MRI & MRS Flashcards
- What is CT best used for?
Bony structures
Can be used for soft tissue though
- What is MRI best used for and why?
Soft Tissue and signal is being picked up by protons in fat, in water
- What are two types of MRI?
- Proton Density (PD)
- T2- Weighted (T2)
- What is the contrast in CT images due to?
Tissue density density dependent attenuation of x-rays
- What does signal density depend on?
Hounsfield Number (how much of the x-rays are being attenuated)
- What does attenuated mean?
Attenuation is the reduction of the intensity of an x-ray beam as it traverses matter. The reduction may be caused by absorption or by deflection (scatter) of photons from the beam
- In terms of hounsfield number, what would be high and what would be low?
High = Bone Low = Tissue
- What is the range of hounsfield number?
-1000 (Air) to 1000 (Cortical Bone)
- What does the range -100 to +100 cover in hounsfield numbers?
Water
- MRI image contrast shows the relative signal intensities between diff tissue types and pathologies , what does this depend on?
Physical properties of the tissue such as water and fat content, cellular structure and cell density.
- Is MRI sensitive? If so, what to?
MRI contrast is very sensitive to changes in a large variety of the physical properties of tissue water and blood
- What does T1 relaxation show?
Paramagnetic blood breakdown products
- What does T2 relaxation show?
Tissue Fluidity - oedema, deoxyhaemoglobin
- What does water diffusion/movement (micro) ?
Cell membrane integrity, cell size
- What does flow (macro) show?
Blood flow
- What does perfusion show?
Blood flow, blood vessel density
- How does T2 weighted MRI look?
Well, its more fluid bright so CSF shows up bright and fatty tissue shows up light –> indicates pathological changes
- How does T1 weighted MRI look?
CSF shows up dark–> used for anatomical changes due to contrast
- What does anisotropy mean?
It means that something is directional - has different properties depending on the direction its in
- What do anisotropy maps show?
Show anistropic water diffusion ( diff in diff directions) in the brain
- What are the magnets within us?
Proton from H atom of water -> Magnetic moment (spins on its axis) –> protons line up with magnetic field of scanner—> can be manipulated by radio freq to produce signals which can be picked up.
- What does the positive charge of a spinning proton produce?
A magnetic moment (u )
- What magnetic field do the protons line up in?- how do they align?
B0
They don’t align perfectly along compass needle but instead they align with earths magnetic field.
- What do the protons do after they align?
They precess (rotate )
- What is Lamor Frequency V( with a little capital L next to it)
When the freq of precession is proportional to the magnetic field
- BRIEFLY describe process of MR imaging please?
- BO HAS MAGNETIC FIELD
- RF PUT THROUGH COILS
- SIGNALS –> AMPLIFIER–> COMPUTER
- MAGENTIC FIELD GRADIENTS -> ENCODE SIGNALS–> IMAGE
- What is the Bo? What does it contain?
Bo is what the patient lies in
It has a strong and uniform magnetic field
it contains imaging coils that a radio-frequency current is put through
- How do magnetic field gradients encode the signals sent to amplifiers, to create an image on the computer?
Spatial Variation in frequency allows us to see where the signal is coming from
- What is one rule for safety in MRI?
No ferromagnetic objects in the examination room ( Scissors, stethoscopes, wheelchairs, gas cylinders, hearing aids,watches , spectacles )
30 . What are some Contraindications (cant be done because it would be harmful for the patient) for MRI
• Pacemakers
• Infusion pumps
• 1st trimester
pregnancy
• Aneurysm clips
• Metallic foreign
bodies (orbit x-ray,
shrapnel)- What does the strong magnetic field create in all tissues?
The strong magnetic field creates magnetisation (from protons in water/fat) in all the tissue which can be manipulated by radiofrequency pulses to produce an MRI signal to create an image.
- What does the intensity of the image depend on?
Water content Tissue structure Blood Flow Perfusion Diffusion Paramagnetic
- What is it called when the magnetisation from tissue is aligned with the magnetic field
Equilibrium (MO)
- Radiofrequency interacts with and knocks out the magnetism from its previous alignment, into which plane?
The xy plane (Mxy)
- Whats the difference between T2 and T1 relaxation?
T2 relaxation: When the magnetisation dies with time, its referred to as T2 signal decay in xy.
T1 relaxation: When the magnetisation recovers into alignment, it’s referred to as T1 signal recovery along the z plane.
- Explain the process of MRI signals and contrast (T2) ?
o A radiofrequency pulse generates an MRI signal from the tissue, the MR image is built up from a series of signal acquisitions.
o Initially there is a delay in picking up a signal. The time for this (echo time) depends on T2 relaxation.
o Initially, the signal is intense but it decays with time in an exponential function.
o Longer echo time: weaker signal.
- The following questions refer to T2 weighted image of a brain:
- Why would be put in delays
- What has a longer relaxation time, CFG or white/grey matter?
- Do Lesions increase or decrease T2 ?
o We can put in delays so that the signal intensity is dependent on the tissue.
o Relaxation time for CSF is much longer (1000ms) than white or grey matter (80/90ms).
o Lesions increase T2 causing an increase in signal intensity.
- Where does the signal come from in T2 relaxation in tissues?
o The signal comes from all the water protons in tissue (intra- and extracellular water and also cellular structures e.g. cell membranes).
- What decays quicker Free water or bound water
Bound water–> Decays very quickly
Free water—> Decays very slowly
- What will provide a stronger signal - a shorter or longer echo time?
Shorter echo time ( due to less decay)
- If you had a graph measuring signal intensities at diff echo time, what would change the position of the curve on the graph?
Pathological Changes
- What would expect to see on a T2w image of a high grade brain tumour?
more brighter because the signal from the normal brain decays more quickly than the CSF (outside).
The signal intensity is higher at any given echo time
- Whats the difference between proton density and T2 weighted?
o Proton density (short echo time): signal is proportional to the tissue and water.
o T2 weighted (longer echo time): more brighter because the signal from the normal brain decays more quickly than the CSF (outside).
- What does a quantitive T2 map show?
T2 relaxation time
- Summarise T2 ?
o The T2 of tissue determines how quickly the MRI signal decays away after the radiofrequency pulse
- T2 is very dependent on how mobile the water is in the tissue and increases with what two things?
- Oedema, an increase in water content
* Demyelination, a loss of brain tissue structure
- What is T2 reduced by?
presence of paramagnetic ions e.g. Fe from blood breakdown products and Gd from contrast agents.
- How does MRI signals and contrast work for T1?
o An MR image is built up from a series of signal acquisitions from when the magnetisation returns to alignment to the Z plane.
o CSF magnetisation has a long T1 so the magnetisation takes much longer to come back.
o Brain tissue magnetisation has a shorter T1 so it comes back much more quickly.
o Image contrast: comparison of how quickly the magnetisation recovers at a certain point echo time.
- How do we form T1w images?
o Enter a sequence of pulses with short echo time (no T1 recovery) and short repetition time. o CSF (longest T1) shows up dark and fat (shortest T1) shows up bright). o White matter has a shorter T1 as it’s more rigid and myelinated. o Useful for detecting volumetric changes that occur with various diseases
- Does grey matter increase or decrease with age?
Grey matter volume decreases with age.
- What would you see in a T1w image of a dementia patient?
Larger dark space due to more CSF in the area as the hippocampus gets smaller and enlargement of the ventricles.
- What is the taxi driver effect?
o 3D T1 images used to determine an increase volume of hippocampus.
o Taxi drivers have a larger hippocampal volume than normal - neurogenesis proposed to explain need for increased capacity of spatial memory.
- Summarise T1 relaxation?
o Reduced repetition time - magnetisation producing the MRI signal is reduced (“saturated”)
o Tissue with long T1 produces a smaller signal than short T1.
o T1 is also dependent on how mobile the water is in the tissue and T1 increases slightly with oedema.
- What are contrast Agents?
MRI contrast agents are contrast agents used to improve the visibility of internal body structures in magnetic resonance imaging (MRI)
- Give an example of contrast agents used?
Paramagnetic (unpaired electrons) or superparamagnetic (ferrites) are chelated to reduce toxicity and then injected intravenously.
- What happens after the injection of contrast Agents?
o Water in the vicinity of the contrast agent experiences strong fluctuating magnetic fields hence T1 and T2 are reduced.
- What would you expect to see on a T1w image of glioma (tumour of glial cells)?
Tumour core distinguished as contrast enhancement in areas of BBB breakdown – areas of neo-angiogenesis stimulated by tumour growth
- What are MRI characteritstics of MS?
o Active lesion with Gd contrast.
o Number of lesions/lesion type can aid monitoring of disease progression or treatment response.
-Inflammation and Demyelination makes tissue more fluid (T2)
-BBB leakage (T1)
