Imaging techniques Flashcards
What is a preclinical modality?
Related to modalities investigating animal models
Pre-therapeutic
Which pre-clinical modalities can imaging techniques be applied to?
Neurology
Oncology
Cardiology
Cell tracking
What can clinical imaging be used for?
Diagnosis
Prognosis
Determine the location of the disease
Monitor the response to therapy
What can preclinical imaging be used for?
Validating clinical imaging methods - confirm using histology
Interpreting mechanisms of human disease and therapy - look at uptake of therapeutic cells, combine with other imaging technique to look at therapeutic response
Developing new clinical imaging technology
What is light?
A form of electrical radiation
What are the two states light can be found?
At some wavelengths it takes form in visible light
At some wavelengths it takes a form of radiation we cannot see
What type of light are X-rays?
Short wavelengths
Why can X-rays be used to observe the body?
Some X-rays are absorbed by the body
Some X-rays are attenuated by the body
Bones contain calcium, making them denser than other tissues = shadows
What are X-rays usually used for?
Revealing fractures
What are CT scans?
Multiple cross-sectional images of the body using X-rays
Computer processing techniques bring them together
How are CT scans better than X-rays?
More detailed
Can reveal bones, tissues and organs
How do CT scans produce more detailed images than X-rays?
X-rays use a fixed tube that sends X-rays in one direction
CT scanner uses a motorised X-ray source that shoots beams of X-rays as it revolves around the patient
How is image reconstruction of a CT carried out?
Digital X-ray detectors detect the signal
These are found opposite to the X-ray source
As the X-ray passes through the patient, they are picked up by detectors and transmitted to a computer
2D image slices are stacked together into 3D images
What are the advantages of CT scans?
Great bone-soft tissue contrast
High spatial resolution
Whole body coverage
Non-invasive
Cheap compared to MRI
What are the limitations of CT scans?
Ionising
Limited soft tissue contrast
Why are CT scans often used in neurology?
Cheaper than MRI
Broad diagnosis of neurological disorders
What are ways to improve CTs?
Contrast agents
Definition of radioactivity
Process in which unstable atomic nuclei spontaneously emit ionizing radiation
What type of scan is a PET scan?
Nuclear scan
What techniques do PET scans use to obtain an image?
Radioactive techniques
What does PET stand for?
Positron Emission Tomography
What are alpha particles?
He nuclei
Large
Don’t travel far in space
What are gamma rays?
Highly energetic photons
Similar properties to X-rays
What are beta particles?
Electrons
Don’t travel far
Absorbed by the body
What are positrons?
Antimatter of electrons = positive electron
Similar properties to electrons
Don’t cause damage until they collide and cause annihilation
What is annihilation?
When a positron and an electron collide
Causes the release of two gamma rays at 180 degrees from one another
Allows to pinpoint the place in the body where annihilation occurs
How do PET scans produce images?
Uses positrons and electron interactions
Tracers are made up of carrier molecules covalently bonded to radioactive isotopes
The gamma rays 180 degrees apart formed through annihilation are detected by coincidence
Detectors measure these photons and use the information to create a 3D image
How are tracers administered to the body?
Intravenously
Advantages of PET scans
Sensitive (nM-pM)
Half-life of tracers is long (20 minutes - few hours)
Large range of tracers
Disadvantages of PET scans
Single signal
Resolution is relatively low
Ionising radiation
High cost due to the use of tracers
What do radiotracers target?
Biomarkers
What are biomarkers?
Anything biological that is of interest in disease states
What criteria must a target fulfill in order to be considered a biomarker?
The target should be biologically informative
The target must be differentially expressed over the background
The unbound radiotracer should be cleared quickly by the body
The radiotracer must be delivered to and specifically interact with the target
Background activity should clear within the lifetime of the radioactivity : contrast
Examples of multimodal imaging in PET
PET-CT
PET-MRI
Uses of PET-CT and PET-MRI
CT dosen’t show biochemistry of disorder
PET retained in tumour cells show the regions of cancer
CT = anatomy PET = glow of cancer cells
How can PET be used to image cancer?
Through cancer metabolism
Tumour cells have enhanced metabolic activity, as both glycolysis and oxidative phosphorylation are turned on
Example of PET scan tracer used in cancer
FDG
What is FDG?
Modified form of glucose with unstable fluorine 18 used as a radioactive tracer
Since FDG can no longer undergo metabolic degeneration, it is trapped inside the tissues
What is the distribution of FDG?
Brain - takes up 60% of glucose in normal functioning
Salivary
Myocardium
Renal excretion
Applications of FDG in oncology
Diagnosis
Staging
Response monitoring
What are the problems with FDG?
Cannot detect tumour types in organs with glucose as primary source of energy (prostate, brain cancer)
FDG is taken up by inflammatory cells
What is the goal of future biomarkers?
Aim at complex biochemical pathways involved in cancer metabolism
Try to find associated radiotracers
What do MRI use?
Protons which are abundant in the human body
Manipulate the magnetisation of the hydrogen protons to create an image
What is the major source of protons in the body?
Water
What are the three components of an MRI machine?
A big magnet (3T in hospitals, 9.4T preclinically)
Radio frequency pulse transmitter and receiver
Magnetic field
Explain how MRIs work
- All protons spin, creating a small magnetic charge
- When a strong magnetic field is introduced, the protons align with the field
- The MRI generates a stable magnetic field parallel to how the patient lies
- The MRI technician then introduces a radiofrequency pulse using a radiofrequency pulse transmitter in the opposite direction to the magnetic field
- This disrupts the proton and forces it either into a 90 degree or 180 degree realignment
- Since the radiofrequency pulse push the proton against its nature, once the force is turned off, the protons realign with their magnetic field in a process called precessing
- As the nuclei are precessing, there is release of electromagnetic energy
How does MRI distinguish between tissues?
Detection of the electromagnetic energy as they precess
Differentiation of the tissues based on how quickly they release energy after the pulse is turned off
What does the Larmor equation explain?
The magnetic field strength emitted from the CT machine increases with distance
What is the importance of the Larmor equation?
Allows to differentiate between the front and back of the brain
The lower magnetic frequencies come fro the front of the brain
What is another name for precession?
Relaxation
Why do different tissue types have different relaxation?
Relaxation is determined by the environment of the water molecules
What does T1 measure?
The time required for the spins to go back to their original axis (applied magnet)
Water spins quickly = more time needed to go back to axis = darker T1
Lipids spin slowly = more time needed to go back to axis = lighter T1
What does T2 measure?
The coordination of the H+ spins
Water spins quickly = quicker for the spins to be random = lighter T2
Lipids spin slower = more time needed for spins to be random = darker T2
What are the advantages of MRI?
Great soft tissue contrast
High spatial resolution
Non-invasive
Non-ionising
Can image structure and function
What are the disadvantages of MRI?
Expensive
Can lack specificity
Lack of temporal resolution
What is functional MRI?
Indirectly measures neuronal firing
What is measured in fMRI?
NOT measure neuronal firing
But rather the blood flow changes that happens in response through the neurovascular coupling in the brain
More active brain regions = more blood flow
What signal is measured in fMRI?
BOLD
Blood Oxygen Dependent Signal
BOLD increases with neronal firing
How is fMRI useful?
Differences in the T2 signal between oxygenated and deoxygenated blood
T2 of oxygenated is longer than T2 in deoxygenated blood
What is requires to capture the BOLD signal?
Fast imaging
Gradient-Echo Planar Imaging
What type of system underlies the ultrasound?
Acoustic based system
Works between 1-100 mHz
What is needed to carry an fMRI out?
Task - neuronal stimuli
Increased neuronal firing
Vessel dilation
Increased cerebral blood flow from the baseline
Increased BOLD signal
Describe how an ultrasound works
Transducer uses an array of pizoelectrical crystals
These vibrate when an electric signal is applied
Produce a high frequency sound or compressional waves called ultrasounds
Crystals also work in reverse, producing electric signals when sensing high frequency of compressional sounds
What ate sound waves?
Propagating fluctuations in pressure, density, temperature and particle motion
How does an ultrasound use sound waves to create an image?
Some tissues with high densities echo sound waves
Some tissues with lower densities allow sound waves to pass through the body
The echoed sound waves are captured by the pizoelectric crystal, transducing the signal into an electric one
What does ultrasund measure?
The boundaries between materials which have different acoustic impedance
What changes the electric signals into points of brightness on the image?
Computer
What is the image formed by an ultrasound called?
A sonogram
What allows a sonogram to form a real time motion?
Crystals are repeatedly activated many times so a complete image frame is completed 20 times per second
What are sonograms used for?
Looking at babies during pregnancies
Heart during cardiac investigations
What are the advantages of ultrasound?
Cheap
Portable
Non-invasive
Non-ionising
What are the disadvantages of ultrasound?
Spatial resolution and contrast
Depth
Cannot image the adult brain because of the skull
