X-Ray, CT, PET Flashcards
List imaging types with Ionising Radiation
Ionising Radiation Planar X-ray CT Gamma Camera and SPECT PET Hybrid Imaging
What is ionising radiation?
Radiation that causes ionisation when it interacts with matter
Types used for medical imaging are:
Gamma rays
X-rays
Why use ionising radiation?
Why use ionising radiation?
Penetrating
Ionising Radiation Action
Indirect Action –
Majority of the body is water
Production of Free Radicals & Hydrogen Peroxide
FR are highly reactive, so direct action = mutation in cell DNA
= biological response = genetic, death, cancer
Direct effect of radation damage
Only at high radiation dose not noticed at usual diagnostic doses
Threshold effect
e.g. Erythema & hair loss
Indirect effect of radation damage
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
Some typical effective doses (mSv) are:
Some typical effective doses (mSv) are: 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
Risk of fatal cancer induction is:
Risk of fatal cancer induction is:
5% per Sievert
1 in 20000 per mSv
1 in 2000 per PET scan
Annual radiation dose in the UK average = ?
Annual radiation dose in the UK average 2.7 mSv
Positrons - effect
Positive electrons interact with matter to create gamma rays = PET
Gamma rays - effect
Penetrating radiation = Gamma camera imaging
e.g. SPECT
X-rays - effect
X-rays Spectrum of electromagnetic radiation = X-Ray imaging e.g. radiographs, CT
Artificially produced in an X‑ray tube
Relation of positrons’ and gamma rays’ emission
Emitted following the radioactive decay of an unstable nucleus
Attenuation increases with
Attenuation increases with
Higher atomic number
Higher density
X-Rays are essentially an attenuation map
Transmission Imaging - action and advantages
Transmission Imaging
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 - action
The radiation is administered to a patient in the form of a tracer
Emitted radiation is detected outside the patient
Voltage/current function in X-ray tube
High voltage controls the energy of the x-rays
Current control the amount of x-rays
X-ray produced when and how much control do we have
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
Detectors for planar X-ray - Film Hardcopy = describe action
Film processor with tanks of chemicals
High resolution
Detectors for planar X-ray - Computed Radiology computer copy
= describe action
Phosphor plate
Special laser scanner or CR reader that reads and digitizes the image
Digital enhancement and archiving
Detectors for planar X-ray - Digital Radiology (DR)
= describe action
Flat panel detector, fully digitised system
PE X-ray description
Wedge shaped defect 🡪 Pulmonary embolism
Only large PE can be seen on X-ray
X-Ray Screening: Mammography = describe action and advantages
High resolution
Compression plate used to reduce breast thickness
Improves resolution
Lowers radiation dose (used as a screening tool)
Real-time X-ray: Fluoroscopy - action and use
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
X-ray intervention:
Coronary angiography - action
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
Limitations of Planar X-ray
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
Helical scanning - action
Helical scan is a method of recording high-frequency signals on magnetic tape
Continuous rotation
Continuous table feed
Helical MSCT - advantages
Multi-slice
Faster scan
More coverage each rotation
CT acute diagnosis:
Haemorrhage or blood clot? - what is needed and why
Urgent diagnosis required for treatment
Clot busting drugs may increase bleeding
CT: Disease progression - use of imaging and any measurements done
Measurement of the size of the left inguinal lymph node shows progression of disease
Imaging is used for monitoring response to therapy
CT in 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
Functional Imaging - action
Inject radioactive tracer, patient is emitting the gamma rays
Image depends on the metabolism of the tracer: Functional Imaging
Gamma camera - what does it use and +ves
Uses single photon emitting radionuclides
Can operate in 2D (planar) or 3D (SPECT)
PET - uses what and characteristics
PET
Positron Emission Tomography
Uses positron emitting radionuclides
Always 3D
Why is nuclear medicine a functional modality
Nuclear Medicine only shows function
It may reflect anatomy but without metabolism, the tracer will not be taken up
Nuclear Medicine is a functional modality
Half-life- define
Half-life is time taken for the radioactivity to reduce to 50%
Gamma Camera Imaging - function of imaging heads
Gamma cameras have imaging “heads”
For radionuclides that decay with direct emission of gamma rays
Most common radionuclide
Most common radionuclide is Tc-99m (T1/2 = 6 hours)
Tracers used in gamma camera imaging:
Tracers used in gamma camera imaging:
Tc-99m MDP (bone scans)
Tc-99m DTPA (kidneys)
Tc-99m White Cells (infection/inflammation)
Dynamic Renal Transplant Scan - action
Camera positioned above the patient
Tc-99m DTPA injected IV
Camera positioned above the patient
Gamma camera records gamma rays and collects image over time
Functional Time –Activity curves are obtained
SPECT - define + +ves
SPECT is a nuclear imaging scan that integrates computed tomography (CT) and a radioactive tracer (observe BF)
Acquire up to 64 images from around the head
Reconstruction of transaxial slices
DaTSCAN - define and action
A DaTSCAN is a scan of the brain. DaTSCAN contains the active substance ioflupane (I123) which is used to help diagnose abnormalities in the brain.
Binds to the dopamine transporters (DAT), on the neurons
β+ Particle Radiation - effect
Both positron and electron are annihilated
2 gamma rays are created at 180o to each other
β+ Particle - define
A beta particle, also called beta ray or beta radiation, is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay
FDG - define + allows what
FDG is a glucose analogue which enters cells in the same way as glucose
Allows = good reflection of the distribution of glucose uptake and phosphorylation by cells in the body
Metabolism of FDG
Glucose in blood → Glucose in cell → Glucose-6P (by phosphorylation) → Glycolysis
FDG in blood → FDG in cell → FDG-6P by phoshphorylation but then it is metabolically trapped
PET Scanner - action
A ring of scintillation detectors supported in a fixed gantry
Operated in “coincidence mode” - only photons emitted from an annihilation event are recorded
PET Coincidence - describe
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
Alzheimer’s disease characteristics
Alzheimer’s disease (hypometabolism, mostly in temporal and parietal regions)
Pick’s disease characteristics
Pick’s disease (fronto-temporal hypometabolism)
High resolution - types of scans
High resolution
CT
SPECT
PET
Hybrid Imaging - examples
PET-CT
SPECT-CT
PET-MR
Role of CT in localisation
Fused PET & CT show the exact location of the “hot spot”
Attenuation - describe
Gamma rays originating from the centre of the patient will travel through more tissue which mean they are attenuated more
The CT image is used as an attenuation map to correct the PET image
