MRI and MRS Flashcards
What is CT best used for?
CT is best used for bony structures but can also be used for soft tissue
What is MRI best used for?
MRI is best used for soft tissue as signals is being picked up from protons in the fat and water
What is CT contrast due to?
CT contrast is due to tissue density dependent attenuation of x-rays
What does the Hounsfield number measure?
Hounsfield number measures how much X-ray has attenuated as it passes through any material
What is the first call for brain imaging and why?
Ct is the first call for brain imaging as its quick and cheap
What does CT show the difference between?
Clearly shows the difference between bone and tissue but anatomical detail is faint
What is image contrast?
Image contrast is the relative signal intensities between different tissue types and pathologies
What does MRI image contrast depend on?
Depends on physical propterties of the tissue such as water and fat content, cellular structure, cell density
What is T2 weighted MRI more sensitive to and how does CSF show up in it?
T2 weighted MRI is more sensitive to water
-CSF shows up bright
What is T1 weighted MRI more sensitive to and what does it show?
- T1 weighted MRI is more sensitive to structural qualities
- Shows white and grey matter
What is nuclear magnetism?
-Nuclear magnetism is the positive charge of a spinning proton producing a magnetic moment
What is resonance?
Resonance in a magnetic field B0 is the magnetic moment of a proton which processes at the Larmor frequency
Steps involved in MRI
- Patient lies in scanner with powerful magnet
- All protons in their body align with the magnetic field but don’t generate a signal until a radiofrequency pulse is inserted which knocks. Out magnetisation to 90 degrees, generating a MRI signal
- Magnetic field gradients are used to encode the signal in space so computer can generate an image
What do strong magnetic fields create?
Strong magnetic field creates magnetisation in all the tissue
What is magnetisation from in water and tissue?
Magnetisation is from protons in water and fat in tissue
What can magnetisation be manipulated by?
Magnetisation can be manipulated by radiofrequency pulses to produce an MRI signal to create an image
What does intensity in image depend on?
Intensity in image depends on water content, tissue structure, blood flow., perfusion, diffusion, paramagnetic
When is the strongest signal in MRI received?
Strongest signal is received when the first radiofrequency pulse is introduced and gets smaller with time
What does image density depend on and what does it provide?
Image signal intensity depends on T1 and T2 and provides contrast between tissue in a MRI image
What is a pulse sequence?
A pulse sequence is the time of putting in the radiofrequency pulse
What is echo signal?
Echo signal(TE) is the time between radiofrequency pulse and the MRI signal being picked up
What is the repetition time(TR)?
Repetition time(TR) is the time between different radiofrequency pulses
What is repeated several times to create a single image?
Echo signal is repeated several hundred times to create a single image
What determines image contrast?
TE and TR determine image contrast
What does T2 MRI use?
Uses 2 radiofrequency pulses and one knocks it to 90 degrees and one to 180 degrees
What can T2 MRI detect?
Can detect loss of brain structure, tumours, oedema because these cause increased amount of free water and hence decreased number of fixed protons in cell membranes.
How do MRI with longer T2 show up as?
Longer T2 show up as brighter
What does T2 of tissue determine
T2 of tissue determines how quickly the MRI signal decays away after the radiofrequency pulse
What is T2 very dependent on?
T2 very dependent on how mobile the water is in the tissue
What does T2 decrease with?
Decreases with:
- Oedema
- Demyelination
What is T2 reduced by?
T2 is reduced by the presence of paramagnetic ions like Fe
What are the properties of contrast agents?
Properties:
- Paramagnetic - Chelated to reduce toxicity
What does water in the vicinity of contrast agents experience and hence what happens to the T1 and T2 values?
Water in the vicinity of the contrast agent experiences strong fluctuating magnetic fields so T1 and T2 are reduced
What are contrast agents used in and why?
Used in brain tumours as blood vessels created by the tumour are poor quality hence a large contrast of agent molecules can cross the poorly developed vascular wall
Steps in T1 MRI
- First radiofrequency pulse is put in and this causes magnetisation in the XY plane which generates a signal but no magnetisation in the z axis
- Must then wait for magnetisation to come back along the z axis before putting in another radiofrequency pulse
What does intensity of image depend on in T1 MRI?
Intensity of image depends on T1 relaxation time of tissue
What do tissues with long T1 produce?
Tissues with long T1 produce a smaller signal than tissue with short T1
What is T1 MRI useful for looking at?
Useful for looking at pathological changes that relate to neurodegenerative processes where there are changes in volume of the brain tissue, in particular, grey matter
What matter is T1 lower in and why?
-T1 is lower in white matter than grey matter due to myelinated neurons
What is T1 dependent on and what does it increase with?
-T1 is dependent on how mobile the water in the tissue is and increases slightly with oedema
What ions is T1 very dependent on and what does it reduce?
-T1 is very dependent on the presence of paramagnetic ions which reduce T1
What is used to determine the volume of the hippocampus?
-T1 images were used to determine the volume of the hippocampus
What happens the longer someone is a taxi driver and what was proposed to explain this?
- The longer the time spent as a taxi driver, the larger the volume of the hippocampus
- Neurogenesis proposed to explan the need for increased capacity of spatial memory
What can magnetic resonance spectroscopy specify and using what?
-Can specify a small region of the brain(15mm) and develop an MR spectrum of biochemicals using three selective pulses
What does each peak in the spectrum of MRS refer to?
-Each peak on the spectrum refers to a different chemical
What do chemicals have around them?
Electron clouds
What does a stronger shielded nucleus have?
A stronger shielded nucleus has a lower resonant frequency
Why does water have a higher frequency signal than fat as in lipids?
- For instance water:
- Electrons are closer to the oxygen than H
- There’s weaker shielding around protons
- Hence there’s a higher frequency signal than fat as in lipids the electrons are shared
Why is MRS important when MRI looks normal?
-MRS is important when the MRI looks normal as the spectroscopy detects an imbalance in chemicals
Safety issues with MRI
Safety issues:
- No ferromagnetic objects in the exam room
- Contraindications:
- Pacemakers
- Infusion pumps
- 1st trimester pregnancy
- Aneurysm clips
- Metallic foreign bodies
