MRI & MRS Flashcards
What is image contrast?
Ability to see differences in signal between different anatomical / pathological regions
What is colour cryo section?
Full multislice CT & MRI carried out and cadavers frozen and laser dissected to get prosection anatomical info to compare with scans
What do CT scans image?
CT contrast dependent on bony structures - can be altered to see brain and soft tissue
What do MRI show?
MRI images soft tissue and no bony structures: PD and T2 show fat and water tissues
What does the potential difference signal tell us?
PD signal proportional to amount of fat and water tissues present
What does T2 signal tell us?
T2 signal proportional to T2 relaxation time - tissue property: higher T2 the more fluid the tissue is
What creates the CT contrast?
CT contrast is due to tissue density dependent attenuation of x-rays (Hounsfield number)
Compare the contrast of CT to MRI
CT contrast a lot lower than MRI
How does hounsfield no. change depending on tissue type?
Hounsfield no. high in bone as x rays are very highly absorbed - shows up brighter
Fluid / water has lower attenuation (hounsfield No.) shows up darker
How will a haemorrhage look on CT?
Region of haemorrhage - blood breakdown products absorb more x rays∴shows up brighter on CT
Area of oedema; increased water content = attenuation of x rays is lower
What is MRI signal intensity dependent on?
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, cell density
What does MRI get signals from?
MRI - signals from water and fat molecules
MRI contrast is very sensitive to changes in a large variety of the physical properties of tissue water and blood
Describe MRI T2 property
T2: fluid = bright, fatty tissues = dark → good for picking up pathological changes in brain ie. lesions, stroke, tumours etc.
What is the T1 tissue property of MRI?
T1: fluid = dark, deep contrast between rigid structures → good for showing anatomical changes relating to grey matter volume
Where is magnetic signal found fro MRI?
Magnetic signal comes from within us as we are biological magnets
We’re composed of 75% water (H₂O) - H+ has proton at its core and is spinning on its axis creating a magnetic moment
How does our magnetism create an MRI image?
All the protons in our body line up with the strong magnetic field of the scanner and are manipulated using radio frequency particles to generate an image
What creates a high contrast MRI image?
The more water = stronger signal = better image
Also depends on T1 and T2 parameters
What is nuclear magnetism?
The positive charge of a spinning proton produces a magnetic moment μ, dependent on radioactive isotopes
What causes resonance?
In a magnetic field Bo the magnetic moment of a proton precesses at the Larmor frequency νL
Frequency is proportional to the magnetic field
How is MR image formed?
MR Imaging is formed using a radiofrequency pulse to generate an MR signal from a slice of tissue
Outline how an MRI is formed
- Patient lies in bore of magnetic
( 30,000x uniform magnetic field ) - Imaging coil (loops of wire) around body part
- Insert radiofrequency current through imaging coil
- Sends in a radiofrequency variation in magnetic field into
body part - Signal picked up by imaging coil and sent to amplifier
(radiofrequency receiver) → CPU → produces image
What angle does MRI take images?
Can image various slices at any orientation
How does a computer generate an image from MRI signals?
Magnetic field gradients are used to encode the signal in space so that the computer can generate an image
Give examples of types of MRI scanners
The following scanners contain a magnetic field 30,000x higher than the earths
- Chair
- Iv pole
- Oxygen cylinder
- Floor buffer
What safety regulations need to be in check for MRI?
No ferromagnetic objects in the exam room
- Scissors, stethoscopes, wheel chairs, gas cylinders
- Hearing aids, watches, spectacles, (dentures – image
quality
What contraindications need to be considered before doing an MRI?
- Pacemakers
- Infusion pumps
- 1st trimester pregnancy
- Aneurysm clips (refer to manufacturers specifications
- Metallic foreign bodies (orbit x-ray, shrapnel)
What is the MRI signal source?
Strong magnetic field from protons in water and fat in tissue creates magnetisation in all the tissue
How is magnetisation turned into an MRI image
The magnetisation can be manipulated by radiofrequency pulses to produce an MRI signal to create an image
What does image density depend upon?
The intensity in the image depends on water content, tissue structure, blood flow, perfusion, diffusion, paramagnetics etc
Describe the regular magnetic field in all tissues
Magnetisation aligned with strong magnetic field in equilibrium (B0)
What happens to the normal magnetisation when exposed to radiofrequency pulses?
A radiofrequency pulse is fired that interacts with magnetisation
Knocks it out of magnetic field (no longer points within axis of B0 (z axis)
- knocked sideways into x/y plane
What happens to the signal once in the x/y plane?
One of 2 relaxation times can take place determining how strong the signal is:
T1 and T2
What is T2 relaxation time?
T2 signal decay in xy: Magnetisation signal dies away with time (T2 relaxation)
What is T1 relaxation time?
T1 signal recovery along z: Slow recovery of magnetisation along z axis (T1 relaxation)
What factors determine the MR image formed?
These are MR parameters that vary with tissue type e.g. grey or white matter, & with disease → allow images to be made that demonstrate anatomy and pathology
Describe the effect of T2 relaxation time on MR image?
Mxy decays according to T2 which affects how long the MR signal lasts - dependent on tissue structure (higher liquid = quick, more fat = slow)
Outline the effects of T1 relaxation time on an MRI image
Mz recovers according to T1 which affects how much M there is available to be excited to give the next signal
What causes the build up of an MRI image?
An MR image is built up from a series of signal acquisitions
Waves of radiofrequency signals pick up MRI signals from tissues several times to create an image
What is meant by ‘TE’?
TE = echo time delay which can vary
- Signal picked up depends on T2 relaxation time
How does echo time effect the quality of an MRI?
MR signal is strongest after a radiofrequency pulse and then decays with time = T2 relaxation time
Longer the echo time = weaker signal
Give examples of brain structure TE
(TE) T2 relaxation times of brain structures:
White Matter : 90 ms
Grey Matter: 80 ms
CSF >1000 ms
How does T2 effect signal intensity?
As T2 increases, signal intensity also increases
How does TE differ within tissues?
Bound water decays v. quickly
Free water decays slower
ICF and ECF in cells is highly mobile
Membrane proteins also contain H+ within macromolecules or water in hydration complexes of cell membrane - have a shorter relaxation time
How does tissue structure effect T2 signal intensity?
As tissue structure changes amount of bound and free water changes → alters signal intensity and contrast
What does T2 relaxation time tell us?
The T2 of tissue determines how quickly the MRI signal decays away after the radiofrequency pulse
Under which circumstances does T2 signal increase?
T2 is very dependent on how mobile the water is in the tissue and increases with
- Oedema, an increase in water content
- Demyelination, a loss of brain tissue structure
How is T2 reduced?
T2 is reduced by the presence of paramagnetic ions
- Fe from blood breakdown products
- Gd from contrast agents
What is ‘TR’?
Repetition time (TR): how quickly signal returns to z axis
T1 dependent on TR
Give examples of TR of brain structures
(TR) T1 relaxation times of brian structures:
> inverse of T2 image
White matter : 1000 ms
Grey matter : 1800 ms
CSF > 2500 ms
How does repetition time effect an MRI?
When repetition time (TR) between pulses is shorter than T1, the magnetisation is reduced (“saturated”) = T1 weighted
Describe how T1 effects signal intensity
The MR signal is then T1-weighted: Tissue with long T1 produces a smaller signal than tissue with short T1.
Why is T1 shorter in white matter than grey?
T1 is lower in white matter than grey matter because of myelinated neurones
What increases T1?
T1 is also dependent on how mobile the water is in the tissue and T1 increases slightly with oedema
What decreases T1?
T1 is very dependent on the presence of paramagnetic ions which reduce T1
- Fe from blood breakdown products
- Gd from contrast agents
Give examples of contrast agents used in MRI
Paramagnetic (unpaired electrons) or superparamagnetic (ferrites) - Chelated to reduce toxicity
Why do contrast agents reduce T1 / T2?
Water in the vicinity of the contrast agent experiences strong fluctuating magnetic fields hence T1 and T2 are reduced.
Outline the basis of a dynamic contrast agent enhanced MRI
- Contrast agent injected intravenously into the tissue
- Diffuses out into cells decreasing T1
- T1 weighted image of T1 decreases : signal intensity increases
How can we identify characteristics of multiple sclerosis lesions using MRI?
Active lesion with Gd contrast.
Number of lesions/lesion type can aid monitoring of disease progression or treatment response
MRS A 1H spectrum of biochemicals can be obtained from a localised region of the brain using three slice selective pulses.
