MRI and MRS Flashcards by Tooks yh

What is CT best used for?

CT is best used for bony structures but can also be used for soft tissue

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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

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What is CT contrast due to?

CT contrast is due to tissue density dependent attenuation of x-rays

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What does the Hounsfield number measure?

Hounsfield number measures how much X-ray has attenuated as it passes through any material

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What is the first call for brain imaging and why?

Ct is the first call for brain imaging as its quick and cheap

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What does CT show the difference between?

Clearly shows the difference between bone and tissue but anatomical detail is faint

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What is image contrast?

Image contrast is the relative signal intensities between different tissue types and pathologies

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What does MRI image contrast depend on?

Depends on physical propterties of the tissue such as water and fat content, cellular structure, cell density

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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

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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

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What is nuclear magnetism?

-Nuclear magnetism is the positive charge of a spinning proton producing a magnetic moment

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What is resonance?

Resonance in a magnetic field B0 is the magnetic moment of a proton which processes at the Larmor frequency

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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

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What do strong magnetic fields create?

Strong magnetic field creates magnetisation in all the tissue

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What is magnetisation from in water and tissue?

Magnetisation is from protons in water and fat in tissue

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What can magnetisation be manipulated by?

Magnetisation can be manipulated by radiofrequency pulses to produce an MRI signal to create an image

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What does intensity in image depend on?

Intensity in image depends on water content, tissue structure, blood flow., perfusion, diffusion, paramagnetic

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When is the strongest signal in MRI received?

Strongest signal is received when the first radiofrequency pulse is introduced and gets smaller with time

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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

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What is a pulse sequence?

A pulse sequence is the time of putting in the radiofrequency pulse

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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