MRI
= non invasive medical imaging technique used in radiology that produces detailed images of the anatomy and the physiological processes inside the human body without using ionising radiation unlike X-rays
Including organs, bones, muscles and blood
Using a large magnet and radio waves
= Diagnosing medical condition
= Planning a course of treatment
Structural MRI
T1 weight, T2 *
sMRI scan sequences are volumetric meaning
that measurements can be made of specific
brain structures to calculate volume of tissue.
Structures of interest
* thalamus
* substantia nigra pars compacta
* locus coeruleus
Functional MRI
→ Functional connectivity
- fMRI relies on techniques that detect blood
changes in relaxation to brain activity involved during the execution of various tasks or when the subject is at reste (rs-fMRI) - BOLD (Blood-Oxygen-Level-Dependent) contrast when nerve cells are active they consume glucose and oxygen
→ Local increase in blood flow
→ Hyperfusion of the local tissue (more oxygen provided than needed)
→ Blood with and without oxygen have different magnetic susceptibility
Diffusion MRI
→ Anisotropy fraction
- dMRI maps the structure and integrity of major water tracts in 3D using Diffusion Tensor Imaging (DTI) because water molecules tend to diffuse along brain white matter fibers
- Brain’s wiring allows us to understand how different brain regions are connected & how diseases affect white matter and cause neurological problems.
→ Matters
- white matter: transport of nerve impulses (fibers) in PD : connectivity changes in white matter
- grey matter: information processing (neurons)
R2* maps
Maps R2* = 1/T2* Transverse relaxation rate
T2 = spin-spin relaxation time
T2* = function of T2 ; magnetic field inhomogeneities
MRI= measures how long the tissue signal takes to decrease after being excited by a magnetic field
- If tissue contains iron → signal decreases faster
- measurement: signal decrease rate measurement
R2* hight if iron increase and speed decrease
→ Images obtain at different times
→ Obtain a map value where each area has an R2* value
Subthalamic stimulation STS
= High frequency electrical stimulation of the subthalamic nucleus
-High dopa-sensitivity leads to the induction of more severe motor complications which is effectively treated by STS
Exploratory study with an unbiased approach but given what si in the literature we can expect to observe :
1) sMRI
Substantia nigra and striatum shrinkage
SN: death of the dopaminergic neurons of the SN so we expected there is less matter
Striatum: there is a projection of the dopaminergic neurons towards the striatum so if there is a neuronal death there will be necessary be less matter in the striatum
Exploratory study with an unbiased approach but given what si in the literature we can expect to observe :
2) fMRI
Modification of functional connectivity between different areas of interest such as striatum and motor cortex
- because it’s known that the dopaminergic neurons of the SN pass through the striatum before projecting into the motor cortex which causes motor activity
- if less dopamine reaches the striatum to go into the motor cortex, there is a loss of functional connectivity
Exploratory study with an unbiased approach but given what si in the literature we can expect to observe :
3) dMRI
Modification of the diffusion of the nigro-striatal pathway to the motor cortex
Benefit: see precisely the changes in the path of the fibers what fMRI doesn’t allow to know what is due to the loos of connectivity
(For example: a demyelination of axons or neuronal death)
Exploratory study with an unbiased approach but given what si in the literature we can expect to observe :
4) R2* maps
Iron accumulation
- physiopathologically, the PD is characterized by a neuronal death but also an accumulation of iron in the substantia nigra
- so it’s interesting to look at this criterion in relation to dopa-sensitivity
