MRI/MRS Flashcards
CT contrast is due to what?
attenuation of x-rays
-it is tissue density dependent
what does an MR image contrast refer to?
the relative signal intensities between different tissue types and pathologies – depends on physical properties of the tissue such as water and fat content, cellular structure and cell density
MRI gets signals from…
water and fat molecules
what is the MRI contrast very sensitive to?
very sensitive to changes in a large variety of the physical properties of tissue water and blood
what is T1 dependent on?
very dependent on the presence of paramagnetic ions, which reduce T1
Fe from blood breakdown products
Gd from contrast agents
also dependent on how mobile the water is in the tissue and T1 increases slightly with oedema
what is T2 relaxation very dependent on?
mobile the water is in the tissue
cell membrane integrity and cell size affects what?
diffusion of water
what does perfusion refer to?
blood flow, blood vessel density
what does a white matter map show?
diffusion anisotropy
-white bright areas are anisotropic, fluid travelling in different direction
define nuclear magnetism
The positive charge of a spinning proton produces a magnetic moment miu (u)
how is MR imaging formed?
The strong magnetic field creates magnetisation from the protons in water and fat in the tissue
The magnetisation can be manipulated by radiofrequency pulses to produce an MRI signal to create an image (from tissue).
Magnetic field gradients used to encode the signal in space so that the computer can generate an image
what are some safety issues associated with MRI?
Safety
No ferromagnetic objects in the exam room
- Scissors, stethoscopes, wheel chairs, gas cylinders
- Hearing aids, watches, spectacles, (dentures – image quality)
Contraindications
- Pacemakers
- Infusion pumps
- 1st trimester pregnancy
- Aneurysm clips (refer to manufacturers specifications
- Metallic foreign bodies (orbit x-ray, shrapnel)
in MRI, what does the intensity of the image depend on?
on water content, tissue structure, blood flow, perfusion, diffusion, paramagnetics etc
T1 and T2 relaxation times vary between what?
different tissues and pathology
what does the image signal intensity depend on and what does it provide?
The image signal intensity depends on T1 and T2 and provides contrast between tissue in an MR image
what are T1 and T2?
looking more closely at the MR signal we find there are two relaxation times that determine how strong the signal is
mxy and mz?
Mxy decays according to T2 - affects how long the MR signal lasts
Mz recovers according to T1 which affects how much M there is available to be excited to give the next signal
what is an MR image built up from?
a series of signal acquisitions
the T2 of tissue determines what?
how quickly the MRI signal decays away after the radiofrequency pulse
what is T2 very dependent on?
very dependent on how mobile the water is in the tissue
what does T2 increase with?
Oedema, an increase in water content
Demyelination, a loss of brain tissue structure
what is T2 reduced by?
presence of paramagnetic ions:
Fe from blood breakdown products
Gd from contrast agents
what has a long T1?
CSF has a very long T1
explain how saturation occurs
when the repetition time (TR) between pulses is much shorter than T1, the magnetisation that produces the MRI signal is reduced/saturated
how do tissues with long T1 differ to tissues with short T1?
produces a smaller signal
why is T1 lower in white matter than grey matter?
because of myelinated neurones
T1 is dependent on what?
very dependent on the presence of paramagnetic ions which reduce T1
- Fe from blood breakdown products
- Gd from contrast agents
also dependent on how mobile the water is in the tissue
what is the role of contrast agents in MRI?
usedto improve the visibility of internal body structures inmagnetic resonance imaging(MRI)
-water in the vicinity of the contrast agent experiences strong fluctuating magnetic fields, reducing T1 and T2
types of contrast agents in MRI?
Paramagnetic (unpaired electrons) or superparamagnetic (ferrites)
-Chelated to reduce toxicity
what happens to water in the vicinity of contrast agents
it experiences strong fluctuating magnetic fields, reducing T1 and T2
the signal intensity of CT depends on what?
a parameter called the Hounsfield number, which is a measure of how much the x-rays have actually attenuated
High hounsfield number in the bone because the x-rays are highly absorbed, and fluid has a lower attenuation so shows up dark.
when is CT useful?
when looking at haemorrhagic stroke or when looking at traumatic brain injury - this is because blood breakdown products absorb x-rays, so show up brighter on the CT image
CT is the first line of imaging when it comes to trauma – why?
you can get a person straight into a CT scanner after an accident/stroke without any worries, but with MR you have to be careful because it is a strong magnetic field so you have to screen patients before they go into an MR scanner to make sure they haven’t got metal on them
CT is fairly quick and cheap, most routine imaging used in hospitals.
T2 vs T1 weighted image
a T2 weighted image, the more fluid you have, the brighter the signal. They are very good in picking up pathological lesions in the brain, like stroke, tumour, MS.
T1 weighted image, CSF shows up dark, and there is good contrast between deep grey and white matter, and cortical structures. Useful for picking up anatomical changes that affect volume of grey matter in the brain eg. alzheimers, dementia
directionality of diffusion is called?
anisotropy
how would a lesion affect T2?
Lesions means T2 increases, so signal intensity in a T2 weighted image would increase
