13. X-Ray CT PET Flashcards
What is ionising radiation? and why use it?
Radiation that causes ionisation when it interacts with matter
Types used for medical imaging are:
- Gamma rays
- X-rays
=> Because they penetrate through the body (through the different mediums/tissues) and can be captured on our imaging detectors
Describe the two ways in which ionising radiation acts.
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 imapct 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 effects, such as:
- cell death (necrosis)
- cell loses ability to regulate cell growth
- 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 and hair loss
INDIRECT EFFECT:
- 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
- Gamma camera imaging
e. g. SPECT - X-RAYS: Spectrum of electromagnetic radiation
- X-Ray imaging e.g. radiographs, CT
- Positrons and gamma rays are emitted following the radioactive decay of an unstable nucleus
- X-rays are 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: (X-rays)
- Radiation is directed through the patient
- A transmission map collected is essentially an attenuation map
- 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 (patient is injected)
- 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.
- X-rays only produced when tube is in action i.e. can be switched on/off
- We have control over the amount and energy of x-rays produced
List some detectors for planar x-rays
- FILM HARDCOPY:
- Film processor with tanks of chemicals
- High resolution
- COMPUTED RADIOGRAPHY COMPUTER COPY:
- Phosphor plate
- Special laser scanner or CR reader that reads and digitizes the image
- Digital enhancement and archiving
- DIGITAL RADIOLOGY (DR):
- Flat panel detector, fully digitised system
In mammography x-ray screening, why do we need a compression plate on the breast?
- Compression plate used to reduce breast thickness
- Improves resolution
- Lowers radiation dose (used as a screening tool)
Describe fluoroscopy as an x-ray imaging technique.
- Real-time imaging
- A catheter is fed inside an artery and radio opaque dye is injected
- Show blood flow inside vessels and can be used to assist with interventions
Describe coronary angiography as an x-ray imaging technique
- Real-time imaging using an image intensifier called fluoroscopy
- A cardiac catheter is fed inside the aorta
- Radio-opaque contrast agent used to identify areas of occlusion
- Treatment may be either balloon angioplasty or insertion of a stent
What are the limitations with planar x-rays?
- Cannot distinguish between overlying tissues
- Tissues other than those being observed 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” (from Greek “part/slice” - “write”)
- Superseded by Computed Axial Tomography, now abbreviated to CT
What is helical scanning, and how has it improved?
- 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 seconds 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.
- So, x-rays are used for the urgent diagnosis required for treatment
How are x-rays used in disease progression?
- X-ray imaging is used for monitoring response to therapy
