Lecture Three: Contrast MRI Flashcards
What are the types of contrast MRI?
BOLD
DWI
Diffusion Tensor Imaging
What does T1 and T2 weighted imaging show?
T1= CSF is dark, WM is bright T2= CSF is bright
T1 and T2 imaging delineates very well
What is Spin?
Quantum property that gives nuclie a magnetic field
What is DWI?
Diffusion Weighted Imaging;
- Tissue structure based on water movement
- Both anatomy and Physiology
What is important about WMT?
WMT are highly ordered (can cross over)
What is diffusion?
- Molecules are in constant thermal motion (brownian motion)
- Diffusion describes and migration over time
Describe DWI;
1) Bo magnetic field in the longitudinal plane aligns nuclie. Excitation stimulus introduces “wobble” (EVERYTHING IS ALIGNED ATM)
2) Gradient stimulus (+ive or -ive) is introduced (DEPHASING) this alters the spin rate for particles influenced by the gradient, there is a differential spinning rate now (PARTICLES ARE NOT ALIGNED)
3) Particles in the magnetic field are spinning at different rates
4) RE-PHASING, the opposite gradient to before is introduced and particles become re-aligned
5) signal produced (gradient echo)
How is a signal produced in DWI????
If particles have continued to diffuse then they will not be exposed to the magentic field strength (gradient) that is equal and opposite to that of the first gradient that was produced.
Therefore the sum of signal will be altered and indicate the diffusion and with integrated maths it can indicate diffusion over time and direction
What is DWI dependant on?
The signal produced is dependant on the amount of diffusion into the tissue
High signal = low diffusion
Signal loss is proportional to the rate of diffusion
Can measure diffusivity
Describe DWI and stroke?
Stroke = increased signal = poor diffusion as post ischemic event = loss of cellular transport mechanisms.
What does a low diffusion co-efficient mean?
- Spins dont move very far
- Experience small differences in magnetic environment
- little dephasing
- little signal loss
What does a high signal co-efficient mean?
- Spins explore a larger region of space
- Experience greater difference in magnetic field
- More dephasing and significant signal attenuation
What is anisotropic diffusion?
Anisotropic (Not uniform)
- Diffusion is not necessarily isotropic
- Signal attenuation changes for different diffusion gradients
(not the same in all directions)
What restricts diffusion?
Cell boundaries restrict diffusion across the fibres
- Observed in WMT and muscle fibres
Describe anisotropic diffusion;
Diffusion = ellipse (tensor)
- H ions diffuse along axons but not across WMT (Anisotropic diffusion is characteristic of WMT)
In Diffusion Tensor imaging we can measure diffusion and direction
What can several anisotropic diffusion images be used for?
To create a 3d image of diffusion patterns
How does fibre tracking relate to DWI?
- Principle direction of diffusion can be used to track fibre bundles
- Tractography
How can 3D WMT pathways be imaged?
Tissue fibre orientations can be mapped in 3d using the diffusion tensor.
What do neurons require?
Activation of neurons requires energy
- Increased demand for O2 and other nutrients
- Increased cerebral blood flow
How is fMRI generated?
Spectroscopy, the use of Nutrients metabolites to build a functional picture
What can be used in fMRI?
Blood flow (BOLD)
Contrast Agents
Arterial Sin labelling
What is the principle of BOLD?
Hb magnetic properties are dependent on whether O2 is bound.
This is because when O2 is bound there is a conformational change in protein structure which changes Hb’s magnetic properties.
What are the two magentic states of Hb oxygen binding called?
Hb-O2 = Isomagentic Hb = Paramagentic
Write some notes on Hb-O2;
Dimagnetic Hb-O2 has similar properties (isomagnetic) with the surrounding tissues
- Lower magnetisation
- No magnetic field inhomogenities
Write some notes on Hb magnetisation;
Without O2 Hb appears paramagentic compared with the surrounding tissues
- Higher magnetisation
- Field inhomogenities
- Enhanced T2 decay
What sort of effect is BOLD?
Bold is a T2 effect
Describe how BOLD works;
H2O surrounding the Hb has shorter T2 than H2O around H2O (because of the magentic field effect H effect)
What is T2?
Dephasing effect due to differences in local magnetic fields causing differences in precessor Hz, which means the nuclie are going faster or slower than their surrounding mates = signal loss = T2 signal loss
Deoxygenated Hb cause this to occur faster therefore increased rate of signal loss (T2)
This is T2* not intrinsic T2 of the tissue
What creates and removes T2*
Gradient imaging creates T2*
Spin echo removes T2*
Describe BOLD and T2 signalling from active neurons
Neuron Regional O2 demand (increased) Cerebral Blood Flow (increased) Hb (decreased) vs HbO2 (increased) Magentic susceptibility (decreased) T2* (increased) Signal (increased)
Describe active neuron changes in T2 response;
- Increased biomagnetic state
- Increased T2* (~3%)
- Must do several experiments and average
Describe the bold response in an active neuron;
Stimulus (neuron activates)
- Initial dip in BOLD measurement (No increase in blood flow, and O2 is used up
- Increase and overshoot (excess blood)
- Levels
- Post stimulus undershoot
How long does the BOLD response take?
~3 seconds, therefore problem is that we are not seeing the electrical signal , we observe the downstream physiological response
What are the practical considerations for BOLD?
- Not a direct measure of brain activity
(input not output) - Good spatial resolution but poor temporal resolution
- Blood flow response ~5sec (slow)
- Needs to be compared to rest state
- Can combine with other modalities i.e EEG, MEG
- No anatomical information
- can be overlaid with CTs
What is the typical procedure for fMRI?
Typical repated multiple times
- fMRI paradigms
- 30 sec active, 30 sec rest
- Statistical inference
