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