Cellular pathology: X-Ray, CT, PET Flashcards
What is ionising radiation?
- Radiation that causes ionisation, acquiring of a negative or positive charge by an atom as a result of gaining or losing electrons, when it interacts with matter
What are the different types of ionising radiation used in medical imaging?
- Gamma rays - Penetrating electromganetic radiation
- X-rays - Part of the spectrum of electromagnetic radiation
- Positrons - An electron with a positive charge
Why is ionising radaition used in medical imaging?
- It’s penetrating so can travel through tissues easily and provide a clear image of what’s happeneing inside the body
Describe the different effects of ionising radiation on the body
- Indirect effect - ionising radiation can produce free radicals and hydrogen peroxide from the water molecules within our bodies. These species are highly reactive and can cause our cells to mutate
- Direct effect - ionising radiation can break down DNA of our cells leading to these cells to mutate.
- Mutation within our cells can lead to a biological response which include cell death, that cell becoming cancerous or a a chnage in the genetic code within DNA of that cell
What are the risks of the direct effect of ionising radiation?
- Only at high radiation dose, above threshold, will direct effect cause any damage to an individual
- Not noticed at usual diagnostic doses
- Examples of risks of direct effect include:
- Erythema (redness of the skin or mucous membranes due to increased blood flow)
- Hair loss
What are the risks of the indirect effect of ionising radiation?
- Risk of cancer induction
- Risk of genetic change in subsequent population
- Unlike with the direct effect, the risks of the indirect effect of ionising radiation are proportional to radiation dose, no threshold
What are the radaition doses for some of the medical imaging techniques that use ionising radiation?
- Chest X-ray: 0.02 mSv
- CT pelvis: 7 mSv
- CT head: 2 mSv
- PET scan: 10 mSv
- mSv = microsievert
What is the calculated risk of contracting fatal cancer from ionisig radiation?
- 5% per Sievert
- 1 in 20000 per mSv
- 1 in 2000 per PET scan
Briefly explain how the different types of ionising radiation used in medical imgaing are produced?
- Positrons - Emitted following radioactive decay of an unstable nucleus
- Gamma rays - Emitted as a result of positrons interacting with matter
- X-rays - Artificially produced in an X‑ray tube
Give an example of a medical imaging technique that each type of ionising radiation used medical imaging is used for
- Positrons - PET scanning
- Gamma rays - Gamma camera imaging
- X-rays - X-ray imaging, CT scanning
What factors affect the attenuation of X-rays by a particular tissue?
- Atomic number - The higher the atomic number, meaning the higher the no. of protons in the nucleus, the more the X-ray will be attenuated by that tissue
- Density - The higher the density, the more the X-ray will be attenuated
What is the major difference between transmission imaging and emission imaging?
- In transmission imaging the patient lies between radiation source, e.g. X ray tube, and detector and radiation is directed through the patient
- However, in emission imaging the radiation is administered to a patient in the form of a tracer and the radiation emitted from the patient is detected
Explain how an X-ray tube works?
- The X-ray tube is a vaccum and within it there’s a:
- Filament
- Target (anode)
- Between the filament and the target (anode) there’s a high voltage difference - this is used to accelerate electrons
- When X-ray tube is turned on a current will flow through a heating circuit which will cause electrons to be fired from the filament and hit the target (anode).
- As they travel to target these electrons are accelerated by the voltage
- When they hit the target it causes an X-ray to be produced
What elements of an X-ray tube are used to control the X-rays that are produced?
- Voltage controls the speed and therefore the energy of the x-rays
- Current controls the amount of x-rays produced
What are some of the clinical uses of X-rays?
- Diagnostic X-ray - fracture, chest X-ray
- Veterinary X-ray
- Dental radiology
- Mammography
On a chest X-ray if you see a white wedge-shaped area within a lung what might that indicate?
- Indicates that this person may have a pulmonary embolism
- Only large pulmonary embolisms can be seen on X-ray
When performing a mammography why is a compression plate used?
- Compression plate used to reduce breast thickness
- This Improves resolution of image produced
- It also allows for a Lower radiation dose to be used as the X-rays have to travel a shorter distance
Explain how fluoroscopy (real-time X-ray imaging) works?
- Fluoroscopy is usually used during a medical procedure
- A catheter is fed inside an artery and radio opaque dye is injected
- During the procedure the patient is kept within an X-ray imgaing machine which produces X-rays continously throughout the procedure
- The real-time X-ray images produced are usually used to show blood flow inside vessels, which makes it easier to identify areas of occlusion (blockage of a blood vessel)
- Can also be used to assist with interventions
What is coronary angiography?
- Coronary angiography is a technique that uses the process as fluoroscopy to see how blood flows through the coronary arteries
- The cardiac catheter is fed inside the aorta and the dye is first injected into the aorta
What are the treatments available if an area of occlusion is identified in the coronary arteries?
- Balloon angioplasty - specially designed catheter with a tiny balloon is placed in area to of occlusion, then inflated to widen vessel and increase blood flow
- Insertion of a stent - Re-inforces vessel and allows blood flow to re-start
What are some limitations of normal X-rays?
- Cannot distinguish between overlying tissues
- Tissues other than those being observed reduce contrast in the image
- Superseded by Computed Axial Tomography (CT)
Explain how CT scans work?
- Patient lies within the CT scanner between the X-ray tube and detector
- As X-rays are produced by X-ray tube it rotates around the patient so that X-rays travel through patient at different angles
- As this occurs the patient bed also moves up and down
- These 2 processes allow for “slices” of the patient to be produced
- This also allows for overlaying tissues to be distinguished
What are the differences between CT and MSCT (Multi-slice CT)?
- MSCT is able to produce multiple slices of the patient for every rotation around the patient while CT is only able to produce one
- MSCT is faster than CT
- MSCT has more coverage per rotation
What are some of the clinical uses of CT?
- Acute diagnosis - e.g. distinguishing between intracerebral haemorrhages and intracerebral blood clots
- Disease progression - Imaging is used for monitoring response to therapy e.g. chemoterapy in cancer treatment
- Treatment planning for radiotherapy - CT is used to define area to be treated and the direction of the radiotherapy beams that are used in order to avoid damage to normal tissues
What are the 2 types of imaging technique that use 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
Gamma cameras and PET are both used in nuclear medicine imaging. What are the general pronciples of nuclear medicine imaging?
- Nuclear medicine imaging results in the production of an image of the distribution of a radioactive tracer
- Nuclear Medicine usually only shows function of area that’s scanned
- It may reflect anatomy but without metabolism, the tracer will not be taken up
As they are radioactive all tracers used in nuclear medicine imaging have a half-life. What is a half-life?
- The time taken for the radioactivity of a radioactive substance to reduce to 50%
How can the half-life of a radioactive substance be calculated?
- Calculated using the following equation:
-
A = A0e-λt
- Where:
- A = Activity remaining
- A0 = Initial Activity
- t = decay time
- λ = Decay constant
- λ = ln2/half-life (t1/2)
What is the most common radionuclide tracer that’s used in gamma camera imaging?
- Tc-99m (technetium-99m)
What is the half-life of Tc-99m?
- 6 hours
As as well as a radionuclide a radioactive tracer also has a biological marker, why is this?
- When using nuclear medicine imaging you’e scanning a particular organ so the biological marker is used to target the radioactive tracer to a particular organ
Give some examples of radioactive tracers used in gamma camera imaging and the organs they’re used to image
- Tc-99m MDP - bone scans
- Tc-99m DTPA - kidneys
- Tc-99m White Cells - infection/inflammation
How can gamma camera imaging be used to diagnose cancer (metastatic disease)?
- For a cancer that has infiltrated into bone you can administer the Tc-99m MDP radioactive tracer and then place the person into a gamma camera
- The image produced will show the bones as dark which indicates that bones are they only area of the body that metabolise the Tc-99m MDP
- If you see small black spots that show up darker than the normal bones it means the person has metastases within their bones
- This is because the metastases have a greater metabolic rate than normal bone and so metabolise the Tc-99m MDP more and therefore these areas show up darker than normal bone
Explain how a dynamic renal transplant scan works
- Gamma camera positioned above the patient
- Tc-99m DTPA injected Intravenously
- Tc-99m DTPA taken up by kidneys
- Gamma camera records gamma rays produced via radioactive decay of Tc-99m DTPA and collects image over time
- These images are used to produce a functional time curve of the transplanted kidney to see if it will function normally or if it’s rejected
What is SPECT (Single-photon emission computed tomography)?
- It’s a nuclear medicine imaging technique that uses a radioactive tracer administered to the patient to detect gamma rays and produce an image
- The detector used rotates around the patient and produces transaxial slices which are used to produce a 3D image
How can SPECT be used to diagnose someone with parkinson’s disease?
- Administer the radioactive tracer Ioflupane (I-123 FP-CIT) to the patient
- SPECT scan is then performed to view the patients brain
- The image produced looks at the amount of binding to the dopamine transporters on the neurons within the brain by the Ioflupane
- In Parkinson’s disease you will see reduced uptake in the Putamen of the Ioflupane which differentiates it from an essential tremor
What is the major difference between the way radiation is produced by PET and by Gamma cameras?
- In PET a positron within the radioactive tracer will collide with an electron in our bodies annihilating both
- This produces 2 gamma rays at 180o to each other
- However, with gamma cameras the radioactive tracer will only produce 1 gamma ray
What is the most common radioactive tracer used in PET scans?
- FDG (fluorodeoxyglucose)
What is FDG and why is it used as a radioactive tracer in PET scans?
- FDG is a glucose analogue which enters cells in the same way as glucose
- Good reflection of the distribution of glucose uptake and phosphorylation by cells in the body
- This is because after phosphorylation FDG-6P, unlike G-6P, is unable to take part in glycolysis and so stays within the cell
Explain how a PET scanner works
- A ring of scintillation detectors supported in a fixed gantry are used to detect gamma rays produced from radioactive tracer inside the patient
- Operated in “coincidence mode” - only photons emitted from an annihilation event are recorded
Explain the concept of PET coincidence
- 2 gamma rays originate from one annihilation event
- Both are detected within a short time (a few ns)
- Defines ray path for subsequent reconstruction of image
How can a PET scan be used to diagnose types of neurodegenative disease?
- FDG administered to patient and then a PET scan of that patients brain is performed
- FDG is taken up by cells of the brain and reflects metabolic activity
- In Alzheimer’s disease you’ll see hypometabolism, mostly in temporal and parietal regions of brain - FDG taken up more in these areas
- In Pick’s disease you’ll see fronto-temporal hypometabolism - FDG taken up more in these areas
Most modern scanners are hybrid scanners, name some of the different combinations that mae up hybrid scanners
- PET-CT
- SPECT-CT
- PET-MR
Why does attenuation correction need to occur in a PET image?
- Gamma rays originating from the centre of the patient will travel through more tissue which mean they are attenuated more
How is attenuation correction performed?
- A CT image of same area scanned using PET is used as an attenuation map to correct the PET image
