X-Ray, CT, PET Flashcards
What is ionising radiation?
It is radiation that causes ionisation when it interacts with matter.
Types used for medical imaging are:
- Gamma rays
- X-rays
Why do we use ionising radiation?
Because they can penetrate through the body (through different mediums/tissue) and be captured on our imaging detectors.
Describe the two ways in which ionising radiation acts.
There are two ways: an indirect and a direct way.
INDIRECT:
- the radiation interacts with the water in the body
- it will split the water molecules into free radicals
- these free radicals have an impact on the cell
DIRECT:
- the gamma radiation/proton can directly impact the cell DNA strand and cause breaks, and therefore impact and have an effect on the cell
Both these methods may or may not have a biological effect, such as:
- cell death (necrosis)
- cell loses ability to regulate cell growth (cancer)
- causes genetic mutations
What are the two types of radiation damage and risk?
DIRECT EFFECT:
- only at high radiation dose, not noticed at usual diagnostic doses
- threshold effect
e. g. erythema & hair loss
INDIRECT EFFECTS:
- risk of cancer induction
- risk of genetic change in subsequent population
- effect is proportional to radiation dose, no threshold (all radiation has risk)
What are the three types of ionising radiation?
POSITRONS: Positive electrons interact with matter to create gamma rays
- PET scanning
GAMMA RAYS: penetrating radiation
- SPECT
X-RAYS:
- spectrum of electromagnetic radiation
- radiographs, CT
The first two are emitted following radioactive decay of an unstable nucleus.
The third is artificially produced in an x-ray tube.
How is attenuation important in x-rays?
Attenuation means to stop.
X-rays are essentially an attenuation map.
Attenuation increases with:
- higher atomic number
- higher density
Describe emission and transmission imaging.
TRANSMISSION IMAGING:
- radiation is directed through the patient
- a transmission map collected is essentially an attenuation map
- it’s good at showing structure, especially between tissues of different densities or atomic number
EMISSION IMAGING:
- the radiation is administered to a patient in the form of a tracer
- the emitted radiation is detected outside the patient
How does an x-ray tube work?
There are some key components to an x-ray: the evacuated (vacuumed) tube, the target (anode), the filament and the heating circuit. The target and the filament are separated by a potential difference. The heating elements produces electrons, which then hit the target, causing it to emit x-rays.
The high voltage controls the energy of the x-rays.
The current controls the amount of x-rays.
X-rays are only produced when the tube is in action i.e. it can be switched on/off.
We have control over the amount and energy of x-rays produced.
In mammography x-ray screening, why do we need a compression plate on the breast?
The compression plate is used to reduce breast thickness. This:
- improves resolution
- lowers radiation dose (used as a screening tool)
Describe fluoroscopy as an x-ray imaging technique.
It is real-time imaging.
A catheter is fed inside an artery and radio opaque dye is injected.
It shows blood flow inside vessels and can be used to assist with interventions.
Describe coronary angiography as an x-ray imaging technique.
This is real-time imaging using an image intensifier called fluoroscopy.
A cardiac catheter is fed inside the aorta
A radio-opaque contrast agent is used to identify areas of occlusion.
Its treatment may be either balloon angioplasty or insertion of a stent.
What are the limitations with planar x-rays?
- we cannot distinguish between overlying tissues
- tissues other than those being observed reduce contrast in the image
Historically, this was partially solved by moving the film cassette and x-ray relative to the patient to blur out overlying tissues, called “tomography” (from Greek “part/slice” - “write”). - it’s been superseded by Computed Axial Tomography, now abbreviated to CT
What is helical scanning, and how has it improved?
It is also known as spiral scanning. It was introduced in late 1980s
There was continuous rotation of the x-ray and continuous table feed.
Now, we have helical MSCTs. These are multi-slice rotating round, making it more efficient (takes about 30 second for a full-body scan) and gives it more coverage each rotation.
How are x-rays used in A&E?
With the case of a suspected haemorrhage or blood clot, the opposite treatment can be fatal for the condition. For example, clot-busting drugs may increase bleeding, which would be fatal if it was actually a haemorrhage.
Thus, x-rays are used for the urgent diagnosis required for treatment.
How are x-rays used in disease progression?
The measurement of the size of the left inguinal lymph node shows progression of disease (in the particular example on the slide).
X-ray imaging is used for monitoring the response to therapy.
