MRI and MRS Flashcards
What is all medical imaging based on?
Contrast - this is the abillity to distinguish one object from another.
E.g allows us to see differences between anatomical and pathological tissues
Tumour will contain different amounts of densities of water therefore H protons will take longer to return to their resting position hence providing contrast
What provides the contrast in a CT scan?
Contrast is provided due to the tissue density dependant attenuation of X-rays
- The Hounsfield Number is a measure of how much of the X-rays are attenuated
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Attenuation is the reduction of signal intensity of an X-ray beam as it traverses matter
- Signal intensity will decrease in bone because they absorb the X-rays (dense tissue) signal intensity will increase in soft tissue (lung tissue) as they allow more of the X-ray to pass through them
What would the hounsfield number and therefore attenuation be in bone?
High hounsfield number/ attenuation in bone because the denser the tissue the greater the X-ray attenuation.
In bone much of the radiation will be absorbed hence why they appear white on the X-ray
What would the hounsfield number and therefore attenuation be in fluid?
Low hounsfield number/ attenuation in fluid because X-rays are not absorbed = shows up dark
What is MRI (brief)
A technique which uses magnetic fields and radio waves to produce a computer generated image which distinguishes among different types of soft tissue - allows us to see structures in the brain.
NO SIGNAL FROM BONE, however does image fat and water in bone
What provides the contrast in MR imaging?
The relative signal intensities between the tissue types and pathologies, this is due to the differential physical properties of water, fat content, cellular structure and cell density.
What is the difference between T1 weighted MRI and T2 weighted MRIs?
- T1 will highlight fatty tissue within the body
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T2 will highlight fatty tissue AND water within the body
- (CSF will appear lighter on T2 weighted images because it is free fluid and contains no fat)
T1 = good for picking up anatomical changes that might relate to changes in volume of grey matter
T2 = good for picking up pathological changes in the brain that generate lesions
MR imaging and assessing perfusion
- MR can be used to create perfusion maps in patients with frontal meningiomas/ brain tumours
- Increased vascularisation surrounding the tumour hence it turns up red
- Grey matter will also show up green/yellow as there is more blood volume than grey matter
- Perfusion mapping can also be used for strokes
- T2 maps can detect changes in fluidity in brain tissues (in meningoma there is a region surrounding with elevated fluid which is odema)
What is a diffusion map?
This measures how free the water is to diffuse
- In the ventricles there is CSF = water is freee to diffuse so shows up orange and red
- Jelly-like structures of the brain diffusion is hindered by cell membranes
- In the meningoma there is surrounding free water/oedema which diffuses easily
How does diffusion anisotropy work?
- Water in the brain is diffusing but because we have many structures in the brain, the diffusion is limiting.
- In white matter tracts the water will diffuse easily along the tract but restricted to move side to side.
- We can make an MR image which will measure the amount of diffusivity of the water and diffusion in different orientations
- If diffusion is the same in every orientation = isotropic
- Diffusion is varies with direction (diffusion in one direction is very strong, but perpendicular to that it is weak) = anisotropic
What does diffusion anisotropy measure?
It is a marker of how much damage there may be in the white matter tracts
Where does MRI signalling come from?
- The brain is made up of 75% water (H2O)
- Normal H2 atoms will spin on their axis in random directions
- When a magnetic field (B0) is introduced they will align in the same direction = produce a magnetic moment
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When the radiofrequency is added the H atoms will ‘flip’ off their axis
- The radiofrequency must match the frequency at which the protons are spinning (determined by the lamour equation)
- When the radiofrequency is stopped, the protons will return to their original position
- The time taken to return back to its original position will be measured
- Because white matter, grey matter and CSF have different densities of water, hence varied number of hydrogen atoms they will have varied times where they can return to their resting position
- These relaxation time differences will provide contrast (ability to distinguish one object from another)
What are some safety precautions in MRI?
- No ferromagnetic objects in the exam rooms
- Scissors, stethoscopes, wheel chairs, gas cylinders
- Hearing aids, watches, spectacles
What are contraindications of MR imaging?
- Pacemakers
- Infusion pumps
- 1st Trimester pregnancy
- Aneurysm clips
- Metallic foreign bodies
What factors does the signal intensity depend on?
- Water content
- Tissue structure
- Blood flow
- Perfusion
- Diffusion
- Paramagnetics (T1 and T2 relaxation)
What is the difference between T1 and T2 relaxation?
T1 signal recovery along Z = measures the time it takes for the protons to return back to B<strong>0</strong> (Measure of how quickly the magnetisation signal recovers to its neutral state in the direction of the magnetic field (B0) after the application of the radiofrequency pulse)
T2 signal recovery along XY = time it takes for the protons to lose sync with each other ( a measure of how quickly the magnetisation signal decays in the transverse plane)
Explain echo time
In a T2 weighted image
ECHO TIME (TE) delay between when the radiofrequency pulse is added and when the MRI signal is picked up from the tissue by the imaging coil
The echo time will determine how much decay of transverse magnetisation (Mxy signal loss - T2 relaxation) is allowed to occur
When is the MR signal from the tissue the strongest?
The signal you pick up from the tissue is strongest straight after the radiofrequency pulse. Then it decays with time (T2 relaxation/decay)
