X-Ray, CT, PET Flashcards
What is ionising radiation?
Radiation that causes ionisation (loss / gain of e-) when it interacts with matter
Why is ionisation used?
Ionising radiation used due to its ability to penetrate
What are the types of ionising radiation used for imaging?
Types used for medical imaging are:
- Gamma rays
- X-rays
Describe the indirect action of ionising radiation
Indirect action:
Our body is 70% water (H₂O) which reacts with ionising radiation to form free radicals which are highly reactive
What is the direct action of ionising radiation?
Direct Action: Radiation directly ionises cell DNA ⇒ leads to mutation causing biological responses: - Genetic mutation - Cancer - Death
What are the damage and effects caused by direct effect?
Only at high radiation dose not noticed at usual diagnostic doses
Threshold effect
e.g. Erythema & hair loss
What are the indirect damages and effects of radiation?
Risk of cancer induction
Risk of genetic change in subsequent population
Effect is proportional to radiation dose, no threshold - all radiation has risk
Outline radiation doses administered to patients?
Dental intra-oral 0.002 Chest X-ray 0.02 Lumbar Spine 2 NM Bone Scan 3 IVU 4 CT pelvis 7 CT head 2 Barium enema 8 MIBI cardiac scan 10 PET scan 10
What is the risk of cancer induction?
Risk of fatal cancer induction is:
- 5% per Sievert
- 1 in 20000 per mSv
- 1 in 2000 per PET scan
What are the 3 types of ionising radiation?
- Positrons
- Gamma rays
- X rays
Describe how positrons are used for imaging
PET scanning:
positive electrons interact with matter to create gamma rays
How do Gamma rays create an image?
Gamma camera imaging
e.g. SPECT
penetrating radiation
Describe x ray imaging
X ray imaging
e.g. radiographs, CT
spectrum of electromagnetic radiation
How are x rays created?
artificially produced in an x-ray tube
How are positrons and gamma rays produced?
Emitted following radioactive decay of an unstable nucleus
What increases x ray attenuation?
Attenuation increases with
- higher atomic number
- higher density
⇒ produces a brighter image
Describe an x ray image
X-Rays are essentially an attenuation map
Muscle and lung tissue show darker
Describe a transmission image
Radiation is directed through the patient
A transmission map collected is essentially an attenuation map - maps the tissues that either block or allow some / all x rays to pass through
What are the advantages of a transmission image?
Good at showing structure, especially between tissues of different densities or atomic number
How is emission imaging carried out?
The radiation is administered to a patient in the form of a tracer
Emitted gamma radiation is detected outside the patient
What are the important components of an x ray tube?
Important components of X ray Tube:
- Filament
- Target
- Vacuum (tube)
How are electrons accelerated through the xray tube?
Between the two terminals (filament & target) there is a p.d. Voltage difference to accelerate e-
How are x-rays produced in the x-ray tube?
- Current flows through circuit
- Causes e- to fire from filament
- e- are accelerated through p.d. to incidence plate so they can hit the target
- When e- hit the target they produce a beam of x rays
How are x-rays regulated?
- High voltage controls the energy of the x-rays
- Current control the amount of x-rays
- X-rays only produced when tube is in action i.e. can be
switched on/off - We can control amount and energy of x-rays produced
by altering p.d.
Where are common bone fractures found?
Clavicle
Radius / ulna
Proximal phalanx of index finger
Veterinary x rays
How are fillings easily identified in dental radiology?
Fillings identified as they produce a high contrast image as they are composed of metal (have a high atomic no.) compared to the rest of the tooth
Explain what a wedge shaped defect in a diagnostic CXR indicates
Wedge shaped defect Pulmonary embolism
Lungs are perfused; lobed region supply is either reduced or occluded
Only large PE can be seen on X-ray
What is mammography?
Mammography is an x ray of the breast tissue used for screening purposes for minimising cancer
Why is a compression plate used during a mammography?
High resolution
Compression plate used to reduce breast thickness
- Improves resolution
- Lowers radiation dose
How is real time x ray conducted?
Using fluoroscopy
Catheter fed inside artery and radio opaque dye is injected
Show blood flow inside vessels and can be used to assist with interventions
Outline how a coronary angiography is carried out
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 of planar xray?
- Can’t distinguish overlying tissues
- Tissues not being observed, reduce contrast in image
How can we overcome overlying tissues reducing contrast?
Historically partially solved by moving film cassette and X-ray relative to patient to blur out overlying tissues, called “tomography” (from Greek “part/slice” - “write”)
Superseded by Computed Axial Tomography, now abbreviated to CT
Describe a CT scanner
Basically a rotating x ray tube
Fast Gantry Rotation ~0.33
Has the ability to identify and show 3D slices of different tissues
What does a CT scan offer?
Multi-slice
Faster scan
More coverage each rotation
What is the benefit of CT in haemorrhaging and clotting ?
Urgent diagnosis required for treatment
Clot busting drugs may increase bleeding
Allows us to quickly decipher whether patient has a clot or haemorrhage in the brain and can therefore treat accordingly
How is CT used in disease progression?
Can measure size of tumours etc. progression of disease
Imaging is used for monitoring response to therapy
How is CT used in radiotherapy and treatment planning?
External beam radiotherapy irradiates normal tissue as well as tumour
Multiple beams are used to spare normal tissue
CT is used to define area to be treated and the direction of the radiotherapy beams that are used
How is emission imaging carried out?
Nuclear Medicine: Emission Imaging
I
nject radioactive tracer, patient is emitting the gamma rays
Image depends on the metabolism of the tracer
What is the use of emission imaging?
Functional imaging
What are 2 common functional imaging methods?
- PET
- Gamma camera
Describe the gamma camera
Uses single photon emitting radionuclides
Can operate in 2D (planar) or 3D (SPECT)
What is PET scanning?
Positron Emission Tomography
Uses positron emitting radionuclides
Always 3D
What is half life?
Half life - time taken for radioactivity to reduce to 50%
Why is half life significant in emission imaging?
When injecting radioactive tracers need to consider radioactive decay times (half lives) as only a finite amount of radioactivity injected into a patient
Explain how gamma camera imaging works
Gamma cameras have imaging “heads”
For radionuclides that decay with direct emission of gamma rays
Name a common gamma camera radionuclide
Most common radionuclide is Tc-99m (T1/2 = 6 hours)
Give examples of common tracers used in gamma camera imaging
Tc-99m MDP (bone scans)
Tc-99m DTPA (kidneys)
Tc-99m White Cells (infection/inflammation)
How is gamma camera imaging used to identify cancer?
Used to identify functional changes in cancer:
Cancer has higher metabolic rate than normal tissues ∴will take up more tracer
Describe how a dynamic renal transplant scan is carried out using gamma camera imaging
Camera positioned above patient
Tc-99m DTPA injected IV
Gamma camera records gamma rays and collects image over time
Bright area is vascularised shows tracer uptake and normal activity
Functional Time –Activity curves are obtained
What is SPECT?
Single photon emission computed tomography
- uses gamma rays
Acquires <64 images 3D from around the head
Reconstruction of transaxial slices
What is the difference in nuclear and emission imaging?
Nuclear images only detect 1 gamma ray, emission imaging creates 2 gamma rays
What is FDG?
FDG is a glucose analogue which enters cells in the same way as glucose
Why is FDG a used radionuclide in PET scanning?
FDG phosphorylated by hexokinase is “metabolically trapped” and therefore has increased uptake and retention in metabolically active tissue.
Good reflection of distribution and glucose uptake as well as phosphorylation by cells
Describe a PET scanner
A ring of scintillation detectors supported in a fixed gantry
Operated in “coincidence mode” - only photons emitted from an annihilation event are recorded
Outline how a PET signal is formed
2 gamma rays originate from one annihilation event
Both detected within a short time (a few ns)
Defines ray path for subsequent reconstruction of image
Outline the different hybrid imaging techniques used to consolidate findings
PET-CT
SPECT-CT
PET-MR
Why is emission imaging used for attenuation correction?
Gamma rays originating from the centre of the patient will travel through more tissue which mean they are attenuated more
