in them areas of cancer
How us gamma camera imaging used to asses kidney function?
- Camera is positioned above the patient
- Tc-99m DTPA injected IV
- Gamma camera records gamma rays and collects images over time
- Functional Time-Activity curves are obtained
- Compared to normal kidney functioning rates
What is SPECT?
Single Photon Emission Computed Tomography
- 3D version of gamma camera imaging
- Images are taken at different projection angles around the patient (similar concept to CT) but there is delivery of a radionuclide
- Images of high contrast are obtained between layers that are not obscured by all the overlying tissues
How does a PET (B+ particle radiation) scan work?
- Tracer will be injected into patient which releases positrons
- Positrons and electrons will interact and are annihilated, producing two gamma rays at 180º to each other
- PET Scanner detects the gamma rays in a short time
What is PET used in?
- Brain imaging (e.g neurodegenerative dementia)
- FDG (tracer) reflects metabolic activity
How is flurodeoxyglucose used in PET?
- FDG is a glucose analogue,
- concentrations of the tracer imaged will indicate tissue metabolic activity
- can be used to explore cancer metastasis
- FDG will be metabolically trapped because it does not undergo glycolysis like glucose
Application of FDG is neurodegenerative dementias
- FDG will reflect metabolic activity
- Alzheimers disese (hypometabolism, mostly in temporal and parietal regions
- Picks disease (fronto-temporal hypometabolism)
What are examples of hybrid imaging?
PET-CT
SPECT-CT
PET-MR
Combination of different scanners will allow you to get the combination of functional and anatomical elements
How does a PET-CT scan work?
- Patient will go into the bore
- Undergo transmission imaging from the CT
- Patient injected with radioactive tracer (positrons will annihilate with bodily electtrons giving off two gamma rays detected by PET scanner)
How does attenuation correction occur in PET-CT?
- Gamma rays originating frm the centre of the patient will travel through more tissue which means they are attenuated more
- The CT image is used as an attenuation map to correct the PET image
